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XMM-Newton Archival Study of the Ultraluminous X-Ray Population in Nearby Galaxies
We present the results of an archival XMM-Newton study of the brightX-ray point sources (LX>1038 ergss-1) in 32 nearby galaxies. From our list of approximately100 point sources, we attempt to determine if there is a low-statecounterpart to the ultraluminous X-ray (ULX) population, searching for asoft-hard state dichotomy similar to that known for Galactic X-raybinaries and testing the specific predictions of the intermediate-massblack hole (IMBH) hypothesis. To this end, we searched for ``low-state''objects, which we defined as objects within our sample that had aspectrum well fitted by a simple absorbed power law, and ``high-state''objects, which we defined as objects better fitted by a combinedblackbody and a power law. Assuming that low-state objects accrete atapproximately 10% of the Eddington luminosity (as found by Done &Gierlinski) and that high-state objects accrete near the Eddingtonluminosity, we further divided our sample of sources into low- andhigh-state ULX sources. We classify 16 sources as low-state ULXs and 26objects as high-state ULXs. As in Galactic BH systems, the spectralindices, Γ, of the low-state objects, as well as the luminosities,tend to be lower than those of the high-state objects. The observedrange of blackbody temperatures for the high state is 0.1-1 keV, withthe most luminous systems tending toward the lowest temperatures. Wetherefore divide our high-state ULXs into candidate IMBHs (withblackbody temperatures of approximately 0.1 keV) and candidate stellarmass BHs (with blackbody temperatures of approximately 1.0 keV). Asubset of the candidate stellar mass BHs have spectra that are wellfitted by a Comptonization model, a property similar to Galactic BHsradiating in the ``very high'' state near the Eddington limit.
| Ultraviolet through Far-Infrared Spatially Resolved Analysis of the Recent Star Formation in M81 (NGC 3031)
The recent star formation (SF) in the early-type spiral galaxy M81 ischaracterized using imaging observations from the far-ultraviolet to thefar-infrared. We compare these data with models of the stellar, gas, anddust emission for subgalactic regions. Our results suggest the existenceof a diffuse dust emission not directly linked to the recent starformation. We find a radial decrease of the dust temperature and dustmass density, and in the attenuation of the stellar light. The IRemission in M81 can be modeled with three components: (1) cold dust witha temperature c>=18+/-2 K, concentrated near the H II regions butalso presenting a diffuse distribution; (2) warm dust with w>=53+/-7K, directly linked with the H II regions; and (3) aromatic molecules,with diffuse morphology peaking around the H II regions. We deriveseveral relationships to obtain total IR luminosities from IRmonochromatic fluxes, and we compare five different star formation rate(SFR) estimators for H II regions in M81 and M51: the UV, Hα, andthree estimators based on Spitzer data. We find that the Hαluminosity absorbed by dust correlates tightly with the 24 μmemission. The correlation with the total IR luminosity is not as good.Important variations from galaxy to galaxy are found when estimating thetotal SFR with the 24 μm or the total IR emission alone. The mostreliable estimations of the total SFRs are obtained by combining theHα emission (or the UV) and an IR luminosity (especially the 24μm emission), which probe the unobscured and obscured SF,respectively. For the entire M81 galaxy, about 50% of the total SF isobscured by dust. The percentage of obscured SF ranges from 60% in theinner regions of the galaxy to 30% in the outer zones.
| On Extending the Mass-Metallicity Relation of Galaxies by 2.5 Decades in Stellar Mass
We report 4.5 μm luminosities for 27 nearby (D<~5 Mpc) dwarfirregular galaxies measured with the Spitzer Infrared Array Camera. Wehave constructed the 4.5 μm luminosity-metallicity (L-Z) relation for25 dwarf galaxies with secure distance and interstellar medium oxygenabundance measurements. The 4.5 μm L-Z relation is12+log(O/H)=(5.78+/-0.21)+(-0.122+/-0.012)M[4.5], whereM[4.5] is the absolute magnitude at 4.5 μm. The dispersionin the near-infrared L-Z relation is smaller than the correspondingdispersion in the optical L-Z relation. The subsequently derived stellarmass-metallicity (M*-Z) relation is12+log(O/H)=(5.65+/-0.23)+(0.298+/-0.030)logM*, and extendsthe SDSS M*- Z relation to lower mass by about 2.5 dex. Wefind that the dispersion in the M*-Z relation is similar over5 orders of magnitude in stellar mass, and that the relationship betweenstellar mass and interstellar medium metallicity is similarly tight fromhigh-mass to low-mass systems. We find a larger scatter at low mass inthe relation between effective yield and total baryonic mass. In fact,there are a few dwarf galaxies with large yields, which is difficult toexplain if galactic winds are ubiquitous in dwarf galaxies. The lowscatter in the L-Z and M*-Z relationships are difficult tounderstand if galactic superwinds or blowout are responsible for the lowmetallicities at low mass or luminosity. Naively, one would expect anever increasing scatter at lower masses, which is not observed.
| Close Binary Interactions of Intermediate-Mass Black Holes: Possible Ultraluminous X-Ray Sources?
