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A Radio Nebula Surrounding the Ultraluminous X-Ray Source in NGC 5408
New radio observations of the counterpart of the ultraluminous X-raysource in NGC 5408 show for the first time that the radio emission isresolved with an angular size of 1.5"-2.0". This corresponds to aphysical size of 35-46 pc, and rules out interpretation of the radioemission as beamed emission from a relativistic jet. In addition, theradio spectral index of the counterpart is well determined from threefrequencies and found to be α=-0.8+/-0.2. The radio emission islikely to be optically thin synchrotron emission from a nebulasurrounding the X-ray source. The radio luminosity of the counterpart is3.8×1034 erg s-1 and the minimum energyrequired to power the nebula is ~1×1049 erg. Thesevalues are 2 orders of magnitude larger than in any Galactic nebulapowered by an accreting compact object.
| Chemical Properties of Star-Forming Dwarf Galaxies
Recent studies of the near-infrared (NIR) properties of dwarf irregulargalaxies (dIs) and blue compact dwarfs (BCDs) have provided improvedestimates for the NIR luminosity of old stellar populations in thesegalaxies. These can be used to derive gas fractions, and thereby toevaluate how BCDs have evolved with respect to dIs. Oxygen abundanceshave been derived for four BCDs in the Virgo Cluster from a run atGemini-North in 2003. Combining these new abundances with publishedvalues, we study the correlations among the metallicity, Ksluminosity, gas mass, baryonic mass, and gas fraction. Within errors,the two types of dwarfs appear to share a common relation between theoxygen abundance and the luminosity in Ks. The correlationbetween metallicity and the gas fraction is the same for BCDs as fordIs, indicating that BCD evolution has been similar to that of dIs.Since dIs appear to have evolved as isolated systems, the BCD bursts areunlikely to be a consequence of gas infall or merging.
| Dust Masses, PAH Abundances, and Starlight Intensities in the SINGS Galaxy Sample
Physical dust models are presented for 65 galaxies in SINGS that arestrongly detected in the four IRAC bands and three MIPS bands. For eachgalaxy we estimate (1) the total dust mass, (2) the fraction of the dustmass contributed by PAHs, and (3) the intensity of the starlight heatingthe dust grains. We find that spiral galaxies have dust propertiesresembling the dust in the local region of the Milky Way, with similardust-to-gas ratio and similar PAH abundance. The observed SEDs,including galaxies with SCUBA photometry, can be reproduced by dustmodels that do not require ``cold'' (T<~10 K) dust. The dust-to-gasratio is observed to be dependent on metallicity. In the interstellarmedia of galaxies with AO≡12+log10(O/H)>8.1, grainscontain a substantial fraction of interstellar Mg, Si, and Fe. Galaxieswith AO<8.1 and extended H I envelopes in some casesappear to have global dust-to-gas ratios that are low for their measuredoxygen abundance, but the dust-to-gas ratio in the regions whereinfrared emission is detected generally appears to be consistent with asubstantial fraction of interstellar Mg, Si, and Fe being contained indust. The PAH index qPAH, the fraction of the dust mass inthe form of PAHs, correlates with metallicity. The nine galaxies in oursample with AO<8.1 have a median qPAH=1.0%,whereas galaxies with AO>8.1 have a medianqPAH=3.55%. The derived dust masses favor a valueXCO~4×1020 cm-2 (K kms-1)-1 for the CO-to-H2 conversionfactor. Except for some starbursting systems (Mrk 33, Tol 89, NGC 3049),dust in the diffuse ISM dominates the IR power.
