Hickson Compact Groups Research Articles

The properties of the heterogeneous Shakhbazyan groups of galaxies in the SDSS

We present a systematic study of the sub-sample of Shakhbazyan groups (SHKs) covered by the Sloan Digital Sky Survey Data Release--5 (SDSS-5). SHKs probe an environment with characteristics which are intermediate between those of loose and very compact groups. Surprisingly, we found that several groups identifying algorithms (e.g. Berlind et al. 2006, Tago et al. 2008) miss this type of structures. Using the SDSS-5 spectroscopic data and the photometric redshifts derived in D'Abrusco et al. 2007, we identified possible group members in photometric redshift space and derived, for each group, several individual properties. We also combined pointed and stacked Rosat All Sky Survey data to investigate the X-ray luminosities of these systems. Our study confirms that the majority of groups are physical entities with richness in the range 3--13 galaxies, and properties ranging between those of loose and compact groups. We confirm that SHK groups are richer in early-type galaxies than the surrounding environment and the field, as expected from the morphology-density relation and from the selection of groups of red galaxies. Furthermore, our work supports the existence of two sub-classes of structures, the first one being formed by compact and isolated groups and the second formed by extended structures. We suggest that while the first class of objects dwells in less dense regions like the outer parts of clusters or the field, possibly sharing the properties of Hickson Compact Groups, the more extended structures represent a mixture of [core+halo] configurations and cores of rich clusters. X-ray luminosities for SHKs are generally consistent with these results and with the expectations for the L_X-sigma_v relation, but also suggest the velocity dispersions reported in literature are underestimated for some of the richest systems.

Galaxy evolution in Hickson compact groups: the role of ram-pressure stripping and strangulation

Galaxies in compact groups tend to be deficient in neutral hydrogen compared to isolated galaxies of similar optical properties. In order to investigate the role played by a hot intragroup medium (IGM) for the removal and destruction of HI in these systems, we have performed a Chandra and XMM-Newton study of eight of the most HI deficient Hickson compact groups. Diffuse X-ray emission associated with an IGM is detected in four of the groups, suggesting that galaxy-IGM interactions are not the dominant mechanism driving cold gas out of the group members. No clear evidence is seen for any of the members being currently stripped of any hot gas, nor for galaxies to show enhanced nuclear X-ray activity in the X-ray bright or most HI deficient groups. Combining the inferred IGM distributions with analytical models of representative disc galaxies orbiting within each group, we estimate the HI mass loss due to ram pressure and viscous stripping. While these processes are generally insufficient to explain observed HI deficiencies, they could still be important for HI removal in the X-ray bright groups, potentially removing more than half of the ISM in the X-ray bright HCG 97. Ram pressure may also have facilitated strangulation through the removal of galactic coronal gas. In X-ray undetected groups, tidal interactions could be playing a prominent role, but it remains an open question whether they can fully account for the observed HI deficiencies.

The Revealing Dust: Mid‐Infrared Activity in Hickson Compact Group Galaxy Nuclei

We present a sample of 46 galaxy nuclei from 12 nearby (z < 4500 km s−1) Hickson Compact Groups (HCGs) with a complete suite of 1-24 μm 2MASS + Spitzer nuclear photometry. For all objects in the sample, blue emission from stellar photospheres dominates in the near-infrared through the 3.6 μm IRAC band. Of 46 galaxy nuclei, 25 (54%) show red mid-infrared continua characteristic of hot dust powered by ongoing star formation and/or accretion onto a central black hole. We introduce αIRAC, the spectral index of a power-law fit to the 4.5-8.0 μm IRAC data, and demonstrate that it cleanly separates the mid-infrared-active and nonactive HCG nuclei. This parameter is more powerful for identifying low-to moderate-luminosity mid-infrared activity than other measures, which include data at rest-frame λ < 3.6 μm that may be dominated by stellar photospheric emission. While the HCG galaxies clearly have a bimodal distribution in this parameter space, a comparison sample from the Spitzer Nearby Galaxy Survey (SINGS) matched in J-band total galaxy luminosity is continuously distributed. A second diagnostic, the fraction of 24 μm emission in excess of that expected from quiescent galaxies, f24D, reveals an additional three nuclei to be active at 24 μm. Comparing these two mid-infrared diagnostics of nuclear activity to optical spectroscopic identifications from the literature reveals some discrepancies, and we discuss the challenges of distinguishing the source of ionizing radiation in these and other lower luminosity systems. We find a significant correlation between the fraction of mid-infrared-active galaxies and the total H I mass in a group and investigate possible interpretations of these results in light of galaxy evolution in the highly interactive system of a compact group environment.

