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Register a new userIntragroup and Galaxy-Linked Diffuse X-ray Emission in Hickson Compact Groups
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Isolated compact groups of galaxies (CGs) present a range of dynamical states, group velocity dispersions, and galaxy morphologies with which to study galaxy evolution, particularly the properties of gas both within the galaxies and in the intragroup medium. As part of a large, multiwavelength examination of CGs, we present an archival study of diffuse X-ray emission in a subset of nine Hickson compact groups observed with the Chandra X-ray Observatory. We find that seven of the groups in our sample exhibit detectable diffuse emission. However, unlike large-scale emission in galaxy clusters, the diffuse features in the majority of the detected groups are linked to the individual galaxies, in the form of both plumes and halos likely as a result of star formation or AGN activity, as well as in emission from tidal features. Unlike previous studies from earlier X-ray missions, HCGs 31, 42, 59, and 92 are found to be consistent with the Lx-T relationship from clusters within the errors, while HCGs 16 and 31 are consistent with the cluster Lx-sigma relation, though this is likely coincidental given that the hot gas in these two systems is largely due to star formation. We find that Lx increases with decreasing group HI to dynamical-mass ratio with tentative evidence for a dependance in X-ray luminosity on HI morphology whereby systems with intragroup HI indicative of strong interactions are considerably more X-ray luminous than passively evolving groups. We also find a gap in the Lx of groups as a function of the total group specific star formation rate. Our findings suggest that the hot gas in these groups is not in hydrostatic equilibrium and these systems are not low-mass analogs of rich groups or clusters, with the possible exception of HCG 62.
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We present the results of a pilot XMM-$Newton$ and $Chandra$ program aimed at studying the diffuse intragroup medium (DIM) of optically-selected nearby groups from the Zurinch ENvironmental Study (ZENS) catalog. The groups are in a narrow mass range about $10^{13}M_odot$, a mass scale at which the interplay between the DIM and the group member galaxies is still largely unprobed. X-ray emission from the DIM is detected in the energy band 0.5--2 keV with flux $le 10^{-14}$ erg cm$^{-1}$ s$^{-1}$, which is one order of magnitude fainter than for typical ROSAT groups (RASS). For many groups we set upper limits to the X-ray luminosity, indicating that the detections are likely probing the upper envelope of the X-ray emitting groups. We find evidence for our optically selected groups to be under-luminous with respect to predictions from X-ray scaling relations. X-ray mass determinations are in best agreement with those based on the member galaxies bulge luminosity, followed by their total optical luminosity and velocity dispersion. We measure a stellar mass fraction with a median value of about 1$%$, with a contribution from the most massive galaxies between 30 to 50 %. Optical and X-ray data give often complementary answers concerning the dynamical state of the groups, and are essential for a complete picture of the system. Extending this pilot program to a larger sample of groups is mandatory to unveil any imprint of interaction between member galaxies and DIM in halo potentials of key importance for environmentally-driven galactic evolution.
We have found the atomic gas (HI) reservoirs of the blue ultra diffuse galaxy (UDG) candidates identified by Roman and Trujillo in images near Hickson Compact Groups (HCGs). We confirm that all of the objects are indeed UDGs with effective radii R_e > 1.5 kpc. Three of them are likely to be gravitationally bound to the HCG near which they project, one is plausibly gravitationally bound to the nearest HCG, and one is in the background. We measure HI masses and velocity widths for each object directly from the spectra, and use the widths together with the UDG effective radii to estimate dynamical masses and halo spin parameters. The location of the blue UDGs in the HI mass - stellar mass plane is consistent with that of the broader gas-rich galaxy population, and both their HI masses and gas richnesses are correlated with their effective radii. The blue UDGs appear to be low-mass objects with high-spin halos, although their properties are not as extreme as those of the faintest diffuse objects found in HI searches. The data presented here highlight the potential of single-dish radio observations for measuring the physical properties of blue diffuse objects detected in the optical.
