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An essential component of galaxy formation theory is the stellar initial mass function (IMF), that describes the parent distribution of stellar mass in star forming regions. We present observational evidence in a sample of early-type galaxies (ETGs) of a tight correlation between central velocity dispersion and the strength of several absorption features sensitive to the presence of low-mass stars. Our sample comprises ∼ 40, 000 ETGs from the SPIDER survey (z < ∼ 0.1). The data -extracted from the Sloan Digital Sky Survey -are combined, rejecting both noisy data, and spectra with contamination from telluric lines, resulting in a set of 18 stacked spectra at high signal-to-noise ratio (S/N > ∼ 400Å −1 ). A combined analysis of IMF-sensitive line strengths and spectral fitting is performed with the latest state-of the art population synthesis models (an extended version of the MILES models). A significant trend is found between IMF slope and velocity dispersion, towards an excess of low-mass stars in the most massive galaxies. Although we emphasize that accurate values of the IMF slope will require a detailed analysis of chemical composition (such as [α/Fe] or even individual element abundance ratios), the observed trends suggest that low-mass ETGs are better fit by a Kroupalike IMF, whereas massive galaxies require bottom-heavy IMFs, exceeding the Salpeter slope at σ > ∼ 200 km s −1 .

At a fixed stellar mass, the size of low‐redshift early‐type galaxies is found to be a factor of 2 larger than that of their counterparts at z∼ 1, a result with important implications for galaxy formation models. In this paper, we have explored the buildup of the local mass–size relation of elliptical galaxies using two visually classified samples. At low redshift, we compiled a subsample of 2656 elliptical galaxies from the Sloan Digital Sky Survey, whereas at higher redshift (up to z∼ 1), we extracted a sample of 228 objects from the Hubble Space Telescope/Advanced Camera for Surveys images of the Great Observatories Origins Deep Survey. All the galaxies in our study have spectroscopic data, allowing us to determine the age and mass of the stellar component. Contrary to previous claims in the literature, using the fossil record information contained in the stellar populations of our local sample, we do not find any evidence for an age segregation at a given stellar mass, depending on the size of the galaxies. At a fixed dynamical mass, there is only a ≲9 per cent size difference in the two extreme age quartiles of our sample. Consequently, the local evidence does not support a scenario whereby the present‐day mass–size relation has been progressively established via a bottom‐up sequence, where older galaxies occupy the lower part of this relation, remaining in place since their formation. We do not find any age‐segregation difference in our high‐z sample either. Therefore, we find a trend in size that is insensitive to the age of the stellar populations, at least since z∼ 1. This result supports the idea that the stellar mass–size relation is formed at z∼ 1, with all galaxies populating a region which roughly corresponds to 1/2 of the present size distribution. We have explored two possible scenarios for size growth: puffing up or minor merging. The fact that the evolution in size is independent of the stellar age, together with the absence of an increase in the scatter of the relationship with redshift does not support the puffing‐up mechanism. The observational evidence, however, cannot reject at this stage the minor‐merging hypothesis. We have made an estimation of the number of minor‐merger events necessary to bring the high‐z galaxies into the local relation compatible with the observed size evolution. Since z= 0.8, if the mass ratio of the merger is 1:3, then we estimate ∼3 ± 1 minor mergers and if the ratio is 1:10, then we obtain ∼8 ± 2 events.

This is the first paper of a series presenting the Spheroids Panchromatic Investigation in Different Environmental Regions (SPIDER). The sample of spheroids consists of 5080 bright (M r < −20) early-type galaxies (ETGs), in the redshift range of 0.05 to 0.095, with optical (griz) photometry and spectroscopy from the Sloan Digital Sky Survey Data Release 6 (SDSS-DR6) and near-infrared (YJHK) photometry from the UKIRT Infrared Deep Sky Survey-Large Area Survey (UKIDSS-LAS) (DR4). We describe how homogeneous photometric parameters (galaxy colours and structural parameters) are derived using grizYJHK wavebands. We find no systematic steepening of the colour-magnitude relation when probing the baseline from g − r to g − K, implying that internal colour gradients drive most of the mass-metallicity relation in ETGs. As far as structural parameters are concerned we find that the mean effective radius of ETGs smoothly decreases, by 30 per cent, from g through K, while no significant dependence on waveband is detected for the axial ratio, Sersic index and a 4 parameters. Furthermore, velocity dispersions are remeasured for all the ETGs using STARLIGHT and compared to those obtained by SDSS. The velocity dispersions are rederived using a combination of simple stellar population models as templates, hence accounting for the kinematics of different galaxy stellar components. We compare our (2DPHOT) measurements of total magnitude, effective radius and mean surface brightness with those obtained as part of the SDSS pipeline (PHOTO). Significant differences are found and reported, including comparisons with a third and independent part. A full characterization of the sample completeness in all wavebands is presented, establishing the limits of application of the characteristic parameters presented here for the analysis of the global scaling relations of ETGs.

