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Register a new userThe Optical Green Valley vs Mid-IR Canyon in Compact Groups
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Compact groups of galaxies provide conditions similar to those experienced by galaxies in the earlier universe. Recent work on compact groups has led to the discovery of a dearth of mid-infrared transition galaxies (MIRTGs) in IRAC (3.6 - 8.0 micron) color space (Johnson et al. 2007; Walker et al. 2012) as well as at intermediate specific star formation rates (Tzanavaris et al. 2010). However, we find that in compact groups these mid-infrared (mid-IR) transition galaxies in the mid-infrared dearth have already transitioned to the optical ([g-r]) red sequence. We investigate the optical color-magnitude diagram (CMD) of 99 compact groups containing 348 galaxies and compare the optical CMD with mid-IR color space for compact group galaxies. Utilizing redshifts available from SDSS, we identified new galaxy members for 6 groups. By combining optical and mid-IR data, we obtain information on both the dust and the stellar populations in compact group galaxies. We also compare with more isolated galaxies and galaxies in the Coma cluster, which reveals that, similar to clusters, compact groups are dominated by optically red galaxies. While we find that compact group transition galaxies lie on the optical red sequence, LVL+SINGS mid-IR transition galaxies span the range of optical colors. The dearth of mid-IR transition galaxies in compact groups may be due to a lack of moderately star forming low mass galaxies; the relative lack of these galaxies could be due to their relatively small gravitational potential wells. This makes them more susceptible to this dynamic environment, thus causing them to more easily lose gas or be accreted by larger members.
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Hickson Compact Groups (HCGs) are among the densest galaxy environments of the local universe. To examine the effects of the environment on the infrared properties of these systems, we present an analysis of Spitzer and ISO mid-infrared imaging as well as deep ground based near-infrared imaging of 14 HCGs containing a total of 69 galaxies. Based on mid-infrared color diagnostics we identify the galaxies which appear to host an active nucleus, while using a suite of templates, we fit the complete infrared spectral energy distribution for each group member. We compare our estimates of galaxy mass, star formation rate, total infrared luminosities, and specific star formation rates (sSFR) for our HCG sample, to samples of isolated galaxies and interacting pairs and find that overall there is no discernible difference among them. However, HCGs which can be considered as dynamically old, host late-type galaxies with a slightly lower sSFR than the one found in dynamically young groups. This could be attributed to multiple past interactions among the galaxies in old groups, that have led to the build up of their stellar mass. It is also consistent with our prediction for the presence of diffuse cold dust in the intergalactic medium of 9 of the dynamically old groups.
We present a comprehensive study on the impact of the environment of compact galaxy groups on the evolution of their members using a multi-wavelength analysis, from the UV to the infrared, for a sample of 32 Hickson compact groups (HCGs) containing 135 galaxies. Fitting the SEDs of all galaxies with the state-of-the-art model of da Cunha (2008) we can accurately calculate their mass, SFR, and extinction, as well as estimate their infrared luminosity and dust content. We compare our findings with samples of field galaxies, early-stage interacting pairs, and cluster galaxies with similar data. We find that classifying the groups as dynamically old or young, depending on whether or not at least one quarter of their members are early-type systems, is physical and consistent with past classifications of HCGs based on their atomic gas content. [...ABRIDGED...] We also examine their SF properties, UV-optical and mid-IR colors, and we conclude that all the evidence point to an evolutionary scenario in which the effects of the group environment and the properties of the galaxy members are not instantaneous. Early on, the influence of close companions to group galaxies is similar to the one of galaxy pairs in the field. However, as the time progresses, the effects of tidal torques and minor merging, shape the morphology and star formation history of the group galaxies, leading to an increase of the fraction of early-type members and a rapid built up of the stellar mass in the remaining late-type galaxies.
