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Intragroup diffuse light in compact groups of galaxies: HCG 79, 88 and 95
Deep B and R images of three Hickson Compact Groups, HCG 79, 88 and 95,have been analysed using a new wavelet technique to measure possibleintragroup diffuse light present in these systems. The method used,OV_WAV, is a wavelet technique particularly suitable fordetecting low surface brightness extended structures, down to asignal-to-noise ratio (S/N) = 0.1 per pixel, which corresponds to a5σ detection level in wavelet space. The three groups studied arein different evolutionary stages, as can be judged by their verydifferent fractions of the total light contained in their intragrouphaloes: 46 +/- 11 per cent for HCG 79 and 11 +/- 26 per cent for HCG 95,in the B band, and HCG 88 had no component detected down to a limitingsurface brightness of 29.1B mag arcsec-2. For HCG 95, theintragroup light (IGL) is red, similar to the mean colours of the groupgalaxies themselves, suggesting that it is formed by an old populationwith no significant ongoing star formation. For HCG 79, however, theintragroup material has a significantly bluer colour than the meancolour of the group galaxies, suggesting that the diffuse light may, atleast in part, come from stripping of dwarf galaxies which dissolvedinto the group potential well.
| Star-forming Objects in the Tidal Tails of Compact Groups
A search for star-forming objects belonging to tidal tails has beencarried out in a sample of deep Hα images of 16 compact groups ofgalaxies. A total of 36 objects with Hα luminosities greater than1038 ergs s-1 has been detected in five groups.The fraction of the total Hα luminosity of their respective parentgalaxies shown by the tidal objects is always below 5% except for thetidal features of Hickson compact group 95, whose Hα luminosityamounts to 65% of the total luminosity. Out of these 36 objects, ninestar-forming tidal dwarf galaxy candidates finally have been identifiedon the basis of their projected distances to the nuclei of the parentgalaxies and their total Hα luminosities. Overall, the observedproperties of the candidates resemble those previously reported for theso-called tidal dwarf galaxies.
| 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).
| Star formation in distant starburst galaxies
This paper discusses the stellar population content of distant (5 000 kms^{-1} <== V_R<=16 000 km s^{-1}) galaxies with enhancedstar-formation activity. Distinction is made between isolated galaxiesand galaxies morphologically disturbed, with clear signs of interactionsuch as mergers. In these galaxies the International UltravioletExplorer (M_BoxIUE) large aperture samples most of the galaxy's body.Consequently, the resulting integrated spectra arise primarily from bluestellar populations of different ages together with significantcontributions from intermediate and old age components, subject tovarying reddening amounts. Instead of analysing individual, usually lowSignal-to-Noise ratio (S/N) spectra, our approach is to coadd thespectra of objects with similar spectral properties in the UV,considering as well their properties in the visible/near-infraredranges. Consequently, the resulting high (S/N) template spectra containthe average properties of a rather uniform class of objects, andinformation on spectral features can now be analysed with moreprecision. Three groups have been found for the interacting galaxies,corresponding to a red, blue and very blue continuum. Isolated galaxieshave been separated into two groups, one with a flat/red continuum andthe other with a blue continuum. For comparison, we also include in thepresent analysis two groups of nearby disturbed galaxies. Stellarpopulations are analysed by means of a synthesis algorithm based on starcluster spectral components of different ages which fit the observedspectra both in terms of continuum distribution and spectral features.Flux fractions of the different age groups found in the synthesis havebeen transformed into mass fractions, allowing inferences on the starformation histories. Young stellar populations (age <500 Myr) are themain flux contributors, except for the groups with a red spectrum notdue to extinction, arising from the intermediate (age ~1 - 2 Gyr) andold age populations. We also study the reddening values and theextinction law: a Small Magellanic Cloud-like extinction law isappropriate for all cases. As compared to nearby galaxies with enhancedstar-formation, the distant starburst galaxy spectral groups exhibitlarger contributions from the intermediate and old age populations. Thiseffect is mainly accounted for by the larger spatial area sampled by theM_BoxIUE slit in the distant galaxies, including not only the entirebulge but also evolved disk populations. The present results provide aquantitative measure of the star-forming activity in interactinggalaxies, compared to isolated galaxies. Based upon data collected withthe International Ultraviolet Explorer (IUE) Satellite, supported byNASA, SERC and ESA.
