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# NGC 7715

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 HI content in galaxies in loose groupsGas 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. A Chandra ACIS Study of 30 Doradus. I. Superbubbles and Supernova RemnantsWe present an X-ray tour of diffuse emission in the 30 Doradusstar-forming complex in the Large Magellanic Cloud using high spatialresolution X-ray images and spatially resolved spectra obtained with theAdvanced CCD Imaging Spectrometer on board the Chandra X-RayObservatory. The dominant X-ray feature of the 30 Doradus nebula is theintricate network of diffuse emission generated by interacting stellarwinds and supernovae working together to create vast superbubbles filledwith hot plasma. We construct maps of the region showing variations inplasma temperature (T=3-9 million degrees), absorption[NH=(1-6)×1021 cm-2], andabsorption-corrected X-ray surface brightness[SX=(3-126)×1031 ergs s-1pc-2]. Enhanced images reveal the pulsar wind nebula in thecomposite supernova remnant N157B, and the Chandra data show spectralevolution from nonthermal synchrotron emission in the N157B core to athermal plasma in its outer regions. In a companion paper we show thatR136, the central massive star cluster, is resolved at the arcsecondlevel into almost 100 X-ray sources. Through X-ray studies of 30 Doradusthe complete life cycle of such a massive stellar cluster can berevealed. Remarkable Disk and Off-Nuclear Starburst Activity in the Tadpole Galaxy as revealed by the Spitzer Space TelescopeWe present ground-based optical and Spitzer Space Telescope infraredimaging observations of the interacting galaxy UGC 10214, the Tadpolegalaxy (z=0.0310), focusing on the star formation activity in thenuclear, disk, spiral arms, and tidal tail regions. The ground-basedoptical data set spans a wavelength range between 0.3 and 0.8 μm, thenear-IR data set spans 1-2.2 μm, and the Spitzer IR data set spans3-70 μm. The major findings of this study are that the Tadpole isactively forming stars in the main disk outside of the nucleus and inthe tidal plume, with an estimated mean star formation rate of ~2-4Msolar yr-1. The most prominent sites of mid-IRemission define a ring'' morphology that, combined with the overallmorphology of the system, suggests the interaction may belong to therare class of off-center collisional ring systems that form bothshock-induced rings of star formation and tidal plumes. In starkcontrast to the disk star formation, the nuclear emission is solelypowered by older stars, with little evidence for ongoing star formationat the center of the Tadpole. Extranuclear star formation accounts for>50% of the total star formation in the disk and spiral arms,featuring infrared-bright hot spots'' that exhibit strong polycyclicaromatic hydrocarbon (PAH) emission, the band strength of which iscomparable to that of late-type star-forming disk galaxies. The tidaltail, which extends 2' (~75 kpc) into the intergalactic medium, ispopulated by supermassive star clusters, M~106Msolar, likely triggered by the galaxy-galaxy interactionthat has distorted UGC 10214 into its current tadpole'' shape. TheTadpole is therefore an example of an off-nuclear or tidal-tailstarburst, with several large sites of massive star formation in thedisk and in the plume, including the most prominent Hubble SpaceTelescope-revealed cluster, J160616.85+552640.6. The clusters exhibitremarkable IR properties, including exceptionally strong 24 μmemission relative to the underlying starlight, hot dust continuum, andPAH emission, with an estimated current star formation rate of ~0.1-0.4Msolar yr-1, representing >10% of the totalstar formation in the system. We estimate the mass of the largestcluster to be ~(1.4-1.6)×106 Msolar based onthe g'-band (0.5 μm) and near-IR (2.2 μm) integrated fluxes incombination with an assumed mass-to-light ratio appropriate to youngclusters, or large enough to be classified as a nascent dwarf galaxy orglobular cluster. Hierarchical merging, ultraluminous and hyperluminous X-ray sourcesVarious arguments strongly suggest that the population of ultraluminousX-ray sources (ULXs, apparent X-ray luminosity > Eddington limit for10 Msolar~= 1039erg s-1) in nearbygalaxies are mostly stellar-mass X-ray binaries in unusual evolutionarystages. However, there are indications that the very brightest systemsmay be difficult to explain this way. Accordingly, we consider the classof hyperluminous X-ray sources (HLXs; i.e. those with apparentbolometric luminosities >~1041erg s-1). Becausethis class is small (currently only the M82 object is a secure member)we do not need to invoke a new formation mechanism for its black holes.We explore instead the idea that HLXs may be the nuclei of satellitegalaxies captured during hierarchical merging. The observed correlationbetween active galactic nuclei and tidal interactions implies that HLXactivity would switch on during passage through the host galaxy, closeto