SEARCHING FOR STAR FORMATION OUTSIDE GALAXIES: MULTIWAVELENGTH ANALYSIS OF THE INTRAGROUP MEDIUM OF HICKSON COMPACT GROUP 100

We use multiwavelength data (H I, FUV, NUV, R) to search for evidence of star formation in the intragroup medium of the Hickson Compact Group 100. We find that young star-forming regions are located in the intergalactic H I clouds of the compact group which extend to over 130 kpc away from the main galaxies. A tidal dwarf galaxy (TDG) candidate is located in the densest region of the H I tail, 61 kpc from the brightest group member and its age is estimated to be only 3.3 Myr. Fifteen other intragroup H II regions and TDG candidates are detected in the Galaxy Evolution Explorer (GALEX) FUV image and within a field 10' × 10' encompassing the H I tail. They have ages <200 Myr, H I masses of 109.2-10.4 M☉, 0.001 <SFR <0.01 M☉ yr−1, and stellar masses 104.3-106.5 M☉. The H I clouds to which many of them are associated have column densities about one order of magnitude lower than N(H I) ~1021 cm−2.

Decoding the spectra of SDSS early-type galaxies: new indicators of age and recent star formation

We apply Principal Component Analysis (PCA) to a sample of early-type galaxies from the Sloan Digital Sky Survey (SDSS) in order to infer differences in their star formation histories from their unresolved stellar populations. We select a z < 0.1 volume-limited sample comprising � 7000 early-type galaxies from SDSS/Data Release 4. Out of the firs t few principal components (PC), we study four which give information about stellar populations and velocity dispersion. We construct two parameters (η and ζ) as linear combinations of PC1 and PC2. The four components can be presented as “optimal filt ers” to explore in detail the properties of the underlying stellar populations. By comparing various photo-spectroscopic observables ‐ including NUV photometry from GALEX ‐ we find ζ to be most sensitive to recent episodes of star formation, and η to be strongly dependent on the average age of the stellar populations. Both η and ζ also depend on metallicity. We apply these optimal filters to composite spectra assembled by Bernardi et al. The distribution of the η component of the composites appear to be indistinguishable between high and low density regions, whereas the distribution of ζ parameters have a significant skew towards lower values for g alaxies in low density regions. This result suggests that galaxies in l ower density environments are less likely to present weak episodes of recent star formation. In contrast, a significant number of galaxies from our high density subsample ‐ which includes clusters (both outer regions and centres) and groups ‐ underwent small but detectable recent star formation at high metallicity, in agreement with recent estimates targeting elliptical ga laxies in Hickson Compact Groups and in the field (Ferreras et al.).