| Supercritically accreting stellar mass black holes as ultraluminous X-ray sources
We derive the luminosity-temperature relation for the supercriticallyaccreting black holes (BHs) and compare it to the data on ultraluminousX-ray sources (ULXs). At super-Eddington accretion rates, an outflowforms within the spherization radius. We construct the accretion discmodel accounting for the advection and the outflow, and computecharacteristic disc temperatures. The bolometric luminosity exceeds theEddington luminosity LEdd by a logarithmic factor (where isthe accretion rate in Eddington units) and the wind kinetic luminosityis close to LEdd. The apparent luminosity for the face-onobserver is 2-7 times higher because of geometrical beaming. Such anobserver has a direct view of the inner hot accretion disc, which has apeak temperature Tmax of a few keV in stellar mass BHs. Theemitted spectrum extends as a power law FE ~ E-1down to the temperature at the spherization radius . We associateTmax with a few keV spectral components and Tspwith the soft, 0.1-0.2 keV components observed in ULXs. An edge-onobserver sees only the soft emission from the extended envelope, withthe photosphere radius exceeding the spherization radius by orders ofmagnitude. The dependence of the photosphere temperature on luminosityis consistent with that observed in the super-Eddington accreting BHs SS433 and V4641 Sgr. Strong outflows combined with the large intrinsicX-ray luminosity of the central BH explain naturally the presence of thephotoionized nebulae around ULXs. An excellent agreement between themodel and the observational data strongly argues in favour of ULXs beingsupercritically accreting, stellar mass BHs similar to SS 433, butviewed close to the symmetric axis.
| Dust and Atomic Gas in Dwarf Irregular Galaxies of the M81 Group: The SINGS and THINGS View
We present observations of the dust and atomic gas phase in seven dwarfirregular galaxies of the M81 group from the Spitzer SINGS and VLATHINGS surveys. The Spitzer observations provide a first glimpse of thenature of the nonatomic ISM in these metal-poor (Z~0.1Zsolar), quiescent (SFR~0.001-0.1 Msolaryr-1) dwarf galaxies. Most detected dust emission isrestricted to H I column densities >1×1021cm-2, and almost all regions of high H I column density(>2.5×1021 cm-2) have associated dustemission. Spitzer spectroscopy of two regions in the brightest galaxies(IC 2574 and Holmberg II) show distinctly different spectral shapes andaromatic features, although the galaxies have comparable gas-phasemetallicities. This result emphasizes that the strength of the aromaticfeatures is not a simple linear function of metallicity. We estimatedust masses of ~104-106 Msolar for theM81 dwarf galaxies, resulting in an average dust-to-gas ratio(Mdust/MHI) of ~3×10-4(1.5×10-3 if only the H I that is associated with dustemission is considered); this is an order of magnitude lower than thetypical value derived for the SINGS spirals. The dwarf galaxies areunderluminous per unit star formation rate at 70 μm as compared tothe more massive galaxies in SINGS by a factor of ~2. However, theaverage 70/160 μm ratio in the sample dwarf galaxies is higher thanwhat is found in the other galaxies of the SINGS sample. This can beexplained by a combination of a lower dust content in conjunction with ahigher dust temperature in the dwarfs.
| Quasi-periodic Variability in NGC 5408 X-1
We report the discovery with XMM-Newton of quasi-periodic variability inthe 0.2-10 keV X-ray flux from the ULX NGC 5408 X-1. The average powerspectrum of all EPIC-pn data reveals a strong 20 mHz QPO with an averageamplitude (rms) of 9% and a coherenceQ≡ν0/σ~6. In a 33 ks time interval when the 20mHz QPO is strongest we also find evidence for a second QPO peak at 15mHz, the first indication of a close pair of QPOs in a ULX source. Thefrequency ratio of this QPO pair is inconsistent with 3:2 at the 3σ level but is consistent with a 4:3 ratio. A power-law noisecomponent with slope near 1.5 is also present below 0.1 Hz with evidencefor a break to a flatter slope at about 3 mHz. The source showssubstantial broadband variability, with a total amplitude (rms) of about30% in the 0.1-100 mHz frequency band. The power spectrum of hard X-rayphotons (>2 keV) shows a ``classic'' flat-topped continuum breakingto a power law with index 1.5-2. Both the break and 20 mHz QPO aredetected in the hard band, and the 20 mHz QPO is essentially at thebreak. The QPO is both strong and narrow in this band, having anamplitude (rms) of 15%, and Q~25. Both the timing and spectralproperties of NGC 5408 X-1 are strikingly reminiscent of Galactic blackholes at high inferred accretion rates, but with its characteristicfrequencies (QPO and break frequencies) scaled down by a factor of10-100. We discuss the implications of these findings for the object'smass.
