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HCG 31: a multiple merger in progress
Context: HCG 31 is one of the most intriguing compact groups inHickson's catalogue. It contains a central pair of interacting dwarfgalaxies and other small members that are highly peculiar. Aims: Themain goal of this study is to investigate the evolutionary stage of thegroup and measure the internal kinematics of the individual galaxies andof the possible tidal dwarf galaxies. Methods: We obtained newFabry-Perot data cubes, including Hα images, velocity fields,velocity dispersion maps, lambda maps, position-velocity plots androtation curves. We have taken advantage of the high spatial (samplingof ~0.4 arcsec) and spectral (velocity sampling of ~3 km s-1)resolutions of our Fabry-Perot data. Results: Kinematic peculiaritiesand overlapping double velocity components are measurable throughout thevelocity field of A+C, with no signature of either distinct rotatingdisks or a single rotating galaxy. The velocity dispersion map showswidespread low-velocity values throughout the group (consistent with theturbulent velocity of the gas), except in a narrow interface between Aand C, where the merging may be occurring. The velocity curves of fourcandidate tidal dwarf galaxies (regions E, F, A1, and A2) show flatvelocity patterns in two cases (A1 and F) and gradients with amplitudesof A˜ 30-40 km s-1~in two others (E and A2). Ourmeasurements show good agreement with previous optical literature data,but our data set has a much improved velocity sampling and deepercoverage. Moreover, within the star-forming regions of the group, ourdata profit from the higher spatial resolution with respect to HI data. Conclusions: The dynamics of the A+C system, with two main velocitycomponents of approximately the same intensities, indicate that it is ina pre-merger stage. The two disks, in a bound orbit and in the processof merging, have had at least one earlier passage. They rotate withalmost parallel spin axes like a set of gear wheels. This progradeencounter and the high star-formation rates favor the formation of a newdisk. Object F may turn into a tidal dwarf galaxy bound to the group.Based on observations collected at the European Southern Observatory, LaSilla, Chile. Appendix A is only available in electronic form athttp://www.aanda.org

The evolution of HCG 31: Optical and high-resolution HI study
Here we present the results of our new optical imaging and spectroscopicstudy and the analysis of new high-resolution HI images of the HicksonCompact Group HCG 31. Taking advantage of the improved sensitivity andangular resolution of the new optical and HI images, we have identifiedan extensive complex of stellar and HI tidal features and theirkinematics. Our HI study show that H31A and C are not an advanced mergersince their velocity fields can be still separated and have almostorthogonal orientations. All of the current sites of ongoing activestar formation are shown to be associated with the highest columndensity peaks traced in HI. A new companion A0500-0434 located 240 kpcsouth of the group center is also discovered in HI. A detailed scenariofor the tidal interactions involved and the origins of the individualtidal features are constructed using the morphology and kinematics ofthe tidal features. The derived dynamical mass for the entire group isabout 2× 1011 Mȯ, which is a few timeslarger than the sum of the masses of the individual group galaxies. Theultimate fate of the group is that HCG 31 is probably on its way to forma single HI cloud group containing all galaxies.Based on observations made with the VLA operated by the National RadioAstronomy Observatory (the National Radio Astronomy Observatory is afacility of the National Science Foundation operated under cooperativeagreement by Associated Universities, Inc.) and on data taken usingALFOSC, which is owned by the Instituto de Astrofísica deAndalucía (IAA) and operated at the Nordic Optical Telescopeunder agreement between IAA and the NBIfA of the AstronomicalObservatory of Copenhagen.