The Infrared Properties of Hickson Compact Groups

Compact groups of galaxies provide a unique environment to study the mechanisms by which star formation occurs amid continuous gravitational encounters. We present 2MASS (JHK) and Spitzer IRAC (3.5-8 μm) and MIPS (24 μm) observations of a sample of 12 Hickson Compact Groups (HCGs 2, 7, 16, 19, 22, 31, 42, 48, 59, 61, 62, and 90) that includes a total of 45 galaxies. The infrared colors of the galaxies in this sample span a range of parameter space, and some trends are apparent in the data. The near-infrared colors of the sample galaxies are largely consistent with being dominated by slightly reddened normal stellar populations. There is also some evidence for a K-band excess in a few cases, which likely indicates the presence of hot dust at or near the sublimation temperature associated with active galactic nuclei or star formation activity. Galaxies that have the most significant polycyclic aromatic hydrocarbon and/or hot dust emission (as inferred from excess 8 μm flux) also tend to have larger amounts of extinction and/or K-band excess and stronger 24 μm emission, all of which suggest ongoing star formation activity. We separate the 12 HCGs in our sample into three types based on the ratio of the group H I mass to dynamical mass. We find evidence that galaxies in the most gas-rich groups tend to be the most actively star forming. Galaxies in the most gas-poor groups tend to be tightly clustered around a narrow range in colors consistent with the integrated light from a normal stellar population. We interpret these trends as indicating that galaxies in gas-rich groups experience star formation and/or nuclear actively until their neutral gas is consumed, stripped, or ionized. The galaxies in this sample exhibit a between gas-rich and gas-poor groups in infrared color space that is sparsely populated and not seen in the Spitzer First Look Survey sample. This gap may suggest a rapid evolution of galaxy properties in response to dynamical effects. These results suggest that the global properties of the groups and the local properties of the galaxies are connected.

Optical and radio survey of southern compact groups of galaxies

Context. Multi-wavelength observations of Hickson's Compact Groups (HCGs) have shown that many of these groups are physical bound structures and are in different stage of evolution, from spiral-dominated systems to almost merged objects. Very few studies have analysed the Southern Compact Groups (SCGs) sample, which is thought to be younger that HCGs, due to an on average higher number of spiral galaxies. We present here the first results from optical and radio observations on a pilot sample of SCGs. Aims. Previous HI studies of HCGs seems to corroborate an evolutionary sequence from low velocity dispersion, spiral dominated, young and star-forming groups to higher velocity dispersion, mostly early-type, X-ray bright groups. In this scheme, the dominant parameter is most likely the ratio between the galaxy mass vs. the available gas mass. A second parameter, namely the merging history of the group, however might have an important role in the evolution of the intra-group medium. Powerful merging events and/or interactions can contribute significantly to the heating of the intra-group medium by means of supernova explosions or by triggering an AGN. We propose to investigate whether the evolutionary scheme of the intragroup medium found for HCGs also holds in a different compact group sample. In addition to this, we start in this paper to investigate which is the influence of the merging history of groups on their evolution. Methods. Optical observations of SCGs obtained with ESO telescopes and radio data from the ATCA allow us to probe the distribution of the warm intra-group medium and the evolutionary stage of each group, by means of morphological studies and via measurements of star formation and other types of nuclear activity. Results. We present here results from a pilot sample of SCGs: we find an evolutionary trend, based on the properties of the warm intra-group medium, similar to what has been found for HCGs. Both galaxies with Sy2 nuclei in our sample are members of groups in the late stages of their dynamical evolution. However while one group is strongly HI deficient and shows, at the same time a strong ongoing merging, the other does not show any HI deficiency and the galaxies are only interacting with each other.

Truncated Star Formation in Compact Groups of Galaxies: A Stellar Population Study

We present the results of a study comparing the stellar populations in the elliptical galaxies of Hickson compact groups (HCGs) with those in low-density environments. We analyze a sample of intermediate-resolution (4.25 Å FWHM) spectra of 22 galaxies in HCGs and 12 in the field or in loose groups. Three different population-synthesis models and stellar population analyses are used to make the results more robust. Great care has been taken to correct for the emission contamination of the Balmer lines, used to determine the mean luminosity-weighted ages of the galaxies. We find that, on average, galaxies in HCGs are systematically older and more metal-poor than in the field, in agreement with previous works. The most interesting finding is that the low-σ galaxies in HCGs show an enhanced [Mg/Fe] ratio and depleted metallicity [Z/H] with respect to their counterparts in the field. This anomalous behavior is interpreted as evidence for the action of a mechanism that truncates the star formation (SF). Hydrodynamic simulations of galaxy mergers (Di Matteo et al. 2005) support this interpretation by predicting the quenching of SF soon after the merger event. Combining this scenario and the evidence presented here, the HCGs, generally considered to be ideal environments for galaxy-galaxy interactions, become ideal places for SF truncation.

