Katherine Alatalo

The ATLAS3D project – I. A volume‐limited sample of 260 nearby early‐type galaxies: science goals and selection criteria

The ATLAS3D project is a multiwavelength survey combined with a theoretical modelling effort. The observations span from the radio to the millimetre and optical, and provide multicolour imaging, two-dimensional kinematics of the atomic (H i), molecular (CO) and ionized gas (H beta, [O iii] and [N i]), together with the kinematics and population of the stars (H beta, Fe5015 and Mg b), for a carefully selected, volume-limited (1.16 x 105 Mpc3) 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 15 degrees) morphologically selected ETGs extracted from a parent sample of 871 galaxies (8 per cent E, 22 per cent S0 and 70 per cent spirals) brighter than M-K <-21.5 mag (stellar mass M-star greater than or similar to 6 x109 M-circle dot). We analyse 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 find typical 1 Sigma errors of delta V approximate to 6 km s-1, delta Sigma approximate to 7 km s-1, delta h(3) approximate to delta h(4) approximate to 0.03 in the mean velocity, the velocity dispersion and Gauss-Hermite (GH) moments for galaxies with effective dispersion Sigma(e) greater than or similar to 120 km s-1. For galaxies with lower Sigma(e) (approximate to 40 per cent of the sample) the GH moments are gradually penalized by ppxf towards zero to suppress the noise produced by the spectral undersampling and only V and Sigma can be measured. We give an overview of the characteristics of the other main data sets already available for our sample and of the ongoing modelling projects.

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, which depends on the present number of each type of star in the galaxy. The present number depends on the stellar initial mass function (IMF), which describes the distribution of stellar masses when the population formed, and knowledge of it is critical to almost every aspect of galaxy evolution. More than 50 years after the first IMF determination, no consensus has emerged on whether it is universal among different types of galaxies. Previous studies indicated that the IMF and the dark matter fraction in galaxy centres cannot both be universal, but they could not convincingly discriminate between the two possibilities. Only recently were indications found that massive elliptical galaxies may not have the same IMF as the Milky Way. Here we report a study of the two-dimensional stellar kinematics for the large representative ATLAS3D sample of nearby early-type galaxies spanning two orders of magnitude in stellar mass, using detailed dynamical models. We find a strong systematic variation in IMF in early-type galaxies as a function of their stellar mass-to-light ratios, producing differences of a factor of up to three in galactic stellar mass. This implies that a galaxy’s IMF depends intimately on the galaxys formation history.

The ATLAS3D project – III. A census of the stellar angular momentum within the effective radius of early-type galaxies: unveiling the distribution of fast and slow rotators

The definitive version can be found at : http://onlinelibrary.wiley.com/ Copyright Royal Astronomical Society

The ATLAS3D project XV: benchmark for early-type galaxies scaling relations from 260 dynamical models: mass-to-light ratio, dark matter, fundamental plane and mass plane

