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The Relation between Galaxy Activity and the Dynamics of Compact Groups of Galaxies
Using a sample of 91 galaxies distributed over 27 compact groups (CGs)of galaxies, we define an index that allows us to quantify their levelof activity due to an active galactic nucleus (AGN) or star formation.By combining the mean activity index with the mean morphological type ofthe galaxies in a group, we are able to quantify the evolutionary stateof the groups. We find that they span an evolutionary sequence thatcorrelates with the spatial configuration of the galaxies in the CG. Wedistinguish three main configuration types: A, B, and C. Type A CGs showpredominantly low velocity dispersions and are rich in late-type spiralsthat show active star formation or harbor an AGN. Type B groups haveintermediate velocity dispersions and contain a large fraction ofinteracting or merging galaxies. Type C comprises CGs with high velocitydispersions, which are dominated by elliptical galaxies that show noactivity. We suggest that evolution proceeds A==>B==>C. Mappingthe groups with different evolution levels in a diagram of radius versusvelocity dispersion does not reveal the pattern expected based on theconventional fast merger model for CGs, which predicts a direct relationbetween these two parameters. Instead, we observe a trend contrary toexpectation: the evolutionary state of a group increases with velocitydispersion. This trend seems to be related to the masses of thestructures in which CGs are embedded. In general, the evolutionary stateof a group increases with the mass of the structure. This suggestseither that galaxies evolve more rapidly in massive structures or thatthe formation of CGs embedded in massive structures predated theformation of CGs associated with lower mass systems. Our observationsare consistent with the structure formation predicted by the CDM model(or ΛCDM), only if the formation of galaxies is a biased process.
| A Multi-Band Photometric Study of Tidal Debris in a Compact Group of Galaxies: Seyfert's Sextet
In order to investigate the properties of the prominent tidal debrisfeature extending to the northeast of a compact group of galaxies,Seyfert's Sextet, we analyzed multi-band (U, B, V, VR, R, I, J, H, andK') photometric imaging data and obtained the following results: 1) Theradial surface brightness distribution of this tidal debris in Seyfert'sSextet (TDSS) in each band appears to be well approximated by anexponential profile. 2) The observed B-V color of TDSS is similar tothose of dwarf elliptical galaxies in nearby clusters. 3) Comparing thespectral energy distribution (SED) of TDSS with theoretical photometricevolution models and with the SED of the stars in the outer part of HCG79b, we find that its SED is comparable to that of a ~10Gyr-old stellarpopulation with solar metallicity, similar to the stellar population inthe outer part of HCG 79b. This suggests that TDSS consists of starsthat may have been liberated from HCG 79b by strong ga laxyinteractions, not a pre-existing dwarf galaxy as previously thought.
| The Beginning of the End: Hubble Space Telescope Images of Seyfert's Sextet
Hubble Space Telescope (HST) Wide Field Planetary Camera 2 images ofHickson Compact Group (HCG) 79, Seyfert's Sextet, are presented. Bothpoint sources and extended sources detected on the three wide-fieldchips were photometered in four filters: F336W, F439W, F555W, and F814W.Unlike other HCGs that have been imaged with HST, there do not appear tobe any candidate young star clusters among the detected point sources.The majority of the point sources that may be star clusters associatedwith the Sextet have red colors consistent with stellar populationsolder than 1 Gyr. A similar conclusion is drawn with regard to theextended sources. The majority of these appear to be backgroundgalaxies, but a few candidate dwarf galaxies are identified aspotentially associated with Seyfert's Sextet. However, no blue,star-forming objects similar to the tidal dwarf galaxy candidatesidentified in other HCGs are found among the extended objects identifiedin this study. A redshift for one dwarf galaxy candidate was measuredfrom a spectrum obtained with the Hobby-Eberly Telescope, and thisobject was found to have a redshift similar to NGC 6027e, the discordantspiral formerly identified as a member of this compact group. The HSTobservations presented here and previous radio observations of theneutral gas content of this group suggest that the interactions thathave taken place in the Sextet only redistributed the stars from themember galaxies within the group. We speculate that future interactionsmay be strong enough to strip the gas from NGC 6027d and triggerstar-cluster formation.
| The UZC-SSRS2 Group Catalog
We apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.
| Where is the neutral atomic gas in Hickson groups?
