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First Detection of PAHs and Warm Molecular Hydrogen in Tidal Dwarf Galaxies
We observed two faint tidal dwarf galaxies (TDGs), NGC 5291 N and NGC5291 S, with the Infrared Spectrograph on the Spitzer Space Telescope.We detect strong polycyclic aromatic hydrocarbon (PAH) emission at 6.2,7.7, 8.6, 11.3, 12.6, and 16.5 μm, which match models of groups of~100 carbon atoms with an equal mixture of neutral and ionized PAHs. TheTDGs have a dominant warm (~140 K) dust component in marked contrast tothe cooler (40-60 K) dust found in starburst galaxies. For the firsttime we detect the low-J rotational lines from molecular hydrogen.Adopting LTE, there is ~105 Msolar of ~400 K gas,which is <0.1% of the cold gas mass inferred from 12CO(1-0) measurements. The combination of one-third solar metallicity witha recent (<5 million year) episode of star formation is reflected inthe S and Ne ratios. The excitation is higher than typical values forstarburst galaxies and similar to that found in BCDs. Using the InfraredArray Camera, we identify an additional 13 PAH-rich candidate TDGs.These sources occupy a distinct region of IRAC color space with[3.6]-[4.5]<0.4 and [4.5]-[8.0]>3.2. Their disturbed morphologiessuggest past merger events between companions; for example, NGC 5291 Shas a projected 11 kpc tail. NGC 5291 N and S have stellar masses of(1.5 and 3.0)×108 Msolar, which iscomparable to BCDs, although still roughly 10% of the LMC's stellarmass. The candidate TDGs are an order of magnitude less massive. Thissystem appears to be a remarkable TDG nursery.
| The K Luminosity-Metallicity Relation for Dwarf Galaxies and the Tidal Dwarf Galaxies in the Tails of HCG 31
We determine a K-band luminosity-metallicity (L-Z) relation for dwarfirregular galaxies over a large range of magnitudes,-20.5
| Massive HI clouds with no optical counterparts as high-density regions of intragroup HI rings and arcs
We present a new scenario in which massive intragroup HI clouds are thehigh-density parts of large HI rings/arcs formed by dynamicalinteraction between galaxy groups and gas-rich, low surface brightness(LSB) galaxies with extended gas discs. Our hydrodynamical simulationsdemonstrate that the group tidal field is very efficient at strippingthe outer HI gas of the disc if the gaseous disc of the LSB galaxyextends 2-5 times further than the stellar disc. We find that a massive,extended `leading stream' orbiting the centre of the group can form outof the stripped outer HI envelope, while the severely shrunken LSBgalaxy, whose stellar disc remains unaffected, continues on its path.The result is a relatively isolated, massive HI cloud with a ring- orarc-like shape, a very inhomogeneous density distribution(NHI~ 1.0 × 1017-1.1 × 1020atomcm-2), and, initially, no stellar content. Only thehigh-density peaks of the simulated intragroup HI ring/arc can bedetected in many current HI observations. These will appear asrelatively isolated `HI islands' near the group centre. We also findthat star formation can occur within the ring/arc, if the total gas masswithin the intragroup ring/arc is very large (~4 × 109Msolar). We discuss these results in terms of existingobservations of intragroup gas (e.g. the Leo Ring and HIPASS J0731-69)and intergalactic HII regions.
| Principal component analysis of International Ultraviolet Explorer galaxy spectra
We analyse the UV spectral energy distribution of a sample of normalgalaxies listed in the International Ultraviolet Explorer (IUE) NewlyExtracted Spectra (INES) Guide No. 2 - Normal Galaxies using a principalcomponent analysis. The sample consists of the IUE short-wavelength (SW)spectra of the central regions of 118 galaxies, where the IUE apertureincluded more than 1 per cent of the galaxy size. The principalcomponents are associated with the main components observed in theultraviolet (UV) spectra of galaxies. The first component, accountingfor the largest source of diversity, may be associated with the UVcontinuum emission. The second component represents the UV contributionof an underlying evolved stellar population. The third component issensitive to the amount of activity in the central regions of galaxiesand measures the strength of star-formation events.In all the samples analysed here, the principal component representativeof star-forming activity accounts for a significant percentage of thevariance. The fractional contribution to the spectral energydistribution (SED) by the evolved stars and by the young population aresimilar.Projecting the SEDs on to their eigenspectra, we find that none of thecoefficients of the principal components can outline an internalcorrelation or can correlate with the optical morphological types. In asubsample of 43 galaxies, consisting of almost only compact and BCDgalaxies, the third principal component defines a sequence related tothe degree of starburst activity of the galaxy.
