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Accueil > Le Laboratoire > Séminaires et conférences > Résumés

 Résumés

Remarque : les enregistrements des séminaires sont disponibles sur seminars.lam.fr.



09 April 2021 - 11h

  • Giovanni Cresci (INAF/Arcetri)
  • MAVIS : sharper than JWST, deeper than HST

MAVIS (MCAO Assisted Visible Imager and Spectrograph) is a forthcoming instrument for the ESO’s VLT Adaptive Optics Facility, currently starting Phase B with an active participation of LAM. It is made of two main parts : a Multi-Conjugate Adaptive Optics (MCAO) system, that cancels the image blurring induced by atmospheric turbulence in the visible on a large field, and its post focal instrumentation, for which the baseline is a wide field imager and a IFU spectrograph, both covering the visible part of the light spectrum. MAVIS has the potential to be an extremely novel and powerful facility : with an angular resolution of 15 mas (close to 50 times better than the seeing limited conditions) and a powerful and sensitive post-focal instrumentation, MAVIS will be instrumental to bring answers to a number of astrophysical science questions, from solar system planets and moon, stellar evolution, BH seeds, up to the physical composition of high-z galaxies and early galaxy assembly.

02 April 2021 - 11h

  • Atsushi Taruya (Yukawa Institute for Theoretical Physics (YITP) )
  • Intrinsic alignment of galaxies as a novel cosmological probe

Mapping the large-scale structure of the universe with galaxy surveys is one of the main science drivers for cosmology. In doing so, the spatial distribution of galaxies has been the major observable, ignoring the individual shapes and orientations. In this talk, I will consider the intrinsic alignment (IA) of galaxies as a novel cosmological probe. The IA has been treated as a contaminant against weak gravitational lensing experiments. Here, based on cosmological N-body simulations, we show that the spatial correlation of IAs exhibits characteristic anisotropies, in which the baryon acoustic oscillation feature can be detected. Making use of these features, we show that in combination with the conventional galaxy clustering statistics, the large-scale IA correlations help to improve the measurements of the geometric distances and growth of structure.

26 March 2021 - 11h

  • Nathalie Ysard (Intitut d’Astrophysique Spatiale)
  • Dust evolution towards dense interstellar regions

Dust grains are ubiquitous in all astrophysical environments, from the Solar System and protoplanetary disks to interstellar and intergalatic clouds, and their influence on the radiative properties of all these very diverse media is always significant through the absorption, scattering, and (non-)thermal re-emission of starlight. They are also a major player in the determination of the ISM gas temperature and have a great influence on the chemical complexity in the ISM. The grain radiative properties and their catalytic efficiency are, at least, reliant on the grain size distribution, structure and chemical composition, which vary throughout the dust lifecycle.

Observations show that major changes in grain properties occur in cold and dense ISM regions. Grain growth arises in cold molecular clouds and cores as traced by an enhancement of the dust far-IR emissivity, a change in the far-IR SED spectral index and by the effects of cloud-/core-shine from the visible to the mid-IR. The grain surface properties also appear to be altered compared to those found in more diffuse and warmer ISM regions, most certainly through gas accretion in the form of ice layers and possibly through the accretion of the smallest carbonaceous grains onto larger silicate ones. Dust evolution impacts a priori as much on its size distribution as on its structure or chemical composition. The aim of this seminar will be to make a brief inventory of the observations that have allowed us to measure this evolution and to see to what extent we can understand it today in terms of grain property modelling.

19 March 2021 - 11h

  • James Owen (Imperial College London)
  • The origin of close-in exoplanets

The observed exoplanet population unveiled by recent detection programs is billions of years old, distinctly separated in time from the planet formation process that only lasted 10-100 Myr. I will argue that atmospheric escape has been one of the key evolutionary drivers shaping the exoplanet population we observe today. By understanding how these planets evolve in time, I will show we can place some intriguing constraints on how they formed.

12 March 2021 - 11h

  • Thibaut Paumard (Observatoire de Paris)
  • GRAVITY+ : Towards Faint Science, All Sky, High Contrast, Milli‑Arcsecond Optical Interferometric Imaging

GRAVITY and the VLTI have transformed optical interferometry with groundbreaking results on the Galactic Center (see Nobel prize in Physics 2020), active galactic nuclei, and exoplanets. Through its upgrades (off-axis fringe-tracking, extreme adaptive optics (AO) and laser guide stars for the 4 UTs), GRAVITY+ will open up the extragalactic sky for milli-arcsec resolution interferometric imaging, and give access to targets as faint as K = 22 mag. GRAVITY+ will measure the black hole masses of active galactic nuclei across cosmic time, and obtain high quality exoplanet spectra and orbits. I will present the results of GRAVITY as well as the science prospects and instrument concepts of GRAVITY+.

19 February 2021 - 11h

  • Christophe Lovis (University of Geneva)
  • Science with RISTRETTO : combining high contrast and high spectral resolution at the VLT

RISTRETTO will be a novel high-resolution spectrograph fed by a high-performance adaptive optics system at the VLT. It will offer high spatial and spectral resolution in the visible, enabling new exoplanet and solar system science. The main science goal of RISTRETTO is reflected-light spectroscopy of exoplanets orbiting very nearby stars, among which Proxima b. A number of exoplanets of various sizes and irradiation levels will be amenable to atmospheric and surface characterization, opening the way for large population studies in reflected light with ELT-HIRES in the next decade. In addition, RISTRETTO offers several other science opportunities that need to be further developed. It is expected to play an important role in detecting and characterizing accreting protoplanets through spectrally-resolved H-alpha emission. It can also provide spatially-resolved wind and chemical composition measurements of various solar system objects, e.g. the ice giants Uranus and Neptune, and the icy moons of Jupiter and Saturn. In this talk we will present the status of the project and discuss the science cases for RISTRETTO.

12 February 2021 - 11h

  • Laura Kreidberg (MPIA)
  • Hot Takes on Cool Worlds : Exoplanet Atmosphere Characterization in the 2020s

Exoplanets are now known to be common in the Galaxy, and the next step in studying these abundant worlds is to characterize their atmospheres. As new instrumentation and >observing techniques develop, it is rapidly becoming possible to characterize smaller and cooler planets than ever before. In this talk, I will discuss the status quo in >exoplanet atmosphere characterization, including recent measurements of sub-Neptune atmosphere composition and efforts to determine whether Earth-size planets have atmospheres >at all. I will also overview the progress expected from next-generation observing facilities, including unprecedentedly precise measurements of the chemical composition and >cloud properties of gaseous planets, and a first glimpse at the atmospheric and surface properties of rocky worlds.

