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Revisiting the infrared spectra of active galactic nuclei with a new torus emission model
We describe improved modelling of the emission by dust in atoroidal-like structure heated by a central illuminating source withinactive galactic nuclei (AGNs). We have chosen a simple but realistictorus geometry, a flared disc, and a dust grain distribution functionincluding a full range of grain sizes. The optical depth within thetorus is computed in detail taking into account the differentsublimation temperatures of the silicate and graphite grains, whichsolves previously reported inconsistencies in the silicate emissionfeature in type 1 AGNs. We exploit this model to study the spectralenergy distributions (SEDs) of 58 extragalactic (both type 1 and type 2)sources using archival optical and infrared data. We find that both AGNand starburst contributions are often required to reproduce the observedSEDs, although in a few cases they are very well fitted by a pure AGNcomponent. The AGN contribution to the far-infrared luminosity is foundto be higher in type 1 sources, with all the type 2 requiring asubstantial contribution from a circumnuclear starburst. Our resultsappear in agreement with the AGN unified scheme, because thedistributions of key parameters of the torus models turn out to becompatible for type 1 and type 2 AGNs. Further support to theunification concept comes from comparison with medium-resolutioninfrared spectra of type 1 AGNs by the Spitzer observatory, showingevidence for a moderate silicate emission around 10 μm, which ourcode reproduces. From our analysis we infer accretion flows in the innernucleus of local AGNs characterized by high equatorial optical depths(AV~= 100), moderate sizes (Rmax < 100 pc) andvery high covering factors (f~= 80 per cent) on average.
| The Polycyclic Aromatic Hydrocarbon Emission Deficit in Low-Metallicity Galaxies-A Spitzer View
Archival observations of 18 starburst galaxies that span a wide range inmetallicity reveal for the first time a correlation between the ratio ofemission-line fluxes of [Fe II] at 26 μm and [Ne II] at 12.8 μmand the 7.7 μm PAH strength, with the [Fe II]/[Ne II] flux ratiodecreasing with increasing PAH strength. We also find a strongcorrelation between the [Fe II]/[Ne II] flux ratio and the host galaxymetallicity, with the flux ratio decreasing with increasing metallicity.Since [Fe II] emission has been linked primarily to supernova shocks, weattribute the high [Fe II]/[Ne II] ratios in low-metallicity galaxies toenhanced supernova activity. We consider this to be a dominant mechanismfor PAH destruction, rather than grain destruction in photoionizedregions surrounding young massive stars. We also consider whether theextreme youth of the low-metallicity galaxies is responsible for thelack of PAH emission.
| Extragalactic Science with Tunable Filters
Tunable filters provide unique capabilities to carry out a wide array ofextragalactic projects. The emphasis of this review is on sciencerelating to starburst and active galaxies. Future avenues of researchwith 8-meter class telescopes equipped with tunable filters are alsodiscussed briefly.
| The evolution of actively star-forming galaxies in the mid-infrared
In this paper we analyze the evolution of actively star-forming galaxiesin the mid-infrared (MIR). This spectral region, characterized bycontinuum emission by hot dust and by the presence of strong emissionfeatures generally ascribed to polycyclic aromatic hydrocarbon (PAH)molecules, is the most strongly affected by the heating processesassociated with star formation and/or active galactic nuclei (AGNs).Following the detailed observational characterization of galaxies in theMIR by the Infrared Space Observatory (ISO), we have updated themodelling of this spectral region in our spectrophotometric modelGRASIL. In the diffuse component we have updated the treatment of PAHsaccording to the model by Li & Draine. As for the dense phase of theinterstellar medium associated with the star-forming regions, themolecular clouds, we strongly decrease the abundance of PAHs as comparedto that in the cirrus, based on the observational evidence of the lackor weakness of PAH bands close to the newly formed stars, possibly dueto the destruction of the molecules in strong ultraviolet fields. Therobustness of the model is checked by fitting near-infrared to radiobroad-band spectra and the corresponding detailed MIR spectra of a largesample of galaxies, at once. With this model, we have analyzed thelarger sample of actively star-forming galaxies by Dale et al. We showthat the observed trends of galaxies in the ISO-IRAS-radio colour-colourplots can be interpreted in terms of the different evolutionary phasesof star formation activity, and the consequent different dominance inthe spectral energy distribution of the diffuse or dense phase of theISM. We find that the observed colours indicate a surprising homogeneityof the starburst phenomenon, allowing only a limited variation of themost important physical parameters, such as the optical depth of themolecular clouds, the time-scale of the escape of young stars from theirfor mation sites, and the gas consumption time-scale. In this paper wedo not attempt to reproduce the far-infrared coolest region in thecolour-colour plots, as we concentrate on models meant to reproduceactive star-forming galaxies, but we discuss possible requirements of amore complex modelling for the coldest objects.
