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The Hamburg/RASS Catalogue of optical identifications. Northern high-galactic latitude ROSAT Bright Source Catalogue X-ray sources
We present the Hamburg/RASS Catalogue (HRC) of optical identificationsof X-ray sources at high-galactic latitude. The HRC includes all X-raysources from the ROSAT Bright Source Catalogue (RASS-BSC) with galacticlatitude |b| >=30degr and declination delta >=0degr . In thispart of the sky covering ~ 10 000 deg2 the RASS-BSC contains5341 X-ray sources. For the optical identification we used blue Schmidtprism and direct plates taken for the northern hemisphere Hamburg QuasarSurvey (HQS) which are now available in digitized form. The limitingmagnitudes are 18.5 and 20, respectively. For 82% of the selectedRASS-BSC an identification could be given. For the rest either nocounterpart was visible in the error circle or a plausibleidentification was not possible. With ~ 42% AGN represent the largestgroup of X-ray emitters, ~ 31% have a stellar counterpart, whereasgalaxies and cluster of galaxies comprise only ~ 4% and ~ 5%,respectively. In ~ 3% of the RASS-BSC sources no object was visible onour blue direct plates within 40\arcsec around the X-ray sourceposition. The catalogue is used as a source for the selection of(nearly) complete samples of the various classes of X-ray emitters.
| The UZC-SSRS2 Group Catalog
We apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.
| VLBA Observations of a Sample of Nearby FR I Radio Galaxies
We observed 17 nearby low-luminosity FR I radio galaxies using the NRAOVery Long Baseline Array (VLBA) at 1.67 GHz, as part of amultiwavelength study of a complete sample of 21 sources selected byradio flux density from the Uppsala General Catalogue of Galaxies. Wedetected radio emission from all 17 galaxies. At a FWHM resolution of~10×4 mas, five galaxies show only an unresolved radio core, 10galaxies show core-jet structures, and two galaxies show twin-jetstructures. Comparing these VLBA images with images previously obtainedwith the NRAO VLA, we find that all detected VLBA jets are well alignedon parsec scales with the VLA jets on kiloparsec scales and that thejet-to-counterjet surface brightness ratios, or the sidedness, decreasessystematically with increasing distance along the jet. We attribute thesidedness to the Doppler boosting effect and its decline to thedeceleration of the jets. We show that a distribution of Lorentz factorcentered near Γ=5 can reproduce our VLBA detection statistics forcore, core-jet, and twin-jet sources. We also note that the luminosityper unit length, Lj, of the VLBA jets drops quickly withdistance, r, along the jet, approximately asLj~r-2.0. We discuss three different mechanisms toexplain this jet fading: (1) the decrease of Doppler boosting due to jetdeceleration, (2) synchrotron losses, and (3) expansion losses inconstant velocity but adiabatically spreading jets. Mechanisms (1) and(2) are inconsistent with the observations, while mechanism (3) isconsistent with the observations provided the magnetic field lines inthe jets are aligned perpendicular to the jet axis. This implies thatthe deceleration of the jets required by the unified scheme does notoccur on the tens of parsec scales but must occur on larger scales.
| The Dynamics of Poor Systems of Galaxies
We assemble and observe a sample of poor galaxy systems that is suitablefor testing N-body simulations of hierarchical clustering and otherdynamical halo models. We (1) determine the parameters of the densityprofile rho(r) and the velocity dispersion profile sigma_p(R), (2)separate emission-line galaxies from absorption-line galaxies, examiningthe model parameters and as a function of spectroscopic type, and (3)for the best-behaved subsample, constrain the velocity anisotropyparameter, beta, which determines the shapes of the galaxy orbits. Oursample consists of 20 systems, 12 of which have extended X-ray emissionin the ROSAT All-Sky Survey. We measure the 877 optical spectra ofgalaxies brighter than m_R~15.4 within 1.5 h^-1 Mpc of the systemcenters (we take H_0=100 h km s^-1 Mpc^-1). Thus, we sample the systemmembership to a radius typically three times larger than other recentoptical group surveys. The average system population is 30 galaxies, andthe average line-of-sight velocity dispersion is ~300 km s^-1. TheNavarro, Frenk, & White universal profile and the Hernquist modelboth provide good descriptions of the spatial data. In most cases anisothermal sphere is ruled out. Systems with declining sigma_p(R) arewell-matched by theoretical profiles in which the star-forming galaxieshave predominantly radial orbits (beta>0) many of these galaxies areprobably falling in for the first time. There is significant evidencefor spatial segregation of the spectroscopic classes regardless ofsigma_p(R).
