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|On the origin of warps and the role of the intergalactic medium|
There is still no consensus as to what causes galactic discs to becomewarped. Successful models should account for the frequent occurrence ofwarps in quite isolated galaxies, their amplitude as well as theobserved azimuthal and vertical distributions of the HI layer.Intergalactic accretion flows and intergalactic magnetic fields may bendthe outer parts of spiral galaxies. In this paper we consider theviability of these non-gravitational torques to take the gas off theplane. We show that magnetically generated warps are clearly flawedbecause they would wrap up into a spiral in less than two or threegalactic rotations. The inclusion of any magnetic diffusivity to dilutethe wrapping effect causes the amplitude of the warp to damp. We alsoconsider the observational consequences of the accretion of anintergalactic plane-parallel flow at infinity. We have computed theamplitude and warp asymmetry in the accretion model, for a disc embeddedin a flattened dark matter halo, including self-consistently thecontribution of the modes with azimuthal wavenumbers m= 0 and m= 1.Since the m= 0 component, giving a U-shaped profile, is not negligiblecompared to the m= 1 component, this model predicts quite asymmetricwarps, maximum gas displacements on the two sides in the ratio 3 : 2 forthe preferred Galactic parameters, and the presence of a fraction ~3.5per cent of U-shaped warps, at least. The azimuthal dependence of themoment transfer by the ram pressure would produce a strong asymmetry inthe thickness of the HI layer and asymmetric density distributions in z,in conflict with observational data for the warp in our Galaxy and inexternal galaxies. The amount of accretion that is required to explainthe Galactic warp would give gas scaleheights in the far outer disc thatare too small. We conclude that accretion of a flow with no net angularmomentum cannot be the main and only cause of warps.
|The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) Program. III. Optical Spectroscopy|
We present the catalog of 477 spectra from the SerendipitousExtragalactic X-ray Source Identification (SEXSI) program, a surveydesigned to probe the dominant contributors to the 2-10 keV cosmic X-raybackground. Our survey covers 1 deg2 of sky to 2-10 keVfluxes of 1×10-14 ergs cm-2 s-1,and 2 deg2 for fluxes of 3×10-14 ergscm-2 s-1. Our spectra reach to R-band magnitudesof <~24 and have produced identifications and redshifts for 438 hardX-ray sources. Typical completeness levels in the 27 Chandra fieldsstudied are 40%-70%. The vast majority of the 2-10 keV selected sampleare active galactic nuclei (AGNs) with redshifts between 0.1 and 3; ourhighest redshift source lies at z=4.33. We find that few sources atz<1 have high X-ray luminosities, reflecting a dearth of high-mass,high-accretion-rate sources at low redshift, a result consistent withother recent wide-area surveys. We find that half of our sources showsignificant obscuration, with NH>1022cm-2, independent of unobscured luminosity. We classify 168sources as emission-line galaxies; all are X-ray-luminous(LX>1041 ergs s-1) objects withoptical spectra lacking both high-ionization lines and evidence of anonstellar continuum. The redshift distribution of these emission-linegalaxies peaks at a significantly lower redshift than does that of thesources we spectroscopically identify as AGNs. We conclude that few ofthese sources, even at the low-luminosity end, can be powered bystarburst activity. Stacking spectra for a subset of these sources in asimilar redshift range, we detect [Ne V] λ3426 emission, a clearsignature of AGN activity, confirming that the majority of these objectsare Seyfert 2 galaxies in which the high-ionization lines are diluted bystellar emission. We find a total of 33 objects lacking broad lines intheir optical spectra that have quasar X-ray luminosities(LX>1044 ergs s-1), the largestsample of such objects identified to date. In addition, we explore 17AGNs associated with galaxy clusters and find that the cluster-memberAGN sample has a lower fraction of broad-line AGNs than does thebackground sample.The majority of data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andNASA. The Observatory was made possible by the generous financialsupport of the W. M. Keck Foundation.
|Masses of the local group and of the M81 group estimated from distortions in the local velocity field|
Based on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies.
