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Analysis of possible anomalies in the QSO distribution of the Flesch & Hardcastle catalogue
Aims.A recent catalogue by Flesch & Hardcastle presents two majoranomalies in the spatial distribution of QSO candidates: i) an apparentexcess of such objects near bright galaxies, and ii) an excess of verybright QSO candidates compared to random background expectations inseveral regions of the sky. Because anyone of these anomalies would berelevant in a cosmological context, we carried out an extensive analysisof the probabilities quoted in that catalogue. Methods: We determinethe nature and redshift of a subsample of 30 sources in that catalogueby analysing their optical spectra (another 11 candidates wereidentified from existing public databases). These have allowed us tostatistically check the reliability of the probabilities QSO statusquoted by Flesch & Hardcastle for their candidates. Results: Only12 of the 41 candidates turned out QSOs (7 of which have been identifiedhere for the first time). Conclusions: The probabilities of the QSOs'being the candidates given by Flesch & Hardcastle are overestimatedfor mB ≤ 17 and for objects projected near (≤1 arcmin)bright galaxies. This is the cause of the anomalies mentioned above.
| The Revealing Dust: Mid-Infrared Activity in Hickson Compact Group Galaxy Nuclei
We present a sample of 46 galaxy nuclei from 12 nearby (z<4500 kms-1) Hickson Compact Groups (HCGs) with a complete suite of1-24 μm 2MASS + Spitzer nuclear photometry. For all objects in thesample, blue emission from stellar photospheres dominates in thenear-infrared through the 3.6 μm IRAC band. Of 46 galaxy nuclei, 25(54%) show red mid-infrared continua characteristic of hot dust poweredby ongoing star formation and/or accretion onto a central black hole. Weintroduce αIRAC, the spectral index of a power-law fitto the 4.5-8.0 μm IRAC data, and demonstrate that it cleanlyseparates the mid-infrared-active and nonactive HCG nuclei. Thisparameter is more powerful for identifying low-to moderate-luminositymid-infrared activity than other measures, which include data atrest-frame λ<3.6 μm that may be dominated by stellarphotospheric emission. While the HCG galaxies clearly have a bimodaldistribution in this parameter space, a comparison sample from theSpitzer Nearby Galaxy Survey (SINGS) matched in J-band total galaxyluminosity is continuously distributed. A second diagnostic, thefraction of 24 μm emission in excess of that expected from quiescentgalaxies, f24D, reveals an additional three nuclei to beactive at 24 μm. Comparing these two mid-infrared diagnostics ofnuclear activity to optical spectroscopic identifications from theliterature reveals some discrepancies, and we discuss the challenges ofdistinguishing the source of ionizing radiation in these and other lowerluminosity systems. We find a significant correlation between thefraction of mid-infrared-active galaxies and the total H I mass in agroup and investigate possible interpretations of these results in lightof galaxy evolution in the highly interactive system of a compact groupenvironment.
| The Compact Group-Fossil Group Connection: Observations of a Massive Compact Group at z=0.22
It has been suggested that fossil groups could be the cannibalizedremains of compact groups that lost energy through tidal friction.However, in the nearby universe, compact groups that are close to themerging phase and display a wealth of interacting features (such as HCG31 and HCG 79) have very low velocity dispersions and poorneighborhoods, unlike the massive, cluster-like fossil groups studied todate. In fact, known z=0 compact groups are very seldom embedded inmassive enough structures that may have resembled the intergalacticmedium of fossil groups. In this Letter, we study the dynamicalproperties of CG 6, a massive compact group at z=0.220 that has severalproperties in common with known fossil groups. We report on newg' and i' imaging and multislit spectroscopyperformed with GMOS on Gemini South. The system has 20 members within aradius of 1 h-170 Mpc, a velocity dispersion of700 km s-1, and a mass of 1.8×1014h-170 Msolar, similar to that of themost massive fossil groups known. The merging of the four centralgalaxies in this group would form a galaxy with magnitudeMr'~-23.4, typical for first-ranked galaxies offossil groups. Although nearby compact groups with similar properties toCG 6 are rare, we speculate that such systems occurred more frequentlyin the past and they may have been the precursors of fossil groups.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the National Science Foundationon behalf of the Gemini partnership: the National Science Foundation(US), the Particle Physics and Astronomy Research Council (UK), theNational Research Council (Canada), CONICYT (Chile), the AustralianResearch Council (Australia), CNPq (Brazil), and CONICET(Argentina)-program identification: GS-2005B-Q-37.
