Environments of Galaxies and Their Effects on Galaxy Properties

Abstract

In the history of universe, galaxies are consistently affected by surrounding medium or neighbor galaxies. These effects control galaxy evolution, making properties of galaxies diverse and dependent on environments. We investigate environments of various types of galaxies and how they affect galaxy properties, such as bar structures, galaxy sizes, morphology, and star formation, etc.

First, we present the observational evidence that bars can form from a cluster-cluster interaction. Bars are an elongated structure that extends from the center of galaxies, and about one third of disk galaxies are known to possess bars. Bars are thought to be formed through a physical process inherent in galaxies, or through an external process such as galaxy-galaxy interaction. However, there is another plausible mechanism to form bars, namely a cluster-cluster interaction, that still need to be observationally tested. We examined 105 galaxy clusters at $0.015 < z < 0.060$ that are selected from the Sloan Digital Sky Survey (SDSS) data, and identified 16 interacting clusters. We find that the barred disk-dominated galaxy fraction is about 1.5 times higher in interacting clusters than in clusters with no clear signs of ongoing interaction (42\% versus 27\%). This result indicates that bars can form due to a large-scale violent phenomenon, and cluster-cluster interaction should be considered as an important channel for bar formation.

Second, we study environmental dependence of mass--size relation of the most massive early-type galaxies. Under the $\Lambda$ cold dark matter ($\Lambda$CDM) cosmological models, massive galaxies are expected to be larger in denser environments through frequent hierarchical mergers with other galaxies. Yet, observational studies of low-redshift early-type galaxies have shown no such trend, standing as a puzzle to solve during the past decade. We analyzed 73,116 early-type galaxies at $0.1\leq z < 0.15$, adopting a robust nonparametric size measurement technique and extending the analysis to many massive galaxies. We find for the first time that local early-type galaxies heavier than $10^{11.2}M_{\odot}$ show a clear environmental dependence in mass--size relation, in such a way that galaxies are as much as 20 -- 40 \% larger in densest environments than in underdense environments. Splitting the sample into the brightest cluster galaxies (BCGs) and non-BCGs does not affect the result. This result agrees with the $\Lambda$CDM cosmological simulations and suggests that mergers played a significant role in the growth of massive galaxies in dense environments as expected in theory.

Third, we find evidence for the role of environment in quenching and morphological transformation of low-mass galaxies, by investigating S\'{e}rsic indices ($n$) and mid-infrared (MIR) emissions of a large sample of low-mass quiescent galaxies with $9.4\le\log (M_\mathrm{star}/M_{\odot})<10.0$ in the redshift range of $0.01\le z\le0.04$ from SDSS. We find that a median S\'{e}rsic index of the low-mass quiescent galaxies in cluster environments is $n=2.6$, while that of the counterparts in the lowest density environment is $n=3.4$. Moreover, low-mass quiescent galaxies in higher-density environments have higher MIR emissions for a given specific star formation rate (sSFR), compared to the counterparts in lower-density environments, which suggests that low-mass quiescent galaxies in higher-density environments have had higher star formation activities in the past than those with the same current sSFRs in lower-density environments. Using MIR emission with complementary near-ultraviolet emission, we find that $25\pm4\%$ of low-mass quiescent galaxies in cluster environments were star forming late-type galaxies in the past ($\gtrsim 1$ Gyr), but experienced rapid quenching within the last Gyr with morphological transformation in dense environments. However, the effect could not completely change their morphology, so that their S\'{e}rsic indices are $n\sim2.5$, which is the median value of a bulge-dominated light profile and a disk-dominated one. We also find that the mass--size relation of quiescent or early-type galaxies is dependent on environments in the mass range of $9.4\le\log (M_\mathrm{star}/M_{\odot})<10.0$, owing to different structures (S\'{e}rsic indices) for low-mass quiescent galaxies in different environments.

Lastly, we investigate environments of the most massive galaxies and those of extremely massive quasars and compare them. Black hole mass scaling relations suggest that extremely massive black holes (EMBHs) with $M_\mathrm{BH}\gtrsim10^{9.4}\,M_{\odot}$ are found in the most massive galaxies with $M_\mathrm{star}\gtrsim10^{11.6}\,M_{\odot}$, which are commonly found in dense environments, like galaxy clusters. Therefore, one can expect that there is a close connection between active EMBHs and dense environments. We study the environments of 9461 galaxies and 2943 quasars at $0.24 \le z \le 0.40$, among which 52 are extremely massive quasars with $\log(M_\mathrm{BH}/M_{\odot}) \ge 9.4$, using SDSS and MMT Hectospec data. We find that, on average, both massive quasars and massive galaxies reside in environments more than $\sim2$ times as dense as those of their less massive counterparts with $\log(M_\mathrm{BH}/M_{\odot}) \lesssim 9.0$. However, massive quasars reside in environments about half as dense as inactive galaxies with $\log(M_\mathrm{BH}/M_{\odot}) \ge 9.4$, and only about one third of massive quasars are found in galaxy clusters, while about two thirds of massive galaxies reside in such clusters. This indicates that massive galaxies are a much better signpost for galaxy clusters than massive quasars. The prevalence of massive quasars in moderate to low density environments is puzzling, considering that several simulation results show that these quasars appear to prefer dense environments. Several possible reasons for this discrepancy are discussed, although further investigation is needed to obtain a definite explanation.