the pericentre. This suggests that HLXs should appear near the hostgalaxy, be associated with star formation and thus possibly with ULXs. The star-forming environment of an ultraluminous X-ray source in NGC4559: an optical studyWe have studied the candidate optical counterparts and the stellarpopulation in the star-forming complex around the bright ultraluminousX-ray source (ULX) in the western part of the spiral galaxy NGC4559,using the HST Wide Field Planetary Camera 2 (WFPC2), XMM-Newton/OpticalMonitor and ground-based data. We find that the ULX is located near asmall group of OB stars, but is not associated with any massive youngclusters nor with any extraordinary massive stars. The brightest pointsource in the Chandra error circle is consistent with a single bluesupergiant (BSG) of mass ~20Msolar and age ~10 Myr. A fewother stars are resolved inside the error circle: mostly BSGs and redsupergiants (RSGs) with inferred masses ~10-15Msolar and ages~20 Myr. This is consistent with the interpretation of this ULX as ablack hole (BH) accreting from a high-mass donor star in its supergiantphase, with mass transfer occurring via Roche-lobe overflow. Theobserved optical colours and the blue-to-red supergiant ratio suggest alow metal abundance for the stellar population: 0.2<~Z/Zsolar<~ 0.4 (using the Padua tracks), or 0.05<~Z/Zsolar<~ 0.2 (using the Geneva tracks). The age ofthe star-forming complex is <~30 Myr. Hα images show that thisstar-forming region has a ring-like appearance. We propose that it is anexpanding wave of star formation, triggered by an initial densityperturbation, in a region where the gas was only marginally stable togravitational collapse. We also suggest that the most likely trigger wasa collision with a satellite dwarf galaxy going through the gas-richouter disc of NGC4559 less than 30 Myr ago. The culprit could be thedwarf galaxy visible a few arcsec north-west of the complex. If this isthe case, this system is a scaled-down version of the Cartwheel galaxy.The X-ray data favour a BH more massive (M > 50Msolar)than typical Milky Way BH candidates. The optical data favour a young BHoriginating in the recent episode of massive star formation; however,they also rule out an association with young massive star clusters (noneare present in the X7 field). We speculate that other mechanisms maylead to the formation of relatively massive BHs (perhaps M~50-100Msolar) from stellar evolution processes inlow-metallicity environments, or when star formation is triggered bygalactic collisions. Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data AnalysisX-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources. RETROCAM: A Versatile Optical Imager for Synoptic StudiesWe present RETROCAM, an auxiliary CCD camera that can be rapidlyinserted into the optical beam of the MDM 2.4 m telescope. The speed andease of reconfiguring the telescope to use the imager and astraightforward user interface permit the camera to be used during thecourse of other observing programs. This in turn encourages RETROCAM'suse for a variety of monitoring projects. Chandra X-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/7715: Tidal Ultraluminous X-Ray Sources, Emergent Wind, and Resolved H II RegionsWe present high spatial resolution X-ray imaging data for theinteracting galaxy pair NGC 7714/7715 (Arp 284) from the Chandra X-raytelescope. In addition to the unresolved starburst nucleus, a variablepoint source with LX~1040 ergs s-1 wasdetected 1.5" (270 pc) to the northwest of the nucleus, coincident witha blue, extremely optically luminous (MV~-14.1) point sourceon Hubble Space Telescope images. Eleven other candidate pointlikeultraluminous X-ray sources (ULXs) were also detected in the vicinity ofNGC 7714/7715, two of which exceed 1040 ergs s-1.Ten of these appear to be associated with interaction-induced features,but only two are associated with star formation regions. We also founddiffuse emission with LX~3×1040 ergss-1 extending 11" (1.9 kpc) to the north of the nucleus. Itsspectrum can be fitted with either a two-temperature MEKAL function(kT=0.59+0.05-0.06 and8+10-3 keV) or a 0.6 keV MEKAL function plus apower law (Γ=1.8+/-0.2). The hard component may be due tohigh-mass X-ray binaries (HMXBs) with possible contributions frominverse Compton radiation, while the soft component is likely from asuperwind. Superbubble models imply an expansion age of ~15 Myr,supporting previous assertions of an intermediate-age nuclear stellarpopulation in addition to a 5 Myr starburst. We also detected extendedX-ray emission associated with four extranuclear H II region complexes.The emission from these H II regions and the nuclear starburst could bedue to either an enhanced population of HMXBs relative to Local Groupgalactic averages or to diffuse gas heated by winds from supernovae, ifthe X-ray production efficiency LX/Lmech is high(~5%). To estimate LX/Lmech, we collectedpublished data for well-studied H II regions and superbubbles in