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# NGC 4559

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 A dynamical model for the extraplanar gas in spiral galaxiesRecent HI observations reveal that the discs of spiral galaxies aresurrounded by extended gaseous haloes. This extraplanar gas reacheslarge distances (several kiloparsecs) from the disc and shows peculiarkinematics (low rotation and inflow). We have modelled the extraplanargas as a continuous flow of material from the disc of a spiral galaxyinto its halo region. The output of our models is pseudo data cubes thatcan be directly compared to the HI data. We have applied these models totwo spiral galaxies (NGC 891 and NGC 2403) known to have a substantialamount of extraplanar gas. Our models are able to reproduce accuratelythe vertical distribution of extraplanar gas for an energy inputcorresponding to a small fraction (<4 per cent) of the energyreleased by supernovae. However, they fail in two important aspects: (1)they do not reproduce the right gradient in rotation velocity; (2) theypredict a general outflow of the extraplanar gas, contrary to what isobserved. We show that neither of these difficulties can be removed ifclouds are ionized and invisible at 21cm as they leave the disc butbecome visible at some point on their orbits. We speculate that thesefailures indicate the need for accreted material from the intergalacticmedium that could provide the low angular momentum and inflow required. Optical Counterparts of Ultraluminous X-Ray Sources Identified from Archival HST WFPC2 ImagesWe present a systematic analysis of archival HST WFPC2 Association''data sets that correlate with the Chandra positions of a set of 44ultraluminous X-ray sources (ULXs) of nearby galaxies. The mainmotivation is to address the nature of ULXs by searching for opticalcounterparts. Sixteen of the ULXs are found in early-type galaxies (RC3Hubble type <3). We have improved the Chandra/HST relative astrometrywhenever possible, resulting in errors circles of 0.3"-1.7" in size.Disparate numbers of potential ULX counterparts are found, and in somecases none are found. The lack of or low number of counterparts in somecases may be due to insufficient depth in the WFPC2 images. Particularlyin late-type galaxies, the HST image in the ULX region was often complexor crowded, requiring source detection to be performed manually. Wetherefore address various scenarios for the nature of the ULX since itis not known which, if any, of the sources found are true counterparts.The optical luminosities of the sources are typically in the range104-106 Lsolar, with (effective) Vmagnitudes typically in the range 22-24. In several cases colorinformation is available, with the colors roughly tending to be more redin early-type galaxies. This suggests that, in general, the (potential)counterparts found in early-type galaxies are likely to be older stellarpopulations and are probably globular clusters. Several early-typegalaxy counterparts have blue colors, which may be due to youngerstellar populations in the host galaxies, however, these could also bebackground sources. In spiral galaxies the sources may also be due tolocalized structure in the disks rather than bound stellar systems.Alternatively, some of the counterparts in late-type galaxies may beisolated supergiant stars. The observed X-ray/optical flux ratio isdiluted by the optical emission of the cluster in cases where the systemis an X-ray binary in a cluster, particularly in the case of a low-massX-ray binaries in an old cluster. If any of the counterparts are boundsystems with ~104-106 stars and are the truecounterparts to the ULX sources, then the X-ray luminosities of the ULXare generally well below the Eddington limit for a black hole with mass~0.1% of the cluster mass. Finally, we find that the optical flux of thecounterparts is consistent with being dominated by emission from anaccretion disk around an intermediate-mass black hole if the black holehappens to have a mass >~102 Msolar and isaccreting at close to the Eddington rate, unless the accretion disk isirradiated (which would result in high optical disk luminosities atlower black hole 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 NAS 5-26555. This project isassociated with Archival proposal 9545. Close Binary Interactions of Intermediate-Mass Black Holes: Possible Ultraluminous X-Ray Sources?While many observed ultraluminous X-ray sources (ULXs;LX>=1039 ergs s-1) could beextragalactic X-ray binaries (XRBs) emitting close to the Eddingtonlimit, the highest luminosity ULXs(LX>3×1039 ergs s-1) exceedthe isotropic Eddington luminosity for even high-stellar-mass-accretingblack hole XRBs. It has been suggested that these highest luminosityULXs may contain accreting intermediate-mass black hole (IMBH) binaries.We consider this hypothesis for dense, young (~100 Myr) stellar clusterswhere we assume that a 50-500 Msolar central IMBH has formedthrough runaway growth of a massive star. We develop numericalsimulations of such clusters' cores by combining single and binary starevolutionary syntheses with a simple treatment of dynamicalinteractions. We model interactions of the IMBH with single and binarystars, as well as single-binary and binary-binary interactions, but notthe formation of a cusp around the IMBH. The core density and velocitydispersion are assumed to be constant over 100 Myr. We investigate thesuccession of IMBH binary companions and the evolution of their orbitalparameters to obtain estimates of the incidence of mass transfer phasesand possible ULX activity involving the IMBH. We find that although itis common for the central black hole to acquire binary companions, thereis a very low probability that these interacting binaries will becomeobservable ULX sources. Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT HRI Observations. II. Statistical PropertiesThe statistical properties of the nonnuclear X-ray point sources fromthe ROSAT HRI survey of nearby galaxies in Paper I are studied, withparticular attention to the contamination from background and/orforeground objects. This study reveals a statistical preference for theultraluminous X-ray sources (ULXs) to occur in late-type galaxies overearly-type galaxies, and in starburst/H II galaxies over nonstarburstgalaxies. There is a trend of greater occurrence frequencies and ULXrates for galaxies with increasing star formation rates, confirming theconnection between the ULX phenomenon and the star formation. Anonlinear correlation is found between the number of ULXs and the starformation rate, with significantly more ULXs at low star formation ratesthan the ULX population expected from the high-mass X-ray binaries(HMXBs) as an indicator of the star formation and the accompanying youngstellar population, suggestive of another population of ULXs associatedwith the low-mass X-ray binaries (LMXBs) and the old stellar population.There are no breaks around 1039 ergs s-1 in theluminosity functions of ULXs in all galaxies or in late-type galaxies,suggesting the regular ULXs below 1040 ergs s-1are a high-luminosity extension of the ordinary HMXB/LMXB populationsbelow 1039 ergs s-1. There is evidence that theextreme ULXs above 1040 ergs s-1 might be adifferent ULX class from the regular ULXs below 1040 ergss-1, although a larger sample with more ULXs is needed toestablish the statistical properties of the extreme ULXs as a class. Toward a clean sample of ultra-luminous X-ray sourcesContext: .Observational follow-up programmes for the characterization ofultra-luminous X-ray sources (ULXs) require the construction of cleansamples of such sources in which the contamination byforeground/background sources is minimum. Aims: .We calculate thedegree of foreground/background contaminants among the ULX samplecandidates in a published catalogue and compare these computations withavailable spectroscopic identifications. Methods: .We usestatistics based on known densities of X-ray sources and AGN/QSOsselected in the optical. The analysis is done individually for eachparent galaxy. The existing identifications of the optical counterpartsare compiled from the literature. Results: .More than a half ofthe ULXs, within twice the distance of the major axis of the 25mag/arcsec2 isophote from RC3 nearby galaxies and with X-rayluminosities L_X[ 2-10 keV] ≥ 1039 erg/s, are expected tobe high redshift background QSOs. A list of 25 objects (clean sample)confirmed to be real ULXs or to have a low probability of beingcontaminant foreground/background objects is provided. A catalogue of ultra-luminous X-ray source coincidences with FIRST radio sourcesAims.We search for ultra luminous X-ray source (ULXs) radio counterpartslocated in nearby galaxies in order to constrain their physicalnature. Methods: .Our work is based on a systematiccross-identification of the most recent and extensive available ULXcatalogues and archival radio data. Results: .A catalogue of 70positional coincidences is reported. Most of them are located within thegalaxy nucleus. Among them, we find 11 new cases of non-nuclear ULXsources with possibly associated radio emission. Reconciling the local galaxy population with damped Lyman α cross-sections and metal abundancesA comprehensive analysis of 355 high-quality Westerbork Synthesis RadioTelescope (WSRT) HI 21-cm line maps of nearby galaxies shows that theproperties and incident rate of damped Lyman α absorption systems(DLAs) observed in the spectra of high-redshift QSOs are in goodagreement with DLAs originating in gas discs of galaxies like those inthe z~ 0 population. Comparison of low-z DLA statistics with the HIincidence rate and column density distribution f(NHI) for thelocal galaxy sample shows no evidence for evolution in the integralcross-section density'=l-1 (l= mean freepath between absorbers) below z~ 1.5, implying that there is no need fora hidden population of galaxies or HI clouds to contribute significantlyto the DLA cross-section. Compared with z~ 4, our data indicateevolution of a factor of 2 in the comoving density along a line ofsight. We find that dN/dz(z= 0) = 0.045 +/- 0.006. The idea that thelocal galaxy population can explain the DLAs is further strengthened bycomparing the properties of DLAs and DLA galaxies with the expectationsbased on our analysis of local galaxies. The distribution ofluminosities of DLA host galaxies, and of impact parameters