29 January 2021 - 11h

  • Pavel Kroupa (University of Bonn)
  • Latest news from FIRST

Quasars are found to appear a few hundred Myr after the Big Bang, but pressing matter together into super-massive black holes(SMBHs) so quickly appears to be impossible. At a later stage, the spheroidal component of a galaxy (its bulge if it is not an elliptical galaxy) is observed to show a correlation between its mass and that of the central SMBH it harbours, although spheroids with a mass lower than a few 1E9 Msun appear to only host a nuclear star cluster. I will discuss a theory for the formation of SMBHs which accounts for these observations using standard, non-exotic physics. Starting from the observed correlation between the mass of the most massive young star cluster and the star-formation rate (SFR) of the hosting galaxy, at the onset of the formation of a spheroid at very high redshift, the first (normal) stars form from very low-metallicity gas in extremely massive star clusters with a top-heavy stellar initial mass function (IMF). This cluster appears, for about 50 Myr, as a very high-redshift quasar. Once the massive stars die the luminosity drops, leaving a cluster containing more than 1E5 stellar-mass black holes (BHs).This cluster shrinks due to gas-infall from the spheroid which continues to form for the observed down-sizing (Thomas) time-scale. Shrinkage takes the cluster into the relativistic regime where the individual BHs begin to merge due to the emission of gravitational waves through the increasing rate fo BH—BH encounters. After about 100 Myr of the formation of the first stars, a SMBH-seed thus forms efficiently. The SMBH-seed accretes (at or below the Eddington rate) the infalling gas from the forming spheroid until the spheroid quenches after the Thomas time. Applying the IGIMF theory, which physically connects the independently observationally derived correlations between the star-cluster mass, the galaxy-wide SFR, the metallicity- and gas density-dependent stellar IMF, the Thomas times and spheroid masses, leads automatically and naturally to the observed spheroid-mass—SMBH-mass correlation. Spheroids less massive than a few 1E9 Msun cannot form a SMBH in this theory. It thus appears that the solution to the general SMBH problem may be quite straight-forward.

22 January 2021 - 11h

  • Martina Knoop (CNRS)
  • Actions transverses et interdisciplinaires au CNRS

Le CNRS est organisé en dix instituts sur de grands périmètres disciplinaires. La Mission pour les Initiatives Transverses et Interdisciplinaires (MITI) du CNRS coordonne et anime des actions qui visent à augmenter l’interaction entre les disciplines et les instituts sur des sujets scientifiques mais également autour des questions de métier, de technique et de technologie. La MITI met en œuvre aussi bien des appels à projets, que l’accompagnement des actions transverses scientifiques. Elle pilote et anime la plateforme des réseaux transverses et interdisciplinaires. Je présenterai les différents outils de la MITI et des exemples d’actions.

15 January 2021 - 11h

  • Elsa Huby (LESIA - Observatoire de Paris)
  • Latest news from FIRST

FIRST (Fibered Imager foR a Single Telescope) is a visible high angular resolution instrument based on the same principle as aperture masking, which turns a telescope into an interferometer. The goal of sparse aperture masking is to recombine the light from sub-pupils in a non-redundant scheme, to avoid the blurring of the fringes due to the atmospheric turbulence and therefore provide angular resolution down to the limit of diffraction, or even below. Instead of masking the pupil, FIRST relies on the use of single-mode fibers to re-arrange the configuration of the sub-pupils and thus allow the use of the whole pupil.
From Paris Observatory laboratory to the 3m-telescope of Lick Observatory, and then to the Subaru 8m-telescope, the FIRST instrument has been steadily evolving in the past 10 years, recently taking new steps to increase the instrument sensitivity. The latest developments focus on the upgrade of the recombination scheme, making use of stable and robust integrated optics chip, designed in collaboration with IPAG. The detection of substellar companions, the ultimate goal of FIRST, might be within reach in the near future !

18 December 2020 - 11h

  • Faustine Cantalloube (Max Planck Institute for Astronomy)
  • High-contrast imaging of exoplanets and circumstellar disks : from the instrument to the data analysis

High-contrast imaging is one of the most challenging observational techniques in the Vis/IR. It requires the best adaptive optics systems installed on the largest ground-based telescopes, advanced coronagraph design and dedicated image processing techniques. Thanks to the second generation of high-contrast instruments, such as VLT/SPHERE, partially developed at LAM, we now understand subtle effects that stem from the atmospheric turbulence and interacting all the way through the instrument, driving the final performance of the instrument. This knowledge turns out to be of great interest in the context of the design of the future 40-m class telescopes instrument suite. In this talk I will give an overview of my research approach for these topics and the different projects that I will carry on leading once installed at LAM from 2021 on.

11 December 2020 - 11h

  • Luca Malavolta (University of Padova)
  • Measuring the mass of exoplanets : an observer’s perspective

Radial velocity was the first technique used to reveal the existence of planetary systems around other stars, and for a long time it has been the most successful method to discover new exoplanets. The situation has reversed with the arrival of high-precision photometry from space, starting from CoRoT (CNRS/CNES) to Kepler, TESS (NASA) and CHEOPS (ESA) towards PLATO (ESA). While these missions are discovering and/or characterising candidate planets at smaller and smaller radii, radial velocities are struggling in determining their masses, despite the incessant advancements in technology and data analysis. In this seminar I will review the major challenges that the next-generation spectrographs such as ESPRESSO (VLT) and HIRES (E-ELT) will face when attempting to determine the mass of Earth-like planets around solar-type stars and what we can learn from the past.

04 December 2020 - 11h

  • Bernd Sierk (European Space Agency)
  • ESA’s Earth Observation missions for measuring Greenhouse Gases from space

The Earth Observation (EO) directorate of the European Space Agency (ESA) is running various programmes for the development of space missions for science, as well as in support of operational services. The Earth Explorer programme aims at exploring and advancing new observation techniques and technologies with high impact in geo-sciences and the potential for future operational service. In contrast, the European Union’s Copernicus programme, jointly managed by the European Commission and ESA, is developing operational space missions for long-term observations with high reliability, in order to support policy makers with relevant information. In recent years precise measurements of greenhouse gas concentrations, as well as of other critical climate-relevant parameters have gotten into the focus of both, the Earth Explorer and Copernicus programmes.

