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Публикации по объекту
Environmental Effects on Late-Type Galaxies in Nearby Clusters
The transformations that take place in late-type galaxies in theenvironment of rich clusters of galaxies at z=0 are reviewed. From thehandful of late-type galaxies that inhabit local clusters, whether theywere formed in situ and survived as such, avoiding transformation oreven destruction, or if they are newcomers that have recently fallen infrom outside, we can learn an important lesson on the latest stages ofgalaxy evolution. We start by reviewing the observational scenario,covering the broadest possible stretch of the electromagnetic spectrum,from the gas tracers (radio and optical) to the star formation tracers(UV and optical), the old star tracers (near-IR), and the dust (far-IR).Strong emphasis is given to the three nearby, well-studied clustersVirgo, A1367, and Coma, which are representative of differentevolutionary stages, from unrelaxed and spiral-rich (Virgo) to relaxedand spiral-poor (Coma). We continue by providing a review of models ofgalaxy interactions that are relevant to clusters of galaxies.Prototypes of various mechanisms and processes are discussed, and theirtypical timescales are given in an appendix. Observations indicate thepresence of healthy late-type galaxies falling into nearby clustersindividually or as part of massive groups. More rare are infallinggalaxies belonging to compact groups, where significant preprocessingmight take place. Once they have entered the cluster, they lose theirgas and quench their star formation activity, becoming anemic.Observations and theory agree in indicating that the interaction withthe intergalactic medium is responsible for the gas depletion. However,this process cannot be the origin of the cluster lenticular galaxypopulation. Physical and statistical properties of S0 galaxies in nearbyclusters and at higher redshift indicate that they originate from spiralgalaxies that have been transformed by gravitational interactions.
| Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
We estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion.
| Dust in spiral galaxies: global properties
We present and analyse high-quality Submillimetre Common-User BolometerArray (SCUBA) 850- and 450-μm images of 14 local spiral galaxies,including the detection of dust well out into the extended disc in manycases. We use these data in conjunction with published far-infrared fluxdensities from IRAS and ISO, and millimetre-wave measurements fromground-based facilities to deduce the global properties of the dust inthese galaxies, in particular temperature and mass. We find that simpletwo-temperature greybody models of fixed dust emissivity index β= 2and with typical temperatures of 25 < Twarm < 40 K and10 < Tcold < 20 K provide good fits to the overallspectral energy distributions. The dust mass in the cold componentcorrelates with the mass in atomic hydrogen and the mass in the warmcomponent correlates with the mass in molecular hydrogen. These resultsthus fit the simple picture in which the cold dust is heatedpredominantly by the interstellar radiation field, while the hot dust isheated predominantly by OB stars in more active regions, although weargue that there is some mixing. The mean gas-to-dust mass ratio is 120+/- 60, very similar to that found within our own galaxy and roughly afactor of 10 lower than that derived from IRAS data alone. Thegas-to-dust mass ratios in the warm, molecular component are on averagehigher than those in the cold, atomic component. We compare ourmodelling results with similar results for more luminous spiral galaxiesselected at far-infrared wavelengths by the SCUBA Local Universe GalaxySurvey. We find that whilst the total dust mass distributions of the twosamples are indistinguishable, they have significantly different dusttemperature distributions in both the warm and cold components. Wesuggest that this difference might be related to the level of starformation activity in these systems, with the more active galaxieshaving more intense interstellar radiation fields and higher dusttemperatures.
| 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.
| Dust in the outer regions of interacting galaxies.
