arXiv subjects

Frederic Bournaud

Publications and source records attributed to Frederic Bournaud.

At least 73 records · Page 4Linked to original sources

arXiv2012

A systematic variation of the stellar initial mass function in early-type galaxies

Much of our knowledge of galaxies comes from analysing the radiation emitted by their stars. It depends on the stellar initial mass function (IMF) describing the distribution of stellar masses when the population formed. Consequently knowledge of the IMF is critical to virtually every aspect of galaxy evolution. More than half a century after the first IMF determination, no consensus has emerged on whether it is universal in different galaxies. Previous studies indicated that the IMF and the dark matter fraction in galaxy centres cannot be both universal, but they could not break the degeneracy between the two effects. Only recently indications were found that massive elliptical galaxies may not have the same IMF as our Milky Way. Here we report unambiguous evidence for a strong systematic variation of the IMF in early-type galaxies as a function of their stellar mass-to-light ratio, producing differences up to a factor of three in mass. This was inferred from detailed dynamical models of the two-dimensional stellar kinematics for the large Atlas3D representative sample of nearby early-type galaxies spanning two orders of magnitude in stellar mass. Our finding indicates that the IMF depends intimately on a galaxy's formation history.

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arXiv2011

Rotational Support of Giant Clumps in High-z Disc Galaxies

We address the internal support against total free-fall collapse of the giant clumps that form by violent gravitational instability in high-z disc galaxies. Guidance is provided by an analytic model, where the proto-clumps are cut from a rotating disc and collapse to equilibrium while preserving angular momentum. This model predicts prograde clump rotation. This is confirmed in hydro-AMR zoom-in simulations of galaxies in a cosmological context. In most high-z clumps, the centrifugal force dominates the support, R=Vrot^2/Vcirc^2 > 0.5, where Vrot is the rotation velocity and Vcirc is the circular velocity. The clump spin indeed tends to be in the sense of the global disc angular momentum, but substantial tilts are frequent. Most clumps are in Jeans equilibrium, with the rest of the support provided by turbulence. Simulations of isolated gas-rich discs that resolve the clump substructure reveal that the cosmological simulations may overestimate R by ~30%, but the dominance of rotational support at high-z is not a resolution artifact. In turn, isolated gas-poor disc simulations produce at z=0 smaller gaseous non-rotating transient clouds, indicating that the difference in rotational support is associated with the fraction of cold baryons in the disc. In our current cosmological simulations, the clump rotation velocity is typically Vrot~100 km/s, but when beam smearing of \geq 0.1 arcsec is imposed, the rotation signal is reduced to a small gradient of \leq 30 km/s/kpc across the clump. The velocity dispersion in the simulated clumps is comparable to the disc dispersion so it is expected to leave only a marginal signal. Retrograde minor-merging galaxies could lead to massive clumps that do not show rotation.Testable predictions of the scenario as simulated are that the mean stellar age of the clumps, and the stellar fraction, are declining linearly with distance from the disc center.

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arXiv2011

The Atlas3D project -- XIII. Mass and morphology of HI in early-type galaxies as a function of environment