| 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).
| A radio study of the superwind galaxy NGC 1482
We present multifrequency radio continuum as well as HI observations ofthe superwind galaxy NGC 1482, with both the Giant Metrewave RadioTelescope (GMRT) and the Very Large Array (VLA). This galaxy has aremarkable hourglass-shaped optical emission-line outflow as well asbipolar soft X-ray bubbles on opposite sides of the galactic disc. Thelow-frequency, lower-resolution radio observations show a smoothstructure. From the non-thermal emission, we estimate the availableenergy in supernovae, and examine whether this would be adequate todrive the observed superwind outflow. The high-frequency,high-resolution radio image of the central starburst region located atthe base of the superwind bi-cone shows one prominent peak and moreextended emission with substructure. This image has been compared withthe infrared, optical red continuum, Hα, and soft and hard X-rayimages from Chandra to understand the nature and relationship of thevarious features seen at different wavelengths. The peak of the infraredemission is the only feature that is coincident with the prominent radiopeak, and possibly defines the centre of the galaxy.The HI observations with the GMRT show two blobs of emission on oppositesides of the central region. These are rotating about the centre of thegalaxy and are located at ~2.4 kpc from it. In addition, theseobservations also reveal a multicomponent HI absorption profile againstthe central region of the radio source, with a total width of ~250 kms-1. The extreme blue- and redshifted absorption componentsare at 1688 and 1942 km s-1, respectively, while the peakabsorption is at 1836 km s-1. This is consistent with theheliocentric systemic velocity of 1850 +/- 20 km s-1,estimated from a variety of observations. We discuss possibleimplications of these results.
| Radio Continuum and Far-infrared Emission from the Galaxies in the Eridanus Group
The Eridanus galaxies follow the well-known radio-FIR correlation. Themajority (70%) of these galaxies have their star formation rates belowthat of the Milky Way. The galaxies that have a significant excess ofradio emission are identified as low luminosity AGNs based on theirradio morphologies obtained from the GMRT observations. There are nopowerful AGNs (L20 cm>1023WHz-1) in the group. The twomost far-infrared and radio luminous galaxies in the group have opticaland HI morphologies suggestive of recent tidal interactions. TheEridanus group also has two far-infrared luminous but radio-deficientgalaxies. It is believed that these galaxies are observed within a fewMyr of the onset of an intense star formation episode after beingquiescent for at least a 100 Myr. The upper end of the radio luminositydistribution of the Eridanus galaxies (L20 cm1022WHz-1) isconsistent with that of the field galaxies, other groups, and late-typegalaxies in nearby clusters.
| The HI Content of the Eridanus Group of Galaxies
The HI content of galaxies in the Eridanus group is studied using theGMRT observations and the HIPASS data. A significant HI deficiency up toa factor of 2-3 is observed in galaxies in the high galaxy densityregions. The HI deficiency in galaxies is observed to be directlycorrelated to the local projected galaxy density, and inverselycorrelated to the lineof-sight radial velocity. Furthermore, galaxieswith larger optical diameters are predominantly in the lower galaxydensity regions. It is suggested that the HI deficiency in Eridanus isdue to tidal interactions. In some galaxies, evidences of tidalinteractions are seen. An important implication is that significantevolution of galaxies can take place in the group environment. In thehierarchical way of formation of clusters via mergers of groups, afraction of the observed HI deficiency in clusters could have originatedin groups. The co-existence of S0s and severely HI deficient galaxies inthe Eridanus group suggests that tidal interaction is likely to be aneffective mechanism for transforming spirals to S0s.
| GMRT HI Observations of the Eridanus Group of Galaxies I.
