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Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-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.
| Extranuclear X-Ray Emission in the Edge-on Seyfert Galaxy NGC 2992
We observed the edge-on Seyfert 1.9 galaxy NGC 2992 with the ACIS CCDarray on the Chandra X-Ray Observatory and found several extranuclear(r>~3'') X-ray nebulae within 40" (6.3 kpc for our assumeddistance of 32.5 Mpc) of the nucleus. The net X-ray luminosity from theextranuclear sources is ~2-3×1039 ergs s-1in the 0.3-8.0 keV band. The X-ray core itself (r<~1'') ispositioned at R.A. 9h45m41.06s, decl.-14°19'34.8" (J2000.0) and has a remarkably simple power-lawspectrum with photon index Γ=1.86 and intrinsicNH=7×1021 cm-2. The near-nuclear(3''<~r<~18'') Chandra spectrum is bestmodeled by three components: (1) a direct active galactic nucleus (AGN)component from the wings of the point-spread function or anelectron-scattered AGN component, with Γ fixed at 1.86, (2) coldCompton reflection of the AGN component with intrinsic absorptionNH~1022 cm-2, with approximately thesame 0.3-8.0 keV flux as the direct component, and (3) a 0.5 keVlow-abundance (Z<0.03 Zsolar) thermal plasma, with ~10% ofthe flux of either of the first two components. The X-ray luminosity ofthe third component (the ``soft excess'') is ~1.4×1040ergs s-1, or ~5 times that of all of the detectedextranuclear X-ray sources. We suggest that most (~75%-80%) of the softexcess emission originates from a region between radii of 1" and 3",which is not imaged in our observation due to severe CCD pileup. We alsorequire the cold reflector to be positioned at least 1" (158 pc) fromthe nucleus, since there is no reflection component in the X-ray corespectrum. Much of the extranuclear X-ray emission is coincident withradio structures (nuclear radio bubbles and large-scale radio features),and its soft X-ray luminosity is generally consistent with luminositiesexpected from a starburst-driven wind (with the starburst scaled fromLFIR). However, the AGN in NGC 2992 seems equally likely topower the galactic wind in that object. Furthermore, AGN photoionizationand photoexcitation processes could dominate the soft excess, especiallythe ~75%-80% that is not imaged by our observations.
| Mid-infrared imaging of active galaxies. Active nuclei and embedded star clusters
High resolution, mid-infrared (MIR) images of nine nearby activegalaxies are presented. The data were obtained with the TIMMI 2instrument mounted at the ESO 3.6 m telescope using a set of N-bandnarrow filters. The resulting images have an angular resolution of0.6´´-1´´. The MIR emission has been resolved infour galaxies: NGC 253, NGC 1365, NGC 1808 and NGC 7469. The images showa circumnuclear population of unknown MIR sources in NGC 1365 and NGC1808, coincident with radio sources. These MIR/radio sources areinterpreted in terms of embedded young star clusters. A high-resolutionMIR map of NGC 253 is also presented, and enables the identification ofa previously unknown MIR counterpart to the radio nucleus. Extended MIRemission is detected in NGC 7469, and concurs with previous observationsin the NIR and radio. For this source, an interesting morphologicaldifference between the 10.4 μ m and the 11.9 μ m emission isobserved, suggesting the presence of a dust-rich micro-bar. Our MIRimages of Circinus do not show resolved emission from the nucleus downto an angular scale of 0.5´´. In the case of NGC 2992, anupper limit to the extended MIR emission can be set. We provide new MIRflux measurements for the unresolved AGN in NGC 5995, IZw1 and IIZw136.
| Spectro-morphology of galaxies: A multi-wavelength (UV-R) classification method
We present a quantitative method to classify galaxies, based onmulti-wavelength data and constructed from the properties of nearbygalaxies. Our objective is to define an classification method that canbe used for low and high redshift objects. We estimate the concentrationof light (C) at the galaxy center and the 180° rotational asymmetry(A), computed at several wavelengths, from ultraviolet (UV) to I-band.The variation of the indices of concentration and asymmetry with thewavelength reflects the proportion and the distribution of young and oldstellar populations in galaxies. In general C is found to decrease, andA to increase from optical to UV: the patchy appearance of a galaxy inthe UV with no bulge is often very different from its counterpart atoptical wavelengths, with a prominent bulge and a more regular disk. Wequantify the variation of C and A with wavelength. In this way we areable to distinguish five types of galaxies that we callspectro-morphological types: compact, ringed, spiral, irregular andcentral-starburst galaxies, which can be differentiated by thedistribution of their stellar populations. We discuss in detail themorphology of the galaxies of the sample, and describe the morphologicalcharacteristics of each spectro-morphological type. We applyspectro-morphology to three objects at a redshift z1 in the HubbleDeep Field North, which gives encouraging results for applications tolarge samples of high-redshift galaxies. This method of morphologicalclassification could be used to study the evolution of the morphologywith redshift and is expected to put observational constraints onscenarios of galaxy evolution.
