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Radial transport of dust in spiral galaxies
Motivated by recent observations which detect dust at largegalactocentric distances in the disks of spiral galaxies, we propose amechanism of outward radial transport of dust by spiral stellar densitywaves. We consider spiral galaxies in which most of dust formation islocalized inside the corotation radius. We show that in the disks ofsuch spiral galaxies, the dust grains can travel over radial distancesthat exceed the corotation radius by roughly 25%. A fraction of the dustgrains can be trapped on kidney-shaped stable orbits between the stellarspiral arms and thus can escape the destructive effect of supernovaexplosions. These grains form diffuse dusty spiral arms, which stretch 45 kpc from the sites of active star formation. About 10% of dust by massinjected inside corotation can be transported over radial distances 3 4kpc during ≈1.0 Gyr. This is roughly an order of magnitude moreefficient than can be provided by the turbulent motions.
| 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.
| Optical Counterparts of Ultraluminous X-Ray Sources Identified from Archival HST WFPC2 Images
We present a systematic analysis of archival HST WFPC2 ``Association''data sets that correlate with the Chandra positions of a set of 44ultraluminous X-ray sources (ULXs) of nearby galaxies. The mainmotivation is to address the nature of ULXs by searching for opticalcounterparts. Sixteen of the ULXs are found in early-type galaxies (RC3Hubble type <3). We have improved the Chandra/HST relative astrometrywhenever possible, resulting in errors circles of 0.3"-1.7" in size.Disparate numbers of potential ULX counterparts are found, and in somecases none are found. The lack of or low number of counterparts in somecases may be due to insufficient depth in the WFPC2 images. Particularlyin late-type galaxies, the HST image in the ULX region was often complexor crowded, requiring source detection to be performed manually. Wetherefore address various scenarios for the nature of the ULX since itis not known which, if any, of the sources found are true counterparts.The optical luminosities of the sources are typically in the range104-106 Lsolar, with (effective) Vmagnitudes typically in the range 22-24. In several cases colorinformation is available, with the colors roughly tending to be more redin early-type galaxies. This suggests that, in general, the (potential)counterparts found in early-type galaxies are likely to be older stellarpopulations and are probably globular clusters. Several early-typegalaxy counterparts have blue colors, which may be due to youngerstellar populations in the host galaxies, however, these could also bebackground sources. In spiral galaxies the sources may also be due tolocalized structure in the disks rather than bound stellar systems.Alternatively, some of the counterparts in late-type galaxies may beisolated supergiant stars. The observed X-ray/optical flux ratio isdiluted by the optical emission of the cluster in cases where the systemis an X-ray binary in a cluster, particularly in the case of a low-massX-ray binaries in an old cluster. If any of the counterparts are boundsystems with ~104-106 stars and are the truecounterparts to the ULX sources, then the X-ray luminosities of the ULXare generally well below the Eddington limit for a black hole with mass~0.1% of the cluster mass. Finally, we find that the optical flux of thecounterparts is consistent with being dominated by emission from anaccretion disk around an intermediate-mass black hole if the black holehappens to have a mass >~102 Msolar and isaccreting at close to the Eddington rate, unless the accretion disk isirradiated (which would result in high optical disk luminosities atlower black hole masses).Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. This project isassociated with Archival proposal 9545.
| 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.