Not Available
| Dense Cloud Ablation and Ram Pressure Stripping of the Virgo Spiral NGC 4402
We present optical, H I, and radio continuum observations of the highlyinclined Virgo Cluster Sc galaxy NGC 4402, which show evidence for rampressure stripping and dense cloud ablation. Very Large Array H I andradio continuum maps show a truncated gas disk and emission to thenorthwest of the main disk emission. In particular, the radio continuumemission is asymmetrically extended to the north and skewed to the west.The Hα image shows numerous H II complexes along the southern edgeof the gas disk, possibly indicating star formation triggered by theintracluster medium (ICM) pressure. Our BVR images at 0.5" resolutionobtained with the WIYN Tip-Tilt Imager show a remarkable dust lanemorphology: at half the optical radius, the dust lane of the galaxycurves up and out of the disk, matching the H I morphology. Large dustplumes extend upward for ~1.5 kpc from luminous young star clusters atthe southeast edge of the truncated gas disk. These star clusters arevery blue, indicating very little dust reddening, which suggests dustblown away by an ICM wind at the leading edge of the interaction. To thesouth of the main ridge of interstellar material, where the galaxy isrelatively clean of gas and dust, we have discovered 1 kpc long lineardust filaments with a position angle that matches the extraplanar radiocontinuum tail; we interpret this angle as the projected ICM winddirection. One of the observed dust filaments has an H II region at itshead. We interpret these dust filaments as large, dense clouds that wereinitially left behind as the low-density interstellar medium wasstripped but were then ablated by the ICM wind. These results providestriking new evidence on the fate of molecular clouds in strippedcluster galaxies.
| Ram pressure stripping of disk galaxies. From high to low density environments
Galaxies in clusters and groups moving through the intracluster orintragroup medium (abbreviated ICM for both) are expected to lose atleast a part of their interstellar medium (ISM) by the ram pressure theyexperience. We perform high resolution 2D hydrodynamical simulations offace-on ram pressure stripping (RPS) of disk galaxies to compile acomprehensive parameter study varying galaxy properties (mass, verticalstructure of the gas disk) and covering a large range of ICM conditions,reaching from high density environments like in cluster centres to lowdensity environments typical for cluster outskirts or groups. We findthat the ICM-ISM interaction proceeds in three phases: firstly theinstantaneous stripping phase, secondly the dynamic intermediate phase,thirdly the quasi-stable continuous viscous stripping phase. In thefirst phase (time scale 20 to 200~Myr) the outer part of the gas disk isdisplaced but only partially unbound. In the second phase (10 times aslong as the first phase) a part of the displaced gas falls back (about10% of the initial gas mass) despite the constant ICM wind, but mostdisplaced gas is now unbound. In the third phase the galaxy continues tolose gas at a rate of about 1~Mȯ~~yr-1 byturbulent viscous stripping. We find that the stripping efficiencydepends slightly on the Mach number of the flow, however, the mainparameter is the ram pressure. The stripping efficiency does not dependon the vertical structure and thickness of the gas disk. We discussuncertainties in the classic estimate of the stripping radius of Grunn& Gott (1972, ApJ, 176, 1), which compares the ram pressure to thegravitational restoring force. In addition, we adapt the estimate usedby Mori & Burkert (2000, ApJ, 538, 559) for spherical galaxies,namely the comparison of the central pressure with ram pressure. We findthat the latter estimate predicts the radius and mass of the gas diskremaining at the end of the second phase very well, and better than theGrunn & Gott (1972, ApJ, 176, 1) criterion. From our simulations weconclude that gas disks of galaxies in high density environments areheavily truncated or even completely stripped, but also the gas disks ofgalaxies in low density environments are disturbed by the flow andback-falling material, so that they should also be pre-processed.
