Although mergers and starbursts are often invoked in the discussion of quasar activity and its effect on galaxy evolution, several studies have questioned their importance or even their presence in quasar host galaxies. In this talk, I will discuss results from a long campaign of space- and ground-based imaging and spectroscopic observations of z < 0.5 QSO hosts that imply that mergers are indeed essential for the triggering of QSO activity, and that these mergers invariably induce starbursts either during and/or shortly after the merger. In particular, I will discuss observations of a sample of host galaxies previously classified as passively evolving ellipticals. Our results clearly show that these galaxies have undergone major episodes of star formation in the past ~2 Gyr. The morphologies of the host galaxies suggest that these aging starbursts were induced during the early stages of the mergers that resulted in the elliptical-shaped galaxies that we observe today. The current AGN activity likely corresponds to the late episodes of accretion predicted by numerical simulations, which occur near the end of the mergers, whereas earlier episodes may be more difficult to observe due to obscuration. I will discuss numerical simulations that indicate that any potential current star formation or young stellar populations in these hosts would be confined to the central few kiloparsecs, a region that is typically outshined by the bright nucleus. I will also discuss our efforts to search for these starbursts in type-2 QSOs. Finally, I will discuss our ongoing work to probe the co-evolution of black holes and their hosts through scaling relations as a function of time.
The physical nature of the cosmic dark matter remains elusive, but several well-motivated extension of the particle-physics standard model provide candidates that are experimentally searched. The axion, a hypothetical very low-mass boson motivated by quantum chromodynamics (QCD), will be introduced and ongoing experimental searches as well as astrophysical limits will be reviewed. The interest in axion dark matter has recently surged and a number of completely new search initiatives have emerged.
I present results from numerical simulations of star cluster formation, and discuss how the physical processes involved in star formation may lead to the observed properties of stellar systems and whether or not these properties may vary in different environments.
Galactic Archaeology is a coined term to describe the fact that the Milky Way's history is encoded both in the amounts of various chemical elements seen in the spectra of stellar atmospheres (abundances), and in stellar motions. One of the pillars of Galactic Archaeology is the use of stellar abundance ratios as an indirect age estimator, which although imprecise, has been proved useful in providing relative ages between the different galactic components. The lack of more precise age determination for large samples of field stars is one of the main reasons why different scenarios for the formation of our Galaxy can still be accommodated to current observational constraints, thus preventing a clear picture of the Milky Way's assembling history. Another difficulty is that most of the available information (especially on ages) has been confined to a region close to the Sun. As it will be shown in this talk, these two main obstacles can now start to be overcome thanks to a) large spectroscopic and photometric surveys covering larger portions of the Milky Way, and b) the combination of the photometric and spectroscopic information with that coming from asteroseismology. The latter promises a breakthrough in the field of Galactic Archaeology, as it brings the opportunity to, for the first time, measure ages for large samples of distant field giant stars, which cover a large age-baseline. When combining this information with that soon available from Gaia, the field of Galactic Archaeology will be shaken and modelers will certainly have less flexibility in finding models that comply to these precious new observational constraints.
Massive stars are rare and short-lived. Nevertheless, through their extreme
brightness, strong outflows and powerful explosions, they heat and stir
their surroundings, drive outflows on galactic scales, are thought to be
responsible for reionization and the main production the heavy elements in
the Universe. Because of their large impact, evolutionary models of massive
stars are an essential ingredient for a wide variety of astrophysical
problems. Recently it has become clear that the majority of massive stars,
possibly as much as 7 out of 10, will experience severe interaction with a
binary companion. I will discuss several aspects of our quickly increasing
understanding of how this affects (1) the lives of massive stars, (2) our
interpretations of observations of young stellar populations nearby and at
high redshift and transient phenomena and (3) our understanding of the role
that massive stars play through their radiative, mechanical and chemical
The nearly 200000 spectra of z>2 quasars in the Baryon Oscillation Spectroscopic Survey have resulted in measurements of the autocorrelation of the Lyman alpha absorption by the intervening intergalactic medium, and cross-correlations of these absorption with quasars and damped Lyman alpha systems. Results on the measurement of the Baryon Acoustic Oscillation scale at high redshift, the large-scale bias and redshift distortions in the Lyman alpha forest, and the bias factors of quasars and DLAs will be summarized. A new surprising result has been the detection of Lyman alpha emission correlated with high-redshift quasars, using nearly a million spectra of lower redshift galaxies that may contain background emission line galaxies or other diffuse Lyman alpha emission. The detection is still marginal, but if correct it can only be accounted for by known star-forming galaxies if most Lyman alpha photons produced from the stellar ionizing light at z ~ 2.5 manage to escape from low surface brightness halos surrounding star-forming galaxies. Alternatively, some of the diffuse Lyman alpha emission may be due to intergalactic heating associated with quasars.