| VLA HI and OVRO CO Interferometry of a Tidal Dwarf Galaxy
We present high resolution interferometric observations of the coolatomic and cold molecular ISM of the TDG candidate Arp 245N, an objectresembling a dwarf galaxy in the northern tidal tail of the interactingsystem NGC 2992/3. We observed the HI line with the NRAO VLA and theCO(1→0) transition with the OVRO millimeter interferometer at5''-6'' angular resolution (750 pc linear resolution). These datacubesoffer the required spatial and velocity resolution to determine whetherthe mass concentration near the tip of the tail is a genuine feature,and hence a good TDG candidate, or an artefact caused by a fortuitousalignment of our line of sight with the direction of the tail. Apreliminary analysis seems to confirm that Arp 245N is aself-gravitating entity.
| An IRAS High Resolution Image Restoration (HIRES) Atlas of All Interacting Galaxies in the IRAS Revised Bright Galaxy Sample
The 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.
| Stellar populations in Active Galactic Nuclei III
In this paper we apply the stellar population synthesis methodpreviously described in Boisson et al. (\cite{Boisson2000}) to five moreAGN. The analysis of these new data strengthen our previous conclusions:i) homogeneity of the stellar population within a class of nuclearactivity regardless of the morphological type of the host galaxy; ii)populations within the nuclear regions of LINERs and Seyfert 2s aredifferent: LINERs have a very old metal-rich population while in theSeyfert 2s a contribution of a weak burst of star formation is observedtogether with the old high metallicity component; iii) in thecircum-nuclar region (200 pc ≤D≤1 kpc) of all the activegalaxies in our sample, except for NGC 2992, we detect an old burst ofstar formation (0.2-1 Gyr),which is contrary to what is observed innormal galaxies. We note that the broad OIλ8446 Å emissionline detected in the spectrum of the nucleus of NGC 2992 confirms itsclassification as a Seyfert 1.Based on observations collected at the New Technology Telescope of theEuropean Southern Observatory, La Silla, Chile.
| Kinematics of tidal tails in interacting galaxies: Tidal dwarf galaxies and projection effects
The kinematics of tidal tails in colliding galaxies has been studied viaFabry-Pérot observations of the Hα emission. With theirlarge field of view and high spatial resolution, the Fabry-Pérotdata allow us to probe simultaneously, in 2D, two kinematical featuresof the tidal ionized gas: large-scale velocity gradients due tostreaming motions along the tails, and small-scale motions related tothe internal dynamics of giant HII regions within the tails. In severalinteracting systems, massive (109 Mȯ)condensations of HI, CO and stars are observed in the outer regions oftails. Whether they are genuine accumulations of matter or not is stilldebated. Indeed a part of the tidal tail may be aligned with theline-of-sight, and the associated projection effect may result inapparent accumulations of matter that does not exist in the 3D space.Using numerical simulations, we show that studying the large-scalekinematics of tails, it is possible to know whether these accumulationsof matter are the result of projection effects or not. We conclude thatseveral ones (Arp 105-South, Arp 242, NGC 7252, and NGC 5291-North) aregenuine accumulations of matter. We also study the small-scale motionsinside these regions: several small-scale velocity gradients areidentified with projected values as large as 50-100 km s-1accross the observed HII regions. In the case of NGC 5291-North, thespatial resolution of our observations is sufficient to detail thevelocity field; we show that this system is rotating andself-gravitating, and discuss its dark matter content. TheFabry-Pérot observations have thus enabled us to prove that some109 Mȯ condensations of matter are realstructures, and are kinematically decoupled from the rest of the tail.Such massive and self-gravitating objects are the progenitors of theso-called ``Tidal Dwarf Galaxies''.Based on observations collected at the European Southern Observatory, LaSilla, Chile and at the Canada-France-Hawaii Observatory, Hawaii, USA.Appendix is only available in electronic form athttp://www.edpsciences.org
| Classifications of the Host Galaxies of Supernovae, Set II
