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The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) Program. III. Optical Spectroscopy
We present the catalog of 477 spectra from the SerendipitousExtragalactic X-ray Source Identification (SEXSI) program, a surveydesigned to probe the dominant contributors to the 2-10 keV cosmic X-raybackground. Our survey covers 1 deg2 of sky to 2-10 keVfluxes of 1×10-14 ergs cm-2 s-1,and 2 deg2 for fluxes of 3×10-14 ergscm-2 s-1. Our spectra reach to R-band magnitudesof <~24 and have produced identifications and redshifts for 438 hardX-ray sources. Typical completeness levels in the 27 Chandra fieldsstudied are 40%-70%. The vast majority of the 2-10 keV selected sampleare active galactic nuclei (AGNs) with redshifts between 0.1 and 3; ourhighest redshift source lies at z=4.33. We find that few sources atz<1 have high X-ray luminosities, reflecting a dearth of high-mass,high-accretion-rate sources at low redshift, a result consistent withother recent wide-area surveys. We find that half of our sources showsignificant obscuration, with NH>1022cm-2, independent of unobscured luminosity. We classify 168sources as emission-line galaxies; all are X-ray-luminous(LX>1041 ergs s-1) objects withoptical spectra lacking both high-ionization lines and evidence of anonstellar continuum. The redshift distribution of these emission-linegalaxies peaks at a significantly lower redshift than does that of thesources we spectroscopically identify as AGNs. We conclude that few ofthese sources, even at the low-luminosity end, can be powered bystarburst activity. Stacking spectra for a subset of these sources in asimilar redshift range, we detect [Ne V] λ3426 emission, a clearsignature of AGN activity, confirming that the majority of these objectsare Seyfert 2 galaxies in which the high-ionization lines are diluted bystellar emission. We find a total of 33 objects lacking broad lines intheir optical spectra that have quasar X-ray luminosities(LX>1044 ergs s-1), the largestsample of such objects identified to date. In addition, we explore 17AGNs associated with galaxy clusters and find that the cluster-memberAGN sample has a lower fraction of broad-line AGNs than does thebackground sample.The majority of data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andNASA. The Observatory was made possible by the generous financialsupport of the W. M. Keck Foundation.
| Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
We estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion.
| Spitzer and JCMT Observations of the Active Galactic Nucleus in the Sombrero Galaxy (NGC 4594)
We present Spitzer 3.6-160 μm images, Spitzer mid-infrared spectra,and JCMT SCUBA 850 μm images of the Sombrero Galaxy (NGC 4594), an Sagalaxy with a 109 Msolar low-luminosity activegalactic nucleus (AGN). The brightest infrared sources in the galaxy arethe nucleus and the dust ring. The spectral energy distribution of theAGN demonstrates that, while the environment around the AGN is aprominent source of mid-infrared emission, it is a relatively weaksource of far-infrared emission, as had been inferred for AGNs inprevious research. The weak nuclear 160 μm emission and thenegligible polycyclic aromatic hydrocarbon emission from the nucleusalso implies that the nucleus is a site of only weak star formationactivity and the nucleus contains relatively little cool interstellargas needed to fuel such activity. We propose that this galaxy may berepresentative of a subset of low-ionization nuclear emission regiongalaxies that are in a quiescent AGN phase because of the lack of gasneeded to fuel circumnuclear star formation and Seyfert-like AGNactivity. Surprisingly, the AGN is the predominant source of 850 μmemission. We examine the possible emission mechanisms that could giverise to the 850 μm emission and find that neither thermal dustemission, CO line emission, bremsstrahlung emission, nor the synchrotronemission observed at radio wavelengths can adequately explain themeasured 850 μm flux density by themselves. The remainingpossibilities for the source of the 850 μm emission include acombination of known emission mechanisms, synchrotron emission that isself-absorbed at wavelengths longer than 850 μm, or unidentifiedspectral lines in the 850 μm band.
