No abstract

Dynamical analysis of compact groups provides important tests of models of compact group formation and evolution. By compiling 2066 redshifts from FLWO/FAST, from the literature, and from SDSS DR12 in the fields of compact groups in McConnachie et al. (2009), we construct the largest sample of compact groups with complete spectroscopic redshifts in the redshift range 0.01 < z < 0.22. This large redshift sample shows that the interloper fraction in the McConnachie et al. ( 2009) compact group candidates is ∼ 42%. A secure sample of 332 compact groups includes 192 groups with four or more member galaxies and 140 groups with three members. The fraction of early-type galaxies in these compact groups is 62%, slightly higher than for the original Hickson compact groups. The velocity dispersions of early-and late-type galaxies in compact groups change little with groupcentric radius; the radii sampled are less than 100 h −1 kpc, smaller than the radii typically sampled by members of massive clusters of galaxies. The physical properties of our sample compact groups include size, number density, velocity dispersion, and local environment; these properties slightly differ from those derived for the original Hickson compact groups and for the DPOSS II compact groups. Differences result from subtle differences in the way the group candidates were originally selected. The space density of the compact groups changes little with redshift over the range covered by this sample. The approximate constancy of the space density for this sample is a potential constraint on the evolution of compact groups on a few Gigayear timescale.

We investigate the connection between the presence of bars and AGN activity, using a volumelimited sample of ∼9,000 late-type galaxies with axis ratio b/a > 0.6 and M r < −19.5 + 5logh at low redshift (0.02 ≤ z 0.055), selected from Sloan Digital Sky Survey Data Release 7. We find that the bar fraction in AGN-host galaxies (42.6%) is ∼2.5 times higher than in non-AGN galaxies (15.6%), and that the AGN fraction is a factor of two higher in strong-barred galaxies (34.5%) than in non-barred galaxies (15.0%). However, these trends are simply caused by the fact that AGN-host galaxies are on average more massive and redder than non-AGN galaxies because the fraction of strong-barred galaxies (f SB ) increases with u − r color and stellar velocity dispersion. When u − r color and velocity dispersion (or stellar mass) are fixed, both the excess of f SB in AGN-host galaxies and the enhanced AGN fraction in strong-barred galaxies disappears. Among AGN-host galaxies we find no strong difference of the Eddington ratio distributions between barred and non-barred systems.These results indicate that AGN activity is not dominated by the presence of bars, and that AGN power is not enhanced by bars. In conclusion we do not find a clear evidence that bars trigger AGN activity.

We study the mid-infrared (MIR) properties of the galaxies in the A2199 supercluster at z = 0.03 to understand the star formation activity of galaxy groups and clusters in the supercluster environment. Using the Wide-field Infrared Survey Explorer data, we find no dependence of mass-normalized integrated SFRs of galaxy groups/clusters on their virial masses. We classify the supercluster galaxies into three classes in the MIR color-luminosity diagram: MIR blue cloud (massive, quiescent and mostly early-type), MIR star-forming sequence (mostly late-type), and MIR green valley galaxies. These MIR green valley galaxies are distinguishable from the optical green valley galaxies, in the sense that they belong to the optical red sequence. We find that the fraction of each MIR class does not depend on virial mass of each group/cluster. We compare the cumulative distributions of surface galaxy number density and cluster/group-centric distance for the three MIR classes. MIR green valley galaxies show the distribution between MIR blue cloud and MIR SF sequence galaxies. However, if we fix galaxy morphology, early-and late-type MIR green valley galaxies show different distributions. Our results suggest a possible evolutionary scenario of these galaxies: 1) Late-type MIR SF sequence galaxies → 2) Late-type MIR green valley galaxies → 3) Early-type MIR green valley galaxies → 4) Early-type MIR blue cloud galaxies. In this sequence, star formation of galaxies is quenched before the galaxies enter the MIR green valley, and then morphological transformation occurs in the MIR green valley.

We study the star formation activity of nearby galaxies with bars using a sample of late-type galaxies at 0.02≤ z ≤ 0.05489 and M r < −19.5 from the Sloan Digital Sky Survey. We compare the physical properties of strongly and weakly barred galaxies with those of non-barred galaxies that have stellar mass and redshift distributions similar to barred galaxies. We find that the star formation activity of strongly barred galaxies probed by starburstiness, g − r , NUV−r, and mid-infrared [3.4]−[12] colors is, on average, lower than that of non-barred galaxies. However, weakly barred galaxies do not show such a difference between barred and non-barred galaxies. The amounts of atomic and molecular gas in strongly barred galaxies are smaller than those of non-barred galaxies, and the gas metallicity is higher in strongly barred galaxies than in non-barred galaxies. The gas properties of weakly barred galaxies again show no difference from those of non-barred galaxies. We stack the optical spectra of barred and non-barred galaxies in several mass bins and fit to the stacked spectra with a spectral fitting code, STARLIGHT. We find no significant difference in stellar populations between barred and non-barred galaxies for both strongly and weakly barred galaxies. Our results are consistent with the idea that the star formation activity of barred galaxies is enhanced in the past along with significant gas consumption, and is currently lower than or similar to that of non-barred galaxies. The past star formation enhancement depends on the strength of bars.