Classifications on the DDO system are given for an additional 231 hostgalaxies of supernovae that have been discovered during the course ofthe Lick Observatory Supernova Search with the Katzman Automatic ImagingTelescope (KAIT). This brings the total number of hosts of supernovae(SNe) discovered (or independently rediscovered) by KAIT, which have sofar been classified on a homogeneous system, to 408. The probabilitythat SNe Ia and SNe II have a different distribution of host-galaxyHubble types is found to be 99.7%. A significant difference is alsofound between the distributions of the host galaxies of SNe Ia and ofSNe Ibc (defined here to include SNe Ib, Ib/c, and Ic). However, nosignificant difference is detected between the frequency distributionsof the host galaxies of SNe II and SNe IIn. This suggests that SNe IInare generally not SNe Ia embedded in circumstellar material that aremasquerading as SNe II. Furthermore, no significant difference is foundbetween the distribution of the Hubble types of the hosts of SNe Ibc andof SNe II. Additionally, SNe II-P and SNe II-L are found to occur amongsimilar stellar populations. The ratio of the number of SNe Ia-pec tonormal SNe Ia appears to be higher in early-type galaxies than it is ingalaxies of later morphological types. This suggests that the ancestorsof SNe Ia-pec may differ systematically in age or composition from theprogenitors of normal SNe Ia. Unexpectedly, five SNe of Types Ib/c, II,and IIn (all of which are thought to have massive progenitors) are foundin host galaxies that are nominally classified as types E and S0.However, in each case the galaxy classification is uncertain, or newlyinspected images show evidence suggesting a later classification. Amongthese five objects, NGC 3720, the host galaxy of SN 2002at, wasapparently misidentified in the Carnegie Atlas of Galaxies.
| Elusive active galactic nuclei
A fraction of active galactic nuclei do not show the classicalSeyfert-type signatures in their optical spectra, i.e. they areoptically `elusive'. X-ray observations are an optimal tool to identifythis class of objects. We combine new Chandra observations with archivalX-ray data in order to obtain a first estimate of the fraction ofelusive active galactic nuclei (AGN) in local galaxies and to constraintheir nature. Our results suggest that elusive AGN have a local densitycomparable to or even higher than optically classified Seyfert nuclei.Most elusive AGN are heavily absorbed in the X-rays, with gas columndensities exceeding 1024 cm-2, suggesting thattheir peculiar nature is associated with obscuration. It is likely thatin elusive AGN the nuclear UV source is completely embedded and theionizing photons cannot escape, which prevents the formation of aclassical narrow-line region. Elusive AGN may contribute significantlyto the 30-keV bump of the X-ray background.
| Near-infrared spectroscopy of nearby Seyfert galaxies - II. Molecular content and coronal emission
We present subarcsec near-infrared 1.5-2.5 μm moderate resolutionlong-slit spectra of eight nearby Seyfert galaxies (z < 0.01), bothparallel to the ionization cone and perpendicular to it. These spectracomplement similar data on six Seyferts, presented in Reunanen,Kotilainen & Prieto, and are used to study the spatial extent of theline emission, the integrated masses of excited H2 and theexcitation mechanisms of interstellar gas.Large concentrations of molecular gas (H2) are present in thenucleus regardless of the Seyfert type. The spatial extent of theH2 emission is larger perpendicular to the cone than parallelto it in 6/8 (75 per cent) galaxies, in agreement with the unifiedmodels of active galactic nuclei. The full width at half maximum (FWHM)sizes of the nuclear H2 emission range from <20 to ~300pc, and are larger than the predicted sizes for molecular torus (1-100pc). Thus the emission probably arises from the material surrounding thetorus rather than directly from the torus.Broad Brγ was detected in nearly half of the optically classifiedSeyfert 2 galaxies, including two objects with no evidence for a hiddenpolarized broad line region. This high detection rate stresses theimportance of extinction effects as the main cause for the Seyfertdichotomy.Brγ and [Fe II] correlate both spatially and kinematically.Nuclear [Fe II] emission is generally blueshifted which, together withthe high Brγ/[Fe II] ratios, suggests shocks as the dominantexcitation mechanism in Seyfert galaxies.Bright coronal emission lines [SiVI] and [Si VII] are common inSeyferts, as they are detected in ~60 per cent of the galaxies. In threegalaxies the coronal lines are extended only in the direction parallelto the cone. This could be explained by a strongly collimated radiationfield or, most plausibly, by shock excitation due to the jet orsuperwind interacting with the interstellar medium.
| Supernova 2003ao in NGC 2993
IAUC 8070 available at Central Bureau for Astronomical Telegrams.