We present the Atlas3D HI survey of 166 nearby early-type galaxies (ETGs) down to M(HI)~10^7 M_sun. We detect HI in ~40% of all ETGs outside the Virgo cluster and in ~10% of all ETGs inside it. This demonstrates that it is common for non-cluster ETGs to host HI. The HI morphology varies from regular discs/rings (the majority of the detections) to unsettled gas distributions. The former are either small discs (M(HI)<10^8 M_sun) confined within the stellar body and sharing the same kinematics of the stars, or large discs/rings (M(HI) up to 5x10^9 M_sun) extending to tens of kpc from the host galaxy and frequently kinematically decoupled from the stars. Neutral hydrogen provides material for star formation in ETGs. Galaxies with central HI exhibit signatures of star formation in ~70% of the cases, ~5 times more frequently than galaxies without central HI. The central ISM is dominated by molecular gas. In ETGs with a small gas disc the conversion of HI into H_2 is as efficient as in spirals. The ETG HI mass function has M*~2x10^9 M_sun and slope=-0.7. ETGs host much less HI than spirals as a family. However, a significant fraction of them is as HI-rich as spirals. The main difference between ETGs and spirals is that the former lack the high-column-density HI typical of the bright stellar disc of the latter. We find an envelope of decreasing M(HI) with increasing environment density. The gas-richest ETGs live in the poorest environments (where star-formation is more common), galaxies in the centre of Virgo have the lowest HI content, and the cluster outskirts are a transition region. We find an HI morphology-density relation. At low environment density HI is mostly distributed on large discs/rings. More disturbed HI morphologies dominate environment densities typical of rich groups, confirming the importance of processes occurring on a galaxy-group scale for the evolution of ETGs.

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arXiv2011

Black Hole growth and AGN obscuration by instability-driven inflows in high-redshift disk galaxies fed by cold streams

Disk galaxies at high redshift have been predicted to maintain high gas surface densities due to continuous feeding by intense cold streams leading to violent gravitational instability, transient features and giant clumps. Gravitational torques between the perturbations drive angular momentum out and mass in, and the inflow provides the energy for keeping strong turbulence. We use analytic estimates of the inflow for a self-regulated unstable disk at a Toomre stability parameter Q~1, and isolated galaxy simulations capable of resolving the nuclear inflow down to the central parsec. We predict an average inflow rate ~10 Msun/yr through the disk of a 10^11 Msun galaxy, with conditions representative of z~2 stream-fed disks. The inflow rate scales with disk mass and (1+z)^{3/2}. It includes clump migration and inflow of the smoother component, valid even if clumps disrupt. This inflow grows the bulge, while only a fraction ~ 10^-3 of it needs to accrete onto a central black hole (BH), in order to obey the observed BH-bulge relation. A galaxy of 10^11 Msun at z~2 is expected to host a BH of ~10^8 Msun, accreting on average with moderate sub-Eddington luminosity L_X ~ 10^42-43 erg/s, accompanied by brighter episodes when dense clumps coalesce. We note that in rare massive galaxies at z~6, the same process may feed 10^9 Msun BH at the Eddington rate. High central gas column densities can severely obscure AGN in high-redshift disks, possibly hindering their detection in deep X-ray surveys.

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arXiv2011

CANDELS: Constraining the AGN-Merger Connection with Host Morphologies at z~2

Using HST/WFC3 imaging taken as part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), we examine the role that major galaxy mergers play in triggering active galactic nuclei (AGN) activity at z~2. Our sample consists of 72 moderate-luminosity (Lx ~ 1E42-1E44 erg/s) AGN at 1.5<z<2.5 that are selected using the 4 Msec Chandra observations in the Chandra Deep Field South, the deepest X-ray observations to date. Employing visual classifications, we have analyzed the rest-frame optical morphologies of the AGN host galaxies and compared them to a mass-matched control sample of 216 non-active galaxies at the same redshift. We find that most of the AGN reside in disk galaxies (51.4%), while a smaller percentage are found in spheroids (27.8%). Roughly 16.7% of the AGN hosts have highly disturbed morphologies and appear to be involved in a major merger or interaction, while most of the hosts (55.6%) appear relatively relaxed and undisturbed. These fractions are statistically consistent with the fraction of control galaxies that show similar morphological disturbances. These results suggest that the hosts of moderate-luminosity AGN are no more likely to be involved in an ongoing merger or interaction relative to non-active galaxies of similar mass at z~2. The high disk fraction observed among the AGN hosts also appears to be at odds with predictions that merger-driven accretion should be the dominant AGN fueling mode at z~2, even at moderate X-ray luminosities. Although we cannot rule out that minor mergers are responsible for triggering these systems, the presence of a large population of relatively undisturbed disk-like hosts suggests that secular processes play a greater role in fueling AGN activity at z~2 than previously thought.

