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Late-type galaxies observed with SAURON: two-dimensional stellar and emission-line kinematics of 18 spirals
We present the stellar and gas kinematics of a sample of 18 nearbylate-type spiral galaxies (Hubble types ranging from Sb to Sd), observedwith the integral-field spectrograph SAURON at the 4.2-m WilliamHerschel Telescope. SAURON covers the spectral range 4800-5380Å,allowing us to measure the Hβ, Fe, Mgb absorption features and theemission in the Hβ line and the [OIII]λλ4959,5007Å and [NI]λλ5198, 5200Å doublets over a 33× 41-arcsec2 field of view. The maps cover the nuclearregion of these late-type galaxies and in all cases include the entirebulge. In many cases the stellar kinematics suggests the presence of acold inner region, as visible from a central drop in the stellarvelocity dispersion. The ionized gas is almost ubiquitous and behaves ina complicated fashion: the gas velocity fields often display morefeatures than the stellar ones, including wiggles in the zero-velocitylines, irregular distributions, ring-like structures. The line ratio[OIII]/Hβ often takes on low values over most of the field,probably indicating a wide-spread star formation.
| 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.
| A 62 Day X-Ray Periodicity and an X-Ray Flare from the Ultraluminous X-Ray Source in M82
In 240 days of X-ray monitoring of M82, we have discovered an X-rayperiodicity at 62.0+/-2.5 days with a peak-to-peak amplitudecorresponding to an isotropic luminosity of 2.4×1040ergs s-1 in M82 and an X-ray flare reaching a peak luminosityof 9.8×1040 ergs s-1. The periodicity andflare likely originate from the ultraluminous X-ray source (ULX) in M82,which has been identified as a possible intermediate-mass black hole. Wesuggest that the 62 day modulation is due to orbital motion within anX-ray binary with a Roche lobe overflowing companion star, which wouldimply that the average density of the companion star is near5×10-5 g cm-3 and is therefore a giant orsupergiant. Chandra observations just after the flare show an energyspectrum that is consistent with a power law with no evidence of athermal component or line emission. Radio observations made with the VLAduring the flare allow us to rule out a blazar identification for thesource and place strong constraints on relativistically beamed models ofthe X-ray emission. The Chandra observations reveal that a second X-raysource reached a flux of 4.4×10-12 ergs cm-2s-1 in the 0.3-7 keV band, which is dramatically higher thanany flux previously seen from this source and corresponds to anisotropic luminosity of 1.1×1040 ergs s-1.This source is a second ultraluminous X-ray source in M82 and may giverise to the QPOs detected from the central region of M82.
| Hubble Space Telescope Identification of the Optical Counterparts of Ultraluminous X-Ray Sources in M51
We present the results of a search for optical identifications ofultraluminous X-ray sources (ULXs) in M51 by using mosaic images takenwith the Hubble Space Telescope(HST) Advanced Camera for Surveys (ASC)in filters F435W (B), F555W (V), F814W (I), and F658N (Hα). Oursample, consisting of nine ULXs, is defined by analyzing the threeChandra observations of M51 performed in 2000 June, 2001 June, and 2003August. We found that four ULXs have one or two candidates forcounterparts, while two have multiple stars within their error circles.The other three have no candidate counterparts. Four ULXs are locatednear or in a star cluster, while others have no association with acluster. These results indicate that the companion stars, environments,and origins of ULXs are probably heterogeneous.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT HRI Observations. II. Statistical Properties
The statistical properties of the nonnuclear X-ray point sources fromthe ROSAT HRI survey of nearby galaxies in Paper I are studied, withparticular attention to the contamination from background and/orforeground objects. This study reveals a statistical preference for theultraluminous X-ray sources (ULXs) to occur in late-type galaxies overearly-type galaxies, and in starburst/H II galaxies over nonstarburstgalaxies. There is a trend of greater occurrence frequencies and ULXrates for galaxies with increasing star formation rates, confirming theconnection between the ULX phenomenon and the star formation. Anonlinear correlation is found between the number of ULXs and the starformation rate, with significantly more ULXs at low star formation ratesthan the ULX population expected from the high-mass X-ray binaries(HMXBs) as an indicator of the star formation and the accompanying youngstellar population, suggestive of another population of ULXs associatedwith the low-mass X-ray binaries (LMXBs) and the old stellar population.There are no breaks around 1039 ergs s-1 in theluminosity functions of ULXs in all galaxies or in late-type galaxies,suggesting the regular ULXs below 1040 ergs s-1are a high-luminosity extension of the ordinary HMXB/LMXB populationsbelow 1039 ergs s-1. There is evidence that theextreme ULXs above 1040 ergs s-1 might be adifferent ULX class from the regular ULXs below 1040 ergss-1, although a larger sample with more ULXs is needed toestablish the statistical properties of the extreme ULXs as a class.
