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Hickson 62. I. Kinematics of NGC 4778
Context: .Detailed studies of the photometric and kinematical propertiesof compact groups of galaxies are crucial to understand the physics ofgalaxy interactions and to shed light on some aspects of galaxyformation and evolution. In this paper we present a kinematical andphotometrical study of a member, NGC 4778, of the nearest (z=0.0137)compact group: Hickson 62. Aims: .The aim of this work was toinvestigate the existence of kinematical anomalies in the brightestgroup member, NGC 4778 in order to constrain the dynamical status andthe formation history of the group. Methods: .We used long-slitspectra obtained with FORS1 at VLT, to measure line-of-sight velocitydistributions by means of the Fourier Correlation Quotient method, andto derive the galaxy rotation curve and velocity dispersion profile. Results: .Our analysis reveals that Hickson 62a, also known as NGC4778, is an S0 galaxy with kinematical and morphological peculiarities,both in its central regions (r < 5'') and in the outer halo. In thecentral regions, the rotation curve shows the existence of akinematically decoupled stellar component, offset with respect to thephotometric center. In the outer halo we find an asymmetric rotationcurve and a velocity dispersion profile showing a rise on the SW side,in direction of the galaxy NGC 4776. Conclusions: .The nuclearcounterrotation, the distorted kinematics in the outer halo and theX-ray properties of the group suggest that NGC 4778 may be the productof a recent minor merger, more reliable with a small late-type galaxy.
| Evidence for radio-source heating of groups
We report evidence that the gas properties of X-ray groups containingradio galaxies differ from those of radio-quiet groups. For awell-studied sample of ROSAT-observed groups, we found that more thanhalf of the elliptical-dominated groups can be considered `radio-loud',and that radio-loud groups are likely to be hotter at a given X-rayluminosity than radio-quiet groups. We tested three different models forthe origin of the effect and conclude that radio-source heating is themost likely explanation. We found several examples of groups where thereis strong evidence from Chandra or XMM-Newton images for interactionsbetween the radio source and the group gas. A variety of radio-sourceheating processes are important, including shock-heating by youngsources and gentler heating by larger sources. The heating effects canbe longer-lasting than the radio emission. We show that the sample ofX-ray groups used in our study is not significantly biased in thefraction of radio-loud groups that it contains. This allows us toconclude that the energy per particle that low-power radio galaxies caninject over the group lifetime is comparable to the requirements ofstructure formation models.
| The complex kinematics of galaxies in Hickson 62.
Not Available
| K-band Properties of Galaxy Clusters and Groups: Brightest Cluster Galaxies and Intracluster Light
We investigate the near-infrared K-band properties of the brightestcluster galaxies (BCGs) in a sample of 93 X-ray galaxy clusters andgroups, using data from the Two Micron All Sky Survey. Our clustersample spans a factor of 70 in mass, making it sensitive to any clustermass-related trends. We derive the cumulative radial distribution forthe BCGs in the ensemble and find that 70% of the BCGs are centered inthe cluster to within 5% of the virial radius r200; thisquantifies earlier findings that BCG position coincides with the clustercenter as defined by the X-ray emission peak. We study the correlationsbetween the luminosity of the BCGs (Lb) and the mass and theluminosity of the host clusters, finding that BCGs in more massiveclusters are more luminous than their counterparts in less massivesystems and that the BCGs become less important in the overall clusterlight (L200) as cluster mass increases. By examining a largesample of optically selected groups, we find that these correlationshold for galactic systems less massive than our clusters(<3×1013 Msolar). From the differencesbetween luminosity functions in high- and low-mass clusters, we arguethat BCGs grow in luminosity mainly by merging with other luminousgalaxies as the host clusters grow hierarchically; the decreasing BCGluminosity fraction (Lb/L200) with cluster massindicates that the rate of luminosity growth in BCGs is slow compared tothe rate at which clusters acquire galaxy light from the field or othermerging clusters. Utilizing the observed correlation between the clusterluminosity and mass and a merger tree model for cluster formation, weestimate that the amount of intracluster light (ICL) increases withcluster mass; our calculations suggest that in 1015Msolar clusters more than 50% of total stellar mass is inICL, making the role of ICL very important in the evolution andthermodynamic history of clusters. The cluster baryon fractionaccounting for the ICL is in good agreement with the value derived fromcosmic microwave background observations. The inclusion of ICL reducesthe discrepancy between the observed cluster cold baryon fraction andthat found in hydrodynamical simulations. Based on the observed ironabundance in the intracluster medium, we find that the ICL predicted byour model, together with the observed galaxy light, match the ironmass-to-light ratio expected from simple stellar population models,provided that the Salpeter initial mass function is adopted. The ICLalso makes it easier to produce the ``iron excess'' found in the centralregions of cool-core clusters.