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arXiv2011

The ATLAS3D Project-- VIII: Modelling the Formation and Evolution of Fast and Slow Rotator Early-Type Galaxies within $Λ$CDM

We propose a simple model for the origin of fast and slow rotator early-type galaxies (ETG) within the hierarchical $Λ$CDM scenario, that is based on the assumption that the mass fraction of stellar discs in ETGs is a proxy for the specific angular momentum expressed via $λ_R$. Within our model we reproduce the fraction of fast and slow rotators as a function of magnitude in the \atl survey, assuming that fast rotating ETGs have at least 10% of their total stellar mass in a disc component. In agreement with \atl observations we find that slow rotators are predominantly galaxies with $ M_* > 10^{10.5}$ M$_{\odot}$ contributing $\sim 20%$ to the overall ETG population. We show in detail that the growth histories of fast and slow rotators are different, supporting the classification of ETGs into these two categories. Slow rotators accrete between $\sim 50% -90%$ of their stellar mass from satellites and their most massive progenitors have on average up to 3 major mergers during their evolution. Fast rotators in contrast, accrete less than 50% and have on average less than one major merger in their past. We find that the underlying physical reason for the different growth histories is the slowing down and ultimately complete shut-down of gas cooling in massive galaxies. Once cooling and associated star formation in disc stops, galaxies grow via infall from satellites. Frequent minor mergers thereby, destroy existing stellar discs via violent relaxation and also tend to lower the specific angular momentum of the main stellar body, lowering $λ_R$ into the slow rotator regime. Abridged...

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arXiv2011

Star formation in galaxy interactions and mergers

This lecture reviews the fundamental physical processes involved in star formation in galaxy interactions and mergers. Interactions and mergers often drive intense starbursts, but the link between interstellar gas physics, large scale interactions, and active star formation is complex and not fully understood yet. Two processes can drive starbursts: radial inflows of gas can fuel nuclear starbursts, triggered gas turbulence and fragmentation can drive more extended starbursts in massive star clusters with high fractions of dense gas. Both modes are certainly required to account for the observed properties of starbursting mergers. A particular consequence is that star formation scaling laws are not universal, but vary from quiescent disks to starbursting mergers. High-resolution hydrodynamic simulations are used to illustrate the lectures.

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arXiv2011

The Atlas-3D project - IX. The merger origin of a fast and a slow rotating Early-Type Galaxy revealed with deep optical imaging: first results

The mass assembly of galaxies leaves imprints in their outskirts, such as shells and tidal tails. The frequency and properties of such fine structures depend on the main acting mechanisms - secular evolution, minor or major mergers - and on the age of the last substantial accretion event. We use this to constrain the mass assembly history of two apparently relaxed nearby Early-Type Galaxies (ETGs) selected from the Atlas-3D sample, NGC 680 and NGC 5557. Our ultra deep optical images obtained with MegaCam on the Canada-France-Hawaii Telescope reach 29 mag/arcsec^2 in the g-band. They reveal very low-surface brightness (LSB) filamentary structures around these ellipticals. Among them, a gigantic 160 kpc long tail East of NGC 5557 hosts gas-rich star-forming objects. NGC 680 exhibits two major diffuse plumes apparently connected to extended HI tails, as well as a series of arcs and shells. Comparing the outer stellar and gaseous morphology of the two ellipticals with that predicted from models of colliding galaxies, we argue that the LSB features are tidal debris, the star-forming objects near NGC 5557, long lived Tidal Dwarf Galaxies and that each of the two ETGs was assembled during a relatively recent, major wet merger, which likely occurred at a redshift below z = 0.5. The inner kinematics of NGC 680 is typical for fast rotators which make the bulk of nearby ETGs in the Atlas-3D sample. NGC 5557 belongs to the poorly populated class of massive, round, slow rotators that are predicted by semi-analytic models and cosmological simulations to be the end-product of a complex mass accretion history, involving ancient major mergers and more recent minor mergers. Our observations suggest that under specific circumstances a single binary merger may dominate the formation history of such objects and thus that at least some massive ETGs may form at relatively low redshift (abridged).