| HCG 31: a multiple merger in progress
Context: HCG 31 is one of the most intriguing compact groups inHickson's catalogue. It contains a central pair of interacting dwarfgalaxies and other small members that are highly peculiar. Aims: Themain goal of this study is to investigate the evolutionary stage of thegroup and measure the internal kinematics of the individual galaxies andof the possible tidal dwarf galaxies. Methods: We obtained newFabry-Perot data cubes, including Hα images, velocity fields,velocity dispersion maps, lambda maps, position-velocity plots androtation curves. We have taken advantage of the high spatial (samplingof ~0.4 arcsec) and spectral (velocity sampling of ~3 km s-1)resolutions of our Fabry-Perot data. Results: Kinematic peculiaritiesand overlapping double velocity components are measurable throughout thevelocity field of A+C, with no signature of either distinct rotatingdisks or a single rotating galaxy. The velocity dispersion map showswidespread low-velocity values throughout the group (consistent with theturbulent velocity of the gas), except in a narrow interface between Aand C, where the merging may be occurring. The velocity curves of fourcandidate tidal dwarf galaxies (regions E, F, A1, and A2) show flatvelocity patterns in two cases (A1 and F) and gradients with amplitudesof A˜ 30-40 km s-1~in two others (E and A2). Ourmeasurements show good agreement with previous optical literature data,but our data set has a much improved velocity sampling and deepercoverage. Moreover, within the star-forming regions of the group, ourdata profit from the higher spatial resolution with respect to HI data. Conclusions: The dynamics of the A+C system, with two main velocitycomponents of approximately the same intensities, indicate that it is ina pre-merger stage. The two disks, in a bound orbit and in the processof merging, have had at least one earlier passage. They rotate withalmost parallel spin axes like a set of gear wheels. This progradeencounter and the high star-formation rates favor the formation of a newdisk. Object F may turn into a tidal dwarf galaxy bound to the group.Based on observations collected at the European Southern Observatory, LaSilla, Chile. Appendix A is only available in electronic form athttp://www.aanda.org
| The K Luminosity-Metallicity Relation for Dwarf Galaxies and the Tidal Dwarf Galaxies in the Tails of HCG 31
We determine a K-band luminosity-metallicity (L-Z) relation for dwarfirregular galaxies over a large range of magnitudes,-20.5
| Multiwavelength Star Formation Indicators: Observations
We present a compilation of multiwavelength data on different starformation indicators for a sample of nearby star forming galaxies. Herewe discuss the observations, reductions and measurements of ultravioletimages obtained with STIS on board the Hubble Space Telescope (HST),ground-based Hα, and VLA 8.46 GHz radio images. These observationsare complemented with infrared fluxes, as well as large-apertureoptical, radio, and ultraviolet data from the literature. This databasewill be used in a forthcoming paper to compare star formation rates atdifferent wave bands. We also present spectral energy distributions(SEDs) for those galaxies with at least one far-infrared measurementsfrom ISO, longward of 100 μm. These SEDs are divided in two groups,those that are dominated by the far-infrared emission, and those forwhich the contribution from the far-infrared and optical emission iscomparable. These SEDs are useful tools to study the properties ofhigh-redshift galaxies.Based on observations made with the NASA/ESA Hubble Space Telescope,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.Based on observations obtained with the Apache Point Observatory 3.5 mtelescope, which is owned and operated by the Astrophysical ResearchConsortium.
| Star formation and stellar populations in the Wolf-Rayet(?) luminous compact blue galaxy IRAS 08339+6517
Aims.IRAS 08339+6517 is a luminous infrared and Lyα-emittingstarburst galaxy that possesses a dwarf companion object at a projecteddistance of 56 kpc. An H I tidal tail has recently been detected betweenboth galaxies, suggesting that about 70% of the neutral gas has beenejected from them.Methods.We present deep broad-band optical images,together with narrow band Hα CCD images, and opticalintermediate-resolution spectroscopy of both galaxies.Results.The imagesreveal interaction features between both systems and strong Hαemission in the inner part of IRAS 08339+6517. The chemical compositionof the ionized gas of the galaxies is rather similar. The analysis oftheir kinematics also indicates interaction features and reveals anobject that could be a candidate tidal dwarf galaxy or a remnant of anearlier merger. Our data suggest that the H I tail has been mainlyformed from material stripped from the main galaxy. We find weakspectral features that could be attributed to the presence of Wolf-Rayetstars in this starburst galaxy and estimate an age of the most recentburst of around 4-6 Myr. A more evolved underlying stellar population,with a minimal age between 100-200 Myr, is also detected and fits anexponential intensity profile. A model which combines 85% young and 15%old populations can explain both the spectral energy distribution andthe H I Balmer and He I absorption lines presented in our spectrum. Thestar formation rate of the galaxy is consistently derived using severalcalibrations, giving a value of ~9.5 Mȯyr-1.Conclusions.IRAS 08339+6517 does satisfy the criteria ofa luminous compact blue galaxy, rare objects in the local universe butcommon at high redshifts, being a very interesting target for detailedstudies of galaxy evolution and formation.
