SPIDER – I. Sample and galaxy parameters in the grizYJHK wavebands

Depto de Astrofísica, Universidad de La Laguna (ULL), E-38206 La Laguna, Tenerife, Spain
Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.52). 10/2010; 408(3):1313 - 1334. DOI: 10.1111/j.1365-2966.2010.16850.x
Source: arXiv

ABSTRACT 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 (Mr < −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 a4 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.

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    ABSTRACT: The size-luminosity relation of early-type Brightest Cluster Galaxies (BCGs), R_e ~ L^0.88, is steeper than that for the bulk of the early-type galaxy population, for which R_e ~ L^0.68. In addition, although BCGs are hardly offset from the Fundamental Plane defined by the bulk of the early-type population, they show considerably smaller scatter. The larger than expected sizes of BCGs, and the increased homogeneity, are qualitatively consistent with models which seek to explain the colors of the most massive galaxies by invoking dry dissipationless mergers, since dissipation tends to reduce the sizes of galaxies, and wet mergers which result in star formation would tend to increase the scatter in luminosity at fixed size and velocity dispersion. Furthermore, BCGs define the same g-r color-magnitude relation as the bulk of the early-type population. If BCGs formed from dry mergers, then BCG progenitors must have been red for their magnitudes, suggesting that they hosted older stellar populations than typical for their luminosities. Our findings have two other consequences. First, the R_e-L relation of the early-type galaxy population as a whole (i.e., normal plus BCG) exhibits some curvature. Some of this curvature must be a consequence of the fact that an increasing fraction of the most luminous galaxies are BCGs. The second consequence is suggested by the fact that, despite following a steeper size-luminosity relation, BCGs tend to define a tight relation between dynamical mass R_e sigma^2/G and luminosity. As consequence, we find that BCGs define a shallower sigma-L relation than the bulk of the early-type galaxy population.
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