Galaxy growth by merging in the nearby universe

TL;DR

This study measures galaxy mass growth through merging by analyzing cross-correlation functions, revealing that red galaxies accrete mass faster than blue ones, with low but significant rates.

Contribution

It introduces a method to estimate galaxy accretion rates from cross-correlation functions and provides new quantitative insights into galaxy merging in the nearby universe.

Findings

Red galaxies have higher accretion rates than blue galaxies.

Accretion rates are approximately 1.5% per Gyr for red galaxies.

Blue galaxies have accretion rates around 0.5% per Gyr.

Abstract

We measure the mass growth rate by merging for a wide range of galaxy types. We present the small-scale (0.014 < r < 11 h70^{-1} Mpc) projected cross-correlation functions w(rp) of galaxy subsamples from the spectroscopic sample of the NYU VAGC (5 \times 10^5 galaxies of redshifts 0.03 < z < 0.15) with galaxy subsamples from the SDSS imaging (4 \times 10^7 galaxies). We use smooth fits to de-project the two-dimensional functions w(rp) to obtain smooth three-dimensional real-space cross-correlation functions \xi(r) for each of several spectroscopic subsamples with each of several imaging subsamples. Because close pairs are expected to merge, the three-space functions and dynamical evolution time estimates provide galaxy accretion rates. We find that the accretion onto massive blue galaxies and onto red galaxies is dominated by red companions, and that onto small-mass blue galaxies, red and blue galaxies make comparable contributions. We integrate over all types of companions and find that at fixed stellar mass, the total fractional accretion rates onto red galaxies (\sim 1.5 h70 percent per Gyr) is greater than that onto blue galaxies (\sim 0.5 h70 percent per Gyr). Although these rates are very low, they are almost certainly over-estimates because we have assumed that all close pairs merge as quickly as dynamical friction permits.