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Environmental Effects on Late-Type Galaxies in Nearby Clusters
The transformations that take place in late-type galaxies in theenvironment of rich clusters of galaxies at z=0 are reviewed. From thehandful of late-type galaxies that inhabit local clusters, whether theywere formed in situ and survived as such, avoiding transformation oreven destruction, or if they are newcomers that have recently fallen infrom outside, we can learn an important lesson on the latest stages ofgalaxy evolution. We start by reviewing the observational scenario,covering the broadest possible stretch of the electromagnetic spectrum,from the gas tracers (radio and optical) to the star formation tracers(UV and optical), the old star tracers (near-IR), and the dust (far-IR).Strong emphasis is given to the three nearby, well-studied clustersVirgo, A1367, and Coma, which are representative of differentevolutionary stages, from unrelaxed and spiral-rich (Virgo) to relaxedand spiral-poor (Coma). We continue by providing a review of models ofgalaxy interactions that are relevant to clusters of galaxies.Prototypes of various mechanisms and processes are discussed, and theirtypical timescales are given in an appendix. Observations indicate thepresence of healthy late-type galaxies falling into nearby clustersindividually or as part of massive groups. More rare are infallinggalaxies belonging to compact groups, where significant preprocessingmight take place. Once they have entered the cluster, they lose theirgas and quench their star formation activity, becoming anemic.Observations and theory agree in indicating that the interaction withthe intergalactic medium is responsible for the gas depletion. However,this process cannot be the origin of the cluster lenticular galaxypopulation. Physical and statistical properties of S0 galaxies in nearbyclusters and at higher redshift indicate that they originate from spiralgalaxies that have been transformed by gravitational interactions.
| The existence and detection of optically dark galaxies by 21-cm surveys
One explanation for the disparity between cold dark matter (CDM)predictions of galaxy numbers and observations could be that there arenumerous dark galaxies in the Universe. These galaxies may still containbaryons, but no stars, and may be detectable in the 21-cm line of atomichydrogen. The results of surveys for such objects, and simulations thatdo/do not predict their existence, are controversial. In this paper, weuse an analytical model of galaxy formation, consistent with CDM, tofirst show that dark galaxies are certainly a prediction of the model.Secondly, we show that objects like VIRGOHI21, a dark galaxy candidaterecently discovered by us, while rare are predicted by the model.Thirdly, we show that previous `blind' HI surveys have placed fewconstraints on the existence of dark galaxies. This is because they haveeither lacked the sensitivity and/or velocity resolution or have not hadthe required detailed optical follow up. We look forward to new 21-cmblind surveys [Arecibo Legacy Fast ALFA (ALFALFA) survey and AreciboGalactic Environments Survey (AGES)] using the Arecibo multibeaminstrument which should find large numbers of dark galaxies if theyexist.
| On the Fraction of X-Ray-obscured Quasars in the Local Universe
Recent wide-area hard X-ray and soft gamma-ray surveys have shown thatthe fraction of X-ray-obscured active galactic nuclei (AGNs) in thelocal universe significantly decreases with intrinsic luminosity. Inthis Letter we point out that two corrections have to be made to thesamples: (1) radio-loud AGNs have to be excluded, since their X-rayemission might be dominated by the jet component, and (2) Compton-thicksources have to be excluded too, since their hard X-ray and softgamma-ray emission are also strongly attenuated by Compton scattering.The soft gamma-ray-selected AGN samples obtained by Swift and INTEGRALprovide the best opportunity to study the fraction of obscured AGNs inthe local universe in the least biased way. We choose these samples tocheck if the corrections could alter the above result on the fraction ofobscured AGNs. We find that before the corrections both samples showsignificant anticorrelation between LX and NH,indicating an obvious decrease in the fraction of obscured AGNs withluminosity. However, after the corrections, we find only marginalevidence of anticorrelation (at the 98% confidence level) in the Swiftsample and no evidence at all in the INTEGRAL sample, which consists ofa comparable number of objects. We conclude that current samples onlyshow a marginal decrease in the fraction of obscured AGNs in the localuniverse and that much larger samples are required in order to reach amore robust conclusion.