We study the volume-limited and nearly mass-selected (stellar mass M-stars greater than or similar to 6 x 10(9) M circle dot) ATLAS(3D) sample of 260 early-type galaxies (ETGs, ellipticals Es and lenticulars S0s). We construct detailed axisymmetric dynamical models (Jeans Anisotropic MGE), which allow for orbital anisotropy, include a dark matter halo and reproduce in detail both the galaxy images and the high-quality integral-field stellar kinematics out to about 1R(e), the projected half-light radius. We derive accurate total mass-to-light ratios (M/L)(e) and dark matter fractions f(DM), within a sphere of radius centred on the galaxies. We also measure the stellar (M/L)(stars) and derive a median dark matter fraction f(DM) = 13 per cent in our sample. We infer masses M-JAM equivalent to L x (M/L)(e) approximate to 2 x M-1/2, where M-1/2 is the total mass within a sphere enclosing half of the galaxy light. We find that the thin two-dimensional subset spanned by galaxies in the (M-JAM, sigma(e), R-e(maj)) coordinates system, which we call the Mass Plane (MP) has an observed rms scatter of 19 per cent, which implies an intrinsic one of 11 per cent. Here, is the major axis of an isophote enclosing half of the observed galaxy light, while Sigma(e) is measured within that isophote. The MP satisfies the scalar virial relation M-JAM proportional to sigma R-2(e)e(maj) within our tight errors. This show that the larger scatter in the Fundamental Plane (FP) (L, Sigma(e), R-e) is due to stellar population effects [including trends in the stellar initial mass function (IMF)]. It confirms that the FP deviation from the virial exponents is due to a genuine (M/L)(e) variation. However, the details of how both R-e and Sigma(e) are determined are critical in defining the precise deviation from the virial exponents. The main uncertainty in masses or M/L estimates using the scalar virial relation is in the measurement of R-e. This problem is already relevant for nearby galaxies and may cause significant biases in virial mass and size determinations at high redshift. Dynamical models can eliminate these problems. We revisit the (M/L)(e)-Sigma(e) relation, which describes most of the deviations between the MP and the FP. The best-fitting relation is (M/L)(e) sigma(0.72)(e) (r band). It provides an upper limit to any systematic increase of the IMF mass normalization with Sigma(e). The correlation is more shallow and has smaller scatter for slow rotating systems or for galaxies in Virgo. For the latter, when using the best distance estimates, we observe a scatter in (M/L)(e) of 11 per cent, and infer an intrinsic one of 8 per cent. We perform an accurate empirical study of the link between Sigma(e) and the galaxies circular velocity V-circ within 1R(e) (where stars dominate) and find the relation max (V-circ) approximate to 1.76 x Sigma(e), which has an observed scatter of 7 per cent. The accurate parameters described in this paper are used in the companion Paper XX (Cappellari et al.) of this series to explore the variation of global galaxy properties, including the IMF, on the projections of the MP.

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

We use the ATLAS3D sample of 260 early-type galaxies to study the apparent kinematic misalignment angle, Ψ, defined as the angle between the photometric and kinematic major axes. We find that 71 per cent of nearby early-type galaxies are strictly aligned systems (Ψ≤ 5°), an additional 14 per cent have 5° <Ψ≤ 10° and 90 per cent of galaxies have Ψ≤ 15°. Taking into account measurement uncertainties, 90 per cent of galaxies can be considered aligned to better than 5°, suggesting that only a small fraction of early-type galaxies (˜10 per cent) are not consistent with the axisymmetry within the projected half-light radius. We identify morphological features such as bars and rings (30 per cent), dust structures (16 per cent), blue nuclear colours (6 per cent) and evidence of interactions (8 per cent) visible on ATLAS3D galaxies. We use KINEMETRY to analyse the mean velocity maps and separate galaxies into two broad types of regular and non-regular rotators. We find 82 per cent of regular rotators and 17 per cent of non-regular rotators, with two galaxies that we were not able to classify due to the poor data quality. The non-regular rotators are typically found in dense regions and are massive. We characterize the specific features in the mean velocity and velocity dispersion maps. The majority of galaxies do not have any specific features, but we highlight here the frequency of the kinematically distinct cores (7 per cent of galaxies) and the aligned double peaks in the velocity dispersion maps (4 per cent of galaxies). We separate galaxies into five kinematic groups based on the kinemetric features, which are then used to interpret the (Ψ-ɛ) diagram. Most of the galaxies that are misaligned have complex kinematics and are non-regular rotators. In addition, some show evidence of the interaction and might not be in equilibrium, while some are barred. While the trends are weak, there is a tendency that large values of Ψ are found in galaxies at intermediate environmental densities and among the most massive galaxies in the sample. Taking into account the kinematic alignment and the kinemetric analysis, the majority of early-type galaxies have velocity maps more similar to that of the spiral discs than to that of the remnants of equal-mass mergers. We suggest that the most common formation mechanism for early-type galaxies preserves the axisymmetry of the disc progenitors and their general kinematic properties. Less commonly, the formation process results in a triaxial galaxy with much lower net angular momentum.