The GMRT HI 21cm-line observations of galaxies in the Eridanus group arepresented. The Eridanus group, at a distance of ~23 Mpc, is a loosegroup of ~200 galaxies. The group extends to more than 10 Mpc inprojection. The velocity dispersion of the galaxies in the group is ~240km s-1. The galaxies are clustered into different sub-groups. Theoverall population mix of the group is 30% (E + S0) and 70% (Sp + Irr).The observations of 57 Eridanus galaxies were carried out with the GMRTfor ~ 200 h. HI emission was detected from 31 galaxies. The channel rmsof ~1 mJy beam-1 was achieved for most of the image-cubes made with 4 hof data. The corresponding HI column density sensitivity (3σ) is~1 × 1020 cm-2 for a velocity-width of ~ 13.4 km s-1.The 3σ detection lss surface densities, HI disk parameters and HIrotation curves are presented. The velocity fields are analysedseparately for the approaching and the receding sides of the galaxies.These data will be used to study the HI and the radio continuumproperties, the Tully-Fisher relations, the dark matter halos, and thekinematical and HI lopsidedness in galaxies.
| Outflows from three active galaxies: NGC 1482, NGC 4438 and NGC 6764.
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| Highlights from the Observatories
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| 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.
| 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.
| Gemini Near-Infrared Spectrograph Observations of a Red Star-forming Galaxy at z=2.225: Evidence of Shock Ionization Due to a Galactic Wind
Recent studies have shown that K-luminous galaxies at 22 galaxies should be regardedwith caution, especially since the existence of strong galactic winds inthese objects is well established. Based on Sloan Digital Sky Surveydata for nearby galaxies and the limited data available at highredshift, we speculate that the effects of shocks may correlate withdust content. The results presented here demonstrate the importance ofmeasuring the full rest-frame optical spectra of high-redshift galaxiesand showcase the potential of GNIRS for such studies.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership.
| Hα Imaging of Early-Type Sa-Sab Spiral Galaxies. II. Global Properties
New results, based on one of the most comprehensive Hα imagingsurveys of nearby Sa-Sab spirals completed to date, reveals early-typespirals to be a diverse group of galaxies that span a wide range inmassive star formation rates. While the majority of Sa-Sab galaxies inour sample are forming stars at a modest rate, a significant fraction(~29%) exhibit star formation rates greater than 1 Msolaryr-1, rivaling the most prolifically star-forming late-typespirals. A similar diversity is apparent in the star formation historyof Sa-Sab spirals as measured by their Hα equivalent widths.Consistent with our preliminary results presented in the first paper inthis series, we find giant H II regions [L(Hα)>=1039ergs s-1] in the disks of ~37% of early-type spirals. Wesuspect that recent minor mergers or past interactions are responsiblefor the elevated levels of Hα emission and, perhaps, for thepresence of giant H II regions in these galaxies. Our results, however,are not in total agreement with the Hα study of Kennicutt &Kent, who did not find any early-type spirals with Hα equivalentwidths >14 Å. A close examination of the morphologicalclassification of galaxies, however, suggests that systematicdifferences between the Revised Shapley-Ames Catalog and the SecondReference Catalogue may be responsible for the contrasting results.Based on observations obtained with the 3.5 m telescope at Apache PointObservatory (APO) and the 0.9 m telescope at Kitt Peak NationalObservatory (KPNO). The APO 3.5 m telescope is owned and operated by theAstrophysical Research Consortium.
| GMRT observations of the group Holmberg 124: Evolution by tidal forces and ram pressure?