We study the nuclear activity of galaxies in local compact groups. We use a spectroscopic sample of 238 galaxies in 58 compact groups from the Sloan Digital Sky Survey data release 7 to estimate the fraction of AGN-host galaxies in compact groups, and to compare it with those in cluster and field regions. We use emission-line ratio diagrams to identify AGN-host galaxies, and find that the AGN fraction of compact group galaxies is 17-42% depending on the AGN classification method. The AGN fraction in compact groups is not the highest among the galaxy environments. This trend remains even if we use several subsamples segregated by galaxy morphology and optical luminosity. The AGN fraction for early-type galaxies decreases with increasing galaxy number density, but the fraction for late-type galaxies changes little. We find no mid-infrared detected AGN-host galaxies in our sample of compact groups using Wide-field Infrared Survey Explorer data. These results suggest that the nuclear activity of compact group galaxies (mostly early types) is not strong because of lack of gas supply even though they may experience frequent galaxy-galaxy interactions and mergers that could trigger nuclear activity.

Bars are an elongated structure that extends from the centre of galaxies, and about one-third of disk galaxies are known to possess bars 1,2,3 . These bars are thought to form either through a physical process inherent in galaxies 4,5,6 , or through an external process such as galaxygalaxy interactions 7,8,9 . However, there are other plausible mechanisms of bar formation that still need to be observationally tested.Here we present the observational evidence that bars can form via cluster-cluster interaction 10 . We examined 105 galaxy clusters at redshift 0.015 < z < 0.060 that are selected from the Sloan Digital Sky Survey data, and identified 16 interacting clusters. We find that the barred diskdominated galaxy fraction is about 1.5 times higher in interacting clusters than in clusters with no clear signs of ongoing interaction (42% versus 27%). Our result indicates that bars can form through a large-scale violent phenomenon, and cluster-cluster interaction should be considered an important mechanism of bar formation.We used a volume-limited sample of galaxies with log(M star /M ) ≥ 10.0 in the MPA-JHU catalogue, and selected 105 clusters with M 200 > 7 × 10 13 M in the redshift range of 0.015 < z < 0.060. Here, M star is the galaxy stellar mass, M the mass of Sun and M 200 the cluster halo mass (see Methods). Of these, 16 clusters were found to be in pairs or have substructures (see Methods) and are defined here as interacting clusters. Figure 1 shows examples of the surface number density maps, velocities, and spatial distributions for galaxies of clusters in isolation, in pairs, and with substructures.As detailed in Methods, we detected bars in the cluster member galaxies using a quantitative method that searches for an elongated structure that has a large ellipticity for several consecutive isophotal ellipses but a sudden drop beyond a certain radius, and a nearly constant position angle over only the high-ellipticity region. We complemented the bar classification with a visual inspection to exclude false classifications and add bar galaxies that were not detected by the automated method. This process also examines the radial surface brightness profiles of galaxies, from which we derived the bulge-tototal light ratio (B/T ) of each galaxy. Example images of barred and non-barred galaxies are presented in Fig. 2.The bar fraction of interacting clusters and noninteracting clusters as a function of B/T is shown in Fig. 3a. We find that the bar fraction is several times higher for galaxies with B/T ≤ 0.5 (hereafter, disk-dominated galaxies) than those with B/T > 0.5, in agreement with results from previous studies 2,11,12 . The results of previous numerical simulations also support the finding that the formation or maintenance of bars is prohibited in galaxies with high B/T or high central mass concentrations 13,14,15 . It is also known that bar formation can be suppressed in galaxies that are dynamically hot (random motions of stars are dominant) or have high velocity dispersions 4,5 . Given that bars are...

We study the mid-infrared (MIR) properties of galaxies in compact groups and their environmental dependence using the Wide-field Infrared Survey Explorer (WISE) data. We use a volume-limited sample of 670 compact groups and their 2175 member galaxies with M r < −19.77 and 0.01 < z < 0.0741, drawn from Sohn et al. (2016), which were identified using a friends-of-friends algorithm. Among the 2175 galaxies, 1541 galaxies are detected at WISE 12 µm with a signal-to-noise ratio greater than 3. Among the 1541 galaxies, 433 AGN-host galaxies are identified by using both optical and MIR classification scheme. Using the remaining 1108 non-AGN galaxies, we find that the MIR [3.4] − [12] colors of compact group early-type galaxies are on average bluer than those of cluster early-type galaxies. When compact groups have both early-and late-type member galaxies, the MIR colors of the late-type members in those compact groups are bluer than the MIR colors of cluster latetype galaxies. As compact groups are located in denser regions, they tend to have larger early-type galaxy fractions and bluer MIR color galaxies. These trends are also seen for neighboring galaxies around compact groups. However, compact group member galaxies always have larger early-type galaxy fractions and bluer MIR colors than their neighboring galaxies. Our findings suggest that the properties of compact group galaxies depend on both internal and external environments of compact groups, and that galaxy evolution is faster in compact groups than in the central regions of clusters.