Planets are ubiquitous in our Galaxy, in systems far more diverse than predicted by theoretical models that could reproduce the properties of our own Solar System. I will discuss how scaling laws between the properties of protoplanetary disks and exoplanets and the mass of the central star are key to identifying the physical processes that shape planetary systems. I will also examine the spread in disk properties relevant to planet formation and make a link to the diversity of planetary systems.
Mass accretion onto supermassive black holes occurs on scales beyond the diffraction limit of any single optical/infrared (IR) telescope. Thanks to the resolution power of the VLT Interferometer, we are now tapping into the outer accretion structure of active galactic nuclei (AGN) - commonly referred to as the "dusty torus". Several surprising results are challenging our current paradigm: While the bulk of the mid-IR emission originates from perpendicular where models would put the torus, the IR emission as a whole appears to be made of two components. In this talk I will give a basic introduction to IR interferometry and discuss what our recent results tell us about AGN unification and the physical processes that regulate accretion and feedback. I will also give a brief glimpse into how IR interferometry can help us establishing AGN as cosmological distance measures.
During the past decade, the number of known planets has increased explosively, revealing an extreme range in planet compositions. The origin of this diversity is largely unknown. It is also unknown how common access to surface water and organics is on these planets, and thus the frequency of chemically habitable planets. I will present on how these questions can be addressed through a combination of astrophysical observations and laboratory simulations of the chemistry present in protoplanetary disks, the birthplaces of planets. We use spatially and spectrally resolved observations (ALMA and SMA) to explore the organic inventory, isotopic fractionation chemistry, and other chemical structures in disks. In parallel, we use laboratory ice experiment to quantify how these observed gas phase abundances relate to the total volatile reservoirs in disks, which are typically dominated by icy grain mantles. Recent highlights include observations of spectacular chemical ring structures that trace condensation-, temperature- and radiation-regulated chemistry, new constraints on isotopic fractionation during planet formation, and the detection of the first complex molecule in a disk. I will discuss these observations in light of laboratory constraints on ice chemistry, and how they compare with the chemical compositions found at earlier stages of star formation and in the Solar System.
This year, we celebrate the centenary of Einstein's theory of general relativity. When the theory was conceived, the number of experimental tests to confront the theory with was limited. Since then we have come a long way. In particular astronomical observations provide precision tests that were inconceivable even 50 years ago. We use neutron stars observable as pulsars to provide the most precise tests for strongly self-gravitating bodies, to prove that gravitational waves exist or to measure the effects of curvature of space time. We also attempt to determine the properties of black holes, such as their mass and spin to test the description of black holes within general relativity. One of the highlights will be an image of the "Cshadow" of the supermassive black hole in the centre of our Milky Way. Soon we also expect that gravitational wave detectors open up a new window to Einstein's Universe. In all cases, neutron stars or black holes play a crucial role. In this talk I will review some of the current and future tests of general relativity and compare those results with tests of alternative theories.