We have analyzed the total HI contents of 72 Hickson compact groups ofgalaxies (HCGs) and the detailed spatial distributions and kinematics ofHI within a subset of 16 groups using the high angular resolutionobservations obtained with the VLA in order to investigate a possibleevolutionary scenario for these densest systems in the present daygalaxy hierarchy. For the more homogeneous subsample of 48 groups, wefound a mean HI deficiency of Def_HI = 0.40 +/- 0.07, which correspondsto 40% of the expected HI for the optical luminosities and morphologicaltypes of the member galaxies. The individual galaxies show largerdegrees of deficiency than the groups globally, Def_HI = 0.62 +/- 0.09(24% of the expected HI), due in most cases to efficient gas strippingfrom individual galaxies into the group environment visible in the VLAmaps. The degree of deficiency is found to be similar to the centralgalaxies of Virgo and Coma cluster, and Coma I group, in spite of thesignificantly different characteristics (number of galaxies, velocitydispersion) of these environments. It does not seem plausible that asignificant amount of extended HI has been missed by the observations.Hence phase transformation of the atomic gas should explain the HIdeficiency. The groups richer in early type galaxies or more compactwith larger velocity dispersions show a weak tendency to be more HIdeficient. The detection rate of HCGs at X-ray wavelengths is larger forHI deficient groups, although the hot gas distribution and hence itsorigin is only known for a few cases. In the evolutionary scenario wepropose, the amount of detected HI would decrease further withevolution, by continuous tidal stripping and/or heating. The H_2 contentalso tends to be lower than expected for the galaxies in HI deficientgroups, this may suggest that the HI stripping by frequent tidalinteraction breaks the balance between the disruption of molecularclouds by star formation and the replenishment from the ambient HI. Thiswork is partially based on observations made with the VLA operated bythe National Radio Astronomy Observatory, a facility of the NationalScience Foundation operated under cooperative agreement by AssociatedUniversities, Inc., ALFOSC, which is owned by the Instituto deAstrofísica de Andalucía (IAA, CSIC) and operated at theNordic Optical Telescope (NOT) under agreement between IAA and the NBIfAof the Astronomical Observatory of Copenhagen, and 1.5 m telescope ofthe Observatorio de Sierra Nevada, Granada, Spain, which is operated bythe IAA (CSIC).
| Deep Optical Imaging of a Compact Group of Galaxies: Seyfert's Sextet
To investigate the dynamical status of Seyfert's Sextet (SS), we haveobtained a deep optical (VR+I) image of this group. Our image shows thata faint envelope, down to a surface brightnessμoptical(AB)~=27 mag arcsec-2, surrounds themember galaxies. This envelope is irregular in shape. It is likely thatthis shape is attributed either to recent-past or to ongoing galaxyinteractions in SS. If the member galaxies have experienced a number ofmutual interactions over a long timescale, the shape of the envelopeshould be rounder. Therefore, the irregularly shaped morphology suggeststhat SS is in an early phase of dynamical interaction among the membergalaxies. It is interesting to note that the soft X-ray image obtainedwith ROSAT (Pildis, Bregman, & Evrard) is significantly similar inmorphology. We discuss the possible future evolution of SS briefly.
