F. La Barbera

SPIDER VIII – constraints on the stellar initial mass function of early-type galaxies from a variety of spectral features

We perform a spectroscopic study to constrain the stellar initial mass function (IMF) by using a large sample of 24 781 early-type galaxies from the Sloan Digital Sky Survey-based Spheroids Panchromatic Investigation in Different Environmental Regions survey. Clear evidence is found of a trend between IMF and central velocity dispersion (σ0), evolving from a standard Kroupa/Chabrier IMF at σ0 ~ 100 km s-1 towards a more bottom-heavy IMF with increasing σ0, becoming steeper than the Salpeter function at σ0 > 220 km s-1. We analyse a variety of spectral indices, combining gravity-sensitive features, with age- and metallicitysensitive indices, and we also consider the effect of non-solar abundance variations. The indices, corrected to solar scale by means of semi-empirical correlations, are fitted simultaneously with the (nearly solar-scaled) extended MILES (MIUSCAT) stellar population models. Similar conclusions are reached when analysing the spectra with a hybrid approach, combining constraints from direct spectral fitting in the optical with those from IMF-sensitive indices. Our analysis suggests that σ0, rather than [a/Fe], drives the variation of the IMF. Although our analysis cannot discriminate between a single power-law (unimodal) IMF and a low-mass (<0.5Mȯ) tapered (bimodal) IMF, robust constraints can be inferred for the fraction in lowmass stars at birth. This fraction (by mass) is found to increase from ~20 per cent at σ0 ~ 100 km s-1, up to ~80 per cent at σ0 ~ 300 km s-1. However, additional constraints can be provided with stellar mass-to-light (M/L) ratios: unimodal models predict M/L significantly larger than dynamical M/L, across the whole σ0 range, whereas a bimodal IMF is compatible. Our results are robust against individual abundance variations. No significant variation is found in Na and Ca in addition to the expected change from the correlation between [s-1/Fe] and σ0.

The different physical mechanisms that drive the star formation histories of giant and dwarf galaxies

We present an analysis of star formation and nuclear activity in galaxies as a function of both luminosity and environment in the fourth data release of the Sloan Digital Sky Survey. Using a sample of 27 753 galaxies in the redshift range 0.005 < z < 0.037 that is ≥90 per cent complete to M r = -18.0, we find that the Ha equivalent width, EW(Hα), distribution is strongly bimodal, allowing galaxies to be robustly separated into passively evolving and star-forming populations about a value EW(Ha) = 2 A. In high-density regions ∼70 per cent of galaxies are passively evolving independent of luminosity. In the rarefied field, however, the fraction of passively evolving galaxies is a strong function of luminosity, dropping from 50 per cent for M r ≤ -21 to zero by M r ∼ -18. Indeed for the lowest luminosity range covered (-18 < M r < -16) none of the ∼600 galaxies in the lowest-density quartile is passively evolving. The few passively evolving dwarf galaxies in field regions appear as satellites to bright (≥ L*) galaxies. We find a systematic reduction of ∼30 per cent in the Ha emission from dwarf (-19 < M r < - 18) star-forming galaxies in high-density regions with respect to field values, implying that the bulk of star-forming dwarf galaxies in groups and clusters are currently in the process of being slowly transformed into passive galaxies. The fraction of galaxies with the optical signatures of an active galactic nucleus (AGN) decreases steadily from ∼50 per cent at M r ∼ -21 to ∼0 per cent by M r ∼ -18 closely mirroring the luminosity dependence of the passive galaxy fraction in low-density environments. This result reflects the increasing importance of AGN feedback with galaxy mass for their evolution, such that the star formation histories of massive galaxies are primarily determined by their past merger history. In contrast, the complete absence of passively evolving dwarf galaxies more than ∼2 virial radii from the nearest massive halo (i.e. cluster, group or massive galaxy) indicates that internal processes, such as merging, AGN feedback or gas consumption through star formation, are not responsible for terminating star formation in dwarf galaxies. Instead the evolution of dwarf galaxies is primarily driven by the mass of their host halo, probably through the combined effects of tidal forces and ram-pressure stripping.