Variability from black holes - both stellar mass and supermassive - has been studied for many years, but we have only recently pieced together the physical origin of the variability. Developments in multiwavelength monitoring and X-ray spectral-timing now allow us to study the causal relationship between variations of different spectral components, allowing a direct link to the physics of the emitting regions close to the black hole. I will describe the recent advances in this area which reveal some surprises about the role of the turbulent accretion disc in producing the variability, give clues to the nature of the mysterious quasi-periodic oscillations and show the promise of new X-ray spectral timing techniques to map the emitting regions closest to the event horizon.
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? Recent imaging of deep
fields with HST's Wide Field Camera 3 in conjunction with ground-based
spectroscopy has provided important new insight into understanding
when reionization occurred and the role of early galaxies in the
process. Gravitational lensing by foreground clusters is providing
complementary evidence. I will review this rapid progress in our
understanding of what could be considered the last missing piece
in our overall picture of cosmic history and discuss the remaining
challenges ahead of future facilities such as E-ELT/TMT and JWST.
Cosmic rays in Milky Way-like galaxies represent only about a billionth of interstellar particles by number, but carry as much energy as the thermal particles and interstellar magnetic field. Although they are virtually collisionless, they can exchange energy and momentum with the thermal gas through scattering from magnetic fluctuations. This coupling can launch galactic winds, heat interstellar and intracluster gas, enhance magnetic buoyancy, and modify shocks. I will discuss the basis for cosmic ray - thermal gas coupling and some open issues.
Galaxy clusters are the largest and most recently formed cosmological objects in the universe, making them powerful laboratories for both cosmology and astrophysics. The current generation of multi-wavelength cluster surveys (including Planck, ROSAT and SDSS with follow-up observations by Chandra, HST and XMM-Newton space observatories) have dramatically increased the sample size and the image quality of observed galaxy clusters out to high-redshift. However, the statistical power of these surveys are limited by complex and still poorly understood cluster astrophysics that shape their observable properties and evolution. In this talk, I will review recent advances and challenges in our understanding of cluster astrophysics and discuss future prospect for the use of galaxy clusters as a cosmological probe.
Galactic winds are the most dramatic form of feedback provided by massive stars. In the first part of my talk I will summarize the importance of galactic winds for the evolution of galaxies and the inter-galactic medium. I will briefly describe the physical processes that drive these winds, and give a short guided tour of the multi-phase wind driven from the local starburst galaxy M82. I will then describe how the properties of winds are typically incorporated in cosmological simulations and semi-analytic models. In the second part of my talk I report on recent work that has determined the dependence of the basic wind properties (outflow velocities, mass and momentum outflow rates) on the properties of the galaxy/starburst. These results are strongly at odds with some of the most popular wind prescriptions in simulations and models. In the third part of the talk I will describe observations of the impact of galactic winds on the circum-galactic medium. Finally, I will report on new observations that reveal how stellar feedback, including galactic winds, enabled early star-forming galaxies to re-ionize the universe.
Understanding the process responsible for transforming star forming galaxies
into passive and quiescent systems is currently one of the hottest topics
in astronomy. I will discuss recent observational results probing different
mechanisms at work in different galaxies and at different epochs. I will
show that the analysis of the stellar metallicities in large samples of
local galaxies reveals that "strangulation" (i.e. the lack of gas inflows)
is responsible for quenching star formation in most galaxies. I will
discuss the possible mechanisms responsible for such starvation of galaxies.
Very likely the environment in which galaxies evolve plays a role, but it
is not the only culprit. Then I will shortly review multiwavelength
observations that have provided evidence for powerful starburst-driven
and AGN-driven outflows in galaxies. Certainly these outflows have a
profound impact on the evolution of galaxies, both locally and at high-z,
however such outflows are probably uncapable of completely quenching star
formation in galaxies. Yet, I will show that such massive outflows can
have an unexpected, positive effect on the evolution of galaxies, which
has been overlooked so far.