| A Comprehensive Search for Gamma-Ray Lines in the First Year of Data from the INTEGRAL Spectrometer
We have carried out an extensive search for gamma-ray lines in the firstyear of public data from the spectrometer (SPI) on the INTEGRAL mission.INTEGRAL has spent a large fraction of its observing time in theGalactic plane with particular concentration in the Galactic center (GC)region (~3 Ms in the first year). Hence the most sensitive searchregions are in the Galactic plane and center. The phase space of thesearch spans the energy range 20-8000 keV and line widths from 0 to 1000keV (FWHM). It includes both diffuse and pointlike emission. We havesearched for variable emission on timescales down to ~1000 s. Diffuseemission has been searched for on a range of different spatial scalesfrom ~20° (the approximate field of view of the spectrometer) up tothe entire Galactic plane. Our search procedures were verified by therecovery of the known gamma-ray lines at 511 and 1809 keV at theappropriate intensities and significances. We find no evidence for anypreviously unknown gamma-ray lines. The upper limits range from afew×10-5 to a few×10-2 cm-2s-1 depending on line width, energy, and exposure; regions ofstrong instrumental background lines were excluded from the search.Comparison is made between our results and various prior predictions ofastrophysical lines.
| Lens-Aided Multi-Angle Spectroscopy (LAMAS) Reveals Small-Scale Outflow Structure in Quasars
Spectral differences between lensed quasar image components are common.Since lensing is intrinsically achromatic, these differences aretypically explained as the effect of either microlensing, or as lightpath time delays sampling intrinsic quasar spectral variability. Here weadvance a novel third hypothesis: some spectral differences are due tosmall line-of-sight differences through quasar disk wind outflows. Inparticular, we propose that variable spectral differences seen only incomponent A of the widest separation lens SDSS J1004+4112 are due todifferential absorption along the sight lines. The absorber propertiesrequired by this hypothesis are akin to known broad absorption line(BAL) outflows but must have a broader, smoother velocity profile. Weinterpret the observed C IV emission-line variability as furtherevidence for spatial fine structure transverse to the line of sight.Since outflows are likely to be rotating, such absorber fine structurecan consistently explain some of the UV and X-ray variability seen inAGNs. The implications are many: (1) Spectroscopic differences in otherlensed objects may be due to this ``lens-aided multi-anglespectroscopy'' (LAMAS). (2) Outflows have fine structure on size scalesof arcseconds, as seen from the nucleus. (3) Assuming either broadabsorption line region sizes proposed in recent wind models, ortypically assumed continuum emission region sizes, LAMAS and/orvariability provide broadly consistent absorber size scale estimates of~1015 cm. (4) Very broad smooth absorption may be ubiquitousin quasar spectra, even when no obvious troughs are seen.
| Discovery of a 500 Parsec Shell in the Nucleus of Centaurus A
Spitzer Space Telescope mid-infrared images of the radio galaxyCentaurus A reveal a shell-like, bipolar, structure 500 pc to the northand south of the nucleus. This shell is seen in 5.8, 8.0, and 24 μmbroadband images. Such a remarkable shell has not been previouslydetected in a radio galaxy and, if confirmed, would be the firstextragalactic nuclear shell detected at mid-infrared wavelengths.Assuming that it is a coherent expanding structure, we estimate that theshell is a few million years old and has a mass on the order of amillion solar masses. A conservative estimate for the mechanical energyin the wind-driven bubble is 1053 ergs. The shell could havebeen created by a small, few-thousand solar mass, nuclear burst of starformation. Alternatively, the bolometric luminosity of the activenucleus is sufficiently large that it could power the shell. Constraintson the shell's velocity are lacking. However, if the shell is moving at1000 km s-1, then the required mechanical energy would be 100times larger.
| The First INTEGRAL AGN Catalog
We present the first INTEGRAL AGN catalog, based on observationsperformed from launch of the mission in 2002 October until 2004 January.The catalog includes 42 AGNs, of which 10 are Seyfert 1, 17 are Seyfert2, and 9 are intermediate Seyfert 1.5. The fraction of blazars is rathersmall, with five detected objects, and only one galaxy cluster and nostarburst galaxies have been detected so far. A complete subset consistsof 32 AGNs with a significance limit of 7 σ in the INTEGRAL ISGRI20-40 keV data. Although the sample is not flux limited, thedistribution of sources shows a ratio of obscured to unobscured AGNs of1.5-2.0, consistent with luminosity-dependent unified models for AGNs.Only four Compton-thick AGNs are found in the sample. Based on theINTEGRAL data presented here, the Seyfert 2 spectra are slightly harder(Γ=1.95+/-0.01) than Seyfert 1.5 (Γ=2.10+/-0.02) and Seyfert1 (Γ=2.11+/-0.05).
| A 70 Kiloparsec X-Ray Tail in the Cluster A3627
We present the discovery of a 70 kpc X-ray tail behind the smalllate-type galaxy ESO 137-001, in the nearby, hot (T=6.5 keV) mergingcluster A3627, from both Chandra and XMM-Newton observations. The tailhas a length-to-width ratio of ~10. It is luminous(L0.5-2keV~1041 ergs s-1), with atemperature of ~0.7 keV and an X-ray Mgas of ~109Msolar (~10% of the galaxy's stellar mass). We interpret thistail as the stripped interstellar medium of ESO 137-001 mixed with thehot cluster medium, with this blue galaxy being converted into agas-poor galaxy. Three X-ray point sources are detected in the axis ofthe tail, which may imply active star formation there. The straightnessand narrowness of the tail also imply that the turbulence in theintracluster medium is not strong on scales of 20-70 kpc.