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arXiv2011

The Atlas3D Project - VI. Simulations of binary galaxy mergers and the link with Fast Rotators, Slow Rotators, and Kinematically Distinct Cores

We study the formation of early-type galaxies through mergers with a sample of 70 high-resolution (softening length < 60 pc and 12*10^6 particles) numerical simulations of binary mergers of disc galaxies and 16 simulations of ETG remergers. These simulations, designed to accompany observations and models conducted within the Atlas3D project, encompass various mass ratios (from 1:1 to 6:1), initial conditions and orbital parameters. The progenitor disc galaxies are spiral-like with bulge to disc ratios typical of Sb and Sc galaxies. We find that binary mergers of disc galaxies with mass ratios of 3:1 and 6:1 are nearly always classified as Fast Rotators according to the Atlas3D criterion (based on the lambda_R parameter): they preserve the structure of the input fast rotating spiral progenitors. They have intrinsic ellipticities larger than 0.5, cover intrinsic lambda_R values between 0.2 and 0.6, within the range of observed Fast Rotators. Major disc mergers (mass ratios of 2:1 and 1:1) lead to both Fast and Slow Rotators. Most of the Fast Rotators produced in major mergers have intermediate flattening, with ellipticities between 0.4 and 0.6. Most Slow Rotators formed in these binary disc mergers hold a stellar Kinematically Distinct Core (KDC) in their 1-3 central kilo-parsec: these KDCs are built from the stellar components of the progenitors. Besides a handful of specific observed systems -- the counter-rotating discs (2-sigma galaxies) -- these therefore cannot reproduce the observed population of Slow Rotators in the nearby Universe. The mass ratio of the progenitors is a fundamental parameter for the formation of Slow Rotators in these binary mergers, but it also requires a retrograde spin for the earlier-type (Sb) progenitor galaxy with respect to the orbital angular momentum. (Abridged)

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arXiv2011

The Atlas3D project - VII. A new look at the morphology of nearby galaxies: the kinematic morphology-density relation

We look at the morphology of fast and slow rotator early-type galaxies. Edge-on fast rotators are lenticular galaxies. They appear like spiral galaxies with the gas and dust removed, and in some cases are flat ellipticals with disky isophotes. Fast rotators are often barred and span the same full range of bulge fractions as spiral galaxies. The slow rotators are rounder and are generally consistent with being genuine, namely spheroidal-like, elliptical galaxies. We propose a revision to the tuning-fork diagram by Hubble as it gives a misleading description of ETGs. We study for the first time the kinematic morphology-density T-Sigma relation using fast and slow rotators to replace lenticulars and ellipticals. We find that our relation is cleaner than using classic morphology. Slow rotators are nearly absent at the lowest density environments [f(SR)<2%] and generally constitute a small fraction [f(SR)~4%] of the total galaxy population in the environments explored by our survey, with the exception of the densest core of the Virgo cluster [f(SR)~20%]. We find a clean log-linear relation between the fraction f(Sp) of spiral galaxies and the local galaxy surface density. The existence of a smooth kinematic T-Sigma relation in the field excludes processes related to the cluster environment as main contributors to the apparent conversion of spirals into fast-rotators in low-density environments. It shows that the segregation is driven by local effects at the small-group scale. Only at the largest densities in the Virgo core does the f(Sp) relation break down and steepens sharply, while the fraction of slow-rotators starts to significantly increase. This suggests that a different mechanism is at work there. (Abridged)

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arXiv2011

Discovery of an AGN-Driven Molecular Outflow in the Local Early-Type Galaxy NGC 1266