| Completing H I observations of galaxies in the Virgo cluster
High sensitivity (rms noise 0.5 mJy) 21-cm H I line observationswere made of 33 galaxies in the Virgo cluster, using the refurbishedArecibo telescope, which resulted in the detection of 12 objects. Thesedata, combined with the measurements available from the literature,provide the first set of H I data that is complete for all 355 late-type(Sa-Im-BCD) galaxies in the Virgo cluster with mp ≤ 18.0mag. The Virgo cluster H I mass function (HIMF) that was derived forthis optically selected galaxy sample is in agreement with the HIMFderived for the Virgo cluster from the blind HIJASS H I survey and isinconsistent with the Field HIMF. This indicates that both in this richcluster and in the general field, neutral hydrogen is primarilyassociated with late-type galaxies, with marginal contributions fromearly-type galaxies and isolated H I clouds. The inconsistency betweenthe cluster and the field HIMF derives primarily from the difference inthe optical luminosity function of late-type galaxies in the twoenvironments, combined with the HI deficiency that is known to occur ingalaxies in rich clusters.Tables \ref{t1, \ref{sample_dat} and Appendix A are only available inelectronic form at http://www.edpsciences.org
| Dust in the Outskirts of Interacting Galaxies
Not Available
| Spiral Galaxy - ICM Interactions in the Virgo Cluster
We discuss HI and optical evidence for ongoing ICM-ISM interactions in 6HI-deficient Virgo cluster spiral galaxies. One of the clearest cases isthe highly inclined Virgo galaxy NGC 4522, which has a normal stellardisk but a truncated gas disk, and lots of extraplanar gas right next tothe gas truncation radius in the disk. Unusually strong HI, Hα andradio continuum emission are all detected from the extraplanar gas. Theradio continuum polarized flux and spectral index peak on the sideopposite the extraplanar gas, suggesting ongoing pressure by the ICM.Four other HI-deficient edge-on Virgo spirals show evidence ofextraplanar ISM gas or exhibit asymmetries in their disk HIdistributions, but contain much less extraplanar HI than NGC 4522.Comparison with recent simulations suggests this difference may beevolutionary, with large surface densities of extraplanar gas observedonly in early phases of an ICM-ISM interaction. In NGC 4569, theHα image shows 2 effects of ICM pressure on the galaxy ISM. Ananomalous arm of HII regions, possibly extraplanar, emerges from theedge of a truncated Hα disk. This resembles the arms seen insimulations which are formed by the combined effects of wind pressureplus rotation. An extended nebulosity near the minor axis, also in theNW, is interpreted as a starburst outflow bubble disturbed by ICM windpressure.
| Dust in the Intergalactic Medium of Galaxy Clusters
Far-infrared (FIR) observations with ISOPHOT aboard the Infrared SpaceObservatory were carried out to observe the thermal FIR emission fromdiffuse intracluster dust (ICD) in galaxy clusters and outlying duststructures in the elliptical galaxies M86 and NGC 5128 (Centaurus A).Extended FIR emission from ICD has only been detected in the Comacluster. It is likely caused by tidal interactions, since Coma is adynamically young cluster with on-going gravitational interactions inthe centre and infalling galaxy groups. M86 has a complex FIR morphologywith a compact double source near its centre and several off-centresources. The brightest FIR source lies between M86 and the nearby spiralNGC 4402 and represents the first direct detection of a localizedintracluster dust cloud not associated with optical or neutral hydrogenemission. Overall, the FIR data do not support the ram pressure duststripping scenario suggested on the basis of IRAS data, but indicate asignificant influence of tidal interactions. NGC 5128 shows obviousoptical signs of interaction, most notably a central dust lane andstellar shells. The FIR emission of outlying dust 15 kpc from the centrehas been discovered where previously HI and molecular gas closelyassociated with the shells had been found. NGC 5128 is the first case ofa shell galaxy where all components of the interstellar medium (ISM) aredetected far off the nucleus, indicating an inner disk origin of theoutlying material.
| Radio and Far-Infrared Emission as Tracers of Star Formation and Active Galactic Nuclei in Nearby Cluster Galaxies
We have studied the radio and far-infrared (FIR) emission from 114galaxies in the seven nearest clusters (<100 Mpc) with prominentX-ray emission to investigate the impact of the cluster environment onthe star formation and active galactic nucleus (AGN) activity in themember galaxies. The X-ray selection criterion is adopted to focus onthe most massive and dynamically relaxed clusters. A large majority ofcluster galaxies show an excess in radio emission over that predictedfrom the radio-FIR correlation, the fraction of sources with radioexcess increases toward cluster cores, and the radial gradient in theFIR/radio flux ratio is a result of radio enhancement. Of theradio-excess sources, 70% are early-type galaxies, and the same fractionhost an AGN. The galaxy density drops by a factor of 10 from thecomposite cluster center out to 1.5 Mpc, yet galaxies show no change inFIR properties over this region and show no indication of masssegregation. We have examined in detail the physical mechanisms thatmight impact the FIR and radio emission of cluster galaxies. Whilecollisional heating of dust may be important for galaxies in clustercenters, it appears to have a negligible effect on the observed FIRemission for our sample galaxies. The correlations between radio and FIRluminosity and radius could be explained by magnetic compression fromthermal intracluster medium pressure. We also find that simple delayedharassment cannot fully account for the observed radio, FIR, and mid-IRproperties of cluster galaxies.