| INTEGRAL IBIS Extragalactic Survey: Active Galactic Nuclei Selected at 20-100 keV
Analysis of International Gamma-Ray Astrophysics Laboratory (INTEGRAL)Core Programme and public open-time observations performed up to 2005April provides a sample of 62 active galactic nuclei in the 20-100 keVband above a flux limit of ~1.5×10-11 ergscm-2 s-1. Most (42) of the sources in the sampleare Seyfert galaxies, almost equally divided between type 1 and type 2objects; six are blazars, and 14 are still unclassified. Excluding theblazars, the average redshift of our sample is 0.021, while the meanluminosity is logL=43.45. We find that absorption is present in 65% ofthe objects, with 14% of the total sample due to Compton-thick activegalaxies. In agreement with both Swift BAT team results and 2-10 keVstudies, the fraction of absorbed objects decreases with the 20-100 keVluminosity. All Seyfert 2's in our sample are absorbed, as are 33% ofSeyfert 1's. The present data highlight the capability of INTEGRAL toprobe the extragalactic gamma-ray sky and to find new and/or absorbedactive galaxies.Based on observations obtained with INTEGRAL, an ESA project withinstruments and science data center funded by ESA member states(especially the PI countries: Denmark, France, Germany, Italy,Switzerland, Spain), the Czech Republic, and Poland and with theparticipation of Russia and the US.
| X-Ray Emission from Megamaser Galaxy IC 2560
An observation of the H2O megamaser galaxy IC 2560 with theChandra X-Ray Observatory reveals a complex spectrum composed of softX-ray emission due to multitemperature thermal plasma and a hardcontinuum with strong emission lines. The continuum is most likely aCompton reflection (reprocessing) of primary emission that is completelyabsorbed at least up to 7 keV. The lines can be identified withfluorescence from Si, S, and Fe in the lowest ionization stages. Theequivalent widths of the Si and S lines are broadly compatible withthose anticipated for reprocessing by optically thick cold plasma ofsolar abundances, while the large equivalent width of the Fe linerequires some overabundance of iron. A contribution to the line from atransmitted component cannot be ruled out, but the limits on thestrength of the Compton shoulder make it less likely. From thebolometric luminosity of the nuclear region, we infer that the sourceradiates at 1%-10% of its Eddington luminosity for an adopted centralmass of 3×106 Msolar. The overall spectrumis consistent with the hypotheses that the central engines powering thedetected megamasers in accretion disks are obscured from direct view bythe associated accretion disk material itself and that there is acorrelation between the occurrence of megamaser emission andCompton-thick absorption columns. For the 11 known galaxies with bothcolumn density measurements and maser emission believed to arise fromaccretion disks, eight AGNs are Compton thick.
| Swift Observations of the Highly X-Ray Variable Narrow-Line Seyfert 1 Galaxy RX J0148.3-2758
We report on Swift observations of the narrow-line Seyfert 1 galaxy RXJ0148.3-2758. It was observed for 41.6 ks in 2005 May and for 15.8 ks in2005 December. On short as well as on long timescales, RX J0148.3-2758is a highly variable source. It doubles its X-ray flux within 18-25 ks.The observation of 2005 December 9, which had a flux 4 times lower thanduring the 2005 May observations, shows a significant hardening of theX-ray hardness ratio compared with the 2005 May and December 20 and 21observations. A detailed analysis of the X-ray spectra shows that weactually observe two spectral changes in RX J0148.3-2758: first, adecrease of the soft X-ray component between 2005 May and December 9,which is most likely due to an increase of the intrinsic absorbercolumn, and second, a decrease of the hard X-ray flux in the December 20and 21 observations. The soft X-ray spectral slopeαX,soft=2.58+0.15-0.12 during thehigh state in 2005 May agrees well with that measured by ROSAT(αX,soft=2.54+/-0.82). This soft X-ray spectrum issuperposed on a hard X-ray component withαX,hard=0.96+0.16-0.14, which isconsistent with the hard X-ray spectral slopeαX,hard=1.11+0.16-0.19 found byASCA. The soft X-ray slopeαX,soft=1.93+0.58-0.42 measuredduring the December 9 observation agrees well withαX,soft=2.03+0.23-0.20 measuredfrom the ASCA observation when RX J0148.3-2758 was also in a low state.In contrast to the strong X-ray variability, the analysis of the SwiftUV-Optical Telescope (UVOT) photometry from 2005 December of RXJ0148.3-2758 shows no significant variability in any of the six UVOTfilters. From the simultaneous X-ray and UV observations in 2005December we measured the X-ray loudness αox and foundit to vary between αox=1.5 and 1.8. Our Swiftobservations of RX J0148.3-2758 demonstrate the great potential that themultiwavelength observatory Swift has for active galactic nucleusscience.