| 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.
| 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
| Compact Groups of Galaxies: Evolution of the Stellar Population
We present a study focusing on the nature of compact groups (CGs)through the study of their elliptical (E) galaxies. Parametersdescribing the internal dynamics and the stellar populations of thegalaxies are compared with their counterparts in other environments. Oursample has 24 bright E galaxies located in the core of Hickson CGs and11 bright `bona fide' Es, located in the field or very loose groups.Their spectra were obtained with the 2.1 m Telescope at KPNO and have a4.1 Å resolution over the wavelength range 3500-7000 Å. Wehave found that, from the dynamical point of view, E galaxies in CGs areessentially similar to those in dense clusters. Moreover, the stellarpopulations of Es in CGs seem to be older and less metal rich than thosein the field, behaviour which has also been observed by Rose et al.(1994).
| The Fundamental Plane of Elliptical Galaxies in Compact Groups
We present a study focusing on the nature of compact groups through thestudy of their elliptical galaxies. We determine velocity dispersions(σ) for 18 bright elliptical galaxies located in the core ofHickson compact groups and a control sample of 12 bright bona fideelliptical galaxies located in the field or very loose groups. Severaltests are carried out to avoid sources of systematic effects in σmeasurements. We use these velocity dispersions to compare the positionsof 11 compact group galaxies in the fundamental plane with those of alarge and homogeneous sample of elliptical galaxies. We find that littleor no significant difference exists, as far as the fundamental plane isconcerned, between elliptical galaxies in compact groups and theircounterparts in other environments.
| The Optical and Near-Infrared Morphologies of Isolated Early-Type Galaxies
To study early-type galaxies in their simplest environments, we haveconstructed a well-defined sample of 30 isolated galaxies. The samplecontains all early-type galaxies listed in the Third Reference Catalogueof Bright Galaxies (RC3) with no other cataloged galaxy with a knownredshift lying within a projected radius of 1h-1100 Mpc and +/-1000 km s-1 (where weuse the recession velocities in the RC3). We have obtained optical andnear-infrared images of 23 of the galaxies and of a comparison sample of13 early-type galaxies in X-ray-detected poor groups of galaxies. Wehave applied the techniques of unsharp-masking, galaxy model division,and color maps to search for morphological features that might provideclues to the evolution of these galaxies. Evidence for dust features isfound in approximately 75% of both the isolated and group galaxies (17of 22 and 9 of 12, respectively). However, shells or tidal features aremuch more prevalent in our isolated sample than in our group sample (9of 22=41% vs. 1 of 12=8%, respectively). The isolation and colors ofthese shell galaxies make it unlikely that tidal interactions orasymmetric star formation are the causes of such features. One modelthat is not ruled out is that mergers produce the shells. If shells anddust are both merger signatures, the absence of shells in groupelliptical galaxies implies that shells (1) form more easily, (2) areyounger, and/or (3) are longer lived in isolated environments.
| Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, whichis a complete, distance-limited (cz<=6000 km s-1) andmagnitude-limited (B<=14) sample of ~7000 optical galaxies. Thesample covers 2/3 (8.27 sr) of the sky (|b|>20deg) andappears to have a good completeness in redshift (97%). We select thesample on the basis of homogenized corrected total blue magnitudes inorder to minimize systematic effects in galaxy sampling. We identify thegroups in this sample by means of both the hierarchical and thepercolation ``friends-of-friends'' methods. The resulting catalogs ofloose groups appear to be similar and are among the largest catalogs ofgroups currently available. Most of the NOG galaxies (~60%) are found tobe members of galaxy pairs (~580 pairs for a total of ~15% of objects)or groups with at least three members (~500 groups for a total of ~45%of objects). About 40% of galaxies are left ungrouped (field galaxies).We illustrate the main features of the NOG galaxy distribution. Comparedto previous optical and IRAS galaxy samples, the NOG provides a densersampling of the galaxy distribution in the nearby universe. Given itslarge sky coverage, the identification of groups, and its high-densitysampling, the NOG is suited to the analysis of the galaxy density fieldof the nearby universe, especially on small scales.
| 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.