nearbygalaxies. For H II regions with ages less than 3.5 Myr, the medianLX/Lmech~0.02%, while for older star formationregions, LX/Lmech~0.2%-7%. Thus, it is possiblethat gas heating by supernovae may be sufficient to account for theobserved X-rays from these H II regions. In galaxies much more distantthan NGC 7714, for example, the Cartwheel galaxy, H II region complexessimilar to those in NGC 7714 will be unresolved by Chandra and willmimic ULXs. No X-ray emission was detected from the Type Ib supernova SN1999dn, with an upper limit of ~2×1038 ergss-1. Spitzer Infrared Spectrograph Spectroscopy of the Prototypical Starburst Galaxy NGC 7714We present observations of the starburst galaxy NGC 7714 with theInfrared Spectrograph on board the Spitzer Space Telescope. The spectrayield a wealth of ionic and molecular features that allow a detailedcharacterization of its properties. NGC 7714 has an H II region-likespectrum with strong polycyclic aromatic hydrocarbon emission features.We find no evidence for an obscured active galactic nucleus, and with[NeIII]/[NeII]~0.73, NGC 7714 lies near the upper end ofnormal-metallicity starburst galaxies. With very little silicateabsorption and a temperature of the hottest dust component of 340 K, NGC7714 is the perfect template for a young, unobscured starburst. Classification of Spectra from the Infrared Space Observatory PHT-S DatabaseWe have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA). Hubble Space Telescope Spectroscopy of Four Luminous Compact Blue Galaxies at Intermediate RedshiftWe present a spectroscopic study of four blue (B-V<=0.6) compact(μB<=21.0 mag arcsec-2) field galaxies withredshifts 0.095<=z<=0.438 and absolute magnitudes MBbetween -17.5 and -21.1. The spectra, taken with the Space TelescopeImaging Spectrograph on board the Hubble Space Telescope, show prominentemission lines whose spatial profiles reveal, at least in two cases, theexistence of various star-forming regions and an underlying stellarpopulation. The derived star formation rates from Hα luminositiesin these star-forming regions range between 0.5 and 7 Msolaryr-1. These regions are typically 10 times more luminous thanand approximately twice as large as the well-known giant H II region 30Doradus in the LMC. When compared with local starburst galaxies, theobjects presented here turn out to be very similar to the brightest H IIgalaxies in terms of the areal star formation rate and in the ratiobetween the galactic half-light diameter to the line-emitting regionhalf-light diameter (Deff/D0).Based on observations obtained with the NASA/ESA Hubble Space Telescopeobtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555. An IRAS High Resolution Image Restoration (HIRES) Atlas of All Interacting Galaxies in the IRAS Revised Bright Galaxy SampleThe importance of far-infrared observations for our understanding ofextreme activity in interacting and merging galaxies has beenillustrated by many studies. Even though two decades have passed sinceits launch, the most complete all-sky survey to date from which far-IRselected galaxy samples can be chosen is still that of the InfraredAstronomical Satellite (IRAS). However, the spatial resolution of theIRAS all-sky survey is insufficient to resolve the emission fromindividual galaxies in most interacting galaxy pairs, and hence previousstudies of their far-IR properties have had to concentrate either onglobal system properties or on the properties of very widely separatedand weakly interacting pairs. Using the HIRES image reconstructiontechnique, it is possible to achieve a spatial resolution ranging from30" to 1.5m (depending on wavelength and detector coverage), whichis a fourfold improvement over the normal resolution of IRAS. This issufficient to resolve the far-IR emission from the individual galaxiesin many interacting systems detected by IRAS, which is very importantfor meaningful comparisons with single, isolated galaxies. We presenthigh-resolution 12, 25, 60, and 100 μm images of 106 interactinggalaxy systems contained in the IRAS Revised Bright Galaxy Sample (RBGS,Sanders et al.), a complete sample of all galaxies having a 60 μmflux density greater than 5.24 Jy. These systems were selected to haveat least two distinguishable galaxies separated by less than threeaverage galactic diameters, and thus we have excluded very widelyseparated systems and very advanced mergers. Additionally, some systemshave been included that are more than three galactic diameters apart,yet have separations less than 4' and are thus likely to suffer fromconfusion in the RBGS. The new complete survey has the same propertiesas the prototype survey of Surace et al. We find no increased tendencyfor infrared-bright galaxies to be associated with other infrared-brightgalaxies among the widely separated pairs studied here. We find smallenhancements in far-IR activity in multiple galaxy systems relative toRBGS noninteracting galaxies with the same blue luminosity distribution.We also find no differences