|Structural Parameters of Thin and Thick Disks in Edge-on Disk Galaxies|
We analyze the global structure of 34 late-type, edge-on, undisturbed,disk galaxies spanning a wide range of mass. We measure structuralparameters for the galaxies using two-dimensional least-squares fittingto our R-band photometry. The fits require both a thick and a thin diskto adequately fit the data. The thick disks have larger scale heightsand longer scale lengths than the embedded thin disks by factors of ~2and ~1.25, respectively. The observed structural parameters agree wellwith the properties of thick and thin disks derived from star counts inthe Milky Way and from resolved stellar populations in nearby galaxies.We find that massive galaxies' luminosities are dominated by the thindisk. However, in low-mass galaxies (Vc<~120 kms-1) thick disk stars contribute nearly half the luminosityand dominate the stellar mass. Thus, although low-mass dwarf galaxiesappear blue, the majority of their stars are probably quite old.Our data are most easily explained by a formation scenario in which thethick disk is assembled through direct accretion of stellar materialfrom merging satellites while the thin disk is formed from accreted gas.The baryonic fraction in the thin disk therefore constrains the gasrichness of the merging pregalactic fragments. If we include the mass inH I as part of the thin disk, the thick disk contains <~10% of thebaryons in high-mass galaxies and ~25%-30% of the baryons in low-massgalaxies. Our data, therefore, indicate that the fragments were quitegas rich at the time of merging (fgas=75%-90%). However,because low-mass galaxies have a smaller fraction of baryons in theirthin disks, the pregalactic fragments from which they assembled musthave been systematically more gas poor. We believe this trend resultsfrom increased outflow due to supernova-driven winds in the lower masspregalactic fragments. We estimate that ~60% of the total baryonic massin these systems was lost due to outflows. Pushing the episode ofsignificant winds to early times allows the mass-metallicityrelationship for disks to be established early, before the main disk isassembled, and obviates the difficulty in driving winds from diffusedisks with low star formation efficiencies. We discuss otherimplications of this scenario for solving the G dwarf problem, forpredicting abundance trends in thick disks, and for removingdiscrepancies between semianalytic galaxy formation models and theobserved colors of low-mass galaxies.
|A new method to determine the thickness of non-edge-on disk galaxies|
Aims.We present a new method to determine the thickness of non-edge-ondisk galaxies. This method allows us to investigate the mass-to-lightratio of the disk. Methods: .Our method is based on the comparisonof observations and theory of the distribution of the vertical velocitydispersion, which is obtained from the solution of three dimensionalPoisson equations and the galactic dynamical equation. Results:.As examples, the thickness and mass-to-light ratio of two diskgalaxies, NGC 1566 and NGC 5247, which have been extensively studied byspectroscopy, have been calculated. The calculated results areconsistent with observations and support the use of this method.However, due to the small sample size available, the results should beconfirmed on other samples of galaxies.
|X-ray observations of the edge-on star-forming galaxy NGC 891 and its supernova SN1986J|
We present XMM-Newton observations of NGC 891, a nearby edge-on spiralgalaxy. We analyse the extent of the diffuse emission emitted from thedisc of the galaxy, and find that it has a single-temperature profilewith best-fitting temperature of 0.26 keV, though the fit of adual-temperature plasma with temperatures of 0.08 and 0.30 keV is alsoacceptable. There is a considerable amount of diffuse X-ray emissionprotruding from the disc in the north-west direction out toapproximately 6 kpc. We analyse the point-source population using aChandra observation, using a maximum-likelihood method to find that theslope of the cumulative luminosity function of point sources in thegalaxy is -0.77+0.13-0.1. Using a sample of otherlocal galaxies, we compare the X-ray and infrared properties of NGC 891with those of `normal' and starburst spiral galaxies, and conclude thatNGC 891 is most likely a starburst galaxy in a quiescent state. Weestablish that the diffuse X-ray luminosity of spirals scales with thefar-infrared luminosity asLX~L0.87+/-0.07FIR, except for extremestarbursts, and NGC 891 does not fall in the latter category. We studythe supernova SN1986J in both XMM-Newton and Chandra observations, andfind that the X-ray luminosity has been declining with time more steeplythan expected (LX~t-3).
|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.
|The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) Program. II. Optical Imaging|
The Serendipitous Extragalactic X-ray Source Identification (SEXSI)Program is designed to expand significantly the sample of identifiedextragalactic hard X-ray sources at intermediate fluxes,10-13ergscm-2s-1<~S2-10keV<10-15ergscm-2s-1.SEXSI, which includes sources derived from more than 2 deg2of Chandra images, provides the largest hard X-ray-selected sample yetstudied, offering an essential complement to the Chandra Deep Fields(total area ~0.2 deg2). In this paper we describe R-bandoptical imaging of the SEXSI fields from the Palomar, MDM, and Keckobservatories. We have identified counterparts or derived flux limitsfor nearly 1000 hard X-ray sources. Using the optical images, we deriveaccurate source positions. We investigate correlations between opticaland X-ray flux, and optical flux and X-ray hardness ratio. We also studythe density of optical sources surrounding X-ray counterparts, as wellas the properties of optically faint, hard X-ray sources.