The ATLAS3D project - IV. The molecular gas content of early-type galaxies

The definitive version can be found at : http://onlinelibrary.wiley.com/ Copyright Royal Astronomical Society

The ATLAS3D project – VII. A new look at the morphology of nearby galaxies: the kinematic morphology–density relation

In Paper I of this series we introduced a volume-limited parent sample of 871 galaxies from which we extracted the ATLAS(3D) sample of 260 early-type galaxies (ETGs). In Papers II and III we classified the ETGs using their stellar kinematics, in a way that is nearly insensitive to the projection effects, and we separated them into fast and slow rotators. Here we look at galaxy morphology and note that the edge-on fast rotators generally are lenticular galaxies. They appear like spiral galaxies with the gas and dust removed, and in some cases are flat ellipticals (E5 or flatter) with discy isophotes. Fast rotators are often barred and span the same full range of bulge fractions as spiral galaxies. The slow rotators are rounder (E4 or rounder, except for counter-rotating discs) 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 by ignoring the large variation in the bulge sizes of fast rotators. Motivated by the fact that only one third (34 per cent) of the ellipticals in our sample are slow rotators, 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) less than or similar to 2 per cent] and generally constitute a small fraction [f (SR) approximate to 4 per cent] of the total galaxy population in the relatively low-density environments explored by our survey, with the exception of the densest core of the Virgo cluster [f(SR) approximate to 20 per cent]. This contrasts with the classic studies that invariably find significant fractions of (misclassified) ellipticals down to the lowest environmental densities. We find a clean log-linear relation between the fraction f(Sp) of spiral galaxies and the local galaxy surface density Sigma(3), within a cylinder enclosing the three nearest galaxies. This holds for nearly four orders of magnitude in the surface density down to Sigma(3) approximate to 0.01 Mpc(-2), with f(Sp) decreasing by 10 per cent per dex in Sigma(3), while f(FR) correspondingly increases. The existence of a smooth kinematic T-Sigma relation in the field excludes processes related to the cluster environment, like e.g. ram-pressure stripping, 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. This is supported by the relation becoming shallower when using a surface density estimator Sigma(10) with a cluster scale. Only at the largest densities in the Virgo core does the f(Sp) relation break down and steepen sharply, while the fraction of slow rotators starts to significantly increase. This suggests that a different mechanism is at work there, possibly related to the stripping of the gas from spirals by the hot intergalactic medium in the cluster core and the corresponding lack of cold accretion.

The ATLAS3D project - XIII. Mass and morphology of H I in early-type galaxies as a function of environment

We present the ATLAS3D H i survey of a volume-limited, complete sample of 166 nearby early-type galaxies (ETGs) brighter than MK=-21.5. The survey is mostly based on data taken with the Westerbork Synthesis Radio Telescope, which enables us to detect H i down to 5 x 1065 x 107 M? within the survey volume. We detect similar to 40 per cent of all ETGs outside the Virgo galaxy cluster and similar to 10 per cent of all ETGs inside it. This demonstrates that it is common for non-cluster ETGs to host H i. The morphology of the detected gas varies in a continuous way from regular, settled H i discs and rings to unsettled gas distributions (including tidal or accretion tails) and systems of clouds scattered around the galaxy. The majority of the detections consist of H i discs or rings (1/4 of all ETGs outside Virgo) so that if H i is detected in an ETG it is most likely distributed on a settled configuration. These systems come in two main types: small discs [ M?], which are confined within the stellar body and share the same kinematics of the stars; and large discs/rings [M(H i) up to 5 x 109 M?], which extend to tens of kpc from the host galaxy and are in half of the cases kinematically decoupled from the stars. Neutral hydrogen seems to provide material for star formation in ETGs. Galaxies containing H i within similar to 1Re exhibit signatures of on-going star formation in similar to 70 per cent of the cases, approximately five times more frequently than galaxies without central H i. The interstellar medium (ISM) in the centre of these galaxies is dominated by molecular gas, and in ETGs with a small gas disc the conversion of H i into H2 is as efficient as in spirals. The ETG H i mass function is characterized by M*similar to 2 x 109 M? and by a slope a similar to-0.7. Compared to spirals, ETGs host much less H i as a family. However, a significant fraction of all ETGs are as H i-rich as spiral galaxies. The main difference between ETGs and spirals is that the former lack the high-column-density H i typical of the bright stellar disc of the latter. The ETG H i properties vary with environment density in a more continuous way than suggested by the known Virgo versus non-Virgo dichotomy. We find an envelope of decreasing M(H i) and M(H i)/LK with increasing environment density. The gas-richest galaxies live in the poorest environments (as found also with CO observations), where the detection rate of star formation signatures is higher. Galaxies in the centre of Virgo have the lowest H i content, while galaxies at the outskirts of Virgo represent a transition region and can contain significant amounts of H i, indicating that at least a fraction of them has joined the cluster only recently after pre-processing in groups. Finally, we find an H i morphologydensity relation such that at low environment density (measured on a local scale) the detected H i is mostly distributed on large, regular discs and rings, while more disturbed H i morphologies dominate environment densities typical of rich groups. This confirms the importance of processes occurring on a galaxy-group scale for the evolution of ETGs.