| Parameters of bulges and disks of interacting galaxies
The photometric structure of 23 interacting galaxies is analyzed. It isshown that equivalent photometric profiles, if used for statisticalanalyses of the global structure of the disks of spiral galaxies, yieldthe same results as major-axis profiles. When analyzing nonaxisymmetricand faint objects, the equivalent profiles should be preferred to boththe major-axis and elliptically averaged profiles. The studies ofintegral characteristics of bulges of interacting spiral galaxies showedthat the parameters of these objects obey the mu_e-gamma_e relation fornormal spirals over a wide range of surface-brightness (~ 10 mag) andlinear-dimension (~ 3x10^2) values. This fact may imply that stronggravitational perturbations have little effect on dense central galaxyregions, leaving the bulges in a state that is very close toequilibrium. The central parts of disks in interacting spirals are, onthe average, ~ 1 mag brighter than the corresponding regions in isolatedgalaxies, which is probably attributable to enhanced star-formationprocess. Integrated photometric parameters of the disks of localinteracting galaxies are similar to those of spiral galaxies ofcomparable size observed at z ~ 1. The latter are also characterized byhigh surface brightness. Among them, interacting or disturbed objectscomprise a sizable fraction. It may be assumed that gravitationalperturbations and external accretion contribute significantly to theobserved evolution of disk parameters in large spiral galaxies, when z ~1.
| 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.
| 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.
| Detailed Photometric Study of the Merging Group of Galaxies HCG 95
Results from Hα + continuum and Johnson UBVI imaging for HCG 95are presented in order to study the star formation history of thegalaxies in the group. Based on the spatial distribution of the stellarpopulations, as well as the morphology of the three-galaxy systemA-C1-C2, we propose two possible scenarios for the dynamical evolutionof this system: In one of them, each of the nuclei C1 and C2 interactedwith the elliptical galaxy at different times, and we are seeing bothnuclei together as a projection effect. The other scenario suggests thatboth nuclei are involved in a merging process, with the resulting mergerremnant interacting with the elliptical galaxy. The recent starformation of the system is strongly balanced toward the northern tidaltail and the nucleus C1. We note the presence of a candidate of a youngdwarf galaxy which would have been created at the tip of one of thenorthern tidal tails. A very young population and no signs of anunderlying population were detected for this object. Recentspectroscopic observations of the galaxy HCG 95 B yielded a value forits radial velocity of 8000 km s^-1, that is, more than 3000 km s^-1less than the rest of galaxies in the group. This new result would implythat galaxy B is a foreground galaxy instead of being physicallyassociated with the group HCG 95. This galaxy appears distorted probablyas a result of the close passage of a dwarf satellite that has beendetected in the broadband frames and in Hα.
| Starbursts Induced by Interactions in Groups of Galaxies
Not Available
| The Onset of a Three-Galaxy Merger in the Compact Group of Galaxies HCG 95?
In this Letter we report new evidence for a close three-galaxyinteraction in the compact group of galaxies HCG 95. Deep opticalimaging in the Johnson UBVRI bands and in narrowband H alpha have shownfour long tidal tails emanating from the two disk galaxies as well assigns of transfer of material between one of the tidal tails and theelliptical galaxy of the system. An unusual color gradient, likelyrelated to an obscuring patch of dust, has been found in the ellipticalgalaxy. This fact, together with the morphology observed for theextended H alpha emission of the elliptical galaxy, gives furthersupport to the thesis of the transfer of mass between the tidal tail andthis galaxy. We argue that these three galaxies will probably merge in afew orbital periods; therefore, this system may represent a usefulempirical test of what theoretical models have predicted for theevolution of compact groups.
| Tidally-triggered disk thickening. II. Results and interpretations.
We have reported in a previous paper (Paper I, Reshetnikov and Combes,1996A&AS..116..417R) B,V and I band photometric data for a sample of24 edge-on interacting spiral galaxies, together with a control sampleof 7 edge-on isolated galaxies. We discuss here the main result found inthis study: the ratio h/z_0_ of the radial exponential scalelength h tothe constant scaleheight z_0_ is about twice smaller for interactinggalaxies. This is found to be due both to a thickening of the plane, andto a radial stripping or shrinking of the stellar disk. If we believethat any galaxy experienced a tidal interaction in the past, we mustconclude that continuous gas accretion and subsequent star formation canbring back the ratio h/z_0_ to higher values, in a time scale of 1Gyr.
| 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.
| Tidally-triggered disk thickening. I. Observations.
