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The dichotomy of early-type galaxies from their globular cluster systems.
Growing evidence for the existence of two classes of ellipticals callsfor a comparison of the properties of their globular cluster systems. Icompiled information on the properties of 53 globular cluster systems offaint and bright early-type galaxies, and investigated them in the lightof the properties of the parent galaxies. The properties of globularcluster systems appear to separate into two classes rather than tofollow continuous relations with their host galaxy properties. The"faint" systems have a low specific frequency (less than about 5), lessthan roughly 1500 globular clusters, a relatively low metallicity([Fe/H]<-1.2), and a steep density profile that follows the galaxylight. These systems appear essentially unperturbed, and are hosted byfaint (about M_V_>-21.5), disky early-type galaxies with unresolvedcores. On the other hand, "bright" globular cluster systems had a higherefficiency in producing globular clusters and have higher specificfrequencies (higher than 5). They have larger number of globularclusters (more than 2000), have flat density profiles (α>-1.7)and their color distributions are often broad, and show several peaks orgradients in many cases. Finally the mean metallicity is higher than in"faint" globular cluster systems. "Bright" globular cluster systems showall signs predicted for globular cluster systems that experienced amerger event, and are associated with bright (about M_V_<-21.5) boxyellipticals with resolved cores. I conclude that every galaxy is likelyto have formed globular clusters during the early proto-galacticcollapse, but "bright" systems were enriched and disturbed during mergerevents. These two classes of globular cluster systems support the ideathat major merger events could be a cause for the dichotomy ofearly-type galaxies.
| Far infrared properties of Hickson compact groups of galaxies. I. High resolution IRAS maps and fluxes.
The Far Infrared (FIR) properties of galaxies which are members ofcompact groups bear relevant information on the dynamical status and thephysical properties of these structures. All studies published so farhave been undermined by the poor sensitivity and spatial resolution ofthe IRAS-PSC and IRAS Sky Survey data. We used the HIRAS softwareavailable at the IRAS server at the Laboratory for Space Research inGroningen to fully exploit the redundancy of the IRAS data and toapproach the theoretical diffraction limit of IRAS. Among the 97 groupswhich were observed by IRAS, 62 were detected in at least one band,while reliable upper limits were derived for all the others. Among thedetected groups, 49 were fully or partially resolved, i.e. it waspossible to discriminate which member or members emit most of the FIRlight. At 60μm, for instance, 87 individual sources were detected in62 groups. In order to ease the comparison with data obtained at otherwavelengths - and in particular in the X and radio domains - we giveco-added and HIRAS maps for all the detected groups.
| Morphology of galaxies in compact groups
We present the results of an isophotal analysis of 140 early-typegalaxies and a visual inspection of images of an additional 202 galaxiesin compact groups. This is essentially the entire sample of galaxies inthe subset of 92 Hickson compact groups which have at least threeaccordant members. About 12% of the elliptical galaxies have largercharacteristic radii and shallower surface brightness profiles thangalaxies of the same luminosity in less dense environments. The averageellipticity of elliptical galaxies in compact groups is a slowlyincreasing function of the metric radius, as it is for field andloose-group galaxies. No alignment is found among the major axes of thegalaxies and the major axis of the group. When combined with previouslypublished morphological, kinematic, radio, infrared, and colorinformation on the same galaxies, our data show that 43% of the galaxiesin the compact group sample show morphological and/or kinematicaldistortions indicative of interactions and/or mergers. About 32% of thegroups have three or more galaxies which show some sign of interaction.This is a lower limit, since for the great majority of the galaxies inthe groups, only imaging and low-resolution spectra are available. Forthe subsample of 16 groups for which published detailed kinematical dataare also available, the fraction of groups with three or more galaxiesin interaction is 75%. No correlation is found between the number ofinteracting galaxies in a group and the group velocity dispersion orcrossing time. These observations strongly support the view that compactgroups are systems of physically associated galaxies and not chancealignments of field, loose-group, or cluster galaxies. They also confirmthe importance of compact groups for studies of interactions and galaxyevolution. While the lack of a good control sample makes it difficult tomake quantitative comparisons for some aspects of this study, it isclear that the fraction of galaxies showing evidence of interactions ismuch higher in compact groups than in other environments.
| Star formation and merging in compact groups of galaxies
New UBV aperture photometry for 177 galaxies in Hickson compact groupsis presented. The analysis of the color-color diagram indicates that,contrary to what one naively would expect, only a slight enhancement intheir star formation rates is present. The results are confirmed by theanalysis of the IRAS data, taking in particular into account the 25μmdetections. Star formation in compact groups is not found to be higherthan in isolated pairs of galaxies. Earlier results on the absence ofclear candidates for merger products of late type galaxies are confirmedand strengthened. It is pointed out that the relation between velocitydispersion and morphological mixing found for Hickson groups could beone of the clues to understand the evolution of galaxies in them.
| 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.
| A photometric study of interacting galaxies. I - Observations
As part of an investigation on what effect galaxy-galaxy interaction hason the global photometric properties of the involved galaxies, wepresent here new photometric data in the Cousins R-prime system for asample of 35 interacting systems. Isophotes, equivalent profiles, andtotal magnitudes of all sample galaxies (73) are determined. Ourestimates and the comparison with the published results show that theaccuracy of our photometry is about 0.2-0.3 mag. A later paper willpresent the interpretation of the data.
| A photometric study of interacting galaxies. 2: Analysis of the results
We present the results of a detailed photometric study in theRC band of 73 galaxies in close interacting systems. Thesample for this study was selected mainly from the Atlas of PeculiarGalaxies by Arp on the basis of the optical morphology of galaxies. Theobserved systems exhibit: an excess of early type spirals (Sa) anddeficiency of late type ones (Sm/Irr), and excess of brighter galaxiesas compared with field galaxies. From the decomposition of theequivalent profiles we found the following principal results: (a)Interacting ellipticals and bulges of SO galaxies are systematicallydisplaced in the plane mue - Re from the relationfor normal galaxies. This displacement may be explained in terms ofbulges of interacting galaxies being more compact. (b) Bulges ofinteracting spirals are located in the effective parameters plane in thesame region of bulges of non-interacting galaxies. (c) Disks of stronglyinteracting spirals are 2-3 times thicker as compared with disks ofnormal galaxies. (d) The central surface brightnesses (mu0 ofthe disks of interacting galaxies are, on average, brighter than forfield spirals. (e) There is no clear dependence of the globalphotometric characteristics of galaxies on the interaction strength.There is a weak tendency for galactic disks to be slightly brighter andshorter than in strongly interacting systems.
| 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.
| 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.
| Galaxies of the M 51 type.
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