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Gas stripping in galaxy groups - the case of the starburst spiral NGC 2276
Ram-pressure stripping of galactic gas is generally assumed to beinefficient in galaxy groups due to the relatively low density of theintragroup medium (IGM) and the small velocity dispersions of groups. Totest this assumption, we obtained Chandra X-ray data of the starburstingspiral NGC 2276 in the NGC 2300 group of galaxies, a candidate for astrong galaxy interaction with hot intragroup gas. The data reveal ashock-like feature along the western edge of the galaxy and a lowsurface brightness tail extending to the east, similar to the morphologyseen in other wavebands. Spatially resolved spectroscopy shows that thedata are consistent with intragroup gas being pressurized at the leadingwestern edge of NGC 2276 due to the galaxy moving supersonically throughthe IGM at a velocity ~850 km s-1. Detailed modelling of thegravitational potential of NGC 2276 shows that the resulting rampressure could significantly affect the morphology of the outer gas discbut is probably insufficient to strip large amounts of cold gas from thedisc. We estimate the mass-loss rates due to turbulent viscous strippingand starburst outflows being swept back by ram pressure, showing thatboth mechanisms could plausibly explain the presence of the X-ray tail.Comparison to existing HI measurements shows that most of the gasescaping the galaxy is in a hot phase. With a total mass-loss rate of~5Msolaryr-1, the galaxy could be losing itsentire present HI supply within a Gyr. This demonstrates that theremoval of galactic gas through interactions with a hot IGM can occurrapidly enough to transform the morphology of galaxies in groups.Implications of this for galaxy evolution in groups and clusters arebriefly discussed.
| The hot, warm and cold gas in Arp 227 - an evolving poor group
Arp 227 represents a prototypical example of an interacting mixed pairof galaxies located in a low-density environment. We investigate the gasproperties of the pair in the X-ray, Hα, HI and CO bands. We alsodetect two additional members of the group in HI which indicates thatthe pair constitutes the dominant members of a loose group.The HI distribution shows a tail of gas that connects the spiral member,NGC 470, to the lenticular, NGC 474, showing that the two main membersare currently undergoing interaction. The Hα emission reveals thepresence of secondary components at the centre of NGC 470, superposed onthe main component tracing the rotation of the galaxy. This latter mapsa nearly unperturbed velocity field. The dominant, nearly unperturbedtrend of the kinematics is confirmed by CO observations, althoughrestricted to the centre of the galaxy. The X-ray luminosity of NGC 470is comparable with that of a `normal' spiral galaxy. NGC 474 on theother hand is very gas-poor and has not been detected in Hα. ItsX-ray luminosity is consistent with the low end of the expected emissionfrom discrete sources.Arp 227 as a loose group shows several signatures of galaxy-galaxyinteraction. Our observations suggest the presence of signatures ofinteraction in the overall kinematics of the spiral companion. Theongoing interaction is clearly visible only in the outer HI halo of NGC470. While the large shell system of NGC 474 could be associated with anaccretion event, the secondary components in the Hα profile in thecentre of NGC 470 could be due to the interaction with the companion.The low X-ray luminosity of NGC 470 seems to be a characteristic ofdynamically young systems. All the above evidence suggest that Arp 227is an evolving group in the early phase of its evolution and that itsdrivers are the accretion of faint galaxies and the ongoing large-scaleinteraction between NGC 470 and 474.
| Redshift-Distance Survey of Early-Type Galaxies: Spectroscopic Data
We present central velocity dispersions and Mg2 line indicesfor an all-sky sample of ~1178 elliptical and S0 galaxies, of which 984had no previous measures. This sample contains the largest set ofhomogeneous spectroscopic data for a uniform sample of ellipticalgalaxies in the nearby universe. These galaxies were observed as part ofthe ENEAR project, designed to study the peculiar motions and internalproperties of the local early-type galaxies. Using 523 repeatedobservations of 317 galaxies obtained during different runs, the dataare brought to a common zero point. These multiple observations, takenduring the many runs and different instrumental setups employed for thisproject, are used to derive statistical corrections to the data and arefound to be relatively small, typically <~5% of the velocitydispersion and 0.01 mag in the Mg2 line strength. Typicalerrors are about 8% in velocity dispersion and 0.01 mag inMg2, in good agreement with values published elsewhere.