| Hα Kinematics of Tidal Tails in Interacting Systems: Projection Effects and Dark Matter in TDGs
Several interacting systems exhibit at the tip of their long tidal tailsmassive condensations of atomic hydrogen, which may be the progenitorsof Tidal Dwarf Galaxies. Because, quite often, these tails are observededge-on, projection effects have been claimed to account for the largeHI column densities measured there. Here we show that determining thevelocity field all along the tidal features, one may disentangleprojection effects along the line of view from real bound structures.Due to its large field of view, high spectral and 2D spatialresolutions, Fabry-Perot observations of the ionized gas are welladapted to detect a kinematical signature of either streaming motionsalong a bent tidal tail or of infalling/rotating material associatedwith a forming TDG. Spectroscopic observations also allow to measure thedynamical masses of the TDGs that are already relaxed and check theirdark matter content.
| HI Tidal Tails, Bridges and Clouds
There is plenty of intergalactic H I gas without any obvious stellarcontent ranging from (1) extended gas envelopes around some normal andpeculiar galaxies, (2) tidal tails/bridges in interacting or merginggalaxy systems, (3) large-scale rings around early type galaxies, and(4) detached clouds at varying distances from associated galaxies, butthere are few or no isolated H I clouds.The HIPASS Bright Galaxy Catalog, which covers the whole southern sky,contains only one definite extragalactic H I cloud which is locatedclose to the galaxy NGC 2442 whereas it is sensitive to isolated H Iclouds with MHI > 106 × D2Mȯ. The space density of H I clouds is therefore about1/1000th that of galaxies with the same MHI.
| A Nursery of Young Objects: Intergalactic H II Regions in Stephan's Quintet
We have discovered four intergalactic H II regions in Stephan's quintet,which is more than a 25 kpc projected distance from the center of thenearest group galaxy, with no apparent optical connection to it. Theyhave MB ranging from -11.9 to -12.5 mag, colors B-R=0.7-1.1mag, radial velocities from 6565 to 6651 km s-1, and they aresuperposed onto the H I tail east of NGC 7319, with a mean radialvelocity of 6610 km s-1. In addition, they have metallicitiesof the order of 12+log(O/H)=8.58+/-0.25, which suggests that they wereformed from preenriched material. We derive a mean age of 4.6+/-0.6 Myrand a mean stellar mass of (2.9+/-1.4)×104Msolar for the four objects. The masses, ages, colors,velocities, metallicities, and location of the objects suggest that theyare H II regions that were formed far away from the galaxies throughcompression of the intergalactic H I gas by galaxy collisions.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (US), the Particle Physicsand Astronomy Research Council (UK), the National Research Council(Canada), CONICYT (Chile), the Australian Research Council (Australia),CNPq (Brazil), and CONICET (Argentina).