| An Optical Study of Stellar and Interstellar Environments of Seven Luminous and Ultraluminous X-Ray Sources
We have studied the stellar and interstellar environments of twoluminous X-ray sources and five ultraluminous X-ray sources (ULXs) inorder to gain insight into their nature. Archival Hubble Space Telescopeimages were used to identify the optical counterparts of the ULXs Ho IXX-1 and NGC 1313 X-2, and to make photometric measurements of the localstellar populations of these and the luminous source IC 10 X-1. Weobtained high-dispersion spectroscopic observations of the nebulaearound these seven sources to search for He II λ4686 emission andto estimate the expansion velocities and kinetic energies of thesenebulae. Our observations did not detect nebular He II emission from anysource, with the exception of LMC X-1 this is either because we missedthe He III regions or because the nebulae are too diffuse to produce HeII surface brightnesses that lie within our detection limit. We comparethe observed ionization and kinematics of the supershells around theULXs Ho IX X-1 and NGC 1313 X-2 with the energy feedback expected fromthe underlying stellar population to assess whether additional energycontributions from the ULXs are needed. In both cases, we findinsufficient UV fluxes or mechanical energies from the stellarpopulation; thus these ULXs may be partially responsible for theionization and energetics of their supershells. All seven sources thatwe studied are in young stellar environments, and six of them haveoptical counterparts with masses >~7 Msolar thus, thesesources are most likely high-mass X-ray binaries.
| 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.
| Objective Classification of Spiral Galaxies Having Extended Rotation Curves Beyond the Optical Radius
We carry out an objective classification of four samples of spiralgalaxies having extended rotation curves beyond the optical radius. Amultivariate statistical analysis (viz., principal component analysis[PCA]) shows that about 96% of the total variation is due to twocomponents, one being the combination of absolute blue magnitude andmaximum rotational velocity beyond the optical region and the otherbeing the central density of the halo. On the basis of PCA a fundamentalplane has been constructed that reduces the scatter in the Tully-Fisherrelation up to a maximum of 16%. A multiple stepwise regression analysisof the variation of the overall shape of the rotation curves shows thatit is mainly determined by the central surface brightness, while theshape purely in the outer part of the galaxy (beyond the optical radius)is mainly determined by the size of the galactic disk.
| Toward a clean sample of ultra-luminous X-ray sources
Context: .Observational follow-up programmes for the characterization ofultra-luminous X-ray sources (ULXs) require the construction of cleansamples of such sources in which the contamination byforeground/background sources is minimum. Aims: .We calculate thedegree of foreground/background contaminants among the ULX samplecandidates in a published catalogue and compare these computations withavailable spectroscopic identifications. Methods: .We usestatistics based on known densities of X-ray sources and AGN/QSOsselected in the optical. The analysis is done individually for eachparent galaxy. The existing identifications of the optical counterpartsare compiled from the literature. Results: .More than a half ofthe ULXs, within twice the distance of the major axis of the 25mag/arcsec2 isophote from RC3 nearby galaxies and with X-rayluminosities L_X[ 2-10 keV] ≥ 1039 erg/s, are expected tobe high redshift background QSOs. A list of 25 objects (clean sample)confirmed to be real ULXs or to have a low probability of beingcontaminant foreground/background objects is provided.