While many observed ultraluminous X-ray sources (ULXs;LX>=1039 ergs s-1) could beextragalactic X-ray binaries (XRBs) emitting close to the Eddingtonlimit, the highest luminosity ULXs(LX>3×1039 ergs s-1) exceedthe isotropic Eddington luminosity for even high-stellar-mass-accretingblack hole XRBs. It has been suggested that these highest luminosityULXs may contain accreting intermediate-mass black hole (IMBH) binaries.We consider this hypothesis for dense, young (~100 Myr) stellar clusterswhere we assume that a 50-500 Msolar central IMBH has formedthrough runaway growth of a massive star. We develop numericalsimulations of such clusters' cores by combining single and binary starevolutionary syntheses with a simple treatment of dynamicalinteractions. We model interactions of the IMBH with single and binarystars, as well as single-binary and binary-binary interactions, but notthe formation of a cusp around the IMBH. The core density and velocitydispersion are assumed to be constant over 100 Myr. We investigate thesuccession of IMBH binary companions and the evolution of their orbitalparameters to obtain estimates of the incidence of mass transfer phasesand possible ULX activity involving the IMBH. We find that although itis common for the central black hole to acquire binary companions, thereis a very low probability that these interacting binaries will becomeobservable ULX sources.
| Quasi-periodic Oscillations and Strongly Comptonized X-Ray Emission from Holmberg IX X-1
We report the discovery of a 200 mHz quasi-periodic oscillation (QPO) inthe X-ray emission from a bright ultraluminous X-ray source (ULX),Holmberg IX X-1, using a long XMM-Newton observation. The QPO has acentroid at νQPO=202.5+4.9-3.8 mHz,a coherence Q≡νQPO/ΔνFWHM~9.3,and an amplitude (rms) of 6% in the 0.2-10 keV band. This is only thesecond detection of a QPO from a ULX, after M82 X-1, and provides strongevidence against beaming. The power spectrum is well fitted by a powerlaw with an index of ~0.7. The total integrated power (rms) is ~9.4% inthe 0.001-1 Hz range. The X-ray spectrum shows clear evidence of a softX-ray excess component that is well described by a multicolor diskblackbody (kTin~0.3 keV) and a high-energy curvature that canbe modeled either by a cutoff power law (Γ~1 Ecutoff=9keV) or as a strongly Comptonized continuum in an optically thick(τ~7.3) and cool (kTe~3 keV) plasma. Both the presence ofthe QPO and the shape of the X-ray spectrum strongly suggest that theULX is not in the high/soft or thermally dominated state. A truncateddisk and inner optically thick corona may explain the observed X-rayspectrum and the presence of the QPO.
| An Optical Study of Stellar and Interstellar Environments of Seven Luminous and Ultraluminous X-Ray Sources
We have studied the stellar and interstellar environments of twoluminous X-ray sources and five ultraluminous X-ray sources (ULXs) inorder to gain insight into their nature. Archival Hubble Space Telescopeimages were used to identify the optical counterparts of the ULXs Ho IXX-1 and NGC 1313 X-2, and to make photometric measurements of the localstellar populations of these and the luminous source IC 10 X-1. Weobtained high-dispersion spectroscopic observations of the nebulaearound these seven sources to search for He II λ4686 emission andto estimate the expansion velocities and kinetic energies of thesenebulae. Our observations did not detect nebular He II emission from anysource, with the exception of LMC X-1 this is either because we missedthe He III regions or because the nebulae are too diffuse to produce HeII surface brightnesses that lie within our detection limit. We comparethe observed ionization and kinematics of the supershells around theULXs Ho IX X-1 and NGC 1313 X-2 with the energy feedback expected fromthe underlying stellar population to assess whether additional energycontributions from the ULXs are needed. In both cases, we findinsufficient UV fluxes or mechanical energies from the stellarpopulation; thus these ULXs may be partially responsible for theionization and energetics of their supershells. All seven sources thatwe studied are in young stellar environments, and six of them haveoptical counterparts with masses >~7 Msolar thus, thesesources are most likely high-mass X-ray binaries.