The presentation will focus on two missions addressing climate-relevant observations :
The Far-infrared Outgoing Radiation Understanding and Monitoring (FORUM) mission, which was recently selected as ESA’s ninth Earth Explorer, and the upcoming Copernicus Carbon Dioxide Monitoring (CO2M) mission. FORUM is a Fourier Transform Infrared Radiometer (FTIR) targeting the precise measurement of Earth’s far infrared emission spectrum in the spectral region 100 to 1600 cm-¹ (i.e. 6.25 to 100 μm). These observations will close a long-standing gap in spectrally resolved Top-Of-Atmosphere radiance measurements, that are critical to understanding key processes of climate change.
The CO2M mission will be implemented as a constellation of three identical satellites, simultaneously operated over a period > 7 years and measuring carbon dioxide and methane concentration in terms of column-averaged mole fraction (denoted as XCO2 and XCH4). Each satellite will continuously image XCO2 and XCH4 along the satellite track on the sun-illuminated part of the orbit, with a swath width of >250 km. Observations will be provided at a spatial resolution of 2km x 2 km, with high precision (<0.7 ppm) and accuracy (bias <0.5 ppm). The payload comprises a suite of instruments addressing the various aspects of the challenging observation requirements. At its core is a push-broom imaging spectrometer performing co-located measurements of top-of-atmosphere radiances in the Near Infrared (NIR) and Short-Wave Infrared (SWIR) at high to moderate spectral resolution (NIR : 747-773nm@0.1nm, SWIR-1 : 1595-1675nm@0.3nm, SWIR-2 : 1990-2095nm@0.35nm).

The presentation will showcase these two missions and their individual scientific and observational challenges, as well as the instrument concepts with an emphasis on optical design. It will also highlight critical technologies and pre-development activities initiated by ESA, and provide an outlook on the current trends in Earth Observation.

27 November 2020 - 11h

  • Ortwin Gerhard (Max-Planck Institute, Garching)
  • Structure and evolution of the Milky Way and nearby disk galaxies

The inner regions of the Milky Way are dominated by the Galactic bar whose central three-dimensional part constitutes the b/p bulge. There is no compelling evidence for a "classical" bulge. Metallicity and age measurements using Gaia and complementary surveys suggest an early formation of both the stars and the bar structure in the inner Galaxy. The dark matter density in the bulge region is unexpectedly low. Due to it’s relatively large extent the Galactic bar has significant influence on the disk star velocities out to the solar circle. Our neighbour galaxy M31 has had a much more turbulent past, with a fairly major (1:5) merger some 2.5 Gyr ago indicated by the age-velocity dispersion relation in the disk, consistent with the star formation history and the planetary nebula (PN) population properties across Andromeda. The merged satellite would have once been the third-largest galaxy in the Local Group. M31 appears to contain both a classical and a barred bulge. Stars in the bulge region are uniformly old, and metallicities are enhanced along the bar. Dynamical determination of the mass in Galactic disks requires resolving the vertical velocity distribution. Obtaining and modelling high-spectral-resolution and PN velocity data for two nearby face-on disk galaxies results in a significantly higher contribution of baryonic mass to the inner rotation curve than previously inferred (and lower contribution by dark matter).

13 November 2020 - 11h

  • Sergio Cristallo (INAF-osservatorio astronomico d’Abruzzo)
  • Cauldrons in the Cosmos

Stars are the cauldrons where most of the chemical elements are created. In spite of the variety of stellar objects in the Universe, the distribution of heavy elements in stars (included our Sun) can be reproduced with only two neutron capture processes : the slow neutron capture process (s-process, with low neutron densities) and the rapid neutron capture process (r-process, with very large neutron densities). The s-process is active during the Asymptotic Giant Branch (AGB) phase of low and intermediate mass stars. The r-process is thought to work during the explosive phases at the end of massive stars evolution, whether in a single object evolution (Magnetars) or in a binary merger event (Neutron Stars Mergers, NSMs, and/or Black Hole-Neutron Star Mergers). In the epoch of multi-messenger astrophysics, scientists cannot limit their studies to the nucleosynthesis occurring in low mass stars or highly compact interacting objects only, but they have to deal with them at the same time. I will present some recent improvements in our theoretical knowledge of both classes of objects.

06 November 2020 - 14h

  • Lori Glaze (NASA Planetary Science Division)
  • NASA Planetary Science highlights : current status and future plans

This is an exciting time for planetary science exploration at NASA and beyond. We currently have the Perseverance Rover well on its way to Mars and OSIRIS-REx is poised to perform its touch-and-go (TAG) maneuver, which will allow us to sample asteroid Bennu and bring back its precious material to Earth. Alongside these ongoing missions, NASA’s Planetary Science Program is working hand-in-hand with other parts of NASA on the Artemis program as we go forward to the Moon. As we plan to land the first woman and next man on the lunar surface, we are working to ensure science plays an important part in these exploration plans. Our many other active planetary science missions and investments span the breadth of the solar system—with ESA/JAXA’s BepiColombo about to flyby Venus on its way to Mercury and New Horizons continuing to explore the Kuiper Belt. Dr Lori Glaze will discuss all of these activities during her seminar, as well as forthcoming missions that range in size from small cube sat missions (e.g., Q-PACE and LunaH Map), through medium/large Discovery and New Frontiers missions (Lucy, Psyche, Dragonfly), to flagship strategic endeavors (Europa Clipper, Mars Sample Return).

16 October 2020 - 11h

  • Anne Verhamme (University of Geneva)
  • Searching for the sources of cosmic reionization

Cosmic reionization corresponds to the period in the history of the Universe during which the predominantly neutral intergalactic medium was ionised by the emergence of the first luminous sources. Young stars in primeval galaxies may be the sources of reionization, if the ionising radiation, called Lyman continuum (LyC), that they produce can escape their interstellar medium : the escape fraction of LyC photons from galaxies is one of the main unknowns of reionization studies. The increasing opacity of the intergalactic medium with redshift renders direct LyC detections impossible during reionisation. Indirect methods are the only probes of LyC leakage in the distant Universe. I will discuss three indirect diagnostics of LyC leakage that were recently reported in the literature. The first diagnostic for LyC leakage relates the escape of the strongly resonant Lyman-alpha radiation from galaxies to the LyC escape (Verhamme et al. 2015), and was recently validated by observations (Verhamme et al. 2017, Izotov+18, Steidel+18). The second diagnostic proposes that the strength of Oxygen lines ratios can trace density-bounded interstellar regions. It was the selection criterion for the successful detection of 11 strong Lyman Continuum Emitters from our team (Izotov 2016a,b, 2018). The third diagnostic relates the metallic absorption line strengths to the porosity of the absorbing interstellar gas in front of the stars. These diagnostics will soon become observables at the redshifts of interest with JWST.