| Kinematics of tidal tails in interacting galaxies: Tidal dwarf galaxies and projection effects
The kinematics of tidal tails in colliding galaxies has been studied viaFabry-Pérot observations of the Hα emission. With theirlarge field of view and high spatial resolution, the Fabry-Pérotdata allow us to probe simultaneously, in 2D, two kinematical featuresof the tidal ionized gas: large-scale velocity gradients due tostreaming motions along the tails, and small-scale motions related tothe internal dynamics of giant HII regions within the tails. In severalinteracting systems, massive (109 Mȯ)condensations of HI, CO and stars are observed in the outer regions oftails. Whether they are genuine accumulations of matter or not is stilldebated. Indeed a part of the tidal tail may be aligned with theline-of-sight, and the associated projection effect may result inapparent accumulations of matter that does not exist in the 3D space.Using numerical simulations, we show that studying the large-scalekinematics of tails, it is possible to know whether these accumulationsof matter are the result of projection effects or not. We conclude thatseveral ones (Arp 105-South, Arp 242, NGC 7252, and NGC 5291-North) aregenuine accumulations of matter. We also study the small-scale motionsinside these regions: several small-scale velocity gradients areidentified with projected values as large as 50-100 km s-1accross the observed HII regions. In the case of NGC 5291-North, thespatial resolution of our observations is sufficient to detail thevelocity field; we show that this system is rotating andself-gravitating, and discuss its dark matter content. TheFabry-Pérot observations have thus enabled us to prove that some109 Mȯ condensations of matter are realstructures, and are kinematically decoupled from the rest of the tail.Such massive and self-gravitating objects are the progenitors of theso-called ``Tidal Dwarf Galaxies''.Based on observations collected at the European Southern Observatory, LaSilla, Chile and at the Canada-France-Hawaii Observatory, Hawaii, USA.Appendix is only available in electronic form athttp://www.edpsciences.org
| HIDEEP - an extragalactic blind survey for very low column-density neutral hydrogen
We have carried out an extremely long integration time (9000 sbeam-1) 21-cm blind survey of 60 deg2 in Centaurususing the Parkes multibeam system. We find that the noise continues tofall as throughout, enabling us to reach an HI column-density limit of4.2 × 1018 cm-2 for galaxies with a velocitywidth of 200 km s-1 in the central 32 deg2 region,making this the deepest survey to date in terms of column densitysensitivity. The HI data are complemented by very deep opticalobservations from digital stacking of multi-exposure UK SchmidtTelescope R-band films, which reach an isophotal level of 26.5 R magarcsec-2 (~=27.5 B mag arcsec-2). 173 HI sourceshave been found, 96 of which have been uniquely identified with opticalcounterparts in the overlap area. There is not a single source withoutan optical counterpart. Although we have not measured the columndensities directly, we have inferred them from the optical sizes oftheir counterparts. All appear to have a column density ofNHI= 1020.65+/-0.38. This is at least an order ofmagnitude above our sensitivity limit, with a scatter only marginallylarger than the errors on NHI. This needs explaining. Ifconfirmed it means that HI surveys will only find low surface brightness(LSB) galaxies with high MHI/LB. Gas-rich LSBgalaxies with lower HI mass to light ratios do not exist. The paucity oflow column-density galaxies also implies that no significant populationwill be missed by the all-sky HI surveys being carried out at Parkes andJodrell Bank.
| FLASH redshift survey - I. Observations and catalogue
The FLAIR Shapley-Hydra (FLASH) redshift survey catalogue consists of4613 galaxies brighter than bJ= 16.7 (corrected for Galacticextinction) over a 700-deg2 region of sky in the generaldirection of the Local Group motion. The survey region is a70°× 10° strip spanning the sky from the ShapleySupercluster to the Hydra cluster, and contains 3141 galaxies withmeasured redshifts. Designed to explore the effect of the galaxyconcentrations in this direction (in particular the Supergalactic planeand the Shapley Supercluster) upon the Local Group motion, the 68 percent completeness allows us to sample the large-scale structure betterthan similar sparsely-sampled surveys. The survey region does notoverlap with the areas covered by ongoing wide-angle (Sloan or 2dF)complete redshift surveys. In this paper, the first in a series, wedescribe the observation and data reduction procedures, the analysis forthe redshift errors and survey completeness, and present the surveydata.
| 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.
| Tidal dwarf galaxies in the Stephan's Quintet?