We present a complete analysis of the Fundamental Plane (FP) of early-type galaxies (ETGs) in the nearby Universe (z < 0.1). The sample, as defined in Paper I, comprises 39 993 ETGs located in environments covering the entire domain in local density (from field to cluster). We derive the FP in the grizYJHK wavebands with a detailed discussion on fitting procedure, bias due to selection effects and bias due to correlated errors on the effective parameters, r e and μ e , as key factors in obtaining meaningful FP coefficients. Studying the Kormendy relation (KR) we find that its slope varies from g (3.44 ± 0.04) through K (3.80 ± 0.02) implying that smaller size ETGs have a larger ratio of optical to near-infrared (NIR) radii than galaxies with larger r e . We also examine the Faber-Jackson (FJ) relation and find that its slope is similar for all wavebands, within the uncertainties, with a mean value of 0.198 ± 0.007. Writing the FP equation as log r e = a log σ 0 + b μ e + c, we find that the 'a' varies from 1.38 ± 0.02 in g to 1.55 ± 0.02 in K, implying a 12 per cent variation across the grizYJHK wavelength baseline. The corresponding variation of 'b' is negligible (b ∼ 0.316), while 'c' varies by ∼10 per cent. We show that the waveband dependence of the FJ and KR results from the complex variation of the distribution of galaxies in the face-on projection of the FP as well as by the change of FP coefficients with waveband. We find that 'a' and 'b' become smaller for higher Sersic index and larger axial ratios, independent of the waveband. This suggests that these variations are likely to be related to differences in structural and dynamical (rather than stellar population) properties of ETGs. It is noticeable that galaxies with bluer colours and disc-like isophotes have smaller 'b', with the effect decreasing smoothly from g through K.Considering a power-law relation between mass-to-light ratio and (dynamical) mass, M/L ∝ M γ , we estimate γ from the FP coefficients in grizYJHK. The γ decreases from 0.224 ± 0.008 in g to 0.186 ± 0.009 in K band. Using the γ values, we estimate the variation of age and metallicity of the stellar populations present in massive galaxies per decade in stellar mass. This analysis shows that in the NIR the tilt of the FP is not due to stellar population variation, and that ETGs have coeval stellar populations with an age variation of a few per cent per decade in mass, and a corresponding metallicity increase of ∼23 per cent. We also show that current semi-analytical models of galaxy formation reproduce very well these amounts of variation of age and metallicity with respect to stellar mass.

We use SDSS-DR4 photometric and spectroscopic data out to redshift z ∼ 0.1 combined with ROSAT All Sky Survey X-ray data to produce a sample of twenty-five fossil groups (FGs), defined as bound systems dominated by a single, luminous elliptical galaxy with extended X-ray emission. We examine possible biases introduced by varying the parameters used to define the sample and the main pitfalls are discussed. The spatial density of FGs, estimated via the Vikhlinin et al. (1999), who examined an X-ray overluminous elliptical galaxy sample (OLEG). We compare the general properties of FGs identified here with a sample of bright field ellipticals generated from the same dataset. These two samples show no differences in the distribution of neighboring faint galaxy density excess, distance from the red sequence in the color-magnitude diagram, and structural parameters such as a 4 and internal color gradients. Furthermore, examination of stellar populations shows that our twenty-five FGs have similar ages, metallicities, and α-enhancement as the bright field ellipticals, undermining the idea that these systems represent fossils of a physical mechanism that occurred at high redshift.Our study reveals no difference between FGs and field ellipticals, suggesting that FGs might not be a distinct family of true fossils, but rather the final stage of mass assembly in the Universe.

We present an analysis of stellar population gradients in 4,546 Early-Type Galaxies (ETGs) with photometry in grizYHJK along with optical spectroscopy. ETGs were selected as bulge-dominated systems, displaying passive spectra within the SDSS fibers. A new approach is described which utilizes color information to constrain age and metallicity gradients. Defining an effective color gradient, ∇ ⋆ , which incorporates all of the available color indices, we investigate how ∇ ⋆ varies with galaxy mass proxies, i.e. velocity dispersion, stellar (M ⋆ ) and dynamical (M dyn ) masses, as well as age, metallicity, and [α/Fe]. ETGs with M dyn larger than 8.5 × 10 10 M ⊙ have increasing age gradients and decreasing metallicity gradients wrt mass, metallicity, and enhancement. We find that velocity dispersion and [α/Fe] are the main drivers of these correlations. ETGs with 2.5 × 10 10 M ⊙ ≤ M dyn ≤ 8.5 × 10 10 M ⊙ , show no correlation of age, metallicity, and color gradients wrt mass, although color gradients still correlate with stellar population parameters, and these correlations are independent of each other. In both mass regimes, the striking anti-correlation between color gradient and α-enhancement is significant at ∼ 5 σ, and results from the fact that metallicity gradient decreases with [α/Fe]. This anti-correlation may reflect the fact that star formation and metallicity enrichment are regulated by the interplay between the energy input from supernovae, and the temperature and pressure of the hot X-ray gas in ETGs. For all mass ranges, positive age gradients are associated with old galaxies (> 5 − 7 Gyr). For galaxies younger than ∼ 5 Gyr, mostly at low-mass, the age gradient tends to be anti-correlated with the Age parameter, with more positive gradients at younger ages.