09 October 2020 - 11h

  • Sean Raymond (Laboratoire d’Astrophysique de Bordeaux))
  • Solar system formation in the context of Extra-Solar Planets

The past decade of exoplanet observations has confirmed one of humanity’s (and all teenagers’) worst fears : we are weird. Even though Jupiter is the only Solar System planet likely to be detected with present-day technology, the Solar System is quantifiably unusual among exoplanet systems at the 1% level. Instead, roughly half of main sequence stars host close-in "super-Earths", and 10% have Jupiters on non-Jupiter-like, eccentric or close-in orbits. In this talk I will explore how the Solar System fits in a larger context by addressing key steps in planetary system formation. I will present models to explain the diversity of observed planetary systems (including super-Earth systems and giant exoplanets) and the mechanisms that create that diversity. While there is as yet no consensus on exactly how the Solar System formed, Jupiter is likely to have played a decisive role.

12 June 2020 - 11h

  • Guy Perrin
  • Explorations of Sgr A* at the event horizon scale and tests of general relativity with GRAVITY

The existence of black holes has been predicted for a long time, even before general relativity was sketched by Albert Einstein. Their extreme compactness makes them difficult to explore on spatial scales close to the event horizon.
Sagittarius A*, at the center of the Galaxy, is the black hole with the largest angular size. A collaboration of European astronomers has built the GRAVITY instrument for the Very Large Telescope Interferometer of ESO to test the black hole nature of Sgr A* thanks to observations in the infrared at unprecedented spatial scales. General relativity can consequently also be tested at these scales in an extreme regime of gravity.
I will present the detection of gravitational redshift with the star S2, the closest star known to Sgr A*. I will also present the measurements of orbits of infrared flares occurring near the event horizon. I will also describe results obtained on other supermassive black holes. I will conclude on the promising prospects of GRAVITY.

5 June 2020 - 11h

  • Sylvestre Lacour (LESIA)
  • Direct detection of exoplanets with the GRAVITY instrument

In March 2019, the GRAVITY collaboration published the first spectrum and astrometry of an exoplanet obtained by optical interferometry. The technique open the door to a sharper view of imaged exoplanets : a better astrometry, and a better spectrum compared to single telescope observations. But what is the accuracy, and what are the limitations of the technique ? Most importantly, will this door give access to new planets, colder, closer, than the ones that were previously directly imaged ? During this talk, I will present the technique that enabled the high contrast observations. I will discuss the drawbacks and the advantages. From this, I will present the accessible parameter space in contrast versus separation, and its corresponding capabilities in mass versus semi-major axis.

29 May 2020 - 11h

  • Oliver Hahn (Observatoire de la Côte d’Azur)
  • The collisionless dynamics of dark matter and cosmic structure formation

The dynamics of dark matter provides the backbone of studies of cosmic structure formation. Despite our ignorance about the particle physics nature of the elusive dark matter, its microscopic properties leave a distinct imprint on its macroscopic dynamics which can be studied in computer simulations. Such N-body simulations have driven most of our theoretical knowledge about the distribution of matter in the Universe which in turn reflects properties of the dark matter particle. I will review the theoretical assumptions underlying such simulations and how they are used to study the nature of dark matter through its dynamics. I will particularly focus on various recent attempts to model dark matter in the continuum limit. I will discuss how such new methods can be used to overcome known problems of N-body simulations, but also help to gain completely new insights into dark matter dynamics and the formation of the large-scale structure of the Universe.

15 May 2020 - 11h

  • Roser Pello (GECO, LAM)
  • Exploring the faintest population of star-forming galaxies at z 3-7 with MUSE observations of lensing clusters

The formation of the first stars and galaxies marked the end of the dark ages and the beginning of the reionization process. It has been established that cosmic reionization was mostly completed by z∼6 (Fan et al. 2006 ; Becker et al. 2015). The identification and detailed characterization of the sources responsible for this major transition is still a matter of debate. There are two main signatures currently used for the identification of high-z star-forming galaxies : the Lyman “drop-out” signature in the continuum of Lyman break galaxies (LBGs), and the detection of the Lyman-alpha line to target Lyman-alpha emitters (LAEs). While faint low-mass star-forming galaxies (SFGs) are presently considered as the main sources of ionizing photons, the relative contribution of the different populations to this process is unclear.

Regarding LBGs, ultra-deep photometry obtained by the Hubble Space Telescope (HST) on blank fields combined with ground-based photometry and spectroscopy has fundamentally improved our knowledge of the galaxy UV Luminosity Function (LF) up to z 10 (see e.g. Bouwens et al. 2015 ; Finkelstein et al. 2015, and the references therein). The integration of the UV LF is often used to derive the evolution of the cosmic star-formation density, and the density of ionizing radiation (usually assuming a constant star-formation rate), the two key parameters being the slope of the faint-end of the LF and the faint-end integration limit. Up to z 7, current observations reach as deep as M(1500) -17 in blank fields, which is about three magnitudes brighter than the faint-end UV luminosity limit which is needed to reionize the universe at z 6-7. Using lensing clusters as gravitational telescopes make it possible to reach M(1500) -15 at z 7, therefore, improving our constraints on the contribution of SFGs to cosmic reionization. Regarding the population of LAEs, recent studies have found a deficit of “bright” Lyman-alpha galaxies at z≥6.5, whereas no significant evolution is observed below z∼6 ; this trend is attributed to either an increase in the fraction of neutral hydrogen in the IGM or an evolution of the parent population or both. LBGs and LAEs constitute two different observational approaches to select SFGs. The prevalence of Lyman-alpha emission in well-controlled samples of star-forming galaxies is also a test for the reionization history. However, a complete and “as unbiased as possible” census of ionizing sources can only be enabled through 3D/IFU spectroscopy without any photometric preselection.