Massive Star Formation and Tidal Structures in HCG 31
We present new broadband optical and near-infrared CCD imaging togetherwith deep optical intermediate-resolution spectroscopy of the HicksonCompact Group 31. We analyze the morphology and colors of the stellarpopulations of the galaxies, as well as the kinematics, physicalconditions, and chemical composition of the ionized gas in order to geta more complete view on the origin and evolution of the system. Weestimate the ages of the most recent star formation bursts of thesystem, finding an excellent consistency among the values obtained withdifferent indicators and starburst models. We find that member F hoststhe youngest starburst of the group, showing a substantial population ofWolf-Rayet stars. The chemical abundances are fairly similar in all themembers of the group despite their very different absolute magnitudes.We argue that the use of traditional metallicity-luminosity relationsbased on the absolute B-magnitude is not appropriate for dwarf starburstgalaxies, because their luminosity is dominated by the transientcontribution of the starburst to the blue luminosity. We think thatmembers E and F of the group are candidate tidal dwarf galaxies becauseof their high metallicity, their kinematics, and the absence ofunderlying old stellar populations. Finally, we propose that HCG 31 issuffering several almost simultaneous interaction processes. The mostrelevant of these processes are (1) the merging of members A and C,which would have produced two optical tidal tails, and (2) a fly-byencounter between G and the A+C complex, which would have produced an HI tidal tail from the stripping of the external gas of A+C, from whichmembers F and E have originated.ID="FN1"> 1Based on observations made with several telescopesoperated on the islands of La Palma and Tenerife by the Isaac NewtonGroup of Telescopes, Nordic Optical Telescope and Instituto deAstrofísica de Canarias in the Spanish observatories of Roque deLos Muchachos and Teide of the Instituto de Astrofísica deCanarias.

Is HCG 31 undergoing a merger or a fly-by interaction?
We present Fabry-Perot and multi-object spectroscopy of the galaxies inHickson compact group 31 (HCG 31). Based upon our Hα data cubes,galaxies A and C are a single entity, showing no discontinuity in theirkinematics. Kinematically, galaxy E is probably a component of the A+Ccomplex; otherwise it is a recently detached fragment. Galaxy F appears,both kinematically and chemically, to have formed from material tidallyremoved from the A+C complex. Galaxies B and G are kinematicallydistinct from this complex. Galaxy Q also has a radial velocitycompatible with group membership. Galaxies A, B, C, and F have nearlyidentical oxygen abundances, despite spanning a luminosity range of 5mag. Galaxy B's oxygen abundance is normal for its luminosity, whilegalaxy F's abundance is that expected given its origin as a tidalfragment of the A+C complex. The oxygen abundances in galaxies A and Care also understandable if the A+C complex is a late-type spiralsuffering strong gas inflow and star formation as a result of a tidalinteraction. Given the kinematics of both the galaxies and the H I gas,the oxygen abundances, and the position of galaxy G, we propose that aninteraction of galaxy G with the A+C complex, rather than a merger ofgalaxies A and C, is a more complete explanation for the tidal featuresand other properties of HCG 31. In this case, the A+C complex need notbe a merger in progress, though this is not ruled out.

Infrared Space Observatory Observations of Hickson Compact Group 31 with the Central Wolf-Rayet Galaxy NGC 1741
Hickson Compact Group (HCG) 31, consisting of the Wolf-Rayet galaxy NGC1741 and its irregular dwarf companions, was observed using the InfraredSpace Observatory. The deconvolved ISOCAM maps of the galaxies using the7.7 and 14.3 μm (LW6 and LW3) filters are presented, along withISOPHOT spectrometry of the central starburst region of NGC 1741 and thenucleus of galaxy HCG 31A. Strong mid-IR emission was detected from thecentral burst in NGC 1741, along with strong polycyclic aromatichydrocarbon (PAH) features and a blend of features, including [S IV] at10.5 μm. The 14.3/6.75 μm flux ratio, for which the 6.75 μmflux was synthesized from the PHT-S spectrum, and 14.3/7.7 μm fluxratio suggest that the central burst within NGC 1741 may be movingtoward the poststarburst phase. Diagnostic tools including the ratio ofthe integrated PAH luminosity to the 40-120 μm infrared luminosityand the far-infrared colors reveal that despite the high surfacebrightness of the nucleus, the properties of NGC 1741 can be explainedin terms of a starburst and do not require the presence of an activegalactic nucleus. The Tycho catalog star Tyc 04758 466 1, withmV=11.3 and spectral type F6, was detected at 7.7 and 14.3μm.