between QSOsand the centres of DLA galaxies, is in good agreement with what isexpected from local galaxies. Approximately 87 per cent of low-z DLAgalaxies are expected to be fainter than L*, and 37 per centhave impact parameters less than 1 arcsec at z= 0.5. The analysis showsthat some host galaxies with very low impact parameters and lowluminosities are expected to be missed in optical follow-up surveys. Thewell-known metallicity-luminosity relation in galaxies, in combinationwith metallicity gradients in galaxy discs, causes the expected medianmetallicity of low-z DLAs to be low (~1/7 solar), which is also in goodagreement with observations of low-z DLAs. We find thatf(NHI) can be fitted satisfactorily with a gammadistribution, a single power law is not a good fit at the highest columndensities NHI > 1021cm-2. The vastmajority (~81 per cent) of the HI gas in the local Universe resides incolumn densities above the classical DLA limit (NHI > 2× 1020cm-2), with NHI~1021cm-2 dominating the cosmic HI mass density. Optical and infrared signatures of ultra-luminous X-ray sourcesWe have constructed a model to describe the optical emission fromultra-luminous X-ray sources (ULXs). We assume a binary model with ablack hole accreting matter from a Roche lobe filling companion star. Weconsider the effects of radiative transport and radiative equilibrium inthe irradiated surfaces of both the star and a thin accretion disc. Wehave developed this model as a tool with which to positively identifythe optical counterparts of ULXs, and subsequently derive parameterssuch as the black hole mass and the luminosity class and spectral typeof the counterpart. We examine the dependence of the optical emission onthese and other variables. We extend our model to examine the magnitudevariation at infrared wavelengths, and we find that observations atthese wavelengths may have more diagnostic power than in the optical. Weapply our model to existing HST observations of the candidates for theoptical counterpart of ULX X-7 in NGC 4559. All candidates could beconsistent with an irradiated star alone, but we find that a number ofthem are too faint to fit with an irradiated star and disc together.Were one of these the optical counterpart to X-7, it would display asignificant temporal variation. Imprints of spiral arms in the oxygen distribution over the galactic discA theory for the oxygen abundance radial distribution formation in thegalactic disc of a spiral galaxy is developed. We take into account thatthe main sources of oxygen are Type II supernovae (SN II), theprogenitors of which are massive short-lived stars strongly concentratedin the spiral arms. Hence oxygen is the most sensitive indicator ofspiral arms' influence on galactic disc enrichment by heavy elements.Various models for the spiral density waves were analysed. We predictthat the imprints in the oxygen radial distribution will enable us todistinguish between different models for spiral patterns. Among otherparameters, the corotation radius happens to be one of the mostimportant. 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. XMM-Newton View of the Ultraluminous X-Ray Sources in M51We present results based on XMM-Newton observations of the nearby spiralgalaxy M51 (NGC 5194 and NGC 5195). We confirm the presence of the sevenknown ultraluminous X-ray sources (ULXs) with luminosities exceeding theEddington luminosity for a 10 Msolar black hole, alow-luminosity active galactic nucleus (LLAGN) with 2-10 keV luminosityof 1.6×1039 ergs s-1, and soft thermalextended emission from NGC 5194 detected with Chandra. In addition, wealso detected a new ULX with luminosity of ~1039 ergss-1. We have studied the spectral and temporal properties ofthe LLAGN and eight ULXs in NGC 5194 and an ULX in NGC 5195. Two ULXs inNGC 5194 show evidence for short-term variability, and all but two ULXsvary on long timescales (over a baseline of ~2.5 yr), providing strongevidence that these are accreting sources. One ULX in NGC 5194, source69, shows possible periodic behavior in its X-ray flux. We derive aperiod of 5925+/-200 s at a confidence level of 95% on the basis ofthree cycles. This period is lower than the period of 7620+/-500 sderived from a Chandra observation in 2000. The higher effective area ofXMM-Newton enables us to identify multiple components in the spectra ofULXs. Most ULXs require at least two components, a power law and a softX-ray excess component that is modeled by an optically thin plasma or amulticolor disk blackbody (MCD). However, the soft excess emissionsinferred from all ULXs except source 69 are unlikely to be physicallyassociated with the ULXs, as their strengths are comparable to that ofthe surrounding diffuse emission. The soft excess emission of source 69is well described either by a two-temperature MEKAL plasma or asingle-temperature MEKAL plasma (kT~690 eV) and an MCD (kT~170 eV). TheMCD component suggests a cooler accretion disk compared to those inGalactic X-ray binaries, consistent with those expected forintermediate-mass black holes (IMBHs). An iron Kα line (EW~700 eV)or K absorption edge at ~7.1 keV is present in the EPIC pn spectrum ofsource 26. The spectrum of the ULX in NGC 5195, source 