| Redshift-Distance Survey of Early-Type Galaxies: Circular-Aperture Photometry
We present R-band CCD photometry for 1332 early-type galaxies, observedas part of the ENEAR survey of peculiar motions using early-typegalaxies in the nearby universe. Circular apertures are used to tracethe surface brightness profiles, which are then fitted by atwo-component bulge-disk model. From the fits, we obtain the structuralparameters required to estimate galaxy distances using theDn-σ and fundamental plane relations. We find thatabout 12% of the galaxies are well represented by a pure r1/4law, while 87% are best fitted by a two-component model. There are 356repeated observations of 257 galaxies obtained during different runsthat are used to derive statistical corrections and bring the data to acommon system. We also use these repeated observations to estimate ourinternal errors. The accuracy of our measurements are tested by thecomparison of 354 galaxies in common with other authors. Typical errorsin our measurements are 0.011 dex for logDn, 0.064 dex forlogre, 0.086 mag arcsec-2 for<μe>, and 0.09 for mRC,comparable to those estimated by other authors. The photometric datareported here represent one of the largest high-quality and uniformall-sky samples currently available for early-type galaxies in thenearby universe, especially suitable for peculiar motion studies.Based on observations at Cerro Tololo Inter-American Observatory (CTIO),National Optical Astronomy Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation (NSF);European Southern Observatory (ESO); Fred Lawrence Whipple Observatory(FLWO); and the MDM Observatory on Kitt Peak.
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
| Nearby Optical Galaxies: Selection of the Sample and Identification of Groups
In this paper we describe the Nearby Optical Galaxy (NOG) sample, whichis a complete, distance-limited (cz<=6000 km s-1) andmagnitude-limited (B<=14) sample of ~7000 optical galaxies. Thesample covers 2/3 (8.27 sr) of the sky (|b|>20deg) andappears to have a good completeness in redshift (97%). We select thesample on the basis of homogenized corrected total blue magnitudes inorder to minimize systematic effects in galaxy sampling. We identify thegroups in this sample by means of both the hierarchical and thepercolation ``friends-of-friends'' methods. The resulting catalogs ofloose groups appear to be similar and are among the largest catalogs ofgroups currently available. Most of the NOG galaxies (~60%) are found tobe members of galaxy pairs (~580 pairs for a total of ~15% of objects)or groups with at least three members (~500 groups for a total of ~45%of objects). About 40% of galaxies are left ungrouped (field galaxies).We illustrate the main features of the NOG galaxy distribution. Comparedto previous optical and IRAS galaxy samples, the NOG provides a densersampling of the galaxy distribution in the nearby universe. Given itslarge sky coverage, the identification of groups, and its high-densitysampling, the NOG is suited to the analysis of the galaxy density fieldof the nearby universe, especially on small scales.
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| Testing the physical reality of binaries and compact groups. Properties of early-type galaxies in groups with diffuse X-ray emission
We present data on the stellar kinematics of the brightest ellipticalsin HCG 62, HCG 68, NGC 2300 groups. Moreover, we report on ASCA GIS aswell as optical observations of the early-type member of the pair K 416(NGC 5480/5481) similar, in many respects, to the dominant pair in theNGC 2300 group. The kinematics of HCG 62a/b and HCG 68a/b indicate thatthey are not interacting pairs. HCG 62a is instead possibly interactingwith HCG 62c, as suggested by their morphology (see Mendes de Oliveiraand Hickson 1994), In contrast to the results for the NGC 2300/2276group, ASCA observations indicate a significant absence of hot gas inthe K 416 system. Whether the NGC 2300 multiplet is viewed as a loosegroup or as a massive E+S pair, it is clear that similar morphologicalentities do not always show similar X-ray properties. Under thehypothesis that diffuse X-ray emission marks the group potential, weconsider the possibility that K 416 is an unbound encounter. In thisscenario, morphological distortions are indicative of the ongoinginteraction, but are only circumstantially correlated with the physicalreality of a pair/multiplet as a bound system. Based on observationsobtained at 1.93m telescope of Observatoire de Haute Provence operatedby INSU.
| An Einstein X-Ray Survey of Optically Selected Galaxies. I. Data
We present the results of a complete Einstein imaging proportionalcounter X-ray survey of optically selected galaxies from theShapley-Ames Catalog, the Uppsala General Catalogue, and the EuropeanSouthern Observatory Catalog. Well-defined optical criteria are used toselect the galaxies, and X-ray fluxes are measured at the opticallydefined positions. The result is a comprehensive list of X-ray detectionand upper limit measurements for 1018 galaxies. Of these, 827 haveeither independent distance estimates or radial velocities. Associatedoptical, redshift, and distance data have been assembled for thesegalaxies, and their distances come from a combination of directlypredicted distances and those predicted from the Faber-Burstein GreatAttractor/Virgocentric infall model. The accuracy of the X-ray fluxeshas been checked in three different ways; all are consistent with thederived X-ray fluxes being of <=0.1 dex accuracy. In particular,there is agreement with previously published X-ray fluxes for galaxiesin common with a 1991 study by Roberts et al. and a 1992 study byFabbiano et al. The data presented here will be used in further studiesto characterize the X-ray output of galaxies of various morphologicaltypes and thus to enable the determination of the major sourcescontributing to the X-ray emission from galaxies.
