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Optical Counterparts of Ultraluminous X-Ray Sources Identified from Archival HST WFPC2 Images
We present a systematic analysis of archival HST WFPC2 ``Association''data sets that correlate with the Chandra positions of a set of 44ultraluminous X-ray sources (ULXs) of nearby galaxies. The mainmotivation is to address the nature of ULXs by searching for opticalcounterparts. Sixteen of the ULXs are found in early-type galaxies (RC3Hubble type <3). We have improved the Chandra/HST relative astrometrywhenever possible, resulting in errors circles of 0.3"-1.7" in size.Disparate numbers of potential ULX counterparts are found, and in somecases none are found. The lack of or low number of counterparts in somecases may be due to insufficient depth in the WFPC2 images. Particularlyin late-type galaxies, the HST image in the ULX region was often complexor crowded, requiring source detection to be performed manually. Wetherefore address various scenarios for the nature of the ULX since itis not known which, if any, of the sources found are true counterparts.The optical luminosities of the sources are typically in the range104-106 Lsolar, with (effective) Vmagnitudes typically in the range 22-24. In several cases colorinformation is available, with the colors roughly tending to be more redin early-type galaxies. This suggests that, in general, the (potential)counterparts found in early-type galaxies are likely to be older stellarpopulations and are probably globular clusters. Several early-typegalaxy counterparts have blue colors, which may be due to youngerstellar populations in the host galaxies, however, these could also bebackground sources. In spiral galaxies the sources may also be due tolocalized structure in the disks rather than bound stellar systems.Alternatively, some of the counterparts in late-type galaxies may beisolated supergiant stars. The observed X-ray/optical flux ratio isdiluted by the optical emission of the cluster in cases where the systemis an X-ray binary in a cluster, particularly in the case of a low-massX-ray binaries in an old cluster. If any of the counterparts are boundsystems with ~104-106 stars and are the truecounterparts to the ULX sources, then the X-ray luminosities of the ULXare generally well below the Eddington limit for a black hole with mass~0.1% of the cluster mass. Finally, we find that the optical flux of thecounterparts is consistent with being dominated by emission from anaccretion disk around an intermediate-mass black hole if the black holehappens to have a mass >~102 Msolar and isaccreting at close to the Eddington rate, unless the accretion disk isirradiated (which would result in high optical disk luminosities atlower black hole masses).Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. This project isassociated with Archival proposal 9545.
| The ESO-Spitzer Imaging extragalactic Survey (ESIS). I. WFIB, V, R deep observations of ELAIS-S1 and comparison to Spitzer and GALEX data
Context.The ESO-Spitzer extragalactic Imaging Survey (ESIS) is theoptical follow up of the Spitzer Wide-Area InfraRed Extragalactic(SWIRE) survey in the ELAIS-S1 area. Aims.The multiwavelengthstudy of galaxy emission is the key to understand the interplay of thevarious components of galaxies and to trace their role in cosmicevolution. ESIS provides optical identification and colors of Spitzer IRgalaxies and builds the bases for photometric redshift estimates. Methods.This paper presents B, V, R Wide Field Imager observations ofthe first 1.5 square degree of the ESIS survey. Data reduction isdescribed including astrometric calibration, illumination and colorcorrections. Synthetic sources are simulated in scientific andsuper-sky-flat images, with the purpose of estimating completeness andphotometric accuracy for the survey. Number counts and colordistributions are compared to literature observational and theoreticaldata, including non-evolutionary, PLE, evolutionary and semi-analyticΛCDM galaxy models, as well as Milky Way stellar predictions. TheELAIS-S1 area benefits from extensive follow-up from X-ray to radiofrequencies: some potential uses of the multi-wavelength observationsare illustrated. Results.Object coordinates are defined with anaccuracy as good as ~0.15 [arcsec] rms with respect to GSC 2.2; fluxuncertainties are ~2, 10, 20% at mag. 20, 23, 24 respectively (Vega); wereach 95% completeness at B, V 25 and R 24.5. ESIS galaxynumber counts are in good agreement with previous works and are bestreproduced by evolutionary and hierarchical ΛCDM scenarios.Optical-Spitzer color-color plots promise to be very powerful tools todisentangle different classes of sources (e.g. AGNs, starbursts,quiescent galaxies). Ultraviolet GALEX data are matched to optical andSpitzer samples, leading to a discussion of galaxy properties in theUV-to-24 μm color space. The spectral energy distribution of a fewobjects, from the X-rays to the far-IR are presented as examples of themulti-wavelength study of galaxy emission components in differentspectral domains.
| 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.