This seminar presents the first results obtained with MUSE on the faint end of the Lyman-alpha LF, based on deep observations of four lensing clusters. The precise aim of this study is to further constrain the abundance of LAEs by taking advantage of the magnification provided by lensing clusters to build a blindly-selected sample of galaxies, less biased than current blank field samples both in redshift and luminosity. By construction, this sample of LAEs is complementary to those built from deep blank fields, whether observed by MUSE or by other facilities and makes it possible to determine the shape of the LF at fainter levels, as well as its evolution with redshift. The price to pay to benefit from magnification is a reduction of the effective volume of the survey on one hand, together with increased complexity in the analysis of the contribution of each galaxy to the global LF on the other hand. As a result of this analysis, the Lyman-alpha LF has been obtained in four different redshift bins, with constraints down to log(L(Lyα))=40.5. These results will be discussed, together with the implications for the contribution of the LAE population to the star formation rate density at z 6-7.

The intersection between the populations of FGs selected as either LBGs or LAEs in the redshift range z 3-7 will be also discussed, based on MUSE observations of the Hubble Frontier Fields lensing cluster A2744, possibly the best-suited field for this exercise. Three samples of star-forming galaxies have been selected within the same volume of the universe : LBG galaxies with an LAE counterpart, LBG galaxies without LAE counterpart, and LAE galaxies without an LBG counterpart. These galaxies are intrinsically faint due to the lensing nature of the sample (M(1500) > -20.5, reaching as deep as M(1500) -15 to -12 in highly magnified regions). The fraction of LAEs among all selected star-forming galaxies increases with redshift up to z 6 and decreases for higher redshifts, in agreement with previous findings. The interrelation between the different SFG populations is shown for the first time up to the faintest limits, with the limitations imposed by the lensing technique. The relative evolution of these populations will be discussed, as well as the possible implications for future studies.

11 April 2020 - 11h

  • Pierre Vernazza (LAM - CNRS)
  • Constraints on the formation and evolution of the largest main belt asteroids

To make substantial progress in our understanding of the shape, internal compositional structure (i.e., density) and surface topography of large main belt asteroids, we have been carrying out an imaging survey with VLT/SPHERE via an ESO Large program (PI : P. Vernazza ; ID : 199.C-0074) of a statistically significant fraction of all D>100 km main-belt asteroids ( 40 out of 200 asteroids ; our survey covers the major compositional classes).
I will present an overview of the results obtained after 2.5 years of observations. So far, every target has challenged current knowledge, with for instance the linkage between an observed impact crater and a small collisional family (Vernazza et al. 2018), the first high angular resolution images of Psyche, target of a forthcoming NASA discovery mission, implying a density compatible with that of stony-iron meteorites (Viikinkoski et al. 2018), the bluffing view of asteroid (4) Vesta (Fetick et al. 2019), the homogeneous internal structure of CM-like asteroid (41) Daphne (Carry et al. 2019), the heavily cratered surface of Pallas (Marsset et al., Nature Astronomy in press), the basin-free spherical shape of Hygiea while it suffered a giant impact that is at the origin of one of the largest asteroid families (Vernazza et al. 2019), and the nearly hydrostatic equilibrium shape of Interamnia (Hanus et al., A&A in press), to name a few.
I will also present a review of the techniques employed to reach our science objectives (3D reconstruction & deconvolution of the reduced images). In particular, the 3D reconstruction algorithm MPCD (Jorda et al. 2016) developed in the framework of the Rosetta space mission has been successfully adapted to our VLT/SPHERE data whereas our deconvolution algorithm has successfully passed the test in the case of (4) Vesta (Fetick et al. 2019).

3 April 2020 - 11h

  • Benoit Neichel (GRD, LAM)
  • Latest news from HARMONI

In this time where everybody is locked home, I propose to travel to the future : 2026, the Extremely Large Telescope (ELT) first light and the first images of the Universe with HARMONI !
In this presentation I will come back on the ELT development, present the current project status and instrument suites, and then focus on HARMONI - the first light Integral Field Spectrograph instrument of the ELT.

27 March 2020 - 11h

  • Denis Burgarella (GECO, LAM)
  • Observational and theoretical constraints on the formation and early evolution of the first dust grains in galaxies at 5 < z < 10

The first generation of stars were born a few hundred million years after the big bang. These stars synthesized elements heavier than H and He, that are later expelled into the interstellar medium, initiating the rise of metals. Within this enriched medium, the first dust grains formed. This event is cosmological crucial for
molecule formation as dust plays a major role by cooling low-metallicity star-forming clouds which can fragment to create lower mass stars. Collecting information on these first dust grains is difficult because of the negative alliance of large distances and low dust masses. We combine the observational information from galaxies at
redshifts 5 < z < 10 to constrain their dust emission and theoretically understand the first evolutionary phases of the dust cycle. Spectral energy distributions (SEDs) are fitted with CIGALE and the physical parameters and their evolution are modelled. From this SED fitting, we build a dust emission template for this population of galaxies in the epoch of reionization. Our new models explain why some early galaxies are observed and others are not. We follow in time the formation of the first grains by supernovae later destroyed by other supernova blasts and expelled in the circumgalactic and intergalactic media. We have found evidence for the first dust grains formed in the universe. But, above all, this letter underlines the need to collect more data and to develop new facilities to further constrain the dust
cycle in galaxies in the epoch of reionization.

20 March 2020 - 11h

  • Matthieu Bethermin (GECO, LAM)
  • Dusty star formation in the high-redshift Universe

Measuring and understanding the star formation history in the Universe in one of key challenges of modern astrophysics. It can be probed using the UV light from young stellar populations. However, the majority of this UV radiation is absorbed by dust and re-remitted in the far-infrared and millimeter domain. The impressive progress of far-infrared and millimeter astronomy in the last decade (e.g., Herschel and ALMA) allowed us to probe the star-formation hidden in the dust clouds in the high-redshift Universe. We found that an important fraction (20-40%) of the star formation budget at z 2 is hosted by dusty galaxies forming stars at a pace 1.5 orders of magnitude higher than the Milky Way (SFR>100 Msun/yr) and it is now clear that very massive and dusty objects exist already as early as z=7.

How can these systems host such an impressive star formation so early in the Universe ? Their cold gas reservoirs and their efficiency to convert this gas into stars are the key observables to address this question. I will present the latest constraints obtained with ALMA on their interstellar medium (cold gas mass, dense-gas fraction). One of the main results of these study is the presence of incredibly large reservoirs of molecular gas. I will finally explain why it is not so surprising considering the large mass of their host dark-matter halos, which can be constrained through the anisotropies of the cosmic infrared background, i.e. integrated relic emission of all dusty galaxies across cosmic times.