| Globular Clusters in Dwarf Galaxies
Data are currently available on the luminosities and half-light radii of101 globular clusters associated with low-luminosity parent galaxies.The luminosity distribution of globular clusters embedded in dwarfgalaxies having Mv>-16 is found to differ dramaticallyfrom that for globular clusters surrounding giant host galaxies withMv<-16. The luminosity distribution of globular clustersin giant galaxies peaks at Mv~-7.5, whereas that for dwarfgalaxies is found to increase monotonically down to the completenesslimit of the cluster data at Mv~-5.0. Unexpectedly, thepower-law distribution of the luminosities of globular clusters hostedby dwarf galaxies is seen to be much flatter than that of the bright,unevolved part of the luminosity distribution of globular clustersassociated with giant galaxies. The specific frequency of globularclusters fainter than Mv=-7.5 is found to be particularlyhigh in dwarf galaxies. The luminosity distribution of the LMC globularclusters is similar to that found in giant galaxies and differs fromthose of globular clusters in dwarf galaxies. The present data appear toshow no strong dependence of globular cluster luminosity on themorphological type of their parent galaxies. No attempt is made toexplain the unexpected discovery that the luminosity distribution ofglobular clusters is critically dependent on parent galaxy luminosity(or mass) but insensitive to the morphological type of the host galaxy.
| An XMM-Newton survey of the Local Group galaxy M 33 - variability of the detected sources
We present an analysis of the individual observations of a deepXMM-Newton survey of the Local Group spiral galaxy M 33. We detected atotal of 350 sources with fluxes (in the 0.2-4.5 keV energy band) in therange 6.7 × 10-16-1.5 × 10-11 ergs-1. This comprehensive study considers flux variability,spectral characteristics, and classification of the detected objects.Thirty-nine objects in our catalogue are new sources, while 311 werealready detected in a previous analysis of most of the same data usingcombined images. We present improved positions of these sources and theX-ray parameters of each source in each individual observation thatcovers the source. We then used these parameters to systematicallysearch for flux variability on time scales of hours to months or years.The long-term light-curves were generated for the 61 sources showing asignificant variability of the (0.2-4.5) keV flux, by a factor of 1.2 to144. The detected variability was then used to classify 8 new X-raybinary candidates in M 33. Together with the hardness ratio method andcross-correlation with optical, infrared, and radio data, we alsoclassify or confirm previous classification of 25 supernova remnants andcandidates, 2 X-ray binaries, and 11 super-soft source candidates (7 ofwhich are new SSS candidates). In addition, we classify 13 activegalactic nuclei and background galaxies, 6 stars, and 23 foreground starcandidates in the direction of M 33. Further 206 objects are classifiedas "hard", approximately half of which are sources intrinsic to M 33.The relative contribution of the classified XRB and SSS in M 33 is nowcomparable to M 31. The luminosity distribution of SNRs in both spiralgalaxies is almost the same, although the number of the detected SNRs inM 33 remains much higher.
| Global Properties of Nearby Galaxies in Various Environments
We analyze a sample of the Local Volume that contains 451 galaxieswithin 10 Mpc. We compare the various global parameters of thesegalaxies with their tidal index that characterizes the local density ofthe environment. The closest correlation is observed between the densityof the galaxy’s environment and its morphological type. Theabundance of neutral hydrogen in the members of close groups was foundto be, on average, a factor of 3 lower than that in isolated galaxies.However, much of this difference is attributable to differentmorphological composition for the group members and field galaxies. Thetotal mass-to-luminosity ratio is virtually independent of the tidalindex of the galaxy, which indirectly indicates a low percentage oftidal systems among dwarf galaxies. All of the galaxies with three ormore companions in the Local Volume are shown to have masses above thethreshold value of 1010 M ȯ.
| 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.