X-ray luminosities of galaxies in groups
We have derived the X-ray luminosities of a sample of galaxies ingroups, making careful allowance for contaminating intragroup emission.The LX:LB and LX:LFIRrelations of spiral galaxies in groups appear to be indistinguishablefrom those in other environments, however the elliptical galaxies fallinto two distinct classes. The first class is central-dominant groupgalaxies, which are very X-ray luminous and may be the focus of groupcooling flows. All other early-type galaxies in groups belong to thesecond class, which populates an almost constant band ofLX/LB over the range9.8

A Dynamical Study of Galaxies in the Hickson Compact Groups
To investigate dynamical properties of spiral galaxies in the Hicksoncompact groups (HCGs), we present rotation curves of 30 galaxies in 20HCGs. We found as follows: (1) There is no significant relation betweendynamical peculiarity and morphological peculiarity in HCG spiralgalaxies. (2) There is no significant relation between the dynamicalproperties and the frequency distribution of nuclear activities in HCGspiral galaxies. (3) There are no significant correlations between thedynamical properties of HCG spiral galaxies and any group properties(i.e., size, velocity dispersion, galaxy number density, and crossingtime). (4) Asymmetric and peculiar rotation curves are more frequentlyseen in the HCG spiral galaxies than in field spiral galaxies or incluster ones. However, this tendency is more obviously seen in late-typeHCG spiral galaxies. These results suggest that the dynamical propertiesof HCG spiral galaxies do not strongly correlate with the morphology,the nuclear activity, and the group properties. Our results also suggestthat more frequent galaxy collisions occur in the HCGs than in the fieldand in the clusters.

Recent Star Formation in Several Galaxies of the Tidally Disturbed System HCG 31
High-resolution (0.7" to 0.8") Hα images of the Hickson CompactGroup 31 (HCG 31) obtained with the WIYN telescope are used inconjunction with broadband optical images from the Hubble SpaceTelescope (HST) to examine the star formation history and properties ofthe component galaxies. The high spatial resolution of the WIYNtelescope has allowed us to identify a large number of starburst regionsfrom their Hα emission, which traces the recent star birthactivity. The HST images of galaxies E and F reveal more detail withinthe starburst regions in which we have identified numerous super starclusters (SSCs). Photometry of these starburst regions and SSCs in theHα and optical images indicates that there has been a substantialamount of star formation throughout HGC 31 over the past 10 Myr. TheHα equivalent widths suggest activity within the group as recentlyas a few megayears ago. There is evidence that galaxy F, the youngestmember of the group, is possibly undergoing its initial episode of starformation, as no underlying stellar population has yet been detected.

On the Influence of the Environment on the Star Formation Rates of a Sample of Galaxies in Nearby Compact Groups
We present the results of a study of the star formation rates (SFRs) ofa sample of disk galaxies in nearby compact groups compared with theSFRs of a sample of field galaxies. For this purpose, Hαluminosities and equivalent widths were derived for the galaxies of oursample. A direct comparison of the equivalent widths and Hαluminosities, normalized to the B luminosities and estimated area of thegalaxies of both samples, yields the result that the median values ofthese quantities are almost identical for both samples, although thedistributions for the compact-group sample are broader around the meanvalue than was found in the field galaxy sample. This result can beexplained by assuming that although interactions between galaxies incompact groups can alter the SFRs, the median value of the normalizedSFRs is preserved, being almost indistinguishable from the correspondingvalue for field galaxies. Measuring the global L_Hα/L_B of thegroups, including early-type galaxies, we find that most of the groupsthat show the highest level of L_Hα/L_B with respect to a set ofsynthetic groups built out of field galaxies show tidal features in atleast one of their members. Finally, we have explored the relationshipbetween the ratio L_Hα/L_B and several relevant dynamicalparameters of the groups: velocity dispersion, crossing time, radius,and the mass-to-luminosity ratio, finding no clear correlation. Thissuggests that the exact dynamical state of a group does not control theSFR of the group as a whole. Our results are compatible with a scenariofor compact groups of galaxies in which the dark matter of the group isarranged in a common halo, therefore preventing a fast collapse of thegalaxies.