12, is consistentwith a simple power law. The LLAGN in NGC 5194 shows an extremely flathard X-ray power law (Γ~0.7), a narrow iron Kα line at 6.4keV (EW~3 keV), and strong soft X-ray excess emission. The full-bandspectrum is well described by a two-component MEKAL plasma andreflection from cold material such as a putative torus. XMM-Newton Observations of Ultraluminous X-Ray Sources in Nearby GalaxiesWe examined X-ray spectral and timing properties of ultraluminous X-raysources (ULXs) in nearby galaxies in XMM-Newton archival data. Thereappear to be three distinct classes of spectra. One class shows emissionfrom hot, diffuse plasma. This thermal emission is similar to that seenfrom recent supernovae; the temperatures are in the range 0.6-0.8 keV,and the luminosities are the lowest in our sample, near 1039ergs s-1. Three sources have spectra that are strongly curvedat high energies and have the highest temperatures in our sample,1.0-1.4 keV. These spectra are well fitted with a power-law plusmulticolor disk blackbody model with the power law dominant at lowenergies or a Comptonization model. The remainder of the sources arebest fitted with a power-law plus multicolor disk blackbody model, as iscommonly used to describe the spectra of accreting black holes. Thesesources have the lowest thermal component temperatures, 0.1-0.4 keV, andextend to the highest luminosities, above 1040 ergss-1. The temperature of the thermal component is in threedistinct ranges for the three source classes. This diversity of spectralshapes and the fact that the sources lie in three distinct temperatureranges suggests that the ULXs are a diverse population. Two ULXs thatshow state transitions stay within a single class over the course of thetransition. However, we cannot conclude with certainty that the classesrepresent distinct types of objects rather than spectral states of asingle population of objects. More monitoring observations of ULXs withXMM-Newton are required. We also searched for timing noise from thesources and report detection of noise above the Poisson level from fivesources. In three of the sources, the power density spectrum increaseswith decreasing frequency as a power law down to the lowest frequenciesobserved, below 10-4 Hz. Infrared Spectral Energy Distributions of Nearby GalaxiesThe Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out acomprehensive multiwavelength survey on a sample of 75 nearby galaxies.The 1-850 μm spectral energy distributions (SEDs) are presented usingbroadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. Theinfrared colors derived from the globally integrated Spitzer data aregenerally consistent with the previous generation of models that weredeveloped using global data for normal star-forming galaxies, althoughsignificant deviations are observed. Spitzer's excellent sensitivity andresolution also allow a detailed investigation of the infrared SEDs forvarious locations within the three large, nearby galaxies NGC 3031(M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapesis found within each galaxy, especially for NGC 3031, the closest of thethree targets and thus the galaxy for which the smallest spatial scalescan be explored. Strong correlations exist between the local starformation rate and the infrared colors fν(70μm)/fν(160 μm) and fν(24μm)/fν(160 μm), suggesting that the 24 and 70 μmemission are useful tracers of the local star formation activity level.Preliminary evidence indicates that variations in the 24 μm emission,and not variations in the emission from polycyclic aromatic hydrocarbonsat 8 μm, drive the variations in the fν(8.0μm)/fν(24 μm) colors within NGC 3031, NGC 5194, andNGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sampleare representative of the range present at high redshift, thenextrapolations of total infrared luminosities and star formation ratesfrom the observed 24 μm flux will be uncertain at the factor of 5level (total range). The corresponding uncertainties using theredshifted 8.0 μm flux (e.g., observed 24 μm flux for a z=2source) are factors of 10-20. Considerable caution should be used wheninterpreting such extrapolated infrared luminosities. Secular Evolution via Bar-driven Gas Inflow: Results from BIMA SONGWe present an analysis of the molecular gas distributions in the 29barred and 15 unbarred spirals in the BIMA CO (J=1-0) Survey of NearbyGalaxies (SONG). For galaxies that are bright in CO, we confirm theconclusion by Sakamoto et al. that barred spirals have higher moleculargas concentrations in the central kiloparsec. The SONG sample alsoincludes 27 galaxies below the CO brightness limit used by Sakamoto etal. Even in these less CO-bright galaxies we show that high central gasconcentrations are more common in barred galaxies, consistent withradial inflow driven by the bar. However, there is a significantpopulation of early-type (Sa-Sbc) barred spirals (6 of 19) that have nomolecular gas detected in the nuclear region and have very little out tothe bar corotation radius. This suggests that in barred galaxies withgas-deficient nuclear regions, the bar has already driven most of thegas within the bar corotation radius to the nuclear region, where it hasbeen consumed