| A Survey of Kiloparsec-Scale Radio Outflows in Radio-Quiet Active Galactic Nuclei
Seyfert galaxies commonly host compact jets spanning 10-100 pc scales,but larger structures are resolved out in long-baseline aperturesynthesis surveys. Previous, targeted studies showed thatkiloparsec-scale radio structures (KSRs) may be a common feature ofSeyfert and LINER galaxies, and the origin of KSRs may be starbursts oractive galactic nuclei (AGNs). We report a new Very Large Array surveyof a complete sample of Seyfert and LINER galaxies. Out of all of thesurveyed radio-quiet sources, we find that 44% (19 out of 43) showextended radio structures at least 1 kpc in total extent that do notmatch the morphology of the disk or its associated star-forming regions.The detection rate is a lower limit owing to the combined effects ofprojection and resolution. The infrared colors of the KSR host galaxiesare unremarkable compared to other Seyfert galaxies, and the large-scaleoutflows orient randomly with respect to the host galaxy axes. The KSRSeyfert galaxies instead stand out by deviating significantly from thefar-infrared-radio correlation for star-forming galaxies, with tendencytoward radio excess, and they are more likely to have a relativelyluminous, compact radio source in the nucleus; these results argue thatKSRs are powered by the AGNs rather than starbursts. The high detectionrate indicates that Seyfert galaxies generate radio outflows over asignificant fraction of their lifetime, which is much longer than thedynamical timescale of an AGN-powered jet but is comparable instead tothe buoyancy timescale. The likely explanation is that the KSRsoriginate from jet plasma that has been decelerated by interaction withthe nuclear interstellar medium (ISM). Based on a simple ram pressureargument, the kinetic power of the jet on kiloparsec scales is about 3orders of magnitude weaker than the power of the jet on 10-100 pcscales. This result is consistent with the interaction model, in whichcase virtually all of the jet power must be lost to the ISM within theinner kiloparsec.
| Seyfert Galaxies and the Hard X-Ray Background: Artificial Chandra Observations of z=0.3 Active Galaxies
Deep X-ray surveys have resolved much of the X-ray background radiationbelow 2 keV into discrete sources, but the background above 8 keVremains largely unresolved. The obscured (type 2) active galactic nuclei(AGNs) that are expected to dominate the hard X-ray background have notyet been detected in sufficient numbers to account for the observedbackground flux. However, deep X-ray surveys have revealed large numbersof faint quiescent and starburst galaxies at moderate redshifts. Inhopes of recovering the missing AGN population, it has been suggestedthat the defining optical spectral features of low-luminosity Seyfertnuclei at large distances may be overwhelmed by their host galaxies,causing them to appear optically quiescent in deep surveys. We test thispossibility by artificially redshifting a sample of 23 nearby,well-studied active galaxies to z=0.3, testing them for X-ray AGNsignatures, and comparing them to the objects detected in deep X-raysurveys. We find that these redshifted galaxies have propertiesconsistent with the deep-field normal and optically bright, X-ray-faintgalaxy (OBXF) populations, supporting the hypothesis that the numbers ofAGNs in deep X-ray surveys are being underestimated and suggesting thatOBXFs should not be ruled out as candidate AGN hosts that couldcontribute to the hard X-ray background source population.
| On the X-ray, optical emission line and black hole mass properties of local Seyfert galaxies
We investigate the relation between X-ray nuclear emission, opticalemission line luminosities and black hole masses for a sample of 47Seyfert galaxies. The sample, which has been selected from the Palomaroptical spectroscopic survey of nearby galaxies (Ho et al. 1997a, ApJS,112, 315), covers a wide range of nuclear powers, from L2-10keV ~ 1043 erg/s down to very low luminosities(L2-10 keV ~ 1038 erg/s). Best available data fromChandra, XMM-Newton and, in a few cases, ASCA observations have beenconsidered. Thanks to the good spatial resolution available from theseobservations and a proper modeling of the various spectral components,it has been possible to obtain accurate nuclear X-ray luminosities notcontaminated by off-nuclear sources and/or diffuse emission. X-rayluminosities have then been corrected taking into account the likelycandidate Compton thick sources, which are a high fraction (>30%)among type 2 Seyferts in our sample. The main result of this study isthat we confirm strong linear correlations between 2-10 keV,[OIII]λ5007, Hα luminosities which show the same slope asquasars and luminous Seyfert galaxies, independent of the level ofnuclear activity displayed. Moreover, despite the wide range ofEddington ratios (L/L_Edd) tested here (six orders of magnitude, from0.1 down to ~10-7), no correlation is found between the X-rayor optical emission line luminosities and the black hole mass. Ourresults suggest that Seyfert nuclei in our sample are consistent withbeing a scaled-down version of more luminous AGN.