|Halos of Spiral Galaxies. III. Metallicity Distributions|
We report results of a campaign to image the stellar populations in thehalos of highly inclined spiral galaxies, with the fields roughly 10 kpc(projected) from the nuclei. We use the F814W (I) and F606W (V) filtersin the Wide Field Planetary Camera 2 on board the Hubble SpaceTelescope. We unambiguously resolve the stellar halos 1 to 2 mag fainterthan the tip of the red giant branch. Extended halo populations aredetected in all galaxies. The color-magnitude diagrams appear to becompletely dominated by giant branch stars, with no evidence for thepresence of young stellar populations in any of the fields. Themetallicity distribution function for the galaxy sample is derived frominterpolation within an extensive grid of red giant branch loci. Theseloci are derived from theoretical sequences that are calibrated usingthe Galactic globular clusters and from empirical sequences formetal-rich stellar populations. We find that the metallicitydistribution functions are dominated by metal-rich populations, with atail extending toward the metal-poor end. To first order, the overallshapes of the metallicity distribution functions are similar to what ispredicted by a simple, single-component model of chemical evolution withthe effective yields increasing with galaxy luminosity. However,metallicity distributions significantly narrower than the simple modelare observed for a few of the most luminous galaxies in the sample. Thediscrepancies are similar to those previously observed for NGC 5128, thehalo of M31, and the Galactic bulge. Our observations can be used tohelp distinguish between models for the formation of spiral galaxies. Itappears that more luminous spiral galaxies also have more metal-richstellar halos. The increasingly significant departures from theclosed-box model for the more luminous galaxies indicate that aparameter in addition to a single yield is required to describe chemicalevolution. This parameter, which could be related to gas infall oroutflow either in situ or in progenitor dwarf galaxies that later mergeto form the stellar halo, tends to make the metallicity distributionsnarrower at high metallicity.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
|Halos of Spiral Galaxies. II. Halo Metallicity-Luminosity Relation|
Using the Hubble Space Telescope, we have resolved individual red giantbranch stars in the halos of eight nearby spiral galaxies. The fieldslie at projected distances between 2 and 13 kpc along the galaxies'minor axes. The data set allows a first look at the systematic trends inhalo stellar populations. We have found that bright galaxies tend tohave broad red giant branch star color distributions with redder meancolors, suggesting that the heavy-element abundance spread increaseswith the parent galaxy luminosity. The mean metallicity of the stellarhalo, estimated using the mean colors of red giant branch stars,correlates with the parent galaxy luminosity. The metallicity of theMilky Way halo falls nearly 1 dex below this luminosity-metallicityrelation, suggesting that the halo of the Galaxy is more the exceptionthan the rule for spiral galaxies; i.e., massive spirals with metal-poorhalos are unusual. The luminosity-halo stellar abundance relation isconsistent with the scaling relation expected for stellar systemsembedded in dominant halos, suggesting that the bulk of the halo stellarpopulation may have formed in situ.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
|Halos of Spiral Galaxies. I. The Tip of the Red Giant Branch as a Distance Indicator|
We have imaged the halo populations of a sample of nearby spiralgalaxies using the Wide Field Planetary Camera 2 on board the HubbleSpace Telescope with the aim of studying the stellar populationproperties and relating them to those of the host galaxies. In fourgalaxies, the red giant branch is sufficiently well populated to measurethe magnitude of the tip of the red giant branch (TRGB), a well-knowndistance indicator. Using both the Sobel edge-detection technique andmaximum likelihood analysis to measure the I-band magnitude of the TRGB,we determine distances to four nearby galaxies: NGC 253, NGC 4244, NGC4945, and NGC 4258. For the first three galaxies, the TRGB distance ishere determined more directly, and is likely to be more accurate, thanprevious distance estimates. In the case of NGC 4258, our TRGB distanceis in good agreement with the geometrical maser distance, supporting theLarge Magellanic Cloud distance modulus (m-M)0=18.50 that isgenerally adopted in recent estimates of the Hubble constant.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
|A Study of Edge-On Galaxies with the Hubble Space Telescope Advanced Camera for Surveys. II. Vertical Distribution of the Resolved Stellar Population|
We analyze the vertical distribution of the resolved stellar populationsin six low-mass (Vmax=67-131 km s-1), edge-on,spiral galaxies observed with the Hubble Space Telescope Advanced Camerafor Surveys. In each galaxy we find evidence for an extraplanar stellarcomponent extending up to 15 scale heights (3.5 kpc) above the plane,with a scale height typically twice that of two-dimensional fits toKs-band Two Micron All Sky Survey images. We analyze thevertical distribution as a function of stellar age by tracking changesin the color-magnitude diagram. The young stellar component(<~108 yr) is found to have a scale height larger than theyoung component in the Milky Way, suggesting that stars in theselow-mass galaxies form in a thicker disk. We also find that the scaleheight of a stellar population increases with age, with youngmain-sequence stars, intermediate-age asymptotic giant branch stars, andold red giant branch (RGB) stars having successively larger scaleheights in each galaxy. This systematic trend indicates that diskheating must play some role in producing the extraplanar stars. Weconstrain the rate of disk heating using the observed trend betweenscale height and stellar age and find that the observed heating ratesare dramatically smaller than in the Milky Way. The color distributionsof the RGB stars well above the midplane indicate that the extendedstellar components we see are moderately metal-poor, with peakmetallicities around [Fe/H]=-1 and with little or no metallicitygradient with height. The lack of metallicity gradient can be explainedif a majority of extraplanar RGB stars were formed at early times andare not dominated by a younger heated population. Our observationssuggest that, like the Milky Way, low-mass disk galaxies also havemultiple stellar components. In its structure, mean metallicity, and oldage, the RGB component in these galaxies seems analogous to the MilkyWay thick disk. However, without additional kinematic and abundancemeasurements, this association is only circumstantial, particularly inlight of the clear existence of some disk heating at intermediate ages.Finally, we find that the vertical dust distribution has a scale heightsomewhat larger than that of the main-sequence stars.