We report new radio continuum and 21 cm HI observations using the GiantMetrewave Radio Telescope (GMRT) of the group Holmberg 124 (Ho 124)comprising four late-type galaxies, namely NGC 2820, Mrk 108, NGC 2814and NGC 2805. The three galaxies, NGC 2820, Mrk 108 and NGC 2814 whichare closely located in the sky plane have clearly undergone tidalinteractions as seen from the various morphological tidal signatures anddebris. Moreover we note various features in the group members which webelieve might be due to ram pressure. In this paper, we describe fourinteresting results emerging from our observations: a) detection of thetidal radio continuum bridge at 330 MHz connecting the galaxies NGC2820+Mrk 108 with NGC 2814. The radio bridge was discovered at 1465 MHzby van der Hulst & Hummel (1985, A&A, 150, 17). We find that thebridge has a fairly steep spectrum with a spectral index α (S∝ να) of -1.8+0.3-0.2which is much steeper than the -0.8 quoted by van der Hulst & Hummel(1985); b) detection of other tidal features like the tilted HI andradio continuum disk of NGC 2814, a HI streamer and a radio continuumtail arising from the south of NGC 2814. We also report the detection ofa possible tidal dwarf galaxy in HI; c) sharp truncation in the HIdistribution in the south of NGC 2820 and in the HI and radio continuumdistribution in the north of NGC 2814. The optical disks in both thecases look undisturbed. As pointed out by Davis et al. (1997, AJ, 114,613), ram pressure affects different components of the interstellarmedium to varying degrees. Simple estimates of pressure in differentcomponents of the interstellar medium (radio continuum, Hα and HI)in NGC 2820 indicate that ram pressure will significantly influence HI;d) detection of a large one-sided HI loop to the north of NGC 2820. Noradio continuum emission or Hα emission is associated with the HIloop. We discuss various scenarios for the origin of this loop includinga central starburst, ram pressure stripping and tidal interaction. We donot support the central starburst scenario since the loop is notdetected in ionized gas. Using the upper limit on X-ray luminosity of Ho124 (Mulchaey et al. 2003, ApJS, 145, 39), we estimate an upper limit onthe intragroup medium (IGrM) density of 8.8×10-4cm-3. For half this electron density, we estimate the rampressure force of the IGrM to be comparable to the gravitational pull ofthe disk of NGC 2820. Since tidal interaction has obviously influencedthe group, we suggest that the loop could have formed by ram pressurestripping if tidal effects had reduced the surface density of HI in NGC2820. From the complex observational picture of Ho 124 and the numericalestimates, we suggest that the evolution of the Ho 124 group may begoverned by both tidal forces due to the interaction and the rampressure due to motion of the member galaxies in the IGrM and that theIGrM densities should not be too low (i.e. ≥4×10-4). However this needs to be verified by furtherobservations.
| ISO observations of the Galactic center interstellar medium. Ionized gas
We present fine structure and recombination line observations of theionized gas toward a sample of 18 sources located within 300 pc of thecenter of the Galaxy (hereafter Galactic center, GC). The sources wereselected as molecular clouds located far from thermal continuum sources.The fine structure lines from [NII] and [SIII] were detected in 16sources. In 10 sources we even detected the [OIII] 88 μm line.Several techniques have been used to determine lower and upper limits tothe extinction toward each source to correct the observed line fluxes.The derived electron densities of the ionized gas vary from 100 to$≤q$30 cm-3. For some sources we were able to derive N, Sand Ne abundances. We found that they are similar to those measured inthe HII regions in the 5-kpc ring and in the nuclei of starburstgalaxies. The fine structure lines ratios measured for all the sourcescan be explained by photo-ionization with an effective temperature ofthe ionizing radiation of 32 000-37 000 K and an ionization parameter,U, of -1>log U > -3. The highest excitation is found in the RadioArc region but it does not decrease smoothly with distance. There mustbe more ionizing sources distributed over the Galactic center than theknown clusters of massive stars. Most of the clouds are located far (upto 45 pc for M-0.96+0.13) from the prominent continuum complexes (Sgr C,B ...). However, it is possible that the clouds are ionized by escapedphotons from those complexes. The comparison of the effectivetemperatures of the ionizing radiation to the measured Lyman continuumphoton emission rate implies that the clouds are indeed ionized bydistant sources. The excitation ratios, effective temperature andionization parameter measured in the GC are similar to those found insome low excitation starburst galaxies. The [NeIII]/[NeII] line ratiosmeasured in the GC sources are consistent with the results of theThornely et al. (2000) model for a short burst of massive star formationless than 8 Myr ago. We have also found that the [NeII] 13 μm tofar-infrared continuum ratio measured for the GC sources is similar tothat of external galaxies, supporting the idea of Sturm et al.(\cite{Sturm2002}) that the far-infrared continuum in Active Galaxies isdominated by dust heated by stellar radiation rather than by the AGN.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.