| Radio Emission of Shakhbazian Compact Galaxy Groups
We detect 353 radio sources from the NRAO VLA Sky Survey (NVSS) and theFIRST Survey within the areas of 179 Shakhbazian compact groups (ShCGs)of galaxies. Ninety-three of them are identified with galaxies in 74ShCGs. Six radio sources have complex structure. The radio spectra of 22sources are determined. Radio luminosities of galaxies in ShCGs are ingeneral higher than those of galaxies in Hickson compact groups (HCGs).A comparison of radio (at 1.4 GHz) and FIR (at 60 μm) fluxes of ShCGgalaxies with those of HCG galaxies shows that galaxies in ShCGs arerelatively stronger emitters at radio wavelengths, while galaxies inHCGs have relatively stronger FIR emission. The reasons for suchdifferences are discussed.
| Galaxy coordinates. II. Accurate equatorial coordinates for 17298 galaxies
Using images of the Digitized Sky Survey we measured coodinates for17298 galaxies having poorly defined coordinates. As a control, wemeasured with the same method 1522 galaxies having accurate coordinates.The comparison with our own measurements shows that the accuracy of themethod is about 6 arcsec on each axis (RA and DEC).
| The Evolution of Galaxies in Compact Groups
We present an analysis of the spectra of 62 galaxies in 15 compactgroups. The galaxies are classified into four activity classes: galaxieswithout emission, starburst galaxies, luminous AGNs (Seyfert andLINERs), and low-luminosity AGNs (LLAGNs). The star formation in theHickson compact group (HCG) starbursts is more intense than in normalspirals, but comparable to that observed in starburst-nucleus galaxies(SBNGs) in the field. In general, the HCG starbursts have mean solar gasmetallicity and do not follow the metallicity-luminosity relation tracedby the early-type SBNGs in the field, suggesting that most of them arelate-type SBNGs. This morphology preference, coupled with theobservation that the HCG starbursts are predominantly located in thehalos of the groups, is consistent with the idea that compact groups areembedded in sparser structures. The stellar metallicities of thenonstarburst galaxies are comparable to those observed in normalgalaxies with similar morphologies, but are relatively high for theirluminosities. In these galaxies, the metal absorption line equivalentwidths are slightly narrower than normal, while the Balmer absorptionlines are relatively strong. All these observations suggest the presenceof a population of intermediate-age stars. These galaxies could bepoststarburst, but at a very advanced stage of evolution, the lastbursts having happened more than 2 Gyr in the past. Our observationssupport a scenario in which the cores of the groups are slowlycollapsing evolved systems embedded in more extended structures. In thecores of the groups, the interactions were more frequent and thegalaxies evolved at a more rapid rate than in their halos.
| Structural and Dynamical Analysis of the Hickson Compact Groups
Based on the spectroscopic survey of de Carvalho et al., we analyze thestructural and dynamical properties of 17 Hickson compact groups. Thisanalysis probes a region of 0.dg5 x 0.dg5 around each group and showsthat most of them are part of larger structures. Our results alsosuggest that the Hickson sample is composed of different dynamicalstages of the groups" evolution. Specifically, we identify threepossible evolutionary phases among groups in the sample: loose groups,core + halo systems, and compact groups, each one presenting a distinctsurface density profile. This sequence is consistent with thereplenishment scenario for the formation and evolution of compact groupswithin larger and less dense systems.
| The Nature of the Activity in Hickson Compact Groups of Galaxies
We present the results of the spectral classification of the 82brightest galaxies in a sample of 17 compact groups. We verify that theactive galactic nuclei (AGNs) are preferentially located in the mostearly-type and luminous galaxies of the groups, as is usually observedin the field. But these AGNs also appear to be systematicallyconcentrated toward the central parts of the groups. Our observationssuggest a correlation between activity types, morphologies, anddensities of galaxies in the compact groups. This is consistent with ascenario in which galaxies of compact groups evolve by interacting withtheir environment and are currently in a quiet phase of their activity.
| Lopsidedness in Early-Type Disk Galaxies
We quantify the mean asymmetry of 54 face-on, early-type disk galaxies(S0 to Sab) using the amplitude of the m = 1 azimuthal Fourier componentof the R-band surface brightness. We find that the median lopsidedness,, of our sample is 0.11 and that the most lopsided 20% ofour galaxies have >= 0.19. Asymmetries in early-typedisks appear to be of similar frequency and strength as in late-typedisk galaxies. We have observed our early-type disks in a bandpass (Rband) in which the light is dominated by stars with ages greater than10^9 yr and therefore are seeing azimuthal asymmetries in the stellarmass distribution. The similar degree of lopsidedness seen in disks ofvery different star formation rates indicates that the lopsidedness inall galactic disks is primarily due to azimuthal mass asymmetries.Hence, 20% of all disk galaxies (regardless of Hubble type) haveazimuthal asymmetries, >= 0.19, in their stellar diskmass distribution, confirming lopsidedness as a dynamical phenomenon.