| The K-band properties of Seyfert 2 galaxies
Aims. It is well known that the [O iii]λ5007 emission line andhard X-ray (2-10 keV) luminosities are good indicators of AGN activitiesand that the near and mid-infrared emission of AGN originates fromre-radiation of dusty clouds heated by the UV/optical radiation from theaccretion disk. In this paper we present a study of the near-infraredK-band (2.2 μm) properties for a sample of 65 Seyfert 2 galaxies. Methods: .By using the AGN/Bulge/Disk decomposition technique, weanalyzed the 2MASS K_S-band images for Seyfert 2 galaxies in order toderive the K_S-band magnitudes for the central engine, bulge, and diskcomponents. Results: .We find that the K_S-band magnitudes of thecentral AGN component in Seyfert 2 galaxies are tightly correlated withthe [O iii]λ5007 and the hard X-ray luminosities, which suggeststhat the AGN K-band emission is also an excellent indicator of thenuclear activities at least for Seyfert 2 galaxies. We also confirm thegood relation between the central black hole masses and bulge's K-bandmagnitudes for Seyfert 2s.
| Witnessing galaxy preprocessing in the local Universe: the case of a star-bursting group falling into Abell 1367
We present a multiwavelength analysis of a compact group of galaxiesinfalling at high speed into the dynamically young cluster Abell 1367.Peculiar morphologies and unusually high Hα emission areassociated with two giant galaxies and at least ten dwarfs/extragalacticHII regions, making this group the region with the highest density ofstar formation activity ever observed in the local clusters. MoreoverHα imaging observations reveal extraordinary complex trails ofionized gas behind the galaxies, with projected lengths exceeding 150kpc. These unique cometary trails mark the gaseous trajectory ofgalaxies, witnessing their dive into the hot cluster intergalacticmedium. Under the combined action of tidal forces among group membersand the ram-pressure by the cluster ambient medium, the group galaxieswere fragmented and the ionized gas was blown out. The properties ofthis group suggest that environmental effects within infalling groupsmay have represented a preprocessing step of the galaxy evolution duringthe high redshift cluster assembly phase.
| Extragalactic H_2O masers and X-ray absorbing column densities
Having conducted a search for the λ 1.3 cm (22 GHz) water vaporline towards galaxies with nuclear activity, large nuclear columndensities or high infrared luminosities, we present H2O spectra for NGC2273, UGC 5101, and NGC 3393 with isotropic luminosities of 7, 1500, and400 Lȯ. The H2O maser in UGC 5101 is by far the mostluminous yet found in an ultraluminous infrared galaxy. NGC 3393 revealsthe classic spectrum of a "disk maser", represented by three distinctgroups of Doppler components. As in all other known cases except NGC4258, the rotation velocity of the putative masing disk is well below1000 km s-1. Based on the literature and archive data, X-rayabsorbing column densities are compiled for the 64 galaxies withreported maser sources beyond the Magellanic Clouds. For NGC 2782 andNGC 5728, we present Chandra archive data that indicate the presence ofan active galactic nucleus in both galaxies. Modeling the hard nuclearX-ray emission, NGC 2782 is best fit by a high energy reflectionspectrum with NH 1024 cm-2. ForNGC 5728, partial absorption with a power law spectrum indicatesNH 8 × 1023 cm-2. Thecorrelation between absorbing column and H2O emission is analyzed. Thereis a striking difference between kilo- and megamasers with megamasersbeing associated with higher column densities. All kilomasers (L_H_2O< 10 Lȯ) except NGC 2273 and NGC 5194 areCompton-thin, i.e. their absorbing columns are <1024cm-2. Among the H{2}O megamasers, 50% arise fromCompton-thick and 85% from heavily obscured (>1023cm-2) active galactic nuclei. These values are not larger butconsistent with those from samples of Seyfert 2 galaxies not selected onthe basis of maser emission. The similarity in column densities can beexplained by small deviations in position between maser spots andnuclear X-ray source and a high degree of clumpiness in thecircumnuclear interstellar medium.
| The soft X-ray/NLR connection: a single photoionized medium?
