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A principal component analysis approach to the star formation history of elliptical galaxies in compact groups
Environmental differences in the stellar populations of early-typegalaxies are explored using principal component analysis (PCA), focusingon differences between elliptical galaxies in Hickson Compact Groups(HCGs) and in the field. The method is model-independent and purelyrelies on variations between the observed spectra. The projections (PC1,PC2) of the observed spectra on the first and second principalcomponents reveal a difference with respect to environment, with a widerrange in PC1 and PC2 in the group sample. We define a spectral parameter(ζ ≡ 0.36PC1-PC2) which simplifies this result to a singlenumber: field galaxies have a very similar value of ζ, whereas HCGgalaxies span a wide range in this parameter. The segregation is foundregardless of the way the input spectral energy distributions (SEDs) arepresented to PCA (i.e. changing the spectral range; using uncalibrateddata; subtracting the continuum or masking the SED to include only theLick spectral regions). Simple models are applied to give physicalmeaning to the PCs. We obtain a strong correlation between the values ofζ and the mass fraction in younger stars, so that some groupgalaxies present a higher fraction of them, implying a more complex starformation history in groups. Regarding `dynamically related' observablessuch as a4 or velocity dispersion, we find a correlation withPC3, but not with either PC1 or PC2. PCA is more sensitive than othermethods based on a direct analysis of observables such as the structureof the surface brightness profile or the equivalent width of absorptionlines. The latter do not reveal any significant variation between fieldand compact group galaxies. Our results imply that the presence of youngstars only amounts to a fraction of a per cent in its contribution tothe total variance, reflecting the power of PCA as a tool to extractsmall variations in the spectra from unresolved stellar populations.
| Ages, Metallicities, and α-Element Enhancement for Galaxies in Hickson Compact Groups
Central velocity dispersions and eight line-strength Lick indices havebeen determined from 1.3 Å resolution long-slit spectra of 16elliptical galaxies in Hickson compact groups. These data were used todetermine galaxy properties (ages, metallicities, and α-elementenhancements) and allowed a comparison with the parameters determinedfor a sample of galaxies in lower density environments studied byGonzález. The stellar population parameters were derived bycomparison to single stellar population models of Thomas et al. and to anew set of simple stellar population models for the indicesMg2, Fe5270, and Fe5335 based on synthetic spectra. Thesemodels, based on an updated version of the fitting functions presentedin Barbuy et al., are fully described here. Our main results are (1) thetwo samples have similar mean values for the metallicities and[α/Fe] ratios and (2) the majority of the galaxies in compactgroups seem to be old (median age of 14 Gyr for eight galaxies for whichages could be derived), in agreement with recent work by Proctor et al.These findings support two possible scenarios: compact groups are eitheryoung systems whose members have recently assembled and had not enoughtime to experience any merging yet, or they are old systems that haveavoided merging since their time of formation.
| 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.
| 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
| 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).
| The Fundamental Plane of Elliptical Galaxies in Compact Groups
We present a study focusing on the nature of compact groups through thestudy of their elliptical galaxies. We determine velocity dispersions(σ) for 18 bright elliptical galaxies located in the core ofHickson compact groups and a control sample of 12 bright bona fideelliptical galaxies located in the field or very loose groups. Severaltests are carried out to avoid sources of systematic effects in σmeasurements. We use these velocity dispersions to compare the positionsof 11 compact group galaxies in the fundamental plane with those of alarge and homogeneous sample of elliptical galaxies. We find that littleor no significant difference exists, as far as the fundamental plane isconcerned, between elliptical galaxies in compact groups and theircounterparts in other environments.
| 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.
| A ROSAT survey of Hickson's compact galaxy groups.
We report the results of an almost complete survey of the X-rayproperties of Hickson's compact galaxy groups with the ROSAT PSPC.Diffuse X-ray emission is detected from 22 groups. We infer that hotintragroup gas is present in $\gtsimm 75\%$ of these systems and derivetheir X-ray luminosity function. Earlier reports that only spiral-poorsystems exhibit diffuse X-ray emission are found to be incorrect. Strongcorrelations are found between the X-ray luminosity and both the gastemperature and the velocity dispersion of the group galaxies. We arguethat these properties provide strong evidence that most of these groupsare genuinely compact configurations, rather than line-of-sightsuperpositions. Comparison with the X-ray properties of galaxy clustersindicate a significant steepening of the $L:T$ relation below$T\sim1$\keV, which may result from the action of galaxy winds.
| 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.
| The fundamental plane of early-type galaxies: stellar populations and mass-to-light ratio.