by star formation. The median mass of nuclear moleculargas is over 4 times higher in early-type bars than in late-type (Sc-Sdm)bars. Since previous work has shown that the gas consumption rate is anorder of magnitude higher in early-type bars, this implies that theearly types have significantly higher bar-driven inflows. The loweraccretion rates in late-type bars can probably be attributed to theknown differences in bar structure between early and late types. Despitethe evidence for bar-driven inflows in both early and late Hubble-typespirals, the data indicate that it is highly unlikely for a late-typegalaxy to evolve into an early type via bar-induced gas inflow.Nonetheless, secular evolutionary processes are undoubtedly present, andpseudobulges are inevitable; evidence for pseudobulges is likely to beclearest in early-type galaxies because of their high gas inflow ratesand higher star formation activity. Optical Sources near the Bright X-Ray Source in NGC 1073New HST observations show that the bright X-ray source in the face-onspiral galaxy NGC 1073 is located near a ring of recent star formationwith an age of 8-16 Myr. This strengthens the association of X-raysources in spiral galaxies emitting near or above the Eddington limitfor a 20 Msolar black hole with recent star formation events.Two candidate optical counterparts of the X-ray source are found. TheX-ray-to-optical flux ratios of both are consistent with those oflow-mass X-ray binaries and higher than most high-mass X-ray binaries,suggesting that reprocessing of X-rays contributes to the optical light.The optical magnitude and color of one candidate is consistent with thatpredicted for an X-ray binary with an initial donor mass of 6-8Msolar. However, the same X-ray binary evolution modelunderestimates the X-ray luminosity. An X-ray source list for the fieldis presented, which includes detections of the nucleus of NGC 1073,three quasars, and an M3e brown dwarf star with high proper motion. The Opacity of Spiral Galaxy Disks. IV. Radial Extinction Profiles from Counts of Distant Galaxies Seen through Foreground DisksDust extinction can be determined from the number of distant fieldgalaxies seen through a spiral disk. To calibrate this number for thecrowding and confusion introduced by the foreground image,González et al. and Holwerda et al. developed the Synthetic FieldMethod (SFM), which analyzes synthetic fields constructed by addingvarious deep exposures of unobstructed background fields to thecandidate foreground galaxy field. The advantage of the SFM is that itgives the average opacity for the area of a galaxy disk without makingassumptions about either the distribution of absorbers or of the diskstarlight. However, it is limited by poor statistics on the survivingfield galaxies, hence the need to combine a larger sample of fields.This paper presents the first results for a sample of 32 deep HubbleSpace Telescope (HST)/WFPC2 archival fields of 29 spiral galaxies. Theradial profiles of average dust extinction in spiral galaxies based oncalibrated counts of distant field galaxies is presented here, both forindividual galaxies and for composites from our sample. The effects ofinclination, spiral arms, and Hubble type on the radial extinctionprofile are discussed. The dust opacity of the disk apparently arisesfrom two distinct components: an optically thicker (AI=0.5-4mag) but radially dependent component associated with the spiral armsand a relatively constant optically thinner disk (AI~0.5mag). These results are in complete agreement with earlier work onocculted galaxies. The early-type spiral disks in our sample show lessextinction than the later types. Low surface brightness galaxies, andpossibly Sd's, appear effectively transparent. The average color of thefield galaxies seen through foreground disks does not appear to changewith radius or opacity. This gray behavior is most likely due to thepatchy nature of opaque clouds. The average extinction of a radialannulus and its average surface brightness seem to correlate for thebrighter regions. This leads to the conclusion that the brighter partsof the spiral disk, such as spiral arms, are also the ones with the mostextinction associated with them. Active and Star-forming Galaxies and Their SupernovaeTo investigate the extent to which nuclear starbursts or other nuclearactivity may be connected with enhanced star formation activity in thehost galaxy, we perform a statistical investigation of supernovae (SNe)discovered in host galaxies from four samples: the Markarian galaxiessample, the Second Byurakan Survey (SBS) sample, the north Galactic pole(NGP) sample of active or star-forming galaxies, and the NGP sample ofnormal galaxies. Forty-seven SNe in 41 Mrk galaxies, 10 SNe in six SBSgalaxies, 29 SNe in 26 NGP active or star-forming galaxies, and 29 SNein 26 NGP normal galaxies have been studied. We find that the rate ofSNe, particularly core-collapse (Types Ib/c and II) SNe, is higher inactive or star-forming galaxies in comparison with normal galaxies.Active or star-forming host galaxies of SNe are generally of latermorphological type and have lower luminosity and smaller linear sizethan normal host galaxies of SNe. The