| Magnetic fields in halos of spiral galaxies
Observations of magnetic fields in halos of edge-on disk galaxies arediscussed in relation to the interstellar disk-halo interface in diskgalaxies. The distribution of extra-planar diffuse ionized gascorrelates on local and global scales with cosmic rays and magneticfields as inferred from observations of the non-thermal radio continuumradiation and its polarisation. From the polarisation a large-scale andwell-ordered magnetic field in these gaseous halos can be deduced. Forseveral objects a significant poloidal component of the halo field islikely. These observations indicate the presence of physical processeswhich generate and maintain magnetic fields on galactic scales. Theimportance of differential rotation of the gaseous halos for suchprocesses is briefly discussed.
| 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.
| Radio polarization and sub-millimeter observations of the Sombrero galaxy (NGC 4594). Large-scale magnetic field configuration and dust emission
We observed the nearby early-type spiral galaxy NGC 4594 (M 104,Sombrero galaxy) with the Very Large Array at 4.86 GHz, with theEffelsberg 100-m telescope at 8.35 GHz as well as with the HeinrichHertz Telescope at 345 GHz in radio continuum. The 4.86 and 8.35 GHzdata contain polarization information and hence information about themagnetic fields: we detected a large-scale magnetic field which is toour knowledge the first detection of a large-scale magnetic field in anSa galaxy in the radio range. The magnetic field orientation in M 104 ispredominantly parallel to the disk but has also vertical components atlarger z-distances from the disk. This field configuration is typicalfor normal edge-on spiral galaxies. The 345 GHz data pertain to the colddust content of the galaxy. Despite the optical appearance of the objectwith the huge dust lane, its dust content is smaller than that of morelate-type spirals.
| The multi-phase gaseous halos of star forming late-type galaxies. I. XMM-Newton observations of the hot ionized medium
This study presents first results from an X-ray mini-survey carried outwith XMM-Newton to investigate the diffuse Hot Ionized Medium in thehalos of nine nearby star-forming edge-on spiral galaxies. Diffusegaseous X-ray halos are detected in eight of our targets, covering awide range of star formation rates from quiescent to starburst cases.For four edge-on spiral galaxies, namely NGC 3044, NGC 3221, NGC 4634,and NGC 5775, we present the first published high resolution/sensitivitydetections of extended soft X-ray halos. EPIC X-ray contour mapsoverlaid onto Hα imaging data reveals that in all cases thepresence of X-ray halos is correlated with extraplanar Diffuse IonizedGas. Moreover, these halos are also associated with non-thermal cosmicray halos, as evidenced by radio continuum observations. SupplementalUV-data obtained with the OM-telescope at 210 nm show Diffuse IonizedGas to be well associated with UV emission originating in the underlyingdisk. Beside NGC 891, NGC 4634 is the second non-starburst galaxy with adiffuse soft X-ray halo (|z|≤ 4 kpc). In case of NGC 3877, for whichwe also present the first high resolution X-ray imaging data, no haloemission is detectable. EPIC pn spectra (0.3-12 keV) of the diffuseX-ray emission are extracted at different offset positions from thedisk, giving evidence to a significant decrease of gas temperatures,electron densities, and gas masses with increasing distance to theplane. A comparison between dynamical and radiative cooling time scalesimplies that the outflow in all targets is likely to be sustained. Wefind very strong indications that spatially correlated multi-phasegaseous halos are created by star forming activity in the disk plane. Ina forthcoming paper, we will present multi-frequency luminosityrelations and evaluate key parameters which might trigger the formationof multi-phase galaxy halos.
| Some astronomical niches with 3D spectroscopy
An overview of some of the most interesting results obtained with theuse of 3D spectrometers working in 4m-class telescopes is given with thepurpose of taking advantage of those experiences in the definition ofscientific programs for telescopes of larger diameter as the GTC.
| Astrophysical magnetic fields and nonlinear dynamo theory
Electronic Article Available from Elsevier Science.
