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A dynamical model for the extraplanar gas in spiral galaxies
Recent HI observations reveal that the discs of spiral galaxies aresurrounded by extended gaseous haloes. This extraplanar gas reacheslarge distances (several kiloparsecs) from the disc and shows peculiarkinematics (low rotation and inflow). We have modelled the extraplanargas as a continuous flow of material from the disc of a spiral galaxyinto its halo region. The output of our models is pseudo data cubes thatcan be directly compared to the HI data. We have applied these models totwo spiral galaxies (NGC 891 and NGC 2403) known to have a substantialamount of extraplanar gas. Our models are able to reproduce accuratelythe vertical distribution of extraplanar gas for an energy inputcorresponding to a small fraction (<4 per cent) of the energyreleased by supernovae. However, they fail in two important aspects: (1)they do not reproduce the right gradient in rotation velocity; (2) theypredict a general outflow of the extraplanar gas, contrary to what isobserved. We show that neither of these difficulties can be removed ifclouds are ionized and invisible at 21cm as they leave the disc butbecome visible at some point on their orbits. We speculate that thesefailures indicate the need for accreted material from the intergalacticmedium that could provide the low angular momentum and inflow required.
| Mapping Large-Scale Gaseous Outflows in Ultraluminous Infrared Galaxies with Keck II ESI Spectra: Spatial Extent of the Outflow
The kinematics of neutral gas and warm ionized gas have been mappedacross ultraluminous starburst galaxies using the Na Iλλ5890, 5896 absorption-line and Hα emission-lineprofiles, respectively, in Keck II ESI spectra. Blueshifted,interstellar absorption is found over extended regions, exceeding 15 kpcin several systems. An outflow diverging from the nuclear starburstwould have to reach large heights to cover this area in projection. Thescale height of the absorbing material could be lower, however, if theoutflow emanates from a larger region of the galaxy. The large velocitygradient discovered across several outflows is inconsistent with a flowdiverging from the nuclear starburst. Widespread star formation,triggered by the merger, probably drives these extended outflows viamechanical feedback from supernovae, although shocks generated by thegalaxy-galaxy merger may also contribute to the formation of a hot wind.In a typical ULIG, the mass carried by the cool phase of the outflow is~108 Msolar i.e., a few percent of the dynamicalmass in the starburst region. Assuming the starburst activity haspersisted for 10 Myr, the kinetic energy of the cool outflows is a fewpercent of the supernova energy. The cool wind is expected to beaccelerated by momentum deposition, possibly from radiation pressure aswell as the ram pressure of the hot, supernova-induced wind. Theturnaround radii of the cool outflows are at least ~30-90 kpc, whichpresents a significant Na I absorption cross section. If mostL>0.1L* galaxies pass through a luminous starburst phase,then relics of cool outflows will create a significant redshift-pathdensity. Galaxy formation models should include this cool phase of theoutflow in addition to a hot wind in feedback models.Data presented herein were obtained at the W. M. Keck Observatory, whichis operated as a scientific partnership among the California Instituteof Technology, the University of California, and the NationalAeronautics and Space Administration. The Observatory was made possibleby the generous financial support of the W. M. Keck Foundation.
| Imaging Fabry-Perot Spectroscopy of NGC 5775: Kinematics of the Diffuse Ionized Gas Halo
We present imaging Fabry-Perot observations of Hα emission in thenearly edge-on spiral galaxy NGC 5775. We have derived a rotation curveand a radial density profile along the major axis by examiningposition-velocity (PV) diagrams from the Fabry-Perot data cube, as wellas a CO 2-1 data cube from the literature. PV diagrams constructedparallel to the major axis are used to examine changes in azimuthalvelocity as a function of height above the midplane. The results of thisanalysis reveal the presence of a vertical gradient in azimuthalvelocity. The magnitude of this gradient is approximately 1 kms-1 arcsec-1, or about 8 km s-1kpc-1, although a higher value of the gradient may beappropriate in localized regions of the halo. The evidence for anazimuthal velocity gradient is much stronger for the approaching half ofthe galaxy, although earlier slit spectra are consistent with a gradienton both sides. There is evidence for an outward radial redistribution ofgas in the halo. The form of the rotation curve may also change withheight, but this is not certain. We compare these results with those ofan entirely ballistic model of a disk-halo flow. The model predicts avertical gradient in azimuthal velocity that is shallower than theobserved gradient, indicating that an additional mechanism is requiredto further slow the rotation speeds in the halo.
