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Register a new userLuminous Red Galaxies in hierarchical cosmologies
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Luminous red galaxies (LRGs) are much rarer and more massive than L* galaxies. Coupled with their extreme colours, LRGs therefore provide a demanding testing ground for the physics of massive galaxy formation. We present the first self-consistent predictions for the abundance and properties of LRGs in hierarchical structure formation models. We test two published models which use quite different mechanisms to suppress the formation of massive galaxies: the Bower et al. (2006) model, which invokes ``AGN-feedback to prevent gas from cooling in massive haloes, and the Baugh et al. (2005) model which relies upon a ``superwind to eject gas before it is turned into stars. Without adjusting any parameters, the Bower et al. model gives an excellent match to the observed luminosity function of LRGs in the SDSS (with a median redshift of z=0.24) and to their clustering; the Baugh et al. model is less successful in these respects. Both models fail to match the observed abundance of LRGs at z=0.5 to better than a factor of ~2. In the models, LRGs are typically bulge dominated systems with M* of ~2x10^11 h^{-1} M_sun and velocity dispersions of ~250 km s^{-1}. Around half of the stellar mass in the model LRGs is already formed by z~2.2 and is assembled into one main progenitor by z~1.5; on average, only 25% of the mass of the main progenitor is added after z~1. LRGs are predicted to be found in a wide range of halo masses, a conclusion which relies on properly taking into account the scatter in the formation histories of haloes. Remarkably, we find that the correlation function of LRGs is predicted to be a power law down to small pair separations, in excellent agreement with observational estimates. Neither the Bower et al. nor the Baugh et al. model is able to reproduce the observed radii of LRGs.
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We measure the 3D genus topology of large scale structure using Luminous Red Galaxies (LRGs) in the Sloan Digital Sky Survey and find it consistent with the Gaussian random phase initial conditions expected from the simplest scenarios of inflation. This studies 3D topology on the largest scales ever obtained. The topology is sponge-like. We measure topology in two volume-limited samples: a dense shallow sample studied with smoothing length of 21h^{-1}Mpc, and a sparse deep sample studied with a smoothing length of 34h^{-1}Mpc. The amplitude of the genus curve is measured with 4% uncertainty. Small distortions in the genus curve expected from non-linear biasing and gravitational effects are well explained (to about 1-sigma accuracy) by N-body simulations using a subhalo-finding technique to locate LRGs. This suggests the formation of LRGs is a clean problem that can be modeled well without any free fitting parameters. This bodes well for using LRGs to measure the characteristic scales such as the baryon oscillation scale in future deep redshift surveys.
We detect and study the properties of faint radio AGN in Luminous Red Galaxies (LRGs). The LRG sample comprises 760,000 objects from a catalog of LRG photometric redshifts constructed from the Sloan Digital Sky Survey (SDSS) imaging data, and 65,000 LRGs from the SDSS spectroscopic sample. These galaxies have typical 1.4 GHz flux densities in the 10s-100s of microJy, with the contribution from a low-luminosity AGN dominating any contribution from star formation. To probe the radio properties of such faint objects, we employ a stacking technique whereby FIRST survey image cutouts at each optical LRG position are sorted by the parameter of interest and median-combined within bins. We find that median radio luminosity scales with optical luminosity (L_opt) as L_1.4 GHz ~ L_opt^(beta), where beta appears to decrease from beta ~ 1 at z = 0.4 to beta ~ 0 at z = 0.7, a result which could be indicative of AGN cosmic downsizing. We also find that the overall LRG population, which is dominated by low-luminosity AGN, experiences significant cosmic evolution between z = 0.2 and z = 0.7. This implies a considerable increase in total AGN heating for these massive ellipticals with redshift. By matching against the FIRST catalog, we investigate the incidence and properties of LRGs associated with double-lobed (FR I/II) radio galaxies. (Abridged)
We study the role of major and minor mergers in the mass growth of luminous red galaxies. We present small-scale ($0.01<r<8,hMpc$) projected cross-correlation functions of $23043$ luminous early-type galaxies from the Sloan Digital Sky Survey (SDSS) Luminous Red Galaxy (LRG) sample ($0.16<z<0.30$, $MMiapprox -22.75,mag$) with all their companions in the SDSS imaging sample, split into color and luminosity subsamples with $MMi<-18,mag$. We de-project the two-dimensional functions to obtain three-dimensional real-space LRG--galaxy cross-correlation functions for each companion subsample. We find that the cross-correlation functions are not purely power-law and that there is a clear ``one-halo to ``two-halo transition near $1,hMpc$. We convert these results into close pair statistics and estimate the LRG accretion rate from each companion galaxy subsample using timescales from dynamical friction arguments for each subsample of the companions. We find that the accretion onto LRGs is dominated by dry mergers of galaxies more luminous than $Lstar$. We integrate the luminosity accretion rate from mergers over all companion galaxy subsamples and find that LRGs are growing by $[1.7pm 0.1]$ percent per $Gyr$, on average, from merger activity at redshift $zsim 0.25$. This rate is almost certainly an over-estimate because we have assumed that all close pairs are merging as quickly as dynamical friction allows; nonetheless it is on the low side of the panoply of measurements in the literature, and lower than any rate predicted from theory.
We use a stacking method to study the radial light profiles of luminous red galaxies (LRGs) at redshift $sim 0.62$ and $sim 0.25$, out to a radial range of 200 kpc. We do not find noticeable evolution of the profiles at the two redshifts. The LRG profiles appear to be well approximated by a single Sersic profile, although some excess light can be seen outside 60 kpc. We quantify the excess light by measuring the integrated flux and find that the excess is about 10% -- a non-dominant but still nonnegligible component.
In this work I discuss the necessary steps for deriving photometric redshifts for luminous red galaxies (LRGs) and galaxy clusters through simple empirical methods. The data used is from the Sloan Digital Sky Survey (SDSS). I show that with three bands only ({it gri}) it is possible to achieve results as accurate as the ones obtained by other techniques, generally based on more filters. In particular, the use of the $(g-i)$ color helps improving the final redshifts (especially for clusters), as this color monotonically increases up to $z sim 0.8$. For the LRGs I generate a catalog of $sim 1.5$ million objects at $z < 0.70$. The accuracy of this catalog is $sigma = 0.027$ for $z le 0.55$ and $sigma = 0.049$ for $0.55 < z le 0.70$. The photometric redshift technique employed for clusters is independent of a cluster selection algorithm. Thus, it can be applied to systems selected by any method or wavelength, as long as the proper optical photometry is available. When comparing the redshift listed in literature to the photometric estimate, the accuracy achieved for clusters is $sigma = 0.024$ for $z le 0.30$ and $sigma = 0.037$ for $030 < z le 0.55$. However, when considering the spectroscopic redshift as the mean value of SDSS galaxies on each cluster region, the accuracy is at the same level as found by other authors: $sigma = 0.011$ for $z le 0.30$ and $sigma = 0.016$ for $030 < z le 0.55$. The photometric redshift relation derived here is applied to thousands of cluster candidates selected elsewhere. I have also used galaxy photometric redshifts available in SDSS to identify groups in redshift space and then compare the redshift peak of the nearest group to each cluster redshift (ABRIDGED).
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