in infrared activity (as measured byinfrared color and luminosity) between late- and early-type spiralgalaxies. A variable ultra-luminous X-ray source in the colliding galaxy NGC 7714We studied the colliding galaxy NGC 7714 with two XMM-Newtonobservations, six months apart. The galaxy contains two bright X-raysources: we show that they have different physical nature. Theoff-nuclear source is an accreting compact object, one of the brightestultraluminous X-ray sources (ULXs) found to date. It showed spectral andluminosity changes between the two observations, from a low/soft to ahigh/hard state; in the high state, it reached Lx ≈ 6× 1040 erg s-1. Its lightcurve in the highstate suggests variability on a ≈ 2 h timescale. Its peculiarlocation, where the tidal bridge between NGC 7714 and NGC 7715 joins theouter stellar ring of NGC 7714, makes it an interesting example of theconnection between gas flows in colliding galaxies and ULX formation.The nuclear source (Lx ≈ 1041 ergs-1) coincides with a starburst region, and is thecombination of thin thermal plasma emission and a point-sourcecontribution (with a power-law spectrum). Variability in the power-lawcomponent between the two observations hints at the presence of asingle, bright point source (Lx  3 ×1040 erg s-1): either a hidden AGN or another ULX. The Hα galaxy survey. I. The galaxy sample, Hα narrow-band observations and star formation parameters for 334 galaxiesWe discuss the selection and observations of a large sample of nearbygalaxies, which we are using to quantify the star formation activity inthe local Universe. The sample consists of 334 galaxies across allHubble types from S0/a to Im and with recession velocities of between 0and 3000 km s-1. The basic data for each galaxy are narrowband H\alpha +[NII] and R-band imaging, from which we derive starformation rates, H\alpha +[NII] equivalent widths and surfacebrightnesses, and R-band total magnitudes. A strong correlation is foundbetween total star formation rate and Hubble type, with the strongeststar formation in isolated galaxies occurring in Sc and Sbc types. Moresurprisingly, no significant trend is found between H\alpha +[NII]equivalent width and galaxy R-band luminosity. More detailed analyses ofthe data set presented here will be described in subsequent papers.Based on observations made with the Jacobus Kapteyn Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias.The full version of Table \ref{tab3} is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/23 Reduced image datafor this survey can be downloaded fromhttp://www.astro.livjm.ac.uk/HaGS/ Globular Clusters as Candidates for Gravitational Lenses to Explain Quasar-Galaxy AssociationsWe argue that globular clusters (GCs) are good candidates forgravitational lenses in explaining quasar-galaxy associations. Thecatalog of associations (Bukhmastova 2001) compiled from the LEDAcatalog of galaxies (Paturel 1997) and from the catalog of quasars(Veron-Cetty and Veron 1998) is used. Based on the new catalog, we showthat one might expect an increased number of GCs around irregulargalaxies of types 9 and 10 from the hypothesis that distant compactsources are gravitationally lensed by GCs in the halos of foregroundgalaxies. The King model is used to determine the central surfacedensities of 135 GCs in the Milky Way. The distribution of GCs incentral surface density was found to be lognormal. Models of the Morphology, Kinematics, and Star Formation History of the Prototypical Collisional Starburst System NGC 7714/7715 = ARP 284We present new N-body, hydrodynamical simulations of the interactionbetween the starburst galaxy NGC 7714 and its poststarburst companionNGC 7715, focusing on the formation of the collisional features,including (1) the gas-rich star-forming bridge, (2) the large gaseousloop (and stellar tails) to the west of the system, (3) the veryextended H I tail to the west and north of NGC 7714, and (4) the partialstellar ring in NGC 7714. Our simulations confirm the results of earlierwork that an off-center inclined collision between two disk galaxies isalmost certainly responsible for the peculiar morphologies of thissystem. However, we have explored a wider set of initial galaxy andcollisional encounter parameters than previously and have found arelatively narrow range of parameters that reproduce all the majormorphologies of this system. The simulations suggest specific mechanismsfor the development of several unusual structures. We find that thecomplex gas bridge has up to four distinct components, with gascontributed from two sides of NGC 7715, as well as from NGC 7714. Theobserved gas-star offset in this bridge is accounted for in thesimulations by the dissipative evolution of the gas. The models suggestthat the most recently formed gas bridge component from NGC 7715 isinteracting with gas from an older component. This interaction may havestimulated the band of star formation on the north side of the bridge.The models also indicate that the low surface