| Mass and Metallicity of Five X-Ray-bright Galaxy Groups
We present ASCA X-ray observations of a sample of five groups selectedfrom a cross-correlation of the ROSAT All-Sky Survey with the White etal. optical catalog of groups. These X-ray-bright groups significantlyincrease the number of known systems with temperatures between 2 and 3keV. They have element abundances of roughly 0.3solar, which are typicalof clusters, but their favored ratio of Si-to-Fe abundance is lower thanthe cluster value. Combining the ASCA results with ROSAT imaging data,we calculate total masses of a few to several times 10^13 M_solar, gasmass fractions of ~10%, and baryonic mass fractions of at least 15%-20%within a radius of 0.5 Mpc. Upper limits for the ratios of gas to galaxymass and of the iron mass to galaxy luminosity overlap with the rangeobserved in rich clusters and extend to lower values, but not to suchlow values as seen in much poorer groups. These results support the ideathat groups, unlike clusters, are subject to the loss of theirprimordial and processed gas and show that this transition occurs at themass scale of the 2-3 keV groups. A discussion of ASCA calibrationissues and a comparison of ROSAT and ASCA temperatures are included inan Appendix.
| Hubble Space Telescope Observations of Nearby Radio-Loud Early-Type Galaxies
We present and analyze Hubble Space Telescope (HST) WFPC2 broad- andnarrowband observations of the central regions of 19 nearby radio-loudearly-type galaxies. Together with two more galaxies, they form acomplete sample of Fanaroff and Riley Type I galaxies. We obtained V-and I-band images and narrowband images centered on the Hα+[N II]emission lines. We use archival data for six galaxies. We describe thedata reduction, give isophotal fits, and analyze the centralemission-line gas and dust distributions. Our main conclusions are thefollowing. Although obscuration by dust inhibits a direct determinationof central cusp slopes, the data suggest that most but not all galaxieshave shallow cores. Dust is detected in all but two galaxies. There area wide variety of central dust morphologies, ranging from central disksto lanes and irregular distributions. The analysis suggests that thedifference between disks and lanes is intrinsic and not due to differentviewing angles. Central emission-line gas is detected in all galaxies.Extended low surface brightness emission is always associated with thedust features. In a subsequent paper we will present a detailed analysisof the relation between these central properties and the nuclearactivity.
| 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 catalogue of spatially resolved kinematics of galaxies: Bibliography
We present a catalogue of galaxies for which spatially resolved data ontheir internal kinematics have been published; there is no a priorirestriction regarding their morphological type. The catalogue lists thereferences to the articles where the data are published, as well as acoded description of these data: observed emission or absorption lines,velocity or velocity dispersion, radial profile or 2D field, positionangle. Tables 1, 2, and 3 are proposed in electronic form only, and areavailable from the CDS, via anonymous ftp to cdsarc.u-strasbg.fr (to130.79.128.5) or via http://cdsweb.u-strasbg.fr/Abstract.html
| 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.
| A Catalog of Stellar Velocity Dispersions. II. 1994 Update
A catalog of central velocity dispersion measurements is presented,current through 1993 September. The catalog includes 2474 measurementsof 1563 galaxies. A standard set of 86 galaxies is defined, consistingof galaxies with at least three reliable, concordant measurements. It issuggested that future studies observe some of these standard galaxies sothat different studies can be normalized to a consistent system. Allmeasurements are reduced to a normalized system using these standards.