|A Study of Edge-On Galaxies with the Hubble Space Telescope Advanced Camera for Surveys. I. Initial Results|
We present the initial results of a Hubble Space Telescope/AdvancedCamera for Surveys snapshot survey of 16 nearby, edge-on, late-typegalaxies covering a range in distance from 2 to 19 Mpc. The images ofthese galaxies show significant resolved stellar populations. We deriveF606W and F814W photometry for more than 1.2 million stars and presentcolor-magnitude diagrams that show a mixture of young, intermediate, andold stars in each galaxy. In one of the fields we serendipitously detectstars from the Large Magellanic Cloud. We also identify a candidateyoung dwarf galaxy lying ~2 kpc above the plane of NGC 4631. For thenearest six galaxies, we derive tip of the red giant branch distancesand demonstrate that these galaxies fall on the K-band Tully-Fisherrelation established in clusters. From the color of the red giantbranch, we also find evidence that these galaxies possess a metal-poorthick-disk or halo population.
|Red Thick Disks of Nearby Galaxies|
Edge-on systems reveal the properties of disk galaxies as a function ofheight, z, above the plane. Four local edge-on galaxies that are closeenough to have been resolved into stars by the Hubble Space Telescopeshow thick disks composed of a red stellar population that is old andrelatively metal rich. Color gradients, Δ(V-I)/Δz, are zeroor slightly positive. Favored models may have an explicit thick diskformation phase.
|The Local Group and Other Neighboring Galaxy Groups|
Over the last few years, rapid progress has been made in distancemeasurements for nearby galaxies based on the magnitude of stars on thetip of the red giant branch. Current CCD surveys with the Hubble SpaceTelescope (HST) and large ground-based telescopes bring ~10% accuratedistances for roughly a hundred galaxies within 5 Mpc. The new data ondistances to galaxies situated in (and around) the nearest groups-theLocal Group, M81 Group, Cen A/M83 Group, IC 342/Maffei Group, Sculptorfilament, and Canes Venatici cloud-allowed us to determine their totalmass from the radius of the zero-velocity surface, R0, whichseparates a group as bound against the homogeneous cosmic expansion. Thevalues of R0 for the virialized groups turn out to be closeeach other, in the range of 0.9-1.3 Mpc. As a result, the total massesof the groups are close to each other, as well, yielding total mass toblue luminosity ratios of 10-40 MsolarL-1solar. The new total mass estimates are 3-5times lower than old virial mass estimates of these groups. Becauseabout half of galaxies in the Local volume belong to such loose groups,the revision of the amount of dark matter (DM) leads to a low localdensity of matter, Ωm~=0.04, which is comparable withthe global baryonic fraction Ωb but much lower than theglobal density of matter, Ωm=0.27. To remove thediscrepancy between the global and local quantities ofΩm, we assume the existence of two different DMcomponents: (1) compact dark halos around individual galaxies and (2) anonbaryonic dark matter ``ocean'' with ΩDM1~=0.07 andΩDM2~=0.20, respectively.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
|Constraints on the halo density profile using HI flaring in the outer Galaxy|
The observed flaring of HI disk in the outer region of galaxies has beenused in the past to determine the shape of the dark matter halo.Previous studies based on this concept suggest a slightly oblate halo(axis ratio 0.8) for our Galaxy. We reinvestigate this problem bycalculating the HI scaleheight in the outer Galaxy to a larger radialdistance, and by studying its dependence on the shape and the densityprofile of the halo. We find that a simple isothermal infinite halo ofany shape- oblate or prolate, is not able to account for the observedflaring. Instead we show that a spherical halo with density fallingfaster than isothermal halo in the outer region provides a better fit tothe observed HI flaring as well as the observed rotation curve of ourGalaxy. These halos have about 95% of their mass within a few hundredsof kpc for Rcirc = 8.5 kpc and Θcirc = 220km s-1, the central density and core radius can beconstrained to the range ρcirc = 0.035- 0.06Mȯ pc-3 and Rc = 8 - 10 kpc. Ourclaim for such "finite-sized" spherical halos is supported by recentliterature on numerical simulation studies of halo formation as well asanalyses of SDSS data.
