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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.
| 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.
| 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.
| 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 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| Total magnitude, radius, colour indices, colour gradients and photometric type of galaxies
We present a catalogue of aperture photometry of galaxies, in UBVRI,assembled from three different origins: (i) an update of the catalogueof Buta et al. (1995) (ii) published photometric profiles and (iii)aperture photometry performed on CCD images. We explored different setsof growth curves to fit these data: (i) The Sersic law, (ii) The net ofgrowth curves used for the preparation of the RC3 and (iii) A linearinterpolation between the de Vaucouleurs (r(1/4) ) and exponential laws.Finally we adopted the latter solution. Fitting these growth curves, wederive (1) the total magnitude, (2) the effective radius, (3) the colourindices and (4) gradients and (5) the photometric type of 5169 galaxies.The photometric type is defined to statistically match the revisedmorphologic type and parametrizes the shape of the growth curve. It iscoded from -9, for very concentrated galaxies, to +10, for diffusegalaxies. Based in part on observations collected at the Haute-ProvenceObservatory.
| 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.
| 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.
| 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.
| Analysis of optical imagery for Seyfert's Sextet and VV 172
Seyfert's Sextet and VV 172, 5-m photographs have been subjected toimage processing to yield field-galaxy density analysis, redshift-scaledimagery, interaction morphology display and enhancement, colordifference imagery,modeling of the VV 172 halo, and image textureanalysis of the spiral galaxy components of Seyfert's Sextet. An effortis made to evaluate the evidence for physical association of thediscordant redshift components of these groups. An especially noteworthycharacteristic of the groups is their extended luminous halos. The haloof VV 172 cannot be explained by the overlapping envelopes of galaxieswith normal luminosity profiles, and the high redshift spiral galaxy inthe Sextet is found to have an asymmetric internal structure andassociated filament which suggest gravitational perturbation by theother members of the group.
| Atlas of interacting galaxies, Part. II and the concept of fragmentation of galaxies.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1977A&AS...28....1V&db_key=AST
| nests of galaxies, their fragmentation.
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