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|Companions to Isolated Elliptical Galaxies: Revisiting the Bothun-Sullivan Sample|
We investigate the number of physical companion galaxies for a sample ofrelatively isolated elliptical galaxies. The NASA/IPAC ExtragalacticDatabase (NED) has been used to reinvestigate the incidence of satellitegalaxies for a sample of 34 elliptical galaxies, first investigated byBothun & Sullivan using a visual inspection of Palomar Sky Surveyprints out to a projected search radius of 75 kpc. We have repeatedtheir original investigation using data cataloged in NED. Nine of theseelliptical galaxies appear to be members of galaxy clusters; theremaining sample of 25 galaxies reveals an average of +1.0+/-0.5apparent companions per galaxy within a projected search radius of 75kpc, in excess of two equal-area comparison regions displaced by 150-300kpc. This is significantly larger than the +0.12+/-0.42companions/galaxy found by Bothun & Sullivan for the identicalsample. Making use of published radial velocities, mostly availablesince the completion of the Bothun-Sullivan study, identifies thephysical companions and gives a somewhat lower estimate of +0.4companions per elliptical galaxy. This is still 3 times larger than theoriginal statistical study, but given the incomplete and heterogeneousnature of the survey redshifts in NED, it still yields a firm lowerlimit on the number (and identity) of physical companions. An expansionof the search radius out to 300 kpc, again restricted to sampling onlythose objects with known redshifts in NED, gives another lower limit of4.5 physical companions per galaxy. (Excluding five elliptical galaxiesin the Fornax Cluster, this average drops to 3.5 companions perelliptical.) These physical companions are individually identified andlisted, and the ensemble-averaged radial density distribution of theseassociated galaxies is presented. For the ensemble, the radial densitydistribution is found to have a falloff consistent withρ~R-0.5 out to approximately 150 kpc. For non-FornaxCluster companions the falloff continues out to the 300 kpc limit of thesurvey. The velocity dispersion of these companions is found to reach amaximum of 350 km s-1 at around 120 kpc, after which theyfall at a rate consistent with Keplerian falloff. This falloff may thenindicate the detection of a cut-off in the mass-density distribution inthe elliptical galaxies' dark matter halo at ~100 kpc.
|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.
|The RSA survey of dwarf galaxies, 1: Optical photometry|
We present detailed surface photometry, based on broad B-band chargecoupled device (CCD) images, of about 80 dwarf galaxies. Our samplerepresents approximately 10% of all dwarf galaxies identified in thevicinity of Revised Shapley-Ames (RSA) galaxies on high resolution bluephotographic plates, referred to as the RSA survey of dwarf galaxies. Wederive global properties and radial surface brightness profiles, andexamine the morphologies. The radial surface brightness profiles ofdwarf galaxies, whether early or late type, display the same varietiesin shape and complexity as those of classical giant galaxies. Only a feware well described by a pure r1/4 law. Exponential profilesprevail. Features typical of giant disk galaxies, such as exponentialprofiles with a central depression, lenses, and even, in one case (IC2041), a relatively prominent bulge are also found in dwarf galaxies.Our data suggest that the central region evolves from being bulge-like,with an r1/4 law profile, in bright galaxies to a lens-likestructure in dwarf galaxies. We prove detailed surface photometry to bea helpful if not always sufficient tool in investigating the structureof dwarf galaxies. In many cases kinematic information is needed tocomplete the picture. We find the shapes of the surface brightnessprofiles to be loosely associated with morphological type. Our samplecontains several new galaxies with properties intermediate between thoseof giant and dwarf ellipticals (but no M32-like objects). This showsthat such intermediate galaxies exist so that at least a fraction ofearly-type dwarf ellipticals is structurally related to early-typegiants instead of belonging to a totally unrelated, disjunct family.This supports an origin of early-type dwarf galaxies as originally moremassive systems that acquired their current morphology as a result ofsubstantial, presumable supernova-driven, mass loss. On the other hand,several early-type dwarfs in our sample are merger candidates. Mergerevents may lead to anisotropic velocity distributions in systems of anyluminosity, including dwarfs. The RSA sample of dwarf galaxies is morelikely to contain mergers because, in contrast to earlier dwarf galaxysurveys that have focused on clusters and rich groups of galaxies, theRSA dwarfs are typically located in low density environments. Theoccurrence of mergers among dwarf galaxies is of interest in connectionwith the rapid evolution of faint blue galaxy counts at redshift z lessthan 1 which suggests that dwarf galaxies were about five times morenumerous in the recent past.
|Low-luminosity companions of early-type galaxies|
Optical redshifts of nine low-luminosity companions of early-type giantgalaxies are obtained as part of a study of the small-scale clusteringproperties of dwarf galaxies. The velocity differences between thelow-luminosity companions and the nearest giant are typically less than300 km/s in the case of 'field' giants and larger for members of groupsof galaxies. Early-type companions of isolated giant galaxies may begravitationally bound satellites which can be used as dynamical probesof dark halos, while in groups of galaxies they are free-floating in thegeneral potential well of the group. The spectra of one nucleated dwarfelliptical and two dwarfs of composite dE and BCD morphology showevidence of recent star formation.
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