| The ISOPHOT 170 μm Serendipity Survey II. The catalog of optically identified galaxies%
The ISOPHOT Serendipity Sky Survey strip-scanning measurements covering≈15% of the far-infrared (FIR) sky at 170 μm were searched forcompact sources associated with optically identified galaxies. CompactSerendipity Survey sources with a high signal-to-noise ratio in at leasttwo ISOPHOT C200 detector pixels were selected that have a positionalassociation with a galaxy identification in the NED and/or Simbaddatabases and a galaxy counterpart visible on the Digitized Sky Surveyplates. A catalog with 170 μm fluxes for more than 1900 galaxies hasbeen established, 200 of which were measured several times. The faintest170 μm fluxes reach values just below 0.5 Jy, while the brightest,already somewhat extended galaxies have fluxes up to ≈600 Jy. For thevast majority of listed galaxies, the 170 μm fluxes were measured forthe first time. While most of the galaxies are spirals, about 70 of thesources are classified as ellipticals or lenticulars. This is the onlycurrently available large-scale galaxy catalog containing a sufficientnumber of sources with 170 μm fluxes to allow further statisticalstudies of various FIR properties.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Members of the Consortium on the ISOPHOT Serendipity Survey (CISS) areMPIA Heidelberg, ESA ISO SOC Villafranca, AIP Potsdam, IPAC Pasadena,Imperial College London.Full Table 4 and Table 6 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/422/39
| Tracing the star formation history of cluster galaxies using the Hα/UV flux ratio
Since the Hα and UV fluxes from galaxies are sensitive to stellarpopulations of ages <107 and ≈ 108 yrrespectively, their ratio f(Hα)/f(UV) provides us with a tool tostudy the recent t ≤ 108 yr star formation history ofgalaxies, an exercise that we present here applied to 98 galaxies in 4nearby clusters (Virgo, Coma, Abell 1367 and Cancer). The observedf(Hα)/f(UV) ratio is a factor of two smaller than theexpected one as determined from population synthesis models assuming arealistic delayed, exponentially declining star formation history. Wediscuss various mechanisms that may have affected the observedf(Hα)/f(UV) ratio and we propose that the above discrepancy arisesfrom either the absorption of Lyman continuum photons by dust within thestar formation regions or from the occurrence of star formationepisodes. After splitting our sample into different subsamples accordingto evolutionary criteria we find that our reference sample of galaxiesunaffected by the cluster environment show an average value off(Hα)/f(UV) two times lower than the expected one. We argue thatthis difference must be mostly due to absorption of ≈45% of the Lymancontinuum photons within star forming regions. Galaxies with clear signsof an ongoing interaction show average values of f(Hα)/f(UV)slightly higher than the reference value, as expected if those objectshad SFR increased by a factor of ≃4. The accuracy of the currentUV and Hα photometry is not yet sufficient to clearly disentanglethe effect of interactions on the f(Hα)/f(UV) ratio, butsignificant observational improvements are shortly expected to resultfrom the GALEX mission.Tables 1-3 are only available in electronic form athttp://www.edpsciences.org
| Radio continuum spectra of galaxies in the Virgo cluster region
New radio continuum observations of galaxies in the Virgo cluster regionat 4.85, 8.6, and 10.55 GHz are presented. These observations arecombined with existing measurements at 1.4 and 0.325 GHz. The sampleincludes 81 galaxies where spectra with more than two frequencies couldbe derived. Galaxies that show a radio-FIR excess exhibit centralactivity (HII, LINER, AGN). The four Virgo galaxies with the highestabsolute radio excess are found within 2° of the centerof the cluster. Galaxies showing flat radio spectra also host activecenters. There is no clear trend between the spectral index and thegalaxy's distance to the cluster center.Figure 3 is only available in electronic form athttp://www.edpsciences.orgTable 3 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/418/1
| Spectrophotometry of galaxies in the Virgo cluster. II. The data
Drift-scan mode (3600-6800 Å) spectra with 500
| An extragalactic HII region in the Virgo cluster
We present spectroscopic observations for six emission-line objectsprojected onto the Virgo cluster. These sources have been selected fromnarrow band (Hα+[NII]) images showing faint detectable continuumemission and EW > 100 Å. Five of these sources result [OIII]λ 5007 emitters at z 0.31, while one 122603+130724 isconfirmed to be an HII region belonging to the Virgo cluster. Thispoint-like source has a recessional velocity of 200 kms-1, and is associated with the giant galaxy VCC 873 (NGC4402). It has a higher luminosity, star formation rate and metallicitythan the extragalactic HII region recently discovered near the Virgogalaxy VCC 836 by Gerhard et al. (\cite{gerhard}).Based on observations obtained with the Loiano telescope belonging tothe University of Bologna (Italy) and with the Calar Alto observatoryoperated by the Centro Astronomico Hispano Aleman (Spain).
