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|HI content in galaxies in loose groups|
Gas deficiency in cluster spirals is well known and ram-pressurestripping is considered the main gas removal mechanism. In some compactgroups too gas deficiency is reported. However, gas deficiency in loosegroups is not yet well established. Lower dispersion of the membervelocities and the lower density of the intragroup medium in small loosegroups favour tidal stripping as the main gas removal process in them.Recent releases of data from the HI Parkes All-Sky Survey (HIPASS) andcatalogues of nearby loose groups with associated diffuse X-ray emissionhave allowed us to test this notion. In this paper, we address thefollowing questions: (i) do galaxies in groups with diffuse X-rayemission statistically have lower gas content compared to the ones ingroups without diffuse X-ray emission? (ii) does HI deficiency vary withthe X-ray luminosity, LX, of the loose group in a systematicway? We find that (i) galaxies in groups with diffuse X-ray emission, onaverage, are HI deficient, and have lost more gas compared to those ingroups without X-ray emission; the latter are found not to havesignificant HI deficiency; (ii) no systematic dependence of the HIdeficiency with LX is found. Ram-pressure-assisted tidalstripping and evaporation by thermal conduction are the two possiblemechanisms to account for this excess gas loss.
|Hubble Space Telescope STIS Spectra of Nuclear Star Clusters in Spiral Galaxies: Dependence of Age and Mass on Hubble Type|
We study the nuclear star clusters (NCs) in spiral galaxies of variousHubble types using spectra obtained with the STIS on board the HubbleSpace Telescope (HST). We observed the nuclear clusters in 40 galaxies,selected from two previous HST WFPC2 imaging surveys. At a spatialresolution of ~0.2" the spectra provide a better separation of clusterlight from underlying galaxy light than is possible with ground-basedspectra. Approximately half of the spectra have a sufficiently highsignal-to-noise ratio for detailed stellar population analysis. For theother half we only measure the continuum slope, as quantified by the B-Vcolor. To infer the star formation history, metallicity, and dustextinction, we fit weighted superpositions of single-age stellarpopulation templates to the high signal-to-noise ratio spectra. We usethe results to determine the luminosity-weighted age, mass-to-lightratio, and masses of the clusters. Approximately half of the sampleclusters contain a population younger than 1 Gyr. Theluminosity-weighted ages range from 10 Myr to 10 Gyr. The stellarpopulations of NCs are generally best fit as a mixture of populations ofdifferent ages. This indicates that NCs did not form in a single event,but that instead they had additional star formation long after theoldest stars formed. On average, the sample clusters in late-typespirals have a younger luminosity-weighted mean age than those inearly-type spirals (L=8.37+/-0.25 vs.9.23+/-0.21). The average mass-weighted ages are older by ~0.7 dex,indicating that there often is an underlying older population that doesnot contribute much light but does contain most of the mass. The averagecluster masses are smaller in late-type spirals than in early-typespirals (logM=6.25+/-0.21 vs. 7.63+/-0.24) and exceed the masses typicalof globular clusters. The cluster mass correlates loosely with totalgalaxy luminosity. It correlates more strongly with both the Hubble typeof the host galaxy and the luminosity of its bulge. The lattercorrelation has the same slope as the well-known correlation betweensupermassive black hole mass and bulge luminosity. The properties ofboth nuclear clusters and black holes in the centers of spiral galaxiesare therefore intimately connected to the properties of the host galaxy,and in particular its bulge component. Plausible formation scenarioshave to account for this. We discuss various possible selection biasesin our results, but conclude that none of them can explain thedifferences seen between clusters in early- and late-type spirals. Theinability to infer spectroscopically the populations of faint clustersdoes introduce a bias toward younger ages, but not necessarily towardhigher masses.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555. These observations areassociated with proposals 9070 and 9783.