| ROSAT Observations of Five Poor Galaxy Clusters with Extended Radio Sources
We present the results of deep ROSAT PSPC observations of the poorclusters MKW2, N79-299A, S49-128, S49-132, and S49-140. These poorclusters all contain extended radio sources, generally with a bent,head- tail (HT) morphology. It had been previously thought that HTsshould only be found in rich clusters, which have sufficiently highintracluster medium (ICM) densities and velocity dispersions foreffective ram pressure bending of the radio jets. We have found that theX-ray emission associated with these poor clusters is generally quiteclumpy and asymmetrical. Often, the clumps are associated with subgroupsor individual galaxies, as well as with extended regions around theradio sources. Our results also indicate that there is a continuum ofX-ray properties from poor to rich clusters. In many respects, poorclusters seem to be a low-mass extension of rich clusters. We find thatthese poor clusters have baryon fractions ranging from 1% to 25%. Also,the radio sources within these clusters are probably thermally confinedby the ICM. Although four of our clusters have central X-ray luminosityexcesses, the implied cooling times are longer than a Hubble time. Weinterpret the central X-ray luminosity excesses as unresolved galaxyemission. We hypothesize that these poor clusters have recentlycollapsed out of large, loose clouds of galaxies. We believe that manyof the poor cluster properties are understandable in light of thishypothesis. First, four of these five clusters are embedded withinlarger Zwicky clusters. This may indicate that these large Zwickyclusters act as "incubators" of poor clusters. Second, the observedflat, broad velocity distributions may reflect the velocities associatedwith the larger-scale systems from which we believe that these poorclusters have collapsed. Third, some of these galaxies (such as NGC4061, within N79-299A) show signs of interactions with neighboringgalaxies with large relative velocities (~850 km/s). Fourth, theobserved ICM densities, coupled with velocity distributions which aresuggestive of unrelaxed systems, and the peculiar velocities of theradio galaxies may explain the ram pressure bending of the radio jets inthe HTs.
| Stellar dynamics in E+E pairs of galaxies. 2: Simulations and interpretation
We have presented in a companion article a kinematic study of three E+Egalaxy pairs, NGC741/742, 1587/1588 (CPG 99) and 2672/2673 (CPG 175). Wefind some evidence for perturbed velocity dispersion profiles. Theseperturbation features are now reported for 14 galaxies in theliterature. They occur, or require observations for detection, at largeradii where the S/N in the data is low. While observations of individualgalaxies are sometimes uncertain, the large number of objects where suchfeatures are suspected gives confidence that they are real. Theseperturbations can be attributed to projection effects contaminationalong the line of sight, or directly to the tidal interaction. We reportthe results of several self-gravitating simulations of unbound pairs inan effort to better understand these perturbations another genericfeatures of close E+E pairs reported in the literature. The modelsfrequently show off-center envelopes created by the asymmetry of tidalforces during interpenetrating encounters. The envelopes last for a few108 yrs, which explains the frequency of such features inobserved pairs. This phenomenon is stronger in the self-gravitatingsimulations than in the MTBA runs. U-shaped (and an equal number ofinverse U shaped velocity profiles are seen in the simulations, a resultof ablation in the outer envelopes. Simulations including inner galaxyrotation also preserve this feature, irrespective of the spin vectordirection in each galaxy. U-shape velocity structure is found to be arobust indicator of the ongoing interaction. All simulations showevidence for enhanced velocity dispersion between the galaxies even inthe case of simple superposition of two non interacting objects. Wetherefore conclude that this cannot be considered an unambiguousindicator of the interaction.
| Stellar dynamics in E+E pairs of galaxies. 1: NGC 741/742, 1587/88 and 2672/73. The data
We present a kinematic study ofthree E+E galaxy pairs, NGC, 741/642,1587/1588 (CPG 99) and 2672/2673 (CPG 175) All three pairs show asimilar morpological distortion (i.e. the off-centering of inner versusouter isphototes; Davoust & Prungniel 1988) which is ascribed to theongoing interaction. The data was obtained at the CFHT equipped with theHerzberg Spectrograph at a resolution of 0.88 A px-1 NGC741and 2673 show significant rotation along the apparent minor axis. Bothcomponents of CPG 99 rotate very fast (with no evidence for rotationalong the mirror axis of either component). None of the galaxies showabnormally high central velocity dispersion. We report some of the firstclear detections of well defined velocity dispersions curves forinteracting pairs. They show a systematic decrease with distance fromthe center, as expected for normal ellipticals. They do not show obviousheating in the outer parts as was previously reported. NGC 741 and 2672show, respectively, possible U and inverse U-shaped structure in theirvelocity profiles.
| The metallicity of elliptical galaxies.