We report the discovery of a powerful molecular wind from the nucleus of the non-interacting nearby S0 field galaxy NGC 1266. The single-dish CO profile exhibits emission to +/- 400 km/s and requires a nested Gaussian fit to be properly described. Interferometric observations reveal a massive, centrally- concentrated molecular component with a mass of 1.1x10^9 Msuns and a molecular outflow with a molecular mass of 2.4x10^7 Msuns. The molecular gas close to the systemic velocity consists of a rotating, compact nucleus with a mass of about 4.1x108 Msuns within a radius of approximately 60 pc. This compact molecular nucleus has a surface density of \approx 2.7 \times 10^4 Msuns/pc^2, more than two orders of magnitude larger than that of giant molecular clouds in the disk of the Milky Way, and it appears to sit on the Kennicutt-Schmidt relation despite its extreme kinematics and energetic activity. We interpret this nucleus as a disk that confines the outflowing wind. A mass outflow rate of 13 Msuns/yr leads to a depletion timescale of about 85 Myr. The star formation in NGC 1266 is insufficient to drive the outflow, and thus it is likely driven by the active galactic nucleus (AGN). The concentration of the majority of the molecular gas in the central 100 pc requires an extraordinary loss of angular momentum, but no obvious companion or interacting galaxy is present to enable the transfer. NGC 1266 is the first known outflowing molecular system that does not show any evidence of a recent interaction.

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arXiv2011

The ATLAS3D project - II. Morphologies, kinemetric features and alignment between photometric and kinematic axes of early-type galaxies

[Abridged] We use the ATLAS3D sample of 260 early-type galaxies (ETGs) to study the apparent kinematic misalignment angle, Psi, defined as the angle between the photometric and kinematic major axis. We find that 71% of nearby ETGs are strictly aligned systems (Psi > 5 deg), an additional 14% have 5 < Psi < 10 deg and 90% of galaxies have Psi < 15 deg. Taking into account measurement uncertainties, 90% of galaxies can be considered aligned to better than 5 deg, suggesting that only a small fraction of early-type galaxies (~10%) are not consistent with axisymmetry within the projected half-light radius. We identify morphological features such as bars and rings (30%), dust structures (16%), blue nuclear colours (6%) and evidence of interactions (8%). We use kinemetry to analyse the mean velocity maps and separate galaxies in two broad types of regular and non-regular rotators. We find 82% of regular rotators and 17% non-regular rotators, with 2 galaxies that we were not able to classify due to data quality. The non-regular rotators are typically found in dense regions and are massive. The majority of galaxies do not have any specific kinemetric features, but we highlight here the frequency of the kinematically distinct cores (7%) and the aligned double peaks in the velocity dispersion maps (4%). Most of the galaxies that are misaligned have complex kinematics and are non-regular rotators. While the trends are weak, there is a tendency that large values of Psi are found in galaxies at intermediate environmental densities and among the most massive galaxies in the sample. We suggest that the most common formation mechanism for ETGs preserves the axisymmetry of the disk progenitors. Less commonly, the formation process results in a triaxial galaxy with much lower net angular momentum.