| ROSAT observations of compact groups of galaxies
We have systematically analyzed a sample of 13 new and archival ROSATPosition Sensitive Proportional Counter (PSPC) observations of compactgroups of galaxies: 12 Hickson compact groups plus the NCG 2300 group.We find that approximately two-thirds of the groups have extended X-rayemission and, in four of these, the emission is resolved into diffuseemission from gas at a temperature of kT approximately 1 keV in thegroup potential. All but one of the groups with extended emission have aspiral fraction of less than 50%. The baryon fraction of groups withdiffuse emission is 5%-19%, similar to the values in clusters ofgalaxies. However, with a single exception (HCG 62), the gas-to-stellarmass ratio in our groups has a median value near 5%, somewhat greaterthan the values for individual early-type galaxies and two orders ofmagnitude than in clusters of galaxies. The X-ray luminosities ofindividual group galaxies are comparable to those of similar fieldgalaxies, although the LX-LB relation forearly-type galaxies may be flatter in compact groups than in the field.
| Magnetic fields in interacting galaxies: NGC 2276.
We observed the radio continuum emission of the interacting spiralgalaxy NGC 2276 with the VLA and the WSRT. i) The spectrum of thenonthermal emission is steeper than in normal spiral galaxies andreveals a gradual steepening with frequency, as expected for strongsynchrotron losses of cosmic-ray electrons. ii) The radio emission isstronger than can be accounted for by the star formation rate or theassumption of equipartition between cosmic rays and magnetic field sothat NGC 2276 clearly deviates from the far-infrared-radio continuumcorrelation: The radio luminosity is ~2.5 times higher than expectedfrom the far-infrared luminosity. The total field strength over thewhole disk (B_t_) of ~19μG is enhanced by a factor ~1.6 compared towhat is expected from the far-infrared luminosity. iii) The strongestlinearly polarized emission is found south of the line-of-nodes alongthe southwestern sharp edge. The uniform magnetic field component (B_u_)is strongest there (~10μG) and shows a spiral-like structure. iv) Thestrong overall B_t_, the locally strong and aligned B_u_, and the longtail of uniform field towards the southeast are probably caused by theinteraction between NGC 2276 and NGC 2300 by compression of the gas andthe frozen-in field and/or excitation of a strong galactic dynamo,possibly operating in a non-axisymmetric mode.
| Diffuse X-ray emission from the NGC 2300 group of galaxies - Implications for dark matter and galaxy evolution in small groups
The discovery of diffuse X-ray emission from the NGC 2300 group ofgalaxies using the ROSAT Position Sensitive Proportional Counter isreported. The gas distributions is roughly symmetric and extends to aradius of at least 0.2/h(50) Mpc. A Raymond-Smith hot plasma modelprovides an excellent fit the X-ray spectrum with a best-fit valuetemperature of 0.9 + -/15 or - 0.14 keV and abundance 0.06 + 0/.12 or -0.05 solar. The assumption of gravitational confinement leads to a totalmass of the group of 3.0 + 0.4 or - 0.5 x 10 exp 13 solar. Baryons canreasonably account for 4 percent of this mass, and errors could pushthis number not higher than 10-15 percent. This is one of the strongestpieces of evidence that dark matter dominates small groups such as thisone. The intragroup medium in this system has the lowest metal abundanceyet found in diffuse gas in a group or cluster.
| General study of group membership. II - Determination of nearby groups
We present a whole sky catalog of nearby groups of galaxies taken fromthe Lyon-Meudon Extragalactic Database. From the 78,000 objects in thedatabase, we extracted a sample of 6392 galaxies, complete up to thelimiting apparent magnitude B0 = 14.0. Moreover, in order to considersolely the galaxies of the local universe, all the selected galaxieshave a known recession velocity smaller than 5500 km/s. Two methods wereused in group construction: a Huchra-Geller (1982) derived percolationmethod and a Tully (1980) derived hierarchical method. Each method gaveus one catalog. These were then compared and synthesized to obtain asingle catalog containing the most reliable groups. There are 485 groupsof a least three members in the final catalog.
| HI observations of galaxies in nearby Zwicky clusters
The results of a long term project of H I observations of galaxieswithin the boundaries of nearby Zwicky clusters are presented. Thedetection rate is rather low (233 out of 618, i.e., 38 percent) ascompared to other surveys carried out recently. Most of the radialvelocities of the detected galaxies are new determinations. The largespread in radial velocities for many of these clusters is a strongindication for the presence of several foreground and/or backgroundgalaxies.
| Radial velocities of galaxies in the neighborhood of groups of galaxies. III
New determinations of the radial velocities for 31 galaxies in theneighborhood of groups of galaxies are reported. The observations aredescribed, and data are given for the galaxies in a table, including thenumber of spectra used, the radial velocity of each galaxy corrected forthe motion of the sun and the earth, the internal error in themeasurement, estimates of the quality of the spectra, the spectral linesused in the calculation of the velocities, the radial velocitiesdetermined by other authors, and the membership of the objects ingalactic systems. The data from other authors are used to determine thereal (external) accuracy of the present determinations, and goodagreement is found.
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