| The Southern Sky Redshift Survey
We report redshifts, magnitudes, and morphological classifications for5369 galaxies with m_B <= 15.5 and for 57 galaxies fainter than thislimit, in two regions covering a total of 1.70 sr in the southerncelestial hemisphere. The galaxy catalog is drawn primarily from thelist of nonstellar objects identified in the Hubble Space TelescopeGuide Star Catalog (GSC). The galaxies have positions accurate to ~1"and magnitudes with an rms scatter of ~0.3 mag. We compute magnitudes(m_SSRS2) from the relation between instrumental GSC magnitudes and thephotometry by Lauberts & Valentijn. From a comparison with CCDphotometry, we find that our system is homogeneous across the sky andcorresponds to magnitudes measured at the isophotal level ~26 magarcsec^-2. The precision of the radial velocities is ~40 km s^-1, andthe redshift survey is more than 99% complete to the m_SSRS2 = 15.5 maglimit. This sample is in the direction opposite that of the CfA2; incombination the two surveys provide an important database for studies ofthe properties of galaxies and their large-scale distribution in thenearby universe. Based on observations obtained at Cerro TololoInter-American Observatory, National Optical Astronomy Observatories,operated by the Association of Universities for Research in Astronomy,Inc., under cooperative agreement with the National Science Foundation;Complejo Astronomico El Leoncito, operated under agreement between theConsejo Nacional de Investigaciones Científicas de laRepública Argentina and the National Universities of La Plata,Córdoba, and San Juan; the European Southern Observatory, LaSilla, Chile, partially under the bilateral ESO-ObservatórioNacional agreement; Fred Lawrence Whipple Observatory;Laboratório Nacional de Astrofísica, Brazil; and the SouthAfrican Astronomical Observatory.
| Testing the physical reality of binaries and compact groups. Properties of early-type galaxies in groups with diffuse X-ray emission
We present data on the stellar kinematics of the brightest ellipticalsin HCG 62, HCG 68, NGC 2300 groups. Moreover, we report on ASCA GIS aswell as optical observations of the early-type member of the pair K 416(NGC 5480/5481) similar, in many respects, to the dominant pair in theNGC 2300 group. The kinematics of HCG 62a/b and HCG 68a/b indicate thatthey are not interacting pairs. HCG 62a is instead possibly interactingwith HCG 62c, as suggested by their morphology (see Mendes de Oliveiraand Hickson 1994), In contrast to the results for the NGC 2300/2276group, ASCA observations indicate a significant absence of hot gas inthe K 416 system. Whether the NGC 2300 multiplet is viewed as a loosegroup or as a massive E+S pair, it is clear that similar morphologicalentities do not always show similar X-ray properties. Under thehypothesis that diffuse X-ray emission marks the group potential, weconsider the possibility that K 416 is an unbound encounter. In thisscenario, morphological distortions are indicative of the ongoinginteraction, but are only circumstantially correlated with the physicalreality of a pair/multiplet as a bound system. Based on observationsobtained at 1.93m telescope of Observatoire de Haute Provence operatedby INSU.
| Redshift Survey of Galaxies around a Selected Sample of Compact Groups
We report the results of a spectroscopic survey of faint galaxies in theregions surrounding Hickson compact groups. Our sample is composed of 17groups within 9000 km s-1. The spectra were taken at the prime focus ofthe Tololo 4 m telescope, using the ARGUS fiber-fed spectrograph. Fromthese observations, redshifts were determined for the faint galaxiespreviously identified by de Carvalho, Ribeiro, & Zepf in thesurroundings of the groups. Statistical methods were applied to theresultant catalog in order to determine the kinematical structure ofeach group. This analysis confirms the idea that the Hickson sample ofcompact groups contains a wide variety of projection and dynamicalconfigurations. Our results demonstrate the necessity of newspectroscopic surveys around compact groups in order to assess theircomplete velocity distribution.
| A ROSAT survey of Hickson's compact galaxy groups.