| Infrared Spectral Energy Distributions of Nearby Galaxies
The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out acomprehensive multiwavelength survey on a sample of 75 nearby galaxies.The 1-850 μm spectral energy distributions (SEDs) are presented usingbroadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. Theinfrared colors derived from the globally integrated Spitzer data aregenerally consistent with the previous generation of models that weredeveloped using global data for normal star-forming galaxies, althoughsignificant deviations are observed. Spitzer's excellent sensitivity andresolution also allow a detailed investigation of the infrared SEDs forvarious locations within the three large, nearby galaxies NGC 3031(M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapesis found within each galaxy, especially for NGC 3031, the closest of thethree targets and thus the galaxy for which the smallest spatial scalescan be explored. Strong correlations exist between the local starformation rate and the infrared colors fν(70μm)/fν(160 μm) and fν(24μm)/fν(160 μm), suggesting that the 24 and 70 μmemission are useful tracers of the local star formation activity level.Preliminary evidence indicates that variations in the 24 μm emission,and not variations in the emission from polycyclic aromatic hydrocarbonsat 8 μm, drive the variations in the fν(8.0μm)/fν(24 μm) colors within NGC 3031, NGC 5194, andNGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sampleare representative of the range present at high redshift, thenextrapolations of total infrared luminosities and star formation ratesfrom the observed 24 μm flux will be uncertain at the factor of 5level (total range). The corresponding uncertainties using theredshifted 8.0 μm flux (e.g., observed 24 μm flux for a z=2source) are factors of 10-20. Considerable caution should be used wheninterpreting such extrapolated infrared luminosities.
| Intergalactic Stellar Distributions in the Interacting M81/M82 Galaxy Group
Previous H I observations of the M81/M82/NGC 3077 galaxy group clearlyshow a widespread H I distribution within this galaxy group. While thegas is vulnerable to tidal disruption from a galaxy encounter, are therealso stars embedded in this H I distribution? Our deep, 1deg2 exposures of the M81/M82 group in 10 optical bands usingthe Beijing-Arizona-Taipei-Connecticut (BATC) filter set clearly revealwidespread stellar distributions that coincide with the atomic hydrogenclouds-considered to be the relics of the merging process of thegalaxies-splayed over the region. The spectral energy distributions ofthe stellar groups to the east and west of M81 (including the ``ArpLoop'') are similar to that measured at the southeast edge of theoptical disk of M82. This similarity in stellar radiation, combined withthe observed peculiar rotational velocity of M82, suggests that thediffuse stellar population in the intergalactic space around M81 ispossibly a relic of the tidally disrupted disk of M82 during the lastclose encounter. Alternately, the stars could have formed in situ in theH I as it was drawn out of the galaxies. Recent measurements ofdistances to and radial velocities of M81 (3.63 Mpc and 48 kms-1, respectively) and M82 (3.9 Mpc and 296 kms-1) lend further support to the notion of a close passagebetween these two galaxies several hundred million years ago.
| The Galaxy Evolution Explorer: A Space Ultraviolet Survey Mission
We give an overview of the Galaxy Evolution Explorer (GALEX), a NASAExplorer Mission launched on 2003 April 28. GALEX is performing thefirst space UV sky survey, including imaging and grism surveys in twobands (1350-1750 and 1750-2750 Å). The surveys include an all-skyimaging survey (mAB~=20.5), a medium imaging survey of 1000deg2 (mAB~=23), a deep imaging survey of 100deg2 (mAB~=25), and a nearby galaxy survey.Spectroscopic (slitless) grism surveys (R=100-200) are underway withvarious depths and sky coverage. Many targets overlap existing orplanned surveys in other bands. We will use the measured UV propertiesof local galaxies, along with corollary observations, to calibrate therelationship of the UV and global star formation rate in local galaxies.We will apply this calibration to distant galaxies discovered in thedeep imaging and spectroscopic surveys to map the history of starformation in the universe over the redshift range 0
| The Local Group and Other Neighboring Galaxy Groups
Over the last few years, rapid progress has been made in distancemeasurements for nearby galaxies based on the magnitude of stars on thetip of the red giant branch. Current CCD surveys with the Hubble SpaceTelescope (HST) and large ground-based telescopes bring ~10% accuratedistances for roughly a hundred galaxies within 5 Mpc. The new data ondistances to galaxies situated in (and around) the nearest groups-theLocal Group, M81 Group, Cen A/M83 Group, IC 342/Maffei Group, Sculptorfilament, and Canes Venatici cloud-allowed us to determine their totalmass from the radius of the zero-velocity surface, R0, whichseparates a group as bound against the homogeneous cosmic expansion. Thevalues of R0 for the virialized groups turn out to be closeeach other, in the range of 0.9-1.3 Mpc. As a result, the total massesof the groups are close to each other, as well, yielding total mass toblue luminosity ratios of 10-40 MsolarL-1solar. The new total mass estimates are 3-5times lower than old virial mass estimates of these groups. Becauseabout half of galaxies in the Local volume belong to such loose groups,the revision of the amount of dark matter (DM) leads to a low localdensity of matter, Ωm~=0.04, which is comparable withthe global baryonic fraction Ωb but much lower than theglobal density of matter, Ωm=0.27. To remove thediscrepancy between the global and local quantities ofΩm, we assume the existence of two different DMcomponents: (1) compact dark halos around individual galaxies and (2) anonbaryonic dark matter ``ocean'' with ΩDM1~=0.07 andΩDM2~=0.20, respectively.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy,Inc., under NASA contract NAS 5-26555.
| Study of DDO 68: nearest candidate for a young galaxy?