radial distribution of SNe inactive and star-forming galaxies shows a higher concentration toward thecenter of the active host galaxy than is the case for normal hostgalaxies, and this effect is more pronounced for core-collapse SNe.Ib/c-type SNe have been discovered only in active and star-forminggalaxies of our samples. About 78% of these SNe are associated with H IIregions or are located very close to the nuclear regions of these activegalaxies, which are in turn hosting AGNs or starburst nuclei. Besidesthese new results, our study also supports the conclusions of severalother earlier papers. We find that Type Ia SNe occur in all galaxytypes, whereas core-collapse SNe of Types Ib/c and II are found only inspiral and irregular galaxies. The radial distribution of Type Ib SNe intheir host galaxies is more centrally concentrated than that of Type IIand Ia SNe. The radial distances of Types Ib/c and II SNe, from thenuclei of their host galaxies, is larger for barred spiral hosts.Core-collapse SNe are concentrated in spiral arms and are often close toor in the H II regions, whereas Type Ia SNe show only a looseassociation with spiral arms and no clear association with H II regions. Gas and Stars in an H I-Selected Galaxy SampleWe present the results of a J-band study of the H I-selected AreciboDual-Beam Survey and Arecibo Slice Survey galaxy samples using TwoMicron All Sky Survey data. We find that these galaxies span a widerange of stellar and gas properties. However, despite the diversitywithin the samples, we find a very tight correlation between luminosityand size in the J band, similar to that found in a previous paper byRosenberg & Schneider between the H I mass and size. We also findthat the correlation between the baryonic mass and the J-band diameteris even tighter than that between the baryonic mass and the rotationalvelocity. The opacity of spiral galaxy disks. V. Dust opacity, HI distributions and sub-mm emissionThe opacity of spiral galaxy disks, from counts of distant galaxies, iscompared to HI column densities. The opacity measurements are calibratedusing the "Synthetic Field Method" from González et al. (1998,ApJ, 506, 152), Holwerda et al. (2005a, AJ, 129, 1381). When comparedfor individual disks, the HI column density and dust opacity do not seemto be correlated as HI and opacity follow different radial profiles. Toimprove statistics, an average radial opacity profile is compared to anaverage HI profile. Compared to dust-to-HI estimates from theliterature, more extinction is found in this profile. This differencemay be accounted for by an underestimate of the dust in earliermeasurements due to their dependence on dust temperature. Since the SFMis insensitive to the dust temperature, the ratio between the SFMopacity and HI could very well be indicative of the true ratio. Earlierclaims for a radially extended cold dust disk were based on sub-mmobservations. A comparison between sub-mm observations and counts ofdistant galaxies is therefore desirable. We present the best currentexample of such a comparison, M 51, for which the measurements seem toagree. However, this remains an area where improved counts of distantgalaxies, sub-mm observations and our understanding of dust emissivityare needed. Extra-planar gas in the spiral galaxy NGC 4559We present 21-cm line observations of the spiral galaxy NGC 4559, madewith the Westerbork Synthesis Radio Telescope. We have used them tostudy the H I distribution and kinematics, the relative amount anddistribution of luminous and dark matter in this galaxy and, inparticular, the presence of extra-planar gas. Our data do reveal thepresence of such a component, in the form of a thick disk, with a massof 5.9 × 108~Mȯ (one tenth of the totalH I mass) and a mean rotation velocity 25-50 km s-1 lowerthan that of the thin disk. The extra-planar gas may be the result ofgalactic fountains but accretion from the IGM cannot be ruled out. Withthis study we confirm that lagging, thick H I layers are likely to becommon in spiral galaxies. A catalogue of ultraluminous X-ray sources in external galaxiesWe present a catalogue of ultraluminous X-ray sources (ULXs) in externalgalaxies. The aim of this catalogue is to provide easy access to theproperties of ULXs, their possible counterparts at other wavelengths(optical, IR, and radio), and their host galaxies. The cataloguecontains 229 ULXs reported in the literature until April 2004. Most ULXsare stellar-mass-black hole X-ray binaries, but it is not excluded thatsome ULXs could be intermediate-mass black holes. A small fraction ofthe candidate ULXs may be background Active Galactic Nuclei (AGN) andSupernova Remnants (SNRs). ULXs with luminosity above 1040ergs s-1 are found in both starburst galaxies and in thehalos of early-type galaxies.