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arXiv2011

The Atlas3D project -- IV: the molecular gas content of early-type galaxies

We have carried out a survey for 12CO J=1-0 and J=2-1 emission in the 260 early-type galaxies of the volume-limited Atlas3D sample, with the goal of connecting their star formation and assembly histories to their cold gas content. This is the largest volume-limited CO survey of its kind and is the first to include many Virgo Cluster members. Sample members are dynamically hot galaxies with a median stellar mass 3\times 10^{10} Msun; they are selected by morphology rather than colour, and the bulk of them lie on the red sequence. The overall CO detection rate is 56/259 = 0.22 \error 0.03, with no dependence on K luminosity and only a modest dependence on dynamical mass. There are a dozen CO detections among the Virgo Cluster members; statistical analysis of their H_2 mass distributions and their dynamical status within the cluster shows that the cluster's influence on their molecular masses is subtle at best, even though (unlike spirals) they seem to be virialized within the cluster. We suggest that the cluster members have retained their molecular gas through several Gyr residences in the cluster. There are also a few extremely CO-rich early-type galaxies with H_2 masses >= 10^9 Msun, and these are in low density environments. We do find a significant trend between molecular content and the stellar specific angular momentum. The galaxies of low angular momentum also have low CO detection rates, suggesting that their formation processes were more effective at destroying molecular gas or preventing its re-accretion. We speculate on the implications of these data for the formation of various sub-classes of early-type galaxies.

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arXiv2011

The Atlas3D project - V. The CO Tully-Fisher relation of early-type galaxies

We demonstrate here using both single-dish and interferometric observations that CO molecules are an excellent kinematic tracer, even in high-mass galaxies, allowing us to investigate for the first time the CO Tully-Fisher relation of early-type galaxies. We compare the Tully-Fisher relations produced using both single-dish and interferometric data and various inclination estimation methods, and evaluate the use of the velocity profile shape as a criterion for selecting galaxies in which the molecular gas extends beyond the peak of the rotation curve. We show that the gradient and zero-point of the best-fit relations are robust, independent of the velocity measure and inclination used, and agree with those of relations derived using stellar kinematics. We also show that the early-type CO Tully-Fisher (CO-TF) relation is offset from the CO-TF of spirals by 0.98 \pm 0.22 magnitude at Ks-band, in line with other results. The intrinsic scatter of the relation is found to be \approx 0.4 magnitudes, similar to the level found in the spiral galaxy population. Next generation facilities such as the Large Millimeter Telescope (LMT) and the Atacama Large Millimeter/Sub-millimeter Array (ALMA) should allow this technique to be used in higher-redshift systems, providing a simple new tool to trace the mass-to-light ratio evolution of the most massive galaxies over cosmic time.

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arXiv2011

The impact of ISM turbulence, clustered star formation and feedback on galaxy mass assembly through cold flows and mergers

Two of the dominant channels for galaxy mass assembly are cold flows (cold gas supplied via the filaments of the cosmic web) and mergers. How these processes combine in a cosmological setting, at both low and high redshift, to produce the whole zoo of galaxies we observe is largely unknown. Indeed there is still much to understand about the detailed physics of each process in isolation. While these formation channels have been studied using hydrodynamical simulations, here we study their impact on gas properties and star formation (SF) with some of the first simulations that capture the multiphase, cloudy nature of the interstellar medium (ISM), by virtue of their high spatial resolution (and corresponding low temperature threshold). In this regime, we examine the competition between cold flows and a supernovae(SNe)-driven outflow in a very high-redshift galaxy (z {\approx} 9) and study the evolution of equal-mass galaxy mergers at low and high redshift, focusing on the induced SF. We find that SNe-driven outflows cannot reduce the cold accretion at z {\approx} 9 and that SF is actually enhanced due to the ensuing metal enrichment. We demonstrate how several recent observational results on galaxy populations (e.g. enhanced HCN/CO ratios in ULIRGs, a separate Kennicutt Schmidt (KS) sequence for starbursts and the population of compact early type galaxies (ETGs) at high redshift) can be explained with mechanisms captured in galaxy merger simulations, provided that the multiphase nature of the ISM is resolved.