We present the results of optical spectroscopy and imaging with the SAO6 m telescope for the dwarf galaxy DDO 68 (UGC 5340 = VV 542), fallinginto the region of very low density of luminous (L > L*)galaxies (Lynx-Cancer void). Its deep images in V,R bands and in thenarrow Hα-filter show that this galaxy has the very irregularmorphology, with a long curved tail on the South and a ring-likestructure at the Northern edge. The latter consists of 5 separateregions, in three of which we could measure O/H by the classicalTe method. Their weighted mean oxygen abundance correspondsto 12+log (O/H)=7.21 ± 0.03, coincident within uncertainties withthose for I Zw 18. The (V-R) colour of DDO 68 is rather blue all overthe galaxy, indicating the youth of its stellar populations. Comparingthe (V-R)0 colour of the underlying exponential disk of0.12m±0.04 with the PEGASE.2 models for the evolving stellarclusters, we give the first estimate of the ages of the oldest stellarpopulation, which needs confirmation by the other colours and thephotometry of resolved stars. These ages are in the range of 200-900 Myrfor continuous star formation law, and ~100-115 Myr for theinstantaneous starburst. We discuss the properties and the possibleyouth of this nearby object (~2.3 times closer than the famous younggalaxy I Zw 18) in the context of its atypical environment.
| Hubble Space Telescope imaging of globular cluster candidates in low surface brightness dwarf galaxies
Fifty-seven nearby low surface brightness dwarf galaxies (-10MV -16) were searched for globular cluster candidates(GCCs) using Hubble Space Telescope WFPC2 imaging in V and I. The sampleconsists of 18 dwarf spheroidal (dSph), 36 irregular (dIrr), and 3"transition" type (dIrr/dSph) galaxies with angular sizes less than 3.7kpc situated at distances 2{-}6 Mpc in the field and in the nearbygroups: M 81, Centaurus A, Sculptor, Canes Venatici I cloud. We findthat 50% of dSph, dIrr/dSph, and dIrr galaxies contain GCCs. Thefraction of GCCs located near the center of dwarf spheroidal galaxies is2 times higher than for dIrrs. The mean integral color of GCCs indSphs, (V-I)_0=1.04±0.16 mag, coincides with the correspondingvalue for Galactic globular clusters and is similar to the blue globularcluster sub-populations in massive early-type galaxies. The colordistribution for GCCs in dIrrs shows a clear bimodality with peaks near(V-I)0 = 0.5 and 1.0 mag. Blue GCCs are presumably young withages t1 Gyr, while the red GCC population is likely to be older.The detected GCCs have absolute visual magnitudes betweenMV=-10 and -5 mag. We find indications for an excesspopulation of faint GCCs with MV-6.5 mag in both dSphand dIrr galaxies, reminiscent of excess populations of faint globularclusters in nearby Local Group spiral galaxies. The measurement ofstructural parameters using King-profile fitting reveals that most GCCshave structural parameters similar to extended outer halo globularclusters in the Milky Way and M 31, as well as the recently discoveredpopulation of "faint fuzzy" clusters in nearby lenticular galaxies.