| The Virgo High-Resolution CO Survey: II. Rotation Curves and Dynamical Mass Distributions
Based on a high-resolution CO survey of Virgo spirals with the NobeyamaMillimeter-wave Array, we determined the dynamical centers usingvelocity fields, and derived position-velocity diagrams (PVDs) along themajor axes of the galaxies across their dynamical centers. We applied anew iteration method to derive rotation curves (RCs), which reproducethe observed PVDs. The obtained high-accuracy RCs generally show a steeprise in the central 100 to 200pc regions, followed by flatrotation in the disk. We applied a deconvolution method to calculate thesurface-mass density (SMD) using the RCs based on two extremeassumptions that the mass distribution is either spherical or thin-diskshaped. Both assumptions give nearly identical results, agreeing witheach other within a factor of two at any radii. The SMD distributionsrevealed central massive cores with peak SMD of 104 -105 Mȯ pc-2 and a total masswithin 200pc radius of the order of 109Mȯ. A correlation analysis among the derived parametersshows that the central CO-line intensity is positively correlated withthe central SMD, which suggests that the deeper is the gravitationalpotential, the higher is the molecular gas concentration in the nuclei,regardless of the morphological types.
| The Virgo High-Resolution CO Survey: I. CO Atlas
We present the results of the Virgo high-resolution CO survey (ViCS)obtained with the Nobeyama Millimeter-wave Array (NMA). This survey wasmade during the course of a long-term project at Nobeyama from 1999December through 2002 April. The objects were selected from Virgocluster members, while considering the CO richness from the single-dishflux, mild inclination, and a lack of strong tidal perturbations. Thecentral 1' regions ( 4.7 kpc) of 15 spiral galaxies were observedwith resolutions of 2'' - 5'' and 10--20 km s-120 kms-1, and sensitivities of 20 mJy beam-1 fora 10 km s-1 channel. The objects lie at the same distance ofthe Virgo cluster (16.1Mpc), which is advantageous for comparisons amongindividual galaxies. We describe the details of observations and datareduction, and present an atlas of integrated CO intensity maps,velocity fields and position-velocity diagrams along the major axes. Themolecular g as morphology in the Virgo galaxies shows a wealth ofvariety, not specifically depending on the Hubble types. Severalgalaxies show a strong concentration of gas in the central few kpcregion, where the CO morphology shows either a ``single-peak'' or a``twin-peaks''. The morphology of more extended CO components can beclassified into ``arm-type'', ``bar-type'', and ``amorphous-type''.