As part of an investigation on what effect galaxy interactions have onthe z-structure of the disks of involved galaxies, we present here newphotometric data in the B, V, I passbands for a sample of 24 interactingsystems and 7 non-interacting galaxies. Isophotes, total magnitudes andcolours of all sample galaxies (67) are determined. Our estimates andthe comparison with the published results show that the accuracy of ourphotometry is about 0.15mag.
| A Multiwavelength Investigation of the Merging Galaxy HCG 95C
The compact group HCG 95 consists of four galaxies in which two of themembers, HCG 95a and HCG 95c, are interacting. The morphology suggeststhe interaction is well evolved with the Northern tidal arm of HCG 95coverlapping the Eastern edge of HCG 95a. A linear bridge connecting thetwo galaxies hints at nuclear activity within HCG 95c. Our color indicesindicate an older stellar population for both HCG 95a and HCG 95c, withthe exception of the Northern tidal arm of HCG 95c. Long slit spectra ofHCG 95a exhibits red-shifted H-alpha +[N II] and [S II] emission linesindicative of a LINER-type AGN. Spatial profiles indicate non-nuclearstarburst activity triggered by tidal forces, while the nuclear spectrumindicates nonthermal activity. IUE observations of HCG 95c exhibit afeatureless continuum with a transient outburst captured in one IUEobservation which shows characteristic galactic emissions of red-shiftedN V (1240A) and C IV (1550 A). These results suggest that HCG 95c is inan early stage of merging with HCG 95a.
| The spatial distribution of supernovae in paired and interacting galaxies
In order to investigate the location of supernovae (SNe) in paired andinteracting galaxies, the 54 supernovae discovered up to May 1993 in 14Isolated Pairs of Galaxies and the 32 Interacting Systems were taken asa sample and studied. The whole sample of SNe, as well as a subsampleformed of type II, and Ib SNe, whose progenitors are young massivestars, do not appear in any one particluar direction with respect to thecompanion. On paired and interacting galaxies, the radial distributionof type Ib and II SNe peaks more toward the galaxy centers than it doesin isolated galaxies, indicating an enhanced Star Formation Rate (SFR)around the nuclei and in the inner disks. The distribution of SNIaappear similar in both samples, confirming that their progenitors arenot the same as those of SNII and IB. The SN rates are related to galaxyluminosity and, as expected, the discovery of SNe is twice more frequentin the brighter galaxy of the pair than in fainter ones. We did not findany relation between the locations of SNe in parent galaxies, the totalStar Formation activity and the kinematics of paired and interactinggalaxies.
| 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.
| 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.
| Longslit Spectra of HCG 95c: Evidence for H II Regions
We report extended H-alpha emission in the red spectrum of spiral galaxyHCG 95c. The longslit spectra that we obtained for this galaxy extendsfrom 5500 to 9500 Angstroms with a mean spectral resolution of ~ 14Angstroms. The slit length of ~ 200 '' was ideally suited for studyingthe spatial distribution of emission features. Spectra were obtainedwith a slit orientated North-South, centered on the nucleus, parallel tothe galaxy's northern spiral arm. The spectrum of HCG 95c showsredshifted H-alpha+[N II] and [S II] emission lines indicative of aLINER type AGN. Spatial profiles extracted from the spectra showextended H-alpha emission being emitted by H II regions in the northernspiral arm, while the equivalent profiles for strong lower excitationforbidden lines of [N II] and [S II] are confined to the nuclear region.Morphologically, HCG 95c is interacting with the brightest member of thegroup (HCG 95a). The spatial profiles suggest non-nuclear starburstactivity triggered by tidal forces, while the nuclear spectrum indicatesnon-thermal activity. This Hickson Compact Group interacting pair couldbe in the early stages of merging. The observations were obtained at theUAO 61-inch telescope, Mt. Bigelow, AZ using the LPL longslitspectrometer in combination with an off-axis CCD camera guider system.
| The Structure and Dynamics of Elliptical Galaxies in Compact Groups
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1993ApJ...418...72Z&db_key=AST
| An interaction model for the formation of dwarf galaxies and 10 exp 8 solar mass clouds in spiral disks
Ten H I clouds with masses larger than 10 exp 8 solar masses in theinteracting galaxies IC 2163/NGC 2207 are identified. Twenty-eight otherinteracting pairs of galaxies with large knots or star formationstructures in their optical images are also tabulated. It is suggestedthat interactions can lead to the formation of greater than 10 solarmass clouds and young stellar regions in the outer parts of galacticdisks, and that some of these regions may become gravitationally bounddwarf galaxies if they are ejected in tidal arms. It is proposed thatthe key to the origin of clouds of greater than 10 exp 8 solar mass ininteracting systems lies in the high velocity dispersion of theinterstellar gas. Numerical N-body simulations of the interactinggalaxies suggest that the complete detachment of an unbound dwarf galaxyrequires a companion mass comparable to or larger than the galaxy mass.
| 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.
| 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.
| HGC 93c: Spectra of an Interacting Galaxy
Not Available
| 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.
| CCD Photometry of the Compact Galaxy Group Arp 150
Not Available
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