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Register a new userThe Fastest Galaxy Evolution in an Unbiased Compact Group Sample with WISE
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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.
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Galaxies in Hickson Compact Groups (HCGs) are believed to experience morphological transformations from blue, star-forming galaxies to red, early-type galaxies. Galaxies with a high ratio between the luminosities of the warm H2 to the 7.7mu PAH emission (Molecular Hydrogen Emission Galaxies, MOHEGs) are predominantly in an intermediate phase, the green valley. Their enhanced H2 emission suggests that the molecular gas is affected in the transition. We study the properties of the molecular gas traced by CO in galaxies in HCGs with measured warm H2 emission in order to look for evidence of the perturbations affecting the warm H2 in the kinematics, morphology and mass of the molecular gas. We analyzed the molecular gas mass derived from CO(1-0), MH2, and its kinematics, and then compared it to the mass of the warm molecular gas, the stellar mass and star formation rate (SFR). Our results are the following. (i) The mass ratio between the CO-derived and the warm H2 molecular gas is in the same range as for field galaxies. (ii) Some galaxies, mostly MOHEGs, have very broad CO linewidths of up to 1000 kms. The line shapes are irregular and show various components. (iii) The mapped objects show asymmetric distributions of the cold molecular gas. (iv) The star formation efficiency (= SFR/MH2) of galaxies in HCGs is similar to isolated galaxies. No significant difference between MOHEGs and non-MOHEGs or between early-types and spirals has been found. (v) The molecular gas masses, MH2, and MH2/LK are lower in MOHEGs (predominantly early-types) than in non-MOHEGs (predominantly spirals). This trend remains when comparing MOHEGs and non-MOHEGs of the same morphological type. The differences in the molecular gas properties of MOHEGs support the view that they are suffering perturbations of the molecular gas, as well as a decrease in the molecular gas content and associated SFR.
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.
I present observations of the Hickson Compact Group 88 (HCG88) obtained during the commissioning of a new 28-inch telescope at the Wise Observatory. This galaxy group was advertised to be non-interacting, or to be in a very early interaction stage, but this is not the case. The observations reported here were done using a luminance filter, essentially a very broad R filter, reaching a low surface brightness level of about 26 mag per square arcsec. Additional observations were obtained in a narrow spectral band approximately centered on the rest-frame H-alpha line from the group. Contrary to previous studies, my observations show that at least two of the major galaxies have had significant interactions in the past, although probably not between themselves. I report the discovery of a faint extended tail emerging from the brightest of the group galaxies, severe isophote twisting and possible outer shells around another galaxy, and map the HII regions in all the galaxies.
Compact groups of galaxies provide insight into the role of low-mass, dense environments in galaxy evolution because the low velocity dispersions and close proximity of galaxy members result in frequent interactions that take place over extended timescales. We expand the census of star formation in compact group galaxies by citet{tzanavaris10} and collaborators with Swift UVOT, Spitzer IRAC and MIPS 24 micron photometry of a sample of 183 galaxies in 46 compact groups. After correcting luminosities for the contribution from old stellar populations, we estimate the dust-unobscured star formation rate (SFR$_{mathrm{UV}}$) using the UVOT uvw2photometry. Similarly, we use the MIPS 24 micron photometry to estimate the component of the SFR that is obscured by dust (SFR$_{mathrm{IR}}$). We find that galaxies which are MIR-active (MIR-red), also have bluer UV colours, higher specific star formation rates, and tend to lie in H~{sc i}-rich groups, while galaxies that are MIR-inactive (MIR-blue) have redder UV colours, lower specific star formation rates, and tend to lie in H~{sc i}-poor groups. We find the SFRs to be continuously distributed with a peak at about 1 M$_{odot}$ yr$^{-1}$, indicating this might be the most common value in compact groups. In contrast, the specific star formation rate distribution is bimodal, and there is a clear distinction between star-forming and quiescent galaxies. Overall, our results suggest that the specific star formation rate is the best tracer of gas depletion and galaxy evolution in compact groups.
Context. Ultra-compact dwarfs (UCDs) are stellar systems displaying colours and metallicities between those of globular clusters (GCs) and early-type dwarf galaxies, as well as sizes of Reff <= 100 pc and luminosities in the range -13.5 < MV < -11 mag. Although their origin is still subject of debate, the most popular scenarios suggest that they are massive star clusters or the nuclei of tidally stripped dwarf galaxies. Aims. NGC 5044 is the central massive elliptical galaxy of the NGC 5044 group. Its GC/UCD system is completely unexplored. Methods. In Gemini+GMOS deep images of several fields around NGC 5044 and in spectroscopic multi-object data of one of these fields, we detected an unresolved source with g~20.6 mag, compatible with being an UCD. Its radial velocity was obtained with FXCOR and the penalized pixel-fitting (pPXF) code. To study its stellar population content, we measured the Lick/IDS indices and compared them with predictions of single stellar population models, and we used the full spectral fitting technique. Results. The spectroscopic analysis of the UCD revealed a radial velocity that agrees with the velocity of the elliptical galaxy NGC 5044. From the Lick/IDS indices, we have obtained a luminosity-weighted age and metallicity of 11.7+/-1.4 Gyr and [Z/H] = -0.79 +/- 0.04 dex, respectively, as well as [alpha/Fe] = 0.30 +/- 0.06. From the full spectral fitting technique, we measured a lower age (8.52 Gyr) and a similar total metallicity ([Z/H] = -0.86 dex). Conclusions. Our results indicate that NGC 5044-UCD1 is most likely an extreme GC (MV ~ -12.5 mag) belonging to the GC system of the elliptical galaxy NGC 5044.
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