|Neutral hydrogen gas in 7 high-inclination spiral galaxies. I. The data|
High-sensitivity interferometric H i line observations of a small sampleof seven galaxies with limiting column densities of a few times1019 cm-2 are presented. A tilted ring modelfitting routine was used to determine some global characteristics of theH i distribution and kinematics in the galaxy disks. 4 of the 7 galaxieshave low maximum rotation velocities of 125 km s-1,indicating that they are low-mass systems. Visual inspection shows thatat least one galaxy, NGC 4700, exhibits signs of extraplanar H iemission. An in-depth search for H i gas in the galaxy halos and thedetermination of halo gas properties, based on three-dimensionalmodeling, will follow in a separate publication. Companion galaxies weredetected in H i line emission near 3 of the 7 sample galaxies: NGC 1511,NGC 4565 and NGC 4700. One of these, NGC 1511, is found to be stronglyinteracting and is therefore not suitable for a study of the dependenceof its halo properties on the level of star formation activity in theunderlying disk. In the case of NGC 4700 the companion galaxy has novisible influence on its gas kinematics, while NGC 4565 might beaffected by its interaction with two small companions.Figures [see full text] and Appendix A are only available in electronicform at http://www.edpsciences.org
|Thick disks and halos of spiral galaxies M 81, NGC 55 and NGC 300|
By using images from the HST/WFPC2/ACS archive, we have analyzed thespatial distribution of the AGB and RGB stars along the galactocentricradius of nearby spiral galaxies M 81, NGC 300 and NGC 55. Examiningcolor-magnitude diagrams and stellar luminosity functions, we gauge thestellar contents of the surroundings of the three galaxies. The redgiant population (RGB) identified at large galactocentric radii yields adistance of 3.85±0.08 Mpc for M 81, 2.12±0.10 Mpc for NGC55, and 2.00±0.13 Mpc for NGC 300, and a mean stellar metallicityof -0.65, -1.25, and -0.87 respectively. We find that there are twonumber density gradients of RGB stars along the radius, which correspondto the thick disk and halo components of the galaxies. We confirm thepresence of a metallicity gradient of evolved stars in these galaxies,based on the systematic changes of the color distribution of red giantstars. These results imply that the thick disk might be a generalfeature of spiral galaxies, and endorse a further investigation of theouter stellar edges of nearby spirals, which is critical in constrainingthe origin and evolution of galaxies.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
|A catalogue of ultraluminous X-ray sources in external galaxies|
We 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 (126.96.36.199) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/1125
|Dynamical friction in flattened systems: a numerical test of Binney's approach|
We carry out a set of self-consistent N-body calculations to investigatehow important the velocity anisotropy in non-spherical dark matterhaloes is for dynamical friction. For this purpose, we allow satellitegalaxies to orbit within flattened and live dark matter haloes (DMHs)and compare the resulting orbit evolution with a semi-analytic code.This code solves the equation of motion of the same satellite orbitswith mass loss and assumes the same DMH, but either employsChandrasekhar's dynamical friction formula, which does not incorporatethe velocity anisotropy, or Binney's description of dynamical frictionin anisotropic systems. In the numerical and the two semi-analyticmodels, the satellites are given different initial orbital inclinationsand orbital eccentricities, whereas the parent galaxy is composed of aDMH with aspect ratio qh= 0.6.We find that Binney's approach successfully describes the overallsatellite decay and orbital inclination decrease for the whole set oforbits, with an averaged discrepancy of less than 4 per cent in orbitalradius during the first three orbits. If Chandrasekhar's expression isused instead, the discrepancy increases to 20 per cent. Binney'streatment therefore appears to provide a significantly improvedtreatment of dynamical friction in anisotropic systems.The velocity anisotropy of the DMH velocity distribution function leadsto a significant decrease with time of the inclination of non-polarsatellite orbits. But, at the same time, it reduces the difference indecay times between polar and coplanar orbits evident in a flattened DMHwhen the anisotropic DMH velocity distribution function is not takeninto account explicitly. Our N-body calculations furthermore indicatethat polar orbits survive about 1.6 times longer than coplanar orbitsand that the orbital eccentricity e remains close to its initial valueif satellites decay slowly towards the galaxy centre. However, orbits ofrapidly decaying satellites modelled with the semi-analytic code show astrong orbital circularization () not present in the N-bodycomputations.
|Haloes around edge-on disc galaxies in the Sloan Digital Sky Survey|
We present a statistical analysis of halo emission for a sample of 1047edge-on disc galaxies imaged in five bands by the Sloan Digital SkySurvey (SDSS). Stacking the homogeneously rescaled images of thegalaxies, we can measure surface brightnesses as deep asμr~ 31 mag arcsec-2. The results stronglysupport the almost ubiquitous presence of stellar haloes around discgalaxies, whose spatial distribution is well described by a power lawρ~r-3, in a moderately flattened spheroid (c/a~ 0.6). Thecolour estimates in g-r and r-i, although uncertain, give a clearindication for extremely red stellar populations, hinting at old agesand/or non-negligible metal enrichment. These results support the ideaof haloes being assembled via early merging of satellite galaxies.
