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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.
| 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 Previously unpublished ROSAT High-ResolutionImager data are also presented.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.
| 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.
| 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.
| 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.
| 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.
| 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.
| 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.
| Cold dust and molecular gas towards the centers of Magellanic type galaxies and irregulars. I. The data
We present 1300 μm continuum emission measurements and observationsof the 12CO (1-0) and (2-1) transition towards the centers of64 Magellanic type galaxies (Sdm/Sm) and irregulars (Im/I0/Irr). Thesources are selected to have IRAS flux densities S100 μm≥1000 mJy and optical diameters mainly below 180 arcsec. We wereable to detect 12CO towards 41 and the continuum emissiontowards 28 galaxies. In addition, we obtained the corresponding data fora set of 6 complementary galaxies of different morphological type.Based on observations collected at ESO, La Silla, Chile and IRAM, PicoVeleta, Spain.The full version of Figs. \ref{spec1.fig} and \ref{spec2.fig} is onlyavailable in electronic form at http://www.edpsciences.org
| 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.
| Infrared Observations of Galaxies in the Local Universe. II. 391 Calibrated Images with Photometric and Structural Measurements
This paper presents empirical results from a deep imaging survey ofgalaxies in the local universe at the J and Ks wavelengths.Three hundred ninety-one images have been obtained and calibrated usingthe same camera and filter set with the Steward Observatory 1.6 m KuiperTelescope on Mount Bigelow and the 2.3 m Bok Telescope on Kitt Peak. Thelimiting magnitude is typically 22 mag arcsec-1 at J and 21mag arcsec-1 at Ks. The central surfacebrightness, apparent magnitudes, sizes, scale lengths, and inclinationsare tabulated from measurements made using these data. The purpose ofthis paper is to provide basic near-infrared data on a variety of galaxytypes.
| A search for Low Surface Brightness galaxies in the near-infrared. I. Selection of the sample
A sample of about 3800 Low Surface Brightness (LSB) galaxies wasselected using the all-sky near-infrared (J, H and Ks-band)2MASS survey. The selected objects have a mean central surfacebrightness within a 5'' radius around their centre fainter than 18 magarcsec-2 in the Ks band, making them the lowestsurface brightness galaxies detected by 2MASS. A description is given ofthe relevant properties of the 2MASS survey and the LSB galaxy selectionprocedure, as well as of basic photometric properties of the selectedobjects. The latter properties are compared to those of other samples ofgalaxies, of both LSBs and ``classical'' high surface brightness (HSB)objects, which were selected in the optical. The 2MASS LSBs have aBT_c-KT colour which is on average 0.9 mag bluerthan that of HSBs from the NGC. The 2MASS sample does not appear tocontain a significant population of red objects.All tables and Figs. 2a-c are only available in electronic form athttp://www.edpsciences.org
| Galaxy flow in the Canes Venatici I cloud
We present an analysis of Hubble Space Telescope/WFPC2 images ofeighteen galaxies in the Canes Venatici I cloud. We derive theirdistances from the luminosity of the tip of the red giant branch starswith a typical accuracy of ~ 12%. The resulting distances are 3.9 Mpc(UGC 6541), 4.9 Mpc (NGC 3738), 3.0 Mpc (NGC 3741), 4.5 Mpc (KK 109),>6.3 Mpc (NGC 4150), 4.2 Mpc (UGC 7298), 4.5 Mpc (NGC 4244), 4.6 Mpc(NGC 4395), 4.9 Mpc (UGC 7559), 4.2 Mpc (NGC 4449), 4.4 Mpc (UGC 7605),4.6 Mpc (IC 3687), 4.7 Mpc (KK 166), 4.7 Mpc (NGC 4736), 4.2 Mpc (UGC8308), 4.3 Mpc (UGC 8320), 4.6 Mpc (NGC 5204), and 3.2 Mpc (UGC 8833).The CVn I cloud has a mean radial velocity