6 March 2020 - 11h

  • Grégoire Danger (Associate Professor - AMU)
  • From Astrochemistry to Planetary Chemistry. An Astrobiological Point of View

Complex organic molecules are detected in gaseous and solid phases of astrophysical objects. The origin of these molecules is still debated, but a large part is supposed to form on the surface of frozen grains. These icy grains observable in dense molecular clouds will be altered by highly energetic processes (VUV photons, ions, electrons) during the formation of planetary systems. These alterations allow the activation of molecules initially present in these ices, resulting in the development of a significant
chemical reactivity. In certain environments such as the solar nebula, these grains can be heated, releasing in the gas phase a large part of the complex organic molecules initially formed on the surface or within the ice bulk. Some of these molecules can also react inside the ice forming nonvolatile molecules that remain on the grains leading to the formation of refractory organic residues. Part of the transformed grains can then accrete, resulting in interplanetary objects such as comets or asteroids. As a result, some of the organic matter present in solar system objects could come from ices found in dense molecular clouds. A part of this material can then be delivered at the surface of telluric planets as observed on Earth through the analysis of meteorites. The question of the role played by this exogenous
organic matter in the emergence of a prebiotic chemistry in planetary environments is then a key issue in astrobiology.
Based on laboratory experiments, we develop analytical strategies to study the chemistry generated by the processing of such ices. We particularly demonstrate the richness of molecules formed. We also investigate the impact of the initial composition of the ice on the abundance of molecules observed in the gas phase as well as on the molecular composition of the remaining residues after the desorption of the most volatile compounds. Furthermore, our analyses show that this ice chemistry leads to the formation of all building blocks needed to start a prebiotic chemistry on telluric planets. We further develop new experiment to investigate the evolution of such matter in the conditions of the primitive Earth.
All results obtained from our experiments suggest that ices of astrophysical objects are key environments to generate a rich molecular diversity and question its potential role in the emergence of a prebiotic chemistry on primitive Earth environments.

14 February 2020 - 11h

  • Elise Vernet (ESO)
  • Design and performance of the ESO ELT adaptive and fast steering mirrors

The optical design of the ELT is based on a novel five-mirror scheme capable of collecting and focusing the light from astronomical sources and feeding state-of-the-art instruments for the purposes of imaging and spectroscopy. Thanks to the combined use of M4 and M5, the optical system is capable of correcting for atmospheric turbulence and the vibration of the telescope structure itself induced by motion and wind.

The presentation will focus on the technological specificities and performance of the two mirrors. The adaptive mirror is using the well-developed adaptive secondary technology but the M4 design has been tuned to reduce risks during the manufacturing and fulfil the ELT needs. In the first part, the technological changes of M4 will be highlighted and the implications discussed. The second part of the talk will be dedicated to M5.
The fast steering mirror is a ultra-light and stiff mirror mounted on a stiff tip tilt stage. After presenting the fast steering requirements and the associated flow down on the two subsystems, I will focus on the mirror design and the criticalities of manufacturing a 2.5 meter class Silicon Carbide mirror for observations at visible wavelengths.

31 January 2020 - 11h

  • Gaitee Hussain (ESO)
  • The evolution of magnetic activity in solar-type stars

Solar-type stars display signs of magnetic activity at all wavelengths, from energetic X-ray and UV flares to photometric variability caused by the presence of large cool spots. The magnetic activity in solar-type stars evolves as they age, affecting the environment in which planets themselves form and evolve. Over the past two decades Doppler imaging techniques have allowed us to detect and map the surface magnetic fields of solar-type stars over a wide range of evolutionary states. In this talk I will explain how we derive and interpret magnetic field maps of solar-type stars and review what they have taught us about the evolution of magnetic activity from the PMS stage through to our own Sun.

17 January 2020 - 11h

  • Stephane Basa & Laurent Martin (LAM)
  • THESEUS

The Transient High-Energy Sky and Early Universe Surveyor (THESEUS) is a mission concept developed in the last years by a large European consortium and currently under study by the European Space Agency (ESA) as one of the three candidates for nextM5 mission (launch in 2032). THESEUS aims at exploiting high-redshift GRBs for getting unique clues to the early Universe and, being an unprecedentedly powerful machine for the detection, accurate location (down to∼arcsec) and redshift determination of all types of GRBs (long, short, high-z, under-luminous, ultra-long) and many other classes of transient sources and phenomena, at providing a substantial contribution to multi-messenger time-domain astrophysics. Under these respects, THESEUS will show a strong synergy with the large observing facilities of the future, like E-ELT, TMT, SKA, CTA, ATHENA, in the electromagnetic domain, as well as with next-generation gravitational-waves and neutrino detectors, thus greatly enhancing their scientific return.
LAM is involved in the development of one of the 3 on-board instruments : the IRT for Infra-Red Telescope. The participation of the LAM consists in the supply of the structure of the instrument as well as the mirrors. The Instrument Scientist, the Optical Architect and the Mechanical Architect are among the key persons of the Project. The LAM is also responsible for the assembly and all of the tests that will take place in Marseille.
The presentation will be in French with slides in English.

10 January 2020 - 11h

  • Astrid Lamberts (OCA)
  • Gravitational wave progenitors near and far

Gravitational waves (GW) have opened a new window on our universe, providing us unique information on fundamental physics, stellar evolution, high energy astrophysics and global star formation in the universe. As the first catalog of GW events observed by LIGO/Virgo has become available, the main question is "where do black hole mergers come from ?" In this talk I will detail in which types of environments progenitors stars of GW events typically form and how the properties of GW events can inform on binary evolution. I will first focus on the mass black holes observed by LIGO/Virgo before presenting predictions for space-based GW detector LISA. LISA will be a detector operating at lower frequencies and will be sensitive to stellar compact binaries in the Milky Way, such as white dwarfs or black holes.

20 December 2019 - 11h

  • Florence Durret (IAP and Sorbonne Universite)
  • Galaxy clusters and their environment

Clusters of galaxies are located at the intersection of cosmic filaments. I will first describe how we have found large extensions around clusters, at optical and/or X-ray wavelengths, suggesting that we are indeed detecting the cosmic web around clusters, and I will discuss some galaxy properties in one of these extended filaments.

At the cluster scale, I will then describe our searches for substructures in X-rays and show how X-ray temperature, pressure, entropy and metallicity maps can give informations on the cluster history by comparing them with hydrodynamical simulations.