| Chandra observations of the interacting galaxies NGC 3395/3396 (Arp 270)
In this paper we present the results of a 20-ks high-resolution ChandraX-ray observation of the peculiar galaxy pair NGC 3395/3396, a system ata very early stage of merging, and less evolved than the famous Antennaeand Mice merging systems. Previously unpublished ROSAT High-ResolutionImager data are also presented. The point-source population and the hotdiffuse gas in this system are investigated and compared with othermerging galaxy pairs.16 X-ray point sources are detected in Arp 270, seven of which areclassified as ultraluminous X-ray sources (ULXs, LX>=1039 erg s-1). From spectral fits and the age ofthe system it seems likely that these are predominantly high-mass X-raybinaries. The diffuse gas emits at a global temperature of ~0.5 keV,consistent with temperatures observed in other interacting systems, andwe see no evidence of the starburst-driven hot gaseous outflows seen inmore evolved systems such as The Mice and The Antennae. It is likelythat these features are absent from Arp 270 as the gas has hadinsufficient time to break out of the galaxy discs. 32 per cent of theluminosity of Arp 270 arises from the diffuse gas in the system, this islow when compared with later stage merging systems and gives furthercredence that this is an early-stage merger.Comparing the ULX population of Arp 270 to other merging systems, wederive a relationship between the star formation rate of the system,indicated by LFIR, and the number [N(ULX)] and luminosity(LULX) of its ULX population. We find N(ULX)~L0.18FIR andLULX~L0.54FIR. These relationships,coupled with the relation of the point-source X-ray luminosity(LXP) to LK and LFIR+UV (Colbert et al.2003), indicate that the ULX sources in an interacting system havecontributions from both the old and young stellar populations.
| XMM-Newton observations of the interacting galaxy pairs NGC 7771/0 and NGC 2342/1
We present XMM-Newton X-ray observations of the interacting galaxy pairsNGC 7771/7770 and NGC 2342/2341. In NGC 7771, for the first time we areable to resolve the X-ray emission into a bright central source plus twobright (LX > 1040 erg s-1)ultraluminous X-ray sources (ULXs) located either end of the bar. In thebright central source (LX~ 1041 ergs-1), the soft emission is well-modelled by a two-temperaturethermal plasma with kT= 0.4/0.7 keV. The hard emission is modelled witha flat absorbed power-law (Γ~ 1.7, NH~ 1022cm-2), and this together with a low-significance (1.7σ)~ 300 eV equivalent width emission line at ~6 keV are the firstindications that NGC 7771 may host a low-luminosity AGN. For the barULXs, a power-law fit to X-1 is improved at the 2.5σ level withthe addition of a thermal plasma component (kT~ 0.3 keV), while X-2 isimproved only at the 1.3σ level with the addition of a discblackbody component with Tin~ 0.2 keV. Both sources arevariable on short time-scales implying that their emission is dominatedby single accreting X-ray binaries (XRBs). The three remaining galaxies,NGC 7770, NGC 2342 and NGC 2341, have observed X-ray luminosities of0.2, 1.8 and 0.9 × 1041 erg s-1,respectively (0.3-10 keV). Their integrated spectra are alsowell-modelled by multi-temperature thermal plasma components with kT=0.2-0.7 keV, plus power-law continua with slopes of Γ= 1.8-2.3that are likely to represent the integrated emission of populations ofXRBs as observed in other nearby merger systems. A comparison with otherisolated, interacting and merging systems shows that all four galaxiesfollow the established correlations for starburst galaxies betweenX-ray, far-infrared and radio luminosities, demonstrating that theirX-ray outputs are dominated by their starburst components.
| 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.