|Circumnuclear Structure and Black Hole Fueling: Hubble Space Telescope NICMOS Imaging of 250 Active and Normal Galaxies|
Why are the nuclei of some galaxies more active than others? If mostgalaxies harbor a central massive black hole, the main difference isprobably in how well it is fueled by its surroundings. We investigatethe hypothesis that such a difference can be seen in the detailedcircumnuclear morphologies of galaxies using several quantitativelydefined features, including bars, isophotal twists, boxy and diskyisophotes, and strong nonaxisymmetric features in unsharp-masked images.These diagnostics are applied to 250 high-resolution images of galaxycenters obtained in the near-infrared with NICMOS on the Hubble SpaceTelescope. To guard against the influence of possible biases andselection effects, we have carefully matched samples of Seyfert 1,Seyfert 2, LINER, starburst, and normal galaxies in their basicproperties, taking particular care to ensure that each was observed witha similar average scale (10-15 pc pixel-1). Severalmorphological differences among our five different spectroscopicclassifications emerge from the analysis. The H II/starburst galaxiesshow the strongest deviations from smooth elliptical isophotes, whilethe normal galaxies and LINERs have the least disturbed morphology. TheSeyfert 2s have significantly more twisted isophotes than any othercategory, and the early-type Seyfert 2s are significantly more disturbedthan the early-type Seyfert 1s. The morphological differences betweenSeyfert 1s and Seyfert 2s suggest that more is at work than simply theviewing angle of the central engine. They may correspond to differentevolutionary stages.
|H I Observations of Barred Magellanic Spirals. II. The Frequency and Impact of Companions|
The results of an H I 21 cm line survey of a sample of Magellanic spiralgalaxies with apparent optical companions reveal that only four of 13systems have confirmed H I-detected neighbors. The current interactionsare affecting the morphology of the main galaxy in only two cases, NGC3664 and NGC 3995. The presence of companions near NGC 2537 and UGC 5391appears to have no effect on the morphology of those galaxies. Overall,there is little difference between the asymmetry of the H I profiles ofthose galaxies with and without companions, and on average, theseMagellanic spirals have H I profiles that are no more asymmetric than arandom sample of spirals in the field. We conclude that currentinteractions cannot be responsible for the lopsided morphology of mostof the galaxies in this sample and that, whatever its original cause,lopsidedness must be a long-lived characteristic of these galaxies.
|Do bulges of early- and late-type spirals have different morphology?|
We study HST/NICMOS H-band images of bulges of two equal-sized samplesof early- (TRC3 <= 3) and late-type spiral (mainly Sbc-Sc)galaxies matched in outer disk axis ratio. We find that bulges oflate-type spirals are more elongated than their counterparts inearly-type spirals. Using a KS-test we find that the two distributionsare different at the 98.4% confidence level. We conclude that the twodata sets are different, i.e. late-type galaxies have a broaderellipticity distribution and contain more elongated features in theinner regions. We discuss the possibility that these would correspond tobars at a later evolutionary stage, i.e. secularly evolved bars.Consequent implications are raised, and we discuss relevant questionsregarding the formation and structure of bulges. Are bulges ofearly-type and late-type spirals different? Are their formationscenarios different? Can we talk about bulges in the same way fordifferent types of galaxies?
|The Nature of Peculiar Stellar Complexes|
The nature of stellar complexes with peculiar populations andmorphologies is investigated. The existence in the LMC of complexes madeup of isolated stars, on the one hand, and consisting exclusively ofclusters, on the other hand, could be due to different turbulencepatterns in the initial gaseous medium. Arc-shaped stellar complexes areunlikely to be the result of star formation in a gaseous shell swept upby a central source of pressure, and instead probably reflect the shapeof a bow shock that develops when a sufficiently dense cloud is subjectto dynamical pressure. A peculiar arc-shaped complex in NGC 6946, whichcontains a young, massive cluster, may be the result of an obliqueinfall of a high-velocity cloud onto a region of the gaseous disk of theGalaxy with a strong, regular magnetic field; the properties of thiscomplex can be explained as the result of a collision of the resultingshocks. The arc-shaped complexes in the LMC were also probably producedby high-velocity clouds moving obliquely through the more tenuous gas ofthe LMC disk. A similar complex in NGC 300 may owe its origin to theeffect produced on a dense cloud by the shock from an extremely powerfulexternal explosion, whose stellar remnant may have survived as an X-raysource now located along the line of symmetry of the arc of the complex.The rareness of such structures can be explained by the narrow range ofconditions under which they can develop.