Shapley Optical Survey – II. The effect of environment on the colour–magnitude relation and galaxy colours

We present an analysis of the effects of environment on the photometric properties of galaxies in the core of the Shapley supercluster at z = 0.05, one of the most massive structures in the local universe. The Shapley Optical Survey (SOS) comprises archive Wide Field Imager (WFI) optical imaging of a 2.0deg 2 region containing the rich clusters A3556, A3558 and A3562 which demonstrate a highly complex dynamical situation including ongoing cluster mergers. The B - R/R colour-magnitude relation has an intrinsic dispersion of 0.045 mag and is 0.015 ± 0.005 mag redder in the highest-density regions, indicative of the red sequence galaxy population being 500-Myr older in the cluster cores than towards the virial radius. The B - R colours of galaxies are dependent on their environment, whereas their luminosities are independent of the local density, except for the very brightest galaxies (M R < -22). The global colours of faint (?M* + 2) galaxies change from the cluster cores where ∼90 per cent of galaxies lie along the cluster red sequence to the virial radius, where the fraction has dropped to just ∼20 per cent. This suggests that processes directly related to the supercluster environment are responsible for transforming faint galaxies, rather than galaxy merging, which should be infrequent in any of the regions studied here. The largest concentrations of faint blue galaxies are found between the clusters, coincident with regions containing high fractions of ∼L* galaxies with radio emission indicating starbursts. Their location suggests star formation triggered by cluster mergers, in particular the merger of A3562 and the poor cluster SC 1329-313, although they may also represent recent arrivals in the supercluster core complex. The effect of the A3562-SC 1329-313 merger is also apparent as a displacement in the spatial distribution of the faint galaxy population from both the centres of X-ray emission and the brightest cluster galaxies for both systems. The cores of each of the clusters/groups are marked by regions that have the lowest blue galaxy fractions and reddest mean galaxy colours over the whole supercluster region, confirming that star formation rates are lowest in the cluster cores. In the cases of A3562 and SC 1329-313, these regions coincide with the centres of X-ray emission rather than the peaks in the local surface density, indicating that ram-pressure stripping may have an important role in terminating any remnant star formation in galaxies that encounter the dense intracluster medium (ICM) of the cluster cores.

Evolutionary stellar population synthesis with MILES – II. Scaled-solar and α-enhanced models

We are indebted to the Padova group for making their isochrone calculations available to us. We are grateful to A. de C. Milone for providing us with his [Mg/Fe] estimates for the stars of the MILES library before publication. We also are grateful to H.-S. Kim, J. Cho, R. M. Sharples and S.-J. Yoon, who provided us with GGC line-strength measurements prior to publication. We are grateful to B. Barbuy, J. L. Cervantes, I. G. de la Rosa, M. Koleva, R. Peletier and A. Sansom for very useful discussions. We thank the referee H.-C. Lee for a careful reading of the manuscript and for very valuable clarifications that helped us to improve the manuscript. We also would like to thank J. A. Perez Prieto for helping us in the construction of the web page for the models. The MILES library was observed at the INT on the island of La Palma, operated by the Isaac Newton Group at the Observatorio del Roque de los Muchachos of the Instituto de Astrofisica de Canarias. This research has made an extensive use of the SIMBAD data base and VizieR catalogue access tool (both operated at CDS, Strasbourg, France), the NASAs Astrophysics Data System Article Service. We made use of data retrieved from the Sloan Digital Sky Survey archives ( http://www.sdss.org ). Funding for the SDSS and SDSS-II has been provided by the Alfred P. Sloan Foundation, the Participating Institutions, the National Science Foundation, the US Department of Energy, the National Aeronautics and Space Administration, the Japanese Monbukagakusho, the Max Planck Society and the Higher Education Funding Council for England. PC acknowledges the financial support by FAPESP via project 2008/58406-4 and fellowship 2009/09465-0. JFB acknowledges support from the Ramon y Cajal programme by the Spanish Ministry of Economy and Competitiveness (MINECO). This work has been supported by the Programa Nacional de Astronomia y Astrofisica of MINECO, under grants AYA2013-48226-C3-1-P, AYA2013-48226-C3-2-P, AYA2013-48226-C3-3-P and by the Generalitat Valenciana under grant PROMETEOII2014-069.