| The Internal Ultraviolet-Optical Color Dispersion: Quantifying the Morphological K-Correction
We present a quantitative measure of the internal color dispersionwithin galaxies, which quantifies differences in galaxy morphology as afunction of observed wavelength. We apply this statistic to a sample oflocal galaxies with archival images at 1500 and 2500 Å from theUltraviolet Imaging Telescope and ground-based B-band observations inorder to investigate how the internal dispersion between these colorsrelates to global galaxy properties (e.g., luminosity, color, andmorphological type). In general, the dispersion in the internal galaxycolors correlates with transformations in the galaxy morphology as afunction of wavelength; i.e., our internal color dispersion statisticquantifies the morphological K-correction. Mid-type spiral galaxiesexhibit the highest dispersion in their ultraviolet-optical internalcolors, which stems from differences in the stellar content thatconstitute the bulge, disk, and spiral-arm components. Irregular andlate-type spiral galaxies show moderate internal color dispersion,although with lower values relative to the mid-type spirals. Thisimplies that young stars generally dominate the ultraviolet-opticalgalaxy colors, modulo variations in the dust, gas, and stellardistributions. Elliptical, lenticular, and early-type spiral galaxiesgenerally have low or negligible internal color dispersion, whichindicates that the stars contributing to the ultraviolet-opticalemission have a very homogeneous distribution. We discuss theapplication of the internal color dispersion to high-redshift galaxiesin deep Hubble Space Telescope images. By simulating the appearance ofthe local galaxy sample at cosmological distances, many of the galaxieshave luminosities that are sufficiently bright at rest-frame opticalwavelengths to be detected within the limits of the currently deepestnear-infrared surveys, even with no evolution. Under the assumption thatthe luminosity and color evolution of the local galaxies conform withthe measured values of high-redshift objects, we show that galaxies'intrinsic internal color dispersion remains measurable out to z~3.
| The IRAS Revised Bright Galaxy Sample
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
| A Multi-Band Photometric Study of Tidal Debris in a Compact Group of Galaxies: Seyfert's Sextet
In order to investigate the properties of the prominent tidal debrisfeature extending to the northeast of a compact group of galaxies,Seyfert's Sextet, we analyzed multi-band (U, B, V, VR, R, I, J, H, andK') photometric imaging data and obtained the following results: 1) Theradial surface brightness distribution of this tidal debris in Seyfert'sSextet (TDSS) in each band appears to be well approximated by anexponential profile. 2) The observed B-V color of TDSS is similar tothose of dwarf elliptical galaxies in nearby clusters. 3) Comparing thespectral energy distribution (SED) of TDSS with theoretical photometricevolution models and with the SED of the stars in the outer part of HCG79b, we find that its SED is comparable to that of a ~10Gyr-old stellarpopulation with solar metallicity, similar to the stellar population inthe outer part of HCG 79b. This suggests that TDSS consists of starsthat may have been liberated from HCG 79b by strong ga laxyinteractions, not a pre-existing dwarf galaxy as previously thought.
| Dust-induced Systematic Errors in Ultraviolet-derived Star Formation Rates
Rest-frame far-ultraviolet (FUV) luminosities form the ``backbone'' ofour understanding of star formation (SF) at all cosmic epochs. Theseluminosities are typically corrected for dust by assuming that the tightrelationship between the UV spectral slopes (β) and the FUVattenuations (AFUV) of starburst galaxies applies to allstar-forming galaxies. Data from seven independent UV experimentsdemonstrate that quiescent, ``normal'' star-forming galaxies deviatesubstantially from the starburst galaxy β-AFUVcorrelation in the sense that normal galaxies are redder thanstarbursts. Spatially resolved data for the Large Magellanic Cloudsuggest that dust geometry and properties, coupled with a smallcontribution from older stellar populations, cause deviations from thestarburst galaxy β-AFUV correlation. Folding in data forstarbursts and ultraluminous infrared galaxies, it is clear that neitherrest-frame UV/optical colors nor UV/Hα colors help significantlyin constraining the UV attenuation. These results argue that theestimation of SF rates from rest-frame UV and optical data alone issubject to large (factors of at least a few) systematic uncertaintiesbecause of dust, which cannot be reliably corrected for using onlyUV/optical diagnostics.
| Bar Galaxies and Their Environments
The prints of the Palomar Sky Survey, luminosity classifications, andradial velocities were used to assign all northern Shapley-Ames galaxiesto either (1) field, (2) group, or (3) cluster environments. Thisinformation for 930 galaxies shows no evidence for a dependence of barfrequency on galaxy environment. This suggests that the formation of abar in a disk galaxy is mainly determined by the properties of theparent galaxy, rather than by the characteristics of its environment.