| ISO observations of the Wolf-Rayet galaxies NGC 5430, NGC 6764, Mrk 309 and VII Zw 19
Observations of four WR galaxies (NGC 5430, NGC 6764, Mrk 309 and VII Zw19) using the Infrared Space Observatory are presented here. ISOCAM mapsof NGC 5430, Mrk 309 and NGC 6764 revealed the location of starformation regions in each of these galaxies. ISOPHOT spectralobservations from 4 to 12 μm detected the ubiquitous PAH bands in thenuclei of the targets and several of the disk star forming regions,while LWS spectroscopy detected [O I] and [C II] emission lines from twogalaxies, NGC 5430 and NGC 6764. Using a combination of ISO and IRASflux densities, a dust model based on the sum of modified blackbodycomponents was successfully fitted to the available data. These modelswere then used to calculate new values for the total IR luminosities foreach galaxy, the size of the various dust populations, and the globalSFR. The derived flux ratios, the SFRs, the high L(PAH)/L(40-120 μm)and F(PAH 7.7 μm)/F(7.7 μm continuum) values suggest that most ofthese galaxies are home to only a compact burst of star formation. Theexception is NGC 6764, whose F(PAH 7.7 μm)/F(7.7 μm continuum)value of 1.22 is consistent with the presence of an AGN, yet theL(PAH)/L(40-120 μm) is more in line with a starburst, a finding inline with a compact low-luminosity AGN dominated by the starburst.
| The evolution of HCG 31: Optical and high-resolution HI study
Here we present the results of our new optical imaging and spectroscopicstudy and the analysis of new high-resolution HI images of the HicksonCompact Group HCG 31. Taking advantage of the improved sensitivity andangular resolution of the new optical and HI images, we have identifiedan extensive complex of stellar and HI tidal features and theirkinematics. Our HI study show that H31A and C are not an advanced mergersince their velocity fields can be still separated and have almostorthogonal orientations. All of the current sites of ongoing activestar formation are shown to be associated with the highest columndensity peaks traced in HI. A new companion A0500-0434 located 240 kpcsouth of the group center is also discovered in HI. A detailed scenariofor the tidal interactions involved and the origins of the individualtidal features are constructed using the morphology and kinematics ofthe tidal features. The derived dynamical mass for the entire group isabout 2× 1011 Mȯ, which is a few timeslarger than the sum of the masses of the individual group galaxies. Theultimate fate of the group is that HCG 31 is probably on its way to forma single HI cloud group containing all galaxies.Based on observations made with the VLA operated by the National RadioAstronomy Observatory (the National Radio Astronomy Observatory is afacility of the National Science Foundation operated under cooperativeagreement by Associated Universities, Inc.) and on data taken usingALFOSC, which is owned by the Instituto de Astrofísica deAndalucía (IAA) and operated at the Nordic Optical Telescopeunder agreement between IAA and the NBIfA of the AstronomicalObservatory of Copenhagen.
| The Compact Group of Galaxies HCG 31 in an Early Phase of Merging
We have obtained high spectral resolution (R=45,900) Fabry-Perotvelocity maps of the Hickson compact group HCG 31 in order to revisitthe important problem of the merger nature of the central object A+C andto derive the internal kinematics of the candidate tidal dwarf galaxiesin this group. Our main findings are as follows: (1) double kinematiccomponents are present throughout the main body of A+C, which stronglysuggests that this complex is an ongoing merger; (2) regions A2 and E,to the east and south of complex A+C, present rotation patterns withvelocity amplitudes of ~25 km s-1, and they counterrotatewith respect to A+C; and (3) region F, which was previously thought tobe the best example of a tidal dwarf galaxy in HCG 31, presents norotation and negligible internal velocity dispersion, as is also thecase for region A1. HCG 31 presents an undergoing merger in its center(A+C), and it is likely that it has suffered additional perturbationsdue to interactions with the nearby galaxies B, G, and Q.Based on observations collected at the European Southern Observatory andGemini North Telescope (project GN-2003-Q-12).