| Dust emission in the far-infrared as a star formation tracer at z= 0: systematic trends with luminosity
We investigate whether dust emission in the far-infrared (far-IR)continuum provides a robust estimate of the star formation rate (SFR)for a nearby, normal late-type galaxy. We focus on the ratio of the40-1000 μm luminosity (Ldust) to the far-ultraviolet(far-UV) (0.165 μm) luminosity, which is connected to recent episodesof star formation. Available total photometry at 0.165, 60, 100 and 170μm limits the statistics to 30 galaxies, which, however, span a largerange in observed (and, thus, attenuated by dust) K-band (2.2 μm)luminosity, morphology and inclination (i). This sample shows that theratio of Ldust to the observed far-UV luminosity depends notonly on i, as expected, but also on morphology and, in a tighter way, onobserved K-band luminosity. We find thatLdust/LFUV~ e-τK (α+0.62)LK0.62, where LFUV andLK are the unattenuated stellar luminosities in far-UV and K,respectively, and α is the ratio of the attenuation optical depthsat 0.165 μm (τFUV) and 2.2 μm (τK).This relation is to zeroth order independent of i and morphology. It maybe further expressed asLdust/LFUV~LδK, whereδ= 0.61 - 0.02α, under the observationally motivatedassumption that, for an average inclination,e-τK~L-0.02K. We adoptcalculations of two different models of attenuation of stellar light byinternal dust to derive solid-angle-averaged values of α. We findthat δ is positive and decreases towards 0 from the more luminousto the less luminous galaxies. This means that there is no universalratio of far-IR luminosity to unattenuated far-UV luminosity for nearby,normal late-type galaxies. The far-IR luminosity systematicallyoverestimates SFR in more luminous, earlier-type spirals, owing to theincreased fractional contribution to dust heating of optical/near-IRphotons in these objects. Conversely, it systematically underestimatesSFR in fainter, later-type galaxies, the τFUV of which isreduced. The limited statistics and the uncertainty affecting theprevious scaling relations do not allow us to establish quantitativeconclusions, but an analogous analysis making use of larger data sets,available in the near future (e.g. from GALEX, ASTRO-F and SIRTF), andof more advanced models will allow a quantitative test of ourconclusions.
| The Contribution of H I-rich Galaxies to the Damped Lyα Absorber Population at z = 0
We present a study of the expected properties of the low-redshift dampedLyα absorber population determined from a sample of H I-selectedgalaxies in the local universe. Because of a tight correlation betweenthe H I mass and H I cross section, which we demonstrate spans allgalaxy types, we can use our H I-selected sample to predict theproperties of the absorption-line systems. We use measurements of thenumber density and H I cross section of galaxies to show that the totalH I cross section at column densities sufficient to produce dampedLyα absorption is consistent with no evolution of the absorberpopulation. We also find that the dN/dz distribution is dominated bygalaxies with H I masses near 109 Msolar. However,because of the large dispersion in the correlation between H I mass andstellar luminosity, we find that the distribution of dN/dz as a functionof LJ is fairly flat. In addition, we examine the line widthsof the H I-selected galaxies and show that there may be evolution in thekinematics of H I-rich galaxies, but it is not necessary for the higherredshift population to contain a greater proportion of high-massgalaxies than we find locally.
| Companions of Bright Barred Shapley-Ames Galaxies
Companion galaxy environment for a subset of 78 bright and nearby barredgalaxies from the Shapley-Ames Catalog is presented. Among the spiralbarred galaxies, there are Seyfert galaxies, galaxies with circumnuclearstructures, galaxies not associated with any large-scale galaxy cloudstructure, galaxies with peculiar disk morphology (crooked arms), andgalaxies with normal disk morphology; the list includes all Hubbletypes. The companion galaxy list includes the number of companiongalaxies within 20 diameters, their Hubble type, and projectedseparation distance. In addition, the companion environment was searchedfor four known active spiral galaxies, three of them are Seyfertgalaxies, namely, NGC 1068, NGC 1097, and NGC 5548, and one is astarburst galaxy, M82. Among the results obtained, it is noted that theonly spiral barred galaxy classified as Seyfert 1 in our list has nocompanions within a projected distance of 20 diameters; six out of 10Seyfert 2 bar galaxies have no companions within 10 diameters, six outof 10 Seyfert 2 galaxies have one or more companions at projectedseparation distances between 10 and 20 diameters; six out of 12 galaxieswith circumnuclear structures have two or more companions within 20diameters.
| 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.