|The Ultraluminous X-Ray Source Population from the Chandra Archive of Galaxies|
One 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 High Spatial Resolution X-Ray and Hα Study of Hot Gas in the Halos of Star-forming Disk Galaxies. I. Spatial and Spectral Properties of the Diffuse X-Ray Emission|
We present arcsecond resolution Chandra X-ray and ground-based opticalHα imaging of a sample of 10 edge-on star-forming disk galaxies(seven starburst and three ``normal'' spiral galaxies), a sample thatcovers the full range of star formation intensity found in diskgalaxies. The X-ray observations make use of the unprecedented spatialresolution of the Chandra X-ray observatory to more robustly than beforeremove X-ray emission from point sources and hence obtain the X-rayproperties of the diffuse thermal emission alone. We have combined theX-ray observations with existing, comparable-resolution, ground-basedHα and R-band imaging and present a mini-atlas of images on acommon spatial and surface brightness scale to aid cross-comparison. Ingeneral, the morphology of the extraplanar diffuse X-ray emission isvery similar to the extraplanar Hα filaments and arcs, on bothsmall and large scales (scales of tens of parsecs and kiloparsecs,respectively). The most spectacular cases of this are found in NGC 1482(for which we provide the first published X-ray observation) and NGC3079. We provide a variety of quantitative measures of how the spectralhardness and surface brightness of the diffuse X-ray emission varieswith increasing height z above the plane of each galaxy. Of the eightgalaxies in which diffuse X-ray emitting halos are found (the starburstsand the normal spiral NGC 891), significant spatial variation in thespectral properties of the extraplanar emission (|z|>=2 kpc) is onlyfound in two cases: NGC 3628 and NGC 4631. In general, the verticaldistribution of the halo-region X-ray surface brightness is bestdescribed as an exponential, with the observed scale heights of thesample galaxies lying in the range Heff~2-4 kpc. The presenceof extraplanar X-ray emission is always associated with the presence ofextraplanar optical line emission of similar vertical extent. No X-rayemission was detected from the halos of the two low-mass normal spiralgalaxies NGC 6503 and NGC 4244. Active galactic nuclei, where present,appear to play no role in powering or shaping the outflows from thestarburst galaxies in this sample. The Chandra ACIS X-ray spectra ofextraplanar emission from all these galaxies can be fitted with a commontwo-temperature spectral model with an enhanced α-to-iron elementratio. This is consistent with the origin of the X-ray emitting gasbeing either metal-enriched merged SN ejecta or shock-heated ambienthalo or disk material with moderate levels of metal depletion onto dust.Our favored model is that SN feedback in the disks of star-forminggalaxies create, via blow-out and venting of hot gas from the disk,tenuous exponential atmospheres of density scale heightHg~4-8 kpc. The soft thermal X-ray emission observed in thehalos of the starburst galaxies is either this preexisting halo medium,which has been swept up and shock-heated by the starburst-driven wind,or wind material compressed near the walls of the outflow by reverseshocks within the wind. In either case, the X-ray emission provides uswith a powerful probe of the properties of gaseous halos aroundstar-forming disk galaxies.
|A High Spatial Resolution X-Ray and Hα Study of Hot Gas in the Halos of Star-forming Disk Galaxies. II. Quantifying Supernova Feedback|
We investigate how the empirical properties of hot X-ray-emitting gas ina sample of seven starburst and three normal edge-on spiral galaxies (asample that covers the full range of star formation intensity found indisk galaxies) correlate with the size, mass, star formation rate, andstar formation intensity in the host galaxies. From this analysis weinvestigate various aspects of mechanical energy ``feedback''-the returnof energy to the interstellar medium from massive star supernovae andstellar winds-on galactic scales. The X-ray observations make use of theunprecedented spatial resolution of the Chandra X-Ray Observatory toremove X-ray emission from point sources more accurately than in anyprevious study and hence obtain the X-ray properties of the diffusethermal emission alone. Intriguingly, the diffuse X-ray properties ofthe normal spirals (in both their disks and halos) fall whereextrapolation of the trends from the starburst galaxies with superwindswould predict. We demonstrate, using a variety of multiwavelength starformation rate and intensity indicators, that the luminosity of diffuseX-ray emission in the disk (and, where detected, in the halo) isdirectly proportional to the rate of mechanical energy feedback frommassive stars in the host galaxies. Accretion of gas from theintergalactic medium (IGM) does not appear to be a significantcontributor to the diffuse X-ray emission in this sample. Nevertheless,with only three nonstarburst normal spiral galaxies it is hard toexclude an accretion-based origin for extraplanar diffuse X-ray emissionaround normal star-forming galaxies. Larger galaxies tend to have moreextended X-ray-emitting halos, but galaxy mass appears to play no rolein determining the properties of the disk or extraplanar X-ray-emittingplasma. The combination of these luminosity and size correlations leadsto a correlation between the surface brightness of the diffuse X-rayemission and the mean star formation rate per unit area in the disk(calculated from the far-infrared luminosity and the optical size of thegalaxy, LFIR/D225). Furtherobservational work of this form will allow empirical constraints to bemade on the critical star formation rate per unit disk area necessary toblow hot gas out of the disk into the halo. We show that a minorgeneralization of standard superbubble theory directly predicts acritical star formation rate per unit area for superbubble blowout fromthe disk and by extension for superwinds to blow out of the gaseoushalos of their host galaxy. At present there are a variety of poorlyknown parameters in this theory that complicate comparison betweenobservation and theory, making it impossible to assess the quantitativeaccuracy of standard superbubble blowout theory. We argue that thecrucial spatial region around a galaxy that controls whether gas instarburst-driven superwinds will escape into the IGM is not the outerhalo ~100 kpc from the host galaxy, but the inner few halo scaleheights, within ~20 kpc of the galaxy plane. Given the properties of thegaseous halos we observe, superwind outflows from disk galaxies of massM~1010-1011 Msolar should still ejectsome fraction of their material into the IGM.