| Massive Star Formation and Tidal Structures in HCG 31
We present new broadband optical and near-infrared CCD imaging togetherwith deep optical intermediate-resolution spectroscopy of the HicksonCompact Group 31. We analyze the morphology and colors of the stellarpopulations of the galaxies, as well as the kinematics, physicalconditions, and chemical composition of the ionized gas in order to geta more complete view on the origin and evolution of the system. Weestimate the ages of the most recent star formation bursts of thesystem, finding an excellent consistency among the values obtained withdifferent indicators and starburst models. We find that member F hoststhe youngest starburst of the group, showing a substantial population ofWolf-Rayet stars. The chemical abundances are fairly similar in all themembers of the group despite their very different absolute magnitudes.We argue that the use of traditional metallicity-luminosity relationsbased on the absolute B-magnitude is not appropriate for dwarf starburstgalaxies, because their luminosity is dominated by the transientcontribution of the starburst to the blue luminosity. We think thatmembers E and F of the group are candidate tidal dwarf galaxies becauseof their high metallicity, their kinematics, and the absence ofunderlying old stellar populations. Finally, we propose that HCG 31 issuffering several almost simultaneous interaction processes. The mostrelevant of these processes are (1) the merging of members A and C,which would have produced two optical tidal tails, and (2) a fly-byencounter between G and the A+C complex, which would have produced an HI tidal tail from the stripping of the external gas of A+C, from whichmembers F and E have originated.ID="FN1"> 1Based on observations made with several telescopesoperated on the islands of La Palma and Tenerife by the Isaac NewtonGroup of Telescopes, Nordic Optical Telescope and Instituto deAstrofísica de Canarias in the Spanish observatories of Roque deLos Muchachos and Teide of the Instituto de Astrofísica deCanarias.
| Is HCG 31 undergoing a merger or a fly-by interaction?
We present Fabry-Perot and multi-object spectroscopy of the galaxies inHickson compact group 31 (HCG 31). Based upon our Hα data cubes,galaxies A and C are a single entity, showing no discontinuity in theirkinematics. Kinematically, galaxy E is probably a component of the A+Ccomplex; otherwise it is a recently detached fragment. Galaxy F appears,both kinematically and chemically, to have formed from material tidallyremoved from the A+C complex. Galaxies B and G are kinematicallydistinct from this complex. Galaxy Q also has a radial velocitycompatible with group membership. Galaxies A, B, C, and F have nearlyidentical oxygen abundances, despite spanning a luminosity range of 5mag. Galaxy B's oxygen abundance is normal for its luminosity, whilegalaxy F's abundance is that expected given its origin as a tidalfragment of the A+C complex. The oxygen abundances in galaxies A and Care also understandable if the A+C complex is a late-type spiralsuffering strong gas inflow and star formation as a result of a tidalinteraction. Given the kinematics of both the galaxies and the H I gas,the oxygen abundances, and the position of galaxy G, we propose that aninteraction of galaxy G with the A+C complex, rather than a merger ofgalaxies A and C, is a more complete explanation for the tidal featuresand other properties of HCG 31. In this case, the A+C complex need notbe a merger in progress, though this is not ruled out.
| Infrared Space Observatory Observations of Hickson Compact Group 31 with the Central Wolf-Rayet Galaxy NGC 1741
Hickson Compact Group (HCG) 31, consisting of the Wolf-Rayet galaxy NGC1741 and its irregular dwarf companions, was observed using the InfraredSpace Observatory. The deconvolved ISOCAM maps of the galaxies using the7.7 and 14.3 μm (LW6 and LW3) filters are presented, along withISOPHOT spectrometry of the central starburst region of NGC 1741 and thenucleus of galaxy HCG 31A. Strong mid-IR emission was detected from thecentral burst in NGC 1741, along with strong polycyclic aromatichydrocarbon (PAH) features and a blend of features, including [S IV] at10.5 μm. The 14.3/6.75 μm flux ratio, for which the 6.75 μmflux was synthesized from the PHT-S spectrum, and 14.3/7.7 μm fluxratio suggest that the central burst within NGC 1741 may be movingtoward the poststarburst phase. Diagnostic tools including the ratio ofthe integrated PAH luminosity to the 40-120 μm infrared luminosityand the far-infrared colors reveal that despite the high surfacebrightness of the nucleus, the properties of NGC 1741 can be explainedin terms of a starburst and do not require the presence of an activegalactic nucleus. The Tycho catalog star Tyc 04758 466 1, withmV=11.3 and spectral type F6, was detected at 7.7 and 14.3μm.