| The Nuclear Activity of Galaxies in the Hickson Compact Groups
In order to investigate the nuclear activity of galaxies residing incompact groups of galaxies, we present results of our opticalspectroscopic program made at Okayama Astrophysical Observatory. We haveperformed optical spectroscopy of 69 galaxies belonging to 31 Hicksoncompact groups (HCGs) of galaxies. Among them, three galaxies havediscordant redshifts and, moreover, spectral quality is too poor toclassify another three galaxies. Therefore, we describe our results forthe remaining 63 galaxies. Our main results are summarized as follows:(1) We have found in our sample 28 active galactic nuclei (AGNs), 16 HII nuclei, and 19 normal galaxies showing no emission line. We used thisHCG sample for statistical analyses. (2) Comparing the frequencydistributions of activity types between the HCGs and the field galaxieswhose data are taken from Ho, Filippenko, & Sargent (382 fieldgalaxies), we find that the frequency of H II nuclei in the HCGs issignificantly less than that in the field. However, this difference maybe due to selection bias to the effect that our HCG sample contains moreearly-type galaxies than the field, because it is known that H II nucleiare rarer in early-type galaxies than in later ones. (3) Applying acorrection to this morphological bias to the HCG sample, we find thatthere is no statistically significant difference in the frequency ofoccurrence of emission-line galaxies between the HCGs and the field.This implies that the dense galaxy environment in the HCGs does notaffect the triggering of either the AGN activity and the nuclearstarburst. We discuss some implications on the nuclear activity in theHCG galaxies.
| 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.
| Kinematics of early-type galaxies in compact groups. HCG 67, HCG 74, and HCG 79
We present measurements of stellar kinematics for seven early-typegalaxies in HCG 67, HCG 74, and HCG 79. These data are aimed at studyingthe relation between the environment and the dynamics, structure andstellar content of early-type galaxies. In the present three groups, thekinematic features we observed cannot be associated unambiguously tophysical interactions. Visible morphological peculiarities do not appearcorrelated with kinematical perturbations. Table \ref{Tab3} and Table 4are available from the CDS, Strasbourg, via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html Based in part on observationsobtained at the Observatoire de Haute-Provence.
| An H_alpha catalogue of galaxies in Hickson compact groups. I. The sample
We present H_alpha photometry for a sample of 95 galaxies in HicksonCompact Groups obtained from observations of 31 groups. The Cataloguelists isophotal and adaptive aperture (Kron aperture) flux measurementsfor about 75% of the accordant galaxies inside the observed HCGs, 22 outof which are upper limits. Non standard data reduction procedures havebeen used to obtain the continuum subtracted H_alpha images for each HCGof the target sample. Flux calibration has also been performed in orderto obtain H_alpha luminosities for the whole sample. Both the datareduction and calibration procedures are carefully described in thispaper. The new data listed in this Catalogue are of great importance inunderstanding the star formation rate inside HCG galaxies and in givingnew insights on its dependence on galaxy interactions. Tables 3 to 10and Figs. 6 to 11 are only available in electronic version athttp://www.edpsciences.com Tables 7, 8, 9 are also available inelectronic version form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| Enhanced Hα Emission in Early-Type Galaxies Belonging to Compact Groups
The results of Hα photometry of a sample of early-type galaxiesbelonging to nearby compact groups are presented. Over 75% of thegalaxies in the sample were detected in Hα, a result that we haveinterpreted as proof of the presence of a warm interstellar mediumwithin them. Comparing the Hα luminosities of the galaxies in oursample with those of a sample of early-type galaxies from the field, wehave found that field early-type galaxies are systematically fainter inHα than the compact group ones for a given B absolute magnitude byDeltalog(L_Hα/L_B)~0.5. We suggest that the excess in the Hαemission within the groups is due to photoionization of gas by Lymancontinuum photons emitted by massive stars. Recent massive starformation would be a consequence of the accretion by early-type galaxiesof gas from the outer envelopes of gas-rich galaxies experiencing closepassages across the group.