|Old and Young X-Ray Point Source Populations in Nearby Galaxies|
We have analyzed Chandra ACIS observations of 32 nearby spiral andelliptical galaxies and present the results of 1441 X-ray point sourcesthat were detected in these galaxies. The total point-source X-ray(0.3-8.0 keV) luminosity LXP is well correlated with theB-band, K-band, and FIR+UV luminosities of spiral host galaxies and iswell correlated with the B-band and K-band luminosities of ellipticalgalaxies. This suggests an intimate connection between LXPand both the old and young stellar populations, for which K and FIR+UVluminosities are reasonable proxies for the galaxy mass M and starformation rate SFR. We derive proportionality constantsα=1.3×1029 ergs s-1M-1solar and β=0.7×1039 ergss-1 (Msolar yr-1)-1, whichcan be used to estimate the old and young components from M and SFR,respectively. The cumulative X-ray luminosity functions for the pointsources have significantly different slopes. For the spiral andstarburst galaxies, γ~0.6-0.8, and for the elliptical galaxies,γ~1.4. This implies that the most luminous point sources-thosewith LX>~1038 ergss-1-dominate LXP for the spiral andstarburst galaxies. Most of the point sources have X-ray colors that areconsistent with soft-spectrum (photon index Γ~1-2) low-mass X-raybinaries, accretion-powered black hole high-mass X-ray binaries (BHHMXBs), or ultraluminous X-ray sources (ULXs, also known as IXOs). Werule out hard-spectrum neutron star HMXBs (e.g., accretion-powered X-raypulsars) as contributing much to LXP. Thus, for spirals,LXP is dominated by ULXs and BH HMXBs. We find no discernibledifference between the X-ray colors of ULXs(LX>=1039 ergs s-1) in spiralgalaxies and point sources withLX~1038-1039 ergs s-1. Weestimate that >~20% of all ULXs found in spirals originate from theolder (Population II) stellar populations, indicating that many of theULXs that have been found in spiral galaxies are in fact Population IIULXs, like those in elliptical galaxies. We find that LXPdepends linearly (within uncertainties) on both M and SFR for our samplegalaxies (M<~1011 Msolar and SFR<~10Msolar yr-1).
|Star Formation Properties of a Large Sample of Irregular Galaxies|
We present the results of Hα imaging of a large sample ofirregular galaxies. Our sample includes 94 galaxies with morphologicalclassifications of Im, 26 blue compact dwarfs (BCDs), and 20 Sm systems.The sample spans a large range in galactic parameters, includingintegrated absolute magnitude (MV of -9 to -19), averagesurface brightness (20-27 mag arcsec-2), current starformation activity (0-1.3 Msolar yr-1kpc-2), and relative gas content(0.02-5Msolar/LB). The Hα images were usedto measure the integrated star formation rates, determine the extents ofstar formation in the disks, and compare azimuthally averaged radialprofiles of current star formation to older starlight. The integratedstar formation rates of Im galaxies normalized to the physical size ofthe galaxy span a range of a factor of 104 with 10% Imgalaxies and one Sm system having no measurable star formation at thepresent time. The BCDs fall, on average, at the high star formation rateend of the range. We find no correlation between star formation activityand proximity to other cataloged galaxies. Two galaxies located in voidsare similar in properties to the Sm group in our sample. The H IIregions in these galaxies are most often found within the Holmbergradius RH, although in a few systems H II regions are tracedas far as 1.7RH. Similarly, most of the star formation isfound within three disk scale lengths RD, but in somegalaxies H II regions are traced as far as 6RD. A comparisonof Hα surface photometry with V-band surface photometry shows thatthe two approximately follow each other with radius in Sm galaxies, butin most BCDs there is an excess of Hα emission in the centers thatdrops with radius. In approximately half of the Im galaxies Hα andV correspond well, and in the rest there are small to large differencesin the relative rate of falloff with radius. The cases with stronggradients in the LHα/LV ratios and with highcentral star formation rate densities, which include most of the BCDs,require a significant fraction of their gas to migrate to the center inthe last gigayear. We discuss possible torques that could have causedthis without leaving an obvious signature, including dark matter barsand past interactions or mergers with small galaxies or H I clouds.There is now a substantial amount of evidence for these processes amongmany surveys of BCDs. We note that such gas migration will also increasethe local pressure and possibly enhance the formation of massive denseclusters but conclude that the star formation process itself does notappear to differ much among BCD, Im, and Sm types. In particular, thereis evidence in the distribution function for Hα surface brightnessthat the turbulent Mach numbers are all about the same in these systems.This follows from the Hα distribution functions corrected forexponential disk gradients, which are log-normal with a nearly constantdispersion. Thus, the influence of shock-triggered star formation isapparently no greater in BCDs than in Im and Sm types.