| Star formation in gaseous galaxy halos. VLT-spectroscopy of extraplanar H II-regions in NGC 55
We present first deep VLT-spectra of a new class of compact extraplanarobjects located in the disk-halo interface of the edge-on spiral galaxyNGC 55. Their spectra reveal continuum emission from stars and showtypical emission-lines as observed in ordinary disk H Ii-regions. Withrespect to emission-line fluxes, these spectra are very similar to thoseobtained for the diffuse ionized gas (DIG), except [O Iii]lambda 5007which is strongly decreased by more than a factor of 3. Similar to theDIG the prominent ionization stage of oxygen is O+, whereasthe corresponding one for low metallicity H Ii-regions isO++. A comparison with CLOUDY model simulations reveals thatthe ionization mechanism of these compact objects is most likelyphotoionization by late OB stars (O9.5 to B0). Further analysis ofdiagnostic diagrams unambiguously confirms the H Ii-region character.This raises the question whether these extraplanar H Ii-regions (EHRs)originated from the prominent extraplanar gas of this galaxy or havejust been expelled from the disk into the halo. From hydrodynamicalconsiderations ejection from the disk can be ruled out. Therefore, theseobjects must have formed within the halo. Compared to the averageabundance of the central disk H Ii-region (45% Zsun) bothEHRs reveal substantially lower [O/H] abundances of about 10%Zsun. We could establish for the first time strongdifferences in the metal content along the minor axis of this galaxy.Oxygen appears to be less abundant in the halo by about a factor of 4.Since both EHRs are located above the central part of NGC 55, it appearslikely that their formation was triggered by star formation activity inthe disk below. In this environment the molecular gas clouds out ofwhich EHRs have formed can survive and collapse only in the periodbetween two successive bursts of star formation.Based on observations collected at the European Southern Observatory,Paranal (Chile); Proposal No.: 64.N-0399(A,B).
| The far-infrared/radio correlation in the ISO era. The warm and cold far-infrared/radio correlations
We present the correlation between the far-infrared (FIR) and radioemissions from a composite sample of 72 nearby normal galaxies observedwith the ISOPHOT instrument on board the Infrared Space Observatory. Thegalaxies in the sample have measurements at three FIR wavelengths (60,100 and 170 mu m), which allowed a direct determination of the warm andcold FIR emission components. This is the first time that thecorrelation has been established for the total FIR luminosity, of whichmost is carried by the cold dust component predominantly emittinglongwards of the spectral coverage of IRAS. The slope of thiscorrelation is slightly non-linear (1.10+/- 0.03). Separate correlationsbetween the warm and cold FIR emission components and the radio emissionhave also been derived. The slope of the warm FIR/radio correlation wasfound to be linear (1.03 +/- 0.03). For the cold FIR/radio correlationwe found a slightly non-linear (1.13 +/- 0.04) slope. We qualitativelyinterpret the correlations in terms of star formation rate and find thatboth the FIR and radio emissions may be consistent with a non-lineardependence on star formation rate for galaxies not undergoing starburstactivity.Based on observations with the Infrared Space Observatory (ISO), an ESAproject with instruments funded by ESA member States (especially the PIcountries: France, Germany, The Netherlands, and the UK) and with theparticipation of ISAS and NASA.Table \ref{Tab2} and Appendices A and B are only available in electronicform at http://www.edpsciences.org
| An Hα survey aiming at the detection of extraplanar diffuse ionized gas in halos of edge-on spiral galaxies. I. How common are gaseous halos among non-starburst galaxies?
In a series of two papers we present results of a new Hα imagingsurvey, aiming at the detection of extraplanar diffuse ionized gas inhalos of late-type spiral galaxies. We have investigated a sample of 74nearby edge-on spirals, covering the northern and southern hemisphere.In 30 galaxies we detected extraplanar diffuse emission at meandistances of |z| ~ 1-2 kpc. Individual filaments can be traced out to|z|<=6 kpc in a few cases. We find a good correlation between the FIRflux ratio (S60/S100) and the SFR per unit area(LFIR/D225), based on thedetections/non-detections. This is actually valid for starburst, normaland for quiescent galaxies. A minimal SFR per unit area for the lowestS60/S100 values, at which extended emission hasbeen detected, was derived, which amounts to dotEA25thres = (3.2+/-0.5)*E40ergs-1 kpc-2. There are galaxies where extraplanaremission was detected at smaller values ofLFIR/D225, however, only in combinationwith a significantly enhanced dust temperature. The results corroboratethe general view that the gaseous halos are a direct consequence of SFactivity in the underlying galactic disk.Based on observations collected at the European Southern Observatory,Chile (ESO No. 63.N-0070, ESO No. 64.N-0034, ESO No. 65.N.-0002).