| XMM-Newton View of the Ultraluminous X-Ray Sources in M51
We present results based on XMM-Newton observations of the nearby spiralgalaxy M51 (NGC 5194 and NGC 5195). We confirm the presence of the sevenknown ultraluminous X-ray sources (ULXs) with luminosities exceeding theEddington luminosity for a 10 Msolar black hole, alow-luminosity active galactic nucleus (LLAGN) with 2-10 keV luminosityof 1.6×1039 ergs s-1, and soft thermalextended emission from NGC 5194 detected with Chandra. In addition, wealso detected a new ULX with luminosity of ~1039 ergss-1. We have studied the spectral and temporal properties ofthe LLAGN and eight ULXs in NGC 5194 and an ULX in NGC 5195. Two ULXs inNGC 5194 show evidence for short-term variability, and all but two ULXsvary on long timescales (over a baseline of ~2.5 yr), providing strongevidence that these are accreting sources. One ULX in NGC 5194, source69, shows possible periodic behavior in its X-ray flux. We derive aperiod of 5925+/-200 s at a confidence level of 95% on the basis ofthree cycles. This period is lower than the period of 7620+/-500 sderived from a Chandra observation in 2000. The higher effective area ofXMM-Newton enables us to identify multiple components in the spectra ofULXs. Most ULXs require at least two components, a power law and a softX-ray excess component that is modeled by an optically thin plasma or amulticolor disk blackbody (MCD). However, the soft excess emissionsinferred from all ULXs except source 69 are unlikely to be physicallyassociated with the ULXs, as their strengths are comparable to that ofthe surrounding diffuse emission. The soft excess emission of source 69is well described either by a two-temperature MEKAL plasma or asingle-temperature MEKAL plasma (kT~690 eV) and an MCD (kT~170 eV). TheMCD component suggests a cooler accretion disk compared to those inGalactic X-ray binaries, consistent with those expected forintermediate-mass black holes (IMBHs). An iron Kα line (EW~700 eV)or K absorption edge at ~7.1 keV is present in the EPIC pn spectrum ofsource 26. The spectrum of the ULX in NGC 5195, source 12, is consistentwith a simple power law. The LLAGN in NGC 5194 shows an extremely flathard X-ray power law (Γ~0.7), a narrow iron Kα line at 6.4keV (EW~3 keV), and strong soft X-ray excess emission. The full-bandspectrum is well described by a two-component MEKAL plasma andreflection from cold material such as a putative torus.
| 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.
| Secular Evolution via Bar-driven Gas Inflow: Results from BIMA SONG
We present an analysis of the molecular gas distributions in the 29barred and 15 unbarred spirals in the BIMA CO (J=1-0) Survey of NearbyGalaxies (SONG). For galaxies that are bright in CO, we confirm theconclusion by Sakamoto et al. that barred spirals have higher moleculargas concentrations in the central kiloparsec. The SONG sample alsoincludes 27 galaxies below the CO brightness limit used by Sakamoto etal. Even in these less CO-bright galaxies we show that high central gasconcentrations are more common in barred galaxies, consistent withradial inflow driven by the bar. However, there is a significantpopulation of early-type (Sa-Sbc) barred spirals (6 of 19) that have nomolecular gas detected in the nuclear region and have very little out tothe bar corotation radius. This suggests that in barred galaxies withgas-deficient nuclear regions, the bar has already driven most of thegas within the bar corotation radius to the nuclear region, where it hasbeen consumed by star formation. The median mass of nuclear moleculargas is over 4 times higher in early-type bars than in late-type (Sc-Sdm)bars. Since previous work has shown that the gas consumption rate is anorder of magnitude higher in early-type bars, this implies that theearly types have significantly higher bar-driven inflows. The loweraccretion rates in late-type bars can probably be attributed to theknown differences in bar structure between early and late types. Despitethe evidence for bar-driven inflows in both early and late Hubble-typespirals, the data indicate that it is highly unlikely for a late-typegalaxy to evolve into an early type via bar-induced gas inflow.Nonetheless, secular evolutionary processes are undoubtedly present, andpseudobulges are inevitable; evidence for pseudobulges is likely to beclearest in early-type galaxies because of their high gas inflow ratesand higher star formation activity.
| Simulating the Spitzer Mid-Infrared Color-Color Diagrams
We use a simple parameterization of the mid-IR spectra of a wide rangeof galaxy types in order to predict their distribution in the InfraredArray Camera (IRAC) 3.6, 4.5, 5.8, and 8.0 μm and MultibandPhotometer for Spitzer 24 μm color-color diagrams. We distinguishthree basic spectral types by the energetically dominant component inthe 3-12 μm regime: stellar-dominated, polycyclic aromatichydrocarbon (PAH)-dominated, and continuum-dominated. We use a Markovchain Monte Carlo approach to arrive at a more systematic and robustrepresentation of the mid-IR spectra of galaxies than do moretraditional approaches. We find that IRAC color-color plots are wellsuited to distinguishing the above spectral types, while the addition of24 μm data allows us to suggest practical three-color cuts thatpreferentially select higher redshift sources of a specific type. Wecompare our simulations with the color-color plot obtained by theSpitzer First Look Survey and find reasonable agreement. Lastly, wediscuss other applications as well as future directions for this work.