| Effects of Interaction-induced Activities in Hickson Compact Groups: CO and Far-Infrared Study
A study of 2.6 mm CO J = 1 --> 0 and far-infrared (FIR) emission in adistance-limited (z < 0.03) complete sample of Hickson compact group(HCG) galaxies was conducted in order to examine the effects of theirunique environment on the interstellar medium of component galaxies andto search for a possible enhancement of star formation and nuclearactivity. Ubiquitous tidal interactions in these dense groups wouldpredict enhanced activities among the HCG galaxies compared to isolatedgalaxies. Instead, their CO and FIR properties (thus, "star formationefficiency") are surprisingly similar to isolated spirals. The CO datafor 80 HCG galaxies presented here (including 10 obtained from theliterature) indicate that the spirals globally show the same H2 contentas the isolated comparison sample, although 20% are deficient in COemission. Because of their large optical luminosity, low metallicity isnot likely the main cause for the low CO luminosity. The CO deficiencyappears linked with the group evolution, and gas exhaustion through paststar formation and removal of the external gas reserve by tidalstripping of the outer H I disk offer a possible explanation. The IRASdata for the entire redshift-limited complete sample of 161 HCG galaxieswere reanalyzed using ADDSCAN/SCANPI, improving the sensitivity by afactor of 3-5 over the existing Point Source Catalog (PSC) and resolvingbetter the contribution from individual galaxies. The new analysis ofthe IRAS data confirms the previous suggestion that FIR emission in HCGgalaxies is similar to isolated, Virgo Cluster, and weakly interactinggalaxies. Their H2 and FIR characteristics yield a star formationefficiency that is similar to that of these comparison samples. A factor2 enhancement in the 25-100 mu m flux ratio among the HCG spirals isfound, which suggests intense localized nuclear starburst activitysimilar to that of H II galaxies. A number of early-type galaxies inHCGs are detected in CO and FIR, lending further support to the ideathat tidal interactions and tidally induced evolution of the groups andmember galaxies are important in our sample.
| 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 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| Molecular gas in galaxies of Hickson compact groups
We have observed 70 galaxies belonging to 45 Hickson compact groups inthe \CO{1}{0} and \CO{2}{1} lines, in order to determine their molecularcontent. We detected 57 galaxies, corresponding to a detection rate of81%. We compare the gas content relative to blue and L_FIR luminositiesof galaxies in compact groups with respect to other samples in theliterature, including various environments and morphological types. Wefind that there is some hint of enhanced MH_2/L_B andM_dust/L_B ratios in the galaxies from compact group with respect to ourcontrol sample, especially for the most compact groups, suggesting thattidal interactions can drive the gas component inwards, by removing itsangular momentum, and concentrating it in the dense central regions,where it is easily detected. The molecular gas content in compact groupgalaxies is similar to that in pairs and starburst samples. However, thetotal L_FIR luminosity of HCGs is quite similar to that of the controlsample, and therefore the star formation efficiency appears lower thanin the control galaxies. However this assumes that the FIR spatialdistributions are similar in both samples which is not the case at radiofrequencies. Higher spatial resolution FIR data are needed to make avalid comparison. Given their short dynamical friction time-scale, it ispossible that some of these systems are in the final stage beforemerging, leading to ultra-luminous starburst phases. We also find forall galaxy samples that the H_2 content (derived from CO luminosity andnormalised to blue luminosity) is strongly correlated to the L_FIRluminosity, while the total gas content (H_2+HI) is not.
| 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.
| Morphology of galaxies in compact groups
We present the results of an isophotal analysis of 140 early-typegalaxies and a visual inspection of images of an additional 202 galaxiesin compact groups. This is essentially the entire sample of galaxies inthe subset of 92 Hickson compact groups which have at least threeaccordant members. About 12% of the elliptical galaxies have largercharacteristic radii and shallower surface brightness profiles thangalaxies of the same luminosity in less dense environments. The averageellipticity of elliptical galaxies in compact groups is a slowlyincreasing function of the metric radius, as it is for field andloose-group galaxies. No alignment is found among the major axes of thegalaxies and the major axis of the group. When combined with previouslypublished morphological, kinematic, radio, infrared, and colorinformation on the same galaxies, our data show that 43% of the galaxiesin the compact group sample show morphological and/or kinematicaldistortions indicative of interactions and/or mergers. About 32% of thegroups have three or more galaxies which show some sign of interaction.This is a lower limit, since for the great majority of the galaxies inthe groups, only imaging and low-resolution spectra are available. Forthe subsample of 16 groups for which published detailed kinematical dataare also available, the fraction of groups with three or more galaxiesin interaction is 75%. No correlation is found between the number ofinteracting galaxies in a group and the group velocity dispersion orcrossing time. These observations strongly support the view that compactgroups are systems of physically associated galaxies and not chancealignments of field, loose-group, or cluster galaxies. They also confirmthe importance of compact groups for studies of interactions and galaxyevolution. While the lack of a good control sample makes it difficult tomake quantitative comparisons for some aspects of this study, it isclear that the fraction of galaxies showing evidence of interactions ismuch higher in compact groups than in other environments.