| UV to radio centimetric spectral energy distributions of optically-selected late-type galaxies in the Virgo cluster
We present a multifrequency dataset for an optically-selected,volume-limited, complete sample of 118 late-type galaxies (>=S0a) inthe Virgo cluster. The database includes UV, visible, near-IR, mid-IR,far-IR, radio continuum photometric data as well as spectroscopic dataof Hα , CO and HI lines, homogeneously reduced, obtained from ourown observations or compiled from the literature. Assuming the energybalance between the absorbed stellar light and that radiated in the IRby dust, we calibarte an empirical attenuation law suitable forcorrecting photometric and spectroscopic data of normal galaxies. Thedata, corrected for internal extinction, are used to construct thespectral energy distribution (SED) of each individual galaxy, andcombined to trace the median SED of galaxies in various classes ofmorphological type and luminosity. Low-luminosity, dwarf galaxies haveon average bluer stellar continua and higher far-IR luminosities perunit galaxy mass than giant, early-type spirals. If compared to nearbystarburst galaxies such as M 82 and Arp 220, normal spirals haverelatively similar observed stellar spectra but 10-100 times lower IRluminosities. The temperature of the cold dust component increases withthe far-IR luminosity, from giant spirals to dwarf irregulars. The SEDare used to separate the stellar emission from the dust emission in themid-IR regime. We show that the contribution of the stellar emission at6.75 mu m to the total emission of galaxies is generally important, from~ 80% in Sa to ~ 20% in Sc.Tables 2-5, 7, 8, and Fig. 2 are only available in electronic form athttp://www.edpsciences.orgTables 10-12 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/402/37
| Optical spectroscopy and the UV luminosity function of galaxies in the Abell 1367, Coma and Virgo clusters
Optical spectroscopy of 93 galaxies, 60 projected in the direction ofAbell 1367, 21 onto the Coma cluster and 12 on Virgo, is reported. Thetargets were selected because they were detected in previous Hα ,UV or r' surveys. The present observations bring to 100% the redshiftcompleteness of Hα selected galaxies in the Coma region and to75% in Abell 1367. All observed galaxies except one show Hαemission and belong to the clusters. This confirms previousdeterminations of the Hα luminosity function of the two clustersthat were based on the assumption that all Hα detected galaxieswere cluster members. Using the newly obtained data we re-determine theUV luminosity function of Coma and we compute for the first time the UVluminosity function of A1367. Their faint end slopes remain uncertain(-2.00
| 12CO(J= 2->1) and CO(J= 3->2) observations of Virgo Cluster spiral galaxies with the KOSMA telescope: Global properties
We present 12CO (J= 2->1) and CO (J= 3->2) observationsof quiescent Virgo Cluster spiral galaxies with the KOSMA 3 m submmtelescope. The beam sizes of 80\arcsec at 345 GHz and 120 arcsec at 230GHz are well suited for the investigation of global properties of VirgoCluster galaxies. The observed sample was selected based on previous12CO (J=1->0) detections by Stark et al. (\cite{Stark86}),performed with the AT&T Bell Laboratory 7 m telescope (beam size ~100\arcsec). We were able to detect 18 spiral galaxies in12CO (2->1) and 16 in 12CO (3->2). Beammatched observations of the lowest three 12CO transitionsallow us to compare our results with previous high spatial resolutionstudies of (moderate) starburst galaxies and galactic core regions. Wediscuss the global excitation conditions of the ISM in these quiescentspiral galaxies. The resulting CO (3-2)/(1-0) integrated line ratiosvary over a relatively narrow range of values from 0.35 to 0.14 (on a Kkm s-1-scale) with increasing CO (2-1)/(1-0) ratio (from 0.5to 1.1). The line ratios between the three lowest rotational transitionsof CO cannot be fitted by any radiative transfer model with a singlesource component. A two-component model, assuming a warm, dense nuclearand a cold, less dense disc component allows us to fit the observed lineratios for most of the galaxies individually by selecting suitableparameters. The two-component model, however, fails to explain theobserved correlation of the line ratios. This is due to a variation ofthe relative filling factor of the warm gas alone, assuming a typicalset of parameters for the two components common for all galaxies.
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