Not Available
| Why do head-tail sources exist in poor clusters of galaxies?
In a continuing study of nearby (z approximately 0.02-0.05) radiosources in poor clusters of galaxies, we obtained Very Large Array (VLA)observations of four head-tail (HT) sources as probes of theintracluster environments: NGC 742, NGC 1044, NGC 4061, and NGC 7503.NGC 742 apparently has a companion, NGC 741, in the midst of itsextended tail structure. NGC 7503 and NGC 4061 have horseshoe shapesvery similar to the archetypal HT radio galaxy, NGC 1265. Thesestructures are remarkable because the sources are found in poor groups,where both the average density of the intracluster medium (ICM) and thevelocities of the galaxies (thus the ram pressures) are supposedly muchlower than in the rich clusters. Yet these poor groups have narrow-angletail (NAT) sources with the same general morphologies as those in richclusters. There is not much difference between our poor-cluster NATsources and rich-cluster NAT sources, in terms of jet radii ofcurvature, jet opening angles, internal ram pressures within the jets,jet luminosity as a fraction of total source luminosity, and ICMdensities. It appears that the HT phenomenon is remarkably similarbetween the poor clusters and the rich clusters because the localconditions near these sources within their clusters are similar. An ICMdensity typical of that found in poor clusters (approximately10-4/cc and a galaxy velocity typical of the rich clusters(approximately 600 km/s) provide sufficient ICM ram pressure to bendradio jets into NAT morphologies. One explanation for the high relativevelocities of the poor cluster HT galaxies is that these clusters aredynamically young and are still collapsing.
| Head tail sources in poor clusters of galaxies
Head tail sources in poor clusters of galaxies have been studied usingthe VLA at wavelengths of 6 cm and 20 cm. Four sources (NGC 742, NGC1044, NGC 4061 and NGC 7503) were selected from a larger sample fordetailed study. These sources represent an exceptional opportunity tostudy radio jets and their interaction with the cluster environment, indetail, because of their proximity (z ~ 0.02 to 0.04). NGC 4061 and NGC7503 exhibit narrow-angle-tail (NAT) morphologies similar to the classicrich cluster NAT, NGC 1265. NGC 742 has a complex morphology while NGC1044 shows a wide-angle-tail (WAT) morphology. The NAT morphologies arebelieved to be the result of the intracluster medium (ICM) rampressures. The poor cluster NATs are thus remarkable because the poorcluster ICM ram pressure is expected to be much lower, in general, thanin rich clusters. Comparisons of the results of the poor cluster sourceswith those of NGC 1265 should reveal much about the radio source andcluster environment interaction. Most of the parameters calculated suchas jet luminosity as a fraction of total source luminosity, jet openingangle, jet radius of curvature and the ICM density surrounding thesources are quite similar between NGC 1265 and the poor cluster sources.This is a very surprising result and we conclude that ICM densitiestypical of poor clusters and a galaxy relative velocity ~ 1000 km s(-1)provide sufficient ram pressure to bend radio jets into NATmorphologies. Spectral indices and polarization structures have beencompared between the rich and poor clusters and are found to be similaras well. The spectral indices in the poor cluster sources (alpha_ {jet}~ -0.6 and alpha_ {tail} \approx -0.8) are similar to those of the richcluster sources. The fractional polarizations vary from \sim 10% in thejets to \sim 30%$ in the tails similar to the rich cluster HT sources.The magnetic field structure appears to follow the general structureseen in low luminosity radio sources. The overall result is theremarkable similarity between the HT phenomenon in the rich and poorclusters.
| Why Do Head Tail Sources Exist In Poor Clusters Of Galaxies?