We analyse the residuals to the fundamental plane (FP) of ellipticalgalaxies as a function of stellar-population indicators; these are basedon the line-strength parameter Mg_2_ and on UBVRI broad-band colors, andare partly derived from new observations. The effect of the stellarpopulations accounts for approximately half the observed variation ofthe mass-to-light ratio responsible for the FP tilt. The residual tiltcan be explained by the contribution of two additional effects: thedependence of the rotational support, and possibly that of the spatialstructure, on the luminosity. We conclude to a constancy of thedynamical-to-stellar mass ratio. This probably extends to globularclusters as well, but the dominant factor would be here the luminositydependence of the structure rather than that of the stellar population.This result also implies a constancy of the fraction of dark matter overall the scalelength covered by stellar systems. Our compilation ofinternal stellar kinematics of galaxies is appended.
| ROSAT observations of compact groups of galaxies
We have systematically analyzed a sample of 13 new and archival ROSATPosition Sensitive Proportional Counter (PSPC) observations of compactgroups of galaxies: 12 Hickson compact groups plus the NCG 2300 group.We find that approximately two-thirds of the groups have extended X-rayemission and, in four of these, the emission is resolved into diffuseemission from gas at a temperature of kT approximately 1 keV in thegroup potential. All but one of the groups with extended emission have aspiral fraction of less than 50%. The baryon fraction of groups withdiffuse emission is 5%-19%, similar to the values in clusters ofgalaxies. However, with a single exception (HCG 62), the gas-to-stellarmass ratio in our groups has a median value near 5%, somewhat greaterthan the values for individual early-type galaxies and two orders ofmagnitude than in clusters of galaxies. The X-ray luminosities ofindividual group galaxies are comparable to those of similar fieldgalaxies, although the LX-LB relation forearly-type galaxies may be flatter in compact groups than in the field.
| Deep Optical Observations of Compact Groups of Galaxies
Compact groups of galaxies appear to be extremely dense, making themlikely sites of intense galaxy interaction, while their smallpopulations make them relatively simple to analyze. In order to searchfor optical interaction tracers such as diffuse light and galaxy tidalfeatures in Hickson compact groups (HCGs), we carried out deepphotometry in three filters on a sample of HCGs with ROSAT observations.Using a modeling procedure to subtract the light of bright early-typegalaxies, we found shell systems and extended envelopes around many, butnot all, of those galaxies. Only one group in our sample, HCG 94, hasdiffuse light in the group potential (with a luminosity of 7 L^*^); theother groups do not contain more than 113 L^*^ in diffuse light. Withthe exception of HCG 94 (which is the most x-ray-luminous HCG), we foundno correlation between the presence of shells or other tidal featuresand the x-ray luminosity of a group. Better predictors of detectablegroup x-ray emission are a low spiral fraction and belonging to a largergalaxy condensation-neither of which are correlated with opticaldisturbances in the group galaxies. No elliptical galaxies that areextremely optically luminous but x-ray faint are found to have shellsand very complex color structures. This is likely due to recent infallof gas-rich material into the galaxies, which would produce both thedisruption of stellar orbits and a significant amount of star formation.
| 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.
| 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
| The morphological catalogue of galaxies equatorial survey
We present 865 redshifts of galaxies located in the equatorial stripdelta between -17.5 deg and -2.5 deg in the right ascension rangebetween 20 h and 5 h. Redshifts have been obtained for the completesample of all 833 galaxies in the Morphological Catalog of Galaxies withmagnitudes brighter than m = 14.5 (corresponding approximately tom(Zwicky) = 15.0). This sample also includes three galaxies from othersources with more reliable magnitudes, satisfying this limit, and 29fainter galaxies, usually companions of the galaxies in the magnitudelimited sample. Our maps of a very large volume of nearby spacedemonstrate a variety of coherent large scale structures which includelarge voids, 20-50/h Mpc in diameter and large walls at least 70/h Mpcacross.
| 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.
| Optical colors of early-type galaxies in compact groups
UBVRI colors of a sample of 55 elliptical galaxies as well as 21 S0galaxies in Hickson compact groups are presented. Four of the ellipticalgalaxies and one of the S0 galaxies in the compact groups aresignificantly bluer than normal for early-type galaxies of theirluminosity. A population of elliptical galaxies with blue optical colorsis predicted by standard models of the evolution of galaxies in compactgroups because of the favorable conditions for interactions and mergersin these regions. The brightest of the blue elliptical galaxies has anextensive shell system; however, the other two blue elliptical galaxiesfor which there is sufficient spatial resolution show no unusualmorphological features such as tails or shells. In addition to theircolor, the primary distinguishing feature of the blue ellipticalgalaxies is their color gradient, as they tend to be bluer in the centerthan at large radii, which is contrary to the usual trend in ellipticalgalaxies.
| The luminosity function of compact groups of galaxies
An analysis of the luminosity function of 68 compact groups of galaxiescataloged by Hickson (1982) is presented. The luminosities of compactgroup galaxies are consistent with their being drawn from a Schechterluminosity function. Individual morphological-type luminosity functionsare also determined. Both the total and morphological-type specificluminosity functions of compact group galaxies are significantlydifferent from those of field, loose-group, and cluster galaxies. Inparticular, the luminosity function of HCG elliptical galaxies has amean magnitude which is significantly brighter than the mean magnitudeof Virgo cluster elliptical galaxies. The mean luminosity density ofgalaxies in compact groups is estimated. The obtained result isconsistent with the conventional scenario in which compact groups mergeto form elliptical galaxies on a relatively short time scale.
| 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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