| Integral Field Unit Observations of NGC 891: Kinematics of the Diffuse Ionized Gas Halo
We present high and moderate spectral resolution spectroscopy of diffuseionized gas (DIG) emission in the halo of NGC 891. The data wereobtained with the SparsePak integral field unit at the WIYN Observatory.The wavelength coverage includes the [N II] λλ6548, 6583,Hα, and [S II] λλ6716, 6731 emission lines.Position-velocity (PV) diagrams, constructed using spectra extractedfrom four SparsePak pointings in the halo, are used to examine thekinematics of the DIG. Using two independent methods, a verticalgradient in azimuthal velocity is found to be present in the northeastquadrant of the halo, with magnitude approximately 15-18 kms-1 kpc-1, in agreement with results from H Iobservations. The kinematics of the DIG suggests that this gradientbegins at approximately 1 kpc above the midplane. In another part of thehalo, the southeast quadrant, the kinematics is markedly different andsuggest rotation at about 175 km s-1, much slower than thedisk but with no vertical gradient. We use an entirely ballistic modelof disk-halo flow in an attempt to reproduce the kinematics observed inthe northeast quadrant. Analysis shows that the velocity gradientpredicted by the ballistic model is far too shallow. Based on intensitycuts made parallel to the major axis in the ballistic model and anHα image of NGC 891 from the literature, we conclude that the DIGhalo is much more centrally concentrated than the model, suggesting thathydrodynamics dominate over ballistic motion in shaping the densitystructure of the halo. Velocity dispersion measurements along the minoraxis of NGC 891 seem to indicate a lack of radial motions in the halo,but the uncertainties do not allow us to set firm limits.
| 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.
| 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.
| Massive Coronae of Galaxies
There is reason to suspect that about half of the baryons in theuniverse are in pressure-supported plasma in the halos of normalgalaxies, drawn in by gravity along with about half of the dark matter.We present a model for this substantial baryonic component, the galacticcoronae, that fits the available observational constraints. Thisphenomenological approach requires departures from state-of-the-artnumerical models of galaxy formation, but the adjustments are not solarge as to seem unreasonable. In particular, massive coronae would haveto be hotter than the kinetic temperature of the halo dark matter so asto produce acceptable central electron densities. This highertemperature might result from the difference of the fluid dynamics ofthe baryons and the collisionless dynamics of the dark matter during theassembly of the protogalaxy, in an analogy to what seems to happen incluster formation. The cooling time of a massive corona would be longerthan the gravitational collapse time but, in the inner parts, shorterthan the Hubble time, making the corona thermally unstable to theformation of a cloudy structure that is settling and adding to the massin interstellar matter and stars. Since in this picture the mass in thecorona of a spiral galaxy is much larger than the mass in condensedbaryons, the corona would be a substantial reservoir that could supplymatter for star formation in isolated spirals continuing well past thepresent epoch.
| Evidence for Chimney Breakout in the Galactic Supershell GSH 242-03+37
We present new high-resolution neutral hydrogen (H I) images of theGalactic supershell GSH 242-03+37. These data were obtained with theParkes Radiotelescope as part of the Galactic All-Sky Survey (GASS). GSH242-03+37 is one of the largest and most energetic H I supershells inthe Galaxy, with a radius of 565+/-65 pc and an expansion energy of3×1053 ergs. Our images reveal a complicated shell withmultiple chimney structures on both sides of the Galactic plane. Thesechimneys appear capped by narrow filaments about 1.6 kpc above and belowthe Galactic midplane, confirming structures predicted in simulations ofexpanding supershells. The structure of GSH 242-03+37 is extremelysimilar to the only other Galactic supershell known to have blown out ofboth sides of the plane, GSH 277+00+36. We compare the GASS H I datawith X-ray and Hα images, finding no strong correlations.
| Simulations of Dust in Interacting Galaxies. I. Dust Attenuation
A new Monte Carlo radiative transfer code, SUNRISE, is used inconjunction with hydrodynamic simulations of major galaxy mergers tocalculate the effects of dust in such systems. Dust has a profoundeffect on the emerging radiation, consistent with observations of dustabsorption in starburst galaxies. The dust attenuation increases withluminosity such that at peak luminosities ~90% of the bolometricluminosity is absorbed by dust. We find that our predictions agree withobserved relationships between the UV spectral slope and the fraction oflight absorbed by dust (IRX-β) and observational estimates of theoptical depth as a function of intrinsic B-band or UV luminosity. Ingeneral, the detailed appearance of the merging event depends on thestage of the merger and the geometry of the encounter. The fraction ofbolometric energy absorbed by the dust, however, is a robust quantitythat can be predicted from the intrinsic properties bolometricluminosity, baryonic mass, star formation rate, and metallicity of thesimulated system. This paper presents fitting formulae, valid over awide range of masses and metallicities, from which the absorbed fractionof luminosity (and consequently also the infrared dust luminosity) canbe predicted. The attenuation of the luminosity at specific wavelengthscan also be predicted, albeit with a larger scatter due to the variationwith viewing angle. These formulae for dust attenuation are consistentwith earlier studies and would be suitable for inclusion in theoreticalmodels, e.g., semianalytic models, of galaxy formation and evolution.