|Nested and Single Bars in Seyfert and Non-Seyfert Galaxies|
We analyze the observed properties of nested and single stellar barsystems in disk galaxies. The 112 galaxies in our sample comprise thelargest matched Seyfert versus non-Seyfert galaxy sample of nearbygalaxies with complete near-infrared or optical imaging sensitive tolength scales ranging from tens of parsecs to tens of kiloparsecs. Thepresence of bars is deduced by fitting ellipses to isophotes in HubbleSpace Telescope (HST) H-band images up to 10" radius and in ground-basednear-infrared and optical images outside the H-band images. This is aconservative approach that is likely to result in an underestimate ofthe true bar fraction. We find that a significant fraction of the samplegalaxies, 17%+/-4%, have more than one bar, and that 28%+/-5% of barredgalaxies have nested bars. The bar fractions appear to be stableaccording to reasonable changes in our adopted bar criteria. For thenested bars, we detect a clear division in length between thelarge-scale (primary) bars and small-scale (secondary) bars, in bothabsolute and normalized (to the size of the galaxy) length. We arguethat this bimodal distribution can be understood within the framework ofdisk resonances, specifically the inner Lindblad resonances (ILRs),which are located where the gravitational potential of the innermostgalaxy switches effectively from three-dimensional to two-dimensional.This conclusion is further strengthened by the observed distribution ofthe sizes of nuclear rings which are dynamically associated with theILRs. While primary bar sizes are found to correlate with the hostgalaxy sizes, no such correlation is observed for the secondary bars.Moreover, we find that secondary bars differ morphologically from singlebars. Our matched Seyfert and non-Seyfert samples show a statisticallysignificant excess of bars among the Seyfert galaxies at practically alllength scales. We confirm our previous results that bars are moreabundant in Seyfert hosts than in non-Seyfert galaxies and that Seyfertgalaxies always show a preponderance of ``thick'' bars compared to thebars in non-Seyfert galaxies. Finally, no correlation is observedbetween the presence of a bar and that of companion galaxies, evenrelatively bright ones. Overall, since star formation and dustextinction can be significant even in the H band, the stellar dynamicsof the central kiloparsec cannot always be revealed reliably by the useof near-infrared surface photometry alone.
|Detailed Structural Decomposition of Galaxy Images|
We present a two-dimensional fitting algorithm (GALFIT) designed toextract structural components from galaxy images, with emphasis onclosely modeling light profiles of spatially well-resolved, nearbygalaxies observed with the Hubble Space Telescope. Our algorithmimproves on previous techniques in two areas: by being able tosimultaneously fit a galaxy with an arbitrary number of components andwith optimization in computation speed, suited for working on largegalaxy images. We use two-dimensional models such as the ``Nuker'' law,the Sérsic (de Vaucouleurs) profile, an exponential disk, andGaussian or Moffat functions. The azimuthal shapes are generalizedellipses that can fit disky and boxy components. Some potentialapplications of our program include: standard modeling of global galaxyprofiles; extracting bars, stellar disks, double nuclei, and compactnuclear sources; and measuring absolute dust extinction or surfacebrightness fluctuations after removing the galaxy model. When examinedin detail, we find that even simple looking galaxies generally requireat least three components to be modeled accurately, rather than the oneor two components more often employed. Many galaxies with complexisophotes, ellipticity changes, and position angle twists can be modeledaccurately in two dimensions. We illustrate this by way of 11 casestudies, which include regular and barred spiral galaxies, highly diskylenticular galaxies, and elliptical galaxies displaying various levelsof complexities. A useful extension of this algorithm is to accuratelyextract nuclear point sources in galaxies. We compare two-dimensionaland one-dimensional extraction techniques on simulated images ofgalaxies having nuclear slopes with different degrees of cuspiness, andwe then illustrate the application of the program to several examples ofnearby galaxies with weak nuclei. Based on observations with theNASA/ESA Hubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy (AURA), Inc., under NASA contract NAS 5-26555.
|Environment, Ram Pressure, and Shell Formation in Holmberg II|
Neutral hydrogen VLA D-array observations of the dwarf irregular galaxyHoII, a prototype galaxy for studies of shell formation, are presented.These were extracted from the multiconfiguration data set of Puche andcolleagues. H I is detected to radii over 16' or 4R25, almosta factor of 2 better than previous studies. The total H I massMHI=6.44×108 Msolar. Theintegrated H I map has a comet-like appearance, with a large but faintcomponent extending to the northwest and the H I appearing compressed onthe opposite side. This suggests that HoII is affected by ram pressurefrom an intragroup medium (IGM). The velocity field shows a clearrotating disk pattern, and a rotation curve corrected for asymmetricdrift was derived. However, the gas at large radii may not be inequilibrium. Puche and colleagues' multiconfiguration data were alsoreanalyzed, and it is shown that they overestimated their fluxes by over20%. The rotation curve derived for HoII is well defined for r<~10kpc. For 10<~r<~18 kpc, however, velocities are only defined onthe approaching side, such that this part of the rotation curve shouldbe used with caution. An analysis of the mass distribution, using thewhole extent of this rotation curve, yields a total mass of6.3×109 Msolar, of which ~80% is dark.Similarly to what is seen in many dwarfs, there is more luminous mass inH I than in stars. One peculiarity, however, is that luminous matterdominates within the optical body of the galaxy and dark matter only inthe outer parts, analogous to what is seen in massive spirals ratherthan dwarfs. HoII lies northeast of the M81 Group's core, along with Kar52 (M81 dwarf A) and UGC 4483. No signs of interaction are observed,however, and it is argued that HoII is part of the NGC 2403 subgroup,infalling toward M81. A case is made for ram pressure stripping and anIGM in the M81 Group. Stripping of the outer parts of the disk wouldrequire an IGM density nIGM>~4.0×10-6atoms cm-3 at the location of HoII. This corresponds to ~1%of the virial mass of the group uniformly distributed over a volume justenclosing HoII and is consistent with the known X-ray properties ofsmall groups. The H I tail is consistent with additional turbulentviscous stripping and evaporation, at least for low IGM temperatures. Itis argued that existing observations of HoII do not supportself-propagating star formation scenarios, whereby the H I holes andshells are created by supernova explosions and stellar winds. Many H Iholes are located in low surface density regions of the disk, where nostar formation is expected or observed. Alternative mechanisms arediscussed, and it is suggested that ram pressure can help. Ram pressurehas the capacity to enlarge preexisting holes and lower their creationenergies, helping to bridge the gap between the observed star formationrate and that required to create the holes.