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/1125 A ULX in NGC 4559: a mini-cartwheel'' scenario?We have studied the peculiar environment around a ULX in NGC 4559 (withL[x] ≈ 2×1040 erg s-1 and M[BH] ≳50Mȯ). The X-ray source is located near the rim of ayoung (age < 30 Myr), large (diameter ≈ 700 pc) ring-like starforming complex possibly triggered by the impact of a dwarf satellitegalaxy through the gas-rich outer disk of NGC 4559. We speculate thatgalaxy interactions (including the infall of high-velocity clouds andsatellites on a galactic disk) and low-metallicity environments offerfavourable conditions for the formation of compact remnants more massivethan standard'' X-ray binaries, and accreting from a massiveRoche-lobe filling companion. Chandra observations of five ultraluminous X-ray sources in nearby galaxies We report the results of a programme of dual-epoch Chandra ACIS-Sobservations of five ultraluminous X-ray sources (ULXs) in nearby spiralgalaxies. All five ULXs are detected as unresolved, point-like X-raysources by Chandra, though two have faded below the 1039 ergs-1 luminosity threshold used to first designate thesesources as ULXs. Using this same criterion, we detect three further ULXswithin the imaged regions of the galaxies. The ULXs appear to be relatedto the star-forming regions of the galaxies, indicating that even innormal spiral galaxies the ULX population is predominantly associatedwith young stellar populations. A detailed study of the Chandra ACIS-Sspectra of six of the ULXs shows that five are better described by apower-law continuum than a multicolour disc blackbody model, thoughthere is evidence for additional very soft components to two of thepower-law continua. The measured photon indices in four out of fivecases are consistent with the low/hard state in black hole binaries,contrary to the suggestion that power-law-dominated spectra of ULXsoriginate in the very high state. A simple interpretation of this isthat we are observing accretion on to intermediate-mass black holes,though we might also be observing a spectral state unique to very highmass accretion rates in stellar-mass black hole systems. Short-term fluxvariability is only detected in one of two epochs for two of the ULXs,with the lack of this characteristic arguing that the X-ray emission ofthis sample of ULXs is not dominated by relativistically beamed jets.The observational characteristics of this small sample suggest that ULXsare a distinctly heterogeneous source class. Probable intermediate-mass black holes in NGC 4559: XMM-Newton spectral and timing constraintsWe have examined X-ray and optical observations of two ultraluminousX-ray sources, X7 and X10 in NGC 4559, using XMM-Newton, Chandra and theHubble Space Telescope (HST). The ultraviolet/X-ray luminosity of X7exceeds 2.1 × 1040 erg s-1 in the XMM-Newtonobservation, and that of X10 is > 1.3 × 1040 ergs-1. X7 has both thermal and power-law spectral components,The characteristic temperature of the thermal component is 0.12 keV. Thepower-law components in the two sources both have slopes with photonindex ~=2.1. A timing analysis of X7 indicates a break frequency at 28MHz in the power spectrum, while that for X10 is consistent with anunbroken power law. The luminosity of the blackbody component in theX-ray spectrum of X7 and the nature of its time-variability providesevidence that this object is an intermediate-mass black hole accretingat sub-Eddington rates, but other scenarios which require high advectionefficiencies from a hollowed-out disc might be possible. The emissionfrom X10 can be characterized by a single power-law. This source can beinterpreted either as an intermediate-mass black hole, or as a stellarmass black hole with relativistically beamed Comptonized emission. Thereare four optical counterparts in the error circle of X7. No counterpartsare evident in the error circle for X10. >From Star Formation to Compact Remnants: X-ray Studies of Spiral GalaxiesHigh-resolution multiwavelength studies of nearby galaxies are essentialto understand the full cycle of star formation, and the balance betweengas recycling and creation of compact objects. In particular, we studythis cycle in the environment of ultra-luminous X-ray sources. The Ultraluminous X-Ray Source Population from the Chandra Archive of GalaxiesOne hundred fifty-four discrete non-nuclear ultraluminous X-ray (ULX)sources, with spectroscopically determined intrinsic X-ray luminositiesgreater than 1039 ergs s-1, are identified in 82galaxies observed with Chandra's Advanced CCD Imaging Spectrometer.Source positions, X-ray luminosities, and spectral and timingcharacteristics are tabulated. Statistical comparisons between theseX-ray properties and those of the weaker discrete sources in the samefields (mainly neutron star and stellar-mass black hole binaries) aremade. Sources above ~1038 ergs s-1 display similarspatial, spectral, color, and variability distributions. In particular,there is no compelling evidence in the sample for a new and distinctclass of X-ray object such as the intermediate-mass black holes.Eighty-three percent of ULX candidates have spectra that can bedescribed as absorbed power laws with index <Γ>=1.74 andcolumn density =2.24×1021cm-2, or ~5 times the average Galactic column. About 20% ofthe ULXs have much steeper indices indicative of a soft, and likelythermal, spectrum. The locations of ULXs in their host galaxies arestrongly peaked toward their galaxy centers. The deprojected radialdistribution of the ULX candidates is somewhat steeper than anexponential disk, indistinguishable from that of the weaker sources.About 