| Global perturbation configurations in a composite disc system with an isopedic magnetic field
We construct stationary global configurations of both aligned andunaligned logarithmic spiral perturbations in a composite disc system ofstellar and isopedically magnetized gaseous singular isothermal discs(SIDs) coupled by gravity. Earlier models are generalized to a moregeneral theoretical framework. The thin gaseous SID is threaded acrossby a vertical magnetic field Bz with a constant ratio of thesurface gas mass density to Bz. In reference to SID models ofShu & Li, Shu et al., Lou & Shen, Lou & Zou, Shen and Liu& Lou, there exist two classes of stationary magnetohydrodynamic(MHD) solutions with in-phase and out-of-phase density perturbationshere. Relevant parameter regimes are explored numerically. For bothaligned and unaligned cases with azimuthal periodicities |m| >= 2 (mis an integer), there may be two, one and no solution situations,depending on the chosen parameters. For the transition criteria from anaxisymmetric equilibrium to aligned secular bar-like instabilities, thecorresponding ratio can be much lower than the oft-quoted value of ,where is the total rotational kinetic energy and is the totalgravitational potential energy plus the magnetic energy. The ratios forthe two sets of solutions in different ranges are separated by m/(4m+4). For the unaligned cases, we study marginal stabilities foraxisymmetric (m= 0) and non-axisymmetric (m≠ 0) disturbances. Byincluding the gravitational influence of an axisymmetric dark matterhalo on the background, the case of a composite partial magnetizedsingular isothermal discs (MSID) system is also examined. The globalanalytical solutions and their properties are valuable for testing andbenchmarking numerical MHD codes. For astrophysical applications tolarge-scale galactic dynamics, our model analysis contains morerealistic elements and offers useful insights into the structures anddynamics of disc galaxies consisting of stars and magnetizedinterstellar medium (ISM). In particular, in the presence of star burstactivities, supernovae, hypernovae, superbubbles etc., our open magneticfield geometry in disc galaxies bears strong implications oncircumnuclear and spiral galactic winds.
| The dynamics and high-energy emission of conductive gas clouds in supernova-driven galactic superwinds
Superwinds from starburst galaxies are multiphase outflows that sweep upand incorporate ambient galactic disc and halo gas. The interaction ofthis denser material with the more diffuse hot wind gas is thought togive rise to the OVI emission and absorption in the far ultraviolet(FUV) and the soft thermal X-ray emission observed in superwinds. Inthis paper, we present high-resolution hydrodynamical models of warmionized clouds embedded in a superwind, and compare the OVI and softX-ray properties to the existing observational data. These modelsinclude thermal conduction, which we show plays an important role inshaping both the dynamics and radiative properties of the resultingwind/cloud interaction. Heat conduction stabilizes the cloud byinhibiting the growth of Kelvin-Helmholtz and Rayleigh-Taylorinstabilities, and also generates a shock wave at the cloud's surfacethat compresses the cloud. This dynamical behaviour influences theobservable properties. We find that while OVI emission and absorptionalways arises in cloud material at the periphery of the cloud, most ofthe soft X-ray arises in the region between the wind bow shock and thecloud surface, and probes either wind or cloud material depending on thestrength of conduction and the relative abundances of the wind withrespect to the cloud. In general, only a small fraction (<~1 percent) of the wind mechanical energy intersecting a cloud is radiatedaway at ultraviolet (UV) and X-ray wavelengths, with more wind energygoing into accelerating the cloud. Clouds in relatively slow cool windsradiate a larger fraction of their energy, which are inconsistent withobservational constraints. Models with heat conduction at Spitzer-levelsare found to produce observational properties closer to those observedin superwinds than models with no thermal conduction, in particular, interms of the OVI to X-ray luminosity ratio, but cloud life times areuncomfortably short (<~1 Myr) compared to the dynamical ages of realwinds. We experimented with reducing the thermal conductivity for oneset of model parameters, and found that even when we reduced conductionby a factor of 25 that the simulations retained the beneficialhydrodynamical stability and low OVI to X-ray luminosity ratio found inthe Spitzer-level conductive models, while also having reducedevaporation rates. Although more work is required to simulate clouds forlonger times and to investigate cloud acceleration and thermalconduction at sub-Spitzer levels in a wider range of models, we concludethat thermal conduction can no longer be ignored in superwinds.
