ALBERT

Description: We present a detailed two-dimensional stellar dynamical analysis of a sample of 44 cosmological hydrodynamical simulations of individual central galaxies with stellar masses of 2 10 10 M    M *   6 10 11 M . Kinematic maps of the stellar line-of-sight velocity, velocity dispersion and higher order Gauss–Hermite moments h 3 and h 4 are constructed for each central galaxy and for the most massive satellites. The amount of rotation is quantified using the R -parameter. The velocity, velocity dispersion, h 3 and h 4 fields of the simulated galaxies show a diversity similar to observed kinematic maps of early-type galaxies in the ATLAS 3D survey. This includes fast (regular), slow and misaligned rotation, hot spheroids with embedded cold disc components as well as galaxies with counter-rotating cores or central depressions in the velocity dispersion. We link the present-day kinematic properties to the individual cosmological formation histories of the galaxies. In general, major galaxy mergers have a significant influence on the rotation properties resulting in both a spin-down as well as a spin-up of the merger remnant. Lower mass galaxies with significant (18 per cent) in situ formation of stars since z 2, or with additional gas-rich major mergers – resulting in a spin-up – in their formation history, form elongated ( ~ 0.45) fast rotators ( R  ~ 0.46) with a clear anticorrelation of h 3 and v /. An additional formation path for fast rotators includes gas-poor major mergers leading to a spin-up of the remnants ( R  ~ 0.43). This formation path does not result in anticorrelated h 3 and v /. The formation histories of slow rotators can include late major mergers. If the merger is gas rich, the remnant typically is a less flattened slow rotator with a central dip in the velocity dispersion. If the merger is gas poor, the remnant is very elongated ( ~ 0.43) and slowly rotating ( R  ~ 0.11). The galaxies most consistent with the rare class of non-rotating round early-type galaxies grow by gas-poor minor mergers alone. In general, more massive galaxies have less in situ star formation since z  ~ 2, rotate slower and have older stellar populations. We discuss general implications for the formation of fast and slowly rotating galaxies as well as the weaknesses and strengths of the underlying models.

Description: One quarter of all nearby early-type galaxies (ETGs) outside Virgo host a disc/ring of H i with size from a few to tens of kpc and mass up to ~10 9 M . Here we investigate whether this H i is related to the presence of a stellar disc within the host making use of the classification of ETGs in fast and slow rotators (FR/SR). We find a large diversity of H i masses and morphologies within both families. Surprisingly, SRs are detected as often, host as much H i and have a similar rate of H i discs/rings as FRs. Accretion of H i is therefore not always linked to the growth of an inner stellar disc. The weak relation between H i and stellar disc is confirmed by their frequent kinematical misalignment in FRs, including cases of polar and counterrotating gas. In SRs the H i is usually polar. This complex picture highlights a diversity of ETG formation histories which may be lost in the relative simplicity of their inner structure and emerges when studying their outer regions. We find that CDM hydrodynamical simulations have difficulties reproducing the H i properties of ETGs. The gas discs formed in simulations are either too massive or too small depending on the star formation feedback implementation. Kinematical misalignments match the observations only qualitatively. The main point of conflict is that nearly all simulated FRs and a large fraction of all simulated SRs host corotating H i . This establishes the H i properties of ETGs as a novel challenge to simulations.

Description: We present measurements of the star formation rate (SFR) in the early-type galaxies (ETGs) of the ATLAS 3D sample, based on Wide-field Infrared Survey Explorer ( WISE ) 22 μm and Galaxy Evolution Explorer far-ultraviolet emission. We combine these with gas masses estimated from 12 CO and H i data in order to investigate the star formation efficiency (SFE) in a larger sample of ETGs than previously available. We first recalibrate (based on WISE data) the relation between old stellar populations (traced at K s band) and 22 μm luminosity, allowing us to remove the contribution of 22 μm emission from circumstellar dust. We then go on to investigate the position of ETGs on the Kennicutt–Schmidt (KS) relation. Molecular gas-rich ETGs have comparable star formation surface densities to normal spiral galaxy centres, but they lie systematically offset from the KS relation, having lower SFEs by a factor of 2.5 (in agreement with other authors). This effect is driven by galaxies where a substantial fraction of the molecular material is in the rising part of the rotation curve, and shear is high. We show here for the first time that although the number of stars formed per unit gas mass per unit time is lower in ETGs, it seems that the amount of stars formed per free-fall time is approximately constant. The scatter around this dynamical relation still correlates with galaxy properties such as the shape of the potential in the inner regions. This leads us to suggest that dynamical properties (such as shear or the global stability of the gas) may be important second parameters that regulate star formation and cause much of the scatter around star formation relations.