| Discovery of multiple ultra-luminous X-ray sources in the galaxy KUG 0214-057
We report the serendipitous discovery of several unresolved X-raysources lying in the prominent spiral arms of the galaxy KUG0214-057 in XMM-Newton observations. The location of theseX-ray sources strongly suggests that at least three, and possibly four,of these may be physically related to the galaxy. The luminosity of eachof these sources at the distance of KUG 0214-057 is >5 ×1039~erg s-1 (0.3-10 keV), making each a strongcandidate ultraluminous X-ray source (ULX). Using the ULXs objects as ametric implies that this relatively low-mass galaxy may be experiencingrather intense starburst activity. The serendipitous discovery of theseULXs objects suggests that such objects are not a negligible componentof the overall extragalactic X-ray source population.
| New H2O masers in Seyfert and FIR bright galaxies
Using the Effelsberg 100-m telescope, detections of four extragalacticwater vapor masers are reported. Isotropic luminosities are ~50, 1000, 1and 230 Lȯ for Mrk 1066 (UGC 2456), Mrk 34, NGC 3556 andArp 299, respectively. Mrk 34 contains by far the most distant and oneof the most luminous water vapor megamasers so far reported in a Seyfertgalaxy. The interacting system Arp 299 appears to show two maserhotspots separated by approximately 20´´. With these newresults and even more recent data from Braatz et al. (2004, ApJ, 617,L29), the detection rate in our sample of Seyferts with known jet-NarrowLine Region interactions becomes 50% (7/14), while in star forminggalaxies with high (S100~μ m>50 Jy) far infrared fluxesthe detection rate is 22% (10/45). The jet-NLR interaction sample maynot only contain “jet-masers” but also a significant numberof accretion “disk-masers” like those seen in NGC 4258. Astatistical analysis of 53 extragalactic H2O sources (excluding theGalaxy and the Magellanic Clouds) indicates (1) that the correlationbetween IRAS Point Source and H2O luminosities, established forindividual star forming regions in the galactic disk, also holds forAGN-dominated megamaser galaxies; (2) that maser luminosities are notcorrelated with 60 μm/100 μm color temperatures; and (3) that onlya small fraction of the luminous megamasers (L_H_2O > 100Lȯ) detectable with 100-m sized telescopes have so farbeen identified. The H2O luminosity function (LF) suggests that thenumber of galaxies with 1 Lȯ < L_H_2O < 10Lȯ, the transition range between“kilomasers” (mostly star formation) and“megamasers” (active galactic nuclei), is small. The overallslope of the LF, ~-1.5, indicates that the number of detectable masersis almost independent of their luminosity. If the LF is not steepeningat very high maser luminosities and if it is possible to find suitablecandidate sources, H2O megamasers at significant redshifts should bedetectable even with present day state-of-the-art facilities.
| 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
| 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.
| Bar-induced perturbation strengths of the galaxies in the Ohio State University Bright Galaxy Survey - I
Bar-induced perturbation strengths are calculated for a well-definedmagnitude-limited sample of 180 spiral galaxies, based on the Ohio StateUniversity Bright Galaxy Survey. We use a gravitational torque method,the ratio of the maximal tangential force to the mean axisymmetricradial force, as a quantitative measure of the bar strength. Thegravitational potential is inferred from an H-band light distribution byassuming that the M/L ratio is constant throughout the disc. Galaxiesare deprojected using orientation parameters based on B-band images. Inorder to eliminate artificial stretching of the bulge, two-dimensionalbar-bulge-disc decomposition has been used to derive a reliable bulgemodel. This bulge model is subtracted from an image, the disc isdeprojected assuming it is thin, and then the bulge is added back byassuming that its mass distribution is spherically symmetric. We findthat removing the artificial bulge stretch is important especially forgalaxies having bars inside large bulges. We also find that the massesof the bulges can be significantly overestimated if bars are not takeninto account in the decomposition.Bars are identified using Fourier methods by requiring that the phasesof the main modes (m= 2, m= 4) are maintained nearly constant in the barregion. With such methods, bars are found in 65 per cent of the galaxiesin our sample, most of them being classified as SB-type systems in thenear-infrared by Eskridge and co-workers. We also suggest that as muchas ~70 per cent of the galaxies classified as SAB-types in thenear-infrared might actually be non-barred systems, many of them havingcentral ovals. It is also possible that a small fraction of the SAB-typegalaxies have weak non-classical bars with spiral-like morphologies.