| GHASP: an Hα kinematic survey of spiral and irregular galaxies - IV. 44 new velocity fields. Extension, shape and asymmetry of Hα rotation curves
We present Fabry-Perot observations obtained in the frame of the GHASPsurvey (Gassendi HAlpha survey of SPirals). We have derived the Hαmap, the velocity field and the rotation curve for a new set of 44galaxies. The data presented in this paper are combined with the datapublished in the three previous papers providing a total number of 85 ofthe 96 galaxies observed up to now. This sample of kinematical data hasbeen divided into two groups: isolated (ISO) and softly interacting(SOFT) galaxies. In this paper, the extension of the Hα discs, theshape of the rotation curves, the kinematical asymmetry and theTully-Fisher relation have been investigated for both ISO and SOFTgalaxies. The Hα extension is roughly proportional toR25 for ISO as well as for SOFT galaxies. The smallestextensions of the ionized disc are found for ISO galaxies. The innerslope of the rotation curves is found to be correlated with the centralconcentration of light more clearly than with the type or thekinematical asymmetry, for ISO as well as for SOFT galaxies. The outerslope of the rotation curves increases with the type and with thekinematical asymmetry for ISO galaxies but shows no special trend forSOFT galaxies. No decreasing rotation curve is found for SOFT galaxies.The asymmetry of the rotation curves is correlated with themorphological type, the luminosity, the (B-V) colour and the maximalrotational velocity of galaxies. Our results show that the brightest,the most massive and the reddest galaxies, which are fast rotators, arethe least asymmetric, meaning that they are the most efficient withwhich to average the mass distribution on the whole disc. Asymmetry inthe rotation curves seems to be linked with local star formation,betraying disturbances of the gravitational potential. The Tully-Fisherrelation has a smaller slope for ISO than for SOFT galaxies.
| XMM-Newton EPIC observations of the ultraluminous X-ray source NGC 5204 X-1
We present the results of two XMM-Newton observations of theultraluminous X-ray source NGC 5204 X-1. The EPIC spectra are wellfitted by the standard spectral model of a black hole X-ray binary,comprising a soft multicolour disc blackbody component plus a harderpower-law continuum. The cool (kTin~ 0.2 keV) inner-disctemperature required by this model favours the presence of anintermediate-mass black hole in this system, though we highlight apossible anomaly in the slope of the power-law continuum in such fits.We discuss the interpretation of this and other, non-standard spectralmodelling of the data.
| The star-forming environment of an ultraluminous X-ray source in NGC4559: an optical study
We have studied the candidate optical counterparts and the stellarpopulation in the star-forming complex around the bright ultraluminousX-ray source (ULX) in the western part of the spiral galaxy NGC4559,using the HST Wide Field Planetary Camera 2 (WFPC2), XMM-Newton/OpticalMonitor and ground-based data. We find that the ULX is located near asmall group of OB stars, but is not associated with any massive youngclusters nor with any extraordinary massive stars. The brightest pointsource in the Chandra error circle is consistent with a single bluesupergiant (BSG) of mass ~20Msolar and age ~10 Myr. A fewother stars are resolved inside the error circle: mostly BSGs and redsupergiants (RSGs) with inferred masses ~10-15Msolar and ages~20 Myr. This is consistent with the interpretation of this ULX as ablack hole (BH) accreting from a high-mass donor star in its supergiantphase, with mass transfer occurring via Roche-lobe overflow. Theobserved optical colours and the blue-to-red supergiant ratio suggest alow metal abundance for the stellar population: 0.2<~Z/Zsolar<~ 0.4 (using the Padua tracks), or 0.05<~Z/Zsolar<~ 0.2 (using the Geneva tracks). The age ofthe star-forming complex is <~30 Myr. Hα images show that thisstar-forming region has a ring-like appearance. We propose that it is anexpanding wave of star formation, triggered by an initial densityperturbation, in a region where the gas was only marginally stable togravitational collapse. We also suggest that the most likely trigger wasa collision with a satellite dwarf galaxy going through the gas-richouter disc of NGC4559 less than 30 Myr ago. The culprit could be thedwarf galaxy visible a few arcsec north-west of the complex. If this isthe case, this system is a scaled-down version of the Cartwheel galaxy.The X-ray data favour a BH more massive (M > 50Msolar)than typical Milky Way BH candidates. The optical data favour a young BHoriginating in the recent episode of massive star formation; however,they also rule out an association with young massive star clusters (noneare present in the X7 field). We speculate that other mechanisms maylead to the formation of relatively massive BHs (perhaps M~50-100Msolar) from stellar evolution processes inlow-metallicity environments, or when star formation is triggered bygalactic collisions.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data 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.
