We study the volume-limited and nearly mass selected (stellar mass M stars > ∼ 6 × 10 9 M ) ATLAS 3D sample of 260 early-type galaxies (ETGs, ellipticals Es and lenticulars S0s). We construct detailed axisymmetric dynamical models (JAM), which allow for orbital anisotropy, include a dark matter halo, and reproduce in detail both the galaxy images and the highquality 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 r = R e centred on the galaxies. We also measure the stellar (M/L) stars and derive a median dark matter fraction f DM = 13% in our sample. We infer masses M JAM ≡ L × (M/L) e ≈ 2 × 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 , σ e , R maj e ) coordinates system, which we call the Mass Plane (MP) has an observed rms scatter of 19%, which implies an intrinsic one of 11%. Here R maj e is the major axis of an isophote enclosing half of the observed galaxy light, while σ e is measured within that isophote. The MP satisfies the scalar virial relation M JAM ∝ σ 2 e R maj e within our tight errors. This show that the larger scatter in the Fundamental Plane (FP) (L, σ 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 σ 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 − σ e relation, which describes most of the deviations between the MP and the FP. The best-fitting relation is (M/L) e ∝ σ 0.72 e (r-band). It provides an upper limit to any systematic increase of the IMF mass normalization with σ 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%, and infer an intrinsic one of 8%. We perform an accurate empirical study of the link between σ e and the galaxies circular velocity V circ within 1R e (where stars dominate) and find the relation max(V circ ) ≈ 1.76 × σ e , which has an observed scatter of 7%. The accurate parameters described in this paper are used in the companion Paper XX of this series to explore the variation of global galaxy properties, including the IMF, on the projections of the MP.

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 ATLAS(3D) 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 galaxy's formation history.

We present the stellar population content of early-type galaxies from the ATLAS 3D survey. Using spectra integrated within apertures covering up to one effective radius, we apply two methods: one based on measuring line-strength indices and applying single stellar population (SSP) models to derive SSP-equivalent values of stellar age, metallicity, and alpha enhancement; and one based on spectral fitting to derive nonparametric star-formation histories, mass-weighted average values of age, metallicity, and half-mass formation timescales. Using homogeneously derived effective radii and dynamically-determined galaxy masses, we present the distribution of stellar population parameters on the Mass Plane (M JAM , σ e , R maj e ), showing that at fixed mass, compact early-type galaxies are on average older, more metal-rich, and more alphaenhanced than their larger counterparts.From non-parametric star-formation histories, we find that the duration of star formation is systematically more extended in lower mass objects. Assuming that our sample represents most of the stellar content of today's local Universe, approximately 50% of all stars formed within the first 2 Gyr following the big bang. Most of these stars reside today in the most massive galaxies (> 10 10.5 M ), which themselves formed 90% of their stars by z ∼2. The lower-mass objects, in contrast, have formed barely half their stars in this time interval. Stellar population properties are independent of environment over two orders of magnitude in local density, varying only with galaxy mass. In the highest-density regions of our volume (dominated by the Virgo cluster), galaxies are older, alpha-enhanced and have shorter star-formation histories with respect to lower density regions.This paper proceeds as follows: Section 2 describes the observations and analysis methods; Sections 3 and 4 present our population parameters on the mass-size plane, and against velocity dispersion and mass, respectively; Section 5 presents trends from our empirical star formation histories, and Section 6 concludes. OBSERVATIONS, LINE-STRENGTHS AND ANALYSISHere we describe the optical spectroscopy used in our analysis. The same spectral data cubes were used for deriving the line strengths and for the spectral fitting, so calibration and removal of emission applies to both techniques. SAURON observations and basic calibrationThe SAURON spectrograph and basic data reduction is described in detail in Bacon et al. (2001). Further details of the ATLAS 3D SAURON observations are given in Cappellari et al. (2011a). The details of our calibrations and line-strength measurements follow closely the techniques developed and described by Kuntschner et al. (2006). Most of the 260 ATLAS 3D SAURON spectra were obtained after the spectrograph was upgraded with a volume-phase holographic (VPH) grating in 2004, except for 64 objects observed as part of the SAURON Survey (de Zeeuw et al. 2002) and other related projects. The pre-and post-VPH data are reduced and analysed self-consistently to respect th...

We present H i observations of the edge-on galaxy NGC 891. These are among the deepest ever performed on an external galaxy. They reveal a huge gaseous halo, much more extended than seen previously and containing almost 30% of the H i. This H i halo shows structures on various scales. On one side, there is a filament extending (in projection) up to 22 kpc vertically from the disk. Small (M H i k 10 6 M ) halo clouds, some with forbidden (apparently counterrotating) velocities, are also detected. The overall kinematics of the halo gas is characterized by differential rotation lagging with respect to that of the disk. The lag, more pronounced at small radii, increases with height from the plane. There is evidence that a significant fraction of the halo is due to a galactic fountain. Accretion from intergalactic space may also play a role in building up the halo and providing the low angular momentum material needed to account for the observed rotation lag. The long H i filament and the counterrotating clouds may be direct evidence of such accretion.

