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|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.
|Fundamental plane distances to early-type field galaxies in the South Equatorial Strip. I. The spectroscopic data|
Radial velocities and central velocity dispersions are derived for 238E/S0 galaxies from medium-resolution spectroscopy. New spectroscopicdata have been obtained as part of a study of the Fundamental Planedistances and peculiar motions of early-type galaxies in three selecteddirections of the South Equatorial Strip, undertaken in order toinvestigate the reality of large-scale streaming motion; results of thisstudy have been reported in Müller et al. (1998). The new APM SouthEquatorial Strip Catalog (-17fdg 5 < delta < +2fdg 5) was used toselect the sample of field galaxies in three directions: (1) 15h10 -16h10; (2) 20h30 - 21h50; (3) 00h10 - 01h30. The spectra obtained have amedian S/N per Å of 23, an instrumental resolution (FWHM) of ~ 4Å, and the spectrograph resolution (dispersion) is ~ 100 kms-1. The Fourier cross-correlation method was used to derivethe radial velocities and velocity dispersions. The velocity dispersionshave been corrected for the size of the aperture and for the galaxyeffective radius. Comparisons of the derived radial velocities with datafrom the literature show that our values are accurate to 40 kms-1. A comparison with results from Jo rgensen et al. (1995)shows that the derived central velocity dispersion have an rms scatterof 0.036 in log sigma . There is no offset relative to the velocitydispersions of Davies et al. (1987).
|Sinking Satellite Disk Galaxies. I. Shell Formation Preceded by Cessation of Star Formation|
Detailed numerical simulations have been carried out on the sinking of agas-rich disk galaxy into a large elliptial (spherical) galaxy. Bothelliptical and spiral galaxies have been modeled as self-gravitatingparticle systems. The interstellar gas component in the disk has beenmodeled as a system of inelastic cloud particles dissipating kineticenergy in mutual collisions. Star formation processes and gasconsumption owing to star formation are included in the numerical code.Sinking on a radial or slightly retrograde orbit has been found toproduce regular shell structures (i.e., without loops or filaments) madefrom the disk material. In this shell formation, no significantsegregation of the stellar and gaseous components occurs. Globaldistribution of the gas clouds is similar to that of the stars, thoughthe latter make clearer shells. Star formation is turned off well beforethe shells develop, because gas clouds are widely scattered and the gasdensity is much decreased. We propose that the "poststarburst" nucleioften observed in shell galaxies are not necessarily the result ofpreceding starbursts, as is widely believed, but could be the result ofthis drastic truncation of star formation activity.
|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.
|A search for secondary nuclei in shell galaxies|
We present the results of our search for secondary nuclei in shellgalaxies. The most recent accretion model simulations of shell galaxyformation, by Salmon, Quinn, & Warren, have included the effects ofdynamical friction and tidal stripping on a captured secondary for thefirst time. In their simulation, the secondary galaxy is disruptedslowly throughout the shell forming process, and should therefore bevisible as a secondary nucleus for a significant fraction of the shelllifetime. We have obtained Charge Couple Device (CCD) images of 29 shellgalaxies, often in two colors, in order to search for secondary nuclei.Identification of possible secondary nuclei was carried out bysubtracting a smooth model of the primary elliptical and carefullyexamining the residual image. We found six galaxies with possiblesecondary nuclei that could be responsible for the shells within theframework of the Salmon, Quinn, & Warren (sinking satellite)accretion model. Foreground contamination by stars or galaxies impliesthat this detection is an upper limit. Based on our resutls, we estimatean upper limit of 200 Myr for the typical lifetime for a secondarynucleus and discuss the frequency of secondary nuclei expected from theweak interaction model and the major merger model.
|Stellar populations in shell galaxies|
UBV surface photometry of the shell galaxies Arp 230, NGC 7010, and Arp223 is presented and all are found to be the result of mergers. In Arp230. the merger of two spirals induced a burst of star formation whichhas declined in strength since the collision. The remnant seems likelyto become elliptical. NGC 7010 has very red shells, perhaps redder thanthe galaxy itself. Evolution of the stellar content of these shells maybe important. Arp 223 has shell colors which are slightly bluer than themain body of the galaxy, consistent with an origin in an Sb. Dataindicate that the timescale since the interaction is typicallyapproximately equal to 1 Gyr. Moreover, those ellipticals that have beenformed by mergers should have complex stellar populations with acomponent formed in the collision. This extra component might bedetectable by the methods of empirical population synthesis, at least tosome age limit, making it possible to investigate the relative numbersof ellipticals formed in this manner by methods other than morphology.
|The components of mid- and far-infrared emission from S0 and early-type shell galaxies|
The IRAS database has been used to study detections of about 150early-type elliptical and S0 galaxies exhibiting a shell structure. Nostrong evidence for the expected enhancement of either star formationrates or heating of the interstellar medium is found. It is suggestedthat for some of the sample galaxies either a contribution from warmdust surrounding evolved stars or emission from an active nucleus may besignificant.
|Spectra of shell ellipticals - Redshifts, velocity dispersions and evidence for recent nuclear star formation|
Nuclear spectra are presented for 100 of the galaxies in the Malin andCarter (1983) catalog. Redshifts and velocity dispersions are given forsome of the galaxies. It is found that about 10 percent of the samplegalaxies have colors, absolute magnitudes, and spectra characteristic ofthe poststarburst galaxies in high reshift clusters. It is suggestedthat the more distant poststarburst galaxies may have undergone mergeror accretion events.
|Radio emission from shell elliptical galaxies|
A subset of the Malin & Carter catalogue of 'shell ellipticalgalaxies' has been surveyed at 20 and 6 cm with the VLA. Forty-two percent of the galaxies were detected, down to a 6-cm flux density limit ofabout 0.6 mJy. This detection rate does not differ significantly from anormal population of mixed elliptical and S0 galaxies. Most of the radiodetections, which are sources coincident with the galaxy nuclei, areunresolved by the C-array and so have angular sizes of less than 3arcsec. The sample contains objects with a wide variety of opticalappearances, suggesting that shell galaxies are not a homogenoeus classwith uniform physical characteristics.
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