Nos tutelles



Accueil > Événements > Séminaires


publié le , mis à jour le

Friday 1 December 2017

Title : E=MC2 and the negative pressure of dark energy

Speaker : Bernard Castaing (LEGI Grenoble)

Abstract : (Common seminar with ENS de Lyon Physics) According to recent observations, the Dark Energy would represent 70% of the content of our Universe. The most popular way to account for this Dark Energy make use of the Cosmological Constant introduced by Einstein. However, some uncertainty remain about the theoretical value this constant should have. Alternate approaches have to introduce a fluid whose pressure would be negative. In this paper, we present unusual particles which could form a gaz with positive thermodynamical pressure, while presenting all the gravitational characteristics of Dark Energy.


Friday 17 November 2017

Title : To be announced.

Speaker : Jan J. Ostrowski (CRAL)

Abstract : To be announced


Friday 10 November 2017

Title : Getting ready for JWST with new-generation spectral models and interpretation tools

Speaker : Jacopo Chevallard (IAP)

Abstract : The James Webb Space Telescope (JWST), scheduled for launch in October 2018, will open a largely unexplored observational window at near- to mid-infrared wavelengths (0.6-28 micron). In particular, the Near-InfraRed Spectrograph (NIRSpec) on board JWST will allow the detection of standard UV and optical emission lines out to redshift 10 and beyond for large samples of galaxies. This will provide unique insight into, for example, the star formation activity and early chemical enrichment of galaxies, the co-evolution of galaxies and AGNs, the impact of stellar and AGN feedback, and the main drivers of cosmic reionization. One of the first large programs to exploit the new observational capabilities of JWST will be the NIRSpec Guaranteed-Time-Observations (GTO) program, which includes a multi-layered survey in Multi-Object-Spectroscopy (MOS) mode targeting 104 galaxies at 1.5 < z < 10, as well as an Integral-Field-Unit (IFU) survey targeting 60 galaxies and AGNs out to z 9.

In this talk, after a short review of JWST and the NIRSpec GTO program, I will present a set of physically-motivated simulations of NIRSpec and NIRCam (the Near-Infrared Camera on board JWST) observations, which we have used to optimise the survey design, and which will be made available to the community on time for the preparation of Cycle-1 General-Observer (GO) observations (call for proposals on 30/11/2017). I will also discuss a new set of state-of-the-art population synthesis+photoionization models, developed in collaboration with the NEOGAL team at the IAP, tailored to the interpretation of high-redshift galaxy observations. I will show how these models, incorporated into our new-generation analysis tool "Beagle", successfully reproduce a broad range of UV and optical features from high-quality spectra of nearby galaxies, as well as the high-ionization UV lines observed in galaxies at z > 6. The ability to self-consistently measure the physical properties of gas and stellar populations across a broad range of redshifts and stellar masses will be crucial to maximise the scientific return of JWST and gain unique insight into the evolution of galaxies across cosmic times.


Friday 20 October 2017

Title : To be announced.

Speaker : John Cannon (Macalester College)

Abstract : To be announced


Friday 13 October 2017

Title : To be announced.

Speaker : Johann Cohen-Tanugi (LUPM, Montpellier)

Abstract : To be announced


Friday 30 June 2017

Title : Statistical isotropy of the Cosmos from Planck.

Speaker : Tarun Souradeep (IUCAA)

Abstract : The widely heralded and remarkable progress in cosmology leading to the emergence of a ’standard cosmological model’ has relied on certain key assumptions at various levels. I briefly review work along multiple facets, largely, from the research program of my group at IUCAA, that have all attempted to adopt an agnostic approach in drawing cosmological inference independent of such assumptions, in the context of the exquisite observations of the Cosmic Microwave Background CMB anisotropy by Planck. I dwell on the enigmatic Cosmic Hemispherical Anisotropy that continues to challenge our understanding of the Universe.


Friday 23 June 2017

Title : The Square Kilometre Array

Speaker : Jeff Wagg (SKA)

Abstract : Building on the major scientific achievements of the current generation of centimetre and metre-wavelength telescopes, phase 1 of the Square Kilometre Array (SKA1) will be the next global radio astronomy observatory. It is currently being designed by engineers and scientists from around the world, with the aim of beginning full science operations in the 2020s. SKA1 will conduct key tests of general relativity through surveys and timing of pulsars in our Galaxy. It will help answer fundamental questions related to the evolution of atomic Hydrogen in the Universe, from the present day back to the Cosmic Dawn. I will give an overview of the wide breadth of science objectives that could be addressed with SKA1, along with a description of the planned telescopes and the sites that will host them.


Friday 9 June 2017

Title : The self induced secular evolution of gravitating systems

Speaker : Christophe Pichon (IAP)

Abstract : Since the seminal work of Einstein, physicists have understood in the context of kinetic theory how ink slowly diffuses in a glass of water. The fluctuations of the stochastic forces acting on water molecules drive the diffusion of the ink in the fluid. This is the archetype of a process described by the so-called fluctuation-dissipation theorem, which universally relates the rate of diffusion to the power spectrum of the fluctuating forces. For stars in galaxies, a similar process occurs but with two significant differences, due to the long-range nature of the gravitational interaction : (i) for the diffusion to be effective, stars need to resonate, i.e. present commensurable frequencies, otherwise they only follow the orbit imposed by their mean field ; (ii) the amplitudes of the induced fluctuating forces are significantly boosted by collective effects, i.e. by the fact that, because of self-gravity, each star generates a wake in its neighbours. In the expanding universe, an overdense perturbation passing a critical threshold will collapse onto itself and, through violent relaxation and mergers, rapidly converge towards a stationary, phase-mixed and highly symmetric state, with a partially frozen orbital structure. The object is then locked in a quasi-stationary state imposed by its mean gravitational field. Of particular interests are strongly responsive colder systems which, given time and kicks, find the opportunity to significantly reshuffle their orbital structure towards more likely configurations. This presentation aims to explain this long-term reshuffling called gravity-driven secular evolution on cosmic timescales, described by extended kinetic theory. I will illustrate this with radial migration, disc thickening and the stellar cluster in the galactic centre.


