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Central stellar populations of early-type galaxies in low-density environments
Following the pilot study of Kuntschner et al., we have investigated theproperties of a volume- and magnitude-limited (cz <10000kms-1, bJ < 16) sample of early-typegalaxies that were carefully selected from the Anglo-AustralianObservatory (AAO) two-degree field galaxy redshift survey (2dFGRS) tohave no more than one and five companions within 1 and 2Mpc,respectively. We used images from the Digital Sky Survey (DSS) toconfirm the E/S0 morphologies. We augmented this sample with fieldgalaxies from Colbert et al. selected as having no neighbour within 1Mpcand +/-1000kms-1. We present spectroscopic observations of 22galaxies from the combined sample, from which central velocitydispersions and the Lick stellar population indices were measured. Aftercarefully correcting the spectra for nebular emission, we derivedluminosity-weighted ages, metallicities and α-element abundanceratios. We compare these isolated galaxies with samples of early-typegalaxies in the Virgo and Coma clusters, and also with the previoussample of galaxies in low-density regions of Kuntschner et al. We findthat galaxies in low-density environments are younger and have a greaterspread of ages compared to cluster galaxies. They also show a widerrange of metallicities at a given velocity dispersion than clustergalaxies, which display only supersolar metallicities. On averagecluster, as well as, isolated galaxies show non-solar abundance ratiosin α elements, suggesting that, independent of galacticenvironment, star formation occurred on short time-scales. However, theabundance ratios for our low-density environment sample galaxies do notscale with the stellar velocity dispersion as observed in clusters. Infact we detect a large spread at a given velocity dispersion evenreaching solar abundance ratios. The metallicity of isolated early-typegalaxies is found to correlate weakly with σ. We reason thatearly-type galaxies in low-density environments experiencedmerging-induced star formation episodes over a longer and more recentperiod of time compared to a cluster environment, and speculate that aconsiderable fraction of their stars formed out of low-metallicity halogaseous material during the slow growth of a stellar disc betweenmerging events.
| The Optical and Near-Infrared Morphologies of Isolated Early-Type Galaxies
To study early-type galaxies in their simplest environments, we haveconstructed a well-defined sample of 30 isolated galaxies. The samplecontains all early-type galaxies listed in the Third Reference Catalogueof Bright Galaxies (RC3) with no other cataloged galaxy with a knownredshift lying within a projected radius of 1h-1100 Mpc and +/-1000 km s-1 (where weuse the recession velocities in the RC3). We have obtained optical andnear-infrared images of 23 of the galaxies and of a comparison sample of13 early-type galaxies in X-ray-detected poor groups of galaxies. Wehave applied the techniques of unsharp-masking, galaxy model division,and color maps to search for morphological features that might provideclues to the evolution of these galaxies. Evidence for dust features isfound in approximately 75% of both the isolated and group galaxies (17of 22 and 9 of 12, respectively). However, shells or tidal features aremuch more prevalent in our isolated sample than in our group sample (9of 22=41% vs. 1 of 12=8%, respectively). The isolation and colors ofthese shell galaxies make it unlikely that tidal interactions orasymmetric star formation are the causes of such features. One modelthat is not ruled out is that mergers produce the shells. If shells anddust are both merger signatures, the absence of shells in groupelliptical galaxies implies that shells (1) form more easily, (2) areyounger, and/or (3) are longer lived in isolated environments.
| 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.
| 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.
| Galaxy structures in the Hercules region
216 redshifts have been obtained in a region of 981 sq deg south of theHercules supercluster. 172 of these redshifts are of galaxies withmpg less than or equal to 15.1, 110 of which had no previousvelocity measurement. 44 new redshifts are of galaxies fainter thanmpg = 15.1. With these new data we have been able to define asample in a vast region (approximately 1700 sq deg) around Herculeslimited to mpg less than or equal to 15.1 with a velocitycompleteness of 81.5%. 189 galaxies have been morphologically classifiedso that all galaxies in the sample with known velocity now also haveknown morphology. The magnitude limited sample, including 556 galaxies,is then used to identify and describe galaxy structures in the region.We find that the overdense volume is small, that its overall appearanceis that of a coral branch floating in a sea of nothing and that earlyand late type galaxies defined different structures.
| The distance to the Hercules supercluster. I - Basic data for 220 galaxies in CGCG field 108.
Abstract image available at:http://adsabs.harvard.edu/cgi-bin/nph-bib_query?1986ApJS...62..255B&db_key=AST
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