We present kinematics and photometric evidence for the presence of sevencandidate tidal dwarf galaxies in Stephan's Quintet. The central regionsof the two most probable parent galaxies, NGC 7319 and NGC 7318B,contain little or no gas whereas the intragroup medium and, inparticular, the optical tails that seem to be associated with NGC 7318Bare rich in cold and ionized gas. Two tidal dwarf candidates may belocated at the edge of a tidal tail, another located within a tail, andfor the four others there is no obvious stellar/gaseous bridge betweenthem and the parent galaxy. Two of the candidates are associated with HI clouds, one of which is, in addition, associated with a CO cloud. Allseven regions have low continuum fluxes and high Hα luminositydensities [F(Hα) = (1-60) × 10-14 ergs s-1cm-2]. Their magnitudes (MB = 16.1 to12.6), sizes ( 3.5 h75 -1 kpc), colors(typically B R = 0.7), and gas velocity gradients ( 826 h75 km s-1 kpc-1) are typicalfor tidal dwarf galaxies. In addition, the ratios between their starformation rates determined from Hα and from the B-band luminosityare typical of other tidal dwarf galaxies. The masses of the tidal dwarfgalaxies in Stephan's Quintet range from 2 × 108to 1010 Mȯ, and the median value for theirinferred mass-to-light ratios is 7 (M/L)ȯ. At least twoof the systems may survive possible ‘fallbacks’ ordisruption by the parent galaxies and may already be, or turn into,self-gravitating dwarf galaxies, new members of the group.
| Redshift-Distance Survey of Early-Type Galaxies. I. The ENEARc Cluster Sample
This paper presents data on the ENEARc subsample of the larger ENEARsurvey of nearby early-type galaxies. The ENEARc galaxies belong toclusters and were specifically chosen to be used for the construction ofa Dn-σ template. The ENEARc sample includes newmeasurements of spectroscopic and photometric parameters (redshift,velocity dispersion, line index Mg2, and the angular diameterdn), as well as data from the literature. New spectroscopicdata are given for 229 cluster early-type galaxies, and new photometryis presented for 348 objects. Repeat and overlap observations withexternal data sets are used to construct a final merged catalogconsisting of 640 early-type galaxies in 28 clusters. Objectivecriteria, based on catalogs of groups of galaxies derived from completeredshift surveys of the nearby universe, are used to assign galaxies toclusters. In a companion paper, these data are used to construct thetemplate Dn-σ distance relation for early-typegalaxies, which has been used to estimate galaxy distances and derivepeculiar velocities for the ENEAR all-sky sample. Based on observationsat Complejo Astronomico El Leoncito, operated under agreement betweenthe Consejo Nacional de Investigaciones Científicas de laRepública Argentina and the National Universities of La Plata,Córdoba, and San Juan; Cerro Tololo Inter-American Observatory,National Optical Astronomical Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation; the EuropeanSouthern Observatory (ESO), partially under the ESO-ON agreement; theFred Lawrence Whipple Observatory; the Observatório do Pico dosDias, operated by the Laboratório Nacional de Astrofísicaand the MDM Observatory at Kitt Peak.
| The Role of Mergers in Galaxy Evolution
The evolution of galaxies has been driven by galaxy-galaxy collisions onall cosmological time-scales, from the primordial universe to thepresent. In this context, nearby mergers serve as local analogs to gaininsight into the physical processes that lead to the formation andtrans-formation of galaxies in the more distant universe. Here I reviewmultiwavelength observations -with particular emphasis on recent resultsobtained with ISO- of mergers of massive galaxies driving the formationof: 1) luminous infrared galaxies, 2) elliptical galaxy cores, 3)luminous dust-enshrouded extranuclear starbursts, 4) symbiotic galaxies,and 5) tidal dwarf galaxies. The most important implication for futurestudies on the formation of galaxies at early cosmological timescales isthat the most luminous galaxies in the local universe (z<=0.1) emitthe bulk of their energy in the mid and far-infrared, and therefore,their analogs in the more distant universe would be invisible in theultraviolet and optical wavelength rest-frames.
| Star-forming Objects in the Tidal Tails of Compact Groups
A search for star-forming objects belonging to tidal tails has beencarried out in a sample of deep Hα images of 16 compact groups ofgalaxies. A total of 36 objects with Hα luminosities greater than1038 ergs s-1 has been detected in five groups.The fraction of the total Hα luminosity of their respective parentgalaxies shown by the tidal objects is always below 5% except for thetidal features of Hickson compact group 95, whose Hα luminosityamounts to 65% of the total luminosity. Out of these 36 objects, ninestar-forming tidal dwarf galaxy candidates finally have been identifiedon the basis of their projected distances to the nuclei of the parentgalaxies and their total Hα luminosities. Overall, the observedproperties of the candidates resemble those previously reported for theso-called tidal dwarf galaxies.