| 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.
| Structural Parameters of Thin and Thick Disks in Edge-on Disk Galaxies
We analyze the global structure of 34 late-type, edge-on, undisturbed,disk galaxies spanning a wide range of mass. We measure structuralparameters for the galaxies using two-dimensional least-squares fittingto our R-band photometry. The fits require both a thick and a thin diskto adequately fit the data. The thick disks have larger scale heightsand longer scale lengths than the embedded thin disks by factors of ~2and ~1.25, respectively. The observed structural parameters agree wellwith the properties of thick and thin disks derived from star counts inthe Milky Way and from resolved stellar populations in nearby galaxies.We find that massive galaxies' luminosities are dominated by the thindisk. However, in low-mass galaxies (Vc<~120 kms-1) thick disk stars contribute nearly half the luminosityand dominate the stellar mass. Thus, although low-mass dwarf galaxiesappear blue, the majority of their stars are probably quite old.Our data are most easily explained by a formation scenario in which thethick disk is assembled through direct accretion of stellar materialfrom merging satellites while the thin disk is formed from accreted gas.The baryonic fraction in the thin disk therefore constrains the gasrichness of the merging pregalactic fragments. If we include the mass inH I as part of the thin disk, the thick disk contains <~10% of thebaryons in high-mass galaxies and ~25%-30% of the baryons in low-massgalaxies. Our data, therefore, indicate that the fragments were quitegas rich at the time of merging (fgas=75%-90%). However,because low-mass galaxies have a smaller fraction of baryons in theirthin disks, the pregalactic fragments from which they assembled musthave been systematically more gas poor. We believe this trend resultsfrom increased outflow due to supernova-driven winds in the lower masspregalactic fragments. We estimate that ~60% of the total baryonic massin these systems was lost due to outflows. Pushing the episode ofsignificant winds to early times allows the mass-metallicityrelationship for disks to be established early, before the main disk isassembled, and obviates the difficulty in driving winds from diffusedisks with low star formation efficiencies. We discuss otherimplications of this scenario for solving the G dwarf problem, forpredicting abundance trends in thick disks, and for removingdiscrepancies between semianalytic galaxy formation models and theobserved colors of low-mass 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.
| Molecular gas in the Andromeda galaxy
Aims.We study the distribution of the molecular gas in the Andromedagalaxy (M 31) and compare this with the distributions of the atomic gasand the emission from cold dust at λ 175 μm.Methods.Weobtained a new 12CO(J = 1-0)-line survey of the Andromedagalaxy with the highest resolution to date (23 arcsec, or 85 pc alongthe major axis), observed On-the-Fly with the IRAM 30-m telescope. Wefully sampled an area of 2°× 0.5 ° with a velocityresolution of 2.6{ km s-1}. In several selected regions wealso observed the 12CO(2-1)-line.Results.Emission fromthe 12CO(1-0) line was detected from galactocentric radiusR=3 kpc to R=16 kpc with a maximum in intensity at R 10 kpc. Themolecular gas traced by the (velocity-integrated) (1-0)-line intensityis concentrated in narrow arm-like filaments, which often coincide withthe dark dust lanes visible at optical wavelengths. Between R=4 kpc andR=12 kpc the brightest CO filaments define a two-armed spiral patternthat is described well by two logarithmic spirals with a pitch angle of7°-8°. The arm-interarm brightness ratio averaged over a lengthof 15 kpc along the western arms reaches about 20 compared to 4 for H Iat an angular resolution of 45 arcsec. For a constant conversion factorX_CO, the molecular fraction of the neutral gas is enhanced in thespiral arms and decreases radially from 0.6 on the inner arms to 0.3 onthe arms at R≃ 10 kpc. The apparent gas-to-dust ratios N(HI)/I175 and (N(H I)+2N(H_2))/I175 increase by afactor of 20 between the centre and R≃ 14{ kpc}, whereas theratio 2N(H_2)/I175 only increases by a factor of 4.Conclusions.Either the atomic and total gas-to-dust ratios increase bya factor of 20 or the dust becomes colder towards larger radii. Astrong variation of X_CO with radius seems unlikely. The observedgradients affect the cross-correlations between gas and dust. In theradial range R=8-14 kpc total gas and cold dust are well correlated;molecular gas correlates better with cold dust than atomic gas. The massof the molecular gas in M 31 within a radius of 18 kpc is M(H2) = 3.6× 108 {M}ȯ at theadopted distance of 780 kpc. This is 7% of the total neutral gas mass inM 31.