| The Chandra view of NGC1800 and the X-ray scaling properties of dwarf starbursts
The superb spatial resolution of Chandra is utilized to study the X-raymorphology of the dwarf starburst galaxy NGC1800 embedded in a smallgroup of galaxies. Diffuse galactic emission is detected, extendingseveral kiloparsec above the galactic plane, with an overall morphologysimilar to the galactic winds seen in nearby X-ray-bright starburstgalaxies. This makes NGC1800 the most distant dwarf starburst with aclear detection of diffuse X-ray emission. The diffuse X-ray luminosityof 1.3 +/- 0.3 × 1038ergs-1 accounts for atleast 60 per cent of the total soft X-ray output of the galaxy. A hotgas temperature of kT= 0.25 keV and metallicity Z~ 0.05Zsolarare derived, the latter being consistent with results from opticalspectroscopy of the interstellar medium. Our failure to detect any hotgas associated with the embedding galaxy group translates into an upperlimit to the group X-ray luminosity of LX <1041ergs-1. There is no convincing evidence thatthe outflowing wind of NGC1800 is currently interacting with anyintragroup gas, and mechanical considerations indicate that the wind canescape the galaxy and its surrounding HI halo, eventually deliveringenergy and metals to the intragroup gas. Properties of NGC1800 arecompared to those of other dwarf starburst galaxies, and a firstdetailed discussion of the X-ray scaling properties of this populationof objects is given, set against the equivalent results obtained fornormal starburst galaxies. Results indicate that dwarf starbursts to alarge degree behave as down-scaled versions of normal starburstgalaxies.
| XMM-Newton observations of the starburst merger galaxies NGC 3256 and NGC 3310
We present XMM-Newton EPIC observations of the two nearby starburstmerger galaxies NGC 3256 and NGC 3310. The broad-band (0.3-10 keV)integrated X-ray emission from both galaxies shows evidence ofmultiphase thermal plasmas plus an underlying hard non-thermal power-lawcontinuum. NGC 3256 is well fitted with a model comprising two MEKALcomponents (kT= 0.6/0.9 keV) plus a hard power law (Γ= 2), whileNGC 3310 has cooler MEKAL components (kT= 0.3/0.6 keV) and a harderpower-law tail (Γ= 1.8). Chandra observations of both galaxiesreveal the presence of numerous discrete sources embedded in the diffuseemission, which dominate the emission above ~2 keV and are likely to bethe source of the power-law emission. The thermal components show atrend of increasing absorption with higher temperature, suggesting thatthe hottest plasmas arise from supernova-heated gas within the discs ofthe galaxies, while the cooler components arise from outflowing galacticwinds interacting with the ambient interstellar medium. We find nostrong evidence for an active galactic nucleus in either galaxy.
| Multifrequency radio-continuum observations of NGC 1569: evidence for a convective wind
We present high-sensitivity radio-continuum observations with the VeryLarge Array (VLA) and Ryle Telescope at 1.5, 4.9, 8.4 and 15.4 GHz ofthe dwarf irregular galaxy NGC 1569. The radio data show an extended,irregularly shaped halo with filamentary structure around the galaxy.The spectral index maps reveal an unusually patchy distribution withregions of flat spectral index extending into the halo. The data allowus to perform a spatially resolved spectral-fitting analysis of thecontinuum emission from which we derive maps of the thermal andsynchrotron emission. The thermal radio emission is concentrated towardsthe brightest H II region west of the super star clusters A and B,whereas the distribution of the synchrotron emission peaks in a bar-likestructure in the disc extending between the two clusters. The total fluxdensity of the thermal radio emission allows us to derive the integratedsynchrotron spectrum and we confirm the break in the spectrum that wasfound by Israel & de Bruyn. We discuss various possibilities thatcould produce such a break and conclude that the only mechanism able tofit the radio data and remain consistent with data at other wavelengthsis a convective wind allowing cosmic ray electrons to escape from thehalo.
