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Close Binary Interactions of Intermediate-Mass Black Holes: Possible Ultraluminous X-Ray Sources?
While many observed ultraluminous X-ray sources (ULXs;LX>=1039 ergs s-1) could beextragalactic X-ray binaries (XRBs) emitting close to the Eddingtonlimit, the highest luminosity ULXs(LX>3×1039 ergs s-1) exceedthe isotropic Eddington luminosity for even high-stellar-mass-accretingblack hole XRBs. It has been suggested that these highest luminosityULXs may contain accreting intermediate-mass black hole (IMBH) binaries.We consider this hypothesis for dense, young (~100 Myr) stellar clusterswhere we assume that a 50-500 Msolar central IMBH has formedthrough runaway growth of a massive star. We develop numericalsimulations of such clusters' cores by combining single and binary starevolutionary syntheses with a simple treatment of dynamicalinteractions. We model interactions of the IMBH with single and binarystars, as well as single-binary and binary-binary interactions, but notthe formation of a cusp around the IMBH. The core density and velocitydispersion are assumed to be constant over 100 Myr. We investigate thesuccession of IMBH binary companions and the evolution of their orbitalparameters to obtain estimates of the incidence of mass transfer phasesand possible ULX activity involving the IMBH. We find that although itis common for the central black hole to acquire binary companions, thereis a very low probability that these interacting binaries will becomeobservable ULX sources.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT HRI Observations. II. Statistical 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.
| Quasi-periodic Oscillations and Strongly Comptonized X-Ray Emission from Holmberg IX X-1
We report the discovery of a 200 mHz quasi-periodic oscillation (QPO) inthe X-ray emission from a bright ultraluminous X-ray source (ULX),Holmberg IX X-1, using a long XMM-Newton observation. The QPO has acentroid at νQPO=202.5+4.9-3.8 mHz,a coherence Q≡νQPO/ΔνFWHM~9.3,and an amplitude (rms) of 6% in the 0.2-10 keV band. This is only thesecond detection of a QPO from a ULX, after M82 X-1, and provides strongevidence against beaming. The power spectrum is well fitted by a powerlaw with an index of ~0.7. The total integrated power (rms) is ~9.4% inthe 0.001-1 Hz range. The X-ray spectrum shows clear evidence of a softX-ray excess component that is well described by a multicolor diskblackbody (kTin~0.3 keV) and a high-energy curvature that canbe modeled either by a cutoff power law (Γ~1 Ecutoff=9keV) or as a strongly Comptonized continuum in an optically thick(τ~7.3) and cool (kTe~3 keV) plasma. Both the presence ofthe QPO and the shape of the X-ray spectrum strongly suggest that theULX is not in the high/soft or thermally dominated state. A truncateddisk and inner optically thick corona may explain the observed X-rayspectrum and the presence of the QPO.
| 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.
| The Unusual Spectrum of the Ultraluminous X-Ray Source M82 X-1
The results of a spectral analysis, using XMM-Newton and Chandra data ofthe brightest ultraluminous X-ray source in the nearby galaxy M82, arepresented. The spectrum of M82 X-1 was found to be unusually hard(photon spectral index Γ~1) with a sharp cutoff at ~6 keV. Thedisk blackbody emission model requires a nonphysically high temperature.Instead, the spectrum is better described, with a lower reducedχ2, as emission due to the nearly saturatedComptonization of photons in an optically thick (τ~10-30, dependingon the geometry) plasma having a temperature kT~2 keV. This is incontrast to the high-energy spectra of other black hole systems, whichare relatively steeper (Γ>1.5) and hence are modeled as theunsaturated thermal and/or nonthermal Comptonization of soft photons, inan optically thin (τ~1) high-temperature plasma. An iron lineemission that is marginally resolved (σ~0.2 keV) is required tofit the data. We argue that the standard geometry for theX-ray-producing region, which consists of an optically thin inner diskor a uniform/patchy corona on top of a cold disk, is not applicable tothis source. Alternatively, the geometry of the X-ray-producing regioncould be a large sphere surrounding a cold accretion disk or anoptically thick inner disk region that cools by bremsstrahlungself-Comptonization. For the latter scenario, such an inner disk region,whose effective optical depth to absorption is less than unity, isexpected in the standard accretion disk theory for near-Eddingtonaccretion rates.