The use of [Mg/Fe] to trace truncated star formation in elliptical galaxies

The present study tries to identify the mechanism responsible for the hostility to star formation (SF) found in galaxies of dense environments. Among the several candidate mechanisms, the gas-rich merger of galaxies stands as a plausible option since the realisation that the AGN-feedback, which follows the merger, can deplete the gas and truncate the SF of the resulting spheroid. Our strategy is to look for traces of SF truncation in the elliptical galaxies of Hickson compact groups (HCGs), because these are the ideal environments for mergers to occur. The stellar populations of 22 elliptical galaxies in HCGs and 12 in the field are compared using three different SSP models and two different emission correction procedures. Along the study, an extensive use of the [Mg/Fe] abundance ratio is made, because it contains useful information on the timescale of the SF history of the galaxies. Systematic differences of [Mg/Fe] and [Z/H] between both galaxy subsamples (field and HCGs) suggest that SF truncation events have affected the intermediate-mass galaxies of the HCGs. Our interpretation is that when a field galaxy enters the dense HCG environment it merges with another galaxy and ends up as a passive elliptical with traces of SF truncation.

Diffuse light in Hickson compact groups: the dynamically young system HCG 44

Context. Compact groups are associations of a few galaxies in which the environment plays an important role in galaxy evolution. The low group velocity dispersion favors tidal interactions and mergers, which may bring stars from galaxies to the diffuse intragroup light. Numerical simulations of galaxy clusters in hierarchical cosmologies show that the amount of the diffuse light increases with the dynamical evolution of the cluster. Aims. We search for diffuse light in the galaxy group HCG 44 in order to determine its luminosity and luminosity fraction. Combining with literature data, we aim to constrain the dynamical status of Hickson compact groups. Methods. We use Intra Group planetary nebulae (IGPNe) as tracers of diffuse light. These are detected by the so-called on band-off band technique. Results. We found 12 emission line objects in HCG 44, none of them associated with the galaxies of the group. The absence of PNe in the elliptical galaxy, NGC 3193, implies that this galaxy is located behind the group, leaving only three spiral galaxy members in HCG 44. 6/12 emission line objects are consistent with being IGPNe in HCG 44, but are also consistent with being Lyα background galaxies. Thus we derive an upper limit to the diffuse light fraction in HCG 44 of 4.7%, corresponding to 1.06 × 10 9 L� ,B and mean surface brightness of µB = 30.04 mag arcsec −2 . We find a correlation between the fraction of elliptical galaxies and the amount of diffuse light in Hickson compact groups. Those with large fraction of diffuse light are those with large fractions in number and luminosity of E/S0 galaxies. This indicates that the diffuse light is mainly created in dynamical processes during the formation of bright elliptical galaxies in major mergers. Conclusions. We propose an evolutionary sequence for Hickson compact groups in which the amount of diffuse light increases with the dynamical evolution of the group.

The faint end luminosity function of compact galaxy groups

We present the combined Luminosity Function of five nearby Hickson Compact Groups. We identify at the faint end the dwarf galaxy group members using the red sequence of the Color Magnitude Diagram from our deep 2.2 m WFI photometry of Hickson Compact Groups 16, 19, 30, 31 and 42. We find that the Luminosity Function of compact groups shows a bimodal distribution of bright galaxies and a dwarf galaxy population as it is known for galaxy clusters, but not for field galaxies. From measuring the field to field variance of background galaxies fulfilling the red sequence criterion coincidentally, a lower limit for the significance of the dwarf galaxy peak in the Luminosity Function was derived. With this method we determined the dwarf galaxy peak to have a significance of more than $2.6\sigma$. Thus, compact groups can be classified as bound systems like galaxy clusters, which also accommodate two galaxy populations. Hence, it seems less unlikely that compact groups are chance configurations of individual galaxies. Our result shows that compact groups are building blocks of galaxy clusters underlining a hierarchical model of structure formation and galaxy evolution. Further we note that the existence of a significant number of satellites in bound systems like compact groups is predicted by Λ Cold Dark Matter theory.