In a continuing study of radio sources in poor clusters of galaxies, wehave obtained VLA observations of four head-tail sources as probes ofthe intracluster environments: NGC 742, NGC 1044, NGC 4061, and NGC7503. These sources represent an exceptional opportunity for the studyof radio sources (and radio jets in particular). They are very nearby (z~ 0.02 to 0.04) and thus can be examined in great detail, much like NGC1265 in Perseus. The four sources contain some remarkable structures.NGC 742 apparently has a companion, NGC 741, in the midst of itsextended tail structure. NGC 7503 has a horseshoe shape very similar toNGC 1265. The NGC 4061 source maintains a very linear structure out toequal distances on either side of the galaxy center, where the lobes aresharply `swept back.' These structures are remarkable because thesources are found in poor groups, where both the density of the ICM andthe velocities of the galaxies (thus the ram pressures) are supposedlymuch lower than in the rich clusters. Yet these poor groups havenarrow-angle-tail sources (NATs) with the same general characteristicsas those in rich clusters. Our calculations indicate that there is notmuch difference between the poor cluster sources and NGC 1265 in ICMdensities, jet luminosity as a fraction of total source luminosity, andradii of curvature of the jets. These are very surprising results. Whilethe jets of two sources (NGC 4061 and NGC 7503) appear to have roughlytwice the opening angle of the NGC 1265 jet (which suggests lesserpressure-confinement in poor clusters), most of our evidence pointstoward remarkably similar environments for rich and poor cluster jets.
| A list of some corrections to Zwicky's Catalogue of Galaxies and Clusters of Galaxies
Not Available
| The shape of central regions in elliptical galaxies
Isophote analysis of high-resolution CCD images of a large sample of 75early-type, mostly elliptical, galaxies indicates that 25 of theseobjects harbor central bodies that appear separate from the rest of thegalaxy, and whose sizes are in the 2-10-arcsec range. Fifteen of these25 central substructures exhibit clearly 'pointed' isophotes, and allobjects known to have decoupled core kinematics exhibit a photometricsubstructure coinciding in radius with the kinematic substructure.Several arguments suggest that these central substructures are disks.
| Head-tail radio sources in poor clusters of galaxies : VLA Observations and comparisons to rich cluster HTs.
Not Available
| The low-mass extension of the fundamental plane of elliptical galaxies
A sample of 17 low-mass elliptical and elliptical-like galaxies wasbuilt with accurate photometric and spectroscopic data. This sample,covering a wide range in surface brightness, is in or near the low-massextension of the 'fundamental plane' defined by bright ellipticals, butshows a scatter which cannot be accounted for by measurement errors andwhich is thus probably due to a large variety of internal structures.Extending the analysis to globular clusters and dwarf spheroidals, it isfound that they are also near or within the fundamental plane, exceptfor two dwarfs suspected of having a high M/L. The range thus covered is20 in absolute magnitude. These results suggest that the fundamentalplane is a robust representation of the virial theorem, valid forellipticals and for other types of pressure-supported stellar systems aswell. It also indicates that the standard parameter relations are littlesensitive to specific formation processes, local environment, andstructural details, that may have an effect at a subtler level.
| Spectrophotometry of Normal Galaxies Nuclei
Not Available
| Hierarchical pairs and the evolution of elliptical galaxies
Consideration is given to the photometric and kinematic characteristicsof galaxies in eight close pairs of ellipticals consisting of a brightgalaxy and a compact faint companion. Redshifts and central velocitydispersions are determined from high-dispersion spectra. Evidence ispresented for interaction in the pairs. The mean surface brightness andcentral velocity dispersions of the galaxies are tightly correlated withluminosity. Two sequences of ellipticals are defined, based on the workof Michard (1979). The galaxies in this study are sequence I galaxies.It is suggested that sequence II galaxies evolve toward sequence Igalaxies through dissipation of internal energy and loss of angularmomentum.