| Dust in spiral galaxies: global properties
We present and analyse high-quality Submillimetre Common-User BolometerArray (SCUBA) 850- and 450-μm images of 14 local spiral galaxies,including the detection of dust well out into the extended disc in manycases. We use these data in conjunction with published far-infrared fluxdensities from IRAS and ISO, and millimetre-wave measurements fromground-based facilities to deduce the global properties of the dust inthese galaxies, in particular temperature and mass. We find that simpletwo-temperature greybody models of fixed dust emissivity index β= 2and with typical temperatures of 25 < Twarm < 40 K and10 < Tcold < 20 K provide good fits to the overallspectral energy distributions. The dust mass in the cold componentcorrelates with the mass in atomic hydrogen and the mass in the warmcomponent correlates with the mass in molecular hydrogen. These resultsthus fit the simple picture in which the cold dust is heatedpredominantly by the interstellar radiation field, while the hot dust isheated predominantly by OB stars in more active regions, although weargue that there is some mixing. The mean gas-to-dust mass ratio is 120+/- 60, very similar to that found within our own galaxy and roughly afactor of 10 lower than that derived from IRAS data alone. Thegas-to-dust mass ratios in the warm, molecular component are on averagehigher than those in the cold, atomic component. We compare ourmodelling results with similar results for more luminous spiral galaxiesselected at far-infrared wavelengths by the SCUBA Local Universe GalaxySurvey. We find that whilst the total dust mass distributions of the twosamples are indistinguishable, they have significantly different dusttemperature distributions in both the warm and cold components. Wesuggest that this difference might be related to the level of starformation activity in these systems, with the more active galaxieshaving more intense interstellar radiation fields and higher dusttemperatures.
| An Analytic Model of Galactic Winds and Mass Outflows
Galactic winds and mass outflows are observed both in nearby starburstgalaxies and in high-redshift star-forming galaxies. We develop a simpleanalytic model to understand the observed superwind phenomenon with adiscussion of the model uncertainties. Our model is built upon the modelof McKee & Ostriker for the interstellar medium. It allows one topredict how properties of a superwind, such as wind velocity and massoutflow rate, are related to properties of its star-forming host galaxy,such as size, gas density and star formation rate. The model predicts athreshold of star formation rate density for the generation ofobservable galactic winds. Galaxies with more concentrated starformation activities produce superwinds with higher velocities. Thepredicted mass outflow rates are comparable to (or slightly larger than)the corresponding star formation rates. We apply our model to both localstarburst galaxies and high-redshift Lyman break galaxies, and find itspredictions to be in good agreement with current observations. Our modelis simple and so can be easily incorporated into numerical simulationsand semi-analytical models of galaxy formation.
| Galactic Winds
Galactic winds are the primary mechanism by which energy and metals arerecycled in galaxies and are deposited into the intergalactic medium.New observations are revealing the ubiquity of this process,particularly at high redshift. We describe the physics behind thesewinds, discuss the observational evidence for them in nearbystar-forming and active galaxies and in the high-redshift universe, andconsider the implications of energetic winds for the formation andevolution of galaxies and the intergalactic medium. To inspire futureresearch, we conclude with a set of observational and theoreticalchallenges.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| The Serendipitous Extragalactic X-Ray Source Identification (SEXSI) Program. II. Optical Imaging
The Serendipitous Extragalactic X-ray Source Identification (SEXSI)Program is designed to expand significantly the sample of identifiedextragalactic hard X-ray sources at intermediate fluxes,10-13ergscm-2s-1<~S2-10keV<10-15ergscm-2s-1.SEXSI, which includes sources derived from more than 2 deg2of Chandra images, provides the largest hard X-ray-selected sample yetstudied, offering an essential complement to the Chandra Deep Fields(total area ~0.2 deg2). In this paper we describe R-bandoptical imaging of the SEXSI fields from the Palomar, MDM, and Keckobservatories. We have identified counterparts or derived flux limitsfor nearly 1000 hard X-ray sources. Using the optical images, we deriveaccurate source positions. We investigate correlations between opticaland X-ray flux, and optical flux and X-ray hardness ratio. We also studythe density of optical sources surrounding X-ray counterparts, as wellas the properties of optically faint, hard X-ray sources.
| 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.