Physical properties of Hickson compact groups and of the loose groups within which they are embedded

Using new data and an enlarged group sample we verify some of our previously published results and present a number of new facts that suggest that compact groups could be casual concentrations in prolate-like looser groups, and thus the nature of compact and ordinary poor groups is probably the same. To this end we used the Sloan Digital Sky Survey (SDSS) redshift catalogue to look for galaxies with accordant redshifts in the nearby environment (up to ∼2 Mpc) of 15 Hickson compact groups (HCG). We also used known member redshifts of looser groups in the environment of 7 other HCGs. From this sample of 22 HCGs we find that: (a) HCG's tend to be aligned with the overall galaxy distribution in their ∼1 Mpc environment; (b) the well-established orientation effect by which the group velocity dispersion σ v correlates with group axial ratio q is present and particularly strong also in the HCG + environment systems; (c) the velocity dispersion σ v of the HCG +environment systems as well as of ordinary poor groups, depends only weakly on the group richness, i.e. on the mass; (d) the mean absolute K-band magnitude of E/S0 galaxies in HCGs is similar to the corresponding one in ordinary poor groups and is brighter than that of isolated E/SO's, indicating that they were formed by the merging of two galaxies of similar luminosity; (e) the fraction of E/S0 galaxies in these HCGs depends, albeit weakly, on the group's richness and on σ v ; (f) the fraction of AGNs is similar in the HCGs and their close environment, while the fraction of starburst galaxies is significantly higher in the HCGs; (g) the fraction of active galaxies (AGNs and starbursts) is anti-correlated with the velocity dispersion of the HCG + environment systems. The combination of all the above facts constitutes a picture in which compact groups are condensations within looser prolate-like, elongated systems, and they appear to be compact when their member galaxies, moving in radial orbits along the group elongation, happen to come close to each other (in which case dynamical interactions among these galaxies become even more probable) or when the group is oriented close to the line of sight, so that many of its members are projected over a small solid angle. The probability of either case is small, so the number of CGs should be much smaller than that of ordinary groups, as observed. Furthermore, the observed fractions of early-type and active galaxies, as well as their correlations with the group velocity dispersion suggests a picture by which nuclear activity and galaxy transformation by merging is instigated by effective gravitational interactions in the low-velocity dispersion groups, which then dynamically evolve via virialization processes to higher velocity dispersion groups, which thus have a higher fraction of early-type galaxies.

A principal component analysis approach to the star formation history of elliptical galaxies in compact groups

Environmental differences in the stellar populations of early-type galaxies are explored using principal component analysis (PCA), focusing on differences between elliptical galaxies in Hickson Compact Groups (HCGs) and in the field. The method is model-independent and purely relies on variations between the observed spectra. The projections (PC1, PC2) of the observed spectra on the first and second principal components reveal a difference with respect to environment, with a wider range in PC1 and PC2 in the group sample. We define a spectral parameter (zeta equivalent to 0.36PC1 - PC2) which simplifies this result to a single number: field galaxies have a very similar value of zeta, whereas HCG galaxies span a wide range in this parameter. The segregation is found regardless of the way the input spectral energy distributions (SEDs) are presented to PCA (i.e. changing the spectral range; using uncalibrated data; subtracting the continuum or masking the SED to include only the Lick spectral regions). Simple models are applied to give physical meaning to the PCs. We obtain a strong correlation between the values of zeta and the mass fraction in younger stars, so that some group galaxies present a higher fraction of them, implying a more complex star formation history in groups. Regarding 'dynamically related' observables such as a(4) or velocity dispersion, we find a correlation with PC3, but not with either PC1 or PC2. PCA is more sensitive than other methods based on a direct analysis of observables such as the structure of the surface brightness profile or the equivalent width of absorption lines. The latter do not reveal any significant variation between field and compact group galaxies. Our results imply that the presence of young stars only amounts to a fraction of a per cent in its contribution to the total variance, reflecting the power of PCA as a tool to extract small variations in the spectra from unresolved stellar populations.