One of the primary motivations for Herschel was to explore star
formation in the distant Universe. To address this topic Herschel
invested significant fraction of its time in undertaking surveys
including the multi-tiered extragalactic survey, HerMES. HerMES
mapped around 400 sq. degrees in the best studied extragalactic fields
on the sky and has uncovered 100s of thousands of distant star
forming galaxies. In this talk I will review some of the key results
from HerMES. In particular the Herschel maps reveal most of the
cosmic infrared background and these and other basic statistical
measurements have constrained our view on galaxy evolution models. I
will summarise what we have learned about the cosmic history of star
formation. I will show how clustering measurements have been used to
reason that these distant star forming galaxies are the progenitors
of present day, massive galaxy, descendants. I will show how Herschel
has revealed new insights into the relation between star formation and
central supermassive +black hole accretion activity. Finally I'll
illustrate how Herschel is uncovering starburst galaxies when the
Universe was less than a billion years old.
The formation of the most massive stars affects our Milky Way
as a whole but at the same time, many of the physical processes
take place on very small spatial scales. This talk will highlight
recent results in that field covering large-scale Milky Way structures
and the locations of the high-mass star formation sites within our
home galaxy, as well as the small-scale physical and chemical
processes at place to actually build and form these high-mass stars.
Magnetars are believed to be the strongest magnets in the Universe, and show themselves via powerful X/gamma-ray steady and flaring emission. The energetics of such flares in our Galaxy second only the supernova explosions. In this talk I will first review the observational characteristics of these magnetic pulsars, their evolutionary relation with the "typical" neutron star population, and recent news in the field (i.e. the low-field magnetars and the Galactic center magnetar). I will then finish with some considerations on how the study of the Galactic population of magnetars might constrain their possible connection with Gamma Ray Bursts.
Over the past 15 years, detailed analyses of optical quasar spectra from the
Keck telescope and VLT have suggested possible evidence for cosmological
variations in the fine-structure constant, alpha. The joint dataset even
contained self-consistent evidence for variations across the sky, or large
spatial scales in the universe. It was long understood that, if these
effects were not real, not due to a varying alpha, then a complicated
combination of systematic effects was required to explain them. At the same
time, very similar analysis techniques, applied to molecular hydrogen
absorption in quasar spectra from the same VLT instrument, indicated no
variation in the proton-to-electron mass ratio. I will describe the recent
development of "supercalibration" techniques to precisely check the
wavelength calibration accuracy of such spectrographs, even using archival
data. These have uncovered subtle but ubiquitous systematic effects which
substantially weaken the previous evidence for variation in alpha. Similar
"supercalibration" checks may be vital for ensuring the accuracy of the next
generation of high-resolution spectrographs on the VLT and the 30-40-m
telescopes of the future.
In past years, large and deep photometric and spectroscopic surveys have significantly advanced our understanding of galaxy growth, from the most active time in the universe (z~2) to the present day. In particular, the evolution in stellar mass, star formation rate, and structure of complete galaxy samples have provided independent and complementary insights into their formation histories. In addition, detailed studies of the properties of distant galaxies have led to a better apprehension of the physical processes which govern galaxy growth. Nonetheless, many outstanding questions remain. In this talk I will give an overview of our current picture of galaxy growth over the past 11 billion years, discuss current challenges and outstanding questions, and introduce new and ongoing efforts to further unravel the formation histories of massive galaxies.
In recent years, precision measurements across cosmic time have led to a widely accepted cosmological paradigm for galaxy assembly and evolution, the cold dark matter (ΛCDM) model. Within this theory, galaxies form "bottom-up," with low-mass objects ("halos") collapsing earlier and merging to form larger and larger systems over time. Ordinary matter follows the dynamics dictated by the dominant dark matter until radiative, hydrodynamic, and star-formation processes take over. Although ΛCDM has had great success in explaining the observed large-scale distribution of mass in the universe, the nature of the dark matter particle is best tested on small scales, where its physical characteristics manifest themselves by modifying the structure of galaxy halos and their lumpiness. It is on these scale that detailed comparisons between observations and theory have revealed several discrepancies and challenged our understanding of the mapping between dark matter halos and their baryonic components. In this talk I will review some of the triumphs and tribulations of the theory. While the latter may indicate the need for more complex physics in the dark sector itself, emerging evidence suggests that a poor understanding of the baryonic processes involved in galaxy formation may be at the origin of these controversies.