| 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.
| The structure of galactic disks. Studying late-type spiral galaxies using SDSS
Using imaging data from the SDSS survey, we present the g' and r' radialstellar light distribution of a complete sample of ~90 face-on tointermediate inclined, nearby, late-type (Sb-Sdm) spiral galaxies. Thesurface brightness profiles are reliable (1 σ uncertainty lessthan 0.2 mag) down to μ27 mag/''. Only ~10% of all galaxies havea normal/standard purely exponential disk down to our noise limit. Thesurface brightness distribution of the rest of the galaxies is betterdescribed as a broken exponential. About 60% of the galaxies have abreak in the exponential profile between 1.5-4.5 times thescalelength followed by a downbending, steeper outer region. Another~30% shows also a clear break between 4.0-6.0 times thescalelength but followed by an upbending, shallower outer region. A fewgalaxies have even a more complex surface brightness distribution. Theshape of the profiles correlates with Hubble type. Downbending breaksare more frequent in later Hubble types while the fraction of upbendingbreaks rises towards earlier types. No clear relation is found betweenthe environment, as characterised by the number of neighbours, and theshape of the profiles of the galaxies.
| 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.
| The Hα Galaxy Survey . III. Constraints on supernova progenitors from spatial correlations with Hα emission
Aims.We attempt to constrain progenitors of the different types ofsupernovae from their spatial distributions relative to star formationregions in their host galaxies, as traced by Hα + [Nii] lineemission. Methods: .We analyse 63 supernovae which have occurredwithin galaxies from our Hα survey of the local Universe. Threestatistical tests are used, based on pixel statistics, Hα radialgrowth curves, and total galaxy emission-line fluxes. Results:.Many type II supernovae come from regions of low or zero emission lineflux, and more than would be expected if the latter accurately traceshigh-mass star formation. We interpret this excess as a 40% "Runaway"fraction in the progenitor stars. Supernovae of types Ib and Ic doappear to trace star formation activity, with a much higher fractioncoming from the centres of bright star formation regions than is thecase for the type II supernovae. Type Ia supernovae overall show a weakcorrelation with locations of current star formation, but there isevidence that a significant minority, up to about 40%, may be linked tothe young stellar population. The radial distribution of allcore-collapse supernovae (types Ib, Ic and II) closely follows that ofthe line emission and hence star formation in their host galaxies, apartfrom a central deficiency which is less marked for supernovae of typesIb and Ic than for those of type II. Core-collapse supernova ratesoverall are consistent with being proportional to galaxy totalluminosities and star formation rates; however, within this total thetype Ib and Ic supernovae show a moderate bias towards more luminoushost galaxies, and type II supernovae a slight bias towardslower-luminosity hosts.
| 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.
| Hydrostatic models for the rotation of extra-planar gas in disk galaxies
We show that fluid stationary models are able to reproduce the observednegative vertical gradient of the rotation velocity of the extra-planargas in spiral galaxies. We have constructed models based on the simplecondition that the pressure of the medium does not depend on densityalone (baroclinic instead of barotropic solutions: isodensity andisothermal surfaces do not coincide). As an illustration, we havesuccessfully applied our method to reproduce the observed velocitygradient of the lagging gaseous halo of NGC 891. Thefluid stationary models discussed here can describe a hot homogeneousmedium as well as a "gas" made of discrete, cold H I clouds with anisotropic velocity dispersion distribution. Although the methodpresented here generates a density and velocity field consistent withobservational constraints, the stability of these configurations remainsunresolved.