We report the results of an almost complete survey of the X-rayproperties of Hickson's compact galaxy groups with the ROSAT PSPC.Diffuse X-ray emission is detected from 22 groups. We infer that hotintragroup gas is present in $\gtsimm 75\%$ of these systems and derivetheir X-ray luminosity function. Earlier reports that only spiral-poorsystems exhibit diffuse X-ray emission are found to be incorrect. Strongcorrelations are found between the X-ray luminosity and both the gastemperature and the velocity dispersion of the group galaxies. We arguethat these properties provide strong evidence that most of these groupsare genuinely compact configurations, rather than line-of-sightsuperpositions. Comparison with the X-ray properties of galaxy clustersindicate a significant steepening of the $L:T$ relation below$T\sim1$\keV, which may result from the action of galaxy winds.
| H(alpha) Imaging of the Hickson Compact Group 62
We report results of a search for optical emission line gas in theHickson compact group 62. From narrow band (Hα) imagingobservations we estimate 10^4^ M_sun_ of ionized hydrogen gas within thecentral regions of three of the galaxies in the group. There has beenconsiderable interest in this group since ROSAT detected an extendedhalo of diffuse x-ray emission, possibly with a cooling flow at thecenter. The optical emission line luminosities from the three galaxiesare comparable to each other (~10^39^ erg s^-1^) and resemble those seenin normal x-ray luminous early type galaxies. There is no evidence forany excess emission in the galaxy which lies at the center of the x-raycooling flow. We find that the most likely source of excitation for theemission line nebulosities is photoionization by young stars.
| ROSAT observations of compact groups of galaxies
We have systematically analyzed a sample of 13 new and archival ROSATPosition Sensitive Proportional Counter (PSPC) observations of compactgroups of galaxies: 12 Hickson compact groups plus the NCG 2300 group.We find that approximately two-thirds of the groups have extended X-rayemission and, in four of these, the emission is resolved into diffuseemission from gas at a temperature of kT approximately 1 keV in thegroup potential. All but one of the groups with extended emission have aspiral fraction of less than 50%. The baryon fraction of groups withdiffuse emission is 5%-19%, similar to the values in clusters ofgalaxies. However, with a single exception (HCG 62), the gas-to-stellarmass ratio in our groups has a median value near 5%, somewhat greaterthan the values for individual early-type galaxies and two orders ofmagnitude than in clusters of galaxies. The X-ray luminosities ofindividual group galaxies are comparable to those of similar fieldgalaxies, although the LX-LB relation forearly-type galaxies may be flatter in compact groups than in the field.
| Deep Optical Observations of Compact Groups of Galaxies
Compact groups of galaxies appear to be extremely dense, making themlikely sites of intense galaxy interaction, while their smallpopulations make them relatively simple to analyze. In order to searchfor optical interaction tracers such as diffuse light and galaxy tidalfeatures in Hickson compact groups (HCGs), we carried out deepphotometry in three filters on a sample of HCGs with ROSAT observations.Using a modeling procedure to subtract the light of bright early-typegalaxies, we found shell systems and extended envelopes around many, butnot all, of those galaxies. Only one group in our sample, HCG 94, hasdiffuse light in the group potential (with a luminosity of 7 L^*^); theother groups do not contain more than 113 L^*^ in diffuse light. Withthe exception of HCG 94 (which is the most x-ray-luminous HCG), we foundno correlation between the presence of shells or other tidal featuresand the x-ray luminosity of a group. Better predictors of detectablegroup x-ray emission are a low spiral fraction and belonging to a largergalaxy condensation-neither of which are correlated with opticaldisturbances in the group galaxies. No elliptical galaxies that areextremely optically luminous but x-ray faint are found to have shellsand very complex color structures. This is likely due to recent infallof gas-rich material into the galaxies, which would produce both thedisruption of stellar orbits and a significant amount of star formation.
| 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.
| 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.
| Redshift observations in the Hydra-Centaurus region
The paper reports 406 redshifts for galaxies in the northern galactichemisphere, south of delta = 0 deg. A substantial fraction of theobserved galaxies are located in the equatorial zone between deltavalues of -17.5 and 0 deg. By combining these new data with thoseavailable in the literature, it is possible to extend the original CfAredshift survey of galaxies brighter than m(B(0)) = 14.5 to b = 30 deg,south of delta = 0. New data taken at lower galactic latitudes alsocontribute to the existing surveys of the Hydra-Centaurus complex.
| 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.
| 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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