| A new method to determine the thickness of non-edge-on disk galaxies
Aims.We present a new method to determine the thickness of non-edge-ondisk galaxies. This method allows us to investigate the mass-to-lightratio of the disk. Methods: .Our method is based on the comparisonof observations and theory of the distribution of the vertical velocitydispersion, which is obtained from the solution of three dimensionalPoisson equations and the galactic dynamical equation. Results:.As examples, the thickness and mass-to-light ratio of two diskgalaxies, NGC 1566 and NGC 5247, which have been extensively studied byspectroscopy, have been calculated. The calculated results areconsistent with observations and support the use of this method.However, due to the small sample size available, the results should beconfirmed on other samples of galaxies.
| Outer structure of the Galactic warp and flare: explaining the Canis Major over-density
Aims.In this paper we derive the structure of the Galactic stellar warpand flare. Methods: .We use 2MASS red clump and red giant stars,selected at mean and fixed heliocentric distances ofRȯ≃3, 7 and 17 kpc. Results: .Our resultscan be summarized as follows: (i) a clear stellar warp signature isderived for the 3 selected rings, proving that the warp starts alreadywithin the solar circle; (ii) the derived stellar warp is consistent(both in amplitude and phase-angle) with that for the Galacticinterstellar dust and neutral atomic hydrogen; (iii) the consistency andregularity of the stellar-gaseous warp is traced out to aboutR_GC20 kpc; (iv) the Sun seems not to fall on the line of nodes.The stellar warp phase-angle orientation (φ15°) is closeto the orientation angle of the Galactic bar and this, most importantly,produces an asymmetric warp for the inner Rȯ≃3 and7 kpc rings; (v) a Northern/Southern warp symmetry is observed only forthe ring at Rȯ≃17 kpc, at which the dependency onφ is weakened; (vi) treating a mixture of thin and thick diskstellar populations, we trace the variation with R_GC of the diskthickness (flaring) and derive an almost constant scale-height (~0.65kpc) within R_GC15 kpc. Further out, the disk flaring increasegradually reaching a mean scale-height of ~1.5 kpc at R_GC23 kpc;(vii) the derived outer disk warping and flaring provide further robustevidence that there is no disk radial truncation at R_GC14 kpc. Conclusions: .In the particular case of the Canis Major (CMa)over-density we confirm its coincidence with the Southern stellarmaximum warp occurring near l240° (forRȯ≃7 kpc) which brings down the Milky Waymid-plane by ~3° in this direction. The regularity and consistencyof the stellar, gaseous and dust warp argues strongly against a recentmerger scenario for Canis Major. We present evidence to conclude thatall observed parameters (e.g. number density, radial velocities, propermotion etc) of CMa are consistent with it being a normal Milky Wayouter-disk population, thereby leaving no justification for more complexinterpretations of its origin. The present analysis or outer diskstructure does not provide a conclusive test of the structure or originof the Monoceros Ring. Nevertheless, we show that a warped flared MilkyWay contributes significantly at the locations of the Monoceros Ring.Comparison of outer Milky Way H I and CO properties with those of othergalaxies favors the suggestion that complex structures close to planarin outer disks are common, and are a natural aspect of warped andflaring disks.