Friday 2 June 2017

Title : Dusty spirals triggered by shadows in protoplanetary disks

Speaker : Nicolás Cuello (Pontifica University)

Abstract : Spirals waves form in protoplanetary disks (PPDs) around young stars due to different physical mechanisms : planet torques, gravitational perturbations and illumination effects. Although recent near-infrared and sub-millimetric observations revealed astonishing spiral-shaped features in PPDs, there is an active discussion to understand how they formed. Regardless of their origin, spirals are characterized by a local increase in pressure, which translates into radial and azimuthal asymmetries in the gas and in the dust distribution. In this work, we focus on the spirals formed in the gaseous phase due to shadows cast at fixed disk locations (Montesinos et al. 2016). In particular, we study their effect on the dynamics of the dust particles in PPDs. Our main finding is that dust particles of different sizes and densities are efficiently trapped inside these shadow-triggered spirals. We also perform radiative transfer calculations to model the thermal emission of the dust, which reveal that the resulting dust structures are in good agreement with the recent observations. Finally, we emulate millimetric-wavelength observations and find that these dusty spirals are detectable with ALMA in principle. Interestingly, the detection of the spirals depends on the size of the inner disk cavity. Given that these dusty features lower the relative velocities among solids, they constitute "sweet spots" for dust growth and result in an efficient mixing of different dust species. These aspects will be discussed in the context of planet formation.


Friday 19 May 2017

Title : Etats des lieux de la région transneptunienne avant l’ère JWST

Speaker : Aurélie Guilbert-Lepoutre (UTINAM)

Abstract : Les objets transneptuniens (OTNs) sont les parents de nombreuses populations de petits corps dans le système solaire : comètes, Centaures, et sans doute aussi une partie des astéroïdes. Au moment de leur découverte, ils ont fourni des diagnostiques qui ont permis de revoir complètement notre paradigme du système solaire, passant d’une vision très statique à une vision selon laquelle les planètes auraient migré pour former les structures dynamiques observées aujourd’hui. Pourtant, étant les objets les plus distants, étudier leurs propriétés physiques (qui permettraient de confirmer les scenarii d’évolution du système solaire) et leur lien avec les autres populations de petits corps est encore aujourd’hui extrêmement difficile. Les études photométriques et spectroscopiques effectuées depuis le sol et l’espace ont de plus dressé un tableau bien difficile à interpréter. Au cours de ma présentation, j’établierai un état des lieux de nos connaissances actuelles sur la région transneptunienne, en lien avec les résultats de missions spatiales telles que NASA/New Horizons et ESA/Rosetta, ainsi que des grandes avancées que nous espérons faire avec le futur télescope spatial JWST.


Friday 12 May 2017

Title : Dark energy without dark energy : Observational tests and theoretical challenges

Speaker : David L. Wiltshire (University of Canterbury)

Abstract : I will give an overview of the timescape cosmology. It is assumed that inhomogeneities - voids, walls and filaments - modify the average background geometry of the universe, which is no longer a simple solution of Einstein’s equations with homogeneous dust. To obtain a viable phenomenology without dark energy, I provide a framework for interpreting Buchert’s backreaction formalism, by revisiting fundamental issues relating to the definition of gravitational energy in a complex geometry.

Cosmic acceleration is realized as an apparent effect due both to backreaction and the calibration of the clocks of observers in gravitationally bound structures relative to the time parameter that best describes the average statistical evolution. The cosmic coincidence problem is solved directly in relation to the growth of the void fraction.

Predictions of the timescape phenomenology are very close to the standard cosmology, but with differences which can be tested. I will outline current observational constraints, future tests (e.g., with the Euclid satellite), and also theoretical challenges that need to be overcome for backreaction models to fully compete with the Lambda Cold Dark Matter cosmology.


Friday 05 May 2017

Title : Planetesimal Formation through the Streaming Instability

Speaker : Chao-Chin Yang (Lund Observatory)

Abstract : The formation of kilometer-scale planetesimals is an important stage in the course of planet formation around young stars. It is faced with several major barriers, though. Direct dust growth by coagulation is limited, up to mm to cm in size, due to inefficient sticking, bouncing, and fragmentation at collision. Even if the solid particles manage to grow past cm in size, they continually lose angular momentum to their surrounding gas due to constant head wind, leading to rapid orbital decay to the star. One promising mechanism for circumventing these barriers is the streaming instability, in which the solids actively participate in the particle-gas dynamics to concentrate themselves to high density, leading to direct gravitational collapse and the formation of planetesimals.

I will review our current understanding of the streaming instability and planetesimal formation. Specifically, how the instability operates, under what conditions it drives strong concentration, the initial mass function of the resulting planetesimals, and its possible interaction with turbulent gas will be examined. Unexplored topics that require further study will also be discussed.


Friday 14 April 2017

Title : Dwarf Galaxies at the Peak Epoch of Star Formation

Speaker : Brian Siana (UC Riverside)

Abstract : Low mass galaxies become more and more prevalent in the early universe, and are thought to be responsible for a number of important phenomena (e.g. reionization and enrichment of the intergalactic medium). I will present our work determining the number density of low mass (6 < log(M*) < 9) galaxies at 1 < z < 3 behind strong lensing clusters using deep near-ultraviolet imaging with the Hubble Space Telescope. I will finish by discussing a number of current investigations into the dust properties, star formation histories, and Lyman continuum escape fractions of these galaxies to foster further discussion during my stay at the Observatory.


Friday 07 April 2017

Title : Dust evolution in circumstellar disks

Speaker : Joanna Drążkowska (University of Zurich)

Abstract : There are more than 3000 known exoplanets and statistical analysis show that every star in our Galaxy should have at least one planet on average. However, the process of planet formation remains poorly understood. Early stages of this process can be constrained thanks to the interferometric observations of circumstellar disks and there is evidence that they have to do with evolution of small solid grains : the protoplanetary dust. In this talk, will give a broad introduction to the protoplanetary dust evolution field, including key physical processes, possible modelling techniques as well as the growth barriers problem. I will also discuss possible scenarios of overcoming the growth barriers and forming planetesimals and planets. I will present my recent work on planetesimal formation in so-called pebble pile-ups and highlight importance of the water snow-line in triggering planet formation.


Wednesday 22 March 2017

Title : The Evanescent wave coronagraph : Principle, Theoretical performance and preliminary results

Speaker : Christophe Buisset (NARIT, Thailand)

Abstract : NARIT is a leading organization in Thailand and South-East Asia based in Chiang Mai. In particular, NARIT manages the Thai National Observatory located at a 2,457 m site on Doi Inthanon, with the new 2.4-meter Thai National Telescope (TNT) which is currently the largest in South East Asia. NARIT also comprises an optical laboratory which includes all the essential equipment to perform cutting edge research in optics. During this seminar, I will describe and present the preliminary the results obtained on the so-called “Evanescent wave coronagraph” (EvaWaCo). This coronagraph is a “band-limited” Lyot coronagraph which mask involves the frustrated total internal reflection phenomenon to produce the coronographic effect. The first part will be dedicated to the description of this coronagraph and to the calculation of the mask transmission. In particular, I will demonstrate that this mask transmission adapts itself to wavelength due to the evanescent wave properties. The second part will be focused on the results of the experimental demonstration of the principle. I will describe the setup we have used and the results we have obtained. The third part of this talk will be focused on the future development of this setup : installation of apodization mask to improve the rejection in the Airy ring area.