The Very Young Starburst Merger System NGC 1741
We use Hubble Space Telescope (HST) Faint Object Camera (FOC)ultraviolet (UV) and WFPC2 optical images in conjunction with UVspectroscopic observations taken with the Goddard High ResolutionSpectrograph to examine the star formation history and properties of theinteracting galaxy system NGC 1741 in the Hickson Compact Group 31. Thehigh spatial resolution afforded by HST has allowed us to identify alarge number of starburst knots, or ``super-star clusters'' (SSCs), inthe starburst regions of this system. Photometry of these SSCs in the UVand optical bands indicates that most of these objects have ages of afew Myr, with a few up to ~100 Myr, and masses between 10^4 and 10^6M_solar. The estimated age is confirmed by a spectral synthesis analysisof one knot for which we have obtained a UV spectrum. The V-bandluminosity function of the SSCs is well represented by a power lawphi(L)~L^-alpha with an index of -1.85, with no evidence of a turnoverbrighter than the completeness limit. These properties are in goodagreement with those found for SSCs in other starburst galaxies. Ourresults support the suggestion that some of these SSCs may be extremelyyoung globular clusters formed in a relatively recent starburst episodethat has been triggered by a merger event.

Atlas of H alpha Emission of a Sample of Nearby Hickson Compact Groups of Galaxies
H alpha and adjacent continuum images are presented for a sample ofnearby groups of galaxies extracted from the Atlas of Compact Groups ofGalaxies. Also, more detailed H alpha maps of the most remarkablegalaxies are shown in this paper. A short description of the H alphaemission for each of the galaxies with accordant redshift is presentedtogether with a morphological classification of the accordant galaxiesin the sample. A large fraction of ellipticals and lenticulars weredetected in H alpha . Also, clear signs of interactions were found inseven of the groups, but in only in three of them was H alpha emissiondetected along the tidal features. Candidates of dwarf galaxies werefound at the tips of the tidal tails developed during the interactionsin these three groups.

Catalogue of HI maps of galaxies. I.
A catalogue is presented of galaxies having large-scale observations inthe HI line. This catalogue collects from the literature the informationthat characterizes the observations in the 21-cm line and the way thatthese data were presented by means of maps, graphics and tables, forshowing the distribution and kinematics of the gas. It containsfurthermore a measure of the HI extension that is detected at the levelof the maximum sensitivity reached in the observations. This catalogueis intended as a guide for references on the HI maps published in theliterature from 1953 to 1995 and is the basis for the analysis of thedata presented in Paper II. The catalogue is only available inelectronic form at the CDS via anonymous ftp orhttp://cdsweb.u-strasbg.fr/Abstract.html

The Apparent Morphology of Peculiar Galaxies at Intermediate to High Redshifts
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....114.1741H

Recent Star Formation in Groups of Galaxies: HCG 31
Broadband UBVRI and narrowband H alpha and continuum images have beenobtained for the Hickson compact group HCG 31 as part of an ongoingprogram devoted to the study of the star formation properties ofgalaxies in these groups. A spatially resolved analysis for the (U-B),(V-I), and (B-I) color maps has been performed, as well as for the Uband, H alpha luminosity, and H alpha equivalent width maps. Integratedluminosities in all the bands have been derived for all the galaxies inthe group. Our results show that all the galaxies appear to haveexperienced their most recent star formation episode less than 107 yearsago, in good agreement with previous values found in the literature.From the morphological point of view, three different kinds of starformation processes are taking place in the group: central starburstinduced by a strong interaction, star formation induced by stripping ofthe gas due to the presence of a medium of higher pressure, andformation of a dwarf galaxy at the tip of one of the tidal tails.Overall, the photometric properties of the central starburst in HCG 31are consistent with model predictions for the early stages of a mergerbetween late-type galaxies.