The formation of ultra-compact dwarf galaxies (UCDs) is believed to be interaction driven, and UCDs are abundant in the cores of galaxy clusters, environments that mark the end-point of galaxy evolution. Nothing is known about the properties of UCDs in compact groups of galaxies, environments where most of galaxy evolution and interaction is believed to occur and where UCDs in intermediate state of evolution may be expected. The main goal of this study is to detect and characterize, for the first time, the UCD population of compact groups. For that, 2 groups in different evolutionary stages, HCG 22 and HCG 90, were targeted with VLT/FORS2/MXU. We detect 16 and 5 objects belonging to HCG 22 and HCG 90, respectively, covering the magnitude range -10.0 > M_R > -11.5 mag. Their colours are consistent with old ages covering a broad range in metallicities. Photometric mass estimates put 4 objects in HCG 90 and 9 in HCG 22 in the mass range of UCDs (>2x10^6 M_Sun) for an assumed age of 12 Gyr. These UCDs are on average 2-3 times larger than typical Galactic GCs, covering a range of 2 >~ r_h >~ 21 pc. The UCDs in HCG 22 are more concentrated around the central galaxy than in HCG 90, at the 99% confidence level. They cover a broad range in [alpha/Fe] abundances from sub- to super-solar. The spectra of 3 UCDs show tentative evidence for intermediate age stellar populations. We calculate the specific frequency (S_N) of UCDs for both groups, finding that HCG 22 has about three times higher S_N than HCG 90. The ensemble properties of the detected UCDs supports 2 co-existing formation channels: a star cluster origin and an origin as tidally stripped dwarf nuclei. Our results imply that the UCDs detected in both groups do not, in their majority, originate from relatively recent galaxy interactions. Most of the detected UCDs have likely been brought into the group with their host galaxies.[abridged]
This continuing study of intragroup light in compact groups of galaxies aims to establish new constraints to models of formation and evolution of galaxy groups, specially of compact groups, which are a key part in the evolution of larger structures, such as clusters. In this paper we present three additional groups (HCG 15, 35 and 51) using deep wide field $B$ and $R$ band images observed with the LAICA camera at the 3.5m telescope at the Calar Alto observatory (CAHA). This instrument provides us with very stable flatfielding, a mandatory condition for reliably measuring intragroup diffuse light. The images were analyzed with the OV_WAV package, a wavelet technique that allows us to uncover the intragroup component in an unprecedented way. We have detected that 19, 15 and 26% of the total light of HCG 15, 35 and 51, respectively, is in the diffuse component, with colours that are compatible with old stellar populations and with mean surface brightness that can be as low as $28.4 {rm B mag arcsec^{-2}}$. Dynamical masses, crossing times and mass to light ratios were recalculated using the new group parameters. Also tidal features were analyzed using the wavelet technique.
Chandra X-ray imaging spectroscopy of the starburst galaxy Henize 2-10 reveals a strong nuclear point source and at least two fainter compact sources embedded within a more luminous diffuse thermal component. Spectral fits to the nuclear X-ray source imply an unabsorbed X-ray luminosity L_x >10^40 erg/s for reasonable power law or blackbody models, consistent with accretion onto a >50 solar mass black hole behind a foreground absorbing column of N_H>10^23 /cm^2. Two of these point sources have L_x=2-5 x 10^38 erg/s, comparable to luminous X-ray binaries. These compact sources constitute a small fraction (<16%) of the total X-ray flux from He~2-10 in the 0.3--6.0 keV band and just 31% of the X-rays in the hard 1.1--6.0 keV band which is dominated by diffuse emission. Two-temperature solar-composition plasmas (kT~0.2 keV and kT~0.7 keV) fit the diffuse X-ray component as well as single-temperature plasmas with enhanced alpha/Fe ratios. Since the observed radial gradient of the X-ray surface brightness closely follows that of the Halpha emission, the composition of the X-ray plasma likely reflects mixing of the ambient cool/warm ISM with an even hotter, low emission measure plasma, thereby explaining the ~solar ISM composition. Aperture synthesis 21-cm maps show an extended neutral medium to radii of 60 so that the warm and hot phases of the ISM, which extend to ~30, are enveloped within the 8x10^20 /cm^2 contour of the cool neutral medium. This extended neutral halo may serve to inhibit a starburst-driven outflow unless it is predominantly along the line of sight. The high areal density of star formation can also be reconciled with the lack of prominent outflow signatures if Henize 2-10 is in the very early stages of developing a galactic wind.
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