| X-ray emission from radiative shocks in type II supernovae
The X-ray emission from the circumstellar interaction in type IIsupernovae with a dense circumstellar medium is calculated. In type IILand type IIn supernovae, mass loss rates are generally high enough forthe region behind the reverse shock to be radiative, producing strongradiation, particularly in X-rays. We present a model for the emissionfrom the cooling region in the case of a radiative reverse shock. Underthe assumption of a stationary flow, a hydrodynamic model is combinedwith a time-dependent ionization balance and multilevel calculations.The applicability of the steady state approximation is discussed forvarious values of the ejecta density gradient and different sets ofchemical composition. We show how the emerging spectrum strongly dependson the reverse shock velocity and the composition of the shocked gas. Wediscuss differences between a spectrum produced by this model and asingle-temperature spectrum. Large differences are found especially forthe line emission, which can seriously affect abundance estimates. Wealso illustrate the effects of absorption in the cool shocked ejecta.The applicability of our model to various types of supernovae isdiscussed.
| 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.
| A catalog of edge-on disk galaxies. From galaxies with a bulge to superthin galaxies
Spiral galaxies range from bulge-dominated early-type galaxies to latetypes with little or no bulge. Cosmological models do not predict theformation of disk-dominated, essentially bulgeless galaxies, yet theseobjects exist. A particularly striking and poorly understood example ofbulgeless galaxies are flat or superthin galaxies with large axisratios. We therefore embarked on a study aimed at a better understandingof these enigmatic objects, starting by compiling a statisticallymeaningful sample with well-defined properties. The disk axis ratios canbe most easily measured when galaxies are seen edge-on. We used datafrom the Sloan Digital Sky Survey (SDSS) in order to identify edge-ongalaxies with disks in a uniform, reproducible, automated fashion. Inthe five-color photometric database of the SDSS Data Release 1 (2099deg^2) we identified 3169 edge-on disk galaxies, which we subdividedinto disk galaxies with bulge, intermediate types, and simple diskgalaxies without any obvious bulge component. We subdivided these typesfurther into subclasses: Sa(f), Sb(f), Sc(f), Scd(f), Sd(f), Irr(f),where the (f) indicates that these galaxies are seen edge-on. Here wepresent our selection algorithm and the resulting catalogs of the 3169edge-on disk galaxies including the photometric, morphological, andstructural parameters of our targets. A number of incompleteness effectsaffect our catalog, but it contains almost a factor of four morebulgeless galaxies with prominent simple disks (flat galaxies) withinthe area covered here than previous optical catalogs, which were basedon the visual selection from photographic plates (cf. Karachentsev etal. 1999, Bull. Special Astrophys. Obs., 47, 5). We find thatapproximately 15% of the edge-on disk galaxies in our catalog are flatgalaxies, demonstrating that these galaxies are fairly common,especially among intermediate-mass star-forming galaxies. Bulgelessdisks account for roughly one third of our galaxies when also puffydisks and edge-on irregulars are included. Our catalog provides auniform database for a multitude of follow-up studies of bulgelessgalaxies in order to constrain their intrinsic and environmentalproperties and their evolutionary status.
| 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.
| The Diffuse Ionized Gas in the large telescopes era
In this workshop we summarize the ``state of the art'' of the DiffuseIonized Gas. We present all the possible situations which can produceionization outside an H II region, as well as some of the observationsthat can be performed with the GTC instrumentation and how relevant theycan be in the undestanding of the ionization mechanisms of the DIG.
