Article

# An Upper Limit to the Dry Merger Rate at z ~ 0.55

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(Impact Factor: 4.02). 01/2010; 139(2):794. DOI: 10.1088/0004-6256/139/2/794
Source: arXiv

ABSTRACT

We measure the fraction of Luminous Red Galaxies (LRGs) in dynamically close
pairs (with projected separation less than 20 $h^{-1}$ kpc and velocity
difference less than 500 km s$^{-1}$) to estimate the dry merger rate for
galaxies with $-23 < M(r)_{k+e,z=0.2} +5 \log h < -21.5$ and $0.45 < z < 0.65$
in the 2dF-SDSS LRG and QSO (2SLAQ) redshift survey. For galaxies with a
luminosity ratio of $1:4$ or greater we determine a $5\sigma$ upper limit to
the merger fraction of 1.0% and a merger rate of $< 0.8 \times 10^{-5}$
Mpc$^{-3}$ Gyr$^{-1}$ (assuming that all pairs merge on the shortest possible
timescale set by dynamical friction). This is significantly smaller than
predicted by theoretical models and suggests that major dry mergers do not
contribute to the formation of the red sequence at $z < 0.7$.

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##### Article: The evolution of Luminous Red Galaxies in the Sloan Digital Sky Survey 7th data release
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ABSTRACT: We present a comprehensive study of the evolution of luminous red galaxies (LRGs) in the latest and final spectroscopic data release of the Sloan Digital Sky Survey. We test the scenario of passive evolution of LRGs in 0.15 < z < 0.5, by looking at the evolution of the number and luminosity density of LRGs, as well as of their clustering. A new weighting scheme is introduced that allows us to keep a large number of galaxies in our sample and put stringent constraints on the growth and merging allowed by the data as a function of galaxy luminosity. Introducing additional luminosity-dependent weighting for our clustering analysis allows us to additionally constrain the nature of the mergers. We find that, in the redshift range probed, the population of LRGs grows in luminosity by 1.5–6 per cent Gyr−1 depending on their luminosity. This growth is predominantly happening in objects that reside in the lowest-mass haloes probed by this study, and cannot be explained by satellite accretion into massive LRGs nor by LRG–LRG merging. We find that the evolution of the brightest objects (with a K+e-corrected Mr,0.1≲−22.8) is consistent with that expected from passive evolution.
Preview · Article · Jan 2010 · Monthly Notices of the Royal Astronomical Society
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##### Article: Spectro-photometric close pairs in GOODS-S: Major and minor companions of intermediate-mass galaxies
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ABSTRACT: Aims. Recent work has shown that major mergers of disc galaxies can only account for ∼20% of the growth of the galaxy red sequence between z = 1 and z = 0. Our goal here is to provide merger frequencies that encompass both major and minor mergers, derived from close pair statistics. We aim to show that reliable close pair statistics can be derived from galaxy catalogues with mixed spectroscopic and photometric redshifts. Methods. We use B−band luminosity-and mass-limited samples from a Spitzer/IRAC-selected catalogue of GOODS-S. We present a new methodology for computing the number of close companions, N c , when spectroscopic redshift information is partial. The methodology extends the one used in spectroscopic surveys to make use of photometric redshift information. We select as close companions those galaxies separated by 6h −1 kpc < r p < 21h −1 kpc in the sky plane and with a difference ∆v ≤ 500 km s −1 in redshift space. Results. We provide N c for four different B−band-selected samples. It increases with luminosity, in good agreement with previous estimations from spectroscopic surveys. The evolution of N c with redshift is faster in more luminous samples. We provide N c of M ≥ 10 10 M galaxies, finding that the number including minor companions (N m c , mass ratio µ ≥ 1/10) is roughly two times the number of major companions alone (N M c , mass ratio µ ≥ 1/3) in the range 0.2 ≤ z < 1.1. We compare the major merger rate derived by close pairs with the one computed by morphological criteria, finding that both approaches provide similar merger rates for field galaxies when the progenitor bias is taken into account. Finally, we estimate that the total (major+minor) merger rate is ∼1.7 times the major merger rate. Conclusions. Only 30% to 50% of the M ≥ 10 10 M early-type (E/S0/Sa) galaxies that appear between z = 1 and z = 0 may have undergone a major or a minor merger. Half of the red sequence growth since z = 1 is therefore unrelated to mergers.
Full-text · Article · Apr 2010 · Astronomy and Astrophysics
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##### Article: The stellar evolution of Luminous Red Galaxies, and its dependence on colour, redshift, luminosity and modelling
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ABSTRACT: We present a series of colour-evolution models for luminous red galaxies (LRGs) in the seventh spectroscopic data release of the Sloan Digital Sky Survey (SDSS), computed using the full-spectrum fitting code vespa on high signal-to-noise ratio stacked spectra. The colour-evolution models are computed as a function of colour, luminosity and redshift, and we do not a priori assume that LRGs constitute a uniform population of galaxies in terms of stellar evolution. By computing star formation histories from the fossil record, the measured stellar evolution of the galaxies is decoupled from the survey’s selection function, which also evolves with redshift. We present these evolutionary models computed using three different sets of stellar population synthesis (SPS) codes. We show that the traditional fiducial model of purely passive stellar evolution of LRGs is broadly correct, but it is not sufficient to explain the full-spectral signature. We also find that higher-order corrections to this model are dependent on the SPS used, particularly when calculating the amount of recent star formation. The amount of young stars can be non-negligible in some cases, and has important implications for the interpretation of the number density of LRGs within the selection box as a function of redshift. Dust extinction, however, is more robust to the SPS modelling: extinction increases with decreasing luminosity, increasing redshift and increasing r − i colour. We are making the colour-evolution tracks publicly available at http://www.icg.port.ac.uk/~tojeiror/lrg_evolution/.
Full-text · Article · Nov 2010 · Monthly Notices of the Royal Astronomical Society