|Spiral Galaxies with HST/NICMOS. II. Isophotal Fits and Nuclear Cusp Slopes|
We present surface brightness profiles for 56 of the 78 spiral galaxiesobserved in the HST/NICMOS2 F160W snapshot survey introduced in Paper Iof this series, as well as surface brightness profiles for 23 objectsout of the 41 that were also observed in the F110W filter. We fit thesesurface brightness profiles with the Nuker law of Lauer et al. and usethe smooth analytical descriptions of the data to compute the averagenuclear stellar cusp slopes <γ> in the 0.1"-0.5" radialrange. Our main result is the startling similarity between the nuclearstellar cusp slopes <γ> in the near-infrared compared withthose derived in the visual passband. This similarity has severalimplications: (1) Despite the significant local color variations thatare found in the nuclear regions of spirals and that are documented inPaper I, there are typically little or no optical-NIR global colorgradients, and thus no global stellar population variations, inside~50-100 pc from the nucleus in nearby spirals. (2) The large observedrange of the strength of the nuclear stellar cusps seen in the HSToptical study of spiral galaxies reflects a physical difference betweengalaxies and is not an artifact caused by nuclear dust and/or recentstar formation. (3) The dichotomy between R1/4 bulges, withsteep nuclear stellar cusps <γ>~1, and exponential bulges,with shallow nuclear stellar cusps <γ><0.3, is also notan artifact of the effects of dust or recent star formation. (4) Thepresence of a surrounding massive disk appears to have no effect on therise of the stellar density distribution within the innermost hundredparsecs of the R1/4 spheroids. These results imply abreakdown within the family of exponential bulges of the nuclear versusglobal relationships that have been found for the R1/4spheroids. Such a breakdown is likely to have significant implicationsconcerning the formation of exponential bulges and their connection withthe R1/4 spheroids. Based on observations with the NASA/ESAHubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
|Spiral Galaxies with HST/NICMOS. I. Nuclear Morphologies, Color Maps, and Distinct Nuclei|
This is the first of two papers where we present the analysis of anHST/NICMOS2 near-infrared (NIR) snapshot survey in the F160W (H) filterfor a sample of 78 spiral galaxies selected from the UGC and ESOLVcatalogs. For 69 of these objects we provide nuclear color informationderived by combining the H data either with additional NICMOS F110W (J)images or with V WFPC2/HST data. Here we present the NIR images and theoptical-NIR color maps. We focus our attention on the properties of thephotometrically distinct ``nuclei'' which are found embedded in most ofthe galaxies and provide measurements of their half-light radii andmagnitudes in the H (and when available in the J) band. We find that (1)in the NIR the nuclei embedded in the bright early- to intermediate-typegalaxies span a much larger range in brightness than the nuclei whichare typically found embedded in bulgeless late-type disks: the nucleiembedded in the early- to intermediate-type galaxies reach, on thebright end, values up to HAB~-17.7 mag; (2) nuclei are foundin both nonbarred and barred hosts, in large-scale (>~1 kpc) as wellas in nuclear (up to a few 100 pc) bars; (3) there is a significantincrease in half-light radius with increasing luminosity of the nucleusin the early/intermediate types (a decade in radius for ~8 magbrightening), a correlation which was found in the V band and which isalso seen in the NIR data; (4) the nuclei of early/intermediate-typespirals cover a large range of optical-NIR colors, from V-H~-0.5 to 3.Some nuclei are bluer and others redder than the surroundinggalaxy,indicating the presence of activity or reddening by dust in many ofthese systems; (5) someearly/intermediate nuclei are elongated and/orslightly offset from the isophotal center of the host galaxy. Onaverage, however, these nuclei appear as centered, star-cluster-likestructures similar to those whichare found in the late-type disks. Basedon observations with the NASA/ESA Hubble Space Telescope, obtained atthe Space Telescope Science Institute, which is operated by Associationof Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