| X-Ray and Optical Eclipses in Ultraluminous X-Ray Sources as Possible Indicators of Black Hole Mass
Ultraluminous X-ray sources (ULXs) with 1039 ergss-1 <~ LX < 1041 ergss-1 have been discovered in great numbers in externalgalaxies with ROSAT, Chandra, and XMM-Newton. The central questionregarding this important class of sources is whether they represent anextension to the luminosity function of binary X-ray sources containingneutron stars and stellar-mass black holes (BHs), or a new class ofobjects, e.g., systems containing intermediate-mass black holes(100-1000 Msolar). We suggest searching for X-ray and opticaleclipses in these systems to provide another diagnostic to helpdistinguish between these two possibilities. The sense of the effect isthat ULXs with stellar-mass black hole accretors should be at leasttwice as likely to exhibit eclipses as intermediate-mass black holesystems-and perhaps much more than a factor of 2. Among other systemparameters, the orbital period would follow. This would provideconsiderable insight as to the nature of the binary.
| XMM-Newton Observations of Ultraluminous X-Ray Sources in Nearby Galaxies
We examined X-ray spectral and timing properties of ultraluminous X-raysources (ULXs) in nearby galaxies in XMM-Newton archival data. Thereappear to be three distinct classes of spectra. One class shows emissionfrom hot, diffuse plasma. This thermal emission is similar to that seenfrom recent supernovae; the temperatures are in the range 0.6-0.8 keV,and the luminosities are the lowest in our sample, near 1039ergs s-1. Three sources have spectra that are strongly curvedat high energies and have the highest temperatures in our sample,1.0-1.4 keV. These spectra are well fitted with a power-law plusmulticolor disk blackbody model with the power law dominant at lowenergies or a Comptonization model. The remainder of the sources arebest fitted with a power-law plus multicolor disk blackbody model, as iscommonly used to describe the spectra of accreting black holes. Thesesources have the lowest thermal component temperatures, 0.1-0.4 keV, andextend to the highest luminosities, above 1040 ergss-1. The temperature of the thermal component is in threedistinct ranges for the three source classes. This diversity of spectralshapes and the fact that the sources lie in three distinct temperatureranges suggests that the ULXs are a diverse population. Two ULXs thatshow state transitions stay within a single class over the course of thetransition. However, we cannot conclude with certainty that the classesrepresent distinct types of objects rather than spectral states of asingle population of objects. More monitoring observations of ULXs withXMM-Newton are required. We also searched for timing noise from thesources and report detection of noise above the Poisson level from fivesources. In three of the sources, the power density spectrum increaseswith decreasing frequency as a power law down to the lowest frequenciesobserved, below 10-4 Hz.
| The Molecular Interstellar Medium of Dwarf Galaxies on Kiloparsec Scales: A New Survey for CO in Northern, IRAS-detected Dwarf Galaxies
We present a new survey for CO in dwarf galaxies using the ARO Kitt Peak12 m telescope. This survey consists of observations of the centralregions of 121 northern dwarfs with IRAS detections and no known COemission. We detect CO in 28 of these galaxies and marginally detectanother 16, increasing by about 50% the number of such galaxies known tohave significant CO emission. The galaxies we detect are comparable instellar and dynamical mass to the Large Magellanic Cloud, althoughsomewhat brighter in CO and fainter in the far-IR. Within dwarfs, wefind that the CO luminosity LCO is most strongly correlatedwith the K-band and the far-infrared luminosities. There are also strongcorrelations with the radio continuum (RC) and B-band luminosities andlinear diameter. Conversely, we find that far-IR dust temperature is apoor predictor of CO emission within the dwarfs alone, although a goodpredictor of normalized CO content among a larger sample of galaxies. Wesuggest that LCO and LK correlate well because thestellar component of a galaxy dominates the midplane gravitational fieldand thus sets the pressure and density of the atomic gas, which controlthe formation of H2 from H I. We compare our sample with moremassive galaxies and find that dwarfs and large galaxies obey the samerelationship between CO and the 1.4 GHz RC surface brightness. Thisrelationship is well described by a Schmidt law withΣRC~Σ1.3CO. Therefore,dwarf galaxies and large spirals exhibit the same relationship betweenmolecular gas and star formation rate (SFR). We find that this result isrobust to moderate changes in the RC-to-SFR and CO-to-H2conversion factors. Our data appear to be inconsistent with large (orderof magnitude) variations in the CO-to-H2 conversion factor inthe star-forming molecular gas.