SPIDER – VII. Revealing the stellar population content of massive early-type galaxies out to 8Re

Radial trends of stellar populations in galaxies provide a valuable tool to understand the mechanisms of galaxy growth. In this paper, we present the first comprehensive analysis of optical–optical and optical–NIR colours, as a function of galaxy mass, out to the halo region (8Re) of early-type galaxies (ETGs). We select a sample of 674 massive ETGs (M� 3 × 10 10 M� ) from the Sloan Digital Sky Survey (SDSS)-based SPIDER survey. By comparing with a large range of population synthesis models, we derive robust constraints on the radial trends in age and metallicity. Metallicity is unambiguously found to decrease outwards, with a measurable steepening of the slope in the outer regions (Re < R < 8Re). The gradients in stellar age are found to be more sensitive to the models used, but in general, the outer regions of ETGs feature older populations compared to the cores. This trend is strongest for the most massive galaxies in our sample (M� 10 11 M� ). Furthermore, when segregating with respect to large-scale environment, the age gradient is more significant in ETGs residing in higher density regions. These results shed light on the processes leading from the formation of the central core to the growth of the stellar envelope of massive galaxies. The fact that the populations in the outer regions are older and more metal-poor than in the core suggests a process whereby the envelope of massive galaxies is made up of accreted small satellites (i.e. minor mergers) whose stars were born during the first stages of galaxy formation.

The Different Environmental Dependencies of Star Formation for Giant and Dwarf Galaxies

We examine the origins of the bimodality observed in the global properties of galaxies around a stellar mass of 3 × 1010 M☉ by comparing the environmental dependencies of star formation for the giant and dwarf galaxy populations. The Sloan Digital Sky Survey DR4 spectroscopic data set is used to produce a sample of galaxies in the vicinity of the supercluster centered on the cluster A2199 at z = 0.03 that is 90% complete to M + 3.3. From this we measure global trends with environment for both giant (Mr 7 Gyr) or passive (EW [Hα] ≤ 4 A) falls gradually from 80% in the cluster cores to ~40% in field regions beyond 3-4Rvir, as found in previous studies. In contrast, we find that the dwarf galaxy population shows a sharp transition at ~1Rvir, from being predominantly old/passive within the cluster, to outside where virtually all galaxies are forming stars and old/passive galaxies are only found as satellites to more massive galaxies. These results imply fundamental differences in the evolution of giant and dwarf galaxies: whereas the star formation histories of giant galaxies are determined primarily by their merger history, star formation in dwarf galaxies is much more resilient to the effects of major mergers. Instead, dwarf galaxies become passive only once they become satellites within a more massive halo either by losing their halo gas reservoir to the host halo or through other environment-related processes such as galaxy harassment and ram pressure stripping.

On the Invariant Distribution of Galaxies in the re--mie plane out to z = 0.64

We study the evolution of the relation between half-light (effective) radius, re, and mean surface brightness, μe, known as the Kormendy relation, out to redshift z = 0.64 in the V-band rest frame on the basis of a large sample of spheroidal galaxies (N = 228) belonging to three clusters of galaxies. The present sample constitutes the largest data set for which the Kormendy relation is investigated up to a look-back time of ~6 Gyr (H0 = 70 km s-1 Mpc-1, Ωm = 0.3, ΩΛ = 0.7). A new fitting procedure, which suitably accounts for selection criteria effects, makes it possible for the first time to study the trend of the slope (β) and of the intrinsic dispersion (σ) of the Kormendy relation, and the properties of the whole distribution in the re-μe plane as a function of look-back time. The slope β of the relation does not change from z = 0.64 to the present epoch: β = 2.92 ± 0.08, implying a tight constraint of 18%-28% on the variation of the stellar formation epoch along the sequence of spheroidal galaxies per decade of radius. The intrinsic dispersion of the relation, σ = 0.40 ± 0.03, does not vary with redshift, and the distribution of galaxy sizes, as well as the distribution in the plane of the effective parameters, does not vary among the clusters, as proven by the Kolmogorov-Smirnov tests. We conclude that whatever the mechanism driving galaxy evolution is, it does not significantly affect the properties of bright galaxies in the log re-μe plane at least since z = 0.64. The evolution of the zero point of the Kormendy relation is fully explained by the cosmological dimming in an expanding universe plus the passive luminosity evolution of stellar populations with high formation redshift (zf > 2).

