Galaxy formation and evolution

Abstract:

In Paper I of this series we introduced a volume-limited parent sample of 871 galaxies from which we extracted the ATLAS3D 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-Σ 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 [per cent] and generally constitute a small fraction [f(SR) ≈ 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) ≈ 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 Σ3, within a cylinder enclosing the three nearest galaxies. This holds for nearly four orders of magnitude in the surface density down to Σ3≈ 0.01Mpc-2, with f(Sp) decreasing by 10 per cent per dex in Σ3, while f(FR) correspondingly increases. The existence of a smooth kinematic T-Σ 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 Σ10 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. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Abstract:

Using far-ultraviolet (FUV) and near-ultraviolet (NUV) photometry from guest investigator programmes on the Galaxy Evolution Explorer (GALEX) satellite, optical photometry from the MDM Observatory and optical integral-field spectroscopy from SAURON, we explore the UV-line-strength relations of the 48 nearby early-type galaxies in the SAURON sample. Identical apertures are used for all quantities, avoiding aperture mismatch. We show that galaxies with purely old stellar populations show well-defined correlations of the integrated FUV -V and FUV - NUV colours with the integrated Mg b and Hβ absorption line-strength indices, strongest for FUV - NUV. Correlations with the NUV -V colour, Fe5015 index and stellar velocity dispersion σ are much weaker. These correlations put stringent constraints on the origin of the UV-upturn phenomenon in early-type galaxies and highlight its dependence on age and metallicity. In particular, despite recent debate, we recover the negative correlation between FUV -V colour and Mg line strength originally publicized by Burstein et al., which we refer to as the 'Burstein relation', suggesting a positive dependence of the UV upturn on metallicity. We argue that the scatter in the correlations is real and present mild evidence that a strong UV excess is preferentially present in slow-rotating galaxies. We also demonstrate that most outliers in the correlations are galaxies with current or recent star formation, some at very low levels. We believe that this sensitivity to weak star formation, afforded by the deep and varied data available for the SAURON sample, explains why our results are occasionally at odds with other recent but shallower surveys. This is supported by the analysis of a large, carefully crafted sample of more distant early-type galaxies from the Sloan Digital Sky Survey (SDSS), more easily comparable with current and future large surveys. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.

Abstract:

We present deep spectroscopic data for a 24-object subsample of our full 41-object z~ 0.5 radio galaxy sample in order to investigate the evolution of the Fundamental Plane of radio galaxies. We find that the low-luminosity, Fanaroff-Riley type I (FRI), radio galaxies in our sample are consistent with the local Fundamental Plane of radio galaxies defined by Bettoni et al. when corrected for simple passive evolution of their stellar populations. However, we find that the higher luminosity, Fanaroff-Riley type II (FRII), radio galaxies are inconsistent with the local Fundamental Plane if only passive evolution is considered, and find evidence for a rotation in the Fundamental Plane at z~ 0.5 when compared with the local relation. We show that neither passive evolution, nor a mass-dependent evolution in the mass-to-light ratio, nor an evolution in the size of the host galaxies can, by themselves, plausibly explain the observed tilt. However, we suggest that some combination of all three effects, with size evolution as the dominant factor, may be sufficient to explain the difference between the planes.We also find evidence for a correlation between host galaxy velocity dispersion and radio luminosity at the 97 per cent significance level within our subsample, although further observations are required in order to determine whether this is different for the FRI and FRII radio sources. Assuming that the MBH-σ relation still holds at z~ 0.5, this implies that radio luminosity scales with black hole mass, in agreement with previous studies. © 2010 The Authors. Journal compilation © 2010 RAS.

Abstract:

The Galaxies in Cosmological Simulations (galics) semi-analytical model of hierarchical galaxy formation is used to investigate the effects of different galactic properties, including star formation rate (SFR) and outflows, on the shape of the mass-metallicity relation and to predict the relation for galaxies at redshift z= 2.27 and 3.54. Our version of galics has the chemical evolution implemented in great detail and is less heavily reliant on approximations, such as instantaneous recycling. We vary the model parameters controlling both the efficiency and redshift dependence of the SFR as well as the efficiency of supernova feedback. We find that the factors controlling the SFR influence the relation significantly at all redshifts and require a strong redshift dependence, proportional to 1 +z, in order to reproduce the observed relation at the low-mass end. Indeed, at any redshift, the predicted relation flattens out at the high-mass end resulting in a poorer agreement with observations in this regime. We also find that variation in the parameters associated with outflows has a minimal effect on the relation at high redshift but does serve to alter its shape in the more recent past. We thus conclude that the relation is one between the SFR and mass and that outflows are only important in shaping the relation at late times. When the relation is stratified by the SFR, it is apparent that the predicted galaxies with increasing stellar masses have higher SFRs, supporting the view that galaxy downsizing is the origin of the relation. Attempting to reproduce the observed relation, we vary the parameters controlling the efficiency of star formation and its redshift dependence and compare the predicted relations with those of Erb et al. at z= 2.27 and Maiolino et al. at z= 3.54 in order to find the best-fitting parameters. We succeed in fitting the relation at z= 3.54 reasonably well; however, we fail at z= 2.27, our relation lying on average below the observed one at the one standard deviation level. We do, however, predict the observed evolution between z= 3.54 and 0. Finally, we discuss the reasons for the above failure and the flattening at high masses, with regards to both the comparability of our predictions with observations and the possible lack of underlying physics. Several of these problems are common to many semi-analytic/hybrid models and so we discuss possible improvements and set the stage for future work by considering how the predictions and physics in these models can be made more robust in light of our results. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.

Abstract:

Extragalactic planetary nebulae (PNe) are not only useful as distance signposts or as tracers of the dark matter content of their host galaxies, but constitute also good indicators of the main properties of their parent stellar populations. Yet, so far, the properties of PNe in the optical regions of galaxies where stellar population gradients can be more extreme have remained largely unexplored, mainly because the detection of PNe with narrow-band imaging or slitless spectroscopy is considerably hampered by the presence of a strong stellar background. Integral field spectroscopy (IFS) can overcome this limitation, and here we present a study of the PN population in the nearby compact elliptical M32. Using SAURON data taken with just two 10-min-long pointings we have doubled the number of known PNe within the effective radius of M32, detecting PNe five times fainter than previously found in narrow-band images that collected nearly the same number of photons. We have carefully assessed the incompleteness limit of our survey, and accounting for it across the entire range of luminosity values spanned by our detected PNe, we could conclude despite having at our disposal only 15 sources that the central PNe population of M32 is consistent with the generally adopted shape for the PNe Luminosity Function and its typical normalization observed in early-type galaxies. Furthermore, owing to the proximity of M32 and to ultraviolet images taken with the Hubble Space Telescope, we could identify the most likely candidates for the central star of a subset of our detected PNe and conclude that these stars are affected by substantial amounts of circumstellar dust extinction, a finding that could reconcile the intriguing discrepancy previously reported in M32 between the model predictions and the observations for the later stages of stellar evolution. Considering the modest time investment on a 4-m-class telescope that delivered these results, this study illustrates the potential of future IFS investigations for the central PNe population of early-type galaxies, either with existing SAURON data for many more, albeit more distant, objects, or from campaigns that will use the future generations of integral field spectrographs that will be mounted on 8-m-class telescopes, such as MUSE on the Very Large Telescope. © 2011 The Authors Monthly Notices of the Royal Astronomical Society © 2011 RAS.