| The Optical Counterpart of M101 ULX-1
We have identified the optical counterpart of the ultraluminous X-raysource M101 ULX-1 (CXOKM101 J140332.37+542102), by comparing HubbleSpace Telescope Advanced Camera for Surveys images with Chandra ACIS-Simages. The optical counterpart has V=23.75 and colors consistent withthose for a mid-B supergiant. Archival Wide Field Planetary Camera 2observations show that the source brightness is constant to within ~0.1mag. The physical association of this source with the ULX is confirmedby Gemini Multi-Object Spectrograph observations that show spatiallyunresolved He II λ4686 and He I λ5876 emission. Theseresults suggest that M101 ULX-1 is a high-mass X-ray binary, but deepspectroscopic monitoring observations are needed to determine thedetailed properties of this system.
| Revealing the Supernova Remnant Population of M33 with Chandra
We present results of a search for supernova remnants (SNRs) in archivalChandra images of M33. We have identified X-ray SNRs by comparing thelist of Chandra X-ray sources in M33 with tabulations of SNR candidatesidentified from (1) elevated [S II]/Hα ratios in the optical and(2) radio spectral indices. In addition, we have searched for opticalcounterparts to soft sources in the Chandra images and X-ray SNRcandidates identified in the XMM-Newton survey of M33. Of the 98optically known SNRs in M33, 22 have been detected at >3 σlevel in the soft band (0.35-1.1 keV). At least four of these SNRcandidates are spatially extended based on a comparison of the data tosimulated images of point sources. Aside from the optically matchingSNRs, we have found one soft X-ray source in M33 that exhibits nooptical emission and is coincident with a known radio source. The radiospectral index of this source is consistent with particle accelerationin shocks, leading us to suggest that it is a nonradiative SNR. We havealso found new optical counterparts to two soft X-ray SNRs in M33. Thesecounterparts exhibit enhanced [S II]/Hα ratios characteristic ofradiative shocks. Pending confirmation from optical spectroscopy, theidentification of these two optical counterparts increases the totalnumber of known optically emitting SNRs in M33 to 100. This brings thetotal number of identified SNRs with X-ray counterparts, including thoseexclusively detected by the XMM-Newton survey of M33, to 37 SNRs. Wefind that while there are a similar number of confirmed X-ray SNRs inM33 and the LMC with X-ray luminosities in excess of 1035ergs s-1, nearly 40% of the LMC SNRs are brighter than1036 ergs s-1, while only 13% of the M33 sampleexceed this luminosity. Including X-ray SNR candidates from theXMM-Newton survey (objects lacking optical counterparts) increases thefraction of M33 SNRs brighter than 1036 ergs s-1to 22%, still only half the LMC fraction. The differences in luminositydistributions cannot be fully explained by uncertainty in spectral modelparameters and are not fully accounted for by abundance differencesbetween the galaxies.
