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Register a new userRecent galaxy mergers and residual star formation of red sequence galaxies in galaxy clusters
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This study explored the GALEX ultraviolet (UV) properties of optical red sequence galaxies in 4 rich Abell clusters at z leq 0.1. In particular, we tried to find a hint of merger-induced recent star formation (RSF) in red sequence galaxies. Using the NUV - r colors of the galaxies, RSF fractions were derived based on various criteria for post-merger galaxies and normal galaxies. Following k-correction, about 36% of the post-merger galaxies were classified as RSF galaxies with a conservative criterion (NUV - r leq 5), and that number was doubled (~ 72%) when using a generous criterion (NUV - r leq 5.4). The trend was the same when we restricted the sample to galaxies within 0.5xR_{200}. Post-merger galaxies with strong UV emission showed more violent, asymmetric features in the deep optical images. The RSF fractions did not show any trend along the clustocentric distance within R_{200}. We performed a Dressler-Shectman test to check whether the RSF galaxies had any correlation with the sub-structures in the galaxy clusters. Within R_{200} of each cluster, the RSF galaxies did not appear to be preferentially related to the clusters sub-structures. Our results suggested that only 30% of RSF red sequence galaxies show morphological hints of recent galaxy mergers. This implies that internal processes (e.g., stellar mass-loss or hot gas cooling) for the supply of cold gas to early-type galaxies may play a significant role in the residual star formation of early-type galaxies at a recent epoch.
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We investigate the origin of the color-magnitude relation (CMR) observed in cluster galaxies by using a combination of a cosmological N-body simulation of a cluster of galaxies and a semi-analytic model of galaxy formation. The departure of galaxies in the bright end of the CMR with respect to the trend defined by less luminous galaxies could be explained by the influence of minor mergers.
This lecture reviews the fundamental physical processes involved in star formation in galaxy interactions and mergers. Interactions and mergers often drive intense starbursts, but the link between interstellar gas physics, large scale interactions, and active star formation is complex and not fully understood yet. Two processes can drive starbursts: radial inflows of gas can fuel nuclear starbursts, triggered gas turbulence and fragmentation can drive more extended starbursts in massive star clusters with high fractions of dense gas. Both modes are certainly required to account for the observed properties of starbursting mergers. A particular consequence is that star formation scaling laws are not universal, but vary from quiescent disks to starbursting mergers. High-resolution hydrodynamic simulations are used to illustrate the lectures.
Galaxy mergers and interactions are an integral part of our basic understanding of how galaxies grow and evolve over time. However, the effect that galaxy mergers have on star formation rates (SFR) is contested, with observations of galaxy mergers showing reduced, enhanced and highly enhanced star formation. We aim to determine the effect of galaxy mergers on the SFR of galaxies using statistically large samples of galaxies, totalling over 200,000, over a large redshift range, 0.0 to 4.0. We train and use convolutional neural networks to create binary merger identifications (merger or non-merger) in the SDSS, KiDS and CANDELS imaging surveys. We then compare the galaxy main sequence subtracted SFR of the merging and non-merging galaxies to determine what effect, if any, a galaxy merger has on SFR. We find that the SFR of merging galaxies are not significantly different from the SFR of non-merging systems. The changes in the average SFR seen in the star forming population when a galaxy is merging are small, of the order of a factor of 1.2. However, the higher the SFR above the galaxy main sequence, the higher the fraction of galaxy mergers. Galaxy mergers have little effect on the SFR of the majority of merging galaxies compared to the non-merging galaxies. The typical change in SFR is less than 0.1~dex in either direction. Larger changes in SFR can be seen but are less common. The increase in merger fraction as the distance above the galaxy main sequence increases demonstrates that galaxy mergers can induce starbursts.
A recent study shows that bars can be induced via interaction of galaxy clusters, but it has been unclear if the bar formation by the interaction between clusters is related to the enhancement of star formation. We study galaxies in 105 galaxy clusters at $0.015<z<0.060$ detected from Sloan Digital Sky Survey data, in order to examine whether the fraction of star-forming galaxies ($f_mathrm{sf}$) in 16 interacting clusters is enhanced compared with that of the other non-interacting clusters and to investigate the possible connection between the $f_mathrm{sf}$ enhancement and the bar formation in interacting clusters. We find that $f_mathrm{sf}$ is moderately higher ($sim20%$) in interacting clusters than in non-interacting clusters and that the enhancement of star formation in interacting clusters occurs only in moderate-mass disk-dominated galaxies ($10^{10.0} le M_mathrm{star}/M_{odot} < 10^{10.4}$ and the bulge-to-total light ratio is $le0.5$). We also find that the enhancement of $f_mathrm{sf}$ in moderate-mass disk-dominated galaxies in interacting clusters is mostly due to the increase of the number of barred galaxies. Our result suggests that the cluster-cluster interaction can simultaneously induce bars and star formation in disk galaxies.
In this letter we construct a large sample of early-type galaxies with measured gas-phase metallicities from the Sloan Digital Sky Survey and Galaxy Zoo in order to investigate the origin of the gas that fuels their residual star formation. We use this sample to show that star forming elliptical galaxies have a substantially different gas-phase metallicity distribution from spiral galaxies, with ~7.4% having a very low gas-phase metallicity for their mass. These systems typically have fewer metals in the gas phase than they do in their stellar photospheres, which strongly suggests that the material fuelling their recent star formation was accreted from an external source. We use a chemical evolution model to show that the enrichment timescale for low-metallicity gas is very short, and thus that cosmological accretion and minor mergers are likely to supply the gas in >37% of star-forming ETGs, in good agreement with estimates derived from other independent techniques.
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