brightness H I tail to thefar west of NGC 7714 is the end of the NGC 7715 countertail, curvedbehind the two galaxies. The sensitivity of the tidal structures tocollision parameters is demonstrated by comparisons between models withslightly different parameter values. Comparison of model andobservational (H I) kinematics provides an important check that themorphological matches are not merely fortuitous. Line-of-sight velocityand dispersion fields from the model are found to match those of theobservations reasonably well at current resolutions. Spectralevolutionary models of the NGC 7714 core by Lançon et al. suggestthe possibility of multiple starbursts in the last 300 Myr. Ourhydrodynamic models suggest that bursts could be triggered by inducedringlike waves and a postcollision buildup of gas in the core of thegalaxy. Star Formation Across the Taffy Bridge: UGC 12914/15We present Berkeley-Illinois-Maryland Association (BIMA) two-fieldmosaic CO (1-0) images of the Taffy galaxies (UGC 12914/15), which showthe distinct taffy-like radio continuum emission bridging the two spiraldisks. Large amounts of molecular gas (1.4×1010Msolar, using the standard Galactic CO-to-H2conversion applicable to Galactic disk giant molecular clouds) wereclearly detected throughout the taffy bridge between the two galaxies,which, as in the more extreme case of H I, presumably results from ahead-on collision between the two galaxies. The highest CO concentrationbetween the two galaxies corresponds to the Hα source in the taffybridge near the intruder galaxy UGC 12915. This H II region is alsoassociated with the strongest source of radio continuum in the bridgeand shows both morphological and kinematic connections to UGC 12915. Theoverall CO distribution of the entire system agrees well with that ofthe radio continuum emission, particularly in the taffy bridge. Thisargues for the star formation origin of a significant portion of theradio continuum emission. Compared with the H I morphology andkinematics, which are strongly distorted owing to the high-speedcollision, CO better defines the orbital geometry and impact parameterof the interaction, as well as the disk properties (e.g., rotation,orientation) of the progenitor galaxies. Based on the 20 cm-to-CO ratiomaps, we conclude that the starburst sites are primarily located in UGC12915 and the Hα source in the bridge and show that the moleculargas in the taffy bridge is forming into stars with star formationefficiency comparable to that of the target galaxy UGC 12914 and similarto that in the Galactic disk. The distribution of atomic gas and dust in nearby galaxies - I. Presentation of matched-resolution VLA H I and SCUBA 850-μm mapsWe present matched-resolution VLA HI and SCUBA 850-μm maps of 20IRAS-bright galaxies. Of the galaxies observed, two were not detected inHI and two were detected in absorption. The HI distributions of thegalaxies have a range of morphologies. Some of the systems appear HIdeficient in the central regions which could be due to a high conversionrate of HI into molecules or HI absorption. In contrast to the HI, the850-μm emission has a smooth distribution which is concentratedtowards the optical centre of each galaxy. We also find evidence for850-μm emission extending to the periphery of the optical disc insome of the galaxies. Finally, we note that the relative lack of850-μm emission when compared with HI does not necessarily mean thatthe atomic gas and dust do not have similar mass distributions. An inverse method to recover the SFR and reddening properties from spectra of galaxiesWe develop a non-parametric inverse method to investigate the starformation rate, the metallicity evolution and the reddening propertiesof galaxies based on their spectral energy distributions (SEDs). Thisapproach allows us to clarify the level of information present in thedata, depending on its signal-to-noise ratio. When low resolution SEDsare available in the ultraviolet, optical and near-IR wavelength rangestogether, we conclude that it is possible to constrain the starformation rate and the effective dust optical depth simultaneously witha signal-to-noise ratio of 25. With excellent signal-to-noise ratios,the age-metallicity relation can also be constrained. We apply thismethod to the well-known nuclear starburst in the interacting galaxy NGC7714. We focus on deriving the star formation history and the reddeninglaw. We confirm that classical extinction models cannot provide anacceptable simultaneous fit of the SED and the lines. We also confirmthat, with the adopted population synthesis models and in addition tothe current starburst, an episode of enhanced star formation thatstarted more than 200 Myr ago is required. As the time elapsed since thelast interaction with NGC 7715, based on dynamical studies, is about 100Myr, our result reinforces the suggestion that this interaction mightnot have been the most important event in the life of NGC 7714. Multiwavelength Study of the Starburst Galaxy NGC 