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How Do We See a Relativistic Accretion Disk during a Thermal Instability?
We calculated bolometric images of relativistic slim disks during aradiation-pressure-driven thermal instability. When the mass-accretionrate exceeds the critical one, an inner region of the standard accretiondisk bursts to change to a slim disk state having a large scale height.That is, the inner region of the disk becomes high temperature, and thethickness of the disk increases due to an increase of the radiationpressure. As a result, we found that the observed image of the diskduring the burst strongly depends on the inclination angle. That is,radiation from the innermost disk would be occulted by the disk outerrim for high inclination angles (i ≳ 70°). We also calculatedthe spectral energy distribution during a thermal instability. The Wienpeak of the spectrum of high inclination angles becomes softer than thatof low inclination angles due to the geometrical thickness. From thesefacts, even if a burst occurs in a black-hole candidate, we may notobserve the burst when the inclination angle is large. We may suggestthat numerous luminous black-hole candidates are still hidden in ourGalaxy.
| Nuclear embedded star clusters in NGC 7582*
We report on the discovery of several compact regions of mid-infraredemission in the star-forming circumnuclear disc of the starburst/Seyfert2 galaxy NGC 7582. The compact sources do not have counterparts in theoptical and near-infrared, suggesting that they are deeply embedded indust. We use the [NeII] 12.8-μm line emission to estimate theemission measure of the ionized gas, which in turn is used to assess thenumber of ionizing photons. Two of the brighter sources are found tohave ionizing fluxes of ~2.5 × 1052, whereas thefainter ones have ~1 × 1052 photon s-1.Comparing with a 1-Myr-old starburst, we derive stellar masses in therange 3-5 × 105Msolar and find that thenumber of O stars in each compact source is typically 0.6-1.6 ×103. We conclude that the compact mid-infrared sources arelikely to be young, embedded star clusters, of which only a few areknown so far. Our observation highlights the need for high-resolutionmid-infrared imaging to discover and study embedded star clusters in theproximity of active galactic nuclei.
| HI content in galaxies in loose groups
Gas deficiency in cluster spirals is well known and ram-pressurestripping is considered the main gas removal mechanism. In some compactgroups too gas deficiency is reported. However, gas deficiency in loosegroups is not yet well established. Lower dispersion of the membervelocities and the lower density of the intragroup medium in small loosegroups favour tidal stripping as the main gas removal process in them.Recent releases of data from the HI Parkes All-Sky Survey (HIPASS) andcatalogues of nearby loose groups with associated diffuse X-ray emissionhave allowed us to test this notion. In this paper, we address thefollowing questions: (i) do galaxies in groups with diffuse X-rayemission statistically have lower gas content compared to the ones ingroups without diffuse X-ray emission? (ii) does HI deficiency vary withthe X-ray luminosity, LX, of the loose group in a systematicway? We find that (i) galaxies in groups with diffuse X-ray emission, onaverage, are HI deficient, and have lost more gas compared to those ingroups without X-ray emission; the latter are found not to havesignificant HI deficiency; (ii) no systematic dependence of the HIdeficiency with LX is found. Ram-pressure-assisted tidalstripping and evaporation by thermal conduction are the two possiblemechanisms to account for this excess gas loss.
| 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.
| 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.
| The Two-dimensional XMM-Newton Group Survey: z < 0.012 Groups
We present the results of the two-dimensional XMM-Newton Group Survey(2dXGS), an archival study of nearby galaxy groups. In this paper weconsider 11 nearby systems (z<0.012) in Mulchaey et al., which span abroad range in X-ray luminosity from 1040 to 1043ergs s-1. We measure the iron abundance and temperaturedistribution in these systems and derive pressure and entropy maps. Wefind statistically significant evidence for structure in the entropy andpressure of the gas component of seven groups on the 10%-20% level. TheXMM-Newton data for the three groups with best statistics also suggestpatchy metallicity distributions within the central 20-50 kpc of thebrightest group galaxy, probed with 2-10 kpc resolution. This providesinsights into the processes associated with thermalization of thestellar mass loss. Analysis of the global properties of the groupsreveals a subclass of X-ray-faint groups, which are characterized byboth higher entropy and lower pressure. We suggest that the mergerhistory of the central elliptical is responsible for both the source andthe observed thermodynamical properties of the hot gas of theX-ray-faint groups.
| Variability Study of Seyfert 2 Galaxies with XMM-Newton
We present the results of timing analysis of XMM-Newton observations ofSeyfert 2 galaxies in order to search for differences in the meanproperties of Seyfert 1 galaxies and Seyfert 2 galaxies. We selected 13Seyfert 2 galaxies from the XMM-Newton archive that have hard X-raycomponents in their spectra and calculated the excess variance(σ2rms) in the 2-10 keV band. We found thatsix Seyfert 2 galaxies (3C 98, IRAS 05189-2524, MCG -5-23-16, NGC 6300,UGC 4203, and PKS 1814-637) have buried luminous nuclei and that thenuclei have timing properties similar to those of Seyfert 1 nuclei. Thisindicates that these galaxies are candidates for having buried Seyfert 1nuclei as expected by the unified Seyfert model. The first five galaxiesshow significant time variability. The amplitude of the time variabilityof IRAS 05189-2524 is similar to that of narrow-line Seyfert 1 galaxies.In contrast, the amplitude of variability of the seven other galaxies isquite small, much smaller than that of Seyfert 1 galaxies with similarX-ray luminosity. The lack of short time variability in these objects isexplained by the dominance of the reflection component in three galaxies(Mrk 3, Mrk 463, and NGC 7582), and by the presence of very massiveblack holes and an inferred low accretion rate in the other threegalaxies (NGC 1052, NGC 4507, and NGC 7172). For Mrk 348, thesignificant time variability that is expected based on the estimate ofthe central black hole mass was not detected.
| 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).
| 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.
| VISIR, a taste of scientific potential.