|High-Resolution H I Observations of the Galaxy NGC 404: A Dwarf S0 with Abundant Interstellar Gas|
As part of a detailed study of the gas content in a sample of early-typegalaxies, we present 21 cm H I line maps of the S0 galaxy NGC 404 at aspatial resolution of 15.2"'×14.4"(α×δ) and a velocity resolution of 2.6 kms-1. The H I has been traced out to a radiusR~8R25 or 48 disk scale lengths, making it one of the largestH I extents reported (800" or 12.6 kpc at the assumed distance of 3.3Mpc). Approximately 75% of the H I resides in a doughnut, which is seenclose to face-on with inner and outer radii of ~R25 and~4R25, respectively. The optical galaxy fits snugly withinthe hole of the doughnut. The remaining 25% of the neutral gas is foundin an annulus concentric with the doughnut and with a somewhat largerellipticity, extending from a radius of ~4R25 to~8R25. A total H I mass of1.52+/-0.04×108Msolar is found, whichimplies an MHI/LB=0.22 in solar units. We arguethat most, if not all, of this gas is of external origin, most likelyfrom the merger of a dwarf irregular galaxy with MB~-15.5mag. The velocity field shows a steeply declining observed rotationcurve, compatible with Keplerian decline. However, because the galaxy isclose to face-on, there is a degeneracy in the determination of theintrinsic rotation curve and inclination. We therefore analyzed twoextreme cases, producing tilted-ring model fits forcing either aKeplerian or a flat rotation curve through the observations; bothapproaches result in equally plausible fits. In both model fits, theposition angle of the kinematical major axis of the annulus is distinctfrom that of the doughnut and ranges from 160° to 120° (for thedoughnut these values are 100° to 60°). Assuming a distance of3.3 Mpc, a total mass of 3×1010Msolar isfound on the basis of the Keplerian rotation curve. This implies aglobal MT/LB ratio of ~44 in solar units, which ishigh and likely a reflection of the low blue luminosity of the galaxy(~15 times lower than the average S0 luminosity). Values for a flatrotation curve are a factor of 4 higher.
|Nuclear Stellar Populations in the Infrared Space Observatory Atlas of Bright Spiral Galaxies|
To understand the nuclear stellar populations and star formationhistories of the nuclei of spiral galaxies, we have obtained K-bandnuclear spectra for 41 galaxies and H-band spectra for 20 galaxies inthe Infrared Space Observatory's Atlas of Bright Spiral Galaxies. In thevast majority of the subsample (80%), the near-infrared spectra suggestthat evolved red stars completely dominate the nuclear stellarpopulations and that hot young stars are virtually nonexistent. Thesignatures of recent star formation activity are only found in 20% ofthe subsample, even though older red stars still dominate the stellarpopulations in these galaxies. Given the dominance of evolved stars inmost galaxy nuclei and the nature of the emission lines in the galaxieswhere they were detected, we suggest that nuclear star formationproceeds in the form of instantaneous bursts. The stars produced bythese bursts comprise only ~2% of the total nuclear stellar mass inthese galaxies, but we demonstrate how the nuclear stellar populationsof normal spiral galaxies can be built up through a series of thesebursts. The bursts were detected only in Sbc galaxies and later, andboth bars and interactions appeared to be sufficient, but not necessary,triggers for the nuclear star formation activity. The vast majority ofgalaxies with nuclear star formation were classified as H II galaxies.With one exception, LINERs and transition objects were dominated byolder red stars, which suggested that star formation was not responsiblefor generating these galaxies' optical line emission.
|A Catalog of Neighboring Galaxies|
We present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its ``global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra.
|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 (188.8.131.52) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/422/39
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