| Chandra X-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/7715: Tidal Ultraluminous X-Ray Sources, Emergent Wind, and Resolved H II Regions
We present high spatial resolution X-ray imaging data for theinteracting galaxy pair NGC 7714/7715 (Arp 284) from the Chandra X-raytelescope. In addition to the unresolved starburst nucleus, a variablepoint source with LX~1040 ergs s-1 wasdetected 1.5" (270 pc) to the northwest of the nucleus, coincident witha blue, extremely optically luminous (MV~-14.1) point sourceon Hubble Space Telescope images. Eleven other candidate pointlikeultraluminous X-ray sources (ULXs) were also detected in the vicinity ofNGC 7714/7715, two of which exceed 1040 ergs s-1.Ten of these appear to be associated with interaction-induced features,but only two are associated with star formation regions. We also founddiffuse emission with LX~3×1040 ergss-1 extending 11" (1.9 kpc) to the north of the nucleus. Itsspectrum can be fitted with either a two-temperature MEKAL function(kT=0.59+0.05-0.06 and8+10-3 keV) or a 0.6 keV MEKAL function plus apower law (Γ=1.8+/-0.2). The hard component may be due tohigh-mass X-ray binaries (HMXBs) with possible contributions frominverse Compton radiation, while the soft component is likely from asuperwind. Superbubble models imply an expansion age of ~15 Myr,supporting previous assertions of an intermediate-age nuclear stellarpopulation in addition to a 5 Myr starburst. We also detected extendedX-ray emission associated with four extranuclear H II region complexes.The emission from these H II regions and the nuclear starburst could bedue to either an enhanced population of HMXBs relative to Local Groupgalactic averages or to diffuse gas heated by winds from supernovae, ifthe X-ray production efficiency LX/Lmech is high(~5%). To estimate LX/Lmech, we collectedpublished data for well-studied H II regions and superbubbles in nearbygalaxies. For H II regions with ages less than 3.5 Myr, the medianLX/Lmech~0.02%, while for older star formationregions, LX/Lmech~0.2%-7%. Thus, it is possiblethat gas heating by supernovae may be sufficient to account for theobserved X-rays from these H II regions. In galaxies much more distantthan NGC 7714, for example, the Cartwheel galaxy, H II region complexessimilar to those in NGC 7714 will be unresolved by Chandra and willmimic ULXs. No X-ray emission was detected from the Type Ib supernova SN1999dn, with an upper limit of ~2×1038 ergss-1.
| A radio monitoring survey of ultra-luminous X-ray sources
We present the results of a radio monitoring campaign to search forradio emission from nearby ultra-luminous X-ray sources (ULXs). Thesesources are bright off-nuclear X-ray point sources with luminositiesexceeding LX > 1039 erg s-1. Awell-defined sample of the 9 nearest ULXs has been monitored eight timesover 5 months with the Very Large Array in A and B configuration. Ourlimiting sensitivity is ≈0.15 mJy (4σ) for radio flares and≈60 μJy for continuous emission. In M 82 two ULXs seem to havecoincident compact radio sources, which are probably supernova remnants.No continuous or flaring radio emission has been detected from any otherULX. Thus, ULXs do not generally emit steady-state radio emission aboveradio powers of 1.5 × 1017 W/Hz. The non-detections ofthe continuous emission are consistent with beamed or unbeamed radioemission from accreting black holes of ≤ 103 Mȯ based on the radio/X-ray correlation. Other publishedradio detections (M 82, NGC 5408) are also discussed in this context.Both detections are significantly above our detection limit. If ULXshave flaring radio emission above 4 × 1017 W/Hz we cangive an upper limit on the duty cycle of the flares of 6%. This upperlimit is in agreement with the observed number of flares in Galacticradio transients. Additionally we present a yet unreported radio doublestructure in the nearby low-luminosity AGN NGC 4736.
| 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 (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/1125
| Chandra Observation of Luminous Sources in the Nearby Irregular Galaxy NGC 4449
Using archival Chandra ACIS-S data, 0.5 8.0 keV X-ray spectra of twoluminous X-ray sources in the nearby dwarf irregular galaxy NGC 4449were studied. One, with an extremely high luminosity of 1.3 ×1039 erg s-1 in the 0.5 8.0 keV band, shows aspectrum that is well described with a power-law model of photon index 2. Its properties are consistent with those of ultraluminouscompact X-ray sources observed in nearby galaxies. The spectrum of theother, with a luminosity of 2.7 × 1038 ergs-1 in the same band, is successfully represented with aso-called multi-color disk blackbody emission model with an inner-mostdisk temperature of 0.59 keV. Its spectral parameters are typicalof ordinary black hole binaries observed in our Galaxy and the LargeMagellanic Cloud. These young population objects, together with a brightsupernova remnant and diffuse hot gas already reported, suggest that theX-ray emission from irregular galaxies is generally enhanced by theirrecent star-forming activities.