| A morphological effect in pairs of elliptical galaxies
A morphological signature of gravitational interaction has been found ina sample of 50 close pairs of elliptical galaxies. The halos of bothgalaxies in five, and possibly more, pairs are off-center and thisoffset is symmetric with respect to the center of the pair. Among thephysical mechanisms that could be responsible for this morphologicaleffect, the displacement of the nucleus with respect to the halo in eachgalaxy or the formation of an asymmetric tidal bulge are the mostlikely. The observation of this effect should provide constraints on theinternal dynamics of elliptical galaxies, as well as on the dynamics ofpairs.
| Spectroscopy and photometry of elliptical galaxies. III - UBV aperture photometry, CCD photometry, and magnitude-related parameters
Photoelectric aperture photometry of nearly 2000 individual observationsof 449 elliptical galaxies combined with published measurements usingthe self-consistent UVB color catalog developed by Burstein et al.(1987) are presented. The data are placed on a standard magnitude andcolor system, and 'total' magnitudes and effective diameters are derivedby comparison with the standard elliptical magnitude growth curve. Agraphical representation of the standard growth curve and the residualsfrom it for each galaxy are given, and a new diameter measurement Dn ispresented which can be measured reliably for elliptical galaxies andserves as an accurate distance indicator when combined with centralvelocity dispersion. Individual magnitudes, surface brightnesses,effective diameters, and values of Dn are summarized for each galaxy incatalog form.
| A VLA 20 CM survey of poor groups of galaxies
The paper reports on VLA 20 cm observations of an extensive sample ofgalaxies in 139 poor groups. These groups, composed of galaxies down tothe limit of the Zwicky et al. (CGCG) catalog, were chosen using apercolation algorithm set at a high surface-density threshold.Approximately 50 percent of the groups have measured redshifts. Thesegroups were surveyed using a 'snapshot' mode of the VLA with aresolution of about 13 arcsec. Analysis of the resulting radio andoptical properties reveals that the presence of a nearby companiongalaxy has an important role in generating radio emission in a galaxy.CCD observations of two radio-loud, disturbed galaxies with companionsare presented and are used to discuss models of radio-source production.Nine tailed radio galaxies are found in the poor groups, which is muchmore than had been expected from previous work on rich clusters and fromtheoretical models. The paper discusses previous statistical biases andproposes a method for bending head-tail sources in poor groups. From theconfinement of extended radio features associated with tailed sources,the presence of a substantial intracluster medium that should radiatesignificantly at soft-X-ray energies is predicted.
| Gravitational amplification of brightest cluster galaxies by foreground clusters
It is suggested that the Hubble diagram of brightest cluster galaxies(BCG) is strongly affected by gravitational amplifications due toforeground clusters of galaxies. Galaxies from Kristian et al.'s (1978)sample are placed with respect to foreground Zwicky clusters and theirdeviations from the mean magnitude-redshift relation are compared to thepredictions of Ricci gravitational amplification formula. It is foundthat the gravitational brightness increase reaches some tenth ofmagnitudes for these BCGs and that it has induced a strong selectioneffect artificially increasing the deceleration parameter q0.Once these effects are accounted for the Hubble diagram value ofq0 might agree with the low values obtained for the densityparameter sigma0 (0.1-0.2) without any need for rapidevolution.
| The nuclear colours of weakly active elliptical galaxies
Long-slit spectroscopy has been obtained of 15 elliptical radiogalaxies. Although many of these show radio emission from their nuclei,and might be expected to have weak H-alpha emission lines, this does notseem to be associated with a nonthermal ionizing continuum extending tothe optical region. Rather, most nuclear regions are redder than thesurrounding regions by a larger amount than expected from thecolor-aperture effect, although about 20 percent of the nuclei arebluer. Evidence from the magnesium index suggests that the reddening isdue to abundance effects, and the colors then imply excess metallicitiesby factors of about 1.5. These colors and metallicities are not asexpected from straightforward predictions by Bailey's (1982) model foractive nuclei. It seems more likely that the apparent excess rednessover the standard color aperture effect only shows that the colorgradient is generally steeper in the nucleus than previously thought,rather than a distinct stellar population being responsible. The bluenuclei may contain either a power-law continuum source, or young stars.
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