7714. II. The Balance between Young, Intermediate-Age, and Old StarsWe combine existing multiwavelength data (including an HST/GHRS UVspectrum and a ground-based optical spectrum) with unpublished HST/WFPC2images, near-IR photometry, and K-band spectroscopy. We use these datato constrain the young, the intermediate-age, and the old stellarpopulations in the central regions of the starburst galaxy NGC 7714. Ina previous paper the stellar features in the HST/GHRS ultraviolet (UV)spectrum and the optical emission lines were used to identify an ~5 Myrold, very little reddened stellar population as the main source of UVlight in the central ~330 pc. The optical data indicated the existenceof an older population. The nature of the latter is investigated here.Stellar absorption features in the optical and the near-IR are used tobreak partly the strong degeneracy between the effects of aging andthose of the inhomogeneous dust distribution on the UV-optical-IRcolors. Consistency with far-IR, X-ray, and radio data is alsoaddressed. The successful models have essential features in common. Wefind that the young burst responsible for the UV light represents only asmall part of an extended episode of enhanced star formation, initiateda few times 108 yr ago. The star formation rate is likely tohave varied on this timescale, averaging about 1 Msolaryr-1. The mass of young and intermediate-age stars thusformed equals at least 10% of the mass locked in preexisting stars ofthe underlying spiral galaxy nucleus, and fractions around 25% arefavored. The spectrophotometric star formation timescale is longcompared to the ~110 Myr elapsed since closest contact with theneighboring NGC 7715, according to the 1992 dynamical models of Smith& Wallin. The initial trigger of the starburst thus remains elusive.NGC 7714 owes its brightness in the UV to a few low extinction lines ofsight toward young stars. Our results based on the integratedspectrophotometry of the central ~330 pc are supported byhigh-resolution images of this area. The different extinction valuesobtained when different spectral indicators are used result naturallyfrom the coexistence of populations with various ages and obscurations.The near-IR continuum image looks smoothest, as a consequence of lowersensitivity to extinction and of a larger contribution of old stars. Wecompare the nuclear properties of NGC 7714 with results from studies inlarger apertures. We emphasize that the global properties of starburstgalaxies are the result of the averaging over many lines of sight withvery diverse properties in terms of obscuration and stellar ages. Theoverall picture is strongly reminiscent of the other nearbyprototypical'' starburst, M82. The Development of Gas/Star Offsets in Tidal TailsWe present models of interacting galaxies in order to study thedevelopment of spatial offsets between the gaseous and stellarcomponents in tidal tails. Observationally, such large-scale offsets areobserved in several systems-most notably NGC 3690-suggesting aninteraction between the tidal gas and some (unseen) hot interstellarmedium (ISM). Instead, our models show that these offsets are a naturalconsequence of the radially extended H I spatial distribution in diskgalaxies, coupled with internal dissipation in the gaseous componentdriven by the interaction. This mechanism is most effective in systemsinvolved in very prograde interactions and explains the observedgas/star offsets in interacting galaxies without invoking interactionswith a hot ISM, starburst ionization, or dust obscuration within thetails. The Nuclear and Circumnuclear Stellar Population in Seyfert 2 Galaxies: Implications for the Starburst-Active Galactic Nucleus ConnectionWe report the results of a spectroscopic investigation of a sample of 20of the brightest type 2 Seyfert nuclei. Our goal is to search for thedirect spectroscopic signature of massive stars and thereby probe therole of circumnuclear starbursts in the Seyfert phenomenon. The methodused is based on the detection of the higher order Balmer lines and He Ilines in absorption and the Wolf-Rayet feature at ~4680 Å inemission. These lines are strong indicators of the presence of young (afew Myr) and intermediate-age (a few 100 Myr) stellar populations. Inover half the sample, we have detected He I and/or strong stellarabsorption features in the high-order (near-UV) Balmer series togetherwith relatively weak lines from an old stellar population. In threeothers we detect a broad emission feature near 4680 Å that is mostplausibly ascribed to a population of Wolf-Rayet stars (the evolveddescendants of the most massive stars). We therefore conclude that theblue and near-UV light of over half of the sample is dominated by youngand/or intermediate age stars. The young'' Seyfert 2 galaxies havelarger far-IR luminosities, cooler mid/far-IR colors, and smaller [OIII]/Hβ flux ratios than the old'' ones. These differences areconsistent with a starburst playing a significant energetic role in theformer