| Subarcsecond radio observations of the dwarf starburst galaxy NGC 3077
We present the first subarcsecond radio observations of the nearby dwarfstarburst galaxy NGC 3077 obtained with the MERLIN interferometer. Wehave detected two resolved sources which are coincident with thepositions of two discrete X-ray sources detected by Chandra. One of theradio sources is associated with a supernova remnant (SNR) and theobserved radio flux is consistent with having a non-thermal origin. Theage of the SNRs of about 760 yr is between the average age of the SNRsdetected in M82 and those detected in the Milky Way and the LargeMagellanic Cloud. We use this detection to calculate a star formationrate (SFR) of 0.28 Msolar yr-1, which is similarto the SFR calculated by using far-infrared and millimetre observationsbut larger than the SFR given by optical recombination lines correctedfor extinction. The other compact radio source detected by MERLIN, whichis coincident with the position of an X-ray binary, has the propertiesof an HII region with a flux density of about 747 μJy, whichcorresponds to an ionizing flux of 6.8 × 1050s-1. A young massive stellar cluster with a mass of ~2× 105 Msolar detected by the Hubble SpaceTelescope could be responsible for the production of the ionizing flux.
| Cold dust in (some) high-z supernova host galaxies
We present deep submillimetre photometry for 14 galaxies at z= 0.5 thatare hosts of Type Ia supernovae, with the aim of examining the evolutionof dust mass and extinction in normal galaxies. We combine these resultswith our previous observations of 17 z~ 0.5 SN1a hosts to look for anyevolution in the dust content of normal galaxies between z= 0 and 0.5.The average observed-frame 850-μm flux of SN1a hosts in the fullsample, excluding two bright individually detected objects, is 0.44 +/-0.22 mJy. This flux level is consistent with there being little or noevolution in the dust content, or optical extinction, of normal galaxiesfrom z= 0 to 0.5. One galaxy, the host of SN1996cf, is detectedindividually, and we also present a deep Hubble Space Telescope (HST)Space Telescope Imaging Spectrograph (STIS) image for this object. Itappears to be an edge-on disc system, similar to the submillimetrebright host of SN1997ey. We thus examine the dust properties of theseand one other individually detected object. Flux ratios and limits of450-850 μm suggest that the dust in the two brightest submillimetresources, SN1996cf and SN1997ey, is cold, T~ 20 K, implying that theycontain a substantial mass of dust ~109Msolar. Thepresence of two bright (F850 > 7 mJy) submillimetresources at z~ 0.5 in a sample of ostensibly normal galaxies issurprising, and has important implications. It supports the idea that asubstantial part of the cosmic infrared background (CIB) may be producedat z < 1, while also suggesting that `foreground' objects such asthese may be a significant `contaminant' in submillimetre surveys.Finally, we examine the overall submillimetre luminosity distribution atz= 0.5 implied by our results, and conclude that either there issubstantial evolution in the submillimetre luminosity function from z= 0to 0.5, or our submillimetre-detected sources are somehow notrepresentative of the bulk of galaxies at this redshift.
| 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.
| X-ray observations of the edge-on star-forming galaxy NGC 891 and its supernova SN1986J
We present XMM-Newton observations of NGC 891, a nearby edge-on spiralgalaxy. We analyse the extent of the diffuse emission emitted from thedisc of the galaxy, and find that it has a single-temperature profilewith best-fitting temperature of 0.26 keV, though the fit of adual-temperature plasma with temperatures of 0.08 and 0.30 keV is alsoacceptable. There is a considerable amount of diffuse X-ray emissionprotruding from the disc in the north-west direction out toapproximately 6 kpc. We analyse the point-source population using aChandra observation, using a maximum-likelihood method to find that theslope of the cumulative luminosity function of point sources in thegalaxy is -0.77+0.13-0.1. Using a sample of otherlocal galaxies, we compare the X-ray and infrared properties of NGC 891with those of `normal' and starburst spiral galaxies, and conclude thatNGC 891 is most likely a starburst galaxy in a quiescent state. Weestablish that the diffuse X-ray luminosity of spirals scales with thefar-infrared luminosity asLX~L0.87+/-0.07FIR, except for extremestarbursts, and NGC 891 does not fall in the latter category. We studythe supernova SN1986J in both XMM-Newton and Chandra observations, andfind that the X-ray luminosity has been declining with time more steeplythan expected (LX~t-3).