Discovery of an active galactic nucleus 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?1 and requires a nested Gaussian fit to be properly described. Interferometric observations reveal a massive, centrally concentrated molecular component with a mass of 1.1 ? 109 M ? and a molecular outflow with a molecular mass of 2.4 ? 107 M ?. The molecular gas close to the systemic velocity consists of a rotating, compact nucleus with a mass of about 4.1 ? 108 M ? within a radius of 60?pc. This compact molecular nucleus has a surface density of 2.7 ? 104 M ??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 M ? yr?1 leads to a depletion timescale of 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. 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.

The ATLAS3D project – X. On the origin of the molecular and ionized gas in early-type galaxies

We make use of interferometric CO and HI observations, and optical integral-field spectroscopy from the ATLAS 3D survey to probe the origin of the molecular and ionised interstellar medium (ISM) in local early-type galaxies. We find th at 36±5% of our sample of fast rotating early-type galaxies have their ionised gas kinematically misaligned with respect to the stars, setting a strong lower limit on the importance of e xternally acquired gas (e.g. from mergers and cold accretion). Slow rotators have a flat distri bution of misalignments, indicating that the dominant source of gas is external. The molecular, ionised and atomic gas in all the detected galaxies are always kinematically aligned, even when they are misaligned from the stars, suggesting that all these three phases of the interst ellar medium share a common origin. In addition, we find that the origin of the cold and warm ga s in fast-rotating early-type galaxies is strongly affected by environment, despite the molecular gas detection rate and mass fractions being fairly independent of group/cluster membership. Galaxies in dense groups and the Virgo cluster nearly always have their molecular gas kinematically aligned with the stellar kinematics, consistent with a purely internal origin (pres umably stellar mass loss). In the field, however, kinematic misalignments between the stellar and gaseous components indicate that at least 42±5% of local fast-rotating early-type galaxies have their ga s supplied from external sources. When one also considers evidence of accretion present in the galaxies’ atomic gas distributions, > 46% of fast-rotating field ETGs are likely to have acquired a d etectable amount of ISM from accretion and mergers. We discuss several scenarios which could explain the environmental dichotomy, including preprocessing in galaxy groups/cluster outskirts and the morphological transformation of spiral galaxies, but we find it difficult to simultaneously explain the kinematic misalignment difference and the constant detection rate. Furthermore, our results suggest that galaxy mass may be an important independent factor associated with the origin of the gas, with the most massive fast-rotating ga laxies in our sample (MK < 24 mag; stellar mass of �8×10 10 M⊙) always having kinematically aligned gas. This mass dependence appears to be independent of environment, suggesting it is caused by a separate physical mechanism.