| The Spitzer Spirals, Bridges, and Tails Interacting Galaxy Survey: Interaction-Induced Star Formation in the Mid-Infrared
We present Spitzer mid-infrared imaging of a sample of 35 tidallydistorted premerger interacting galaxy pairs selected from the ArpAtlas. We compare their global mid-infrared properties with those ofnormal galaxies from the SINGS Spitzer Legacy survey, and separate thedisk emission from that of the tidal features. The [8.0 μm]-[24μm], [3.6 μm]-[24 μm], and [5.8 μm]-[8.0 μm] colors ofthese optically selected interacting galaxies are redder on average thanthose of spirals, implying enhancements to the mass-normalized starformation rates (SFRs) of a factor of ~2. Furthermore, the 24 μmemission in the Arp galaxies is more centrally concentrated than that inthe spirals, suggesting that gas is being concentrated into the innerregions and fueling central star formation. No significant differencescan be discerned in the shorter wavelength Spitzer colors of the Arpgalaxies compared to the spirals, and thus these quantities are lesssensitive to star formation enhancements. No strong trend of Spitzercolor with pair separation is visible in our sample; this may be becauseour sample was selected to be tidally disturbed. The tidal featurescontribute <=10% of the total Spitzer fluxes on average. The SFRsimplied for the Arp galaxies by the Spitzer 24 μm luminosities arerelatively modest, ~1 Msolar yr-1 on average.
| An Ultraviolet-to-Radio Broadband Spectral Atlas of Nearby Galaxies
The ultraviolet-to-radio continuum spectral energy distributions arepresented for all 75 galaxies in the Spitzer Infrared Nearby GalaxiesSurvey (SINGS). A principal component analysis of the sample shows thatmost of the sample's spectral variations stem from two underlyingcomponents, one representative of a galaxy with a lowinfrared-to-ultraviolet ratio and one representative of a galaxy with ahigh infrared-to-ultraviolet ratio. The influence of several parameterson the infrared-to-ultraviolet ratio is studied (e.g., opticalmorphology, disk inclination, far-infrared color, ultraviolet spectralslope, and star formation history). Consistent with our understanding ofnormal star-forming galaxies, the SINGS sample of galaxies in comparisonto more actively star-forming galaxies exhibits a larger dispersion inthe infrared-to-ultraviolet versus ultraviolet spectral slopecorrelation. Early-type galaxies, exhibiting low star formation ratesand high optical surface brightnesses, have the most discrepantinfrared-to-ultraviolet correlation. These results suggest that the starformation history may be the dominant regulator of the broadbandspectral variations between galaxies. Finally, a new discovery showsthat the 24 μm morphology can be a useful tool for parameterizing theglobal dust temperature and ultraviolet extinction in nearby galaxies.The dust emission in dwarf/irregular galaxies is clumpy and warmaccompanied by low ultraviolet extinction, while in spiral galaxiesthere is typically a much larger diffuse component of cooler dust andaverage ultraviolet extinction. For galaxies with nuclear 24 μmemission, the dust temperature and ultraviolet extinction are relativelyhigh compared to disk galaxies.
| Elemental Abundances of Nearby Galaxies through High Signal-to-Noise Ratio XMM-Newton Observations of Ultraluminous X-Ray Sources
In this paper we examined XMM-Newton EPIC spectra of 14 ultraluminousX-ray sources (ULXs) in addition to the XMM-Newton RGS spectra of twosources (Holmberg II X-1 and Holmberg IX X-1). We determined oxygen andiron abundances of the host galaxy's ISM using K-shell (O) and L-shell(Fe) X-ray photoionization edges toward these ULXs. We found that theoxygen abundances closely matched recent solar abundances for all of oursources, implying that ULXs live in similar local environments despitethe wide range of galaxy host properties. Also, we compare the X-rayhydrogen column densities (nH) for eight ULXs with columndensities obtained from radio H I observations. The X-ray modelnH values are in good agreement with the H I nHvalues, implying that the hydrogen absorption toward the ULXs is notlocal to the source (with the exception of the source M81 XMM1). Inorder to obtain the column density and abundance values, we fitted theX-ray spectra of the ULXs with a combined power law and one of severalaccretion disk models. We tested the abundances obtained from the XSPECmodels bbody, diskbb, grad, and diskpn along with a power law, findingthat the abundances were independent of the thermal model used. Wecomment on the physical implications of these different model fits. Wealso note that very deep observations allow a breaking of the degeneracynoted by Stobbart and coworkers favoring a high-mass solution for theabsorbed grad+power-law model.