| Hubble Space Telescope Observations of Star Clusters in M101
Hubble Space Telescope Advanced Camera for Surveys (ACS) images are usedto identify and study star cluster candidates in the nearby spiralgalaxy M101. About 3000 round, slightly resolved cluster candidates areidentified in 10 ACS pointings covering an area of 106arcmin2. The cluster candidates' color and size distributionsare consistent with those of star clusters in other nearby spirals. Themajority of the M101 candidates are blue and more likely to beassociated with the galaxy's spiral arms, implying that they are young.The galaxy-luminosity-normalized number of young massive clusters inM101 is similar to that found in other spirals, as is the clusterdensity at a fiducial absolute magnitude. We confirm a previous findingthat M101 has a large number of faint red star clusters: if these areold globular clusters, then this galaxy has a very large globularcluster population. More plausible is that the faint red clusters arereddened young clusters; their colors and luminosities are alsoconsistent with this explanation.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. These observations are associated withprograms 8640 and 9490.
| Associations of Dwarf Galaxies
The Hubble Space Telescope Advanced Camera for Surveys has been used todetermine accurate distances for 20 galaxies from measurements of theluminosity of the brightest red giant branch stars. Five associations ofdwarf galaxies that had originally been identified based on strongcorrelations on the plane of the sky and in velocity are shown to beequally well correlated in distance. Two more associations with similarproperties have been discovered. Another association is identified thatis suggested to be unbound through tidal disruption. The associationshave the spatial and kinematic properties expected of bound structureswith (1-10)×1011 Msolar. However, theseentities have little light, with the consequence that the mass-to-lightratios are in the range 100-1000 MsolarL-1solar. Within a well-surveyed volume extendingto a 3 Mpc radius, all but one known galaxy lie within one of the groupsor associations that have been identified.
| 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.
| [CII] emission and star formation in the spiral arms of M 31
Context: .The [Cii] 158 μm line is the most important coolant of theinterstellar medium in galaxies but substantial variations are seen fromobject to object. The main source of the emission at a galactic scale isstill poorly understood and candidates range from photodissociationregions (PDRs) to the cold neutral or diffuse warm ionized medium.Previous studies of the [Cii] emission in galaxies have a resolution ofseveral kpc or more so the observed emission is an average of differentISM components. Aims: .The aim of this work is to study, for thefirst time, the [Cii] emission at the scale of a spiral arm. We want toinvestigate the origin of this line and its use as a tracer of starformation. Methods: . We present [Cii] and [Oi] observations of asegment of a spiral arm of M 31 using the Infrared Space Observatory.The [Cii] emission is compared with tracers of neutral gas (CO, Hi) andstar formation (Hα, Spitzer 24 μm). Results: . Thesimilarity of the [Cii] emission with the Hα and 24 μm imagesis striking when smoothed to the same resolution, whereas thecorrelation with the neutral gas is much weaker. The [Cii] cooling rateper H atom increases dramatically from 2.7 ×10-26 erg s-1 atom-1 in the border ofthe map to 1.4 × 10-25 erg s-1atom-1 in the regions of star formation. The[Cii]/FIR{42-122} ratio is almost constant at 2%, a factor 3 higher thantypically quoted. However, we do not believe that M 31 is unusual.Rather, the whole-galaxy fluxes used for the comparisons include thecentral regions where the [Cii]/FIR ratio is known to be lower and theresolved observations neither isolate a spiral arm nor include data asfar out in the galactic disk as the observations presented here. A fitto published PDR models yields a plausible average solution ofG0 100 and n 3000 for the PDR emission in theregions of star formation in the arm of M 31.
| [CII] 158 μm emission and metallicity in photon dominated regions
We study the effects of a metallicity variation on the thermal balanceand [CII] fine-structure line strengths in interstellar photon dominatedregions (PDRs). We find that a reduction in the dust-to-gas ratio andthe abundance of heavy elements in the gas phase changes the heatbalance of the gas in PDRs. The surface temperature of PDRs decreases asthe metallicity decreases except for high density (n>106cm-3) clouds exposed to weak (χ< 100) FUV fields wherevibrational H2-deexcitation heating dominates over photoelectric heatingof the gas. We incorporate the metallicity dependence in our KOSMA-τPDR model to study the metallicity dependence of [CII]/CO line ratios inlow metallicity galaxies. We find that the main trend in the variationof the observed CII/CO ratio with metallicity is well reproduced by asingle spherical clump, and does not necessarily require an ensemble ofclumps as in the semi-analytical model presented by Bolatto et al.(1999).
| Disturbed isolated galaxies: indicators of a dark galaxy population?