| Thick disks and halos of spiral galaxies M 81, NGC 55 and NGC 300
By using images from the HST/WFPC2/ACS archive, we have analyzed thespatial distribution of the AGB and RGB stars along the galactocentricradius of nearby spiral galaxies M 81, NGC 300 and NGC 55. Examiningcolor-magnitude diagrams and stellar luminosity functions, we gauge thestellar contents of the surroundings of the three galaxies. The redgiant population (RGB) identified at large galactocentric radii yields adistance of 3.85±0.08 Mpc for M 81, 2.12±0.10 Mpc for NGC55, and 2.00±0.13 Mpc for NGC 300, and a mean stellar metallicityof -0.65, -1.25, and -0.87 respectively. We find that there are twonumber density gradients of RGB stars along the radius, which correspondto the thick disk and halo components of the galaxies. We confirm thepresence of a metallicity gradient of evolved stars in these galaxies,based on the systematic changes of the color distribution of red giantstars. These results imply that the thick disk might be a generalfeature of spiral galaxies, and endorse a further investigation of theouter stellar edges of nearby spirals, which is critical in constrainingthe origin and evolution of galaxies.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| Properties and environment of the molecular complex near Holmberg IX
This paper is aimed at providing new insight into the nature and originof the molecular complex situated near the line of sight toward HolmbergIX in the M 81 group of galaxies. The first high resolution CO maps ofthe complex as well as single dish 13CO(1-0),12CO(3-2) and millimeter continuum observations and theresults of a survey of 12CO in the region are presented.These data together with the available HI, optical and X-rayobservations are analyzed to study the properties and environment of thecomplex. We confirm there is no unobscured massive star formation insidethe complex, and from the millimeter constraint on the extinction itmust have a low star formation rate or be forming only low mass stars.According to the CO line ratios the abundances and physical conditionscould be similar to that of cold gas in spirals. We find from itsdynamics (no rotation) and its mass (2-6 million solar masses) that itresembles a massive GMC. Also, re-inspecting N-body simulations of the M81 group and the H I data we find that it might be located inside theextreme outer disk of M 81 and be cospatial with the H I feature knownas Concentration I. The negative result of the CO survey suggests thatthe complex is unique in this region and calls for a peculiar localformation process. We find that the distribution of the CO emission inthe data cube is asymmetrical in a way similar to a cometary object. Theoptical observations of the nearby supershell MH9/10 suggest theexistence of an outflow toward the complex. We consider the possibilitythat the molecular complex is related to this hypothetical outflow.Based on observations carried out with the IRAM Plateau de BureInterferometer and 30 m telescope, the 10 m Heinrich-Hertz-Telescope(HHT), and the NRAO 12 m telecsope. IRAM (Institut de Radio-AstronomieMillimétrique) is supported by INSU/CNRS (France), MPG (Germany)and IGN (Spain). The HHT was operated by the Submillimeter TelescopeObservatory on behalf of Steward Observatory and the Max-Planck-Institutfür Radioastronomie. The NRAO (National Radio AstronomyObservatory) is a facility of the National Science Foundation, operatedunder cooperative agreement by Associated Universities, Inc.Guest User, Canadian Astronomy Data Centre, that is operated by theDominion Astrophysical Observatory for the National Research Council ofCanada's Herzberg Institute of Astrophysics.
| The nature of ultraluminous sources: X-ray and optical clues.
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| Cosmic magnetic fields - as observed in the Universe, in galactic dynamos, and in the Milky Way
Cosmic magnetism has that exotic ``Je ne sais quoi''! Magnetism has beenobserved in various objects, located near the edge of the Universe andall the way down to the Milky Way's center. The observed magnetic fieldcan take the cell-type shape in randomly-oriented large blobs found inintracluster gas or outside of clusters of galaxies, the helix shape insynchrotron jets, the longitudinal shape in ram-pressured shocks inradio lobes near elliptical galaxies, the spiral shape of logarithmicarms in spiral galaxies, or the egg shape of an enlarged interstellarbubble. In strength, the magnetic field varies from 0.1 nG(cosmological), to 20 μG (galaxies, jets, superbubbles), and to 1 mGin the Milky Way filaments.Magnetism plays a small physical role in the formation of largestructures. It acts as a tracer of the dynamical histories ofcosmological and intracluster events, it guides the motion of theinterstellar ionised gas, and it aligns the charged dust particles.Batteries and dynamos are often employed in models to create and amplifyseed magnetic fields. Starting soon after the Big Bang (redshiftz>2000), this review covers the cosmological background surface(z~1100, distance ~4.3 Gpc), the epoch of first stars (z~20 distance~4.1 Gpc), the currently observable Universe (z~10, distance ~3.9 Gpc),superclusters of galaxies (size ~50 Mpc), intracluster gas (size ~10Mpc), galaxies (~30 kpc), spiral arms (~10 kpc), interstellarsuperbubbles (~100 pc), synchrotron filaments (~10 pc), and the MilkyWay's center.
| XMM-Newton Spectra of Intermediate-Mass Black Hole Candidates: Application of a Monte Carlo Simulated Model
We present a systematic spectral analysis of six ultraluminous X-raysources (NGC 1313 X-1 and X-2, IC 342 X-1, Ho IX X-1, NGC 5408 X-1, andNGC 3628 X-1) observed with the XMM-Newton observatory. Theseextranuclear X-ray sources in nearby late-type galaxies have beenconsidered as intermediate-mass black hole candidates. We have performedMonte Carlo simulations of Comptonized multicolor blackbody accretiondisks. This unified and self-consistent spectral model assumes aspherically symmetric, thermal corona around each disk and accounts forthe radiation transfer in the Comptonization. We find that the modelprovides satisfactory fits to the XMM-Newton spectra of the sources. Thecharacteristic temperatures of the accretion disks (Tin), forexample, are in the range of ~0.05-0.3 keV, consistent with theintermediate-mass black hole interpretation. We find that the black holemass is typically about a few times 103 Msolar andhas an accretion rate of ~10-6 to 10-5 Msolar yr-1. For the spectra considered here, we find thatthe commonly used multicolor blackbody accretion disk model with anadditive power-law component, although not physical, provides a goodmathematical approximation to the Monte Carlo simulated model. However,the latter model provides additional constraints on the properties ofthe accretion systems, such as the disk inclination angles and coronaoptical depths.