| A Chandra X-Ray Investigation of the Violent Interstellar Medium: From Dwarf Starbursts to Ultraluminous Infrared Galaxies
We have analyzed observations with the Chandra X-Ray Observatory of thediffuse emission by hot gas in seven dwarf starburst galaxies, sixedge-on starburst galaxies, and nine ultraluminous infrared galaxies.These systems cover ranges of ~104 in X-ray luminosity, andseveral thousand in star formation rate and K-band luminosity (a proxyfor stellar mass). Despite this range in fundamental parameters, we findthat the properties of the diffuse X-ray emission are very similar inall three classes of starburst galaxies. The spectrum of the diffuseemission is well fitted by thermal emission from gas with kT~0.25-0.8keV and with several times solar abundance ratios of α-elements toFe. The ratio of the thermal X-ray to far-infrared luminosity is roughlyconstant, as is the characteristic surface brightness of the diffuseX-ray emission. The size of the diffuse X-ray source increasessystematically with both far-infrared and K-band luminosity. All threeclasses show strong morphological relationships between the regions ofhot gas probed by the diffuse X-ray emission and the warm gas probed byoptical line emission. These findings suggest that the same physicalmechanism is producing the diffuse X-ray emission in the three types ofstarbursts. These results are consistent with that mechanism beingshocks driven by a galactic ``superwind,'' which is powered by thekinetic energy collectively supplied by stellar winds and supernovae inthe starburst.
| EGRET Upper Limits and Stacking Searches of Gamma-Ray Observations of Luminous and Ultraluminous Infrared Galaxies
We present a stacking analysis of EGRET γ-ray observations at thepositions of luminous and ultraluminous infrared galaxies. The latterwere selected from the recently presented HCN survey, which is thoughtto contain the most active star-forming regions of the universe.Different sorting criteria are used, and since there is no positivecollective detection of γ-ray emission from these objects, wedetermined both collective and individual upper limits. The uppermostexcess we find appears in the case of ULIRGs ordered by redshift, at avalue of 1.8 σ.
| A Study of Edge-On Galaxies with the Hubble Space Telescope Advanced Camera for Surveys. II. Vertical Distribution of the Resolved Stellar Population
We analyze the vertical distribution of the resolved stellar populationsin six low-mass (Vmax=67-131 km s-1), edge-on,spiral galaxies observed with the Hubble Space Telescope Advanced Camerafor Surveys. In each galaxy we find evidence for an extraplanar stellarcomponent extending up to 15 scale heights (3.5 kpc) above the plane,with a scale height typically twice that of two-dimensional fits toKs-band Two Micron All Sky Survey images. We analyze thevertical distribution as a function of stellar age by tracking changesin the color-magnitude diagram. The young stellar component(<~108 yr) is found to have a scale height larger than theyoung component in the Milky Way, suggesting that stars in theselow-mass galaxies form in a thicker disk. We also find that the scaleheight of a stellar population increases with age, with youngmain-sequence stars, intermediate-age asymptotic giant branch stars, andold red giant branch (RGB) stars having successively larger scaleheights in each galaxy. This systematic trend indicates that diskheating must play some role in producing the extraplanar stars. Weconstrain the rate of disk heating using the observed trend betweenscale height and stellar age and find that the observed heating ratesare dramatically smaller than in the Milky Way. The color distributionsof the RGB stars well above the midplane indicate that the extendedstellar components we see are moderately metal-poor, with peakmetallicities around [Fe/H]=-1 and with little or no metallicitygradient with height. The lack of metallicity gradient can be explainedif a majority of extraplanar RGB stars were formed at early times andare not dominated by a younger heated population. Our observationssuggest that, like the Milky Way, low-mass disk galaxies also havemultiple stellar components. In its structure, mean metallicity, and oldage, the RGB component in these galaxies seems analogous to the MilkyWay thick disk. However, without additional kinematic and abundancemeasurements, this association is only circumstantial, particularly inlight of the clear existence of some disk heating at intermediate ages.Finally, we find that the vertical dust distribution has a scale heightsomewhat larger than that of the main-sequence stars.