We report the results of our search for disturbed (interacting) objectsamong very isolated galaxies. The inspections of 1050 northern isolatedgalaxies from KIG and 500 nearby, very isolated galaxies situated in theLocal Supercluster yielded five and four strongly disturbed galaxies,respectively. We suggest that the existence of "dark" galaxies explainsthe observed signs of interaction. This assumption leads to a cosmicabundance of dark galaxies (with the typical masses for luminousgalaxies) that is less than ~1/20 the population of visible galaxies.
| The evolution of actively star-forming galaxies in the mid-infrared
In this paper we analyze the evolution of actively star-forming galaxiesin the mid-infrared (MIR). This spectral region, characterized bycontinuum emission by hot dust and by the presence of strong emissionfeatures generally ascribed to polycyclic aromatic hydrocarbon (PAH)molecules, is the most strongly affected by the heating processesassociated with star formation and/or active galactic nuclei (AGNs).Following the detailed observational characterization of galaxies in theMIR by the Infrared Space Observatory (ISO), we have updated themodelling of this spectral region in our spectrophotometric modelGRASIL. In the diffuse component we have updated the treatment of PAHsaccording to the model by Li & Draine. As for the dense phase of theinterstellar medium associated with the star-forming regions, themolecular clouds, we strongly decrease the abundance of PAHs as comparedto that in the cirrus, based on the observational evidence of the lackor weakness of PAH bands close to the newly formed stars, possibly dueto the destruction of the molecules in strong ultraviolet fields. Therobustness of the model is checked by fitting near-infrared to radiobroad-band spectra and the corresponding detailed MIR spectra of a largesample of galaxies, at once. With this model, we have analyzed thelarger sample of actively star-forming galaxies by Dale et al. We showthat the observed trends of galaxies in the ISO-IRAS-radio colour-colourplots can be interpreted in terms of the different evolutionary phasesof star formation activity, and the consequent different dominance inthe spectral energy distribution of the diffuse or dense phase of theISM. We find that the observed colours indicate a surprising homogeneityof the starburst phenomenon, allowing only a limited variation of themost important physical parameters, such as the optical depth of themolecular clouds, the time-scale of the escape of young stars from theirfor mation sites, and the gas consumption time-scale. In this paper wedo not attempt to reproduce the far-infrared coolest region in thecolour-colour plots, as we concentrate on models meant to reproduceactive star-forming galaxies, but we discuss possible requirements of amore complex modelling for the coldest objects.
| The Evolution of Close Binary Systems with Intermediate-Mass Black Holes and Ultra-Luminous X-Ray Sources
The results of numerical studies of the evolution of a close binarysystem containing a black hole with a mass of 3000M ȯ arepresented. Such a black hole could form in the center of a sufficientlyrich and massive globular cluster. The secondary could be amain-sequence star, giant, or degenerate dwarf that fills or nearlyfills its Roche lobe. The numerical simulations of the evolution of sucha system take into account the magnetic wind of the donor together withthe wind induced by X-ray irradiation from the primary, the radiation ofgravitational waves by the system, and the nuclear evolution of thedonor. Mass transfer between the components is possible when the donorfills its Roche lobe, and also via the black hole’s capture ofsome material from the induced stellar wind. The computations show thatthe evolution of systems with solar-mass donors depends only weakly onthe mass of the accretor. We conclude that the observed ultra-luminousX-ray sources (L X ≳ 1038 erg/s) in nearby galaxies could includeaccreting black holes with masses of 102∓104 M ȯ. Threescenarios for the formation of black holes with such masses in the coresof globular clusters are considered: the collapse of superstars with thecorresponding masses, the accretion of gas by a black hole with astellar initial mass (<100M ȯ), and the tidal accumulation ofstellar black holes. We conclude that the tidal accumulation ofstellar-mass black holes is the main scenario for the formation ofintermediate-mass black holes (102∓104 M ȯ) in the cores ofglobular clusters.