Friday 17 March 2017

Title : The Life and Death of Star-Forming Molecular Clouds.

Speaker : Sam Geen (University of Heidelberg)

Abstract : Stars are born from dense gas in molecular clouds. These stars produce energy in radiation, winds and supernovae, which drive outflows and reduce or stop additional star formation in the cloud. In this talk I will present our recent results from radiative MHD simulations of star-forming clouds. In particular I will focus on the effectiveness of each of the stellar processes in driving outflows in a cloud environment, and on how observations of local star-forming clouds can be interpreted in comparison to simulations.


Friday 10 March 2017

Title : Cosmology with large spectroscopy surveys

Speaker : Anand Raichoor (EPFL)

Abstract : Cosmology has now entered a new era with the advent of large, international experiments to constrain the dark energy. Baryon Acoustic Oscillation (BAO), one of the main cosmological probe for dark energy, requires the spectroscopic observation of a large number (10^5-10^7) of galaxies. I will present the SDSS/eBOSS survey, the currently largest on going BAO experiment, with focusing on the z 0.8 tracer, the Emission Line Galaxies (ELGs). I will then introduce the 4MOST/Cosmology survey, 4MOST being a major ESO massive spectroscopic survey. Additionnaly to BAO measurements, the 4MOST/Cosmology survey will enable key cosmological projects, with cross-correlating with other experiments, such as weak lensing ones.


Friday 24 February 2017

Title : The MESSIER surveyor : lifting the veil on the ultra-low surface brightness universe

Speaker : David Valls-Gabaud (Paris Observatory)

Abstract : The S-class MESSIER satellite has been designed to explore the extremely low surface brightness universe at UV and optical wavelengths. The two driving science cases target the mildly- and highly non-linear regimes of structure formation to test two key predictions of the LCDM scenario : (1) the detection of the putative large number of galaxy satellites, and (2) the identification of the filaments of the cosmic web. The satellite will drift scan the entire sky in 6 bands covering the 200-1000 nm wavelength range to reach the unprecedented surface brightness levels of 34 mag/arcsec^2 in the optical and 37 mag/arcsec^2 in the UV. Many important secondary science cases will result as free by-products and will be discussed in some detail, such as the luminosity function of galaxies, the contribution and role of intracluster light, the cosmological background radiation at UV and optical wavelengths, the molecular hydrogen content of galaxies at z=0.25, time domain studies of supernovae, GRBs and tidal disruption events, the chemical enrichment of the interstellar medium through mass loss of red giant stars and the accurate measure of the BAO scale at z=0.7 with over 30 million galaxies detected in Lyman-alpha at this redshift. It will provide the astronomical community the first space-based reference UV-optical photometric catalogue of the entire sky, and synergies with GAIA, EUCLID and WFIRST will also be discussed. Technical issues will likewise be addressed for possible improvements on the current design.


Friday 20 January 2017

Title : Science in the SKA era

Speaker : Mamta Pommier (CRAL)

Abstract : The Square Kilometre Array (SKA) is the 21st century world’s largest radio telescope which will answer fundamental questions about the origin and evolution of the Universe. It is an aperture synthesis telescope with a collecting area of up to one million square metres spread over at least 3000 km, providing a sensitivity 50 times higher than the Expanded Very Large Array and an instantaneous FOV of several 10s of degrees and above. Five key science projects for the SKA have been identified, focused on unsolved problems in fundamental physics, cosmology, galaxy evolution, planet formation and astrobiology. In addition the vast increase in sensitivity provided by the SKA will also almost certainly lead to the discovery of new and totally unexpected celestial phenomena. In this talk, I will describe the main features of the SKA, outline plans for the final design and phased implementation of the telescope, On-going key projects with the SKA path finders/precursors around the world and highlight new results on extra galactic projects (Clusters, galaxies).


Friday 16 December 2016

Title : Kinematics of COSMOS star-forming galaxies over the last billion years

Speaker : Debora Pelliccia (LAM, Marseille)

Abstract : In the local Universe it exists a tight relation between galaxy morphology and other physical parameters, like the galaxy internal kinematics, but at higher redshift it is not clear if this relation still exists. The galaxy kinematics is one of the best tool to study the physical processes that govern the galaxy formation, since it traces the galaxy internal distributions of luminous and dark matter and their evolution with time. I will present our survey HR-COSMOS aimed to obtain the first statistical and representative sample to study the kinematics of star-forming galaxies in the treasury HST/ACS COSMOS deep field at redshifts between 0 and 1.2. About 800 emission-line galaxies were observed with the multi-slit spectrograph ESO-VLT/VIMOS in high-resolution spectral mode (R=2500). I performed a kinematic analysis of the sub-sample of 82 galaxies in the highest redshift range, 0.75 and 1.2, and I established the stellar-mass Tully Fisher scaling relation at z approximately 0.9. Using the same methodology and data analysis, I analyzed the sub-sample of 186 galaxies at the lowest redshift range 0.01 and 0.375 and I confirmed beyond a doubt the non evolution of the stellar mass Tully-Fisher relation since the last 8 billion year. I have moreover derived the dynamical masses of those two sub-samples and investigated the comparison with the stellar masses as function of time and galaxy stellar mass.


Friday 09 December 2016

Title : Spectral Energy Distribution of Galaxies : a tool to probe rapid star formation quenching.

Speaker : Laure Ciesla (CEA, Paris)

Abstract : With the wealth of ancillary data available in large domains of the spectrum (GALEX, HST, CFHT, Spitzer, Herschel, etc), panchromatic studies of galaxies are thus one of the keys to understand how galaxies evolved since their formation. After describing the assets of spectral energy distributions (SED) analysis and explaining how we can model them, I will show with a concrete case how we can identify and characterize galaxies than underwent a rapid star formation quenching with a sample of well-known local galaxies, the Herschel Reference Survey. This sample contains galaxies from the field but also from the dense environment of the Virgo cluster. From this pilot study, I will go to higher redshifts in order to blindly identify sources that have just been quenched using CANDELS/GOODS-South data and try to identify possible causes for this.