Finally, I will talk about several properties of galaxies in clusters, such as galaxy luminosity functions, brightest cluster galaxies, jellyfish galaxies, and asymmetric galaxies.

13 December 2019 - 11h

  • Elsa Huby (LESIA)
  • Spectroscopy below the diffraction limit with the FIRST pupil remapper at the Subaru Telescope

FIRST (Fibered Imager foR a Single Telescope instrument) is a post-AO instrument that enables high contrast imaging and spectroscopy at spatial scales below the diffraction limit. FIRST achieves sensitivity and accuracy by a unique combination of sparse aperture masking, spatial filtering by single-mode fibers and cross-dispersion in the visible. Currently integrated as a module of the Subaru Coronagraphic Extreme AO system (SCExAO), on-sky commissioning data show the detection of several stellar companions, and in particular of the Alpha Equu binary system angularly separated by 0.6 λ/D (11mas). Even at such a separation, FIRST delivers information on the companion spectrum, providing valuable constraints on the stellar parameters of the system such as the effective temperatures and surface gravity. On-going developments based on photonics technology focus on improving the instrument’s stability and sensitivity, and hence increasing the dynamic range. By coupling high angular resolution and spectral resolution in the visible, FIRST offers unique capabilities in the context of the detection and spectral characterization of close companions, especially on the future 30m-class telescopes.

29 November 2019 - 11h

  • Mélanie Heil (ESA)
  • ESA’s Enhanced Space Weather Monitoring System

ESA’s Space Situational Awareness Programme aims at protecting space and ground assets against adverse effects from space. The Space Weather Segment is focussing on such effects due to the activity of our Sun.

Monitoring of the Earth’s and Sun’s environment is an essential task for the now- and forecasting of Space Weather and the modelling of interactions between the Sun and the Earth. Due to the asymmetry and complexity of Earth’s magnetosphere, the involved particle environment and its dynamics, it is necessary to capture the state of the magnetic field and the particle distribution in a sufficiently large number of sampling points around the Earth, such that it allows state-monitoring and modelling of the involved processes with sufficient accuracy and timeliness.

ESA is implementing an enhanced space weather monitoring system, which includes the Lagrange mission to observe solar activity from the 5th Lagrange point of the Sun-Earth system and the establishment of a Distributed Space Weather Sensor System (D3S) to observe the effects of solar activity within Earth’s vicinity. Space weather instrumentation traditionally is highly miniaturised, and therefore appears to be well suited for small satellite systems, which could become competitive to the usually followed hosted payload approach while allowing for more flexibility with respect to the flown payload and orbital requirements.

I will present the current status and future plans of ESA’s Space Weather activities particularly focussing on D3S.

22 November 2019 - 11h

  • Françoise Combes (IRAP)
  • Molecular tori, black hole fueling, and feedback in nearby AGN

Recent molecular line observations with ALMA in several nearby Seyferts have revealed the existence of molecular tori, and the nature of gas flows at 10-20pc scale. At 100pc scale or kpc-scale, previous work on gravitational torques had shown that only about one-third of Seyfert galaxies experienced molecular inflow and central fueling, while in most cases the gas was stalled in rings. At higher resolution, i.e. 10-20pc scale, it is possible now to see in some cases AGN fueling due to nuclear trailing spirals, influenced by the black hole (BH) potential. This brings smoking-gun evidence for nuclear fueling. In our sample galaxies, the angular resolution of up to 80mas allows us to reach the BH-zone of influence and the BH mass can be derived more directly than with the M-sigma relation.

15 November 2019 - 11h

  • Pier Stefano Corasaniti (LUTH)
  • Galaxy Cluster Cosmology Beyond Cluster Number Counts

In the past years, dedicated survey programs have provided increasingly large samples of galaxy cluster observations which have opened the way to probing cosmology with galaxy clusters. These carry a wealth of cosmological information encoded in the cluster abundance, spatial clustering and density profile. To date cosmological constraints have been mainly inferred from cluster number count measurements. In this talk I will review recent results and motivate the use of complementary galaxy cluster observables. In particular, I will discuss the use of cluster sparsity measurements as a novel cosmological probe.

8 November 2019 - 11h

  • Pascal Tremblin (CEA Saclay)
  • Thermo-compositional diabatic convection in the atmospheres of brown dwarfs, exoplanets, and in Earth’s atmosphere and oceans

By generalizing the theory of convection to any type of thermal and compositional source terms (diabatic processes), we show that thermohaline convection in Earth’s oceans, fingering convection in stellar atmospheres, and moist convection in Earth’s atmosphere are derived from the same general diabatic convective instability. We also show that "radiative convection" triggered by the CO/CH4 transition with radiative transfer in the atmospheres of brown dwarfs is analogous to moist and thermohaline convection. We derive a generalization of the mixing-length theory to include the effect of source terms in 1D codes. We show that CO/CH4 "radiative" convection could significantly reduce the temperature gradient in the atmospheres of brown dwarfs similarly to moist convection in Earth’s atmosphere, thus possibly explaining the reddening in brown dwarf spectra. By using idealized 2D hydrodynamic simulations in the Ledoux unstable regime, we show that compositional source terms can indeed provoke a reduction of the temperature gradient. The L/T transition could be explained by a bifurcation between the adiabatic and diabatic convective transports and seen as a giant cooling crisis : an analog of the boiling crisis in liquid/steam-water convective flows.

The study of the impact of different parameters (effective temperature, compositional changes) on CO/CH4 radiative convection and the analogy with Earth moist and thermohaline convection is opening the possibility of using brown dwarfs to better understand some aspects of the physics at play in the climate of our own planet.

This mechanism, with other chemical transitions, could be present in many giant and Earth-like exoplanets. We present first results towards the simulation of this process for the CO/CO2 transition in secondary atmospheres of hot rocky exoplanets (young or irradiated) which could be applied to the primitive stages of the atmospheres of Earth, Mars or Venus.

18 October 2019 - 11h

  • Frédéric GALLIANO (AIM, CEA/Saclay, France)
  • Dust Evolution from the Perspective of Nearby Galaxies

A precise characterization of the grain properties is crucial for understanding the life cycle of the interstellar medium (ISM) and the evolution of galaxies. Nowadays, most of our knowledge of dust physics comes from studies of the Milky Way (MW). However, an increasing number of results on nearby galaxies provide unique discriminating constraints on fundamental grain processes. Indeed, nearby galaxies harbor a wider diversity of environmental conditions (metallicity, star-formation activity, etc.) than can be found in the MW. In particular, these nearby systems allow us to observe dust in extreme conditions, providing us valuable empirical information on grain evolution. They also constitute a necessary intermediate step toward understanding distant galaxies, as they are spatially resolved and have better wavelength coverage.