| 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 Diffuse Emission and a Variable Ultraluminous X-Ray Point Source in the Elliptical Galaxy NGC 3379
A Chandra observation of the intermediate-luminosity (MB=-20)elliptical galaxy NGC 3379 resolves 75% of the X-ray emission within thecentral 5 kpc into point sources. Spectral analysis of the remainingunresolved emission within the central 770 pc indicates that 90% of theemission probably arises from undetected point sources, while 10% arisesfrom thermal emission from kT=0.6 keV gas. Assuming a uniform densitydistribution in the central region of the galaxy gives a gas mass of5×105 Msolar. Such a small amount of gas canbe supplied by stellar mass loss in only 107 yr. Thus, thegas must be accreting into the central supermassive black hole at a verylow radiative efficiency as in the ADAF or RIAF models, or it is beingexpelled in a galactic wind driven by the same AGN feedback mechanism asthat observed in cluster cooling flows. If the gas is being expelled inan AGN-driven wind, then the ratio of mechanical to radio power of theAGN must be 104, which is comparable to that measured incluster cooling flows that have recently been perturbed by radiooutbursts. Only 8% of the detected point sources are coincident withglobular cluster positions, which is significantly less than that foundamong other elliptical galaxies observed by Chandra. The low specificfrequency of globular clusters and the small fraction of X-ray pointsources associated with globular clusters in NGC 3379 is more similar tothe properties of lenticular galaxies rather than elliptical galaxies.The brightest point source in NGC 3379 is located 360 pc from thecentral AGN with a peak luminosity of 3.5×1039 ergss-1, which places it in the class of ultraluminous X-raypoint sources (ULXs). Analysis of an archival ROSAT HRI observation ofNGC 3379 shows that this source was at a comparable luminosity 5 yrprior to the Chandra observation. The spectrum of the ULX is welldescribed by a power-law model with Γ=1.6+/-0.1 and galacticabsorption, similar to other ULXs observed by Chandra and XMM-Newton andto the low-hard state observed in Galactic black hole binaries. Duringthe Chandra observation, the source intensity smoothly varies by afactor of 2 with the suggestion of an 8-10 hr period. No changes inhardness ratio are detected as the intensity of the source varies. Whileperiodic behavior has recently been detected in several ULXs, all ofthese reside within spiral galaxies. The ULX in NGC 3379 is the onlyknown ULX in an elliptical galaxy with a smoothly varying light curvesuggestive of an eclipsing binary system.
| VLT Observations of the Ultraluminous X-Ray Source NGC 1313 X-2
We present archive ESO VLT photometric and spectroscopic data of theultraluminous X-ray source NGC 1313 X-2. The superb quality of the VLTimages reveals that two distinct objects, with R magnitudes 23.7 and23.6, are visible inside the Chandra error box. The two objects,separated by 0.75", were unresolved in our previous ESO 3.6 m+EFOSCimage. We show that both are stars in NGC 1313, the first a B0-O9main-sequence star of ~20 Msolar, the second a G supergiantof ~10 Msolar. Irrespective of which of the two objects theactual counterpart is, this implies that NGC 1313 X-2 is a high-massX-ray binary with a very massive donor.
| 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 Ultraluminous X-Ray Source X-37 Is a Background Quasar in the Antennae Galaxies
In this Letter we report that a bright, X-ray source in the Antennaegalaxies (NGC 4038/9), previously identified as an ultraluminous X-raysource (ULX), is in fact a background quasar. We identify an isolatedinfrared and optical counterpart within 0.3" +/- 0.5" of the X-raysource X-37. After acquiring an optical spectrum of its counterpart, weuse the narrow [O III] and broad Hα emission lines to identifyX-37 as a quasar at a redshift of z=0.26. Through a U, V, andKs photometric analysis, we demonstrate that most of theobservable light along this line of sight is from the quasar. We discussthe implications of this discovery and the importance of acquiringspectra for optical and IR counterparts to ULXs.
| Star Formation in H I-selected Galaxies. II. H II Region Properties
A sample of 69 galaxies with radial velocities less than 2500 kms-1 was selected from the H I Parkes All Sky Survey (HIPASS)to deduce details about star formation in nearby disk galaxies selectedwith no bias to optical surface brightness selection effects. Broadband(B and R) and narrowband (Hα) images were obtained for all ofthese objects. More than half of the sample galaxies are late-type,dwarf disks (mostly Sc and Sm galaxies). We have measured the propertiesof the H II regions on Hα continuum-subtracted images, using theHIIphot package developed by Thilker et al. All but one of the galaxiescontained at least one detectable H II region. Examination of theproperties of the H II regions in each galaxy revealed that thebrightest regions in higher surface brightness galaxies tend to be moreluminous than those in lower surface brightness galaxies. A higherfraction (referred to as the diffuse fraction) of the Hα emissionfrom lower surface brightness galaxies comes from diffuse ionized gas. HII region luminosity functions (LFs) co-added according to surfacebrightness show that the shapes of the LFs for the lowest surfacebrightness galaxies are different from those for typical spiralgalaxies. This discrepancy could be caused by the lowest surfacebrightness galaxies having somewhat episodic star formation or by themforming a relatively larger fraction of their stars outside of dense,massive molecular clouds. In general, the results imply that theconditions under which star formation occurs in lower surface brightnessgalaxies are different than in more typical, higher surface brightnessspiral galaxies.