| Revealing a Cool Accretion Disk in the Ultraluminous X-Ray Source M81 X-9 (Holmberg IX X-1): Evidence for an Intermediate-Mass Black Hole
We report the results of an analysis of two XMM-Newton EPIC-pn spectraof the bright ultraluminous X-ray source M81 X-9 (Holmberg IX X-1)obtained in snapshot observations. Soft thermal emission is clearlyrevealed in spectra dominated by hard power-law components. Depending onthe model used, M81 X-9 was observed at a luminosity ofLX=(1.0-1.6)×1040ergss-1(0.3-10.0 keV). The variability previously observed in this sourcesignals that it is an accreting source that likely harbors a black hole.Remarkably, accretion disk models for the soft thermal emission yieldvery low inner disk temperatures (kT=0.17-0.29 keV, including 90%confidence errors and variations between observations and disk models)and improve the fit statistic over any single-component continuum modelat the 6 σ level of confidence. This represents much strongerevidence for a cool disk than prior evidence that combined spectra fromdifferent observatories, and the strongest evidence of a cool disk in anultraluminous X-ray source apart from that for NGC 1313 X-1. In commonwith NGC 1313 X-1, scaling the temperatures measured in M81 X-9 to thosecommonly seen in stellar-mass Galactic black holes at their highestobserved fluxes (kT~=1 keV) may imply that M81 X-9 harbors a black holewith a mass on the order of 103 Msolar themeasured disk component normalization and broadband luminosity implyblack hole masses on the order of 102 Msolar. Itis therefore possible that these sources harbor 103Msolar black holes accreting atLX~=0.1×LEdd. While these results do notrepresent proof that M81 X-9 harbors an intermediate-mass black hole,radio and optical observations suggest that beaming and anisotropicemission from a stellar-mass black hole are unlikely to account for theimplied luminosity. We further argue that the strength of the hardemission in these sources and well-established phenomena frequentlyobserved in stellar-mass black holes near to the Eddington limit suggestthat optically thick photospheres are unlikely to be the origin of thecool thermal emission in bright ultraluminous X-ray sources. Forcomparison to M81 X-9, we have also analyzed the previously unpublishedEPIC-pn spectrum of NGC 1313 X-1 cool disk emission is again observed,and refined spectral fit parameters and mass estimates are reported.
| A Catalog of Neighboring Galaxies
We present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its ``global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra.
| BVR photometry of the resolved dwarf galaxy Ho IX
We present BVR CCD photometry down to limiting magnitude B=23.5 mag for232 starlike objects and 11 diffuse objects in a 5.4 arcmin × 5.4arcmin field of Ho IX. The galaxy is a gas-rich irregular dwarf galaxypossibly very close to M 81, which makes it especially interesting inthe context of the evolution of satellite galaxies and the accretion ofdwarf galaxies. Investigations of Ho IX were hampered by relativelylarge contradictions in the magnitude scale between earlier studies.With our new photometry we resolved these discrepancies. The colormagnitude diagram (CMD) of Ho IX is fairly typical of a star-formingdwarf irregular, consistent with earlier results. Distance estimatesfrom our new CMD are consistent with Ho IX being very close to M 81 andtherefore being a definite member of the M 81 group, apparently in veryclose physical proximity to M 81.This work is a part of a joint project between the AstronomicalInstitute of the Ruhr-University in Bochum and the Institute ofAstronomy of the Bulgarian Academy of Sciences for the study of nearbydwarf galaxies.
