Environmental Dependence of the Galaxy Merger Rate in a ΛCDM Universe

  • Chiueh H
ISSN: 0004-637X
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We make use of four galaxy catalogs based on four different semi-analytical models (SAMs)<br />implemented in the Millennium Simulation to study the environmental effects and the model dependence<br />of the galaxy merger rate. We begin the analyses by finding that the galaxy merger rate in SAMs has<br />a mild redshift evolution with luminosity-selected samples in the evolution-corrected B -band<br />magnitude range,–21 ≤ M e B ≤ –19, consistent with the results of previous works. To study the<br />environmental dependence of the galaxy merger rate, we adopt two estimators, the local overdensity<br />(1 + δ n ), defined as the surface density from the n th nearest neighbor ( n = 6 is chosen in this<br />study), and the host halo mass M h . We find that the galaxy merger rate F mg shows a strong<br />dependence on the local overdensity (1 + δ n ) and the dependence is similar at all redshifts. For<br />the overdensity estimator, the merger rate F mg is found to be about twenty times larger in the<br />densest regions than in underdense ones in two of the four SAMs, while it is roughly four times<br />higher in the other two. In other words, the discrepancies of the merger rate difference between the<br />two extremes can differ by a factor of ~5 depending on the SAMs adopted. On the other hand, for the<br />halo mass estimator, F mg does not monotonically increase with the host halo mass M h but peaks in<br />the M h range between 10 12 and 10 13 h –1 M ☉ , which corresponds to group environments. The high<br />merger rate in high local density regions corresponds primarily to the high merger rate in group<br />environments. In addition, we also study the merger probability of "close pairs" identified using<br />the projected separation and the line-of-sight velocity difference C mg and the merger timescale T<br />mg ; these are two important quantities for observations to convert the pair fraction N c into the<br />galaxy merger rate. We discover that T mg has a weak dependence on environment and different SAMs,<br />and is about 2 Gyr old at z ~ 1. In contrast, C mg depends on both environment (declining with<br />density) and different SAMs; its environmental dependence is primarily due to the projection effect.<br />At z ~ 1, it is found that only ~31% of projected close pairs will eventually merge by z = 0. We<br />find that the projection effect is the dominant factor in accounting for the low merger probability<br />of projected close pairs.




Chiueh, H.-Y. J. and L. L. and T. (2012). Environmental Dependence of the Galaxy Merger Rate in a ΛCDM Universe. The Astrophysical Journal, 754(1), 26. Retrieved from http://stacks.iop.org/0004-637X/754/i=1/a=26

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