of 286 +/- 9 kms-1, a mean distance of 4.1 +/- 0.2 Mpc, a radial velocitydispersion of 50 km s-1, a mean projected radius of 760 kpc,and a total blue luminosity of 2.2 x 1010 Lsun .Assuming virial or closed orbital motions for the galaxies, we estimatedtheir virial and their orbital mass-to-luminosity ratio to be 176 and 88Msun /Lsun , respectively. However, the CVn Icloud is characterized by a crossing time of 15 Gyr, and is thus farfrom a state of dynamical equilibrium. The large crossing time for thecloud, its low content of dSph galaxies (<6%), and the almost``primordial'' shape of its luminosity function show that the CVn Icomplex is in a transient dynamical state, driven rather by the freeHubble expansion than by galaxy interactions.Based on observations made with the NASA/ESA Hubble Space Telescope. TheSpace Telescope Science Institute is operated by the Association ofUniversities for Research in Astronomy, Inc. under NASA contract NAS5-26555.Figures 1 and 2 are only available in electronic form athttp://www.edpsciences.org
| A Chandra observation of the interacting pair of galaxies NGC 4485/4490
We report the results of a 20-ks Chandra ACIS-S observation of thegalaxy pair NGC 4485/4490. This is an interacting system containing alate-type spiral with an enhanced star formation rate (NGC 4490), and anirregular companion that possesses a disturbed morphology. A total of 29discrete X-ray sources are found coincident with NGC 4490, but only oneis found within NGC 4485. The sources range in observed X-ray luminosityfrom ~2 × 1037 to 4 × 1039 ergs-1. The more luminous sources appear, on average, to bespectrally harder than the fainter sources, an effect that isattributable to increased absorption in their spectra. Extensive diffuseX-ray emission is detected coincident with the disc of NGC 4490, and inthe tidal tail of NGC 4485, which appears to be thermal in nature andhence the signature of a hot interstellar medium in both galaxies.However, the diffuse component accounts for only ~10 per cent of thetotal X-ray luminosity of the system (2 × 1040 ergs-1, 0.5-8 keV), which arises predominantly in a handful ofthe brightest discrete sources. This diffuse emission fraction isunusually low for a galaxy pair which has many characteristics thatwould lead it to be classified as a starburst system, possibly as aconsequence of the small gravitational potential well of the system. Thediscrete source population, on the other hand, is similar to thatobserved in other starburst systems, possessing a flat luminosityfunction slope of ~-0.6 and a total of six ultraluminous X-ray sources(ULX). Five of the ULX are identified as probable black hole X-raybinary systems, and the sixth (which is coincident with a radiocontinuum source) is identified as an X-ray luminous supernova remnant.The ULX all lie in star formation regions, providing further evidence ofthe link between the ULX phenomenon and active star formation.Importantly, this shows that even in star-forming regions, the ULXpopulation is dominated by accreting systems. We discuss theimplications of this work for physical models of the nature of ULX, andin particular how it argues against the intermediate-mass black holehypothesis.
| Hα, SCUBA and MERLIN imaging of NGC 4490
We describe Hα, SCUBA and MERLIN imaging of the interacting galaxypair NGC 4490 and 4485. We detect an Hα filament emerging from thedisc of NGC 4490 to a projected distance of 3kpc which has counterpartsin both the radio continuum and HI. The HI counterpart extends to aprojected distance of ~30kpc from NGC 4490 and we argue that this isevidence that the giant HI envelope in this system has its origins instar formation. We use SCUBA and radio continuum data to attempt toplace constraints on the distribution of dust with respect to the starforming regions. This analysis is limited by the lack of an independentestimate of the dust temperature, something that both `SIRTF' and`SOFIA' will be able to provide, however we find some evidence that mostobscuring dust is not located within HII regions themselves.