| An XMM-Newton view of M101 - I. The luminous X-ray source population
We present the first results of an XMM-Newton EPIC observation of theluminous X-ray source population in the face-on supergiant spiral galaxyM101. We have studied the spectral and temporal properties of the 14most luminous sources, all of which have intrinsic X-ray luminositiesexceeding the Eddington limit for a 1.4-Msolar neutron star,with a subset in the ultraluminous X-ray source (ULX) regime(LX>= 1039 erg s-1). Eleven sourcesshow evidence of short-term variability, and most vary by a factor of~2-4 over a baseline of 11-24 yr, providing strong evidence that thesesources are accreting X-ray binary (XRB) systems. Our resultsdemonstrate that these sources are a heterogeneous population, showing avariety of spectral shapes. Interestingly, there is no apparent spectraldistinction between those sources above and below the ULX luminositythreshold. Nine sources are well fitted with either simple absorbed discblackbody or power-law models. However, in three of the four sourcesbest fitted with power-law models, we cannot exclude the disc blackbodyfits and therefore conclude that, coupled with their high luminosities,eight out of nine single-component sources are possibly high-state XRBs.The nuclear source (XMM-10) has the only unambiguous power-law spectrum(Γ~ 2.3), which may be evidence for the presence of alow-luminosity active galactic nucleus (LLAGN). The remaining fivesources require at least two-component spectral fits, with an underlyinghard component that can be modelled by a power-law continuum or, inthree cases, a hot disc blackbody (Tin= 0.9-1.5 keV), plus asoft component modelled as a cool blackbody/disc blackbody/thermalplasma. We have compared the spectral shapes of nine sources covered byboth this observation and an archival 100-ks Chandra observation ofM101; eight show behaviour typical of Galactic XRBs (i.e. softening withincreasing luminosity), the only exception being a transient source(XMM-2) which shows little change in spectral hardness despite a factorof ~30 increase in luminosity. We find no definitive spectral signaturesto indicate that these sources contain neutron star primaries, andconclude that they are likely to be stellar-mass black hole XRBs(BHXBs), with black hole masses of ~2-23 Msolar if accretingat the Eddington limit.
| LX-SFR relation in star-forming galaxies
We compare the results of Grimm, Gilfanov & Sunyaev and Ranalli,Comastri & Seti on the LX-SFR (X-ray luminosity-starformation rate) relation in normal galaxies. Based on theLX-stellar mass dependence for low-mass X-ray binaries(LMXBs), we show that low-SFR (<~1 Msolar yr-1)galaxies in the Ranalli et al. sample are contaminated by the X-rayemission from LMXBs, unrelated to the current star formation activity.However, the most important conclusion from our comparison is that,after the data are corrected for the `LMXB contamination', the two datasets become consistent with each other, despite differences in theircontent, variability effects, adopted source distances, X-ray fluxes andSFR determinations, and also in the cosmological parameters used ininterpreting the Hubble Deep Field North (HDF-N) data. They also agreewell, both in the low- and high-SFR regimes, with the predictedLX-SFR dependence derived from the parameters of the`universal' high-mass X-ray binary (HMXB) luminosity function. Thisencouraging result emphasizes the potential of the X-ray luminosity asan independent SFR indicator for normal 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.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| XMM-Newton Spectroscopy of Four Bright Ultraluminous X-Ray Sources in the Antennae Galaxies (NGC 4038/4039)
We report the results of spectral fits to four bright ultraluminousX-ray sources (ULXs) in the Antennae galaxies (NGC 4038/4039) observedfor 41 ks with XMM-Newton. Although emission regions are not resolved aswell as in prior Chandra observations, at least four ULXs (X-11, X-16,X-37, and X-44 in the Zezas and Fabbiano scheme) are sufficiently brightand well separated with XMM-Newton that reliable extractions andspectral analyses are possible. We find that the single-componentmulticolor disk blackbody models cannot describe any of the spectra.Sources X-11 and X-16 are acceptably fitted with simple power-lawmodels. A thermal bremsstrahlung model provides a better fit to thespectrum of X-44. Including a disk blackbody component to the spectrumof X-37 improves the fit and reveals an apparently cool disk(kT=0.13+/-0.02 keV). This would suggest a parallel to cool disksrecently found in other very luminous ULXs, which may containintermediate-mass black holes; however, the complex diffuse emission ofthe Antennae demands that this finding be regarded cautiously.
| An IRAS High Resolution Image Restoration (HIRES) Atlas of All Interacting Galaxies in the IRAS Revised Bright Galaxy Sample
The importance of far-infrared observations for our understanding ofextreme activity in interacting and merging galaxies has beenillustrated by many studies. Even though two decades have passed sinceits launch, the most complete all-sky survey to date from which far-IRselected galaxy samples can be chosen is still that of the InfraredAstronomical Satellite (IRAS). However, the spatial resolution of theIRAS all-sky survey is insufficient to resolve the emission fromindividual galaxies in most interacting galaxy pairs, and hence previousstudies of their far-IR properties have had to concentrate either onglobal system properties or on the properties of very widely separatedand weakly interacting pairs. Using the HIRES image reconstructiontechnique, it is possible to achieve a spatial resolution ranging from30" to 1.5m (depending on wavelength and detector coverage), whichis a fourfold improvement over the normal resolution of IRAS. This issufficient to resolve the far-IR emission from the individual galaxiesin many interacting systems detected by IRAS, which is very importantfor meaningful comparisons with single, isolated galaxies. We presenthigh-resolution 12, 25, 60, and 100 μm images of 106 interactinggalaxy systems contained in the IRAS Revised Bright Galaxy Sample (RBGS,Sanders et al.), a complete sample of all galaxies having a 60 μmflux density greater than 5.24 Jy. These systems were selected to haveat least two distinguishable galaxies separated by less than threeaverage galactic diameters, and thus we have excluded very widelyseparated systems and very advanced mergers. Additionally, some systemshave been included that are more than three galactic diameters apart,yet have separations less than 4' and are thus likely to suffer fromconfusion in the RBGS. The new complete survey has the same propertiesas the prototype survey of Surace et al. We find no increased tendencyfor infrared-bright galaxies to be associated with other infrared-brightgalaxies among the widely separated pairs studied here. We find smallenhancements in far-IR activity in multiple galaxy systems relative toRBGS noninteracting galaxies with the same blue luminosity distribution.We also find no differences in infrared activity (as measured byinfrared color and luminosity) between late- and early-type spiralgalaxies.