| The inner structure of ΛCDM haloes - II. Halo mass profiles and low surface brightness galaxy rotation curves
We use a set of high-resolution cosmological N-body simulations toinvestigate the inner mass profile of galaxy-sized cold dark matter(CDM) haloes. These simulations extend the numerical convergence studypresented in Paper I of this series, and demonstrate that the massprofile of CDM galaxy haloes can be robustly estimated beyond a minimumconverged radius of order rconv~ 1h-1 kpc in ourhighest-resolution runs. The density profiles of simulated haloes becomeprogressively shallower from the virial radius inwards, and show no signof approaching a well-defined power law near the centre. Atrconv, the density profile is steeper than expected from theformula proposed by Navarro, Frenk & White, which has aρ~r-1 cusp, but significantly shallower than the steeplydivergent ρ~r-1.5 cusp proposed by Moore et al. Weperform a direct comparison of the spherically averaged dark mattercircular velocity profiles with Hα rotation curves of a sample oflow surface brightness (LSB) galaxies. We find that most galaxies in thesample (about 70 per cent) have rotation curves that are consistent withthe structure of CDM haloes. Of the remainder, 20 per cent have rotationcurves which cannot be fit by any smooth fitting function with few freeparameters, and 10 per cent are inconsistent with CDM haloes. However,the latter consist mostly of rotation curves that do not extend to largeenough radii to accurately determine their shapes and maximumvelocities. We conclude that the inner structure of CDM haloes is notmanifestly inconsistent with the rotation curves of LSB galaxies.
| Cores of dark matter haloes correlate with stellar scalelengths
We investigate in detail the mass distribution obtained by means ofhigh-resolution rotation curves of 25 galaxies of differentmorphological types. The dark matter contribution to the circularrotation velocity is well-described by resorting to a dark component,the density of which shows an inner core, i.e. a central constantdensity region. We find a very strong correlation between the coreradius size RC and the stellar exponential scalelengthRD: RC~=13[RD/(5kpc)]1.05kpc, and between RCand the galaxy dynamical mass at this distance,Mdyn(RC). These relationships would not beexpected if the core radii were the product of an incorrectdecomposition procedure, or the biased result of wrong or misunderstoodobservational data. The very strong correlation between the dark andluminous scalelengths found here seems to hold also for different Hubbletypes and opens new scenarios for the nature of the dark matter ingalaxies.
| High-resolution imaging of the HeII λ4686 emission line nebula associated with the ultraluminous X-ray source in Holmberg II
We present Hubble Space Telescope images of the HeIII region surroundingthe bright X-ray source in the dwarf irregular galaxy Holmberg II. UsingChandra, we find a position for the X-ray source of (J2000) with anuncertainty of 0.6 arcsec. We identify a bright, point-like opticalcounterpart centred in the nebula with the X-ray source. The opticalmagnitude and colour of the counterpart are consistent with a star withspectral type between O4V and B3 Ib at a distance of 3.05 Mpc orreprocessed emission from an X-ray illuminated accretion disc. Thenebular HeII luminosity is 2.7 × 1036 ergs-1. The morphology of the HeII, Hβ and [OI] emission isconsistent with being due to X-ray photoionization and is inconsistentwith narrow beaming of the X-ray emission. A spectral model consistingof a multicolour disc blackbody with inverse-Compton emission from a hotcorona gives a good fit to X-ray spectra obtained with XMM-Newton. Usingthe fitted X-ray spectrum, we calculate the relation between the HeIIand X-ray luminosity and find that the HeII flux implies a lower boundon the X-ray luminosity in the range 4 to 6 × 1039 ergs-1 if the extrapolation of the X-ray spectrum between 54 and300 eV is accurate. A compact object mass of at least 25 to 40Msolar would be required to avoid violating the Eddingtonlimit.