We present a sample of 8 nearby Seyfert 2 galaxies observed by HST andChandra. All of the sources present soft X-ray emission which iscoincident in extension and overall morphology with the [O iii]emission. The spectral analysis reveals that the soft X-ray emission ofall the objects is likely to be dominated by a photoionized gas. This isstrongly supported by the 190 ks combined XMM-Newton/RGS spectrum of Mrk3, which different diagnostic tools confirm as being produced in a gasin photoionization equilibrium with an important contribution fromresonant scattering. We tested with the code cloudy a simple scenariowhere the same gas photoionized by the nuclear continuum produces boththe soft X-ray and the [O iii] emission. Solutions satisfying theobserved ratio between the two components exist, and require the densityto decrease with radius roughly like r-2, similarly to whatoften found for the Narrow Line Region.
| X-ray spectral survey with XMM-Newton of a complete sample of nearby Seyfert galaxies
Results obtained from an X-ray spectral survey of nearby Seyfertgalaxies using XMM-Newton are reported. The sample was opticallyselected, well defined, complete in B magnitude, and distance limited:it consists of the nearest (D 22 Mpc) 27 Seyfert galaxies (9 oftype 1, 18 of type 2) taken from the Ho et al. (1997a, ApJS, 112, 315)sample. This is one of the largest atlases of hard X-ray spectra oflow-luminosity active galaxies ever assembled. All nuclear sourcesexcept two Seyfert 2s are detected between 2 and 10 keV, half for thefirst time ever, and average spectra are obtained for all of them.Nuclear luminosities reach values down to 1038 ergs-1. The shape of the distribution of X-ray parameters isaffected by the presence of Compton-thick objects (30% among type2s). The latter have been identified either directly from their intenseFeK line and flat X-ray spectra, or indirectly with flux diagnosticdiagrams which use isotropic indicators. After taking into account thesehighly absorbed sources, we find that (i) the intrinsic X-ray spectralproperties (i.e., spectral shapes and luminosities above 2 keV) areconsistent between type 1 and type 2 Seyferts, as expected from "unifiedmodels"; (ii) Seyfert galaxies as a whole are distributed fairlycontinuously over the entire range of N_H, between 1020 and1025 cm-2; and (iii) while Seyfert 1s tend to havelower NH and Seyfert 2s tend to have the highest, we find 30%and 10% exceptions, respectively. Overall the sample is of sufficientquality to well represent the average intrinsic X-ray spectralproperties of nearby active galactic nuclei, including a proper estimateof the distribution of their absorbing columns. Finally, we concludethat, with the exception of a few cases, the present study agrees withpredictions of unified models of Seyfert galaxies, and extends theirvalidity down to very low luminosities.
| 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.
| Dark galaxies or tidal debris? Kinematical clues to the origin of massive isolated HI clouds
An extended HI cloud (VIRGOHI21) with an HI mass of~108Msolar and no apparent optical counterpart wasrecently discovered in the Virgo cluster. In order to understand theorigin of physical properties of apparently isolated HI clouds likeVIRGOHI21, we numerically investigate physical properties of tidal HIdebris that were formed by galaxy-galaxy interactions in clusters ofgalaxies. Our hydrodynamical simulations demonstrate that tidal debriswith total HI masses of 108-109Msolarcan have (1) a wide spread of HI velocities (>200kms-1),(2) a small mass fraction of stars (~10 per cent), and (3) a mean B-bandsurface brightness of the stellar components fainter than 30magarcsec-2. These results suggest that VIRGOHI21, which liesat a projected distance of ~150 kpc from the one-armed, HI-rich spiralgalaxy M99 (NGC 4254), is tidal debris. We propose that the comparisonbetween the simulated and the observed velocity fields of HI cloudsallows us to better understand their nature and origin (e.g. whetherthey are just tidal debris or are `dark galaxies' that have HI gas onlyand are embedded within dark matter haloes).
| Fe K emission in the ultraluminous infrared galaxy Arp 220
Prominent Fe Kα line emission is detected in the XMM-Newtonspectrum of the ultraluminous infrared galaxy Arp 220. The centroid ofthe line is found at an energy of 6.7 keV and the equivalent width ofthe line is EW ~ 1.9 keV (at 3.5σ significance). A few otherspectral features are found at various degrees of significance in thelower energy range on a hard 2.5-10 keV continuum (Γ~ 1). Thelarge EW of the Fe K line poses a problem with interpreting the hardX-ray emission as integrated X-ray binary emission. A thermal emissionspectrum with a temperature of kT~ 7 keV modified by absorption ofNH~= 3 × 1022cm-2, can describethe 2.5-10 keV continuum shape and the Fe K emission. A hot bubble thatis shocked internally in a starburst region would have a similartemperature and gives a good explanation for the observed X-rayproperties with a high star formation rate. An ensemble of radiosupernovae in a dense environment, as suggested from VLBI imaging, couldbe another possibility, if such powerful supernovae are producedcontinuously at a high rate. However, the apparent lack of emission fromX-ray binaries is incompatible with the high supernova rate (~2 SNeyr-1) required by both interpretations. Highly photoionized,low-density gas illuminated by a hidden Compton-thick active galacticnucleus is a possible alternative for the hard X-ray emission, which canbe tested by examining whether radiative recombination continua fromhighly ionized Ca and Fe are present in better quality data from aforthcoming observation.