| Radio and X-ray properties of relativistic beaming models for ultraluminous X-ray sources
We calculate the broad-band radio-X-ray spectra predicted by microblazarand microquasar models for ultraluminous X-ray sources (ULXs), exploringthe possibility that their dominant power-law component is produced by arelativistic jet, even at near-Eddington mass accretion rates. We dothis by first constructing a generalized disc-jet theoretical frameworkin which some fraction of the total accretion power, Pa, isefficiently removed from the accretion disc by a magnetic torqueresponsible for jet formation. Thus, for different black hole masses,mass accretion rates and magnetic coupling strength, weself-consistently calculate the relative importance of the modified discspectrum, as well as the overall jet emission due to synchrotron andCompton processes. In general, transferring accretion power to a jetmakes the disc fainter and cooler than a standard disc at the same massaccretion rate; this may explain why the soft spectral component appearsless prominent than the dominant power-law component in most brightULXs. We show that the apparent X-ray luminosity and spectrum predictedby the microquasar model are consistent with the observed properties ofmost ULXs. We predict that the radio synchrotron jet emission is toofaint to be detected at the typical threshold of radio surveys to date.This is consistent with the high rate of non-detections over detectionsin radio counterpart searches. Conversely, we conclude that the observedradio emission found associated with a few ULXs cannot be due to beamedsynchrotron emission from a relativistic jet.
| 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.
| Multiband study of NGC7424 and its two newly discovered ultraluminous X-ray sources
We have studied the face-on, barred spiral NGC7424 (site of the rareType IIb SN2001ig) with Chandra, Gemini and the Australia TelescopeCompact Array. After giving revised X-ray colours and luminosity of thesupernova, here we focus on some other interesting sources in thegalaxy: in particular, our serendipitous discovery of two ultraluminousX-ray sources (ULXs). The brighter one (~1040 ergs-1) has a power-law-like spectrum with photon index Γ~ 1.8. The other ULX shows a spectral state transition or outburstbetween the two Chandra observations, 20-d apart. Optical data show thatthis ULX is located in a young (age ~7-10 Myr), bright complex rich withOB stars and clusters. An exceptionally bright, unresolved radio source(0.14 mJy at 4.79 GHz, implying a radio luminosity twice as high as CasA) is found slightly offset from the ULX (~80 pc). Its radio spectralindex α ~ -0.7 suggests optically thin synchrotron emission,either from a young supernova remnant or from a radio lobe powered by aULX jet. An even brighter, unresolved radio source (0.22 mJy at 4.79GHz) is found in another young, massive stellar complex, not associatedwith any X-ray sources: based on its flatter radio spectral index(α ~ -0.3), we suggest that it is a young pulsar wind nebula, afactor of 10 more radio luminous than the Crab.
| 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.
| Radio Emission on Subparsec Scales from the Intermediate-Mass Black Hole in NGC 4395
The Seyfert 1 nucleus of NGC 4395 is energized by a black hole of mass3.6×105 Msolar, making it one of only twonuclear black holes of intermediate mass, 103-106Msolar, detected in the radio regime. Building on UV andX-ray evidence for outflows from this Seyfert nucleus, the VLBI HighSensitivity Array was used at 1.4 GHz to search for extended structureon scales greater than 5 mas (0.1 pc). Elongated emission wasdiscovered, extending over 15 mas (0.3 pc) and suggesting an outflow onsubparsec scales from this intermediate-mass black hole. The Seyfertnucleus is located at the center of an elliptical star cluster, and theelongation position angle of the subparsec radio structure is only19° from the star cluster's minor axis.