Intra-group Light in Hickson Compact Groups

We have analyzed the intra-group light component of 3 Hickson Compact Groups (HCG 79, HCG 88 and HCG 95) with detections in two of them: HCG 79, with 46±11% of the total B band luminosity and HCG 95 with 11±26%. HCG 88 had no component detected. This component is presumably due to tidally stripped stellar material trapped in the group potential and represents an efficient tool to determine the stage of dynamical evolution and to map its gravitational potential. To detect this low surface brightness structure we have applied the wavelet technique OV_WAV, which separates the different components of the image according to their spatial characteristic sizes.

The DPOSS II compact group survey: first spectroscopically confirmed candidates

This paper presents the results of a pilot redshift survey of 18 candidate compact groups from the distant DPOSS survey that extends to redshift ∼0.2 the available surveys of compact groups of galaxies, mainly Hickson Compact Groups and Southern Compact Groups. The goal of our survey was to confirm group membership via redshift information and to measure the characteristic parameters of a representative, albeit small, sample of DPOSS survey groups. Of the 18 candidates observed, seven are found to be indeed isolated compact groups, i.e. groups with 3 or more concordant members and with no neighbouring known cluster, while 7 are chance projection configurations on the sky. Three remaining candidates, despite having 3 or more concordant member galaxies, are located in the neighbourhood of known clusters, while another candidate turned out to be a dense sub-condensation within Abell 0952. The median redshift of our 7 confirmed groups is z ∼ 0.12, to be compared with a median redshift of 0.03 for the local sample of compact groups by Hickson. The typical group size is ∼50 kpc, and the median radial velocity dispersion is 167 km s −1 , while typical crossing times range from 0.005 H −1 0 to 0.03 H −1 0 with a median value of 0.018 H −1 0 , all similar to the values usually found in the literature for such structures in the local universe. The average mass-to-light ratio for our groups, M/LB ,i s 92h, higher than the value found for nearby Hickson compact groups but lower than that found for loose groups. Our results suggest that, once full redshift information for its members becomes available, the DPOSS sample will provide a reference sample to study the properties of compact groups beyond the local universe.

Intragroup diffuse light in compact groups of galaxies: HCG 79, 88 and 95

Deep $B$ and $R$ images of three Hickson Compact Groups, HCG 79, HCG 88 and HCG 95, were analyzed using a new wavelet technic to measure possible intra-group diffuse light present in these systems. The method used, OV\_WAV, is a wavelet technic particularly suitable to detect low-surface brightness extended structures, down to a $S/N = 0.1$ per pixel, which corresponds to a 5-$\sigma$-detection level in wavelet space. The three groups studied are in different evolutionary stages, as can be judged by their very different fractions of the total light contained in their intra-group halos: $46\pm11$% for HCG 79 and $11\pm26$% for HCG 95, in the $B$ band, and HCG 88 had no component detected down to a limiting surface brightness of $29.1 B mag arcsec^{-2}$. For HCG 95 the intra-group light is red, similar to the mean colors of the group galaxies themselves, suggesting that it is formed by an old population with no significant on-going star formation. For HCG 79, however, the intra-group material has significantly bluer color than the mean color of the group galaxies, suggesting that the diffuse light may, at least in part, come from stripping of dwarf galaxies which dissolved into the group potential well.