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arXiv2011

Hydrodynamics of high-redshift galaxy collisions: From gas-rich disks to dispersion-dominated mergers and compact spheroids

Disk galaxies at high redshift (z~2) are characterized by high fractions of cold gas, strong turbulence, and giant star-forming clumps. Major mergers of disk galaxies at high redshift should then generally involve such turbulent clumpy disks. Merger simulations, however, model the ISM as a stable, homogeneous, and thermally pressurized medium. We present the first merger simulations with high fractions of cold, turbulent, and clumpy gas. We discuss the major new features of these models compared to models where the gas is artificially stabilized and warmed. Gas turbulence, which is already strong in high-redshift disks, is further enhanced in mergers. Some phases are dispersion-dominated, with most of the gas kinetic energy in the form of velocity dispersion and very chaotic velocity fields, unlike merger models using a thermally stabilized gas. These mergers can reach very high star formation rates, and have multi-component gas spectra consistent with SubMillimeter Galaxies. Major mergers with high fractions of cold turbulent gas are also characterized by highly dissipative gas collapse to the center of mass, with the stellar component following in a global contraction. The final galaxies are early-type with relatively small radii and high Sersic indices, like high-redshift compact spheroids. The mass fraction in a disk component that survives or re-forms after a merger is severely reduced compared to models with stabilized gas, and the formation of a massive disk component would require significant accretion of external baryons afterwards. Mergers thus appear to destroy extended disks even when the gas fraction is high, and this lends further support to smooth infall as the main formation mechanism for massive disk galaxies.

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arXiv2010

Star formation in galaxy mergers: ISM turbulence, dense gas excess, and scaling relations for disks and starbusts

Galaxy interactions and mergers play a significant, but still debated and poorly understood role in the star formation history of galaxies. Numerical and theoretical models cannot yet explain the main properties of merger-induced starbursts, including their intensity and their spatial extent. Usually, the mechanism invoked in merger-induced starbursts is a global inflow of gas towards the central kpc, resulting in a nuclear starburst. We show here, using high-resolution AMR simulations and comparing to observations of the gas component in mergers, that the triggering of starbursts also results from increased ISM turbulence and velocity dispersions in interacting systems. This forms cold gas that are denser and more massive than in quiescent disk galaxies. The fraction of dense cold gas largely increases, modifying the global density distribution of these systems, and efficient star formation results. Because the starbursting activity is not just from a global compacting of the gas to higher average surface densities, but also from higher turbulence and fragmentation into massive and dense clouds, merging systems can enter a different regime of star formation compared to quiescent disk galaxies. This is in quantitative agreement with recent observations suggesting that disk galaxies and starbursting systems are not the low-activity end and high-activity end of a single regime, but actually follow different scaling relations for their star formation.

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arXiv2010

The Atlas3D project -- I. A volume-limited sample of 260 nearby early-type galaxies: science goals and selection criteria

The Atlas3D project is a multi-wavelength survey combined with a theoretical modeling effort. The observations span from the radio to the millimeter and optical, and provide multi-colour imaging, two-dimensional kinematics of the atomic (HI), molecular (CO) and ionized gas (Hbeta, [OIII] and [NI]), together with the kinematics and population of the stars (Hbeta, Fe5015 and Mgb), for a carefully selected, volume-limited (1.16*10^5 Mpc^3) sample of 260 early-type (elliptical E and lenticular S0) galaxies (ETGs). The models include semi-analytic, N-body binary mergers and cosmological simulations of galaxy formation. Here we present the science goals for the project and introduce the galaxy sample and the selection criteria. The sample consists of nearby (D<42 Mpc) morphologically-selected ETGs extracted from a parent sample of 871 galaxies (8% E, 22% S0 and 70% spirals) brighter than M_K<-21.5 mag (stellar mass M_Star>6*10^9 M_Sun). We analyze possible selection biases and we conclude that the parent sample is essentially complete and statistically representative of the nearby galaxy population. We present the size-luminosity relation for the spirals and ETGs and show that the ETGs in the Atlas3D sample define a tight red sequence in a colour-magnitude diagram, with few objects in the transition from the blue cloud. We describe the strategy of the SAURON integral-field observations and the extraction of the stellar kinematics with the pPXF method. We give an overview of the characteristics of the other main datasets already available for our sample and of the ongoing modelling projects.

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