Friday 02 December 2016

Title : Hubble Frontiers Fields : A new Era for Gravitational Lensing & Cosmology

Speaker : Mathilde Jauzac (Durham University)

Abstract : The Hubble Frontier Fields (HFF) initiative constitutes the largest commitment ever of HST time to the exploration of the distant Universe via gravitational lensing by massive galaxy clusters. This program devotes 140 orbits of HST time to deep imaging observations of each of six cluster lenses reaching m 29 (AB) uniformly from the optical to the near-infrared. These clusters were chosen for their strong lens properties, and are all highly disturbed objects, showing major and minor merging on-going processes, making them ideal targets to trace the Cosmic Web assembly. While combining strong and weak-lensing regimes to map the total mass with X-rays observations of the hot gas and spectroscopy of cluster galaxies to look at their direction of motion, we can thus study the dynamical scenarios in place within these massive galaxy clusters, and trace the sub-structures engaged. I will present a new multi-wavelength picture of the first two HFF clusters. The depth of these dataset makes these clusters amazing Cosmic Telescopes, but also enables us to get an unprecedented understanding of the cluster physics. I will present a comparison of the dark matter, light and gas distributions, that will lead us to the distribution of substructures within the MACSJ0416, and Abell 2744 vicinities. Finally I will discuss the different clues that these observables provide on the evolution processes in massive galaxy clusters, and more globally on Lambda-CDM. If time permits, I will discuss one of the most beautiful HFF discovery, SN Refsdal, the first multiply-lensed supernovae discovered by Kelly et al. (2015) in MACSJ1149. This particular event was a unique chance to test our mass modeling techniques, and improve our methods. I will thus give an overview of the lensing community’s work on SN Refsdal, and discuss the appearance of its last multiple image.


Friday 25 November 2016

Title : What do numerical simulations tell us about the stellar magnetic field ?

Speaker : Laurène Jouve (IRAP, Toulouse)

Abstract : In this talk, we will review some aspects of the stellar magnetism and in particular what numerical simulations tell us about the physical processes underlying the observations. In cool Sun-like stars, a convective dynamo is thought to be responsible for the presence and evolution of magnetic fields. One important aspect of stellar dynamos is the possible presence of magnetic cycles, whose caracteristics depend on the stellar parameters. Another question is how magnetic flux is transported from the interior to the surface to produce starspots and whether those spots play a role in the dynamo process. In hotter and more massive stars, the manifestations of a magnetic field aredrastically different. Only 5 to 10% of those stars seem to exhibit a strong dipolar magnetic field while most of the other stars are thought to possess a weak complex field. The reason for the existence of this magnetic dichotomy could be linked to the development of magnetic instabilities. We will show in this talk that numerical simulations may help us tackle several of the above key questions about stellar magnetism.


Friday 18 November 2016

Title : Galaxy Evolution in the first billion years with the JWST

Speaker : Andrew Bunker (Oxford University)

Abstract : Over the past ten years, we have identified star-forming galaxies at z>6, within the epoch of reionzation, using the Lyman break technique with deep fields from the Hubble Space Telescope and ground-based imaging. From the inferred rest-frame UV luminosity function, we can address the potential role of star formation in the reionization of the Universe. However, spectroscopic follow-up has been challenging, with Lyman-alpha (the main feature accessible from ground-based observations) often weak or absent. James Webb Space Telescope will revolutionise this - the near-infrared spectrograph NIRSpec works out to 5microns with huge multiplex, sampling the rest-frame optical out to z approximately 6, and potentially obtaining redshifts from [OII] out to z approximately 12 (and much higher with the Lyman break). I will describe the Guaranteed Observing Time programme with my colleagues on the NIRSpec instrument science team. Through emission lines and SED fitting, these observations have the potential to chart the evolution of the star formation rate, dust extinction, metallicity and stellar initial mass function within the first billion years.


Friday 4 November 2016

Title : The Physical Conditions of Galactic Outflows

Speaker : John Chisholm (Geneva University)

Abstract : Strangely, a majority of the baryons in the universe are not in galactic disks ; rather they reside in metal-enriched halos that extend more than 100 kpc from star forming regions. Metal enrichment implies that a portion of the gas in these halos originated in star-forming regions, and has been transported into the halo through galactic outflows. Historically, it has been challenging to observe these galactic outflows because they are diffuse, multiphase structures. Here, I present results from a recent complication of nearby star-forming galaxies with ultraviolet spectra from the Hubble Space Telescope. I use a diverse set of absorption lines to find shallow correlations between the outflow velocity and stellar mass of the host galaxy. Additionally, the equivalent widths of the outflows imply that the observed outflows are photoionized, and I use these photoionization models to estimate the density, metallicity, ionization structure, and size of the outflows. In particular, I find that the outflows are metal-rich and originate much closer to star-forming regions than typically assumed. Finally, I use the line profiles to explore the acceleration of outflow, radial density profile, and the mass outflow rate. I conclude with implications for the enrichment of the circum-galactic medium by these outflows.


Friday 28 October 2016

Title : Galaxy stellar mass assembly from deep imaging surveys

Speaker : Olivier Ilbert (LAM Marseille)

Abstract : A clear and comprehensive picture describing the physical processes which regulate the star formation is still missing in galaxy formation scenario. Stellar mass assembly in galaxies is believed to result from several physical processes such as star formation from in-situ or accreted gas, major or minor mergers, as well as supernovae and AGN feedbacks. The relative contribution and operating timescales of these different processes are still a matter of debate. I will present some observational constrains that we established using large and deep imaging surveys, in particular in the COSMOS field. I will show how these data help us to better constrain the quenching processes and the galaxy growth rate out to z approximately 6.


Friday 14 October 2016

Title : Highlights from GLASS : Probing Galaxy Evolution from Redshift 0 to 8 with Slitless HST Spectroscopy of Lensing Clusters

Speaker : Kasper Borello Schmidt (AIP Potsdam)

Abstract : The Grism Lens-Amplified Survey from Space (GLASS) is a large HST cycle-21 program which targeted 10 massive galaxy clusters all with extensive HST imaging from CLASH and the Hubble Frontier Field Initiative. GLASS has obtained 140 orbits of primary near-infrared WFC3 grism and 140 orbits of parallel optical ACS grism observations. This has resulted in spatially resolved spectroscopy of thousands of galaxies in and behind the 10 galaxy clusters. I will present GLASS and a selection of our most recent results including a census of Ly alpha emission at z>6, spatially resolved star formation maps and metallicity gradients at z approximately 2, and an assessment of the environmental effect on cluster members at z approximately 0.5. These results all rely on high fidelity cluster lens models and have spawned a variety of ground-based follow-up campaigns. I will conclude by highlighting results from our follow-up campaigns which include the detection of the most distant multiply imaged galaxy, the first spectroscopic confirmation of a typical galaxy at z>7.5, and the discovery of [CII] emission from a faint low-metallicity system at z approximately 7.