Far infrared properties of Hickson compact groups of galaxies. I. High resolution IRAS maps and fluxes.
The Far Infrared (FIR) properties of galaxies which are members ofcompact groups bear relevant information on the dynamical status and thephysical properties of these structures. All studies published so farhave been undermined by the poor sensitivity and spatial resolution ofthe IRAS-PSC and IRAS Sky Survey data. We used the HIRAS softwareavailable at the IRAS server at the Laboratory for Space Research inGroningen to fully exploit the redundancy of the IRAS data and toapproach the theoretical diffraction limit of IRAS. Among the 97 groupswhich were observed by IRAS, 62 were detected in at least one band,while reliable upper limits were derived for all the others. Among thedetected groups, 49 were fully or partially resolved, i.e. it waspossible to discriminate which member or members emit most of the FIRlight. At 60μm, for instance, 87 individual sources were detected in62 groups. In order to ease the comparison with data obtained at otherwavelengths - and in particular in the X and radio domains - we giveco-added and HIRAS maps for all the detected groups.

The nature of discordant redshift galaxies in compact groups
We have analysed the sample of 92 compact groups in Hickson's revisedcatalogue to investigate whether the numbers and properties of thediscordant redshift galaxies in these groups are consistent with theprojection hypothesis. The main conclusion of our analysis is that thenumbers, sizes, magnitudes and morphological types of the discordantredshift galaxies in the Hickson compact groups are as expected if theseare due to chance projections of unrelated galaxies on the groups. Thedistribution of positions for the discordant galaxies in quintets,however, is more centrally concentrated than that predicted for auniform distribution of field interlopers. One of the possibleexplanations for this discrepancy is that compact groups can act asgravitational lenses, causing the brightening of background galaxiesthat would otherwise fall below the magnitude cut-off of the group.Distances to two galaxies in the Hickson compact group 61, one of them adiscordant redshift galaxy, have been determined using the Tully-Fisherrelation. Both objects were found to be at distances consistent withtheir redshifts and an expanding Universe. All these results support theview of a cosmological origin for galaxy redshifts.

On actual presence of discordant-redshift galaxies in compact groups
Hickson's compact galaxy groups were classified using the statisticalcriterion which includes the radial velocities of galaxies as well astheir relative positions. These groups on the whole and their componentsare identified as the confident and probable non-chance ones as well asprobable and confident chance ones. All confident chance objects havethe discordant radial velocities with the differences of radialvelocities (DV) Epsilon between 1,000 km/s and 20,000 km/s. The specialclass of objects 'bright discordants' is selected. These galaxies havethe discordant radial velocities with DV Epsilon between 825 km/s and8440 km/s and have a strong tendency to be the brightest components oftheir groups. The lowest difference of radial velocities for the lastclass of objects mean value of DV = (1.0 +/- 0.2) x 103 km/sand we accept this value of DV as the lowest value of discordant radialvelocities. It is found that the biggest part of Hickson's compactgroups consist of non-chance aggregations of galaxies and some of thecases of discordant-redshifts require a special study in order toexplain their origin from a dynamic or some other point of view.

Searching for a far-infrared enhancement in compact groups of galaxies
We test the claim that an FIR enhancement is observed from galaxies incompact groups. We present three kinds of evidence that the FIR sourcesin many groups are likely the combined contribution of two or moremembers. We conclude that the level of FIR emission from group galaxieshas been overestimated in previous work. The overestimate arises becauseof the limited resolution of the IRAS survey. Correction for this effectwill lessen the already weak evidence for an FIR enhancement in groups.This result poses difficulties for models that see the groups as compactconfigurations in the process of merger.

Dynamical properties of compact groups of galaxies
Radial velocities are presented for 457 galaxies in the 100 Hicksoncompact groups. More than 84 percent of the galaxies measured havevelocities within 1000 km/s of the median velocity in the group.Ninety-two groups have at least three accordant members, and 69 groupshave at least four. The radial velocities of these groups range from1380 to 42,731 km/s with a median of 8889 km/s, corresponding to amedian distance of 89/h Mpc. The apparent space density of these systemsranges from 300 to as much as 10 exp 8 sq h/sq Mpc, which exceeds thedensities in the centers of rich clusters. The median projectedseparation between galaxies is 39/h kpc, comparable to the sizes of thegalaxies themselves. A significant correlation is found between crossingtime and the fraction of gas-rich galaxies in the groups, and a weakanticorrelation is found between crossing time and the luminositycontrast of the first-ranked galaxy.

