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|Ages and metallicities of Hickson compact group galaxies|
Hickson compact groups (HCGs) constitute an interesting extreme in therange of environments in which galaxies are located, as the spacedensity of galaxies in these small groups are otherwise only found inthe centres of much larger clusters. The work presented here uses Lickindices to make a comparison of ages and chemical compositions ofgalaxies in HCGs with those in other environments (clusters, loosegroups and the field). The metallicity and relative abundance of`α-elements' show strong correlations with galaxy age and centralvelocity dispersion, with similar trends found in all environments.However, we show that the previously reported correlation betweenα-element abundance ratios and velocity dispersion disappears whena full account is taken of the abundance ratio pattern in thecalibration stars. This correlation is thus found to be an artefact ofincomplete calibration to the Lick system.Variations are seen in the ranges and average values of age, metallicityand α-element abundance ratios for galaxies in differentenvironments. Age distributions support the hierarchical formationprediction that field galaxies are on average younger than their clustercounterparts. However, the ages of HCG galaxies are shown to be moresimilar to those of cluster galaxies than those in the field, contraryto the expectations of current hierarchical models. A trend for lowervelocity dispersion galaxies to be younger was also seen. This is againinconsistent with hierarchical collapse models, but is qualitativelyconsistent with the latest N-body smoothed particle hydrodynamics modelsbased on monolithic collapse in which star formation continues for manyGyr in low-mass haloes.
|Near-infrared imaging of ellipticals: surface brightness profiles and photometry|
We present near-infrared K-band imaging of a large sample of candidatemerger remnant galaxies and Hickson Compact Group ellipticals. We derivelight profile indices, effective radii and surface brightnesses, as wellas total K-band magnitudes. We find that the light distributions of themerger remnant candidates are consistent with those of `normal'ellipticals, and scatter around a mean profile index of (1/n) = 0.20.Many of our sample galaxies have surface brightness profiles that arenot well described by a de Vaucouleurs law (1/n= 0.25), and we discussthe implications of this on the derived total magnitudes. Comparing thetotal K magnitudes calculated by extrapolating a de Vaucouleurs profileand those derived using a generalized Sérsic form, we find that asignificant bias is introduced if the de Vaucouleurs law is not a gooddescription of the actual light profile.
|An X-Ray Atlas of Groups of Galaxies|
A search was conducted for a hot intragroup medium in 109 low-redshiftgalaxy groups observed with the ROSAT PSPC. Evidence for diffuse,extended X-ray emission is found in at least 61 groups. Approximatelyone-third of these detections have not been previously reported in theliterature. Most of the groups are detected out to less than half of thevirial radius with ROSAT. Although some spiral-rich groups do contain anintragroup medium, diffuse emission is restricted to groups that containat least one early-type galaxy.
|Globular Cluster Population of Hickson Compact Group 22a and 90c|
We present the first measurement of the globular cluster populations ofgalaxies in Hickson compact groups, in order to investigate the effectof these high-density environments on the formation and evolution ofglobular cluster systems. Based on V- and R-band images that we obtainedof HCG 22a and HCG 90c with the ESO New Technology Telescope (NTT), wefind a total globular cluster population of 1590+/-854 for HCG 22a and2136+/-718 for 90c. The specific frequency was found to beSN=1.9+/-1.0 for HCG 22a and SN=3.4+/-1.1 for HCG90c. A power-law fit to the globular cluster radial profile of HCG 22ayields σ~R-2.01+/-0.30, and for HCG 90c we foundσ~R-1.20+/-0.16. A comparison of the globular clusterradial profiles with the surface brightness of the parent galaxy showsthat the globular cluster systems are at least as extended as the halolight. The measured values for the specific frequency are consistentwith a scenario in which the host galaxies were in a low-density``fieldlike'' environment when they formed their globular clustersystems. Based on observations collected at the European SouthernObservatory, Chile.
|Structural and Dynamical Analysis of the Hickson Compact Groups|
Based on the spectroscopic survey of de Carvalho et al., we analyze thestructural and dynamical properties of 17 Hickson compact groups. Thisanalysis probes a region of 0.dg5 x 0.dg5 around each group and showsthat most of them are part of larger structures. Our results alsosuggest that the Hickson sample is composed of different dynamicalstages of the groups" evolution. Specifically, we identify threepossible evolutionary phases among groups in the sample: loose groups,core + halo systems, and compact groups, each one presenting a distinctsurface density profile. This sequence is consistent with thereplenishment scenario for the formation and evolution of compact groupswithin larger and less dense systems.
|The Nature of the Activity in Hickson Compact Groups of Galaxies|
We present the results of the spectral classification of the 82brightest galaxies in a sample of 17 compact groups. We verify that theactive galactic nuclei (AGNs) are preferentially located in the mostearly-type and luminous galaxies of the groups, as is usually observedin the field. But these AGNs also appear to be systematicallyconcentrated toward the central parts of the groups. Our observationssuggest a correlation between activity types, morphologies, anddensities of galaxies in the compact groups. This is consistent with ascenario in which galaxies of compact groups evolve by interacting withtheir environment and are currently in a quiet phase of their activity.
|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.
|Redshift Survey of Galaxies around a Selected Sample of Compact Groups|
We report the results of a spectroscopic survey of faint galaxies in theregions surrounding Hickson compact groups. Our sample is composed of 17groups within 9000 km s-1. The spectra were taken at the prime focus ofthe Tololo 4 m telescope, using the ARGUS fiber-fed spectrograph. Fromthese observations, redshifts were determined for the faint galaxiespreviously identified by de Carvalho, Ribeiro, & Zepf in thesurroundings of the groups. Statistical methods were applied to theresultant catalog in order to determine the kinematical structure ofeach group. This analysis confirms the idea that the Hickson sample ofcompact groups contains a wide variety of projection and dynamicalconfigurations. Our results demonstrate the necessity of newspectroscopic surveys around compact groups in order to assess theircomplete velocity distribution.
|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.
|Multicolor surface photometry of early-type galaxies. I.|
We have obtained v,g,r and i CCD surface photometry for a sample of 109early-type galaxies. Many of the galaxies covered have no previouslypublished CCD or aperture photometry. Our surface brightness profilestypically extend down to a surface brightness ofμ_r_~24mag/arcsec^2^. Detailed comparisons with previously publishedwork, and internal and external error estimates for all quantities areprovided. The complete surface photometry data set is made available ina computer-readable form.
|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.
|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.
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