Binary compact object mergers are among the primary gravitational wave sources expected to be observed by the next generation of ground-based gravitational wave detectors. Mergers where one or both compact objects are neutron stars will further produce electromagnetic emission, and coincident observation of this together with gravitational wave emission could teach us much about the progenitor systems, test general relativity in the dynamical strong field regime, and help elucidate the nature of matter at nuclear density. I will discuss ongoing work modeling such mergers within the context of general relativity coupled to ideal hydrodynamics, focusing on black hole-neutron star and binary neutron star systems merging with sizable eccentricity. Large eccentricity is expected for mergers that occur following dynamical capture or 3-body interactions in dense cluster environments, and though they may be rarer than the traditional quasi-circular inspiral, they could exhibit strikingly different behavior, including zoom-whirl orbital dynamics and large amounts of unbound material for cases where the neutron star is tidally disrupted.
Understanding the atmospheric and evolutive properties of Very Low Mass stars, 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). Early studies have provided rich insights into atmospheric physics, with discoveries ranging from cloud formation (Tsuji et al. 1996), methane bands (Oppenheimer et al. 1995) and ammonia bands formation (Delorme et al. 2008), to the formation of quasi-molecular KI-H2 absorption (Allard et al. 2007), and to disequilibrium chemistry (Yelle & Griffith 2001). New classical 1D models yield spectral energy distribution (SED) that match relatively well that of M dwarfs, brown dwarfs and young gas giant exoplanets despite these complexities. These models have for instance explained the spectral transition from M to L, T and now Y brown dwarf spectral types (Allard et al. 2013). However, in presence of surface inhomogeneities as revealed recently for a nearby brown dwarf (Crossfield et al. 2014), the SED may well fit exactly, but the model parameters could be far from exact, e.g. with the effective temperature by several hundred kelvins too cool in the case of dusty brown dwarfs and young gas giant exoplanets!
On the contrary, recent developments (revises more complete molecular opacities, revised solar abundances, calibration of the mixing length) have led the to a spectacular improvement of M Dwarfs model atmospheres and synthetic spectra, and therefore to a better knowledge of the Teff-scale of M Dwarfs (Rajpurohit et al. 2013) which has recently been confirmed by the study of 160 M dwarfs (Mann et al. 2015)! This is promising at a time where planets are being searched around M dwarfs. New evolution models for M Dwarfs have therefore been recently published (Baraffe et al. 2015).
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.
Acceleration of the universe has now been confirmed by a number of independent cosmological probes and is firmly established as a basic ingredient in the present-day universe. Yet the physical mechanism behind the acceleration - that is, the nature of dark energy - remains one of the great mysteries of modern physics. Given the increasingly impressive precision of cosmological measurements, an obvious question is whether we can improve our understanding of what drives the acceleration. I will review the current state of affairs, and present some recent developments on how to use measurements to rule out whole classes of explanations for dark energy.
The salience of satellite dwarf galaxies for understanding galaxy formation in a cosmological context has been made all the more evident in the past decade with the discovery of numerous faint Local Group galaxies. These faint systems are not only important to understand the faint end of galaxy formation but also their distribution around their host can test the hierarchical formation induced by the favored cosmological paradigm. I will review our successful effort to mine the two most ambitious surveys of the surroundings of the Milky Way and Andromeda galaxies, Pan-STARRS1 and PAndAS, for numerous new dwarf galaxies, before presenting the updated tally of MW/M31 satellites and what they are starting to tell us about galaxy formation in a LCDM universe.
