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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.
| Serendipitous detection of galaxies behind the Milky Way from the DENIS survey
A search has been undertaken at Lyon Observatory to identify, by eye,galaxy candidates at galactic latitudes lower than +/-15 degrees on theDENIS J and K-band images. This paper presents a list of 2028 objectsthat were serendipitously detected throughout the DENIS survey.Cross-identification with galaxy entries in the LEDA Database has beenperformed. Comparison with the second release of the 2MASS survey led toa satisfactory agreement of J band magnitudes (std. dev. = 0.3 mag). Thedistribution of galaxy candidates along the Galactic Plane shows aconcentration near the galactic longitude l=305 deg . As a by product ofthis inspection of J and K images some interesting galactic objects werefound (star clusters and nebulae). Based on observations collected atthe European Southern Observatory, La Silla, Chile. The catalog (Table1) is only available in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.188.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/387/1
| 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.
| Galaxy coordinates. II. Accurate equatorial coordinates for 17298 galaxies
Using images of the Digitized Sky Survey we measured coodinates for17298 galaxies having poorly defined coordinates. As a control, wemeasured with the same method 1522 galaxies having accurate coordinates.The comparison with our own measurements shows that the accuracy of themethod is about 6 arcsec on each axis (RA and DEC).
| Southern Galaxy Catalogue.
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