|HIPASS Detection of an Intergalactic Gas Cloud in the NGC 2442 Group|
We report the discovery from the H I Parkes All-Sky Survey (HIPASS) of agas cloud associated with the asymmetric spiral galaxy NGC 2442. Thisobject, designated HIPASS J0731-69, contains ~109Msolar of H I, or nearly one-third as much atomic gas as NGC2442 itself. No optical counterpart to any part of HIPASS J0731-69 hasyet been identified, consistent with the gas being diffuse and itsstreamlike kinematics. If the gas in HIPASS J0731-69 was once part ofNGC 2442, then it was most likely a fairly recent tidal encounter with amoderately massive companion that tore it loose, although thepossibility of ram-pressure stripping cannot be ruled out. Thisdiscovery highlights the potential of the HIPASS data for yielding newclues to the nature of some of the best-known galaxies in the localuniverse.
|Gone with the Wind: The Origin of S0 Galaxies in Clusters|
We present three-dimensional, high-resolution hydrodynamical simulationsof the interaction between the hot ionized intracluster medium and thecold interstellar medium of spiral galaxies. Ram pressure andturbulent/viscous stripping remove 100% of the atomic hydrogen contentof luminous galaxies like the Milky Way within 100 million years. Thesemechanisms naturally account for the morphology of S0 galaxies and therapid truncation of star formation implied by spectroscopicobservations, as well as a host of observational data on the neutralhydrogen (HI) morphology of galaxies in clusters.
|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.
|The Centers of Early- to Intermediate-Type Spiral Galaxies: A Structural Analysis|
A recent Hubble Space Telescope (HST)/WFPC2 visual survey of early- andintermediate-type spiral galaxies has unveiled a great complexity in theinner regions of these systems, which include a high fraction ofphotometrically distinct compact sources sitting at the galactic centers(``nuclei''). The faint nuclei (M_V>~-12) are typically hosted byrather amorphous, quiescent, bulgelike structures with an exponential(rather than the classical R^1/4) light profile. These ``exponentialbulges'' are commonly found inside the intermediate-type disks,consistent with previous studies. Brighter nuclei (M_V<~-12) aretypically found instead in the centers of galaxies with circumnuclearrings/arms of star formation or dust and an active, i.e., H II- orAGN-type, central spectrum at ground-based resolution. On the structuralplane of half-light radius (R_e) versus mean surface brightness withinthe half-light radius (mu_e), faint and bright nuclei overlap with, andfill the region of parameter space between, the old Milky Way globularclusters and the young star clusters, respectively, with typical R_e ofabout a few up to ~20 pc. On the same plane, the exponential bulges havesignificantly fainter mu_e than R^1/4 bulges for any given radius andfollow a mu_e-R_e relation typical of disks, which strengthens thesuggestion that the exponential bulges grow inside the disks as a resultof the secular evolution of the latter. Under the likely assumption thatthe visual light from the faint nuclei embedded in the quiescentexponential bulges is of stellar origin and of a similar (>~1 Gyr)age for the central star clusters and their host bulges, the massesinferred for the former agree with those required to disrupt barscomparable in size to the latter. This offers support to scenarios inwhich the exponential bulges grow inside the disks owing to the orbitaldisruption of progenitor bars caused by the growth of a centralconcentration of mass and suggests that this specific mode of bulgeformation is (still) active in the present-day universe. On the otherhand, the presence of the massive clusters at the very center of thelow-density exponential bulges should prevent any other ``nuclear'' barfrom forming, thereby preventing further infall of dissipative fuel tothe nuclear regions. This may argue against the possibility of evolvingthe exponential bulges into denser, R^1/4 bulges by a simple looping forseveral cycles of the bar formation/disruption mechanism.