| A Catalog of Candidate Intermediate-Luminosity X-Ray Objects
ROSAT, and now Chandra, X-ray images allow studies of extranuclear X-raypoint sources in galaxies other than our own. X-ray observations ofnormal galaxies with ROSAT and Chandra have revealed that off-nuclear,compact, intermediate-luminosity(LX[2-10keV]>=1039.0 ergs s-1) X-rayobjects (IXOs, a.k.a. ULXs [ultraluminous X-ray sources]) are quitecommon. Here we present a catalog and finding charts for 87 IXOs in 54galaxies, derived from all of the ROSAT HRI imaging data for galaxieswith cz<=5000 km s-1 from the Third Reference Catalog ofBright Galaxies. We have defined the cutoff LX for IXOs sothat it is well above the Eddington luminosity of a 1.4Msolar black hole (1038.3 ergs s-1), soas not to confuse IXOs with ``normal'' black hole X-ray binaries. Thiscatalog is intended to provide a baseline for follow-up work withChandra and XMM-Newton, and with space- and ground-based survey work atwavelengths other than X-ray. We demonstrate that elliptical galaxieswith IXOs have a larger number of IXOs per galaxy than nonellipticalgalaxies with IXOs and note that they are not likely to be merelyhigh-mass X-ray binaries with beamed X-ray emission, as may be the casefor IXOs in starburst galaxies. Approximately half of the IXOs withmultiple observations show X-ray variability, and many (19) of the IXOshave faint optical counterparts in DSS optical B-band images. Follow-upobservations of these objects should be helpful in identifying theirnature.
| The Nature of Ultraluminous X-Ray Sources in NGC 4565
We report the optical identifications of two X-ray-luminous pointsources in the spiral galaxy NGC 4565 based on archive data of Chandraand the Hubble Space Telescope. The central X-ray point source, RXJ1236.3+2559, is found to be the nucleus of NGC 4565 with an X-rayluminosity of LX~4.3×1039 ergss-1. We show that its multiband properties are consistentwith its being a low-luminosity active galactic nucleus. A faint opticalcounterpart with B~25.1 and I~24.0 was identified for the off-nucleusX-ray point source, RX J1236.2+2558. Its extinction-corrected Bmagnitude is estimated to be 24.5. The X-ray-to-optical flux ratio(fX/fB) is about 540. From the optical and X-rayproperties, we argue that RX J1236.2+2558 is an ultraluminous X-raycompact source with LX~6.5×1039 ergss-1. The source is probably located in a faint globularcluster at the outer edge of NGC 4565's bulge.
| Bar Galaxies and Their Environments
The prints of the Palomar Sky Survey, luminosity classifications, andradial velocities were used to assign all northern Shapley-Ames galaxiesto either (1) field, (2) group, or (3) cluster environments. Thisinformation for 930 galaxies shows no evidence for a dependence of barfrequency on galaxy environment. This suggests that the formation of abar in a disk galaxy is mainly determined by the properties of theparent galaxy, rather than by the characteristics of its environment.
| The UZC-SSRS2 Group Catalog
We apply a friends-of-friends algorithm to the combined Updated ZwickyCatalog and Southern Sky Redshift Survey to construct a catalog of 1168groups of galaxies; 411 of these groups have five or more members withinthe redshift survey. The group catalog covers 4.69 sr, and all groupsexceed the number density contrast threshold, δρ/ρ=80. Wedemonstrate that the groups catalog is homogeneous across the twounderlying redshift surveys; the catalog of groups and their membersthus provides a basis for other statistical studies of the large-scaledistribution of groups and their physical properties. The medianphysical properties of the groups are similar to those for groupsderived from independent surveys, including the ESO Key Programme andthe Las Campanas Redshift Survey. We include tables of groups and theirmembers.