Description: We present a study of the cold gas contents of the Atlas 3D early-type galaxies, in the context of their optical colours, near-ultraviolet colours and Hβ absorption line strengths. Early-type (elliptical and lenticular) galaxies are not as gas poor as previously thought, and at least 40 per cent of local early-type galaxies are now known to contain molecular and/or atomic gas. This cold gas offers the opportunity to study recent galaxy evolution through the processes of cold gas acquisition, consumption (star formation) and removal. Molecular and atomic gas detection rates range from 10 to 34 per cent in red sequence early-type galaxies, depending on how the red sequence is defined, and from 50 to 70 per cent in blue early-type galaxies. Notably, massive red sequence early-type galaxies (stellar masses 〉5 10 10 M , derived from dynamical models) are found to have H i masses up to M (H i )/ M * ~ 0.06 and H 2 masses up to M (H 2 )/ M * ~ 0.01. Some 20 per cent of all massive early-type galaxies may have retained atomic and/or molecular gas through their transition to the red sequence. However, kinematic and metallicity signatures of external gas accretion (either from satellite galaxies or the intergalactic medium) are also common, particularly at stellar masses ≤5 10 10 M , where such signatures are found in ~50 per cent of H 2 -rich early-type galaxies. Our data are thus consistent with a scenario in which fast rotator early-type galaxies are quenched former spiral galaxies which have undergone some bulge growth processes, and in addition, some of them also experience cold gas accretion which can initiate a period of modest star formation activity. We discuss implications for the interpretation of colour–magnitude diagrams.

Description: 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 non-parametric star formation histories, mass-weighted average values of age, metallicity, and half-mass formation time-scales. Using homogeneously derived effective radii and dynamically determined galaxy masses, we present the distribution of stellar population parameters on the Mass Plane ( M JAM , e , $R^{\rm maj}_{\rm e}$ ), showing that at fixed mass, compact early-type galaxies are on average older, more metal-rich, and more alpha-enhanced 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 per cent 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 per cent 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.

Description: We investigate nuclear light profiles in 135 ATLAS 3D galaxies for which the Hubble Space Telescope ( HST ) imaging is available and compare them to the large-scale kinematics obtained with the SAURON integral-field spectrograph. Specific angular momentum, R , correlates with the shape of nuclear light profiles, where, as suggested by previous studies, cores are typically found in slow rotators and core-less galaxies are fast rotators. As also shown before, cores are found only in massive galaxies and only in systems with the stellar mass (measured via dynamical models) M   8  x 10 10 M . Based on our sample, we, however, see no evidence for a bimodal distribution of nuclear slopes. The best predictor for finding a core is based on the stellar velocity dispersion within an effective radius, e , and specific angular momentum, where cores are found for R   0.25 and e   160 km s –1 . We estimate that only about 10 per cent of nearby early-type galaxies contain cores. Furthermore, we show that there is a genuine population of fast rotators with cores. We also show that core fast rotators are morphologically, kinematically and dynamically different from core slow rotators. The cores of fast rotators, however, could harbour black holes of similar masses to those in core slow rotators, but typically more massive than those found in core-less fast rotators. Cores of both fast and slow rotators are made of old stars and found in galaxies typically lacking molecular or atomic gas (with a few exceptions). Core-less galaxies, and especially core-less fast rotators, are underluminous in the diffuse X-ray emission, but the presence of a core does not imply high X-ray luminosities. Additionally, we postulate (as many of these galaxies lack HST imaging) a possible population of core-less galaxies among slow rotators, which cannot be explained as face-on discs, but comprise a genuine sub-population of slow rotators. These galaxies are typically less massive and flatter than core slow rotators, and show evidence for dynamical cold structures and exponential photometric components. Based on our findings, major non-dissipative (gas-poor) mergers together with black hole binary evolution may not be the only path for formation of cores in early-type galaxies. We discuss possible processes for formation of cores and their subsequent preservation.