A High‐Resolution Mosaic of Molecular Gas in Stephan’s Quintet

We present high resolution CO J=1-0 observations of the molecular gas in the Hickson Compact Group Stephan's Quintet (HCG92). Our observations consist of multiple pointing and mosaics covering all the regions where CO and star formation has been detected. Within the 100'' field of view centered on the eastern-most tidal tail, we detect three clumps of emission that may be partially resolved at our resolution of 8''; two of these are new detections not previously seen in ISM studies of this region. Two of these clumps lie in the optical tidal tail, while the third lies to the southeast and is coincident with a large HI feature, but does not correspond to any features at other wavelengths. We also tentatively detect CO emission from the star forming regions in the ``Old Tail'' corresponding to recent star formation activity detected in recent UV and H$\alpha$ observations. Observations of the rest of the compact group do not show detections even though strong emission was detected with single dish telescopes, which suggests the CO emission originates from a diffuse molecular gas cloud or from more at least three separate clumps with separations of greater than around 3 kpc.

Ages, Metallicities, and α-Element Enhancement for Galaxies in Hickson Compact Groups

Central velocity dispersions and eight line-strength Lick indices have been determined from 1.3 Å resolution long-slit spectra of 16 elliptical galaxies in Hickson compact groups. These data were used to determine galaxy properties (ages, metallicities, and α-element enhancements) and allowed a comparison with the parameters determined for a sample of galaxies in lower density environments studied by González. The stellar population parameters were derived by comparison to single stellar population models of Thomas et al. and to a new set of simple stellar population models for the indices Mg2, Fe5270, and Fe5335 based on synthetic spectra. These models, based on an updated version of the fitting functions presented in Barbuy et al., are fully described here. Our main results are (1) the two samples have similar mean values for the metallicities and [α/Fe] ratios and (2) the majority of the galaxies in compact groups seem to be old (median age of 14 Gyr for eight galaxies for which ages could be derived), in agreement with recent work by Proctor et al. These findings support two possible scenarios: compact groups are either young systems whose members have recently assembled and had not enough time to experience any merging yet, or they are old systems that have avoided merging since their time of formation.

The evolution of HCG 31: Optical and high-resolution HI study

Here we present the results of our new optical imaging and spectroscopic study and the analysis of new high- resolution HI images of the Hickson Compact Group HCG 31. Taking advantage of the improved sensitivity and angular resolution of the new optical and HI images, we have identified an extensive complex of stellar and HI tidal features and their kinematics. Our HI study show that H31A and C are not an advanced merger since their velocity fields can be still separated and have almost orthogonal orientations. All of the current sites of ongoing active star formation are shown to be associated with the highest column density peaks traced in HI. A new companion A0500−0434 located 240 kpc south of the group center is also discovered in HI. A detailed scenario for the tidal interactions involved and the origins of the individual tidal features are constructed using the morphology and kinematics of the tidal features. The derived dynamical mass for the entire group is about 2 × 10 11 M� , which is a few times larger than the sum of the masses of the individual group galaxies. The ultimate fate of the group is that HCG 31 is probably on its way to form a single HI cloud group containing all galaxies.

The Compact Group of Galaxies HCG 31 in an Early Phase of Merging

We have obtained high spectral resolution (R = 45,900) Fabry-Perot velocity maps of the Hickson compact group HCG 31 in order to revisit the important problem of the merger nature of the central object A+C and to derive the internal kinematics of the candidate tidal dwarf galaxies in this group. Our main findings are as follows: (1) double kinematic components are present throughout the main body of A+C, which strongly suggests that this complex is an ongoing merger; (2) regions A2 and E, to the east and south of complex A+C, present rotation patterns with velocity amplitudes of ~25 km s-1, and they counterrotate with respect to A+C; and (3) region F, which was previously thought to be the best example of a tidal dwarf galaxy in HCG 31, presents no rotation and negligible internal velocity dispersion, as is also the case for region A1. HCG 31 presents an undergoing merger in its center (A+C), and it is likely that it has suffered additional perturbations due to interactions with the nearby galaxies B, G, and Q.