| The ISOPHOT 170 μm Serendipity Survey II. The catalog of optically identified galaxies%
The ISOPHOT Serendipity Sky Survey strip-scanning measurements covering≈15% of the far-infrared (FIR) sky at 170 μm were searched forcompact sources associated with optically identified galaxies. CompactSerendipity Survey sources with a high signal-to-noise ratio in at leasttwo ISOPHOT C200 detector pixels were selected that have a positionalassociation with a galaxy identification in the NED and/or Simbaddatabases and a galaxy counterpart visible on the Digitized Sky Surveyplates. A catalog with 170 μm fluxes for more than 1900 galaxies hasbeen established, 200 of which were measured several times. The faintest170 μm fluxes reach values just below 0.5 Jy, while the brightest,already somewhat extended galaxies have fluxes up to ≈600 Jy. For thevast majority of listed galaxies, the 170 μm fluxes were measured forthe first time. While most of the galaxies are spirals, about 70 of thesources are classified as ellipticals or lenticulars. This is the onlycurrently available large-scale galaxy catalog containing a sufficientnumber of sources with 170 μm fluxes to allow further statisticalstudies of various FIR properties.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Members of the Consortium on the ISOPHOT Serendipity Survey (CISS) areMPIA Heidelberg, ESA ISO SOC Villafranca, AIP Potsdam, IPAC Pasadena,Imperial College London.Full Table 4 and Table 6 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/422/39
| Minor-axis velocity gradients in disk galaxies
We present the ionized-gas kinematics and photometry of a sample of 4spiral galaxies which are characterized by a zero-velocity plateau alongthe major axis and a velocity gradient along the minor axis,respectively. By combining these new kinematical data with thoseavailable in the literature for the ionized-gas component of the S0s andspirals listed in the Revised Shapley-Ames Catalog of Bright Galaxies werealized that about 50% of unbarred galaxies show a remarkable gasvelocity gradient along the optical minor axis. This fraction rises toabout 60% if we include unbarred galaxies with an irregular velocityprofile along the minor axis. This phenomenon is observed all along theHubble sequence of disk galaxies, and it is particularly frequent inearly-type spirals. Since minor-axis velocity gradients are unexpectedif the gas is moving onto circular orbits in a disk coplanar to thestellar one, we conclude that non-circular and off-plane gas motions arenot rare in the inner regions of disk galaxies.Based on observations carried out at the European Southern Observatoryin La Silla (Chile) (ESO 69.B-0706 and 70.B-0338), with the MultipleMirror Telescope which is a joint facility of the SmithsonianInstitution and the University of Arizona, and with the ItalianTelescopio Nazionale Galileo (AOT-5, 3-18) at the Observatorio del Roquede los Muchachos in La Palma (Spain).Table 1 is only available in electronic form athttp://www.edpsciences.org. Table 5 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/507
| High-mass X-ray binaries as a star formation rate indicator in distant galaxies
Based on Chandra and ASCA observations of nearby starburst galaxies andRXTE/ASM, ASCA and MIR-KVANT/TTM studies of high-mass X-ray binary(HMXB) populations in the Milky Way and Magellanic Clouds, we proposethat the number and/or the collective X-ray luminosity of HMXBs can beused to measure the star formation rate (SFR) of a galaxy. We show that,within the accuracy of the presently available data, a linear relationbetween HMXB number and star formation rate exists. The relation betweenSFR and collective luminosity of HMXBs is non-linear in the low-SFRregime, LX~ SFR~ 1.7, and becomes linear only fora sufficiently high star formation rate, SFR >~ 4.5 Msolaryr-1 (for M* > 8 Msolar). Thenon-linear LX-SFR dependence in the low-SFR limit is notrelated to non-linear SFR-dependent effects in the population of HMXBsources. It is rather caused by the fact that we measure the collectiveluminosity of a population of discrete sources, which might be dominatedby the few brightest sources. Although more subtle SFR-dependent effectsare likely to exist, over the entire range of SFRs the data are broadlyconsistent with the existence of a universal luminosity function ofHMXBs that can be roughly described as a power law with a differentialslope of ~1.6, a cut-off at LX~ few × 1040erg s-1 and a normalization proportional to the starformation rate.We apply our results to (spatially unresolved) starburst galaxiesobserved by Chandra at redshifts up to z~ 1.2 in the Hubble Deep FieldNorth and show that the calibration of the collective luminosity ofHMXBs as an SFR indicator based on the local sample agrees well with theSFR estimates obtained for these distant galaxies with conventionalmethods.