| The K Luminosity-Metallicity Relation for Dwarf Galaxies and the Tidal Dwarf Galaxies in the Tails of HCG 31
We determine a K-band luminosity-metallicity (L-Z) relation for dwarfirregular galaxies over a large range of magnitudes,-20.5
| A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds
We present a Far Ultraviolet Spectroscopic Explorer survey of highlyionized high-velocity clouds (HVCs) in 66 extragalactic sight lines with(S/N)1030>8. We search the spectra for high-velocity (100km s-1<|vLSR|<400 km s-1) O VIabsorption and find a total of 63 absorbers, 16 with 21 cm emitting H Icounterparts and 47 ``highly ionized'' absorbers without 21 cm emission.The highly ionized HVC population is characterized by =38+/-10 km s-1 and =13.83+/-0.36, with negative-velocity clouds generally found atl<180deg and positive-velocity clouds found atl>180deg. Eleven of these highly ionized HVCs arepositive-velocity wings (broad O VI features extending asymmetrically tovelocities of up to 300 km s-1). We find that 81% (30 of 37)of highly ionized HVCs have clear accompanying C III absorption, and 76%(29 of 38) have accompanying H I absorption in the Lyman series. Wepresent the first (O VI selected) sample of C III and H I absorptionline HVCs and find =30+/-8 km s-1,logNa(C III) ranges from <12.5 to >14.4, =22+/-5 km s-1, and log Na(H I) ranges from<14.7 to >16.9. The lower average width of the high-velocity H Iabsorbers implies the H I lines arise in a separate, lower temperaturephase than the O VI. The ratio Na(C III)/Na(O VI)is generally constant with velocity in highly ionized HVCs, suggestingthat at least some C III resides in the same gas as the O VI.Collisional ionization equilibrium models with solar abundances canexplain the O VI/C III ratios for temperatures near1.7×105 K; nonequilibrium models with the O VI ``frozenin'' at lower temperatures are also possible. Photoionization models arenot viable since they underpredict O VI by several orders of magnitude.The presence of associated C III and H I strongly suggests the highlyionized HVCs are not formed in the hotter plasma that gives rise to OVII and O VIII X-ray absorption. We find that the shape of the O VIpositive-velocity wing profiles is well reproduced by a radiativelycooling, vertical outflow moving with ballistic dynamics, withT0=106 K, n0~2×10-3cm-3, and v0~250 km s-1. However, theoutflow has to be patchy and out of ionization equilibrium to explainthe sky distribution and the simultaneous presence of O VI, C III, and HI. We found that a spherical outflow can produce high-velocity O VIcomponents (as opposed to the wings), showing that the possible range ofoutflow model results is too broad to conclusively identify whether ornot an outflow has left its signature in the data. An alternative model,supported by the similar multiphase structure and similar O VIproperties of highly ionized and 21 cm HVCs, is one where the highlyionized HVCs represent the low N(H I) tail of the HVC population, withthe O VI formed at the interfaces around the embedded H I cores.Although we cannot rule out the possibility that some highly ionizedHVCs exist in the Local Group or beyond, we favor a Galactic origin.This is based on the recent evidence that both H I HVCs and themillion-degree gas detected in X-ray absorption are Galactic phenomena.Since the highly ionized HVCs appear to trace the interface betweenthese two Galactic phases, it follows that highly ionized HVCs areGalactic themselves. However, the nondetection of high-velocity O VI inhalo star spectra implies that any Galactic high-velocity O VI exists atz distances beyond a few kpc.
| Mid-Infrared Spectral Diagnostics of Nuclear and Extranuclear Regions in Nearby Galaxies
Mid-infrared diagnostics are presented for a large portion of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archivaldata from ISO and Spitzer. The SINGS data set includes low- andhigh-resolution spectral maps and broadband imaging in the infrared forover 160 nuclear and extranuclear regions within 75 nearby galaxiesspanning a wide range of morphologies, metallicities, luminosities, andstar formation rates. Our main result is that these mid-infrareddiagnostics effectively constrain a target's dominant power source. Thecombination of a high-ionization line index and PAH strength serves asan efficient discriminant between AGNs and star-forming nuclei,confirming progress made with ISO spectroscopy on starbursting andultraluminous infrared galaxies. The sensitivity of Spitzer allows us toprobe fainter nuclear and star-forming regions within galaxy disks. Wefind that both star-forming nuclei and extranuclear regions stand apartfrom nuclei that are powered by Seyfert or LINER activity. In fact, weidentify areas within four diagnostic diagrams containing >90%Seyfert/LINER nuclei or >90% H II regions/H II nuclei. We also findthat, compared to starbursting nuclei, extranuclear regions typicallyseparate even further from AGNs, especially for low-metallicityextranuclear environments. In addition, instead of the traditionalmid-infrared approach to differentiating between AGNs and star-formingsources that utilizes relatively weak high-ionization lines, we showthat strong low-ionization cooling lines of X-ray-dominated regions like[Si II] 34.82 μm can alternatively be used as excellentdiscriminants. Finally, the typical target in this sample showsrelatively modest interstellar electron density (~400 cm-3)and obscuration (AV~1.0 mag for a foreground screen),consistent with a lack of dense clumps of highly obscured gas and dustresiding in the emitting regions.