| Abundant molecular gas in tidal dwarf galaxies: On-going galaxy formation
We investigate the process of galaxy formation as can be observed in theonly currently forming galaxies - the so-called Tidal Dwarf Galaxies,hereafter TDGs - through observations of the molecular gas detected viaits CO (Carbon Monoxide) emission. These objects are formed of materialtorn off of the outer parts of a spiral disk due to tidal forces in acollision between two massive galaxies. Molecular gas is a key elementin the galaxy formation process, providing the link between a cloud ofgas and a bona fide galaxy. We have detected CO in 8 TDGs (two of themhave already been published in Braine et al. 2000, hereafter Paper I),with an overall detection rate of 80%, showing that molecular gas isabundant in TDGs, up to a few 108 Msun. The COemission coincides both spatially and kinematically with the HIemission, indicating that the molecular gas forms from the atomichydrogen where the HI column density is high. A possible trend of moreevolved TDGs having greater molecular gas masses is observed, in accordwith the transformation of HI into H2. Although TDGs sharemany of the properties of small irregulars, their CO luminosity is muchgreater (factor ~ 100) than that of standard dwarf galaxies ofcomparable luminosity. This is most likely a consequence of the highermetallicity (gtrsim 1/3 solar) of TDGs which makes CO a good tracer ofmolecular gas. This allows us to study star formation in environmentsordinarily inaccessible due to the extreme difficulty of measuring themolecular gas mass. The star formation efficiency, measured by the COluminosity per Hα flux, is the same in TDGs and full-sizedspirals. CO is likely the best tracer of the dynamics of these objectsbecause some fraction of the HI near the TDGs may be part of the tidaltail and not bound to the TDG. Although uncertainties are large forindividual objects, as the geometry is unknown, our sample is now ofeight detected objects and we find that the ``dynamical" masses of TDGs,estimated from the CO line widths, seem not to be greater than the``visible" masses (HI + H2 + a stellar component). Althoughhigher spatial resolution CO (and HI) observations would help reduce theuncertainties, we find that TDGs require no dark matter, which wouldmake them the only galaxy-sized systems where this is the case. Darkmatter in spirals should then be in a halo and not a rotating disk. Mostdwarf galaxies are dark matter-rich, implying that they are not of tidalorigin. We provide strong evidence that TDGs are self-gravitatingentities, implying that we are witnessing the ensemble of processes ingalaxy formation: concentration of large amounts of gas in a boundobject, condensation of the gas, which is atomic at this point, to formmolecular gas and the subsequent star formation from the dense molecularcomponent.
| ROSAT-HRI observations of six southern galaxy pairs
We present the detailed analysis of the X-ray data for 6 pairs, isolatedor in poor groups, observed at high resolution with the ROSAT HRI . Inall cases, the stronger X-ray source is associated with the brighterearly-type member and is extended. The extent varies from galactic togroup scale, from 3 (RR 210b) to 182 kpc( RR 22a). The fainter membersare detected only in two pairs, RR 210 and RR 259. Except for one case,no significant substructures have been detected in the X-ray maps,possibly also as a consequence of the poor statistics. The core radii ofthe X-ray surface brightness profiles are in the range 1-3 kpc. Thedistribution of the luminosities of galaxies in pairs encompasses a verywide range of both luminosities and LX / LBratios, in spite of the very small number of objects studied so far. Ourdata provide no evidence that pair membership affects the X-rayproperties of galaxies. Observation are discussed in the context of thepair/group evolution.
| FAUST observations in the Fourth Galactic Quadrant*
We analyse UV observations with FAUST of four sky fields in the generaldirection of the Fourth Galactic Quadrant, in which we detect 777 UVsources. This is ~50 per cent more than detected originally by Bowyer etal. We discuss the source detection process and the identification of UVsources with optical counterparts. For the first time in this project weuse ground-based objective-prism information for two of the fields, toselect the best-matching optical objects with which to identify the UVsources. Using this, and correlations with existing catalogues, wepresent reliable identifications for ~75 per cent of the sources. Mostof the remaining sources have assigned optical counterparts but, lackingadditional information, we offer only plausible identifications. Wediscuss the types of objects found, and compare the observed populationwith predictions of our UV Galaxy model.