We present an analysis of the mid-infrared emission lines for a sample of 12 low metallicity Blue Compact Dwarf (BCD) galaxies based on high resolution observations obtained with Infrared Spectrograph on board the {rm Spitzer} Space Telescope. We compare our sample with a local sample of typical starburst galaxies and active galactic nuclei (AGNs), to study the ionization field of starbursts over a broad range of physical parameters and examine its difference from the one produced by AGN. The high-ionization line [OIV]25.89$mu$m is detected in most of the BCDs, starbursts, and AGNs in our sample. We propose a diagnostic diagram of the line ratios [OIV]25.89$mu$m/[SIII]33.48$mu$m as a function of [NeIII]15.56$mu$m/[NeII]12.81$mu$m which can be useful in identifying the principal excitation mechanism in a galaxy. Galaxies in this diagram split naturally into two branches. Classic AGNs as well as starburst galaxies with an AGN component populate the upper branch, with stronger AGNs displaying higher [NeIII]/[NeII] ratios. BCDs and pure starbursts are located in the lower branch. We find that overall the placement of galaxies on this diagram correlates well with their corresponding locations in the log([NII]/H$alpha$) vs. log([OIII]/H$beta$) diagnostic diagram, which has been widely used in the optical. The two diagrams provide consistent classifications of the excitation mechanism in a galaxy. On the other hand, the diagram of [NeIII]15.56$mu$m/[NeII]12.81$mu$m vs. [SIV]10.51$mu$m/[SIII]18.71$mu$m is not as efficient in separating AGNs from BCDs and pure starbursts. (abridged)
We present Swift UVOT (1600-3000A) 3-band photometry for 41 galaxies in 11 nearby (<4500km/s) Hickson Compact Groups (HCGs) of galaxies. We use the uvw2-band (2000A) to estimate the dust-unobscured component, SFR_UV, of the total star-formation rate, SFR_T. We use Spitzer MIPS 24-micron photometry to estimate SFR_IR, the dust-obscured component of SFR_T. We obtain SFR_T=SFR_UV+SFR_IR. Using 2MASS K_s band based stellar mass, M*, estimates, we calculate specific SFRs, SSFR=SFR_T/M*. SSFR values show a clear and significant bimodality, with a gap between low (<~3.2x10^-11 / yr) and high SSFR (>~1.2x10^-10 / yr) systems. All galaxies with MIR activity index a_IRAC <= 0 (>0) are in the high- (low-) SSFR locus, as expected if high levels of star-formation power MIR emission from polycyclic aromatic hydrocarbon molecules and a hot dust continuum. All elliptical/S0 galaxies are in the low-SSFR locus, while 22 out of 24 spirals/irregulars are in the high-SSFR locus, with two borderline cases. We divide our sample into three subsamples (I, II and III) according to decreasing HI-richness of the parent galaxy group to which a galaxy belongs. Consistent with the SSFR and a_IRAC bimodality, 12 out of 15 type-I (11 out of 12 type-III) galaxies are in the high- (low-) SSFR locus, while type II galaxies span almost the full range of SSFR values. Unlike HCG galaxies, galaxies in a comparison quiescent SINGS sub-sample are continuously distributed both in SSFR and a_IRAC. Any uncertainties can only further enhance the SSFR bimodality. These results suggest that an environment characterized by high galaxy number-densities and low galaxy velocity-dispersions, such as the one found in compact groups, plays a key role in accelerating galaxy evolution by enhancing star-formation processes in galaxies and favoring a fast transition to quiescence.(abridged)
We present a clustering analysis of near ultraviolet (NUV) - optical color selected luminosity bin samples of green valley galaxies. These galaxy samples are constructed by matching the Sloan Digital Sky Survey Data Release 7 with the latest Galaxy Evolution Explorer source catalog which provides NUV photometry. We present cross-correlation function measurements and determine the halo occupation distribution of these transitional galaxies using a new multiple tracer analysis technique. We extend the halo-occupation formalism to model the cross-correlation function between a galaxy sample of interest and multiple tracer populations simultaneously. This method can be applied to commonly used luminosity threshold samples as well as to color and luminosity bin selected galaxy samples, and improves the accuracy of clustering analyses for sparse galaxy populations. We confirm the previously observed trend that red galaxies reside in more massive halos and are more likely to be satellite galaxies than average galaxies of similar luminosity. While the change in central galaxy host mass as a function of color is only weakly constrained, the satellite fraction and characteristic halo masses of green satellite galaxies are found to be intermediate between those of blue and red satellite galaxies.
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