| Light-to-Mass Variations with Environment
Large and well-defined variations exist between the distribution of massand the light of stars on extragalactic scales. Mass concentrations inthe range 1012-1013 Msolar manifest themost light per unit mass. Group halos in this range are typically thehosts of spiral and irregular galaxies with ongoing star formation. Onaverage M/LB~90 Msolar/Lsolar in thesegroups . More massive halos have less light per unit mass. Within agiven mass range, halos that are dynamically old as measured by crossingtimes and galaxy morphologies have distinctly less light per unit mass.At the other end of the mass spectrum, below 1012Msolar, there is a cutoff in the manifestation of light.Group halos in the range 1011-1012Msolar can host dwarf galaxies but with such low luminositiesthat M/LB values can range from several hundred to severalthousand. It is suspected that there must be completely dark halos atlower masses. Given the form of the halo mass function, the low relativeluminosities of the high-mass halos have the greatest cosmologicalimplications. Of order half the clustered mass may reside in halos withgreater than 1014 Msolar. By contrast, only 5%-10%of clustered mass would lie in entities with less than 1012Msolar.
| Dust properties of UV bright galaxies at z ~ 2
We investigate the properties of the extinction curve in the rest-frameUV for a sample of 34 UV-luminous galaxies at 2 < z < 2.5,selected from the FORS Deep Field (FDF) spectroscopic survey. A newparametric description of the rest-frame UV spectral energy distributionis adopted; its sensitivity to properties of the stellar populations orof dust attenuation is established with the use of models. The latterare computed by combining composite stellar population models andcalculations of radiative transfer of the stellar and scatteredradiation through the dusty interstellar medium (ISM) for a dust/starsconfiguration describing dust attenuation in local starbursts. In thefavoured configuration the stars are enveloped by a shell with atwo-phase, clumpy, dusty ISM. The distribution of the z 2UV-luminous FDF galaxies in several diagnostic diagrams shows that theirextinction curves range between those typical of the Small and LargeMagellanic Clouds (SMC and LMC, respectively). For the majority ofstrongly reddened objects having a UV continuum slope β > -0.4 asignificant 2175 Å absorption feature (or "UV bump") is inferred,indicating an LMC-like extinction curve. On the other hand, the UVcontinua of the least reddened objects are mostly consistent withSMC-like extinction curves, lacking a significant UV bump, as for thesample of local starbursts investigated by Calzetti and collaborators.Furthermore, the most opaque (β 0) and, thus (for ourmodels), dustiest UV-luminous FDF galaxies tend to be among the mostmetal-rich, most massive, and largest systems at z 2, indicating< Z > 0.5 {-} 1 Zȯ, < Mstars> 6 × 1010 Mȯ, and 4 kpc, respectively. The presence of the UVbump does not seem to depend on the total metallicity, as given by theequivalent width (EW) of the C IV doublet. Conversely, it seems to beassociated with a large average EW of the six most prominentinterstellar low-ionisation absorption lines falling in the FORSspectra. The average EW of these saturated lines offers a proxy for theISM topology. We interpret these results as the evidence for adifference in the properties of the dusty ISM among the most evolvedUV-luminous, massive galaxies at z 2.