| Classification of Spectra from the Infrared Space Observatory PHT-S Database
We have classified over 1500 infrared spectra obtained with the PHT-Sspectrometer aboard the Infrared Space Observatory according to thesystem developed for the Short Wavelength Spectrometer (SWS) spectra byKraemer et al. The majority of these spectra contribute to subclassesthat are either underrepresented in the SWS spectral database or containsources that are too faint, such as M dwarfs, to have been observed byeither the SWS or the Infrared Astronomical Satellite Low ResolutionSpectrometer. There is strong overall agreement about the chemistry ofobjects observed with both instruments. Discrepancies can usually betraced to the different wavelength ranges and sensitivities of theinstruments. Finally, a large subset of the observations (~=250 spectra)exhibit a featureless, red continuum that is consistent with emissionfrom zodiacal dust and suggest directions for further analysis of thisserendipitous measurement of the zodiacal background.Based on observations with the Infrared Space Observatory (ISO), aEuropean Space Agency (ESA) project with instruments funded by ESAMember States (especially the Principle Investigator countries: France,Germany, Netherlands, and United Kingdom) and with the participation ofthe Institute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| A High Spatial Resolution X-Ray and Hα Study of Hot Gas in the Halos of Star-forming Disk Galaxies. I. Spatial and Spectral Properties of the Diffuse X-Ray Emission
We present arcsecond resolution Chandra X-ray and ground-based opticalHα imaging of a sample of 10 edge-on star-forming disk galaxies(seven starburst and three ``normal'' spiral galaxies), a sample thatcovers the full range of star formation intensity found in diskgalaxies. The X-ray observations make use of the unprecedented spatialresolution of the Chandra X-ray observatory to more robustly than beforeremove X-ray emission from point sources and hence obtain the X-rayproperties of the diffuse thermal emission alone. We have combined theX-ray observations with existing, comparable-resolution, ground-basedHα and R-band imaging and present a mini-atlas of images on acommon spatial and surface brightness scale to aid cross-comparison. Ingeneral, the morphology of the extraplanar diffuse X-ray emission isvery similar to the extraplanar Hα filaments and arcs, on bothsmall and large scales (scales of tens of parsecs and kiloparsecs,respectively). The most spectacular cases of this are found in NGC 1482(for which we provide the first published X-ray observation) and NGC3079. We provide a variety of quantitative measures of how the spectralhardness and surface brightness of the diffuse X-ray emission varieswith increasing height z above the plane of each galaxy. Of the eightgalaxies in which diffuse X-ray emitting halos are found (the starburstsand the normal spiral NGC 891), significant spatial variation in thespectral properties of the extraplanar emission (|z|>=2 kpc) is onlyfound in two cases: NGC 3628 and NGC 4631. In general, the verticaldistribution of the halo-region X-ray surface brightness is bestdescribed as an exponential, with the observed scale heights of thesample galaxies lying in the range Heff~2-4 kpc. The presenceof extraplanar X-ray emission is always associated with the presence ofextraplanar optical line emission of similar vertical extent. No X-rayemission was detected from the halos of the two low-mass normal spiralgalaxies NGC 6503 and NGC 4244. Active galactic nuclei, where present,appear to play no role in powering or shaping the outflows from thestarburst galaxies in this sample. The Chandra ACIS X-ray spectra ofextraplanar emission from all these galaxies can be fitted with a commontwo-temperature spectral model with an enhanced α-to-iron elementratio. This is consistent with the origin of the X-ray emitting gasbeing either metal-enriched merged SN ejecta or shock-heated ambienthalo or disk material with moderate levels of metal depletion onto dust.Our favored model is that SN feedback in the disks of star-forminggalaxies create, via blow-out and venting of hot gas from the disk,tenuous exponential atmospheres of density scale heightHg~4-8 kpc. The soft thermal X-ray emission observed in thehalos of the starburst galaxies is either this preexisting halo medium,which has been swept up and shock-heated by the starburst-driven wind,or wind material compressed near the walls of the outflow by reverseshocks within the wind. In either case, the X-ray emission provides uswith a powerful probe of the properties of gaseous halos aroundstar-forming disk galaxies.