class. We consider the possibility that there may be two distinctsubclasses of Seyfert 2 nuclei (starbursts'' and hidden broadlineregions'' [BLRs]). However, the fact that hidden BLRs have been found inthree of the young'' nuclei argues against this and suggests thatnuclear starbursts may be a more general part of the Seyfert phenomenon. New Observations of Extra-Disk Molecular Gas in Interacting Galaxy Systems, Including a Two-Component System in Stephan's QuintetWe present new CO (1-0) observations of 11 extragalactic tails andbridges in nine interacting galaxy systems, almost doubling the numberof such features with sensitive CO measurements. Eight of these 11features were undetected in CO to very low CO/H I limits, with the mostextreme case being the NGC 7714/5 bridge. This bridge contains luminousH II regions and has a very high H I column density(1.6×1021 cm-2 in the 55" CO beam), yet wasundetected in CO to rms T*R=2.4 mK. The H I columndensity is higher than standard H2 and CO self-shieldinglimits for solar-metallicity gas, suggesting that the gas in this bridgeis metal-poor and has an enhancedNH2/ICO ratio compared with theGalactic value. Only one of the 11 features in our sample wasunambiguously detected in CO, a luminous H I-rich star formation regionnear an optical tail in the compact group Stephan's Quintet. We detectCO at two widely separated velocities in this feature, at ~6000 and~6700 km s-1. Both of these components have H I and Hαcounterparts. These velocities correspond to those of galaxies in thegroup, suggesting that this gas is material that has been removed fromtwo galaxies in the group. The CO/H I/Hα ratios for bothcomponents are similar to global values for spiral galaxies. Near-infrared line imaging of the starburst galaxies NGC 520, NGC 1614 and NGC 7714We present high spatial resolution ( ~ 0\farcs6) near-infraredbroad-band JHK images and Brgamma 2.1661 mu m and H_2 1-0 S(1) 2.122 mum emission line images of the nuclear regions in the interactingstarburst galaxies NGC 520, NGC 1614 and NGC 7714. The near-infraredemission line and radio morphologies are in general agreement, althoughthere are differences in details. In NGC 1614, we detect a nucleardouble structure in \BG, in agreement with the radio double structure.We derive average extinctions of A_K = 0.41 and A_K = 0.18 toward thenuclear regions of NGC 1614 and NGC 7714, respectively. For NGC 520, theextinction is much higher, A_K = 1.2-1.6. The observed H_2/Brgammaratios indicate that the main excitation mechanism of the molecular gasis fluorescence by intense UV radiation from clusters of hot youngstars, while shock excitation can be ruled out. The starburst regions inall galaxies exhibit small Brgamma equivalent widths. Assuming aconstant star formation model, even with a lowered upper mass cutoff ofM_u = 30 M_sun, results in rather old ages (10-40 Myr), in disagreementwith the clumpy near-infrared morphologies. We prefer a model of aninstantaneous burst of star formation with M_u = 100 M_sun, occurring ~6-7 Myr ago, in agreement with previous determinations and with thedetection of W-R features in NGC 1614 and NGC 7714. Finally, we note apossible systematic difference in the amount of hot molecular gasbetween starburst and Seyfert galaxies. Nearby Optical Galaxies: Selection of the Sample and Identification of GroupsIn 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. Tidally Triggered Star Formation in Close Pairs of GalaxiesWe analyze optical spectra of a sample of 502 galaxies in close pairsand N-tuples, separated by <=50 h-1 kpc. We extracted thesample objectively from the CfA2 redshift survey, without regard to thesurroundings of the tight systems; we remeasure the spectra with longerexposures, to explore the spectral characteristics of the galaxies. Weuse the new spectra to probe the relationship between star formation andthe dynamics of the systems of galaxies. The equivalent widths ofHα [EW(Hα)] and other emission lines anticorrelate stronglywith pair spatial separation (ΔD) and velocity separation; theanticorrelations do not result from any large-scale environmentaleffects that we detect. We use the measured EW(Hα) and thestarburst models of Leitherer et al. to estimate the time since the mostrecent burst of star formation began for galaxies in our sample. In theabsence of a large contribution from an old stellar population to thecontinuum around Hα that correlates with the orbit parameters, theobserved ΔD-EW(Hα) correlation signifies that starburstswith larger separations on the sky are, on average, older. We also finda population of galaxies with small to moderate amounts of Balmerabsorption. These galaxies support our conclusion that the sampleincludes many aging bursts of star formation; they have a narrowerdistribution of velocity separations, consistent with a population oforbiting galaxies near apogalacticon. By matching the dynamicaltimescale to the burst timescale, we show that the data support a simplepicture in which a