| The environment of young massive clusters
We observed a sample of Blue Dwarf Galaxies in the Ks (2.2 μm) andLÕ (3.7 μm) IR bands at the ESO VLT with ISAAC. Thepurpose of the observations was to study the population of young massiveclusters and the conditions under which they are formed. The samplegalaxies included: Tol 1924-416, Tol 35, Pox 36, UM 462, He 2-10, II Zw40, Tol 3, NGC 1705, NGC 5408, IC 4662, NGC 5253. They were selected tohave evidence for star formation and firm detection by IRAS. Allgalaxies observed turned to be very rich of young massive clusters inKs. Only few clusters, about 8%, showed counterparts in LÕ.Most L' sources can be associated to radio thermal sources, with theonly exception of the NGC 1705's one. For two galaxies, NGC 5408 and IC4662, we derived the cluster luminosity functions finding themconsistent with a power law of index about -2. We compared the numbersand luminosities of the clusters with the star formation rate of thehost galaxy and could not find any evidence of a relation.
| An ultraluminous X-ray microquasar in NGC5408?
We studied the radio source associated with the ultraluminous X-raysource in NGC5408 (LX ~ 1040ergs-1).The radio spectrum is steep (index ~ -1), consistent with optically thinsynchrotron emission, not with flat-spectrum core emission. Its fluxdensity (~0.28 mJy at 4.8 GHz, at a distance of 4.8 Mpc) was the same inthe March 2000 and December 2004 observations, suggesting steadyemission rather than a transient outburst. However, it is orders ofmagnitude higher than expected from steady jets in stellar-massmicroquasar. Based on its radio flux and spectral index, we suggest thatthe radio source is either an unusually bright supernova remnant, or,more likely, a radio lobe powered by a jet from the black hole (BH).Moreover, there is speculative evidence that the source is marginallyresolved with a radius ~30 pc. A faint HII region of similar sizeappears to coincide with the radio and X-ray sources, but its ionizationmechanism remains unclear. Using a self-similar solution for theexpansion of a jet-powered electron-positron plasma bubble, in theminimum-energy approximation, we show that the observed flux and(speculative) size are consistent with an average jet power ~ 7 ×1038ergs-1 ~ 0.1LX ~0.1LEdd, an age ~105 yr, a current velocity ofexpansion ~80 km s-1. We briefly discuss the importance ofthis source as a key to understand the balance between luminosity andjet power in accreting BHs.
| A catalogue of ultra-luminous X-ray source coincidences with FIRST radio sources
Aims.We search for ultra luminous X-ray source (ULXs) radio counterpartslocated in nearby galaxies in order to constrain their physicalnature. Methods: .Our work is based on a systematiccross-identification of the most recent and extensive available ULXcatalogues and archival radio data. Results: .A catalogue of 70positional coincidences is reported. Most of them are located within thegalaxy nucleus. Among them, we find 11 new cases of non-nuclear ULXsources with possibly associated radio emission.
| XMM-Newton observations of the brightest ultraluminous X-ray sources
We present an analysis of 13 of the best quality ultraluminous X-raysource (ULX) data sets available from XMM-Newton European Photon ImagingCamera (EPIC) observations. We utilize the high signal-to-noise in theseULX spectra to investigate the best descriptions of their spectral shapein the 0.3-10keV range. Simple models of an absorbed power law ormulticolour disc blackbody prove inadequate at describing the spectra.Better fits are found using a combination of these two components, withboth variants of this model - a cool (~0.2keV) disc blackbody plus hardpower-law continuum, and a soft power-law continuum, dominant at lowenergies, plus a warm (~1.7keV) disc blackbody - providing good fits to8/13 ULX spectra. However, by examining the data above 2keV, we findevidence for curvature in the majority of data sets (8/13 with at leastmarginal detections), inconsistent with the dominance of a power law inthis regime. In fact, the most successful empirical description of thespectra proved to be a combination of a cool (~0.2keV) classic blackbodyspectrum, plus a warm disc blackbody that fits acceptably to 10/13 ULXs.The best overall fits are provided by a physically self-consistentaccretion disc plus Comptonized corona model (DISKPN + EQPAIR), whichfits acceptably to 11/13 ULXs. This model provides a physicalexplanation for the spectral curvature, namely that it originates in anoptically thick corona, though the accretion disc photons seeding thiscorona still originate in an apparently cool disc. We note similaritiesbetween this fit and models of Galactic black hole binaries at highaccretion rates, most notably the model of Done & Kubota. In thisscenario the inner disc and corona become energetically coupled at highaccretion rates, resulting in a cooled accretion disc and opticallythick corona. We conclude that this analysis of the best spectral datafor ULXs shows it to be plausible that the majority of the populationare high accretion rate stellar-mass (perhaps up to 80Msolar)black holes, though we cannot categorically rule out the presence oflarger, ~1000-Msolar intermediate-mass black holes (IMBHs) inindividual sources with the current X-ray data.