Friday 7 October 2016

Title : ALMA Observations of the Frontier Fields

Speaker : Franz Bauer (Pontifica University)

Abstract : The Frontier Fields Legacy Program targets six strong lensing clusters with deep HST and Spitzer imaging to detect and characterize the faint background galaxy population, particularly the first galaxies at z between 6-10. Given the wealth of sensitive ancillary data, we initiated an ALMA survey to produce “shallow” roughly 2’x2’ maps of the Frontier Fields at 1.1mm to study intrinsically faint dusty star-forming galaxies (DSFGs) and place constraints on the star formation from a variety of interesting extragalactic source populations (including those at z>6). The first three (A2744, MACS0416, and MACS1149) have now been completed, yielding relatively uniform maps that pinpoint cool dust emission from powerful DSFGs at z>1. I will present a census of the detected objects thus far, as well as some early constraints our team is able to place on the average emission from undetected source populations.


Friday 23 September 2016

Title : Outflows from high redshift quasars : winds and radiation

Speaker : Tiago Costa (Leiden)

Abstract : The detection of bright quasars at redshift 6 - 7 indicates that supermassive black holes of masses in excess of one billion solar masses were already in place when the Universe was less than 1 Gyr old. The rapid growth of such massive systems puts constraints on models of black hole growth and feeding, which must proceed efficiently despite AGN and stellar feedback processes. In this talk, I will present a suite of cosmological hydrodynamic simulations performed with a variety of codes (AREPO/RAMSES-RT/GADGET3) addressing black hole growth and AGN feedback in a range of cosmological environments at z = 6, ranging from average to the most over-dense regions in the Universe. I will argue that the formation of bright quasars at z = 6 is possible provided the black holes grow in very massive haloes, where gas inflow is so efficient that the black holes are able to sustain growth at their Eddington rate. Focus will then be placed on the effects of AGN radiation and winds on the medium surrounding the accreting black holes with an emphasis on the generation of large-scale galactic outflows. Specifically, I will argue that if AGN are to regulate their growth and significantly suppress star formation in their host galaxies, the outflows they launch must be energy-, as opposed to momentum-driven. The theoretical findings will be compared with existing observational detections of molecular outflows at z > 6.


Friday 16 September 2016

Title : VLT/SPHERE deep planet search in the transitional disk of SAO 206462

Speaker : Anne-Lise Maire (MPIA)

Abstract : The SAO 206462 (HD 135344B) disk is one of the few known transitional disks showing asymmetric features in both scattered light and thermal emission. Giant planets in formation have been suggested for accounting for the disk morphology. We obtained new high-contrast and high-resolution near-infrared images of the target with the VLT planet finder instrument SPHERE. The spiral arms and the outer rim of the inner cavity are revealed at high sensitivities without the need for image post-processing. We do not detect any close-in companion candidates. As for the derivation of the detection limits on putative giant planets embedded in the disk, we show that the knowledge of the disk aspect ratio and viscosity is critical for the estimation of the attenuation of a planet signal by the protoplanetary dust because of the gaps that these putative planets can open. Given the assumptions on these parameters, the degradation of the mass limits can vary from negligible to several Jupiter masses at separations beyond the disk spiral arms. Our deep detection limits provide new constraints on a few recent predictions of massive planets in the outer disk based on the spiral density wave theory.


Friday 09 September 2016

Title : Correction of apertures discontinuities for the direct imaging of exoplanets and circumstellar disks

Speaker : Johan Mazoyer (STScI)

Abstract : More than 3000 thousands exoplanets have been discovered to date, but only a few have been imaged directly. However, by allowing the observation of circumstellar disks and planets (sometimes simultaneously around the same star, as in the case of β-pictoris), this method is a fundamental tool for the understanding the process of planetary formation. In addition, direct access to the light of the detected objects allows spectroscopy, paving the way to the full chemical analysis of exoplanets’ atmosphere and disks grains. Several coronagraphic instruments are currently observing to images of young Jupiters and/or Kuiper like disks. These instruments use coronagraphs optimized for circular, often un-obstructed apertures. Indeed, the remaining aberrations created by the atmosphere or optics defaults is limiting the contrast at levels far above the ones created by apertures discontinuities (inter-segment gap or secondary mirror mounts). However, the next generation of ground and space based telescopes will have to address the problem of apertures discontinuities in coronagraphy, if we want to obtain images and spectra of earth sized planets or dust grains below the snow line. In this talk I will present my current research at the Space Telescope Science Institute to improve the contrast level of coronagraphs in the presence of apertures discontinuities using deformable mirrors. I will also quickly present my work in the field of the post processing of high contrast images of circumstellar disks.


Friday 22 July 2016

Title : Nebular diagnostics with MUSE

Speaker : Fernando Selman (ESO)

Abstract : I will present a survey of the literature about different emission-line diagnostic for low-redshift galaxies relevant for the MUSE spectral range, with an application for IC3418, one of the prototypical ram-pressure stripped disk galaxies.


Friday 01 July 2016

Title : Two problems of IPT 2016 : Popsicle Stick Cobra and Sultry Day

Speaker : Jeremy Sautel (ENS)

Abstract : International Physicists Tournament is a world-wide competition which involves undergraduate and master students. A list of 17 open physics problems ( was proposed to the different teams this year. Jérémy Sautel, from the team "France - ENS Lyon", which won IPT 2016, will present his team’s work on two topics : 1 - Popsicle Stick Cobra : speed and hight of a popsicle stick bomb creted by a weave made of wood and PVC sticks, and 9 - Sultry Day : how to determine the temperature of a hot plate by optics ?.


Friday 24 June 2016

Title : Deriving the gas and dust structure of transition disks from multi-wavelength observations.

Speaker : Andres Carmona (IRAP, Toulouse)

Abstract : Transition disks are protoplanetary disks that display cavities and gaps in the dust distribution. The origin of those features is matter of intense debate in the literature, particularly because there is the exciting possibility that the dust cavities and gaps could be the signpost of planets inside the disk. One key aspect to distinguish between different proposed scenarios for the origin of transition disks is to determine the content and distribution of the gas in the disk, most specifically the gas inside the dust cavity.
In this talk, I will discuss the strengths and limitations of different observational methods that we use to investigate the gas and dust in protoplanetary disks, and I will present how multi-wavelength (IR to sub-mm) multi-technique (imaging, interferometry, high-resolution spectroscopy) observations coupled to radiative transfer modeling can be used to derive the gas and dust disk’s structure. I will illustrate the discussion with the case studies of two transition disks (HD 135344B and HD 139614). Finally, I will conclude with exposing open issues and future perspectives.