| Corrections and additions to the third reference catalogue of bright galaxies
List of corrections and additions to the Third Reference Catalogue ofBright Galaxies (RC3) are given. The corrected version of the catalogue(RC3.9b), dated April 1994, is currently available through the nationaldata centers.
| New evidence for mergers in compact groups of galaxies
In the (logR_e_, μ_e_) plane defined by the logarithm of theeffective radius and the effective surface brightness, ellipticalgalaxies that are members of Hickson's compact groups turn out to belongto the bright family, thus suggesting that most of them have likelyexperiences subtantial secondary evolution.
| Star formation and merging in compact groups of galaxies
New UBV aperture photometry for 177 galaxies in Hickson compact groupsis presented. The analysis of the color-color diagram indicates that,contrary to what one naively would expect, only a slight enhancement intheir star formation rates is present. The results are confirmed by theanalysis of the IRAS data, taking in particular into account the 25μmdetections. Star formation in compact groups is not found to be higherthan in isolated pairs of galaxies. Earlier results on the absence ofclear candidates for merger products of late type galaxies are confirmedand strengthened. It is pointed out that the relation between velocitydispersion and morphological mixing found for Hickson groups could beone of the clues to understand the evolution of galaxies in them.
| The frequency of mergers in compact groups
The far-infrared colors, optical morphologies, and emission-linerotation curves of galaxies in compact groups are used to estimate thenumber of these galaxies which are currently merging. Each of theseindependent methods indicates that roughly 7 percent of the galaxies incompact groups are in the process of merging. Although each technique issomewhat uncertain, the agreement among the independent methods for thefraction of ongoing mergers and the generally good correlation betweendifferent measures of merging for individual systems lend added weightto the results. The combination of this fraction of galaxies in compactgroups which are currently merging with previous studies of theproperties of elliptical galaxies in compact groups leads to theconclusion that the dynamical evolution of compact groups throughmerging occurs on a time scale significantly longer than the observedcrossing time.
| Morphology of early-type galaxies in compact groups.
Investigations on the morphology of early-type galaxies in compactgroups should help us to understand if we are looking at objects whichhave jet experienced encounters and merging events. In this paper, wepresent geometrical and luminosity profiles of a sample of 55 early-typegalaxies belonging to 18 Hickson Compact Groups. Although acomprehensive discussion of the morphological properties of this classof objects cannot be tried at this stage, some morphological trends aresuggested by the data. In particular, an indication that boxyellipticals are less frequent in compact groups than in differentenvironments is found.
| 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.
| 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.
| The luminosity function of compact groups of galaxies
An analysis of the luminosity function of 68 compact groups of galaxiescataloged by Hickson (1982) is presented. The luminosities of compactgroup galaxies are consistent with their being drawn from a Schechterluminosity function. Individual morphological-type luminosity functionsare also determined. Both the total and morphological-type specificluminosity functions of compact group galaxies are significantlydifferent from those of field, loose-group, and cluster galaxies. Inparticular, the luminosity function of HCG elliptical galaxies has amean magnitude which is significantly brighter than the mean magnitudeof Virgo cluster elliptical galaxies. The mean luminosity density ofgalaxies in compact groups is estimated. The obtained result isconsistent with the conventional scenario in which compact groups mergeto form elliptical galaxies on a relatively short time scale.
| 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.
| Photometric properties of interacting galaxies.
Not Available
| 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.
| A 21 centimeter line survey of a complete sample of interacting and isolated galaxies
The paper presents 21 cm line observations of a complete sample ofinteracting and isolated galaxies made with the National Radio AstronomyObservatory 91 and 43 m telescopes and the Arecibo 3035 m telescope. The21 cm line data are combined with a homogeneous set of optical data onangular diameters, axial ratios, magnitudes, and colors, and integralproperties are calculated for the galaxies in both samples. In thispaper, the sample selection procedures, the method of observation, thedata reduction, and the observational errors are described. Thedetection percentages are presented for both samples.
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