| Stellar clusters in dwarf galaxies
We present new observations in the Ks (2.2 μm) and L' (3.7μm) infrared bands of a sample of blue dwarf galaxies with the largeraim of studying the population of massive stellar clusters, theoccurrence of dust-embedded stellar clusters, and their properties. AllKs images show a rich population of clusters, but only asmall fraction of them is bright in L'. Most L' sources have radiocounterparts. We derived the luminosity function in Ks forthe galaxies IC 4661 and NGC 5408, finding both to be consistent withthose of similar galaxies. We also compared the number of clusters andtheir luminosities with the star-formation rate of the host galaxies andfound no compelling evidence of correlation. We conclude that youngclusters and embedded clusters are a common feature of blue dwarfgalaxies and possibly of galaxies in general, we suggest that theiroccurrence is due to purely statistical effects rather than a phenomenonrelated to specific physical conditions. In this sense we expect theseobjects to be abundant at high red-shift.
| Black Hole Mass of the Ultraluminous X-Ray Source M82 X-1
We report the first clear evidence for the simultaneous presence of alow-frequency break and a QPO in the fluctuation power spectrum of awell-known ultraluminous X-ray source (ULX) in M82 using long XMM-Newtonobservations. The break occurs at a frequency of34.2+6-3 mHz. The QPO has a centroid atνQPO=114.3+/-1.5 mHz, a coherenceQ≡νQPO/ΔνFWHM~=3.5, and anamplitude (rms) of 19% in the 2-10 keV band. The power spectrum isapproximately flat below the break frequency and then falls off abovethe break frequency as a power law with the QPO superposed. This form ofthe power spectrum is characteristic of the Galactic X-ray binaries(XRBs) in their high or intermediate states. M82 X-1 was likely in anintermediate state during the observation. The EPIC pn spectrum is welldescribed by a model comprising an absorbed power law (Γ~2) and aniron line at ~6.6 keV with a width σ~0.2 keV and an equivalentwidth of ~180 eV. Using the well-established correlations between thepower and energy spectral parameters for XRBs, we estimate a black holemass for M82 X-1 in the range of ~25-520 Msolar, includingsystematic errors that arise due to the uncertainty in the calibrationof the photon spectral index versus QPO frequency relation.
| Oxygen and Nitrogen in Leo A and GR 8
We present elemental abundances for multiple H II regions in Leo A andGR 8 obtained from long-slit optical spectroscopy of these two nearbylow-luminosity dwarf irregular galaxies. As expected from theirluminosities, and in agreement with previous observations, the derivedoxygen abundances are extremely low in both galaxies. Highsignal-to-noise ratio (S/N) observations of a planetary nebula in Leo Ayield 12+log(O/H)=7.30+/-0.05 semiempirical calculations of the oxygenabundance in four H II regions in Leo A indicate12+log(O/H)=7.38+/-0.10. These results confirm that Leo A has one of thelowest ISM metal abundances of known nearby galaxies. Based on resultsfrom two H II regions with high S/N measurements of the weak [O III]λ4363 line, the mean oxygen abundance of GR 8 is12+log(O/H)=7.65+/-0.06 using ``empirical'' and ``semiempirical''methods, similar abundances are derived for six other GR 8 H II regions.Similar to previous results in other low-metallicity galaxies, the meanlog(N/O)=-1.53+/-0.09 for Leo A and -1.51+/-0.07 for GR 8. There is noevidence of significant variations in either O/H or N/O in the H IIregions. The metallicity-luminosity relation for nearby (D<5 Mpc)dwarf irregular galaxies with measured oxygen abundances has a meancorrelation of 12+log(O/H)=5.67MB-0.151MB, with adispersion in oxygen about the relationship of σ=0.21. Theseobservations confirm that gas-rich, low-luminosity galaxies haveextremely low elemental abundances in the ionized gas phase of theirinterstellar media. Although Leo A has one of the lowest metalabundances of known nearby galaxies, detection of tracers of an olderstellar population (RR Lyrae variable stars, horizontal branch stars,and a well-populated red giant branch) indicate that it is not a newlyformed galaxy, as has been proposed for some other similarlow-metallicity star-forming galaxies.
