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Register a new userHierarchical Formation in Action: Characterizing Accelerated Galaxy Evolution in Compact Groups Using Whole-Sky WISE Data
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Compact groups provide an environment to study the growth of galaxies amid multiple prolonged interactions. With their dense galaxy concentrations and relatively low velocity dispersions, compact groups mimic the conditions of hierarchical galaxy assembly. Compact group galaxies are known to show a bimodality in $Spitzer$ IRAC infrared colorspace: galaxies are preferentially either quiescent with low specific star formation rates, or are prolifically forming stars---galaxies with moderate levels of specific star formation are rare. Previous $Spitzer$ IRAC studies identifying this canyon have been limited by small number statistics. We utilize whole-sky WISE data to study 163 compact groups, thereby tripling our previous sample and including more galaxies with intermediate mid-IR colors indicative of moderate specific star formation rates (SSFRs). We define a distinct WISE mid-IR color-space ($log[{frac{rm f_{12}}{rm f_{4.6}}}]$ vs. $log[{frac{rm f_{22}}{rm f_{3.4}}}]$) that we use to identify canyon galaxies from the larger sample. We confirm that compact group galaxies show a bimodal distribution in the mid-infrared and identify 37 canyon galaxies with reliable photometry and intermediate mid-IR colors. Morphologically, we find that the canyon harbors a large population of both Sa-Sbc and E/S0 type galaxies, and that they fall on the optical red sequence rather than the green valley. Finally, we provide a catalog of WISE photometry for 567 of 652 galaxies selected from the sample of 163 compact groups.
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We study the mid-infrared (MIR) properties of galaxies in compact groups and their environmental dependence using the textit{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 citet{sohn+16}, which were identified using a friends-of-friends algorithm. Among the 2175 galaxies, 1541 galaxies are detected at textit{WISE} 12 $micron$ 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 late-type 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.
We study the evolution of the total star formation (SF) activity, total stellar mass and halo occupation distribution in massive halos by using one of the largest X-ray selected sample of galaxy groups with secure spectroscopic identification in the major blank field surveys (ECDFS, CDFN, COSMOS, AEGIS). We provide an accurate measurement of SFR for the bulk of the star-forming galaxies using very deep mid-infrared Spitzer MIPS and far-infrared Herschel PACS observations. For undetected IR sources, we provide a well-calibrated SFR from SED fitting. We observe a clear evolution in the level of SF activity in galaxy groups. The total SF activity in the high redshift groups (0.5<z<1.1) is higher with respect to the low redshift (0.15<z<0.5) sample at any mass by 0.8+/-0.12 dex. A milder difference (0.35+/-0.1 dex) is observed between the low redshift bin and the groups at z~0. We show that the level of SF activity is declining more rapidly in the more massive halos than in the more common lower mass halos. We do not observe any evolution in the halo occupation distribution and total stellar mass- halo mass relations in groups. The picture emerging from our findings suggests that the galaxy population in the most massive systems is evolving faster than galaxies in lower mass halos, consistently with a halo downsizing scenario.
We compare the mean mass assembly histories of compact and fossil galaxy groups in the Millennium dark matter simulation and an associated semi-analytic galaxy formation model. Tracing the halo mass of compact groups (CGs) from z=0 to z=1 shows that, on average, 55 per cent of the halo mass in compact groups is assembled since z~1, compared to 40 per cent of the halo mass in fossil groups (FGs) in the same time interval, indicating that compared to FGs, CGs are relatively younger galaxy systems. At z=0, for a given halo mass, fossil groups tend to have a larger concentration than compact groups. Investigating the evolution of CGs parameters show that they become more compact with time. CGs at z=0.5 see their magnitude gaps increase exponentially, but it takes ~10 Gyr for them to reach a magnitude gap of 2 magnitudes. The slow growth of the magnitude gap leads to only a minority (~41 per cent) of CGs selected at z=0.5 turning into a FG by z=0. Also, while three-quarters of FGs go through a compact phase, most fail to meet the CG isolation criterion, leaving only ~30 per cent of FGs fully satisfying the CG selection criteria. Therefore, there is no strong link of CGs turning into FGs or FGs originating from CGs. The relation between CGs and FGs is thus more complex, and in most cases, FGs and CGs follow different evolutionary tracks.
We analyzed the Einstein radius, $theta_E$, in our sample of SL2S galaxy groups, and compared it with $R_A$ (the distance from the arcs to the center of the lens), using three different approaches: 1.- the velocity dispersion obtained from weak lensing assuming a Singular Isothermal Sphere profile ($theta_{E,I}$), 2.- a strong lensing analytical method ($theta_{E,II}$) combined with a velocity dispersion-concentration relation derived from numerical simulations designed to mimic our group sample, 3.- strong lensing modeling ($theta_{E,III}$) of eleven groups (with four new models presented in this work) using HST and CFHT images. Finally, $R_A$ was analyzed as a function of redshift $z$ to investigate possible correlations with L, N, and the richness-to-luminosity ratio (N/L). We found a correlation between $theta_{E}$ and $R_A$, but with large scatter. We estimate $theta_{E,I}$ = (2.2 $pm$ 0.9) + (0.7 $pm$ 0.2)$R_A$, $theta_{E,II}$ = (0.4 $pm$ 1.5) + (1.1 $pm$ 0.4)$R_A$, and $theta_{E,III}$ = (0.4 $pm$ 1.5) + (0.9 $pm$ 0.3)$R_A$ for each method respectively. We found a weak evidence of anti-correlation between $R_A$ and $z$, with Log$R_A$ = (0.58$pm$0.06) - (0.04$pm$0.1)$z$, suggesting a possible evolution of the Einstein radius with $z$, as reported previously by other authors. Our results also show that $R_A$ is correlated with L and N (more luminous and richer groups have greater $R_A$), and a possible correlation between $R_A$ and the N/L ratio. Our analysis indicates that $R_A$ is correlated with $theta_E$ in our sample, making $R_A$ useful to characterize properties like L and N (and possible N/L) in galaxy groups. Additionally, we present evidence suggesting that the Einstein radius evolves with $z$.
The optical spectroscopic and radio interferometric HI 21 cm-line observations of the blue compact dwarf galaxy Mrk 22 are presented. The Wolf-Rayet (WR) emission line features corresponding to high ionization lines of HeII $lambda$4686 and CIV $lambda$5808 from young massive stars are detected. The ages of two prominent star forming regions in the galaxy are estimated as $sim$10 Myr and $sim$ 4 Myr. The galaxy has non-thermal radio deficiency, which also indicates a young star-burst and lack of supernovae events from the current star formation activities, consistent with the detection of WR emission lines features. A significant N/O enrichment is seen in the fainter star forming region. The gas-phase metallicities [12 + log(O/H)] for the bright and faint regions are estimated as 7.98$pm$0.07 and 7.46$pm$0.09 respectively. The galaxy has a large diffuse HI envelop. The HI images reveal disturbed gas kinematics and HI clouds outside the optical extent of the galaxy, indicating recent tidal interaction or merger in the system. The results strongly indicate that Mrk 22 is undergoing a chemical and morphological evolution due to ongoing star formation, most likely triggered by a merger.
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