| Photon dominated regions in the spiral arms of M 83 and M 51
We present [C I] 3P1-3P0 spectra at four spiralarm positions and the nuclei of the nearby galaxies M 83 and M 51obtained at the JCMT. The spiral arm positions lie at galacto-centricdistances of between 2 kpc and 6 kpc. This data is complemented withmaps of CO 1-0, 2-1, and 3-2, and ISO/LWS far-infrared data of [C II](158 μm), [O I] (63 μm), and [N II] (122 μm) allowing for theinvestigation of a complete set of all major gas cooling lines. From theintensity of the [N II] line, we estimate that between 15% and 30% ofthe observed [C II] emission originates from the dense ionized phase ofthe ISM. The analysis indicates that emission from the diffuse ionizedmedium is negligible. In combination with the FIR dust continuum, wefind gas heating efficiencies below 0.21% in the nuclei, and between0.25 and 0.36% at the outer positions. Comparison with models ofphoton-dominated regions (PDRs) with the standard ratios [O I](63)/[CII]PDR and ([O I](63)+[C II]PDR) vs. TIR, the total infrared intensity,yields two solutions. The physically most plausible solution exhibitsslightly lower densities and higher FUV fields than found when using afull set of line ratios, [C II]{PDR}/[C I](1-0), [C I](1-0)/CO(3-2),CO(3-2)/CO(1-0), [C II]/CO(3-2), and, [O I](63)/[C II]PDR. The best fitsto the latter ratios yield densities of 104 cm-3and FUV fields of G0=20-30 times the average interstellarfield without much variation. At the outer positions, the observed totalinfrared intensities are in agreement with the derived best fitting FUVintensities. The ratio of the two intensities lies at 4-5 at the nuclei,indicating the presence of other mechanisms heating the dust. The [C I]area filling factors lie below 2% at all positions, consistent with lowvolume filling factors of the emitting gas. The fit of the model to theline ratios improves significantly if we assume that [C I] stems from alarger region than CO 2-1. Improved modelling would need to address thefilling factors of the various submm and FIR tracers, taking intoconsideration the presence of density gradients of the emitting gas byincluding cloud mass and size distributions within the beam.
| Spectro-morphology of galaxies: A multi-wavelength (UV-R) classification method
We present a quantitative method to classify galaxies, based onmulti-wavelength data and constructed from the properties of nearbygalaxies. Our objective is to define an classification method that canbe used for low and high redshift objects. We estimate the concentrationof light (C) at the galaxy center and the 180° rotational asymmetry(A), computed at several wavelengths, from ultraviolet (UV) to I-band.The variation of the indices of concentration and asymmetry with thewavelength reflects the proportion and the distribution of young and oldstellar populations in galaxies. In general C is found to decrease, andA to increase from optical to UV: the patchy appearance of a galaxy inthe UV with no bulge is often very different from its counterpart atoptical wavelengths, with a prominent bulge and a more regular disk. Wequantify the variation of C and A with wavelength. In this way we areable to distinguish five types of galaxies that we callspectro-morphological types: compact, ringed, spiral, irregular andcentral-starburst galaxies, which can be differentiated by thedistribution of their stellar populations. We discuss in detail themorphology of the galaxies of the sample, and describe the morphologicalcharacteristics of each spectro-morphological type. We applyspectro-morphology to three objects at a redshift z1 in the HubbleDeep Field North, which gives encouraging results for applications tolarge samples of high-redshift galaxies. This method of morphologicalclassification could be used to study the evolution of the morphologywith redshift and is expected to put observational constraints onscenarios of galaxy evolution.