| Dust in spiral galaxies: global properties
We present and analyse high-quality Submillimetre Common-User BolometerArray (SCUBA) 850- and 450-μm images of 14 local spiral galaxies,including the detection of dust well out into the extended disc in manycases. We use these data in conjunction with published far-infrared fluxdensities from IRAS and ISO, and millimetre-wave measurements fromground-based facilities to deduce the global properties of the dust inthese galaxies, in particular temperature and mass. We find that simpletwo-temperature greybody models of fixed dust emissivity index β= 2and with typical temperatures of 25 < Twarm < 40 K and10 < Tcold < 20 K provide good fits to the overallspectral energy distributions. The dust mass in the cold componentcorrelates with the mass in atomic hydrogen and the mass in the warmcomponent correlates with the mass in molecular hydrogen. These resultsthus fit the simple picture in which the cold dust is heatedpredominantly by the interstellar radiation field, while the hot dust isheated predominantly by OB stars in more active regions, although weargue that there is some mixing. The mean gas-to-dust mass ratio is 120+/- 60, very similar to that found within our own galaxy and roughly afactor of 10 lower than that derived from IRAS data alone. Thegas-to-dust mass ratios in the warm, molecular component are on averagehigher than those in the cold, atomic component. We compare ourmodelling results with similar results for more luminous spiral galaxiesselected at far-infrared wavelengths by the SCUBA Local Universe GalaxySurvey. We find that whilst the total dust mass distributions of the twosamples are indistinguishable, they have significantly different dusttemperature distributions in both the warm and cold components. Wesuggest that this difference might be related to the level of starformation activity in these systems, with the more active galaxieshaving more intense interstellar radiation fields and higher dusttemperatures.
| Supermassive Black Holes: Relation to Dark Halos
Estimates of the masses of supermassive black holes (M bh ) in thenuclei of disk galaxies with known rotation curves are compared withestimates of the rotational velocities V m and the“indicative” masses of the galaxies M i . Although there isa correlation between M bh and V m or M i , it is appreciably weakerthan the correlation with the central velocity dispersion. The values ofM bh for early-type galaxies (S0-Sab), which have more massive bulges,are, on average, higher than the values for late-type galaxies with thesame rotational velocities. We conclude that the black-hole masses aredetermined primarily by the properties of the bulge and not therotational velocity or the mass of the galaxy.
| Galactic Winds
Galactic winds are the primary mechanism by which energy and metals arerecycled in galaxies and are deposited into the intergalactic medium.New observations are revealing the ubiquity of this process,particularly at high redshift. We describe the physics behind thesewinds, discuss the observational evidence for them in nearbystar-forming and active galaxies and in the high-redshift universe, andconsider the implications of energetic winds for the formation andevolution of galaxies and the intergalactic medium. To inspire futureresearch, we conclude with a set of observational and theoreticalchallenges.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| The High-Energy Spectrum of NGC 4151
We present the first INTEGRAL observations of the type 1.5 Seyfertgalaxy NGC 4151. Combining several INTEGRAL observations performedduring 2003, totaling ~400 ks of exposure time, allows us to study thespectrum in the 2-300 keV range. The measurements presented here revealan overall spectrum from X-rays up to soft gamma-rays that can bedescribed by an absorbed (NH=6.9×1022cm-2) model based on a Compton continuum from a hot electronpopulation (kTe=94 keV) from an optically thick (τ=1.3)corona, reflected on cold material (R=0.7), consistent with earlierclaims. The time-resolved analysis shows little variation of thespectral parameters over the duration of the INTEGRAL observations. Thecomparison with CGRO OSSE data shows that the same spectral model can beapplied over a time span of 15 yr, with flux variations of the order ofa factor of 2 and changes in the underlying continuum reflected by thetemperature of the electron population (kTe=50-100 keV). Whenmodeled with an exponential cutoff power law plus Compton reflection,this results in photon indices ranging from Γ=1.5 to 1.9 and acutoff energy in the range 100-500 keV.