close pass initiates a starburst; EW(Hα)decreases with time as the pair separation increases, accounting for theanticorrelation. Recent N-body/smoothed particle hydrodynamicssimulations of interacting pairs suggest a physical basis for thecorrelation-for galaxies with shallow central potentials, they predictgas infall before the final merger. This picture leads to a method formeasuring the duration and the initial mass function ofinteraction-induced starbursts: our data are compatible with thestarburst models and orbit models in many respects, as long as thestarburst lasts longer than ~108 yr and the delay between theclose pass and the initiation of the starburst is less than a few times107 yr. If there is no large contribution from an old stellarpopulation to the continuum around Hα, the Miller-Scalo and cutoff(M<=30 Msolar) Salpeter initial mass functions (IMFs) fitthe data much better than a standard Salpeter IMF. The Neutral Hydrogen Distribution in Merging Galaxies: Differences between Stellar and Gaseous Tidal MorphologiesAs part of several H I synthesis-mapping studies of merging galaxies, wehave mapped the tidal gas in the three disk-disk merger systems Arp 157(NGC 520), Arp 220, and Arp 299 (NGC 3690). These systems differ fromthe majority of the mergers mapped in H I in that their stellar andgaseous tidal features do not coincide. In particular, they exhibitlarge stellar tidal features with little if any accompanying neutral gasand large gas-rich tidal features with little if any accompanyingstarlight. On smaller scales, there are striking anticorrelations inwhich the gaseous and stellar tidal features appear to cross. We exploreseveral possible causes for these differences, including dustobscuration, ram pressure stripping, and ionization effects. No singleexplanation can account for all of the observed differences. The factthat each of these systems shows evidence for a starburst-drivensuperwind expanding in the direction of the most strikinganticorrelations leads us to suggest that the superwind is primarilyresponsible for the observed differences, either by sweeping thefeatures clear of gas via ram pressure or by excavating a clearsightline toward the starburst and allowing UV photons to ionize regionsof the tails. If this suggestion is correct, only systems hosting agalactic superwind and experiencing a high-inclination encountergeometry (such that tidal gas is lifted high above the starburstregions) should exhibit such extreme differences between their H I andoptical tidal morphologies. H I observations of emission-line galaxiesWe present single-dish Lovell telescope H i observations of a sample of67 emission-line and UV-excess galaxies, of which 52 are taken from theUniversity of Michigan (UM) catalogue. In addition, H i observations of24 gas-rich irregular galaxies are presented. We find that emission-linegalaxies are H i-rich with a median H i mass to blue luminosity ratioMHI/LB of ~ 0.45 Msun/Lsun.Within the UM galaxy sample the MHI/LB ratio tendsto increase with decreasing luminosity. Finally, it is found that themost H i-rich UM galaxies are the most metal deficient, implying thatthese objects are less evolved. ISO observations of the Wolf-Rayet galaxy NGC 7714 and its companion NGC 7715The interacting system Arp 284 consisting of the Wolf-Rayet galaxy NGC7714 and its irregular companion NGC 7715 was observed using theInfrared Space Observatory . Deconvolved ISOCAM maps of the galaxiesusing the 14.3 μm, 7.7 μm and 15 μm LW3, LW6 and LW9 filters,along with ISOPHOT spectrometry of the nuclear region of NGC 7714 wereobtained and are presented. Strong ISOCAM emission was detected from thecentral source in NGC 7714, along with strong PAH features, the emissionline [Ar II], molecular hydrogen at 9.66 μm and a blend of featuresincluding [S IV] at 10.6 μm. IR emission was not detected from thecompanion galaxy NGC 7715, the bridge linking the two galaxies or fromthe partial stellar ring in NGC 7714 where emission ceases abruptly atthe interface between the disk and the ring. The morphology of thesystem can be well described by an off-centre collision between the twogalaxies. The LW3/LW2 ratio, where the LW2 flux was synthesized from thePHT-S spectrum, LW9/LW6 and LW3/LW6 ratios suggest that the centralburst within NGC 7714 is moving towards the post-starburst phase, inagreement with the age of the burst. Diagnostic tools including theratio of the integrated PAH luminosity to the 40 to 120 μm infraredluminosity and the far-infrared colours reveal that despite the highsurface brightness of the nucleus, the properties of NGC 7714 can beexplained in terms of a starburst and do not require the prescence anAGN. Based on observations with ISO, an ESA project with instrumentsfunded by ESA member States (especially the PI countries: France,Germany, the Netherlands and the United Kingdom) and with the .Deconvolved ISOCAM 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.
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