|Ongoing Gas Stripping in the Virgo Cluster Spiral Galaxy NGC 4522|
The Virgo Cluster galaxy NGC 4522 is one of the best spiral candidatesfor intracluster medium-interstellar medium (ICM-ISM) stripping inaction. Optical broadband and Hα images from the WIYN telescope ofthe highly inclined galaxy reveal a relatively undisturbed stellar diskand a peculiar distribution of Hα emission. Ten percent of theHα emission arises from extraplanar H II regions which appear tolie within filamentary structures >=3 kpc long above one side of thedisk. The filaments emerge from the outer edge of a disk of brightHα emission which is abruptly truncated beyond 0.35R_25. Togetherthe truncated Hα disk and extraplanar Hα filaments arereminiscent of a bow shock morphology, which strongly suggests that ISMof NGC 4522 is being stripped by the gas pressure of the ICM. The galaxyhas a line-of-sight velocity of ~=1300 km s^-1 with respect to the meanVirgo Cluster velocity and thus is expected to experience a stronginteraction with the intracluster gas. The existence of H II regionsapparently located above the disk plane suggests that star formation isoccurring in the stripped gas, and that newly formed stars will enterthe galaxy halo and/or intracluster space. The absence of H II regionsin the disk beyond 0.35R_25 and the existence of H II regions in thestripped gas suggest that even molecular gas has been effectivelyremoved from the disk of the galaxy.
|3D self-consistent N-body barred models of the Milky Way. II. Gas dynamics|
The gas dynamics in the Galactic disc is modeled by releasing aninitially axisymmetric SPH component in a completely self-consistent andsymmetry-free 3D N-body simulation of the Milky Way in which the stellarcomponents display a COBE-like bar. The density centre of the stellarbar wanders around the centre of mass and the resulting gas flow isasymmetric and non-stationary, reproducing the HI and CO l-V diagramsonly at specific times and thus suggesting a transient nature of theobserved inner gas kinematics. The best matching models allow a new andcoherent interpretation of the main features standing out of the l-Vdata within the bar region. In particular, the l-V traces of theprominent offset dustlanes leading the bar major axis in early-typebarred spirals can be unambiguously identified, and the 3-kpc arm andits non-symmetric galactocentric opposite counterarm are the innerprolongations of disc spiral arms passing round the bar and joining thedustlanes at very different galactocentric distances. Bania's clump 1and 2, and another velocity-elongated feature near l= 5.5degr , areinterpreted as gas lumps crossing the dustlane shocks. The terminalvelocity peaks near l=+/- 2.5degr are produced by gas along thedustlanes and not by the trace of the cusped x_1 orbit, which passesfarther away from the Galactic centre. According to these models and torelated geometrical constraints, the Galactic bar must have aninclination angle of 25degr +/- 4degr , a corotation radius of 4.0-4.5kpc and a face-on axis ratio b/a~ 0.6.
|Very Wide Galaxy Pairs of the Northern and Southern Sky|
We present highly accurate observations of the 21 cm line of hydrogen ingalaxies made at the Arecibo and Parkes Observatories. The galaxiesobserved have been identified, through rigorous selection criteriaapplied to the CfA and SSRS catalogs, as being members of pairs withprojected separations of up to 1.5 Mpc (H0 = 75 km s-1 Mpc-1). Theseobservations form the completion of the Chengalur-Nordgren galaxy pairsample with data previously published by Chengalur, Nordgren andcolleagues. The new selection criteria used in this paper are anextension to larger projected separations of the criteria usedpreviously. Forty-nine new galaxies are observed, while H I is detectedin 41 of them. With the addition of these galaxies, the completed samplehas highly accurate H I velocities for a total of 219 galaxies.