What is the nature of dark energy and dark matter? I will describe two astronomical observations based on strong gravitational lensing that can address this question in a novel way. In the first part of talk, I use as cosmic "standard rods" strong gravitational lenses where the background source is variable in time and the foreground deflector is a massive galaxy. I will illustrate recent advances in modelling techniques and data quality that enable a 6-7% measurement of absolute distance from a single gravitational lens. I will show that results from just two systems yield constraints on the equation of state of dark energy and flatness comparable to those obtained with the best probes. In the second part of the talk I will discuss the use of strongly lensed galaxies and quasars to detect the presence of dark subhalos independent of their stellar content. This observation tests a fundamental prediction of the cold dark matter model, i.e. that galaxies should be surrounded by large numbers of dark satellite subhalos. Proof that such satellites do not exist would force a revision of the model in favor of more exotic alternatives like warm dark matter. I will then conclude by discussing the bright prospects of studies of the dark sector using strong gravitational lensing.
A picture arises where galaxy formation is fed by inflows of gas from
the inter-galactic medium (IGM), counteracted by strong galactic
winds, which in concert establish the growth rate of gas and stars
within galaxies at all cosmic epochs. These processes can be
collectively described as a “baryon cycle". I will present results on
UVES observations of the neutral gas reservoir for star formation, the
kinematics of gas around galaxies probed with IFU SINFONI and
X-Shooter and prospects to detect the circumgalactic medium (CGM) in
emission with purpose-built facilities. I will finally cover future
prospects in this field with the forthcoming Extremely Large optical
The Planck collaboration releases the results and data from the full mission including polarisation. The Planck space mission has fulfilled its initial goal of extracting essentially all the cosmological information in the temperature map of the Cosmic Microwave Background. It has also detected the polarisation cosmological signals with unprecedented sensitivity over the whole sky. The results of the Planck collaboration papers to appear soon will be presented. They bring a determination of the cosmological parameters and support for the standard Lambda CDM model independent from the intensity data. They also bring results relevant for the physics of the primodial universe as well as particle physics. The polarised foreground emission from interstellar dust has been mapped with a spectacular accuracy. The claim for detection of primordial gravity waves from the BICEP2 team using CMB data aquired at south pole will be discussed in the light of the the dust B modes signal observed by Planck.
The magnetic field strength at birth has long been considered a fundamental property in determining the evolutionary path of a neutron star (NS). Objects with very high fields, collectively known as magnetars, are characterized by high X-ray quiescent luminosities, outbursts, and, for some of them, sporadic giant flares. While the magnetic field strength is believed to drive their collective behaviour, however, the diversity of their properties, and the observation of magnetar-like bursts from 'low-field' pulsars, has been a theoretical puzzle. In this talk, I will discuss results of long-term MHD simulations which, by following the evolution of magnetic stresses with in the NS crust, have allowed to relate the observed magnetar phenomenology to the physical properties of the NSs, and in particular to their age and magnetic field strength and topology. The dichotomy of 'high-B' field pulsars versus magnetars is naturally explained, and occasional outbursts from old, low B-field NSs are predicted.
The appearance of the first stars about 100 million years after the Big Bang marked the beginning of the Reionization Epoch, an extended process in which the cosmic gas was ionized by the UV photons from the existing luminous sources. Most likely, in addition to stars, black holes also formed during the same epoch as end-products of massive star evolution, from direct collapse of gas clouds, or by stellar merging in dense stellar clusters. These black holes represent the "seeds" out of which observed super-massive black holes powering the most distant quasars were built. I will review the properties of first stars and black holes, their role for reionization, and the tight physical relationships between these two types of sources. I will put particular emphasis on the critical current and future experiments that could allow us to understand in detail these initial phases of cosmic structure formation.
Deep color-magnitude diagrams reaching the oldest main
sequence turnoffs offer the possibility to derive reliable, precise
star formation histories, and even, chemical enrichment histories for
the galaxies in our Local Group. I will discuss these results for a
variety or Local Group galaxies, as well as their implications in a
number of astrophysics topics such as i) the early conditions of the
Universe: reionization and feedback; ii) the dwarf galaxy
classification and the origin of the different dwarf galaxy types;
iii) the effects in interactions in the evolution of the galaxies.