| Discovery of PAHs in the halo of NGC 5907
We have used sensitive archival data from the Infrared Space Observatory(ISO) to make maps of the edge-on low SFR galaxy, NGC 5907, in 6different MIR bands: LW2, LW5, LW6, LW7, LW8, and LW10, covering thespectrum from 6.5 to 15.0 μm and including several narrow bands thatisolate the infrared aromatic spectral features commonly referred to asPAHs. Most of the MIR emission is dominated by PAHs and it is likelythat emission from VSGs contribute only negligibly except in the broadIRAS-equivalent band. The flux ratios are typical of galaxies with lowSFRs or quiesent regions within galaxies (e.g. M 83) and a very highPAH/continuum ratio is observed. The PAH emission follows the COdistribution and also shows some correlation within the disk with theλ850 μm distribution. However, the PAH emission also reacheslarger galactocentric radii than the CO and other correlations suggestthat the PAHs are also more widespread. A significant new discovery isthe presence of PAHs in the halo of the galaxy. In the narrow bands thatisolate single PAH features, the emission shows structure similar tohigh latitude features seen in other galaxies in other tracers. Thefeatures extend as far as 6.5 kpc from the plane but scale heights of3.5 kpc are more typical. The λ11.3/λ7.7 ratio alsoappears to increase with distance from the major axis. To our knowledge,this is the first time PAHs have been seen in the halo of an externalgalaxy. Just as significantly, they are seen in a low SFR galaxy,suggesting that strong SNe and winds are not necessary for these largemolecules to reach high latitudes.
| 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 Nature of Ultraluminous X-Ray Sources
We present spectroscopic observations of six optical counterparts ofultraluminous X-ray sources (ULXs) around nearby galaxies. The spectraof the six objects show the presence of broad emission features. Theidentification of these allow us to classify all of the objects asquasars at higher redshift than their assigned parent galaxy. This isone of the first and largest identifications of such objects usingunambigous optical spectral features. These results, in conjuction withprevious similar identifications of other sources, indicate thathigh-redshift quasars represent an important fraction of cataloged ULXsources. We estimate the density of such sources and compare this withexpectations for a population of randomly distributed backgroundquasars.
| 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.
| Rotational Widths for Use in the Tully-Fisher Relation. I. Long-Slit Spectroscopic Data
We present new long-slit Hα spectroscopy for 403 noninteractingspiral galaxies, obtained at the Palomar Observatory 5 m Hale telescope,which is used to derive well-sampled optical rotation curves. Becausemany of the galaxies show optical emission features that aresignificantly extended along the spectrograph slit, a technique wasdevised to separate and subtract the night sky lines from the galaxyemission. We exploit a functional fit to the rotation curve to identifyits center of symmetry; this method minimizes the asymmetry in thefinal, folded rotation curve. We derive rotational widths using bothvelocity histograms and the Polyex model fit. The final rotational widthis measured at a radius containing 83% of the total light as derivedfrom I-band images. In addition to presenting the new data, we use alarge sample of 742 galaxies for which both optical long-slit and radioH I line spectroscopy are available to investigate the relation betweenthe H I content of the disks and the extent of their rotation curves.Our results show that the correlation between those quantities, which iswell established in the case of H I-poor galaxies in clusters, ispresent also in H I-normal objects: for a given optical size, starformation can be traced farther out in the disks of galaxies with largerH I mass.
| The Structural Properties of Isolated Galaxies, Spiral-Spiral Pairs, and Mergers: The Robustness of Galaxy Morphology during Secular Evolution
We present a structural analysis of nearby galaxies in spiral-spiralpairs in optical BVRI bands and compare them with the structures ofisolated spiral galaxies and galaxies in ongoing mergers. We use thesecomparisons to determine how galaxy structure changes during galaxyinteractions and mergers. We analyze light concentration (C), asymmetry(A), and clumpiness (S) parameters, and use the projections of CASparameter space to compare these samples. We find that the CASparameters of paired galaxies are correlated with the projectedseparations of the pair. For the widest and closest pairs, the CASparameters tend to be similar to those of isolated and ongoing majormergers (e.g., ultraluminous infrared galaxies), respectively. Ourresults imply that galaxy morphology is a robust property that onlychanges significantly during a strong interaction or major merger. Thetypical timescale for this change in our paired sample, based ondynamical friction arguments, is short, τ~0.1-0.5 Gyr. We findaverage enhancement factors for the spiral-pair asymmetries andclumpiness values of ~2.2 and 1.5. The S parameter, which is related tostar formation (SF) activity, has a moderate level of enhancement,suggesting that this activity in modern spirals depends more on internalprocesses than on external conditions. We further test the statisticalcriterion for picking up interacting galaxies in an automated way byusing the A-S projection plane. The diversity of our spiral-pair samplein the CAS space suggests that structural/SF/morphological properties ofinteracting galaxies change abruptly only when the interaction becomesvery strong and the criteria given previously by Conselice for findinggalaxies involved in major mergers are effective.