| Streaming motions of galaxy clusters within 12000kms-1 - I. New spectroscopic data
We present new spectroscopic data for 532 early-type galaxies,predominantly Abell cluster members with cz<12000kms-1. Wetabulate 919 individual measurements, from six observing runs, forrecession velocity, cz, central velocity dispersion, σ, andmagnesium line-strength indices, Mg2 and Mgb. The medianestimated error, per measurement, in σ is 5 per cent. TheMg2 line-strength data have median errors of 0.009mag. Theobserved scatter between repeat measurements is largely accounted for byphoton-counting errors. These data will be employed in forthcomingpapers, where they will form part of a merged and standardized catalogueof Fundamental Plane data for use in cluster distance estimates andpeculiar velocity analyses.
| 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.
| Dwarf Galaxy Formation Induced by Galaxy Interactions
Growing evidence is being accumulated that some gas-rich dwarf galaxiesare formed from material liberated by galaxy collisions and/or mergers.Also, gas-poor dwarf elliptical galaxies are often found in the centralregions of clusters of galaxies. These observations suggest stronglythat the formation of most dwarf galaxies is linked to galaxyinteractions. Therefore, now seems like the right time to investigatethe formation efficiency of such tidal dwarf galaxies. Adopting thegalaxy interaction scenario proposed by Silk & Norman, we find thatif only a few dwarf galaxies are formed in each galaxy collision, we areable to explain the observed morphology-density relations for both dwarfand giant galaxies in the field, groups of galaxies, and clusters ofgalaxies. It seems worthwhile noting that tidal dwarf formation may becoupled with the transformation from gas-rich disk galaxies toearly-type galaxies such as S0 and elliptical galaxies.
| Formation of a Tidal Dwarf Galaxy in the Interacting System Arp 245 (NGC 2992/93)
Among the various phenomena observed in interacting galaxies is theejection due to tidal forces of stellar and gaseous material into theintergalactic medium and its subsequent rearranging which can lead tothe formation of self-gravitating tidal dwarf galaxies (TDGs). Weinvestigate this process with a detailed multiwavelength study of theinteracting system Arp 245 and a numerical model of the collisioncomputed with a Tree-SPH code. Our observations consist ofoptical/near-infrared broadband imaging, Hα imaging, opticalspectroscopy, H I VLA cartography and CO line mapping. The system,composed of the two spiral galaxies NGC 2992 and NGC 2993, is observedat an early stage of the interaction, about 100 Myr afterperigalacticon, though at a time when tidal tails have alreadydeveloped. The VLA observations disclose a third partner to theinteraction: an edge-on, flat galaxy, FGC 0938, which looks strikinglyundisturbed and might just be falling toward the NGC 2992/93 system. OurH I map shows prominent counterparts to the optical tails. Whereas thestellar and gaseous components of the plume that originates from NGC2992 match, the stellar and H I tails emanating from NGC 2993 have adifferent morphology. In particular, the H I forms a ring, a featurethat has been successfully reproduced by our numerical simulations. TheH I emission in the system as a whole peaks at the tip of the NGC 2992tail where a gas reservoir of about 109 Msolar, about 60% ofthe H I toward NGC 2992, coincides with a star-forming opticalcondensation, A245N. The latter tidal object exhibits properties rangingbetween those of dwarf irregular galaxies (structural parameters, gascontent, star formation rate) and those of spiral disks (metallicity,star formation efficiency, stellar population). Although it is likely,based on our analysis of the H I and model data cube, that A245N mightbecome an independent dwarf galaxy, the dynamical evidence is still opento debate. Prompted by the questions raised for this particular object,we discuss some issues related to the definition and identification ofTDGs and highlight some specific conditions which seem required to formthem. Finally, we outline what is needed in terms of future numericalsimulations in order to further our understanding of these objects.