| An Overview of the Performance of the Chandra X-ray Observatory
The Chandra X-ray Observatory is the X-ray component of NASA’sGreat Observatory Program which includes the recently launched SpitzerInfrared Telescope, the Hubble Space Telescope (HST) for observations inthe visible, and the Compton Gamma-Ray Observatory (CGRO) which, afterproviding years of useful data has reentered the atmosphere. All thesefacilities provide, or provided, scientific data to the internationalastronomical community in response to peer-reviewed proposals for theiruse. The Chandra X-ray Observatory was the result of the efforts of manyacademic, commercial, and government organizations primarily in theUnited States but also in Europe. NASA’s Marshall Space FlightCenter (MSFC) manages the project and provides project science; NorthropGrumman Space Technology (NGST formerly TRW) served as primecontractor responsible for providing the spacecraft, the telescope, andassembling and testing the observatory; and the SmithsonianAstrophysical Observatory (SAO) provides technical support and isresponsible for ground operations including the Chandra X-ray Center(CXC). Telescope and instrument teams at SAO, the MassachusettsInstitute of Technology (MIT), the Pennsylvania State University (PSU),the Space Research Institute of the Netherlands (SRON), the Max-PlanckInstitüt für extraterrestrische Physik (MPE), and theUniversity of Kiel also provide technical support to the ChandraProject. We present here a detailed description of the hardware, itson-orbit performance, and a brief overview of some of the remarkablediscoveries that illustrate that performance.
| The BIMA Survey of Nearby Galaxies (BIMA SONG). II. The CO Data
The BIMA Survey of Nearby Galaxies is a systematic imaging study of the3 mm CO J=1-0 molecular emission within the centers and disks of 44nearby spiral galaxies. The typical spatial resolution of the survey is6" or 360 pc at the average distance (12 Mpc) of the sample. Thevelocity resolution of the CO observations is 4 km s-1,though most maps are smoothed to 10 km s-1 resolution. For 33galaxies, multifield observations ensured that a region >~190"(=10 kpc) in diameter was imaged. For the remaining 11galaxies, which had smaller optical diameters and were on averagefarther away, single-pointing observations imaged a 100" diameter(=11 kpc) region. The sample was not chosen based on CO orinfrared brightness; instead, all spirals were included that met theselection criteria of vsolar<=2000 km s-1,δ>=-20deg, i<=70deg,D25<70', and BT<11.0. Thedetection rate was 41/44 sources or 93%; of the three nondetections, one(M81) is known to have CO emission at locations outside the survey fieldof view. Fully sampled single-dish CO data were incorporated into themaps for 24 galaxies; these single-dish data comprise the most extensivecollection of fully sampled, two-dimensional single-dish CO maps ofexternal galaxies to date. We also tabulate direct measurements of theglobal CO flux densities for these 24 sources. For the remaining 20sources, we collected sensitive single-dish spectra in order to evaluatethe large-scale flux recovery. We demonstrate that the measured ratiosof flux density recovered are a function of the signal-to-noise of theinterferometric data. We examine the degree of central peakedness of themolecular surface density distributions and show that the distributionsexhibit their brightest CO emission within the central 6" in only 20/44or 45% of the sample. We show that all three Local Group spiral galaxieshave CO morphologies that are represented in SONG, though the Milky WayCO luminosity is somewhat below the SONG average, and M31 and M33 arewell below average. This survey provides a unique public database ofintegrated intensity maps, channel maps, spectra, and velocity fields ofmolecular emission in nearby galaxies. It also lays the groundwork forextragalactic surveys by more powerful future millimeter-wavelengthinterferometers like CARMA and ALMA.
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