| Extra-planar gas in the spiral galaxy NGC 4559
We present 21-cm line observations of the spiral galaxy NGC 4559, madewith the Westerbork Synthesis Radio Telescope. We have used them tostudy the H I distribution and kinematics, the relative amount anddistribution of luminous and dark matter in this galaxy and, inparticular, the presence of extra-planar gas. Our data do reveal thepresence of such a component, in the form of a thick disk, with a massof 5.9 × 108~Mȯ (one tenth of the totalH I mass) and a mean rotation velocity 25-50 km s-1 lowerthan that of the thin disk. The extra-planar gas may be the result ofgalactic fountains but accretion from the IGM cannot be ruled out. Withthis study we confirm that lagging, thick H I layers are likely to becommon in spiral 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.
| HI in NGC 5433 and its environment: high-latitude emission in a small galaxy group
We present HI synthesis maps of the edge-on starburst NGC 5433 and itsenvironment, obtained with the Very Large Array in its C and Dconfigurations. The observations and spectral model residuals of themain disc emission in NGC 5433 reveal three extraplanar features. Weassociate two of these features with coherent extraplanar extensionsacross multiple spectral channels in our data, including a complete loopin position-velocity space. Interpreting the latter as an expandingshell we derive a corresponding input energy of 2 ×1054 erg, comparable to that for the largest supershellsfound in the Galaxy and those in other edge-on systems. NGC 5433 is in aricher environment than previously thought. We confirm that KUG 1359+326is a physical companion to NGC 5433 and find two new faint companions,both with Minnesota Automated Plate Scanner identifications, which welabel SIS-1 and SIS-2. Including the more distant IC 4357, NGC 5433 isthe dominant member of a group of at least five galaxies, spanning over750 kpc in a filamentary structure. A variety of evidence suggests thatinteractions are occurring in this group. While a number of underlyingmechanisms are consistent with the morphology of the high-latitudefeatures in NGC 5433, we argue that environmental effects may play arole in their generation.
| Multifrequency radio-continuum observations of NGC 1569: evidence for a convective wind
We present high-sensitivity radio-continuum observations with the VeryLarge Array (VLA) and Ryle Telescope at 1.5, 4.9, 8.4 and 15.4 GHz ofthe dwarf irregular galaxy NGC 1569. The radio data show an extended,irregularly shaped halo with filamentary structure around the galaxy.The spectral index maps reveal an unusually patchy distribution withregions of flat spectral index extending into the halo. The data allowus to perform a spatially resolved spectral-fitting analysis of thecontinuum emission from which we derive maps of the thermal andsynchrotron emission. The thermal radio emission is concentrated towardsthe brightest H II region west of the super star clusters A and B,whereas the distribution of the synchrotron emission peaks in a bar-likestructure in the disc extending between the two clusters. The total fluxdensity of the thermal radio emission allows us to derive the integratedsynchrotron spectrum and we confirm the break in the spectrum that wasfound by Israel & de Bruyn. We discuss various possibilities thatcould produce such a break and conclude that the only mechanism able tofit the radio data and remain consistent with data at other wavelengthsis a convective wind allowing cosmic ray electrons to escape from thehalo.