*Monday* 20 June 2016

Title : Energy dependence of aperiodic variability in low mass X-ray binaries

Speaker : Holger Stiele (National Tsing Hua University, Taiwan)

Abstract : Black hole and neutron star X-ray binaries show variability on time-scales ranging from milliseconds to years. While variability on the time scale of years is related to the accretion rate of matter, the origin of variability on short time scales is still under debate. In the last two decades a detailed phenomenological picture of short-term variability in low-mass X-ray binaries has emerged, mainly based on RXTE observations that cover energies above 3 keV. This picture comprises periodic or quasi-periodic variability, seen as spikes or humps in power density spectra, that are superposed on broad noise components. The overall shape of the noise components as well as the occurrence of quasi-periodic oscillations is known to vary with the state of the X-ray binary. Quasi-periodic oscillations can be related to the relativistic precession of the accretion flow, instabilities in the accretion disc, or to nuclear burning on the neutron star surface. We are accomplishing a comprehensive s
tudy of archival XMM-Newton observations of black hole and neutron star low-mass X-ray binaries to investigate the variability properties of these sources at softer energies where the thermal disc component starts to emerge.
Here we present the results of a comprehensive variability study of a sample of black hole X-ray binaries, including a discussion of the energy dependence of the variability. We will discuss the implications of these findings for the picture of the accretion geometry in black hole X-ray binaries.
Regarding neutron star binaries, we performed a variability study of archival XMM-Newton data of 4U 1636-536 and investigated the energy dependence of the low frequency variability. Here we present the results of our waveform analysis and phase resolved spectral investigations of the low frequency variability in this source and discuss implications on the thermonuclear burning mode, neutron star size and equation of state.


*Monday* 13 June 2016

Title : Balancing the Baryon Budget

Speaker : Ann Zabludoff (Arizona)

Abstract : The deep gravitational potential wells of clusters of galaxies should capture fair samples of the total baryon fraction of the Universe, unless other physical processes drive baryons out of clusters. Thus precision measurements of the baryon fraction, particularly as a function of cluster mass, can reveal the history of baryon flux into and out of clusters. How those baryons are then apportioned between stars and intracluster gas---the star formation efficiency---informs models of cluster assembly and massive galaxy evolution, as well as efforts to use the cluster gas fraction to constrain the mass density and dark energy equation of state parameters. Even the partitioning of the stellar baryons alone, in and out of galaxies, tests models of cluster galaxy evolution, as intracluster stars are the final, unambiguous signature of stars stripped from cluster galaxies during tidal encounters. We have discovered that intracluster stars are a significant part of the stellar baryons in clusters and poorer groups of galaxies. I will present new work characterizing the properties of this previously unexplored component, as well as the consequences for the cluster baryon budget and its relationship to the Universal value.


Friday 10 June 2016

Title : Growing black holes in growing galaxies

Speaker : Marta Volonteri (IAP)

Abstract : Massive black holes, weighing millions to billions of solar masses, inhabit the centers of today’s galaxies. Black hole masses typically scale with properties of their hosts, such as bulge mass and velocity dispersion. The progenitors of these black holes powered luminous quasars within the first billion years of the Universe. The first massive black holes must therefore have formed around the time the first stars and galaxies appeared, and then evolved along with their hosts for the past thirteen billion years. I will discuss some aspects of the cosmic evolution of massive black holes, from their formation to their growth and how different physical processes shape the relation between black holes and galaxies.


Friday 03 June 2016

Title : The impact of energetic phenomena on the evolution of galaxies and
their black holes — a theoretical perspective

Speaker : Michaela Hirschmann (IAP)

Abstract : We present new insights from advanced semi-analytic models (SAMs) and modern cosmological simulations indicating a much more complex interplay between different feedback processes and galaxy/black hole (BH) growth than traditionally anticipated. Both, SAMs and large-scale simulations are successful in producing realistic global BH populations e.g. capturing the observed anti-hierarchical trend in BH growth. For that, stellar feedback can play e.g. an important role for the gas accretion onto low-mass BHs at low redshifts. Employing advanced high-resolution zoom simulations, we can confirm and extend previous results that different AGN feedback mechanisms can strongly affect the kinematics and stellar content of massive galaxies, e.g. by regulating their insitu star formation. But contrary to the traditional picture, galactic winds from SN explosions and massive stars are also crucial for the stellar mass assembly of massive galaxies, not only significantly delaying early insitu star formation but also strongly affecting the accreted stellar populations at large radii. Successes and limitations of most recent models and perspectives for future improvements will be given.


Friday 27 May 2016

Title : The SCUBA-2 Cosmology Legacy Survey

Speaker : Dan Smith (University of Hertfordshire)

Abstract : I will present my results from using the SCUBA-2 Cosmology Legacy Survey (CLS) 850 um observations over the UKIDSS Ultra-Deep Survey field to study the evolution of the obscured component of the cosmic star formation rate density over 1 < z < 4. The 850 um data benefit from negative k-correction, and as a result, allow us to probe for star formation in a virtually redshift-independent manner, provided that some assumptions about the choice of far-infrared SED are made. Using a mass-complete sample of K < 24 galaxies, classified as star-forming or passive using their UVJ rest-frame colours, we are able not only to study evolution in the so-called star formation rate "main sequence", but also (by convolving the main sequence with the stellar mass function) the contribution of different galaxy populations to the cosmic star formation rate density.
In addition, I will say a few words about the WEAVE-LOFAR survey, which — starting in 2018 — will obtain more than a million spectra of low-frequency selected radio sources, with a wide range of science goals.


Friday 20 May 2016

Title : Observations of inhomogeneity and backreaction : A status report

Speaker : David Wiltshire, University of Canterbury (New Zealand)

Abstract : The possibility of backreaction - that inhomogeneous structures on small (< 100/h Mpc) scales change average cosmic evolution relative to a standard homogeneous isotropic FLRW model - can be tested in different ways.
1. In our own < 100/h Mpc environment the assumption of a FLRW geometry can be tested model-independently. We present very strong Bayesian evidence that the standard CMB rest frame is not the frame in which the spherically averaged variation of the Hubble expansion is minimized. Using large ray tracing simulations in exact solutions of Einstein’s equations we further show that observations are consistent with a differential expansion of space on < 70/h Mpc scales that is not included in the standard cosmology. This may have testable implications for CMB anomalies.
2. On > 100/h Mpc scales detailed models of backreaction are required for cosmological tests. The Timescape Cosmology provides such a phenomenology. We report on the status of observational tests of this model, including the fitting of the acoustic peaks in the CMB anisotropy spectrum. The Timescape Cosmology passes current tests, and can be distinguished from the standard cosmology with future Euclid satellite data. In terms of the CMB we find that the effects of backreaction in the primordial plasma still need to be accounted for to deal with systematic uncertainties of 8-13% in particular cosmological parameters.