| Chandra observations of five ultraluminous X-ray sources in nearby galaxies
We report the results of a programme of dual-epoch Chandra ACIS-Sobservations of five ultraluminous X-ray sources (ULXs) in nearby spiralgalaxies. All five ULXs are detected as unresolved, point-like X-raysources by Chandra, though two have faded below the 1039 ergs-1 luminosity threshold used to first designate thesesources as ULXs. Using this same criterion, we detect three further ULXswithin the imaged regions of the galaxies. The ULXs appear to be relatedto the star-forming regions of the galaxies, indicating that even innormal spiral galaxies the ULX population is predominantly associatedwith young stellar populations. A detailed study of the Chandra ACIS-Sspectra of six of the ULXs shows that five are better described by apower-law continuum than a multicolour disc blackbody model, thoughthere is evidence for additional very soft components to two of thepower-law continua. The measured photon indices in four out of fivecases are consistent with the low/hard state in black hole binaries,contrary to the suggestion that power-law-dominated spectra of ULXsoriginate in the very high state. A simple interpretation of this isthat we are observing accretion on to intermediate-mass black holes,though we might also be observing a spectral state unique to very highmass accretion rates in stellar-mass black hole systems. Short-term fluxvariability is only detected in one of two epochs for two of the ULXs,with the lack of this characteristic arguing that the X-ray emission ofthis sample of ULXs is not dominated by relativistically beamed jets.The observational characteristics of this small sample suggest that ULXsare a distinctly heterogeneous source class.
| Observational Constraints on the Physical Parameters of Dark Matter Halos
After looking at the difference in the mass distribution between massivespiral and dwarf irregular (dIrr) and low surface brightness (LSB)galaxies, the central Dark Matter (DM) concentration (flat vs cuspy) indwarf and LSB galaxies, derived from observations, will be examined. Wewill then present what kind of observational constraints can be put onthe total mass and total extent of DM halos from the studies ofindividual galaxies, small groups, satellites' dynamics and tidal tailsof interacting systems. Finally, we will discuss how limits on thephysical parameters of DM halos could be set by deriving extendedrotation curves beyond the HI radius (r > rHI), usingeither Lyα absorption or Hα emission observations.
| 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.
| Mid-Infrared Galaxy Morphology along the Hubble Sequence
The mid-infrared emission from 18 nearby galaxies were imaged with theInfrared Array Camera (IRAC) on the Spitzer Space Telescope, samplingthe spatial distributions of the reddening-free stellar photosphericemission and the warm dust in the interstellar medium. These twocomponents provide a new framework for galaxy morphology classificationin which the presence of spiral arms and their emission strengthrelative to the starlight can be measured directly and with highcontrast. Four mid-infrared classification methods are explored, threeof which are based on quantitative global parameters (colors andbulge-to-disk ratio) that are similar to those used in the past foroptical studies; in this limited sample, all correlate well withtraditional B-band classification. We suggest reasons why infraredclassification may be superior to optical classification.
| The Optical Counterpart of an Ultraluminous X-Ray Source in NGC 5204
Ultraluminous X-ray (ULX) sources are extranuclear point sources inexternal galaxies with LX=1039-1041ergs s-1 and are among the most poorly understood X-raysources. To help understand their nature, we are trying to identifytheir optical counterparts by combining images from the Hubble SpaceTelescope (HST) and the Chandra X-Ray Observatory. Here we report on theoptical counterpart for the ULX in NGC 5204, which has average X-rayluminosity of ~3×1039 ergs s-1 and hasvaried by a factor of 50% over the last 10 years. A unique opticalcounterpart to this ULX is found by carefully comparing the Chandra ACISimages and HST WFPC2 and ACS/HRC images. The spectral energydistribution and the HST/STIS far-ultraviolet (FUV) spectrum of thisobject show that it is a B0 Ib supergiant star with peculiarities,including the N V λ1240 emission line, which is uncommon in Bstellar spectra but has been predicted for X-ray-illuminated accretiondisks and seen in some X-ray binaries. Study of its FUV spectrum leadsto a binary model for this ULX in which the B0 Ib supergiant isoverflowing its Roche lobe and accreting onto the compact primary,probably a black hole. This picture predicts an orbital period of ~10days for different black hole masses, which can be tested by futureobservations.
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