| Near IR spectroscopy of active galactic nuclei
Using the VLT together with the near infrared instrument ISAAC, we haveobtained medium spectral and high spatial resolution observations of asample of nearby Seyfert galaxies in the H-band. This band isparticularly suited for stellar population studies since the stellarcomponent dominates over the AGN nucleus. The H-band also includes thepeak contribution from cool stars. The AGN spectra are very rich instrong metallic lines which are sensitive to stellar luminosity class.For 4 out of 5 galaxies the central velocity dispersions are found to besignificantly lower than reported in previous studies. Gradients in thestellar population within the central regions were searched for,together with evidence for dilution of the stellar spectral featureswithin the nucleus. Based on observations collected at the Very LargeTelescope (UT1) of the European Southern Observatory, Paranal, Chile,ESO NO 63.A-0366.
| UITBOC 1574: A Very Distant Helium-poor Subdwarf O Star
We have obtained the optical spectrum (3750-7500 Å) andultraviolet fluxes at 1521 and 2260 Å for the quasar candidateUITBOC 1574. The optical spectrum shows strong Balmer absorption linesthrough at least n=9 and the He II line at 4686 Å. We compared theoptical spectrum with non-LTE stellar atmosphere models and findTeff=46,900+/-3,000 K, logg=5.6+/-0.3, and helium abundancelog(He/H)=-2.0+/-0.4. We classify the object as a hot subdwarf O star(sdO). With its high effective temperature and low helium abundance,UITBOC 1574 may be considered as belonging to the hot end of thesubdwarf B population. The location of this object on the theoreticalTeff-g diagram suggests that it is most likely in apost-extreme horizontal branch evolutionary stage. However, we cannotexclude the possibility that UITBOC 1574 may be a low-mass helium whitedwarf precursor. We estimate a spectroscopic distance of 5.6+/-1.7 kpcwith a height of 2.7 +/- 0.8 kpc above the Galactic plane if it is ahelium-poor sdO. The heliocentric radial velocity of the star is 89 +/-18 km s-1. Based on observations obtained with the ApachePoint Observatory 3.5 m telescope, which is owned and operated by theAstrophysical Research Consortium.
| The infrared luminosity of the torus and the visibility of scattered broad line emission in Seyfert 2 galaxies
A number of studies have shown that the visibility of scattered broademission lines in Seyfert 2 galaxies is strongly dependent on the IRASf60/f25 flux ratio, where those Seyfert 2 galaxieswith `warm' IRAS colours show polarized broad line emission. It is nowclear that this effect is owing to the increasing dominance of thegalactic rather than the active galactic nucleus (AGN) emission at60μm in less-luminous `cool' Seyfert 2 galaxies. However, we presentevidence that the 25-μm emission is a good measure of the AGNluminosity for most Seyfert 2 galaxies. Using this result, we show thatthe visibility of scattered broad line emission has a dependence on theAGN luminosity. The observations can be interpreted self-consistently ifthe scaleheight of the scattering zone varies with central sourceluminosity whilst the scaleheight of the obscuring torus isapproximately constant.
| The Formation of Tidal Dwarf Galaxies in Interacting Systems: the Case of Arp 245 (NGC 2992/93)
We present some highlights of our multi-wavelength study, which involvesoptical broad- and narrow-band imaging, long-slit spectroscopy,high-resolution HI and CO observations, of the interacting system Arp245. This object consists of the galaxies NGC 2992 and NGC 2993. Basedon a numerical model of the collision, which was computed with aTree-SPH code, we derive that Arp 245 is observed at an early stage ofthe interaction, about 100 Myr after perigalacticon, though at a timewhen tidal tails have already developed. At the tip of the NGC 2992 tailwe find a gas reservoir of about 10^9M_odot, or about 60% of the HIwhich is seen towards NGC 2992, which coincides with what appears to bea star-forming tidal dwarf galaxy, A245N. The TDG A245N exhibitsproperties ranging between those of dwarf irregular galaxies (structuralparameters, gas content, star formation rate) and those of spiral disks(metallicity, star formation efficiency, stellar population). Wespeculate what the required conditions are to form a TDG, and how theycan be distinguished from field dwarf irregulars.
