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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.
|The Relation between Galaxy Activity and the Dynamics of Compact Groups of Galaxies|
Using a sample of 91 galaxies distributed over 27 compact groups (CGs)of galaxies, we define an index that allows us to quantify their levelof activity due to an active galactic nucleus (AGN) or star formation.By combining the mean activity index with the mean morphological type ofthe galaxies in a group, we are able to quantify the evolutionary stateof the groups. We find that they span an evolutionary sequence thatcorrelates with the spatial configuration of the galaxies in the CG. Wedistinguish three main configuration types: A, B, and C. Type A CGs showpredominantly low velocity dispersions and are rich in late-type spiralsthat show active star formation or harbor an AGN. Type B groups haveintermediate velocity dispersions and contain a large fraction ofinteracting or merging galaxies. Type C comprises CGs with high velocitydispersions, which are dominated by elliptical galaxies that show noactivity. We suggest that evolution proceeds A==>B==>C. Mappingthe groups with different evolution levels in a diagram of radius versusvelocity dispersion does not reveal the pattern expected based on theconventional fast merger model for CGs, which predicts a direct relationbetween these two parameters. Instead, we observe a trend contrary toexpectation: the evolutionary state of a group increases with velocitydispersion. This trend seems to be related to the masses of thestructures in which CGs are embedded. In general, the evolutionary stateof a group increases with the mass of the structure. This suggestseither that galaxies evolve more rapidly in massive structures or thatthe formation of CGs embedded in massive structures predated theformation of CGs associated with lower mass systems. Our observationsare consistent with the structure formation predicted by the CDM model(or ΛCDM), only if the formation of galaxies is a biased process.
|Globular Clusters around Galaxies in Groups|
We have obtained deep photometry of NGC 1199 (in the compact group HCG22) and NGC 6868 (in the Telescopium loose group) with the Keck II andthe ESO VLT-I telescopes. Each galaxy is the optically brightest galaxyof its group. NGC 1199 has two companion galaxies at a median projecteddistance of only 33 kpc and, based on its peculiar internal structureand large X-ray halo, NGC 6868 has been proposed to be a merger remnant.Our analysis of B and R images uncovered a population of globularclusters around both galaxies, with total (and local) specific frequencySN=3.6+/-1.8 (3.4+/-1.5) for NGC 1199 andSN=1.8+/-1.1 (0.8+/-0.4) for NGC 6868. The radial profile ofthe globular clusters of NGC 1199 follows the light distribution of thegalaxy and can be fitted by a power law and a core model with a verysteep slope (α=2.5+/-0.3). In the case of NGC 6868, the profile ofthe globular clusters is well fitted by a power law and a core modelprofile of slope 1.4+/-0.3 and is shallower than the galaxy lightdistribution. Maximum likelihood fitting of two Gaussians to theglobular cluster color distribution yields a high significance formultimodality, with peaks centered at (B-R)0=1.13+/-0.04 and1.42+/-0.04 (NGC 1199) and (B-R)0=1.12+/-0.07 and 1.42+/-0.07(NGC 6868). NGC 1199 and NGC 6868 are good examples of galaxies in whichthe group environment is likely to have affected their dynamicalevolution. We find that for NGC 1199 the properties of the globularcluster system are similar to those for other systems around externalelliptical galaxies located in less dense environments but with a verysteep radial profile. In the case of NGC 6868, we find a regular radialprofile and color distribution and a comparatively low specificfrequency for the globular cluster system of the galaxy.
|The Evolution of Galaxies in Compact Groups|
We present an analysis of the spectra of 62 galaxies in 15 compactgroups. The galaxies are classified into four activity classes: galaxieswithout emission, starburst galaxies, luminous AGNs (Seyfert andLINERs), and low-luminosity AGNs (LLAGNs). The star formation in theHickson compact group (HCG) starbursts is more intense than in normalspirals, but comparable to that observed in starburst-nucleus galaxies(SBNGs) in the field. In general, the HCG starbursts have mean solar gasmetallicity and do not follow the metallicity-luminosity relation tracedby the early-type SBNGs in the field, suggesting that most of them arelate-type SBNGs. This morphology preference, coupled with theobservation that the HCG starbursts are predominantly located in thehalos of the groups, is consistent with the idea that compact groups areembedded in sparser structures. The stellar metallicities of thenonstarburst galaxies are comparable to those observed in normalgalaxies with similar morphologies, but are relatively high for theirluminosities. In these galaxies, the metal absorption line equivalentwidths are slightly narrower than normal, while the Balmer absorptionlines are relatively strong. All these observations suggest the presenceof a population of intermediate-age stars. These galaxies could bepoststarburst, but at a very advanced stage of evolution, the lastbursts having happened more than 2 Gyr in the past. Our observationssupport a scenario in which the cores of the groups are slowlycollapsing evolved systems embedded in more extended structures. In thecores of the groups, the interactions were more frequent and thegalaxies evolved at a more rapid rate than in their halos.
|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.
|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 Southern Sky Redshift Survey|
We report redshifts, magnitudes, and morphological classifications for5369 galaxies with m_B <= 15.5 and for 57 galaxies fainter than thislimit, in two regions covering a total of 1.70 sr in the southerncelestial hemisphere. The galaxy catalog is drawn primarily from thelist of nonstellar objects identified in the Hubble Space TelescopeGuide Star Catalog (GSC). The galaxies have positions accurate to ~1"and magnitudes with an rms scatter of ~0.3 mag. We compute magnitudes(m_SSRS2) from the relation between instrumental GSC magnitudes and thephotometry by Lauberts & Valentijn. From a comparison with CCDphotometry, we find that our system is homogeneous across the sky andcorresponds to magnitudes measured at the isophotal level ~26 magarcsec^-2. The precision of the radial velocities is ~40 km s^-1, andthe redshift survey is more than 99% complete to the m_SSRS2 = 15.5 maglimit. This sample is in the direction opposite that of the CfA2; incombination the two surveys provide an important database for studies ofthe properties of galaxies and their large-scale distribution in thenearby universe. Based on observations obtained at Cerro TololoInter-American Observatory, National Optical Astronomy Observatories,operated by the Association of Universities for Research in Astronomy,Inc., under cooperative agreement with the National Science Foundation;Complejo Astronomico El Leoncito, operated under agreement between theConsejo Nacional de Investigaciones Científicas de laRepública Argentina and the National Universities of La Plata,Córdoba, and San Juan; the European Southern Observatory, LaSilla, Chile, partially under the bilateral ESO-ObservatórioNacional agreement; Fred Lawrence Whipple Observatory;Laboratório Nacional de Astrofísica, Brazil; and the SouthAfrican Astronomical Observatory.
|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.
|An image database. II. Catalogue between δ=-30deg and δ=70deg.|
A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect.
|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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