Description: Galactic archaeology based on star counts is instrumental to reconstruct the past mass assembly of Local Group galaxies. The development of new observing techniques and data reduction, coupled with the use of sensitive large field of view cameras, now allows us to pursue this technique in more distant galaxies exploiting their diffuse low surface brightness (LSB) light. As part of the ATLAS 3D project, we have obtained with the MegaCam camera at the Canada–France–Hawaii Telescope extremely deep, multiband images of nearby early-type galaxies (ETGs). We present here a catalogue of 92 galaxies from the ATLAS 3D sample, which are located in low- to medium-density environments. The observing strategy and data reduction pipeline, which achieve a gain of several magnitudes in the limiting surface brightness with respect to classical imaging surveys, are presented. The size and depth of the survey are compared to other recent deep imaging projects. The paper highlights the capability of LSB-optimized surveys at detecting new prominent structures that change the apparent morphology of galaxies. The intrinsic limitations of deep imaging observations are also discussed, among those, the contamination of the stellar haloes of galaxies by extended ghost reflections, and the cirrus emission from Galactic dust. The detection and systematic census of fine structures that trace the present and past mass assembly of ETGs are one of the prime goals of the project. We provide specific examples of each type of observed structures – tidal tails, stellar streams and shells – and explain how they were identified and classified. We give an overview of the initial results. The detailed statistical analysis will be presented in future papers.

Description: We study the volume-limited and nearly mass-selected (stellar mass M stars 6 x 10 9 M ) 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 1 R 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 $r={R_{\rm e}}$ 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 L x (M/L) e 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 $(M_{\rm JAM},\sigma _e,R_{\rm e}^{\rm maj})$ 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, $R_{\rm e}^{\rm maj}$ is the major axis of an isophote enclosing half of the observed galaxy light, while e is measured within that isophote. The MP satisfies the scalar virial relation $M_{\rm JAM}\propto \sigma _e^2 R_{\rm e}^{\rm maj}$ within our tight errors. This show that the larger scatter in the Fundamental Plane (FP) ( L , 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 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 - e relation, which describes most of the deviations between the MP and the FP. The best-fitting relation is $({\rm M/L})_e\propto \sigma _e^{0.72}$ ( r band). It provides an upper limit to any systematic increase of the IMF mass normalization with 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 e and the galaxies circular velocity V circ within 1 R e (where stars dominate) and find the relation max ( V circ )  1.76  x 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.

Description: We study the global efficiency of star formation in high-resolution hydrodynamical simulations of gas discs embedded in isolated early-type and spiral galaxies. Despite using a universal local law to form stars in the simulations, we find that the early-type galaxies are offset from the spirals on the large-scale Kennicutt relation, and form stars two to five times less efficiently. This offset is in agreement with previous results on morphological quenching: gas discs are more stable against star formation when embedded in early-type galaxies due to the lower disc self-gravity and increased shear. As a result, these gas discs do not fragment into dense clumps and do not reach as high densities as in the spiral galaxies. Even if some molecular gas is present, the fraction of very dense gas (typically above 10 4 cm –3 ) is significantly reduced, which explains the overall lower star formation efficiency. We also analyse a sample of local early-type and spiral galaxies, measuring their CO and H i surface densities and their star formation rates as determined by their non-stellar 8 μm emission. As predicted by the simulations, we find that the early-type galaxies are offset from the Kennicutt relation compared to the spirals, with a twice lower efficiency. Finally, we validate our approach by performing a direct comparison between models and observations. We run a simulation designed to mimic the stellar and gaseous properties of NGC 524, a local lenticular galaxy, and find a gas disc structure and global star formation rate in good agreement with the observations. Morphological quenching thus seems to be a robust mechanism, and is also consistent with other observations of a reduced star formation efficiency in early-type galaxies in the COLD GASS survey. This lower efficiency of star formation is not enough to explain the formation of the whole red sequence, but can contribute to the reddening of some galaxies.