| A close look into an intermediate redshift galaxy using STIS
We present a detailed view of a galaxy at z=0.4 which is part of a largedatabase of intermediate redshifts using high resolution images. We usedthe STIS parallel images and spectra to identify the object and obtainthe redshift. The high resolution STIS image (0.05'') enabled us toanalyse the internal structures of this galaxy. A bar along the majoraxis and hot-spots of star formation separated by 0.37'' (1.6 kpc) arefound along the inner region of the galaxy. The analysis of themorphology of faint galaxies like this one is an important step towardsestimating the epoch of formation of the Hubble classification sequence.
| 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).
| 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
| 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.
| 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.
| A Dynamical Study of Galaxies in the Hickson Compact Groups
To investigate dynamical properties of spiral galaxies in the Hicksoncompact groups (HCGs), we present rotation curves of 30 galaxies in 20HCGs. We found as follows: (1) There is no significant relation betweendynamical peculiarity and morphological peculiarity in HCG spiralgalaxies. (2) There is no significant relation between the dynamicalproperties and the frequency distribution of nuclear activities in HCGspiral galaxies. (3) There are no significant correlations between thedynamical properties of HCG spiral galaxies and any group properties(i.e., size, velocity dispersion, galaxy number density, and crossingtime). (4) Asymmetric and peculiar rotation curves are more frequentlyseen in the HCG spiral galaxies than in field spiral galaxies or incluster ones. However, this tendency is more obviously seen in late-typeHCG spiral galaxies. These results suggest that the dynamical propertiesof HCG spiral galaxies do not strongly correlate with the morphology,the nuclear activity, and the group properties. Our results also suggestthat more frequent galaxy collisions occur in the HCGs than in the fieldand in the clusters.
| 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.
| Recent Star Formation in Several Galaxies of the Tidally Disturbed System HCG 31
High-resolution (0.7" to 0.8") Hα images of the Hickson CompactGroup 31 (HCG 31) obtained with the WIYN telescope are used inconjunction with broadband optical images from the Hubble SpaceTelescope (HST) to examine the star formation history and properties ofthe component galaxies. The high spatial resolution of the WIYNtelescope has allowed us to identify a large number of starburst regionsfrom their Hα emission, which traces the recent star birthactivity. The HST images of galaxies E and F reveal more detail withinthe starburst regions in which we have identified numerous super starclusters (SSCs). Photometry of these starburst regions and SSCs in theHα and optical images indicates that there has been a substantialamount of star formation throughout HGC 31 over the past 10 Myr. TheHα equivalent widths suggest activity within the group as recentlyas a few megayears ago. There is evidence that galaxy F, the youngestmember of the group, is possibly undergoing its initial episode of starformation, as no underlying stellar population has yet been detected.
| On the Influence of the Environment on the Star Formation Rates of a Sample of Galaxies in Nearby Compact Groups
We present the results of a study of the star formation rates (SFRs) ofa sample of disk galaxies in nearby compact groups compared with theSFRs of a sample of field galaxies. For this purpose, Hαluminosities and equivalent widths were derived for the galaxies of oursample. A direct comparison of the equivalent widths and Hαluminosities, normalized to the B luminosities and estimated area of thegalaxies of both samples, yields the result that the median values ofthese quantities are almost identical for both samples, although thedistributions for the compact-group sample are broader around the meanvalue than was found in the field galaxy sample. This result can beexplained by assuming that although interactions between galaxies incompact groups can alter the SFRs, the median value of the normalizedSFRs is preserved, being almost indistinguishable from the correspondingvalue for field galaxies. Measuring the global L_Hα/L_B of thegroups, including early-type galaxies, we find that most of the groupsthat show the highest level of L_Hα/L_B with respect to a set ofsynthetic groups built out of field galaxies show tidal features in atleast one of their members. Finally, we have explored the relationshipbetween the ratio L_Hα/L_B and several relevant dynamicalparameters of the groups: velocity dispersion, crossing time, radius,and the mass-to-luminosity ratio, finding no clear correlation. Thissuggests that the exact dynamical state of a group does not control theSFR of the group as a whole. Our results are compatible with a scenariofor compact groups of galaxies in which the dark matter of the group isarranged in a common halo, therefore preventing a fast collapse of thegalaxies.