| The Hα galaxy survey. I. The galaxy sample, Hα narrow-band observations and star formation parameters for 334 galaxies
We discuss the selection and observations of a large sample of nearbygalaxies, which we are using to quantify the star formation activity inthe local Universe. The sample consists of 334 galaxies across allHubble types from S0/a to Im and with recession velocities of between 0and 3000 km s-1. The basic data for each galaxy are narrowband H\alpha +[NII] and R-band imaging, from which we derive starformation rates, H\alpha +[NII] equivalent widths and surfacebrightnesses, and R-band total magnitudes. A strong correlation is foundbetween total star formation rate and Hubble type, with the strongeststar formation in isolated galaxies occurring in Sc and Sbc types. Moresurprisingly, no significant trend is found between H\alpha +[NII]equivalent width and galaxy R-band luminosity. More detailed analyses ofthe data set presented here will be described in subsequent papers.Based on observations made with the Jacobus Kapteyn Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias.The full version of Table \ref{tab3} is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/23 Reduced image datafor this survey can be downloaded fromhttp://www.astro.livjm.ac.uk/HaGS/
| Bubble Nebulae around Ultraluminous X-Ray Sources
The nature of extra-nuclear ultraluminous X-ray sources (ULX) in nearbygalaxies continues to be an enigma, since their adopted isotropichigh-energy output would surpass the Eddington limit of even the mostmassive stellar black holes. Many ultraluminous X-ray sources aresurrounded by emission nebulae that show indications of both shockionization and X-ray ionization. Relatively compact X-ray ionizednebulae can be used to independently infer the luminosities, and thus toexclude possible beaming effects into our line of sight. Largerbubble-like nebulae reach several hundred parsec diameters and provideimportant information on the formation and/or mass loss history of ULX.We point out the close relationship to microquasars and the previouslyunique SS 433 system with its radio nebula W 50.
| SINGS: The SIRTF Nearby Galaxies Survey
The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging andspectroscopic survey of 75 nearby galaxies. Its primary goal is tocharacterize the infrared emission of galaxies and their principalinfrared-emitting components, across a broad range of galaxy propertiesand star formation environments. SINGS will provide new insights intothe physical processes connecting star formation to the interstellarmedium properties of galaxies and provide a vital foundation forunderstanding infrared observations of the distant universe andultraluminous and active galaxies. The galaxy sample and observingstrategy have been designed to maximize the scientific and archivalvalue of the data set for the SIRTF user community at large. The SIRTFimages and spectra will be supplemented by a comprehensivemultiwavelength library of ancillary and complementary observations,including radio continuum, H I, CO, submillimeter, BVRIJHK, Hα,Paα, ultraviolet, and X-ray data. This paper describes the mainastrophysical issues to be addressed by SINGS, the galaxy sample and theobserving strategy, and the SIRTF and other ancillary data products.
| First results from the HI Jodrell All Sky Survey: inclination-dependent selection effects in a 21-cm blind survey
Details are presented of the HI Jodrell All Sky Survey (HIJASS). HIJASSis a blind neutral hydrogen (HI) survey of the northern sky (δ> 22°), being conducted using the multibeam receiver on theLovell Telescope (full width at half-maximum beamwidth 12 arcmin) atJodrell Bank. HIJASS covers the velocity range -3500 to 10 000 kms-1, with a velocity resolution of 18.1 km s-1 andspatial positional accuracy of ~2.5 arcmin. Thus far about 1115deg2 of sky have been surveyed. The average rms noise duringthe early part of the survey was around 16 mJy beam-1.Following the first phase of the Lovell Telescope upgrade (in 2001), therms noise is now around 13 mJy beam-1. We describe themethods of detecting galaxies within the HIJASS data and of measuringtheir HI parameters. The properties of the resulting HI-selected sampleof galaxies are described. Of the 222 sources so far confirmed, 170 (77per cent) are clearly associated with a previously catalogued galaxy. Afurther 23 sources (10 per cent) lie close (within 6 arcmin) to apreviously catalogued galaxy for which no previous redshift exists. Afurther 29 sources (13 per cent) do not appear to be associated with anypreviously catalogued galaxy. The distributions of peak flux, integratedflux, HI mass and cz are discussed. We show, using the HIJASS data, thatHI self-absorption is a significant, but often overlooked, effect ingalaxies with large inclination angles to the line of sight. Properlyaccounting for it could increase the derived HI mass density of thelocal Universe by at least 25 per cent. The effect that this will haveon the shape of the HI mass function will depend on how self-absorptionaffects galaxies of different morphological types and HI masses. We alsoshow that galaxies with small inclinations to the line of sight may alsobe excluded from HI-selected samples, since many such galaxies will haveobserved velocity widths that are too narrow for them to bedistinguished from narrow-band radio-frequency interference. This effectwill become progressively more serious for galaxies with smallerintrinsic velocity widths. If, as we might expect, galaxies with smallerintrinsic velocity widths have smaller HI masses, then compensating forthis effect could significantly steepen the faint-end slope of thederived HI mass function.
| Spectroscopy of the optical counterpart to the IXO in Holmberg 9.
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