Description: In the companion Paper XV of this series, we derive accurate total mass-to-light ratios $(\rm M/L)_{\rm JAM}\approx ({\rm M/L})({\it r}= {R_{\rm e}})$ within a sphere of radius $r= {R_{\rm e}}$ 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 $M_\star \gtrsim 6\times 10^9 {\,\mathrm{M}_{\odot }}$ ) 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 $({M_{\rm JAM}}, {\sigma _{\rm e}})$ and $({M_{\rm JAM}}, {R_{\rm e}^{\rm maj}})$ of the thin Mass Plane (MP) $({M_{\rm JAM}}, {\sigma _{\rm e}}, {R_{\rm e}^{\rm maj}})$ which describes the distribution of the galaxy population, where $ {M_{\rm JAM}}\equiv L\times ({\rm M/L})_{\rm JAM}\approx M_\star$ . 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_{\rm JAM}}\approx 3\times 10^{10} {\,\mathrm{M}_{\odot }}$ , which corresponds to the minimum R e and maximum stellar density. This results in a break in the mean $ {M_{\rm JAM}}\text{--} {\sigma _{\rm e}}$ relation with trends $ {M_{\rm JAM}}\propto \sigma _{\rm e}^{2.3}$ and $ {M_{\rm JAM}}\propto \sigma _{\rm e}^{4.7}$ at small and large e , respectively; (ii) a characteristic mass $ {M_{\rm JAM}}\approx 2\times 10^{11} {\,\mathrm{M}_{\odot }}$ which separates a population dominated by flat fast rotator with discs and spiral galaxies at lower masses, from one dominated by quite round slow rotators at larger masses; (iii) below that mass the distribution of ETGs’ properties on the two projections of the MP tends to be constant along lines of roughly constant e , or equivalently along lines with $ {R_{\rm e}^{\rm maj}}\propto {M_{\rm JAM}}$ , respectively (or even better parallel to the ZOE: $ {R_{\rm e}^{\rm maj}}\propto M_{\rm JAM}^{0.75}$ ); (iv) it forms a continuous and parallel sequence with the distribution of spiral galaxies; (v) at even lower masses, the distribution of fast-rotator ETGs and late spirals naturally extends to that of dwarf ETGs (Sph) and dwarf irregulars (Im), respectively. We use dynamical models to analyse our kinematic maps. We show that e traces the bulge fraction, which appears to be the main driver for the observed trends in the dynamical (M/L) JAM and in indicators of the (M/L) pop of the stellar population like Hβ and colour, as well as in the molecular gas fraction. A similar variation along contours of e is also observed for the mass normalization of the stellar initial mass function (IMF), which was recently shown to vary systematically within the ETGs’ population. Our preferred relation has the form $\log _{10} [({\rm M/L})_{\rm stars}/({\rm M/L})_{\rm Salp}]=a+b\times \log _{10}({\sigma _{\rm e}}/130\, {km s^{-1}})$ with a  = –0.12 ± 0.01 and b  = 0.35 ± 0.06. Unless there are major flaws in all stellar population models, this trend implies a transition of the mean IMF from Kroupa to Salpeter in the interval $\log _{10}({\sigma _{\rm e}}/{\rm km\, s}^{-1})\approx 1.9\text{--}2.5$ (or $ {\sigma _{\rm e}}\approx 90\text{--}290$ km s –1 ), with a smooth variation in between, consistently with what was shown in Cappellari et al. The observed distribution of galaxy properties on the MP provides a clean and novel view for a number of previously reported trends, which constitute special two-dimensional projections of the more general four-dimensional parameters trends on the MP. We interpret it as due to a combination of two main effects: (i) an increase of the bulge fraction, which increases e , decreases R e , and greatly enhance the likelihood for a galaxy to have its star formation quenched, and (ii) dry merging, increasing galaxy mass and R e by moving galaxies along lines of roughly constant e (or steeper), while leaving the population nearly unchanged.