| High Velocity Gas in the Halos of Spiral Galaxies
Recent, high sensitivity, H I observations of nearby spiral galaxiesshow that their thin `cold' disks are surrounded by thick layers (halos)of neutral gas with anomalous kinematics. We present results for threegalaxies viewed at different inclination angles: NGC 891 (edge-on), NGC2403 (i=60 °), and NGC 6946 (almost face-on). These studies show thepresence of halo gas up to distances of 10-15 kpc from the plane. Suchgas has a mean rotation 25-50 km s-1 lower than that of thegas in the plane, and some complexes are detected at very highvelocities, up to 200-300 km s-1. The nature and origin ofthis halo gas are poorly understood. It can either be the result of agalactic fountain or of accretion from the intergalactic medium. It isprobably analogous to some of the High Velocity Clouds (HVCs) of theMilky Way.
| Studies of Extragalactic Formaldehyde and Radio Recombination Lines
We present the most sensitive and extensive survey yet performed ofextragalactic H2CO 6 cm (4.829 GHz) emission/absorption.Sixty-two sources were observed with the C-band system of the AreciboTelescope to a 1 σ rms noise level of ~0.3 mJy. We report a newdetection of H2CO 6 cm absorption toward NGC 520 and theconfirmation of H2CO 6 cm absorption toward several sources.We report confirmation of H2CO 6 cm emission toward the OHmegamasers Arp 220, IC 860, and IRAS 15107+0724. At present these arethe only extragalactic H2CO 6 cm emitters independentlyconfirmed. A characterization of the properties of formaldehydeabsorbers and emitters based on infrared properties of the galaxies isdiscussed. We also conducted a simultaneous survey of the H110αhydrogen recombination line toward a sample of 53 objects. We report thedetection of H110α toward the giant extragalactic H II region NGC604 in M33.
| The Ultraluminous X-Ray Source Population from the Chandra Archive of Galaxies
One hundred fifty-four discrete non-nuclear ultraluminous X-ray (ULX)sources, with spectroscopically determined intrinsic X-ray luminositiesgreater than 1039 ergs s-1, are identified in 82galaxies observed with Chandra's Advanced CCD Imaging Spectrometer.Source positions, X-ray luminosities, and spectral and timingcharacteristics are tabulated. Statistical comparisons between theseX-ray properties and those of the weaker discrete sources in the samefields (mainly neutron star and stellar-mass black hole binaries) aremade. Sources above ~1038 ergs s-1 display similarspatial, spectral, color, and variability distributions. In particular,there is no compelling evidence in the sample for a new and distinctclass of X-ray object such as the intermediate-mass black holes.Eighty-three percent of ULX candidates have spectra that can bedescribed as absorbed power laws with index <Γ>=1.74 andcolumn density =2.24×1021cm-2, or ~5 times the average Galactic column. About 20% ofthe ULXs have much steeper indices indicative of a soft, and likelythermal, spectrum. The locations of ULXs in their host galaxies arestrongly peaked toward their galaxy centers. The deprojected radialdistribution of the ULX candidates is somewhat steeper than anexponential disk, indistinguishable from that of the weaker sources.About 5%-15% of ULX candidates are variable during the Chandraobservations (which average 39.5 ks). Comparison of the cumulative X-rayluminosity functions of the ULXs to Chandra Deep Field results suggests~25% of the sources may be background objects, including 14% of the ULXcandidates in the sample of spiral galaxies and 44% of those inelliptical galaxies, implying the elliptical galaxy ULX population isseverely compromised by background active galactic nuclei. Correlationswith host galaxy properties confirm the number and total X-rayluminosity of the ULXs are associated with recent star formation andwith galaxy merging and interactions. The preponderance of ULXs instar-forming galaxies as well as their similarities to less-luminoussources suggest they originate in a young but short-lived populationsuch as the high-mass X-ray binaries with a smaller contribution (basedon spectral slope) from recent supernovae. The number of ULXs inelliptical galaxies scales with host galaxy mass and can be explainedmost simply as the high-luminosity end of the low-mass X-ray binarypopulation.