Friday 29 April 2016

Title : Dust coagulation with porosity evolution in planet formation

Speaker : Akimasa Kataoka

Abstract : Dust coagulation is the first process of planet formation. Dust grains stick to other grains to form porous dust aggregates. This porosity evolution is a key to understand the planet formation. We have theoretically revealed the overall porosity evolution from micron-sized dust grains to km-sized planetesimals based ; dust grains form extremely porous dust aggregates where the filling factor is approximately 10^-4, and then they are compressed by their collisions, the disk gas, and their self-gravity. The mass and porosity of the final product is consistent with the comets, which are believed to be the remnants of planetesimals. We have also calculated the opacity of the fluffy dust aggregates and discuss possible way to distinguish between compact and fluffy dust aggregates.


Friday 22 April 2016

Title : Atmospheric flows in Stellar to Substellar Atmospheres and their impact on dust formation

Speaker : France Allard (CRAL)

Abstract : Understanding the atmospheric and evolutive properties of brown dwarfs and gas giant exoplanets have been important challenges for modelers around the world since the discovery of the first brown dwarfs in the field (Nakajima et al. 1995) and in the Pleiades cluster (Rebolo et al. 1995) and exoplanet (Mayor & Quéloz 1995). The early studies of brown dwarfs have provided insights into atmospheric physics, with discoveries ranging from cloud formation (Tsuji et al. 1996), methane band (Oppenheimer et al. 1995) and ammonia band (Delorme et al. 2008) formation, to the formation of quasi-molecular KI-H2 absorption (Allard et al. 2007), and to disequilibrium chemistry (Yelle & Griffith 2001). Cloud formation is a primary challenge to the understanding of the spectral transition between the stellar and planetary mass regimes, and play a crutial role for the spectral distribution of very low mass stars and brown dwarfs cooler then 3000K, for biosignature detectability, and for planet habitability. Recently, Crossfield et al. (2014) have demonstrated by Doppler imaging the presence of important surface inhomogeneities at the surface of a close-by brown dwarf (2.0 parsecs) brown dwarf in the absorption bands of carbon monoxyde at 2.2 micrometers. And we have been able to determine by radiation hydrodynamical simulations the formation of laminar flows in these atmospheres with periods of a few hours, that could explain the observed spectral and photometric variability of these objects (Freytag, Allard & Homeier 2016, en prép.). I will review the progress achieved in reproducing the spectral properties of very low mass stars, brown dwarfs/gas giant exoplanets, and review progress in modeling more accurately their atmospheres using Radiation HydroDynamical (RHD) simulations.


Friday 8 April 2016

Title : Shear-driven instabilities and shocks in the atmospheres of hot Jupiters

Speaker : Sebastien Fromang (CEA)

Abstract : Hot Jupiters are giants gaseous planets orbiting very close to their central star. Because of the extreme irradiation they receive, the dynamics of the atmosphere is different from that of the solar system giant planets. In the past few years, general circulation models of the atmosphere of hot Jupiter have shown in particular the existence of a supersonic eastward equatorial jet. Yet these results have mostly been obtained using numerical schemes that filter out vertically propagating sound waves and assume vertical hydrostatic equilibrium,raising the issue of the jet stability and the possible presence of shocks.
In this talk, I will present the results of a series of high resolution simulations specifically designed to address these issues. I will show that the jet is indeed destabilized by two instabilities of Kelvin-Helmoltz type respectively associated with the meridional and vertical velocity shears. These instabilities induce significant flow variability and weak shocks in the upper layers of the atmosphere and may affect their temperature-pressure profile as well as the internal structure of hot Jupiters.


Friday 25 March 2016

Title : Spectroscopic Studies of Star-Forming Galaxies in the Reionization Era

Speaker : Richard Ellis (ESO and UCL)

Abstract : Deep exposures with the Hubble Space Telescope (HST) have provided the primary evidence that star-forming galaxies were present in the first billion years of cosmic history. Sometime during this early period the intergalactic medium transitioned from a neutral gas to one that is fully ionized. How did this `cosmic reionization’ occur and were star-forming galaxies responsible ? Imaging of deep fields with HST’s Wide Field Camera 3 in conjunction with Spitzer photometry and Keck spectroscopy has provided important new insight into understanding when reionization occurred and the role of early galaxies in the process. Recent Planck results on the optical depth of electron scattering to CMB photons provide complementary information. I will review this rapid progress in our understanding of the last missing piece in our overall picture of cosmic history and discuss the remaining challenges ahead of future facilities such as JWST and E-ELT.


Friday 18 March 2016

Title : In-situ energetic particle acceleration in young stellar objects : Confronting theory to observations

Speaker : Alexandre Marcowith (Montpellier)

Abstract : Recent observations in some well-observed low to intermediate solar mass young stellar objects conclude to ionization rates that can be several orders of magnitude higher that local interstellar values. Also some non-thermal, likely synchrotron, radiation have been detected from hot spots in association with the object DG-Tau. In these high density column media, such ionization rates appear difficult to explain by only invoking UV or X-ray radiation. Simple energetic arguments show that background interstellar cosmic rays can not penetrate so deep in these environments because they suffer too high ionization losses. We have conducted calculations of in-situ energetic particle acceleration adapting the diffusive shock acceleration theory developed in the context of fast moving supernova remnant shocks to the case of partially ionized environments found in young stellar environments. We show that particle acceleration is possible in some sites, namely, along the jet and at the termination shock and at the shock stellar surface. We have obtained particle distribution spectra, accounted for all types of losses, calculated propagated spectra in the hot spot or in the envelope and tested our calculations with respect to the observations. We found a relatively good agreement between theory and observations. Acceleration appears to be an intermittent process, in some conditions the maximum particle energies can be as high as several hundred of GeV or even a few TeV. One may expect that some of these objects to be targets of the future Cherenkov telescope CTA. Some aspect of the model should also be testable using ALMA high angular resolution observations.