|Spiral Galaxies with WFPC2. II. The Nuclear Properties of 40 Objects|
We report the analysis of Hubble Space Telescope Wide Field PlanetaryCamera 2 F606W images of 40 spiral galaxies belonging to the sampleintroduced in Paper I, where 35 other targets were discussed. Wedescribe the optical morphological properties of the new 40 galaxies,derive the surface brightness profiles for 25 of them, and present theresults of photometric decompositions of these profiles into a ``bulge''(R^1/4 or exponential) and a disk component. The analysis of theenlarged sample of 75 galaxies puts on a statistically more solid groundthe main results presented in Paper I: (1) In ~30% of the galaxies, theinner, morphologically distinct structures have an irregular appearance.Some of these ``irregular bulges'' are likely to be currently formingstars. (2) Resolved, central compact sources are detected in about 50%of the galaxies. (3) The central compact sources in galaxies withnuclear star formation are brighter, for similar sizes, than those innon-star-forming galaxies. (4) The luminosity of the compact sourcescorrelates with the total galactic luminosity. Furthermore, the analysisof the enlarged sample of 75 objects shows the following: (a) Several ofthe nonclassical inner structures are well fitted by an exponentialprofile. These ``exponential bulges'' are typically fainter than R^1/4bulges, for a given total galaxy luminosity and (catalog) Hubble typelater than Sab. (b) Irregular/exponential bulges typically host centralcompact sources. (c) The central sources are present in all types ofdisk galaxies, starting with systems as early as S0a. About 60% of Sb toSc galaxies host a central compact source. Many of the galaxies thathost compact sources contain a barred structure. (d) Galaxies withapparent nuclear star formation, which also host the brightest compactsources, are preferentially the early- and intermediate-type (S0a-Sb)systems. (e) None of the features depend on environment: isolated andnonisolated galaxies show indistinguishable properties. Independent ofthe physical nature of the nonclassical inner structures, our mainconclusion is that a significant fraction of galaxies classified fromthe ground as relatively early-type spirals show a rich variety ofcentral properties and little or no morphological/photometric evidencefor a smooth, R^1/4 law bulge. Based on observations with the NASA/ESAHubble Space Telescope, obtained at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
|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.
|Spiral Galaxies with WFPC2. III. Nuclear Cusp Slopes|
In this paper, the third of a series dedicated to the investigation ofthe nuclear properties of spiral galaxies, we (1) model the Wide FieldPlanetary Camera 2 F606W nuclear surface brightness profiles of 41spiral galaxies presented in Papers I and II with the analytic lawintroduced by Lauer et al. and (2) deconvolve these surface brightnessprofiles and their analytic fits, so as to estimate the nuclear stellardensities of bulges of spiral galaxies. We find that the nuclear stellarcusps (quantified by the average logarithmic slope of the surfacebrightness profiles within 0.1"-0.5") are significantly different forR^1/4 law and exponential bulges. The former have nuclear propertiessimilar to those of early-type galaxies, i.e., similar values of nuclearcusps for comparable luminosities, and increasingly steeper stellarcusps with decreasing luminosity. By contrast, exponential bulges have(underlying the light contribution from photometrically distinct,central compact sources) comparatively shallower stellar cusps, andlikely lower nuclear densities, than R^1/4 law bulges. Based onobservations with the NASA/ESA Hubble Space Telescope, obtained at theSpace Telescope Science Institute, which is operated by the Associationof Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.
|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.
|NGC 7421: Surfing the intracluster medium?|
NGC 7421 is a barred spiral displaced from the centre of its opticalenvelope, with a 'bow-shock'-like western boundary, both suggestive ofinteraction with an intracluster medium. We have used fiveconfigurations of the ATCA to map the distribution and kinematics of HIin NGC 7421, and find supportive evidence for this scenario in the formof an HI 'wake'. When compared with ROSAT PSPC observations of thediffuse hot gas in the surrounding group of galaxies, these new ATCAresults will allow us to place new constraints on the density anddynamics of the intracluster medium.
|Where Do the Disks of Spiral Galaxies End?|
In spiral galaxies, the H I surface density declines with increasingradius to a point at which it is seen to truncate dramatically in thebest observed cases. If the ambient radiation field is sufficientlystrong, there exists a maximum radius beyond which the cold gas isunable to support itself against ionization. We have now succeeded indetecting ionized gas beyond the observed H I disk in spirals. Here wereport on our findings for the Sculptor galaxy NGC 253. The H I disks inSculptor galaxies extend to only about 1.2R25, although we have detectedionized gas to the limits of our survey, out to 1.4R25. This hasimportant ramifications for spiral galaxies in that it now becomespossible to trace the gravitational potential beyond where the H I diskends. The detections confirm that the rotation curve continues to risein NGC 253, as it appears to do for other Sculptor galaxies, from the HI measurements, but there is a hint that the rotation curve may fallabruptly not far beyond the edge of the H I disk. If this is correct, itsuggests that the dark halo of NGC 253 may be truncated near the H Iedge and provides further support for the link between dark matter and HI. The line ratios are anomalous with [N II] lambda 6548 to H alpharatios close to unity. While metallicities at these large radii areuncertain, such enhanced ratios, compared to solar-abundance H IIregions ([N II] lambda 6548/H alpha = 0.05-0.2), are likely to requireselective heating of the electron population without further ionizationof N+. We discuss the most likely sources of ionization and heating, andthe possible role of refractory element depletion (e.g., Ca, Si, and Fe)onto dust grains.