| HCN Survey of Normal Spiral, Infrared-luminous, and Ultraluminous Galaxies
We report systematic HCN J=1-0 (and CO) observations of a sample of 53infrared (IR) and/or CO-bright and/or luminous galaxies, including sevenultraluminous infrared galaxies, nearly 20 luminous infrared galaxies,and more than a dozen of the nearest normal spiral galaxies. This is thelargest and most sensitive HCN survey of galaxies to date. All galaxiesobserved so far follow the tight correlation between the IR luminosityLIR and the HCN luminosity LHCN initially proposedby Solomon, Downes, & Radford, which is detailed in a companionpaper. We also address here the issue of HCN excitation. There is noparticularly strong correlation between LHCN and the 12 μmluminosity; in fact, of all the four IRAS bands, the 12 μm luminosityhas the weakest correlation with the HCN luminosity. There is also noevidence of stronger HCN emission or a higher ratio of HCN and COluminosities LHCN/LCO for galaxies with excess 12μm emission. This result implies that mid-IR radiative pumping, orpopulating, of the J=1 level of HCN by a mid-IR vibrational transitionis not important compared with the collisional excitation by densemolecular hydrogen. Furthermore, large velocity gradient calculationsjustify the use of HCN J=1-0 emission as a tracer of high-densitymolecular gas (>~3×104/τcm-3) andgive an estimate of the mass of dense molecular gas from HCNobservations. Therefore, LHCN may be used as a measure of thetotal mass of dense molecular gas, and the luminosity ratioLHCN/LCO may indicate the fraction of moleculargas that is dense.
| The Star Formation Rate and Dense Molecular Gas in Galaxies
HCN luminosity is a tracer of dense molecular gas,n(H2)>~3×104cm-3, associatedwith star-forming giant molecular cloud (GMC) cores. We present theresults and analysis of our survey of HCN emission from 65 infraredgalaxies, including nine ultraluminous infrared galaxies (ULIGs,LIR>~1012Lsolar), 22 luminousinfrared galaxies (LIGs,1011Lsolar0.06 are LIGs or ULIGs. Normal spiralsall have similar and low dense gas fractionsLHCN/LCO=0.02-0.05. The global star formationefficiency depends on the fraction of the molecular gas in a densephase.
| Kinematics of the Ionized Halo of NGC 5775
Studies of the kinematics of gaseous halos in some spiral galaxies haveshown that the rotational velocity of the gas decreases with heightabove the midplane (z). This vertical velocity gradient has beenobserved in both the neutral and ionized components of halos. We presentimaging Fabry-Perot Hα observations of the edge-on galaxy NGC5775. These observations allow us to study the variation in the rotationcurve (and, to some extent, the radial profile) of the ionized gascomponent as a function of z. We have used an iterative technique toinfer the major axis rotation curve from the ionized gas observations aswell as CO data. A preliminary analysis of position-velocity (PV)diagrams parallel to the major axis implies the presence of a verticalvelocity gradient. The magnitude of this gradient is tentatively foundto be 2 km s-1 arcsec-1, consistent with theresults of Rand (2000) and Tüllmann et al. (2000).This material is based on work partially supported by the NationalScience Foundation under Grant No. AST 99-86113.
| Kinematics of the ionised gas in the spiral galaxy NGC 2403
We present a study of the kinematics of the ionised gas in the nearbyspiral galaxy NGC 2403 using deep long-slit spectra obtained with the4.2-m William Herschel Telescope. The data show the presence of a halocomponent of ionised gas that is rotating more slowly than the gas inthe disk. The kinematics of this ionised halo gas is similar to that ofthe neutral halo gas. On small scales, broad line profiles (up to 300 kms-1 wide) indicate regions of fast outflows of ionised gas.We discuss these new results in the context of galactic fountain models.Based on observations made with the WHT operated on the island of LaPalma by the Isaac Newton Group in the Spanish Observatorio del Roque delos Muchachos of the Instituto de Astrofisica de Canarias.
| 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/
| Emission line ratio mapping with OSIRIS
We outline two complementary applications of the tunable filter systemin OSIRIS for research on external galaxies: diagnostics of the diffuseionized gas in face-on and in edge-on objects, giving illustrations fornearby galaxies, and discussing projected observations at intermediateand high redshift.
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