| Chandra X-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/7715: Tidal Ultraluminous X-Ray Sources, Emergent Wind, and Resolved H II Regions
We present high spatial resolution X-ray imaging data for theinteracting galaxy pair NGC 7714/7715 (Arp 284) from the Chandra X-raytelescope. In addition to the unresolved starburst nucleus, a variablepoint source with LX~1040 ergs s-1 wasdetected 1.5" (270 pc) to the northwest of the nucleus, coincident witha blue, extremely optically luminous (MV~-14.1) point sourceon Hubble Space Telescope images. Eleven other candidate pointlikeultraluminous X-ray sources (ULXs) were also detected in the vicinity ofNGC 7714/7715, two of which exceed 1040 ergs s-1.Ten of these appear to be associated with interaction-induced features,but only two are associated with star formation regions. We also founddiffuse emission with LX~3×1040 ergss-1 extending 11" (1.9 kpc) to the north of the nucleus. Itsspectrum can be fitted with either a two-temperature MEKAL function(kT=0.59+0.05-0.06 and8+10-3 keV) or a 0.6 keV MEKAL function plus apower law (Γ=1.8+/-0.2). The hard component may be due tohigh-mass X-ray binaries (HMXBs) with possible contributions frominverse Compton radiation, while the soft component is likely from asuperwind. Superbubble models imply an expansion age of ~15 Myr,supporting previous assertions of an intermediate-age nuclear stellarpopulation in addition to a 5 Myr starburst. We also detected extendedX-ray emission associated with four extranuclear H II region complexes.The emission from these H II regions and the nuclear starburst could bedue to either an enhanced population of HMXBs relative to Local Groupgalactic averages or to diffuse gas heated by winds from supernovae, ifthe X-ray production efficiency LX/Lmech is high(~5%). To estimate LX/Lmech, we collectedpublished data for well-studied H II regions and superbubbles in nearbygalaxies. For H II regions with ages less than 3.5 Myr, the medianLX/Lmech~0.02%, while for older star formationregions, LX/Lmech~0.2%-7%. Thus, it is possiblethat gas heating by supernovae may be sufficient to account for theobserved X-rays from these H II regions. In galaxies much more distantthan NGC 7714, for example, the Cartwheel galaxy, H II region complexessimilar to those in NGC 7714 will be unresolved by Chandra and willmimic ULXs. No X-ray emission was detected from the Type Ib supernova SN1999dn, with an upper limit of ~2×1038 ergss-1.
| A Study of Edge-On Galaxies with the Hubble Space Telescope Advanced Camera for Surveys. I. Initial Results
We present the initial results of a Hubble Space Telescope/AdvancedCamera for Surveys snapshot survey of 16 nearby, edge-on, late-typegalaxies covering a range in distance from 2 to 19 Mpc. The images ofthese galaxies show significant resolved stellar populations. We deriveF606W and F814W photometry for more than 1.2 million stars and presentcolor-magnitude diagrams that show a mixture of young, intermediate, andold stars in each galaxy. In one of the fields we serendipitously detectstars from the Large Magellanic Cloud. We also identify a candidateyoung dwarf galaxy lying ~2 kpc above the plane of NGC 4631. For thenearest six galaxies, we derive tip of the red giant branch distancesand demonstrate that these galaxies fall on the K-band Tully-Fisherrelation established in clusters. From the color of the red giantbranch, we also find evidence that these galaxies possess a metal-poorthick-disk or halo population.