I will start with a general introduction, presenting the challenges in dust physics, and the recent advances in our understanding of the grain properties of the nearby Universe. I will then review several studies of the DustPedia project. This european collaboration aims to build a reference survey of ≈900 nearby, NIR-selected galaxies, observed with Herschel. I will detail the effort in modelling techniques we have carried out to provide the most accurate analysis possible of our data. I will finally present the various scaling relations and dust evolution trends derived among and within galaxies. I will discuss, in particular, the evolution of the dust-to-gas mass ratio and the fraction of aromatic feature carriers as a function of metallicity and star formation activity.

4 October 2019 - 11h

  • Jean-Luc Beuzit (LAM)
  • SPHERE, five years at the VLT

SPHERE (Spectro-Polarimetric High-contrast Exoplanet Research in Europe) is a second-generation instrument for the ESO Very Large Telescope, dedicated to the direct detection and spectral characterization of giant extra-solar planets. SPHERE combines an extreme adaptive optics system, various coronagraphic devices and a suite of focal instruments providing imaging, integral field spectroscopy and polarimetry capabilities in the visible and near-infrared spectral ranges. The instrument has now been in operation for 5 years and has produced a harvest of leading scientific results. I will give an overview of the instrument, highlight the human adventure behind its development, review its performance and present a selection of the major results.

6 September 2019 - 11h

  • Annie Hughes (IRAP)
  • PHANGS : Star formation, feedback and chemical enrichment in nearby galaxies

The Physics at High Angular resolution in Nearby Galaxies (PHANGS) project is pursuing thee major observing programmes with ALMA, VLT/MUSE and the HST in order to study the interplay of the small-scale physics of gas and star formation with galactic structure and galaxy evolution across a representative sample of star-forming main sequence galaxies in the local Universe (d<20Mpc). With observations at high angular resolution (<100pc), the PHANGS legacy datasets provide a detailed, panchromatic view of ionised gas, stellar light, and the cold molecular gas in nearby galaxies and enable statistical studies of star formation, feedback, and chemical enrichment across different galaxy environments. In this talk, I will give an overview of the PHANGS project, and highlight our latest results for the physical conditions in the molecular gas reservoir, the star formation activity and chemical enrichment in PHANGS galaxies.

28 June 2019 - 11h

  • Peter Capak (Caltech)
  • Developing a Standard Model of Galaxies

The Physics at High Angular resolution in Nearby Galaxies (PHANGS) project is pursuing thee major observing programmes with ALMA, VLT/MUSE and the HST in order to study the interplay of the small-scale physics of gas and star formation with galactic structure and galaxy evolution across a representative sample of star-forming main sequence galaxies in the local Universe (d<20Mpc). With observations at high angular resolution (<100pc), the PHANGS legacy datasets provide a detailed, panchromatic view of ionised gas, stellar light, and the cold molecular gas in nearby galaxies and enable statistical studies of star formation, feedback, and chemical enrichment across different galaxy environments. In this talk, I will give an overview of the PHANGS project, and highlight our latest results for the physical conditions in the molecular gas reservoir, the star formation activity and chemical enrichment in PHANGS galaxies.
In this presentation Peter Capak will argue that a combination of large galaxy surveys and the latest machine learning techniques are allowing astrophysicists to develop a robust statistical model of the extra-galactic universe. If optimally constructed, this model would encapsulate all available information on the likelihood of observing a given type of galaxy as well as its distribution in space and cosmic epoch. The initial motivation for developing elements of this model was improved constraints on dark energy and dark matter, but it also contains significant information on how galaxies form and evolve. He will show how early version of this model have significantly improved photometric redshifts for weak lensing and can be used for spectroscopic target selection. He will conclude with examples of how he is using his models to optimally design observation with facilities in high demand such as Keck, ALMA, and the future JWST and what future techniques need to be developed.

21 June 2019 - 11h

  • Hendrik Hildebrandt (Alfa-Bonn)
  • Cosmological Weak Gravitational Lensing

Gravitational lensing represents a unique tool to study the dark Universe. Small distortions in the images of galaxies caused by the gravitational lensing effect of the matter distribution in the Universe can be detected over the whole sky. Measuring these coherent distortions makes dark matter structures "visible", allows us to study their growth over cosmic time, and yields cosmological insights complementary to other probes like the cosmic microwave background (CMB). Ongoing wide-field imaging surveys exploit this weak gravitational lensing technique to come up with competitive constraints on important cosmological parameters and insights on fundamental physics.

In this talk I will first introduce the basic concepts of weak gravitational lensing, review the history and challenges of weak lensing measurements, and then concentrate on recent results from the ongoing European Kilo Degree Survey (KiDS) and VISTA Kilo-degree Infrared Galaxy Public Survey (VIKING) projects. These KiDS/VIKING measurements show some tension with CMB measurements from the Planck mission when the standard cosmological model is assumed. The results will be put into context and compared to findings from the two other big cosmic shear experiments (HSC and DES). I will also present brand-new results from a Self-Organised-Map-based calibration of the KiDS+VIKING redshifts and how this influences the cosmological conclusions. Through a careful re-assessment of the HSC and DES results I will show that the discrepancies in large-scale-structure parameters we are seeing today are approaching a level of significance that is similar to the tension in the Hubble constant. Taken together this might hint at a serious problem of the standard ΛCDM paradigm in simultaneously explaining early- and late-time cosmic structure formation. I will conclude with an outlook towards the big experiments of the next decade in this field of research, Euclid and the Large Synoptic Survey Telescope that have the potential to yield some definitive answers to these questions.

14 June 2019 - 11h

  • Vianney Lebouteiller (Laboratoire AIM - CEA Saclay)
  • The interstellar medium of nearby primitive galaxies

The lack of detection of cold molecular gas in blue compact dwarf (BCD) galaxies is at variance with their intense star-formation episode. In particular, CO, often used a tracer of H2 through a conversion function, is selectively photodissociated in dust-poor environments. A potentially large fraction of H2 is thus expected to reside in the so-called CO-free gas, where it could be traced by neutral gas observed with infrared cooling lines



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