| A High Spatial Resolution X-Ray and Hα Study of Hot Gas in the Halos of Star-forming Disk Galaxies. II. Quantifying Supernova Feedback
We investigate how the empirical properties of hot X-ray-emitting gas ina sample of seven starburst and three normal edge-on spiral galaxies (asample that covers the full range of star formation intensity found indisk galaxies) correlate with the size, mass, star formation rate, andstar formation intensity in the host galaxies. From this analysis weinvestigate various aspects of mechanical energy ``feedback''-the returnof energy to the interstellar medium from massive star supernovae andstellar winds-on galactic scales. The X-ray observations make use of theunprecedented spatial resolution of the Chandra X-Ray Observatory toremove X-ray emission from point sources more accurately than in anyprevious study and hence obtain the X-ray properties of the diffusethermal emission alone. Intriguingly, the diffuse X-ray properties ofthe normal spirals (in both their disks and halos) fall whereextrapolation of the trends from the starburst galaxies with superwindswould predict. We demonstrate, using a variety of multiwavelength starformation rate and intensity indicators, that the luminosity of diffuseX-ray emission in the disk (and, where detected, in the halo) isdirectly proportional to the rate of mechanical energy feedback frommassive stars in the host galaxies. Accretion of gas from theintergalactic medium (IGM) does not appear to be a significantcontributor to the diffuse X-ray emission in this sample. Nevertheless,with only three nonstarburst normal spiral galaxies it is hard toexclude an accretion-based origin for extraplanar diffuse X-ray emissionaround normal star-forming galaxies. Larger galaxies tend to have moreextended X-ray-emitting halos, but galaxy mass appears to play no rolein determining the properties of the disk or extraplanar X-ray-emittingplasma. The combination of these luminosity and size correlations leadsto a correlation between the surface brightness of the diffuse X-rayemission and the mean star formation rate per unit area in the disk(calculated from the far-infrared luminosity and the optical size of thegalaxy, LFIR/D225). Furtherobservational work of this form will allow empirical constraints to bemade on the critical star formation rate per unit disk area necessary toblow hot gas out of the disk into the halo. We show that a minorgeneralization of standard superbubble theory directly predicts acritical star formation rate per unit area for superbubble blowout fromthe disk and by extension for superwinds to blow out of the gaseoushalos of their host galaxy. At present there are a variety of poorlyknown parameters in this theory that complicate comparison betweenobservation and theory, making it impossible to assess the quantitativeaccuracy of standard superbubble blowout theory. We argue that thecrucial spatial region around a galaxy that controls whether gas instarburst-driven superwinds will escape into the IGM is not the outerhalo ~100 kpc from the host galaxy, but the inner few halo scaleheights, within ~20 kpc of the galaxy plane. Given the properties of thegaseous halos we observe, superwind outflows from disk galaxies of massM~1010-1011 Msolar should still ejectsome fraction of their material into the IGM.