| Global dynamical evolution of the ISM in star forming galaxies. I. High resolution 3D simulations: Effect of the magnetic field
In star forming disk galaxies, matter circulation between stars and theinterstellar gas, and, in particular the energy input by random andclustered supernova explosions, determine the dynamical and chemicalevolution of the ISM, and hence of the galaxy as a whole. Using a 3D MHDcode with adaptive mesh refinement developed for this purpose, we haveinvestigated the rôle of magnetized matter circulation between thegaseous disk and the surrounding galactic halo. Special emphasis hasbeen put on the effect of the magnetic field with respect to the volumeand mass fractions of the different ISM "phases", the relativeimportance of ram, thermal and magnetic pressures, and whether the fieldcan prevent matter transport from the disk into the halo. Thesimulations were performed on a grid with an area of 1 kpc2,centered on the solar circle, extending ± 10 kpc perpendicular tothe galactic disk with a resolution as high as 1.25 pc. The simulationswere run for a time scale of 400 Myr, sufficiently long to avoid memoryeffects of the initial setup, and to allow for a global dynamicalequilibrium to be reached in case of a constant energy input rate. Themain results of our simulations are: (i) The {T}≤ 103 Kgas is mainly concentrated in shock compressed layers, exhibiting thepresence of high density clouds with sizes of a few parsecs and {T}≤200 K. These structures are formed in regions where several large scalestreams of convergent flow (driven by SNe) occur. They have lifetimes ofa few free-fall times, are filamentary in structure, tend to be alignedwith the local field and are associated with the highest fieldstrengths; (ii) the magnetic field has a high variability and it islargely uncorrelated with the density, suggesting that it is driven bysuperalfvenic inertial motions; (iii) ram pressure controls the flow for200<{T}≤ 105.5 K. For {T}≤ 200 K magnetic pressuredominates, while the hot gas ({T}> 105.5 K) in contrast iscontrolled by the thermal pressure, since magnetic field lines are swepttowards the dense compressed walls; (iv) up to 49% of the mass in thedisk is concentrated in the classical thermally unstable regime200<{T}≤ 103.9 K with 65% of the warm neutralmedium (WNM) mass enclosed in the 500≤ {T}≤ 5000 K gas,consistent with recent observations; (v) the volume filling factors ofthe different temperature regimes depend sensitively on the existence ofthe duty cycle between the disk and halo, acting as a pressure releasemechanism for the hot phase in the disk. We find that in general gastransport into the halo in 3D is not prevented by an initial diskparallel magnetic field, but only delayed initially, for as long as itis needed to punch holes into the thick magnetized gas disk. The meanvolume filling factor of the hot phase in the disk is similar in HD andMHD (the latter with a total field strength of 4.4 μG) runs,amounting to 17-21% for the Galactic supernova rate.
| New H2O masers in Seyfert and FIR bright galaxies
Using the Effelsberg 100-m telescope, detections of four extragalacticwater vapor masers are reported. Isotropic luminosities are ~50, 1000, 1and 230 Lȯ for Mrk 1066 (UGC 2456), Mrk 34, NGC 3556 andArp 299, respectively. Mrk 34 contains by far the most distant and oneof the most luminous water vapor megamasers so far reported in a Seyfertgalaxy. The interacting system Arp 299 appears to show two maserhotspots separated by approximately 20´´. With these newresults and even more recent data from Braatz et al. (2004, ApJ, 617,L29), the detection rate in our sample of Seyferts with known jet-NarrowLine Region interactions becomes 50% (7/14), while in star forminggalaxies with high (S100~μ m>50 Jy) far infrared fluxesthe detection rate is 22% (10/45). The jet-NLR interaction sample maynot only contain “jet-masers” but also a significant numberof accretion “disk-masers” like those seen in NGC 4258. Astatistical analysis of 53 extragalactic H2O sources (excluding theGalaxy and the Magellanic Clouds) indicates (1) that the correlationbetween IRAS Point Source and H2O luminosities, established forindividual star forming regions in the galactic disk, also holds forAGN-dominated megamaser galaxies; (2) that maser luminosities are notcorrelated with 60 μm/100 μm color temperatures; and (3) that onlya small fraction of the luminous megamasers (L_H_2O > 100Lȯ) detectable with 100-m sized telescopes have so farbeen identified. The H2O luminosity function (LF) suggests that thenumber of galaxies with 1 Lȯ < L_H_2O < 10Lȯ, the transition range between“kilomasers” (mostly star formation) and“megamasers” (active galactic nuclei), is small. The overallslope of the LF, ~-1.5, indicates that the number of detectable masersis almost independent of their luminosity. If the LF is not steepeningat very high maser luminosities and if it is possible to find suitablecandidate sources, H2O megamasers at significant redshifts should bedetectable even with present day state-of-the-art facilities.