| An Ultraviolet/Optical Atlas of Bright Galaxies
We present wide-field imagery and photometry of 43 selected nearbygalaxies of all morphological types at ultraviolet and opticalwavelengths. The ultraviolet (UV) images, in two broad bands at 1500 and2500 Å, were obtained using the Ultraviolet Imaging Telescope(UIT) during the Astro-1 Spacelab mission. The UV images have ~3"resolution, and the comparison sets of ground-based CCD images (in oneor more of B, V, R, and Hα) have pixel scales and fields of viewclosely matching the UV frames. The atlas consists of multiband imagesand plots of UV/optical surface brightness and color profiles. Otherassociated parameters, such as integrated photometry and half-lightradii, are tabulated. In an appendix, we discuss the sensitivity ofdifferent wavebands to a galaxy's star formation history in the form of``history weighting functions'' and emphasize the importance of UVobservations as probes of evolution during the past 10-1000 Myr. We findthat UV galaxy morphologies are usually significantly different fromvisible band morphologies as a consequence of spatially inhomogeneousstellar populations. Differences are quite pronounced for systems in themiddle range of Hubble types, Sa through Sc, but less so for ellipticalsor late-type disks. Normal ellipticals and large spiral bulges arefainter and more compact in the UV. However, they typically exhibitsmooth UV profiles with far-UV/optical color gradients which are largerthan any at optical/IR wavelengths. The far-UV light in these cases isprobably produced by extreme horizontal branch stars and theirdescendants in the dominant, low-mass, metal-rich population. The coolstars in the large bulges of Sa and Sb spirals fade in the UV while hotOB stars in their disks brighten, such that their Hubble classificationsbecome significantly later. In the far-UV, early-type spirals oftenappear as peculiar, ringlike systems. In some spiral disks, UV-brightstructures closely outline the spiral pattern; in others, the disks canbe much more fragmented and chaotic than at optical wavelengths.Contributions by bright active galactic nuclei (AGNs) to the integratedUV light in our sample range from less than 10% to nearly 100%. A numberof systems have unusual UV-bright structures in their inner disks,including rings, compact knots, and starburst nuclei, which could easilydominate the UV light in high-redshift analogs. A significant butvariable fraction of the far-UV light in spiral disks is diffuse ratherthan closely concentrated to star-forming regions. Dust in normal spiraldisks does not control UV morphologies, even in some highly inclineddisk systems. The heaviest extinction is apparently confined to thinlayers and the immediate vicinity of young H II complexes; the UV lightemerges from thicker star distributions, regions evacuated of dust byphotodestruction or winds, or by virtue of strong dust clumpiness. Onlyin cases where the dust layers are disturbed does dust appear to be amajor factor in UV morphology. The UV-bright plume of M82 indicates thatdust scattering of UV photons can be important in some cases. In acompanion paper, we discuss far-UV data from the Astro-2 mission andoptical comparisons for another 35 galaxies, emphasizing face-onspirals.
| A Comparison of Ultraviolet Imaging Telescope Far-Ultraviolet and Hα Star Formation Rates
We have used archival ultraviolet (UV) imaging of 50 nearby star-forminggalaxies obtained with the Ultraviolet Imaging Telescope (UIT) to deriveintegrated near-UV and far-UV magnitudes, and have combined these datawith Hα, far-infrared, and thermal radio continuum measurements toexplore the consistency of UV and Hα star formation rates (SFRs).In agreement with previous studies, we find that the UV and HαSFRs are qualitatively consistent, even before corrections forextinction are applied. The uncorrected UV SFRs are systematically lowerby a factor of 1.5 (with a factor of 2 scatter) among luminous galaxieswith SFR>~1 Msolar yr-1, indicating a highereffective attenuation of the far-UV radiation. Among less luminousgalaxies there is no significant offset between the Hα and far-UVSFR scales. This behavior is consistent with that of higher redshiftsamples observed by Sullivan et al., Glazebrook et al., and Yan et al.for comparable ranges of galaxy luminosities and absolute SFRs.Far-infrared and thermal radio continuum data available for a subset ofour sample allow us to estimate the attenuation in the UV and atHα independently. The UV and Hα attenuations appear to becorrelated, and confirm systematically higher attenuations in the UV.Although the galaxies in our sample show modest levels of attenuation(with median values of 0.9 mag at Hα and 1.4 mag at 1550 Å),the range across the sample is large, ~4 mag for Hα and >~5 magin the far-UV (1550 Å). This indicates that the application of asingle characteristic extinction correction to Hα or UV SFRs isonly realistic for large, well-defined and well-studied galaxy samples,and that extinction bias may be important for UV oremission-line-selected samples of star-forming galaxies.
| Two-Dimensional Spectroscopy of NGC 2992
We present the preliminary results obtained from two-dimensionalspectroscopy with optical fibres of the circumnuclear region of theSeyfert 2 galaxy NGC 2992. The inner region of NGC 2992 seems not to beperturbed by the interaction with its companion, NGC 2993.