5%-15% of ULX candidates are variable during the Chandraobservations (which average 39.5 ks). Comparison of the cumulative X-rayluminosity functions of the ULXs to Chandra Deep Field results suggests~25% of the sources may be background objects, including 14% of the ULXcandidates in the sample of spiral galaxies and 44% of those inelliptical galaxies, implying the elliptical galaxy ULX population isseverely compromised by background active galactic nuclei. Correlationswith host galaxy properties confirm the number and total X-rayluminosity of the ULXs are associated with recent star formation andwith galaxy merging and interactions. The preponderance of ULXs instar-forming galaxies as well as their similarities to less-luminoussources suggest they originate in a young but short-lived populationsuch as the high-mass X-ray binaries with a smaller contribution (basedon spectral slope) from recent supernovae. The number of ULXs inelliptical galaxies scales with host galaxy mass and can be explainedmost simply as the high-luminosity end of the low-mass X-ray binarypopulation. A New Nonparametric Approach to Galaxy Morphological ClassificationWe present two new nonparametric methods for quantifying galaxymorphology: the relative distribution of the galaxy pixel flux values(the Gini coefficient or G) and the second-order moment of the brightest20% of the galaxy's flux (M20). We test the robustness of Gand M20 to decreasing signal-to-noise ratio (S/N) and spatialresolution and find that both measures are reliable to within 10% forimages with average S/N per pixel greater than 2 and resolutions betterthan 1000 and 500 pc, respectively. We have measured G andM20, as well as concentration (C), asymmetry (A), andclumpiness (S) in the rest-frame near-ultraviolet/optical wavelengthsfor 148 bright local normal'' Hubble-type galaxies (E-Sd) galaxies, 22dwarf irregulars, and 73 0.05 A thorough study of the intriguing X-ray emission from the Cartwheel ringWe present the results from the high resolution Chandra observation ofthe Cartwheel galaxy. Many individual sources areresolved in the image, mostly associated with the outer ring. Alldetected sources have a very high X-ray luminosity (≥1039 erg s-1) that classifies them as UltraLuminous X-ray sources (ULX). The brightest of them is possibly the mostluminous individual non-nuclear source observed so far, withLX  1041 erg s-1 (at D=122 Mpc).The spatial extent of this source is consistent with a point source atthe Chandra resolution. The luminosity function of individual X-raysources extends about an order of magnitude higher than previouslyreported in other galaxies. We discuss this in the context of the`universal'' luminosity function for High Mass X-ray Binaries and wederive a Star Formation Rate higher than in other starburst galaxiesstudied so far. A diffuse component, associated with hot gas, ispresent. However, deeper observations that we will obtain withXMM-Newton are needed to constrain its properties.Appendix is only available in electronic form athttp://www.edpsciences.org Oxygen and nitrogen abundances in nearby galaxies. Correlations between oxygen abundance and macroscopic propertiesWe performed a compilation of more than 1000 published spectra of H IIregions in spiral galaxies. The oxygen and nitrogen abundances in each HII region were recomputed in a homogeneous way, using the P-method. Theradial distributions of oxygen and nitrogen abundances were derived. Thecorrelations between oxygen abundance and macroscopic properties areexamined. We found that the oxygen abundance in spiral galaxiescorrelates with its luminosity, rotation velocity, and morphologicaltype: the correlation with the rotation velocity may be slightlytighter. There is a significant difference between theluminosity-metallicity relationship obtained here and that based on theoxygen abundances determined through the R23-calibrations.The oxygen abundance of NGC 5457 recently determined using directmeasurements of Te (Kennicutt et al. \cite{Kennicutt2003})agrees with the luminosity-metallicity relationship derived in thispaper, but is in conflict with the luminosity-metallicity relationshipderived with the R23-based oxygen abundances. The obtainedluminosity-metallicity relation for spiral galaxies is compared to thatfor irregular galaxies. Our sample of galaxies shows evidence that theslope of the O/H - MB relationship for spirals (-0.079± 0.018) is slightly more shallow than that for irregulargalaxies (-0.139 ± 0.011). The effective oxygen yields wereestimated for spiral and irregular galaxies. The effective oxygen yieldincreases with increasing luminosity from MB  -11 toMB  -18 (or with increasing rotation velocity fromVrot  10 km s-1 to Vrot  100km s-1) and then remains approximately constant. Irregulargalaxies from our sample have effective oxygen yields lowered by afactor of 3 at maximum, i.e. irregular galaxies usually keep at least1/3 of the oxygen they manufactured during their evolution.Appendix, Tables \ref{table:refero}, \ref{table:referV}, and Figs.\ref{figure:sample2}-\ref{figure:sample5} are only available inelectronic form at http://www.edpsciences.org}
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