Friday 11 March 2016

Title : The Star Formation Activity and Its Diversity of Low-Mass Galaxies at Cosmic Noon

Speaker : Haruka Kusakabe (University of Tokyo)

Abstract : Low-mass galaxies at high redshift play a key role in galaxy formation and evolution as building blocks of more massive galaxies seen at later epochs. I will talk about star formation properties and their diversity of Lya Emitters (LAEs) at z = 2.2, a commonly seen low-mass galaxy population. First, by stacking deep Spitzer/MIPS and Herschel/PACS images for 213 LAEs in the GOODS-South, we find for the first time that LAEs typically have very low IR luminosities less than L3σ = 1.1 χ 10^10 Lsun (3 σ upper limit) and that TIR their attenuation curve is consistent not with the Calzetti curve but with the SMC curve (Kusakabe et al. 2015, ApJ, 800, L29). Second, we divide 604 LAEs in the SXDS field into sub-samples based on the distribution of four UV physical parameters : MUV, β , LLya, and EWLya,r. Then, we calculate the halo mass and stellar population parameters for each sub-sample from clustering analysis and SED fitting with SMC curve, respectively. While two thirds of the entire sample are on the SFMS with stellar masses of roughly 10^9 Msun, the remaining one third, which are the lowest-mass objects in our sample, are forming stars burstly with stellar masses of roughly 10^7 Msun with SFRs exceeding the baryon accretion rates of their hosting halos. These low-stellar mass LAEs may be in initial forming phases at cosmic noon (Kusakabe et al. in prep).


Friday 4 March 2016

Title : Can we describe galaxy formation with a merger tree and a set of coupled, ordinary differential equations ?

Speaker : Peter Mitchell (CRAL)

Abstract : Semi-analytic galaxy formation models are an established tool for describing how galaxies assemble their stellar mass within the context of hierarchical structure formation. With the advent of well calibrated hydrodynamical simulations of cosmologically representative volumes, it is timely to critically review the assumptions and resulting predictions of semi-analytic models. In this seminar, I will focus primarily on the relationship between stellar mass and halo mass assembly, and the importance of accurately modelling gas cycling as a consequence of supernovae explosions. I will also discuss the implications of apparent tensions between observational estimates of stellar masses and star formation rates.


Friday 26 February 2016

Title : Image Restoration Methods : From Numerical Optimization Strategies to Blind Deconvolution and Shift-variant Deblurring, which will include three subtopics

Speaker : Rahul Mourya (Université Jean Monnet, Saint-Etienne)

Abstract :
i) Introduction describing image restoration problem as optimization problem
ii) An approach for astronomical image restoration via image decomposition
iii) Shift-variant Image Deblurring.


Friday 12 February 2016

Title : Dark energy as emerging average negative curvature : low-redshift observational challenges

Speaker : Boud Roukema (Torun, Poland)

Abstract : The hypothesis that recently emerging average negative curvature provides the main physical parameter that replaces dark energy is at least seven years old. This hypothesis offers a physically conservative alternative to the cosmological constant and "new physics" hypotheses, since the physical assumption is to apply general relativity during the structure formation epoch rather than assume rigid co-moving space (one of the assumptions underlying LambdaCDM). A brief pedagogical overview of the standard model and of scalar averaging will be presented. Low-redshift observational parameter values implied by a bi-domain implementation of scalar averaging will be derived. Observational prospects for measuring these parameters will be discussed. Conceptual analogies will be presented as tools for shifting from Euclidean co-moving space intuition to curved space intuition.


Friday 29 January 2016

Title : An overview of the mid-infrared spectro-interferometer MATISSE : science, concept, and current status

Speaker : Alexis Matter (Nice)

Abstract : MATISSE is the second generation mid-infrared spectrograph and imager for the Very Large Telescope Interferometer (VLTI) at Paranal. This new interferometric instrument will allow significant advances by opening new avenues in various fundamental research fields : studying the planet-forming region of disks around young stellar objects, understanding the surface structures and mass loss phenomena affecting evolved stars, and probing the environments of black holes in active galactic nuclei. As a first breakthrough, MATISSE will enlarge the spectral domain of current optical interferometers by offering the L & M bands in addition to the N band. This will open a wide wavelength domain, ranging from 2.8 to 13 microns, exploring angular scales as small as 3 mas (L/M band) / 10 mas (N band). As a second breakthrough, MATISSE will allow mid-infrared imaging - closure-phase aperture-synthesis imaging - with up to four Unit Telescopes (UT) or Auxiliary Telescopes (AT) of the VLTI. Moreover, MATISSE will offer a spectral resolution range between R approximately 30 to approximately 5000. Here, i introduce the physical concept of MATISSE including a description of the signal on the detectors and an evaluation of the expected performances. I also present the potential of MATISSE for the study of the inner regions of protoplanetary disks, which is one of the main science cases that has driven the instrument design and motivated several VLTI upgrades (GRA4MAT & NAOMI). Finally, i discuss the current status of the instrument, which is entering its testing phase, and the foreseen schedule for the next two years that will lead to the first scientific exploitation in 2018.


Friday 22 January 2016

Title : The assumptions we make when we talk about attenuation

Speaker : Kyle Penner (CEA)

Abstract : We use rest-frame infrared, ultraviolet, and Ha line luminosities of dust-poor and dusty galaxies at z approximately 0 and z approximately 1 to compare measured Ha attenuation values with those predicted by the Calzetti attenuation formula. The predictions, based on UV attenuation values, overestimate the measured Ha attenuation values for all samples. The interpretation of this result for dust-poor galaxies at z approximately 0 is that regions with ionizing stars have more dust than regions with nonionizing UV-emitting stars. Dust distributions for these galaxies are nonuniform. The interpretation of the overestimates for dusty galaxies at both redshifts is less clear. If the attenuation formula is inapplicable to these galaxies, perhaps the disagreements are unphysical ; perhaps dust distributions in these galaxies are uniform. If the attenuation formula does apply, then dusty galaxies have nonuniform dust distributions ; the distributions are more uniform than they are in dust-poor galaxies. A broad range of Ha attenuation values at a given UV attenuation for dusty galaxies at z approximately 1, if real, indicates diverse dust morphologies and the implausibility of the screen assumption.


Friday 15 January 2016

Title : Low Frequency view of merging galaxy clusters

Speaker : Mamta Pommier (CRAL)

Abstract : Cluster of galaxies are the largest gravitationally bounded systems in our Universe. They emit non-thermal emission during merger events that takes the form of cluster wide (Mpc-size) radio halos, mini-halos and relics, further implying the presence of relativistic particles and magnetic fields within the intracluster medium. These sources reveal a population of ultra-relativistic electrons coexisting with the intracluster thermal gas. The origin of these relativistic electrons is one of the most intriguing problems of cluster astrophysics. Further, the radio observations also show presence of μG level magnetic fields within the galaxy clusters, with values up to tens of μG at the center of cooling core clusters. In this presentation, I will discuss about the new results on the low frequency nature of a few galaxy clusters observed with the GMRT, VLA and LOFAR as well as their statistical properties. I will also outline the important contribution that are expected in this area from sophisticated radio facilities (like LOFAR, SKA, etc.) as well as synergies with ongoing survey with Multi Unit Spectroscopic Explorer (MUSE).