| 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.
| 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.
| M74 X-1 (CXOU J013651.1+154547): An Extremely Variable Ultraluminous X-Ray Source
Ultraluminous X-ray sources (ULXs) have been described variously as themost luminous normal X-ray binaries, hypernovae, and``intermediate-mass'' black holes with masses of hundreds to thousandsof solar masses. We present results on M74 X-1 (CXOU J013651.1+154547),a ULX in the nearby spiral galaxy M74 (NGC 628), from observations byChandra and XMM-Newton. M74 X-1 is a persistent source that exhibitsextreme variability and changes in spectral state on timescales ofseveral thousand seconds. Its variability timescales and behaviorresemble some Galactic microquasars. This suggests that the emissionmechanism may be related to relativistically beamed jets and that M74X-1 could be an extragalactic ``microblazar''-a microquasar whose jetaxis is aligned with our line of sight. We also note that its spectrumis consistent with the presence of a low-temperature disk blackbodycomponent, which, assuming it is due to radiation from an accretiondisk, could indicate that M74 X-1 contains an intermediate-mass blackhole.
| A radio monitoring survey of ultra-luminous X-ray sources
We present the results of a radio monitoring campaign to search forradio emission from nearby ultra-luminous X-ray sources (ULXs). Thesesources are bright off-nuclear X-ray point sources with luminositiesexceeding LX > 1039 erg s-1. Awell-defined sample of the 9 nearest ULXs has been monitored eight timesover 5 months with the Very Large Array in A and B configuration. Ourlimiting sensitivity is ≈0.15 mJy (4σ) for radio flares and≈60 μJy for continuous emission. In M 82 two ULXs seem to havecoincident compact radio sources, which are probably supernova remnants.No continuous or flaring radio emission has been detected from any otherULX. Thus, ULXs do not generally emit steady-state radio emission aboveradio powers of 1.5 × 1017 W/Hz. The non-detections ofthe continuous emission are consistent with beamed or unbeamed radioemission from accreting black holes of ≤ 103 Mȯ based on the radio/X-ray correlation. Other publishedradio detections (M 82, NGC 5408) are also discussed in this context.Both detections are significantly above our detection limit. If ULXshave flaring radio emission above 4 × 1017 W/Hz we cangive an upper limit on the duty cycle of the flares of 6%. This upperlimit is in agreement with the observed number of flares in Galacticradio transients. Additionally we present a yet unreported radio doublestructure in the nearby low-luminosity AGN NGC 4736.
| Radio Emission Associated with the Ultraluminous X-Ray Source in Holmberg II
We report the detection of radio emission coincident with theultraluminous X-ray source (ULX) in Holmberg II. The radio emission isdiffuse and resolved, covering an area ~60×40 pc in extent andwell matched to the recently discovered He II nebula surrounding theX-ray source. Comparison of the radio and optical properties of thisextended radio emission argue against its association with either an HII region or a supernova remnant. This is additional evidence that thisULX is not powered by a stellar mass object whose emission isrelativistically beamed toward the observer, and thus is either asuper-Eddington source or an intermediate-mass black hole as suggestedby optical observations. Implications of this result to future andexisting radio studies of ULXs are discussed.
| Light and Motion in the Local Volume
Using high-quality data on 149 galaxies within 10 Mpc, I find nocorrelation between luminosity and peculiar velocity at all. There is nounequivocal sign on scales of 1-2 Mpc of the expected gravitationaleffect of the brightest galaxies, in particular infall toward groups, orof infall toward the supergalactic plane on any scale. Either darkmatter is not distributed in the same way as luminous matter in thisregion, or peculiar velocities are not due to fluctuations in mass. Thesensitivity of peculiar velocity studies to the background model ishighlighted.
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