| Optical Imaging and Spectroscopy of the Edge-on Sbc Galaxy UGC 10043: Evidence for a Galactic Wind and a Peculiar Triaxial Bulge
We present new optical imaging and spectroscopy of the peculiar, edge-onSbc galaxy UGC 10043. Using the WIYN telescope, we have obtained B, R,and Hα+[NII] images, together with DensePak integral fieldspectroscopic measurements of the stellar Ca II infrared triplet and theHα and [N II] lines from the ionized gas. The imaging observationsshow that the inner bulge of UGC 10043 (a<=7.5") is elongatedperpendicular to the galaxy major axis. At larger r the bulge isophotestwist to become oblate and nearly circular, suggesting the bulge istriaxial. The bulge shows no clear evidence for rotation about eitherits major or minor axis. The inner, southwestern quadrant of the bulgeis girdled by a narrow dust lane parallel to the minor axis; unsharpmasking reveals that this minor-axis dust lane may be part of an innerpolar ring, although we find no unambiguous kinematic evidence oforthogonally rotating material. The stellar disk of UGC 10043 has arather low optical surface brightness [μ(0)R,i~23.2 magarcsec-2], a small scale height (hz=395 pc forD=33.4 Mpc), and a mild integral sign warp. A dusty, inner diskcomponent that appears tilted relative to the outlying disk is alsoseen. The Hα and [N II] emission lines in UGC 10043 resolve intomultiple velocity components, indicating the presence of a large-scalegalactic wind with an outflow velocity of Vout>~104 kms-1. Hα+[NII] imaging reaffirms this picture byrevealing ionized gas extended to |z|~3.5 kpc in the form of a roughlybiconical structure. The [N II]/Hα line intensity ratio increaseswith increasing distance from the plane, reaching values as high as 1.7.Unlike most galaxies with large-scale winds, UGC 10043 has only a modestglobal star formation rate (<~1 Msolar yr-1),implying the wind is powered by a rather feeble central starburst. Wediscuss evolutionary scenarios that could account for both thestructural complexities of UGC 10043 and its large-scale wind. The mostplausible scenarios require a major accretion or merger event at least afew gigayears ago.
| The rotating visible outflow in M 82
M 82's minor axis outflow is seen at visible wavelengths as more or lessregular hollow cones on both sides of the galactic disk. The outflowingmaterial is expected to entrain the rotation, or part of the rotation,of the disk where the outflow originates. From the conservation ofangular momentum it is furthermore expected that the outflowing materialcontinues to rotate at large distances from the disk, although probablywith smaller velocity because of radial divergence of the cones. Weprovide evidence of this kinematic picture from long-slit spectra of thecone wall H\alpha, [NII] and [SII] emission lines taken at±20'' (±300 pc) and ±40'' (±600pc) distance from the center and parallel to the minor axis, from dataextracted from the literature, and from a cone model fit of the data.The angular momentum which is entrained in the outflow and eventuallydissipated is a small fraction of the total angular momentum associatedwith the stars and gas in the central part of the disk. We compare ourobservation of the visible outflow with the outflow of dragged-outmaterial investigated in mm-wavelength CO by other observers. It seemsthat the material observed at visible wavelengths is confined to narrowcones, and blows out at velocities larger than the escape velocity ofthe galaxy. The dragged-out material moves at slower velocities and onwider cones, and may fall back into the galaxy.
| Star formation in disk and spheroidal galaxies
A model of dynamical and chemical evolution for spheroidal and diskgalaxies is discussed. The star formation rate in the model is regulatedby ionization of the interstellar hydrogen by massive O,B stars. Thevolume of a galaxy is determined by the condition of equilibrium betweencollisional dissipation of turbulent energy of interstellar gas andturbulisation of the gas by supernovae. The modern theory of stellarevolution helps to describe evolution of the stellar component of agalaxy in time and to estimate the current frequency of supernovae. Thismodel reproduces some basic observed parameters of spheroidal and diskgalaxies and their evolution on the Hubble timescale. The role ofexternal factors in evolution of galaxies and possible reasons of anonstationary star formation in galaxies and their nuclei are shortlydiscussed.
| Science with Tunable Filters
In just two years, the OSIRIS tunable filter spectrograph on the 10 mGTC will herald a new era in spectrophotometric imaging, from 350 nm to1 μ m. Like its forebear, the TTF at the Anglo-Australian Telescope(AAT), OSIRIS will offer a wide variety of observing modes linked tocharge shuffling in order to achieve exquisite differential imaging. Insome respects, simply repeating the many science cases conducted at theAAT will lead to advances in a number of fields. This is all butguaranteed by the better apparatus, observing conditions and largeraperture. However, the expected improvement in sensitivity suggests manynew avenues: large surveys of extended sources, absorption line imaging,time series and coronographic imaging, to name a few. OSIRIS willprovide some of the deepest photometric ``diffuse light'' images todate, much better than what can be achieved with an integral fieldspectrograph, and over a much wider field of view.