VLA neutral hydrogen imaging of compact groups of galaxies. II - HCG 31, 44, and 79
Neutral hydrogen images of three compact groups of galaxies arepresented: HCG 31, 44, and 79. The images were obtained with the verylarge array (VLA), an on-line Hanning smoothing was applied to the data,and the H I spectral channel was isolated. The images were made on thePipeline, and were produced by means of a method described by Gorkon andEkers (1988). The images of HCG 44 are compared with earlier Areciboobservations. The H I emission in HCG 44 is discovered within thegalaxies, whereas the emission in 31 and 79 can be found throughout thegroup in clouds that are larger than the galaxies. Evidence of arelationship between the compact groups is found in the H I data, andthe groups are considered to be merging into a single object. Some ofthe groups are theorized to be young amorphous galaxies where the H I isstill bound to individual galaxies, and which have just begun tocondense from the intergalactic medium. The kinematics of the gas areshown to vary, and a common gaseous envelope contains the dwarfgalaxies.

Optical properties and dynamics of galaxies in the Hickson compact groups
The way in which galaxy properties in dense galaxy environments comparewith the properties isolated in the field is presently evaluated inlight of broadband R and H-alpha images, as well as H-alpha long-slitspectroscopy, for a set of galaxies (in 21 Hickson compact groups) whoseobserved velocity patterns range from too peculiar for rotation-curveformation, to abnormal, to normal. A surprisingly high correlation isnoted between absolute magnitude and (log) maximum rotation velocity,especially in the case of galaxies with normal rotation curves. Theseobservations support a model in which the compact-group galaxies haveonly recently accumulated from the general galaxy distribution, and inwhich tidal interactions are frequent and persisting.

One galaxy from several - The Hickson compact group H31
The compact group H31 was identified by Hickson in 1982; it consists offour galaxies in close proximity, at least two of which (A and C) areinteracting. Red and H-alpha frames and long-slit spectroscopy identifyat least three other close members of the group; an H I envelopeencloses all galaxies. Nuclear spectra of galaxies A and C show velocitypatterns characteristic of interacting galaxies as well as the broad4650 A feature indicative of W-R stars. These features reveal that largenumbers of massive stars formed more than 10 exp 6.5 yr ago. Otherobserved characteristics all suggest that H31 is in the process ofmerging to a single galaxy. H31 thus supports computations whichindicate that galaxies in compact groups merge to form a single remnant.

A photometric catalog of compact groups of galaxies
The paper presents astrometry, photometry, and morphological types,derived from CCD images, for 463 galaxies in the 100 compact groupsselected by Hickson. Some minor revisions to the membership of theoriginal catalog are made, based on these new images. The completenessof the catalog is considered as a function of group magnitude andGalactic latitude. At high Galactic latitude the catalog is estimated tobe 90 percent complete for groups with total B(T) magnitude 13.0 orless. It is less complete at lower Galactic latitude because ofobscuration and high stellar density.

Neutral hydrogen in compact groups of galaxies
Integrated H I profiles were detected for 34 of 51 Hickson compactgroups (HCGs) of galaxies, and sensitive upper limits to the H I fluxdensity were measured for the other 17. About 60 percent of the galaxieswithin compact groups are spirals, and a significant tendency exists forthe fraction of elliptical galaxies to increase with group surfacebrightness. The amount of dark matter within the compact group region isnegligibly small. An HCG on average contains half as much neutralhydrogen as a loose group with a similar spectrum of galaxy luminositiesand morphological types, implying that compact groups are independentdynamical entities and not transient or projected configurations ofloose groups. The observed fraction of galaxies which are luminousenough to be possible merger products of compact groups is smallcompared with the fraction required by the theory of dynamical friction.A clear discrepancy thus exists between solid empirical evidence and astraightforward prediction of Newtonian dynamical theory in a settingwhich does not permit a dark matter explanation.

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