| The Very Young Starburst Merger System NGC 1741
We use Hubble Space Telescope (HST) Faint Object Camera (FOC)ultraviolet (UV) and WFPC2 optical images in conjunction with UVspectroscopic observations taken with the Goddard High ResolutionSpectrograph to examine the star formation history and properties of theinteracting galaxy system NGC 1741 in the Hickson Compact Group 31. Thehigh spatial resolution afforded by HST has allowed us to identify alarge number of starburst knots, or ``super-star clusters'' (SSCs), inthe starburst regions of this system. Photometry of these SSCs in the UVand optical bands indicates that most of these objects have ages of afew Myr, with a few up to ~100 Myr, and masses between 10^4 and 10^6M_solar. The estimated age is confirmed by a spectral synthesis analysisof one knot for which we have obtained a UV spectrum. The V-bandluminosity function of the SSCs is well represented by a power lawphi(L)~L^-alpha with an index of -1.85, with no evidence of a turnoverbrighter than the completeness limit. These properties are in goodagreement with those found for SSCs in other starburst galaxies. Ourresults support the suggestion that some of these SSCs may be extremelyyoung globular clusters formed in a relatively recent starburst episodethat has been triggered by a merger event.
| Atlas of H alpha Emission of a Sample of Nearby Hickson Compact Groups of Galaxies
H alpha and adjacent continuum images are presented for a sample ofnearby groups of galaxies extracted from the Atlas of Compact Groups ofGalaxies. Also, more detailed H alpha maps of the most remarkablegalaxies are shown in this paper. A short description of the H alphaemission for each of the galaxies with accordant redshift is presentedtogether with a morphological classification of the accordant galaxiesin the sample. A large fraction of ellipticals and lenticulars weredetected in H alpha . Also, clear signs of interactions were found inseven of the groups, but in only in three of them was H alpha emissiondetected along the tidal features. Candidates of dwarf galaxies werefound at the tips of the tidal tails developed during the interactionsin these three groups.
| Catalogue of HI maps of galaxies. I.
A catalogue is presented of galaxies having large-scale observations inthe HI line. This catalogue collects from the literature the informationthat characterizes the observations in the 21-cm line and the way thatthese data were presented by means of maps, graphics and tables, forshowing the distribution and kinematics of the gas. It containsfurthermore a measure of the HI extension that is detected at the levelof the maximum sensitivity reached in the observations. This catalogueis intended as a guide for references on the HI maps published in theliterature from 1953 to 1995 and is the basis for the analysis of thedata presented in Paper II. The catalogue is only available inelectronic form at the CDS via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| A ROSAT survey of Wolf-Rayet galaxies
We present results from a ROSAT Position Sensitive Proportional Counter(PSPC) survey of the X-ray emission from Wolf-Rayet (WR) galaxies. Ofthe 36 WR galaxies listed in the catalogue of Conti, a total of 14 havebeen observed deliberately or serendipitously with the ROSAT PSPC, andof these, seven have been detected. The derived X-ray luminosities of WRgalaxies range over nearly three orders of magnitude. The X-ray spectraof the WR galaxies can typically be well-fitted with a singletemperature Raymond-Smith spectral model, with a temperature in therange kT = 0.3-1.0 keV, with the general trend that the moreX-ray-luminous WR galaxies have hotter spectra. WR galaxies aresignificantly X-ray-overluminous for their blue luminosity, comparedwith a sample of nearby spiral and starburst galaxies. In addition, theX-ray luminosity of WR galaxies correlates well with the FIR luminosityand the number of Lyman continuum photons. No strong correlation wasfound with the equivalent width of the WR emission feature around 4686 Athe presence of which essentially defines the class of galaxies. Variousexplanations for the observed properties of WR galaxies are explored,and we conclude that the X-ray emission provides strong evidence that alarge fraction of the observed X-rays is coming from a hot superbubbleformed by the combined action of stellar winds from massive early-typestars in the central starburst cluster. These results are consistentwith, and add weight to, the view that WR galaxies are young starbursts,in which the duration of the star-forming epoch was very short, and thatwe are viewing them a few Myr after the initiation of the starburst. Assuch, WR galaxies represent an important epoch in the evolution ofstarburst galaxies.
| 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.
| The Apparent Morphology of Peculiar Galaxies at Intermediate to High Redshifts
Abstract image available at:http://adsabs.harvard.edu/abs/1997AJ....114.1741H
| Starbursts Induced by Interactions in Groups of Galaxies
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