| Spectroscopic Observations of the Star Formation Activities in the Central Regions of Early-Type Spiral Galaxies
We study the characteristics of central star-forming regions ofearly-type spiral galaxies by optical spectroscopic observations. Thesample consists of 13 galaxies which have ratios of far-infrared (FIR)to optical B-band luminosity, log(LFIR/LB), largerthan the average of this type, -0.5. Strong line emissions are detectedaround the nucleus, and the line ratios of most regions are H IIregion-like, except for a few shocked regions. There is nolow-ionization nuclear emission-line region (LINER). The [N II] and [SII] lines are somewhat enhanced, compared to the disk H II regions, butthe enhancements cannot be attributed to hidden active galactic nuclei(AGNs) and the shock excitation. The median extinction derived from theHα/Hβ intensity ratio is 1.0 mag in AV, and themedian SFR of our sample galaxies, corrected for extinction, isestimated to be 2 Msolar yr-1. Both values arecomparable to those of late-type spiral galaxies. We find a correlationthat the radii of the central star-forming regions tend to become largerwith the turn-over radii of the rotation curves.
| The Biconical Outflow in the Seyfert Galaxy NGC 2992
We report on a detailed kinematic study of the galactic-scale outflow inthe Seyfert galaxy NGC 2992. The TAURUS-2 imaging Fabry-Perotinterferometer was used on the 3.9 m Anglo-Australian Telescope toderive the two-dimensional velocity field of the Hα-emitting gasover the central arcminute of NGC 2992. The complete two-dimensionalcoverage of the data combined with simple kinematic models of rotatingaxisymmetric disks allow us to differentiate the outflowing materialfrom the line-emitting material associated with the galactic disk. Thekinematics of the disk component out to R=3.0 kpc are well modeled bypure circular rotation in a plane inclined at i=68°+/-3° fromthe plane of the sky and with kinematic major axis alongP.A.=32°+/-3°. The outflow component is distributed into twowide cones with opening angle ~125°-135° and extending ~2.8 kpc(18") on both sides of the nucleus at nearly right angles(φ~116°+/-5°) to the disk kinematic major axis. The outflowon the southeastern side of the nucleus is made of two distinctkinematic components interpreted as the front and back walls of a cone.The azimuthal velocity gradient in the back-wall component reflectsresidual rotational motion, which indicates either that the outflowingmaterial was lifted from the disk or that the underlying galactic diskis contributing slightly to this component. A single outflow componentis detected in the northwestern cone. A biconical outflow model withvelocities ranging from 50 to 200 km s-1 and oriented nearlyperpendicular to the galactic disk can explain the data. The broad-lineprofiles and asymmetries in the velocity fields suggest that some of theentrained line-emitting material may lie inside the biconical structurerather than only on the surface of the bicone. The mass involved in thisoutflow is of order ~1×107 n-1e,2Msolar, and the bulk and ``turbulent'' kinematic energies are~6×1053 n-1e,2 ergs and~3×1054 n-1e,2 ergs,respectively. The most likely energy source is a hot, bipolar, thermalwind powered on a subkiloparsec scale by the active galactic nucleus anddiverted along the galaxy's minor axis by the pressure gradient of theISM in the host galaxy. The data are not consistent with astarburst-driven wind or a collimated outflow powered by radio jets.
| Stellar and ionized gas kinematics of the interacting Seyfert 1.9 galaxy NGC 2992
Integral field spectroscopy in the central 16''x 12'' (2.4 kpc x 1.8kpc, if H0 = 75 km s-1 Mpc-1) of theSeyfert 1.9 galaxy NGC 2992 has been obtained usingthe fibre system INTEGRAL. The data are mainly used to study the stellarand ionized gas kinematics. In spite of the photometric disruptions inthe outer parts (r > 6 kpc) produced by the interaction with itsclose companion (NGC 2993), the present stellar velocity field showsregular rotation. The ionized gas presents several kinematicallydistinct components. Apart from the outflowing component alreadyreported by other authors, we found an additional (high ionization)kinematic component which seems to be associated with the boundaries ofthe figure-of-eight-shaped emission detected in the 6 cm radio map. Welocate the hidden nucleus in the apex of the biconical structure definedby the [O iii] emission, coincident with the outflow origin and with thecenter of the dust lane. We do not find any clear evidence of directinfluence of the interaction in the kinematics of the stars or theionized gas in the circumnuclear region of NGC 2992.