| Neutral hydrogen gas in 7 high-inclination spiral galaxies. I. The data
High-sensitivity interferometric H i line observations of a small sampleof seven galaxies with limiting column densities of a few times1019 cm-2 are presented. A tilted ring modelfitting routine was used to determine some global characteristics of theH i distribution and kinematics in the galaxy disks. 4 of the 7 galaxieshave low maximum rotation velocities of 125 km s-1,indicating that they are low-mass systems. Visual inspection shows thatat least one galaxy, NGC 4700, exhibits signs of extraplanar H iemission. An in-depth search for H i gas in the galaxy halos and thedetermination of halo gas properties, based on three-dimensionalmodeling, will follow in a separate publication. Companion galaxies weredetected in H i line emission near 3 of the 7 sample galaxies: NGC 1511,NGC 4565 and NGC 4700. One of these, NGC 1511, is found to be stronglyinteracting and is therefore not suitable for a study of the dependenceof its halo properties on the level of star formation activity in theunderlying disk. In the case of NGC 4700 the companion galaxy has novisible influence on its gas kinematics, while NGC 4565 might beaffected by its interaction with two small companions.Figures [see full text] and Appendix A are only available in electronicform at http://www.edpsciences.org
| Intergalactic neutral hydrogen gas in the Grus quartet of galaxies
Australia Telescope Compact Array multi-configuration mosaicing of theGrus quartet of galaxies reveals the presence of spectacular tidalstructures. 2.1×109 Mȯ of neutralatomic hydrogen (H I) gas, i.e. 11% of all H I in the group, are foundto be dragged from NGC 7582 into intergalactic space. About1.34×109 Mȯ of H I gas are contained ina tidal tail emanating from the north-western disk of NGC 7582, with aprojected length of about 85 kpc and width of up to 32 kpc and arelative velocity with respect to the centre of NGC 7582 of 130-140 kms-1. 7.7×108 Mȯ of H Ireside in an intergalactic H I cloud 48 kpc West of NGC 7582, whichmight originate from the disk of NGC 7582 as well and has no opticalcounterpart in a red Digital Sky Survey (DSS) image. These observationsprove that tidal stripping is occurring in the Grus quartet and thattidal features in compact groups can be potentially importantcontributors of metal-enriched matter to the intergalactic medium. Thetidal features around NGC 7582 cover an area of about 2000kpc2, almost doubling the group's cross-section forLyman-α absorption of light from background sources compared tothe optical extent of the member galaxies.
| 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
| 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.
| The Chandra view of NGC1800 and the X-ray scaling properties of dwarf starbursts
The superb spatial resolution of Chandra is utilized to study the X-raymorphology of the dwarf starburst galaxy NGC1800 embedded in a smallgroup of galaxies. Diffuse galactic emission is detected, extendingseveral kiloparsec above the galactic plane, with an overall morphologysimilar to the galactic winds seen in nearby X-ray-bright starburstgalaxies. This makes NGC1800 the most distant dwarf starburst with aclear detection of diffuse X-ray emission. The diffuse X-ray luminosityof 1.3 +/- 0.3 × 1038ergs-1 accounts for atleast 60 per cent of the total soft X-ray output of the galaxy. A hotgas temperature of kT= 0.25 keV and metallicity Z~ 0.05Zsolarare derived, the latter being consistent with results from opticalspectroscopy of the interstellar medium. Our failure to detect any hotgas associated with the embedding galaxy group translates into an upperlimit to the group X-ray luminosity of LX <1041ergs-1. There is no convincing evidence thatthe outflowing wind of NGC1800 is currently interacting with anyintragroup gas, and mechanical considerations indicate that the wind canescape the galaxy and its surrounding HI halo, eventually deliveringenergy and metals to the intragroup gas. Properties of NGC1800 arecompared to those of other dwarf starburst galaxies, and a firstdetailed discussion of the X-ray scaling properties of this populationof objects is given, set against the equivalent results obtained fornormal starburst galaxies. Results indicate that dwarf starbursts to alarge degree behave as down-scaled versions of normal starburstgalaxies.
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