| SINGS: The SIRTF Nearby Galaxies Survey
The SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging andspectroscopic survey of 75 nearby galaxies. Its primary goal is tocharacterize the infrared emission of galaxies and their principalinfrared-emitting components, across a broad range of galaxy propertiesand star formation environments. SINGS will provide new insights intothe physical processes connecting star formation to the interstellarmedium properties of galaxies and provide a vital foundation forunderstanding infrared observations of the distant universe andultraluminous and active galaxies. The galaxy sample and observingstrategy have been designed to maximize the scientific and archivalvalue of the data set for the SIRTF user community at large. The SIRTFimages and spectra will be supplemented by a comprehensivemultiwavelength library of ancillary and complementary observations,including radio continuum, H I, CO, submillimeter, BVRIJHK, Hα,Paα, ultraviolet, and X-ray data. This paper describes the mainastrophysical issues to be addressed by SINGS, the galaxy sample and theobserving strategy, and the SIRTF and other ancillary data products.
| Taurus Tunable Filter - Seven Years of Observing
The Taurus Tunable Filter (TTF) has now been in regular use for sevenyears on the Anglo-Australian Telescope (AAT). The instrument was alsoused for three years (1996-1999) on the William Herschel Telescope(WHT). We present a brief review of the different applications in orderto illustrate the versatility of tunable filters in optical/IRspectrophotometric imaging. Tunable filters are now either planned orunder development for 6-10m class telescopes which ensures their use foryears to come.
| The PDS versus Markarian starburst galaxies: comparing strong and weak IRAS emitter at 12 and 25 μm in the nearby Universe
The characteristics of the starburst galaxies from the Pico dos Diassurvey (PDS) are compared with those of the nearby ultraviolet (UV)bright Markarian starburst galaxies, having the same limit in redshift(vh < 7500 km s-1) and absolute B magnitude(MB < -18). An important difference is found: theMarkarian galaxies are generally undetected at 12 and 25 μm in IRAS.This is consistent with the UV excess shown by these galaxies andsuggests that the youngest star-forming regions dominating thesegalaxies are relatively free of dust.The far-infrared selection criteria for the PDS are shown to introduce astrong bias towards massive (luminous) and large size late-type spiralgalaxies. This is contrary to the Markarian galaxies, which are found tobe remarkably rich in smaller size early-type galaxies. These resultssuggest that only late-type spirals with a large and massive disc arestrong emitters at 12 and 25 μm in IRAS in the nearby Universe.The Markarian and PDS starburst galaxies are shown to share the sameenvironment. This rules out an explanation of the differences observedin terms of external parameters. These differences may be explained byassuming two different levels of evolution, the Markarian being lessevolved than the PDS galaxies. This interpretation is fully consistentwith the disc formation hypothesis proposed by Coziol et al. to explainthe special properties of the Markarian SBNG.
| Recent progress in understanding the hot and warm gas phases in the halos of star-forming galaxies
In this contribution we present a few selected examples of how thelatest generation of space-based instrumentation --- NASA's ChandraX-ray Observatory and the Far-Ultraviolet Spectroscopic Explorer (FUSE) --- are finally answering old questions about the influence ofmassive star feedback on the warm and hot phases of the ISM and IGM. Inparticular, we discuss the physical origin of the soft thermal X-rayemission in the halos of star-forming and starburst galaxies, itsrelationship to extra-planar Hα emission, and plasma diagnosticsusing FUSE observations of OVI absorption and emission.
| Radio Study of a Superwind-galaxy: NGC1482
We present GMRT and VLA observations of the superwind galaxy, NGC1482,which has been discovered to have a remarkable hour-glass shaped opticalemission line outflow on both sides of the galactic disk. The centralregion has a prominent compact component, and suggestions of additionalcompact features.
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