In the companion Paper XV of this series we derive accurate total mass-to-light ratios (M/L) JAM ≈ (M/L)(r = R e ) within a sphere of radius r = R e centred on the galaxy, as well as stellar (M/L) stars (with the dark matter removed) for the volume-limited and nearly mass selected (stellar mass M > ∼ 6 × 10 9 M ) ATLAS 3D sample of 260 early-type galaxies (ETGs, ellipticals Es and lenticulars S0s). Here we use those parameters to study the two orthogonal projections (M JAM , σ e ) and (M JAM , R maj e ) of the thin Mass Plane (MP) (M JAM , σ e , R maj e ) which describes the distribution of the galaxy population, where M JAM ≡ L × (M/L) JAM ≈ M . The distribution of galaxy properties on both projections of the MP is characterized by: (i) the same zone of exclusion (ZOE), which can be transformed from one projection to the other using the scalar virial equation. The ZOE is roughly described by two power-laws, joined by a break at a characteristic mass M JAM ≈ 3 × 10 10 M , which corresponds to the minimum R e and maximum stellar density. This results in a break in the mean M JAM − σ e relation with trends M JAM ∝ σ 2.3

High sensitivity H I observations of the nearby spiral galaxy NGC 2403 obtained with the VLA are presented and discussed. The properties of the extended, differentially rotating H I layer with its H I holes, spiral structure and outer warp are described. In addition, these new data reveal the presence of a faint, extended and kinematically anomalous component. This shows up in the H I line profiles as extended wings of emission towards the systemic velocity. In the central regions these wings are very broad (up to 150 km s −1 ) and indicate large deviations from circular motion. We have separated the anomalous gas component from the cold

We present the results of deep Westerbork Synthesis Radio Telescope observations of neutral hydrogen in 12 nearby elliptical and lenticular galaxies. The selected objects come from a representative sample of nearby galaxies earlier studied at optical wavelengths with the integral‐field spectrograph SAURON (Spectrographic Areal Unit for Research on Optical Nebulae). They are field galaxies, or (in two cases) located in poor group environments. We detect H i– both in regular discs as well as in clouds and tails offset from the host galaxy – in 70 per cent of the galaxies. This detection rate is much higher than in previous, shallower single‐dish surveys, and is similar to that for the ionized gas. The results suggest that at faint detection levels the presence of H i is a relatively common characteristic of field early‐type galaxies, confirming what was suggested twenty years ago by Jura based on IRAS observations. The observed total H i masses range between a few times 106 to just over 109 M⊙. The presence of regular disc‐like structures is a situation as common as H i in offset clouds and tails around early‐type galaxies. All galaxies where H i is detected also contain ionized gas, whereas no H i is found around galaxies without ionized gas. Galaxies with regular H i discs tend to have strong emission from ionized gas. In these cases, the similar kinematics of the neutral hydrogen and ionized gas suggest that they form one structure. The kinematical axis of the stellar component is nearly always misaligned with respect to that of the gas. We do not find a clear trend between the presence of H i and the global age of the stellar population or the global dynamical characteristics of the galaxies. More specifically, H i detections are uniformly spread through the (V/σ, ε) diagram. If fast and slow rotators – galaxies with high and low specific angular momentum – represent the relics of different formation paths, this does not appear in the presence and characteristics of the H i. Our observations support the idea that gas accretion is common and does not happen exclusively in peculiar early‐type galaxies. The links observed between the large‐scale gas and the characteristics on the nuclear scale (e.g. the presence of kinematically decoupled cores, radio continuum emission etc.) suggest that for the majority of the cases the gas is acquired through merging, but the lack of correlation with the stellar population age suggests that smooth, cold accretion could be an alternative scenario, at least in some galaxies. In either case, the data suggest that early‐type galaxies continue to build their mass up to the present.

We report the detection of fast (∼1000 km s −1 ), massive outflows of neutral gas observed -using the WSRT -as 21-cm H i absorption against the strong radio continuum of seven radio sources. The neutral outflows occur, in at least somes cases, at kpc distance from the nucleus, and they are most likely driven by the interactions between the expanding radio jets and the gaseous medium enshrouding the central regions. We estimate that the associated mass outflow rates are up to ∼50 M yr −1 , comparable (although at the lower end of the distribution) to the outflow rates found for starburst-driven superwinds in Ultra Luminous IR Galaxies (ULIRG). This suggests that massive, jet-driven outflows of neutral gas in radio-loud AGN can have as large an impact on the evolution of the host galaxies as the outflows associated with starbursts. A radio-loud phase of the AGN is likely a relatively common, albeit short, phase in the life of many (or even all) massive ellipticals. Jet-driven neutral outflows may represent one of the main feedback mechanisms in these galaxies.