On the origin of the scatter around the Fundamental Plane: correlations with stellar population parameters

We present a Fundamental Plane (FP) analysis of 141 early-type galaxies in the Shapley supercluster at z = 0.049 based on spectroscopy from the AAOmega spectrograph at the Anglo-Australian Telescope and photometry from the WFI on the European Southern Observatory/MPI 2.2-m telescope. The key feature of the survey is its coverage of low-mass galaxies down to σ ∼ 50 km s ―1 . We obtain a best-fitting FP relation r e ∝ σ 1.06±:0.06 0 (I) e ―0.82±0.02 in the R band. The shallow exponent of σ 0 is a result of the extension of our sample to low velocity dispersions. Considering the subsample of σ 0 > 100 km s ―1 galaxies, the FP relation is r e ∝ σ 1.35 (I) ―0.81 e , consistent with previous studies in the high-luminosity regime. We investigate the origin of the intrinsic FP scatter, using estimates of age, metallicity and α/Fe. We find that the FP residuals anticorrelate (>3σ) with the mean stellar age in agreement with previous work. However, a stronger (>4σ) correlation with α/Fe is also found. These correlations indicate that galaxies with effective radii smaller than those predicted by the FP have stellar populations systematically older and with α overabundances larger than average, for their σ. Including α/Fe as a fourth parameter in the FP, the total scatter decreases from 0.088 to 0.075 dex and the estimated intrinsic scatter decreases from 0.068 to 0.049 dex. Thus, variations in α/Fe account for ∼30 per cent of the total variance around the FP, and ∼50 per cent of the estimated intrinsic variance. This result indicates that the distribution of galaxies around the FP are tightly related to the enrichment, and hence to the time-scale of star formation. Our results appear to be consistent with the merger hypothesis for the formation of ellipticals which predicts that a significant fraction of the scatter is due to variations in the importance of dissipation in forming merger remnants of a given mass.

2DPHOT: A Multi-Purpose Environment for the Two-Dimensional Analysis of Wide-Field Images

We describe 2DPHOT, a general-purpose analysis environment for source detection and analysis in deep wide-field images. 2DPHOT is an automated tool to obtain both integrated and surface photometry of galaxies in an image, to perform reliable star-galaxy separation with accurate estimates of contamination at faint flux levels, and to estimate completeness of the image catalog. We describe the analysis strategy on which 2DPHOT is based, and provide a detailed description of the different algorithms implemented in the package. This new environment is intended as a dedicated tool to process the wealth of data from wide-field imaging surveys. To this end, the package is complemented by 2DGUI, an environment that allows multiple processing of data using a range of computing architectures.

Shapley Optical Survey – I. Luminosity functions in the supercluster environment

We present the Shapley Optical Survey, a photometric study covering a ∼2-deg 2 region of the Shapley supercluster core at z ∼ 0.05 in two bands (B and R). The galaxy sample is complete to B = 22.5 (>M* + 6, N gal = 16588) and R = 22.0 (>M* + 7, N gal = 28008). The galaxy luminosity function (LF) cannot be described by a single Schechter function due to dips apparent at B ∼ 17.5 (M B ∼ -19.3) and R ∼ 17.0 (M R ∼ -19.8) and the clear upturn in the counts for galaxies fainter than B and R ∼ 18 mag. We find, instead, that the sum of a Gaussian and a Schechter function, for bright and faint galaxies, respectively, is a suitable representation of the data. We study the effects of the environment on the photometric properties of galaxies, deriving the galaxy LFs in three regions selected according to the local galaxy density, and find a marked luminosity segregation, in the sense that the LF faint end is different at more than 3σ confidence level in regions with different densities. In addition, the LFs of red and blue galaxy populations show very different behaviours: while red sequence counts are very similar to those obtained for the global galaxy population, the blue galaxy LFs are well described by a single Schechter function and do not vary with the density. Such large environmentally dependent deviations from a single Schechter function are difficult to produce solely within galaxy merging or suffocation scenarios. Instead the data support the idea that mechanisms related to the cluster environment, such as galaxy harassment or ram-pressure stripping, shape the galaxy LFs by terminating star formation and producing mass-loss in galaxies at ∼M* + 2, a magnitude range where blue late-type spirals used to dominate cluster populations, but are now absent.