| The Westerbork HI survey of spiral and irregular galaxies. II. R-band surface photometry of late-type dwarf galaxies
R-band surface photometry is presented for 171 late-type dwarf andirregular galaxies. For a subsample of 46 galaxies B-band photometry ispresented as well. We present surface brightness profiles as well asisophotal and photometric parameters including magnitudes, diameters andcentral surface brightnesses. Absolute photometry is accurate to 0.1 magor better for 77% of the sample. For over 85% of the galaxies the radialsurface brightness profiles are consistent with published data withinthe measured photometric uncertainty. For most of the galaxies in thesample H I data have been obtained with the Westerbork Synthesis RadioTelescope. The galaxies in our sample are part of the WHISP project(Westerbork H I Survey of Spiral and Irregular Galaxies), which aims atmapping about 500 nearby spiral and irregular galaxies in H I. Theavailability of H I data makes this data set useful for a wide range ofstudies of the structure, dark matter content and kinematics oflate-type dwarf galaxies. Based on observations made with INT operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias. The tables in Appendix A are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/390/863. Thefigures in Appendix B are only available in electronic formhttp://www.edpsciences.org
| The 2dF Galaxy Redshift Survey: spectra and redshifts
The 2dF Galaxy Redshift Survey (2dFGRS) is designed to measure redshiftsfor approximately 250000 galaxies. This paper describes the surveydesign, the spectroscopic observations, the redshift measurements andthe survey data base. The 2dFGRS uses the 2dF multifibre spectrograph onthe Anglo-Australian Telescope, which is capable of observing 400objects simultaneously over a 2° diameter field. The sourcecatalogue for the survey is a revised and extended version of the APMgalaxy catalogue, and the targets are galaxies with extinction-correctedmagnitudes brighter than bJ=19.45. The main survey regionsare two declination strips, one in the southern Galactic hemispherespanning 80°×15° around the SGP, and the other in thenorthern Galactic hemisphere spanning 75°×10° along thecelestial equator; in addition, there are 99 fields spread over thesouthern Galactic cap. The survey covers 2000deg2 and has amedian depth of z=0.11. Adaptive tiling is used to give a highly uniformsampling rate of 93 per cent over the whole survey region. Redshifts aremeasured from spectra covering 3600-8000Å at a two-pixelresolution of 9.0Å and a median S/N of 13pixel-1. Allredshift identifications are visually checked and assigned a qualityparameter Q in the range 1-5 Q>=3 redshifts are 98.4 per centreliable and have an rms uncertainty of 85kms-1. The overallredshift completeness for Q>=3 redshifts is 91.8 per cent, but thisvaries with magnitude from 99 per cent for the brightest galaxies to 90per cent for objects at the survey limit. The 2dFGRS data base isavailable on the World Wide Web at http://www.mso.anu.edu.au/2dFGRS.
| Recovering physical parameters from galaxy spectra using MOPED
We derive physical parameters of galaxies from their observed spectrausing MOPED, the optimized data compression algorithm of Heavens,Jimenez & Lahav. Here we concentrate on parametrizing galaxyproperties, and apply the method to the NGC galaxies in Kennicutt'sspectral atlas. We focus on deriving the star formation history,metallicity and dust content of galaxies. The method is very fast,taking a few seconds of CPU time to estimate ~17 parameters, and istherefore specially suited to studying large data sets, such as theAnglo-Australian two-degree-field (2dF) galaxy survey and the SloanDigital Sky Survey (SDSS). Without the power of MOPED, the recovery ofstar formation histories in these surveys would be impractical. InKennicutt's atlas, we find that for the spheroidals a small recent burstof star formation is required to provide the best fit to the spectrum.There is clearly a need for theoretical stellar atmospheric models withspectral resolution better than 1Å if we are to extract all therich information that large redshift surveys contain in their galaxyspectra.
| A study of the core of the Shapley Concentration - VI. Spectral properties of galaxies*
We present the results of a study of the spectral properties of galaxiesin the central part of the Shapley Concentration, covering an extremelywide range of densities, from the rich cluster cores to the underlyingsupercluster environment. Our sample is homogeneous, in a well definedmagnitude range (17<=bJ<=18.8) and contains ~1300spectra of galaxies at the same distance, covering an area of~26deg2. These characteristics allowed an accurate spectralclassification that we performed using a principal components analysistechnique. This spectral classification, together with the [Oii]equivalent widths and the star formation rates, has been used to studythe properties of galaxies at different densities: cluster, intercluster(i.e. galaxies in the supercluster but outside clusters) and fieldenvironment. No significant differences are present between samples atlow density regimes (i.e. intercluster and field galaxies). Clustergalaxies, instead, not only have values that are significantly differentfrom the field ones, but also show a dependence on the local density.Moreover, a well defined morphology-density relation is present in thecluster complexes, although these structures are known to be involved inmajor merging events. Also the mean equivalent width of [Oii] shows atrend with the local environment, decreasing at increasing densities,even if it is probably induced by the morphology-density relation.Finally we analysed the mean star formation rate as a function of thedensity, finding again a decreasing trend (at ~3σ significancelevel). Our analysis is consistent with the claim of Balogh et al. thatthe star formation in clusters is depressed.
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