| GHASP: an Hα kinematic survey of spiral and irregular galaxies - IV. 44 new velocity fields. Extension, shape and asymmetry of Hα rotation curves
We present Fabry-Perot observations obtained in the frame of the GHASPsurvey (Gassendi HAlpha survey of SPirals). We have derived the Hαmap, the velocity field and the rotation curve for a new set of 44galaxies. The data presented in this paper are combined with the datapublished in the three previous papers providing a total number of 85 ofthe 96 galaxies observed up to now. This sample of kinematical data hasbeen divided into two groups: isolated (ISO) and softly interacting(SOFT) galaxies. In this paper, the extension of the Hα discs, theshape of the rotation curves, the kinematical asymmetry and theTully-Fisher relation have been investigated for both ISO and SOFTgalaxies. The Hα extension is roughly proportional toR25 for ISO as well as for SOFT galaxies. The smallestextensions of the ionized disc are found for ISO galaxies. The innerslope of the rotation curves is found to be correlated with the centralconcentration of light more clearly than with the type or thekinematical asymmetry, for ISO as well as for SOFT galaxies. The outerslope of the rotation curves increases with the type and with thekinematical asymmetry for ISO galaxies but shows no special trend forSOFT galaxies. No decreasing rotation curve is found for SOFT galaxies.The asymmetry of the rotation curves is correlated with themorphological type, the luminosity, the (B-V) colour and the maximalrotational velocity of galaxies. Our results show that the brightest,the most massive and the reddest galaxies, which are fast rotators, arethe least asymmetric, meaning that they are the most efficient withwhich to average the mass distribution on the whole disc. Asymmetry inthe rotation curves seems to be linked with local star formation,betraying disturbances of the gravitational potential. The Tully-Fisherrelation has a smaller slope for ISO than for SOFT galaxies.
| Structure and kinematics of edge-on galaxy discs - V. The dynamics of stellar discs
In earlier papers in this series we determined the intrinsic stellardisc kinematics of 15 intermediate- to late-type edge-on spiral galaxiesusing a dynamical modelling technique. The sample covers a substantialrange in maximum rotation velocity and deprojected face-on surfacebrightness, and contains seven spirals with either a boxy orpeanut-shaped bulge. Here we discuss the structural, kinematical anddynamical properties. From the photometry we find that intrinsicallymore flattened discs tend to have a lower face-on central surfacebrightness and a larger dynamical mass-to-light ratio. This observationsuggests that, at a constant maximum rotational velocity, lower surfacebrightness discs have smaller vertical stellar velocity dispersions.Although the individual uncertainties are large, we find from thedynamical modelling that at least 12 discs are submaximal. The averagedisc contributes 53 +/- 4 per cent to the observed rotation at 2.2 discscalelengths (hR), with a 1σ scatter of 15 per cent.This percentage becomes somewhat lower when effects of finite discflattening and gravity by the dark halo and the gas are taken intoaccount. Since boxy and peanut-shaped bulges are probably associatedwith bars, the result suggests that at 2.2hR the submaximalnature of discs is independent of barredness. The possibility remainsthat very high surface brightness discs are maximal, as these discs areunderrepresented in our sample. We confirm that the radial stellar discvelocity dispersion is related to the galaxy maximum rotationalvelocity. The scatter in this σ versus vmax relationappears to correlate with the disc flattening, face-on central surfacebrightness and dynamical mass-to-light ratio. Low surface brightnessdiscs tend to be more flattened and have smaller stellar velocitydispersions. The findings are consistent with the observed correlationbetween disc flattening and dynamical mass-to-light ratio and cangenerally be reproduced by the simple collapse theory for disc galaxyformation. Finally, the disc mass Tully-Fisher relation is offset fromthe maximum-disc scaled stellar mass Tully-Fisher relation of the UrsaMajor cluster. This offset, -0.3 dex in mass, is naturally explained ifthe discs of the Ursa Major cluster spirals are submaximal.
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