| The Relationship of Hard X-Ray and Optical Line Emission in Low-Redshift Active Galactic Nuclei
In this paper we assess the relationship of the population of activegalactic nuclei (AGNs) selected by hard X-rays to the traditionalpopulation of AGNs with strong optical emission lines. First, we studythe emission-line properties of a new hard-X-ray-selected sample of 47local AGNs (classified optically as Type 1 and 2 AGNs). We find that thehard X-ray (3-20 keV) and [O III] λ5007 optical emission-lineluminosities are well-correlated over a range of about 4 orders ofmagnitude in luminosity (mean luminosity ratio 2.15 dex with a standarddeviation of σ=0.51 dex). Second, we study the hard X-rayproperties of a sample of 55 local AGNs selected from the literature onthe basis of the flux in the [O III] line. The correlation between thehard X-ray (2-10 keV) and [O III] luminosity for the Type 1 AGNs isconsistent with what is seen in the hard-X-ray-selected sample. However,the Type 2 AGNs have a much larger range in the luminosity ratio, andmany are very weak in hard X-rays (as expected for heavily absorbedAGNs). We then compare the hard X-ray (3-20 keV) and [O III] luminosityfunctions of AGNs in the local universe. These have similar faint-endslopes, with a luminosity ratio of 1.60 dex (0.55 dex smaller than themean value for individual hard-X-ray-selected AGNs). We conclude that atlow redshift, selection by narrow optical emission lines will recovermost AGNs selected by hard X-rays (with the exception of BL Lacobjects). However, selection by hard X-rays misses a significantfraction of the local AGN population with strong emission lines.
| The Swift/BAT High-Latitude Survey: First Results
We present preliminary results from the first 3 months of the SwiftBurst Alert Telescope (BAT) high Galactic latitude survey in the 14-195keV band. The survey reaches a flux of ~10-11 ergscm-2 s-1 and has ~2.7 arcmin (90% confidence)positional uncertainties for the faintest sources. This represents themost sensitive survey to date in this energy band. These data confirmthe conjectures that a high-energy-selected active galactic nucleus(AGN) sample would have very different properties from those selected inother bands and that it represents a ``true'' sample of the AGNpopulation. We have identified 86% of the 66 high-latitude sources.Twelve are Galactic-type sources, and 44 can be identified withpreviously known AGNs. All but five of the AGNs have archival X-rayspectra, enabling us to estimate the line-of-sight column densities andother spectral properties. Both of the z>0.11 objects are blazars.The median redshift of the others (excluding radio-loud objects) is0.012. We find that the column density distribution of these AGNs isbimodal, with 64% of the nonblazar sources having column densitiesNH>=1022 cm-2. None of the sourceswith logLX>43.5 (cgs units) show high column densities,and very few of the lower LX sources have low columndensities. Based on these data, we expect the final BAT catalog to have>200 AGNs and reach fluxes of less than ~10-11 ergscm-2 s-1 over the entire sky.
| The Remnants of Intergalactic Supernovae
Intergalactic Type Ia supernovae (SNe) have been discovered recently inrich galaxy clusters. These SNe are likely the descendants of anintergalactic stellar population, which has been discovered in recentyears through a variety of tracers. We estimate the observationalsignatures of the associated SN remnants (SNRs) in the unusualintracluster medium (ICM) environment. We find that if Type Ia SNe stillhave a circumstellar medium (CSM) at the time of explosion, then theirremnants are visible in the optical for ~102-103yr, with properties similar to young galactic SNRs. In contrast withgalactic SNRs, in which the ejecta from the explosion interacts with theinterstellar medium (ISM), intracluster SNRs become undetectable in theoptical band once their ejecta passes beyond the CSM and enters the hotand tenuous ICM. If Type Ia SNe have a CSM, there should be ~150 youngSNRs in the nearby Virgo Cluster, with an Hα luminosity of~1035 ergs s-1 and an angular size of ~0.1". Weinvestigate the possibility that members of this SNR population may haverecently been detected but incorrectly identified as intergalactic H IIregions. Alternatively, if optical intergalactic SNRs do not exist inVirgo, this will constitute evidence that Type Ia SNe are devoid of aCSM, with implications for progenitor scenarios. Regardless of thepresence of a CSM, about 10 older SNRs per square degree should bedetectable in Virgo in the radio band, with fluxes of order 0.1 mJy at 1GHz. Their angular sizes (~1"), morphologies, and lack of opticalassociation with distant galaxies can distinguish them from the muchmore numerous background population. Their detection would provide anaccurate measurement of the intracluster SN rate. Deep pointedobservations toward the site of SN 1980I, a possibly intergalactic TypeIa event in Virgo, could test for the existence of a CSM by comparisonto our predictions for the early-time development of intergalactic SNRs.
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