|The Catalog of Southern Ringed Galaxies|
The Catalog of Southern Ringed Galaxies (CSRG) is a comprehensivecompilation of diameters, axis ratios, relative bar position angles, andmorphologies of inner and outer rings, pseudorings, and lenses in 3692galaxies south of declination -17 deg. The purpose of the catalog is toevaluate the idea that these ring phenomena are related to orbitalresonances with a bar or oval in galaxy potentials. The catalog is basedon visual inspection of most of the 606 fields of the Science ResearchCouncil (SRC) IIIa-J southern sky survey, with the ESO-B, ESO-R, andPalomar Sky surveys used as auxiliaries when needed for overexposed coreregions. The catalog is most complete for SRC fields 1-303 (mostly southof declination -42 deg). In addition to ringed galaxies, a list of 859mostly nonringed galaxies intended for comparison with other catalogs isprovided. Other findings from the CSRG that are not based on statisticsare the identification of intrinsic bar/ring misalignment; bars whichunderfill inner rings; dimpling of R'1pseudorings; pointy, rectangular, or hexagonal inner or outer ringshapes; a peculiar polar-ring-related system; and other extreme examplesof spiral structure and ring morphology.
|A Preliminary Classification Scheme for the Central Regions of Late-Type Galaxies|
The large-scale prints in The Carnegie Atlas of Galaxies have been usedto formulate a classification scheme for the central regions oflate-type galaxies. Systems that exhibit small bright central bulges ordisks (type CB) are found to be of earlier Hubble type and of higherluminosity than galaxies that do not contain nuclei (type NN). Galaxiescontaining nuclear bars, or exhibiting central regions that are resolvedinto individual stars and knots, and galaxies with semistellar nuclei,are seen to have characteristics that are intermediate between those oftypes CB and NN. The presence or absence of a nucleus appears to be auseful criterion for distinguishing between spiral galaxies andmagellanic irregulars.
|Quantitative Morphology of Bars in Spiral Galaxies|
As suggested by numerical simulations, the axis ratio of the bar is afundamental parameter to describe the dynamical evolution of a barredgalaxy. In a first-order approximation considering bars as ellipticalfeatures, visual measurements of bar axis ratios and lengths of 136spiral galaxies were performed on photographs of good linear scale.Despite the limitations affecting such measurements, morphologicalproperties of bars in spirals along the Hubble sequence as well as therelationship between the bar axis ratio and nuclear star formationactivity are studied. It is found that the relative length of bars inearly-type galaxies is, on average, about a factor of 3 larger than thelength observed in late-type spirals. Also, a relation between barlengths and bulge diameters is observed for both early-type andlate-type spirals, confirming results from previous works. Furthermore,although the number of objects is small, there is an apparentcorrelation between the presence of nuclear star formation activity andthe bar axis ratio: about 71% of the starburst galaxies included in thesample have a strong bar (b/a < 0.6). The introduction of thesequantitative parameters in galaxy classification schemes is discussed.
|Galaxy properties in different environments. 1: The sample|
This paper presents two galaxy samples, respectively in a high and in alow local density environments, that were generated from the SouthernSky Redshift Survey (SSRS) catalog using objective criteria. Apreliminary comparison of physical properties in these two samplesreveals that galaxies in high-density environments tend to be under ahigher starbursting activity, have a deficiency of the neutral hydrogencontent, present a higher fractional Seyfert population and a higherfraction of barred spirals as well. The present samples are intended tobe used in future spectroscopic observations for more detailedinvestigation.
|Dynamics of the Pavo-Indus and Grus clouds of galaxies|
A study of groups of galaxies in the above regions was carried out byselecting a sample extending one magnitude deeper than previous work inthe area, complete down to 15 mag. We report new redshift determinationsfor 58 galaxies in the region and 13 other miscellaneous galaxies, basedon La Silla observations. Using a total of 266 galaxies with measuredredshifts in the Pavo-Indus and Grus clouds, we perform a new analysisof groupings following a well-tested algorithm. A total of 18 groups issingled out, most of them known from previous work, even though a fewadditional members are added. For all the groups, we have calculateddynamical parameters and M/L ratios. All groups are found to be boundaggregates, but only one group can be virialized. For the six mostpopulated examples, having at least five members, we also calculateseveral mass estimators and discuss the wide range of observed M/Lratios, which extends from nine to nearly 500 M(solar)/L(solar). Weintroduce two parameters to measure the presence of either a dominantgalaxy or internal subcondensations, respectively, and test whether anycorrelation with the M/L ratios can be detected. No clear correlationsare found.
|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.
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