| Imaging and photometry of nearby dwarf galaxies. II. Southern dwarfs
We carried out CCD photometry in the Johnson-Cousins B and R bands of 23dwarf galaxies: SDIG, ESO 410-17, KK11, ESO 245-05, KKs3, KK27, KK38,KK40, IC 4662, KK244, KK246, KK247, KK248, KK249, KK253, KK255, KK256,KK257, KK258, KK259, UGCA 438, ESO 347-17, and UGCA 442. Both isolatedgalaxies and members of the Sculptor group and the NGC 1313 group wereobserved. The galaxy sample is characterized by a median distance of 9.3Mpc, and median absolute magnitude of -14.8 mag. The central surfacebrightnesses are in the range from 22.2 to 24.4 mag arcsec-2in B.Based on observations obtained with CTIO 1.5-m telescope, which isoperated by the Association of Universities for Research in AstronomyInc. (AURA), under a cooperative agreement with the National ScienceFoundation as part of the National Optical Astronomy Observatories.Tables 1 and 2, complete Figs. 1 and 2 are only available in electronicform at http://www.edpsciences.org
| 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
| Properties and environment of the molecular complex near Holmberg IX
This paper is aimed at providing new insight into the nature and originof the molecular complex situated near the line of sight toward HolmbergIX in the M 81 group of galaxies. The first high resolution CO maps ofthe complex as well as single dish 13CO(1-0),12CO(3-2) and millimeter continuum observations and theresults of a survey of 12CO in the region are presented.These data together with the available HI, optical and X-rayobservations are analyzed to study the properties and environment of thecomplex. We confirm there is no unobscured massive star formation insidethe complex, and from the millimeter constraint on the extinction itmust have a low star formation rate or be forming only low mass stars.According to the CO line ratios the abundances and physical conditionscould be similar to that of cold gas in spirals. We find from itsdynamics (no rotation) and its mass (2-6 million solar masses) that itresembles a massive GMC. Also, re-inspecting N-body simulations of the M81 group and the H I data we find that it might be located inside theextreme outer disk of M 81 and be cospatial with the H I feature knownas Concentration I. The negative result of the CO survey suggests thatthe complex is unique in this region and calls for a peculiar localformation process. We find that the distribution of the CO emission inthe data cube is asymmetrical in a way similar to a cometary object. Theoptical observations of the nearby supershell MH9/10 suggest theexistence of an outflow toward the complex. We consider the possibilitythat the molecular complex is related to this hypothetical outflow.Based on observations carried out with the IRAM Plateau de BureInterferometer and 30 m telescope, the 10 m Heinrich-Hertz-Telescope(HHT), and the NRAO 12 m telecsope. IRAM (Institut de Radio-AstronomieMillimétrique) is supported by INSU/CNRS (France), MPG (Germany)and IGN (Spain). The HHT was operated by the Submillimeter TelescopeObservatory on behalf of Steward Observatory and the Max-Planck-Institutfür Radioastronomie. The NRAO (National Radio AstronomyObservatory) is a facility of the National Science Foundation, operatedunder cooperative agreement by Associated Universities, Inc.Guest User, Canadian Astronomy Data Centre, that is operated by theDominion Astrophysical Observatory for the National Research Council ofCanada's Herzberg Institute of Astrophysics.
| Ultraluminous X-ray Sources: an Observational Review
Ultraluminous X-ray Sources (ULXs) are, as suggested by their name,extremely luminous and rare X-ray emitting objects found in galaxies.Because of their luminosity, it has been suggested that they may bepowered by accretion onto a black hole (BH) of a few 100Mȯ, more massive than what one would expect to originatefrom normal stellar evolution. Alternative models include youngsupernova remnants (SNRs), beamed emission from normal BH X-ray binaries(XRB) with high accretion rates, and relativistically beamed XRBemission. The observational evidence on ULXs suggests that while most ofthem are likely to be compact accreting objects, there is no clearunique evidence pointing either to the beamed XRB model or to accretiononto a very massive BH. It is possible that what we call ULXs are aheterogeneous family of X-ray sources.
| Dwarf and Normal Spiral Galaxies: are they Self-Similar?
The investigation presented here was focused on clarifying the existenceof dwarf spiral galaxies as a separate group from classical spirals.First, a list of spiral galaxies with small sizes was obtained.Information on colors, luminosities, morphologies and chemical contentwas searched for in the literature for these galaxies. Using thisinformation, it can be concluded that dwarf spirals are not likely to bethe tail of the distribution of classical galaxies. On the contrary,significant differences in some of the most important properties ofspiral galaxies, such as the metallicity gradient and the bar frecuency,were found. In any case, further and more accurate observations areneeded for a definitive answer.
| Astrophysics in 2003
Five coherent sections appear this year, addressing solar physics,cosmology (with WMAP highlights), gamma-ray bursters (and theirassociation with Type Ia supernovae), extra-solar-system planets, andthe formation and evolution of galaxies (from reionization to assemblageof Local Group galaxies). There are also eight incoherent sections thatdeal with other topics in stellar, galactic, and planetary astronomy andthe people who study them.
| The neutral hydrogen environments of the nearby galaxies WLM, NGC 1313 and Sextans A
We have mapped the neutral gas content of the surroundings of threegalaxies on the outskirts of the Local Group - Wolf-Lundmark-Melotte(WLM), NGC 1313 and Sextans A - at high velocity resolution andbrightness sensitivity. We present high-quality HI spectra andparameters for the target galaxies, and find no unknown companion HIclouds nearer than ~100 kpc to the targets, despite reaching asensitivity of ~106 Msolar for narrow-linewidthobjects.
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