| Abundant molecular gas in tidal dwarf galaxies: On-going galaxy formation
We investigate the process of galaxy formation as can be observed in theonly currently forming galaxies - the so-called Tidal Dwarf Galaxies,hereafter TDGs - through observations of the molecular gas detected viaits CO (Carbon Monoxide) emission. These objects are formed of materialtorn off of the outer parts of a spiral disk due to tidal forces in acollision between two massive galaxies. Molecular gas is a key elementin the galaxy formation process, providing the link between a cloud ofgas and a bona fide galaxy. We have detected CO in 8 TDGs (two of themhave already been published in Braine et al. 2000, hereafter Paper I),with an overall detection rate of 80%, showing that molecular gas isabundant in TDGs, up to a few 108 Msun. The COemission coincides both spatially and kinematically with the HIemission, indicating that the molecular gas forms from the atomichydrogen where the HI column density is high. A possible trend of moreevolved TDGs having greater molecular gas masses is observed, in accordwith the transformation of HI into H2. Although TDGs sharemany of the properties of small irregulars, their CO luminosity is muchgreater (factor ~ 100) than that of standard dwarf galaxies ofcomparable luminosity. This is most likely a consequence of the highermetallicity (gtrsim 1/3 solar) of TDGs which makes CO a good tracer ofmolecular gas. This allows us to study star formation in environmentsordinarily inaccessible due to the extreme difficulty of measuring themolecular gas mass. The star formation efficiency, measured by the COluminosity per Hα flux, is the same in TDGs and full-sizedspirals. CO is likely the best tracer of the dynamics of these objectsbecause some fraction of the HI near the TDGs may be part of the tidaltail and not bound to the TDG. Although uncertainties are large forindividual objects, as the geometry is unknown, our sample is now ofeight detected objects and we find that the ``dynamical" masses of TDGs,estimated from the CO line widths, seem not to be greater than the``visible" masses (HI + H2 + a stellar component). Althoughhigher spatial resolution CO (and HI) observations would help reduce theuncertainties, we find that TDGs require no dark matter, which wouldmake them the only galaxy-sized systems where this is the case. Darkmatter in spirals should then be in a halo and not a rotating disk. Mostdwarf galaxies are dark matter-rich, implying that they are not of tidalorigin. We provide strong evidence that TDGs are self-gravitatingentities, implying that we are witnessing the ensemble of processes ingalaxy formation: concentration of large amounts of gas in a boundobject, condensation of the gas, which is atomic at this point, to formmolecular gas and the subsequent star formation from the dense molecularcomponent.
| Formation of molecular gas in the tidal debris of violent galaxy-galaxy interactions
In many gravitational interactions between galaxies, gas and stars thathave been torn from the precursor galaxies can collect in tidal `tails'.Star formation begins anew in some of these regions, producing tidaldwarf galaxies. Observations of these new galaxies provides insight intoprocesses relevant to galaxy formation more generally, because thetimescale of the interaction is well defined. But tracking the starformation process has hitherto been difficult because the tidal dwarfgalaxies with young stars showed no evidence of the molecular gas out ofwhich those young stars formed. Here we report the discovery ofmolecular hydrogen (traced by carbon monoxide emission) in two tidaldwarf galaxies. In both cases, the concentration of molecular gas peaksat the same location as the maximum in atomic-hydrogen density, unlikethe situation in most gas-rich galaxies. We infer from this that themolecular gas formed from the atomic hydrogen, rather than being torn inmolecular form from the interacting galaxies. Star formation in thetidal dwarf galaxies therefore appears to mimic the process in normalspiral galaxies like our own.
| Adaptive optics near-infrared imaging of NGC 2992 - unveiling core structures related to radio figure-of-8 loops
We present near-infrared adaptive optics, Very Large Array (VLA) radioand Hubble Space Telescope (HST) optical imaging of the nearby Seyfertgalaxy NGC 2992. Spiral structure and an extension to the west aretraced down to the core region at the limiting resolution of ournear-infrared images. A faint, diffuse loop of near-infrared and radioemission is also observed to the north, embedded within the prominent2-arcsec radio loop previously observed to the north-west. Near-infraredcolour maps and CO narrow-band imaging are then used to identify whichregions may not be purely reddened stellar populations. Our new dataprovide evidence that the VLA radio-loop morphology in the shape of afigure of 8 represents two components superimposed: (1) outflow bubblesout of the plane of the disc, coincident with the extended emission-lineregion (EELR); (2) star formation along the spiral arm within the galaxydisc and through the dust lane. The near-infrared continuum emissionassociated with the outflowing radio bubbles suggests that the radioloops are driven by the active nucleus.
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