| Spectral classification of emission-line galaxies
The main goal of this work is to further investigate the classificationof emission-line galaxies from the ``Spectrophotometric Catalogue of HII galaxies'' by Terlevich et al. (1991) in a homogeneous and objectiveway, using the three line-ratio diagrams, called diagnostic diagrams, ofVeilleux & Osterbrock (1987). On the basis of the resultingcatalogue, we critically discuss the classification methods in theoptical range. In particular we compare our classification scheme to theone done by Rola et al. (1997) which is efficient for the classificationof redshifted galaxies. We also propose a new diagnostic diagraminvolving the known intensity ratio R23=([O II],l 3727+[OIII] l 4959+{[O III] l 5007)/Hb which appears to be a very goodcriterion allowing to discriminate the Seyfert 2 from H ii galaxies. Therevised catalogue including 314 narrow-emission-line galaxies contains HII galaxies, Seyfert 2 galaxies, Low Ionization Nuclear Emission-LineRegions (hereafter LINERs) galaxies and some particular types ofgalaxies with the most intriguing ones, called ``ambiguous'', due to theambiguity of their location in the diagnostic diagrams. These galaxiesappear as H II galaxies and as active galactic nuclei (hereafter AGNs)in different diagrams of Veilleux & Osterbrock and constitutecertainly a sample of particularly interesting candidates for a thoroughstudy of connections between starbursts and AGNs. Available inelectronic form only via anonymous ftp 130.79.128.5 orhttp://cdsweb.u-strasbg.fr/Abstract.html
| Galaxy collisions.
Theories of how galaxies, the fundamental constituents of large-scalestructure, form and evolve have undergone a dramatic paradigm shift inthe last few decades. Earlier views were of rapid, early collapse andformation of basic structures, followed by slow evolution of the stellarpopulations and steady buildup of the chemical elements. Currenttheories emphasize hierarchical buildup via recurrent collisions andmergers, separated by long periods of relaxation and secularrestructuring. Thus, collisions between galaxies are now seen as aprimary process in their evolution. This article begins with a briefhistory; we then tour parts of the vast array of collisional forms thathave been discovered to date. Many examples are provided to illustratehow detailed numerical models and multiwaveband observations haveallowed the general chronological sequence of collisional morphologiesto be deciphered, and how these forms are produced by the processes oftidal kinematics, hypersonic gas dynamics, collective dynamical frictionand violent relaxation. Galaxy collisions may trigger the formation of alarge fraction of all the stars ever formed, and play a key role infueling active galactic nuclei. Current understanding of the processesinvolved is reviewed. The last decade has seen exciting new discoveriesabout how collisions are orchestrated by their environment, howcollisional processes depend on environment, and how these environmentsdepend on redshift or cosmological time.
| Tidal Dwarf Galaxies
It will be reviewed the idea that dwarf galaxies can be formed out ofthe tidal debries of interacting giant galaxies. This class of dwarfgalaxies are called ``Tidal Dwarf Galaxies".
| 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.
| 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).
| New catalogue of Wolf-Rayet galaxies and high-excitation extra-galactic HII regions
We present a new compilation of Wolf-Rayet (WR) galaxies andextra-galactic Hii regions showing broad He ii lambda 4686 emissiondrawn from the literature. Relevant information on the presence of otherbroad emission lines ([N i] lambda 5199ii, C iv lambda 5808 and others)from WR stars of WN and WC subtypes, and other existing broad nebularlines is provided. In total we include 139 known WR galaxies. Amongthese, 57 objects show both broad He ii lambda 4686 and C iv lambda 5808features. In addition to the broad (stellar) He ii lambda 4686 emission,a nebular He ii component is well established (suspected) in 44 (54)objects. We find 19 extra-galatic Hii regions without WR detectionsshowing nebular He ii lambda 4686 emission. The present sample can beused for a variety of studies on massive stars, interactions of massivestars with the ISM, stellar populations, starburst galaxies etc. Thedata is accessible electronically and will be updated periodically. Thecatalogue is available at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/Abstract.html
| 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.
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