Not Available
| An atlas of calcium triplet spectra of active galaxies
We present a spectroscopic atlas of active galactic nuclei covering theregion around the λλ8498, 8542, 8662 calcium triplet(CaT). The sample comprises 78 objects, divided into 43 Seyfert 2s, 26Seyfert 1s, three starburst and six normal galaxies. The spectra pertainto the inner ~300 pc in radius, and thus sample the central kinematicsand stellar populations of active galaxies. The data are used to measurestellar velocity dispersions (σ*) with bothcross-correlation and direct fitting methods. These measurements arefound to be in good agreement with each other and with those in previousstudies for objects in common. The CaT equivalent width is alsomeasured. We find average values and sample dispersions ofWCaT of 4.6 +/- 2.0, 7.0 +/- 1.0 and 7.7 +/- 1.0 Å forSeyfert 1s, Seyfert 2s and normal galaxies, respectively. We furtherpresent an atlas of [SIII]λ9069 emission-line profiles for asubset of 40 galaxies. These data are analysed in a companion paperwhich addresses the connection between stellar and narrow-line regionkinematics, the behaviour of the CaT equivalent width as a function ofσ*, activity type and stellar population properties.
| On the nature of bulges in general and of box/peanut bulges in particular: input from N-body simulations
Objects designated as bulges in disc galaxies do not form a homogeneousclass. I distinguish three types: the classical bulges, the propertiesof which are similar to those of ellipticals and which form by collapseor merging; boxy and peanut bulges, which are seen in near-edge-ongalaxies and which are in fact just a part of the bar seen edge-on; and,finally, disc-like bulges, which result from the inflow of (mainly) gasto the centre-most parts, and subsequent star formation. I make adetailed comparison of the properties of boxy and peanut bulges withthose of N-body bars seen edge-on, and answer previously voicedobjections about the links between the two. I also present and analysesimulations where a boxy/peanut feature is present at the same time as aclassical spheroidal bulge, and compare them with observations. Finally,I propose a nomenclature that can help to distinguish between the threetypes of bulges and avoid considerable confusion.
| 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 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.
| Molecular Hydrogen Excitation around Active Galactic Nuclei
We report R~3000 Very Large Telescope ISAAC K-band spectroscopy of thenuclei (i.e., central 100-300 pc) of nine galaxies hosting an activegalactic nucleus. For five of these we also present spectra of thecircumnuclear region out to 1 kpc. We have measured a number ofmolecular hydrogen lines in the ν=1-0, 2-1, and 3-2 vibrationaltransitions, as well as the Brγ and He I recombination lines andthe Na I stellar absorption feature. Although only three of the galaxiesare classified as type 1 Seyfert galaxies in the literature, broadBrγ (FWHM>~1000 km s-1) is seen in seven of theobjects. The ν=1-0 emission appears thermalized at temperaturesT~1000 K. However, the ν=2-1 and ν=3-2 emission show evidence ofbeing radiatively excited by far-ultraviolet photons. Thephotodissociation region models that fit the data best are, as for theultraluminous infrared galaxies in Davies et al., those for which theH2 emission arises in dense clouds illuminated by intensefar-ultraviolet radiation. The Na I stellar absorption line is clearlyseen in six of the nuclear spectra of these AGNs, indicating thepresence of a significant population of late-type stars. It is possiblethat these stars are a result of the same episode of star formation thatgave rise to the stars heating the photodissociation regions. It seemsunlikely that the AGN is the dominant source of excitation for thenear-infrared H2 emission: in two of the nuclear spectraH2 was not detected at all, and in general we find noevidence of suppression of the 2-1 S(3) line, which may occur inX-ray-irradiated gas. Our data do not reveal any significant differencebetween the nuclear and circumnuclear line ratios, suggesting that thephysical conditions of the dominant excitation mechanism are similarboth near the AGN and in the larger scale environment around it, andthat star formation is an important process even in the central 100 pcaround AGNs.Based on observations at the European Southern Observatory VLT(69.B-0075).
| Dust Morphology of Hidden Broad-Line Region and Non-Hidden Broad-Line Region Seyfert 2 Galaxies
We investigate the nuclear dust properties of hidden broad-line region(HBLR) and non-HBLR Seyfert 2 galaxies. Optical images obtained from theHubble Space Telescope for a selected sample of HBLR and non-HBLRSeyfert 2 galaxies are fitted with the Galfit package to probe the innerstructures of these galaxies within the central 1 kpc regions. Most ofthe galaxies show complicated dust features in these regions. However,the dust morphology shows no significant difference between the HBLR andnon-HBLR Seyfert 2 galaxies. Dust masses inside the 1 kpc nuclearregions (M1kpc) are estimated from the obscuration levels inthe central regions of these galaxies. We compare our results with otherobserved properties, including [O III], far-infrared, and radioemission. We find that the HBLR and non-HBLR Seyfert 2 galaxies showdifferent near-infrared colors and M1kpc-FIR correlations,indicating that these two classes of Seyfert 2 galaxies are dominated bydifferent emission mechanisms. We suggest that they are intrinsicallydifferent and cannot be explained by the standard unification model.
| The Kinematic Signature of Face-On Peanut-shaped Bulges
We present a kinematic diagnostic for peanut-shaped bulges in nearlyface-on galaxies. The face-on view provides a novel perspective onpeanuts that would allow study of their relation to bars and disks ingreater detail than hitherto possible. The diagnostic is based on thefact that peanut shapes are associated with a flat density distributionin the vertical direction. We show that the kinematic signaturecorresponding to such a distribution is a minimum in the fourth-orderGauss-Hermite moment s4. We demonstrate our method on N-bodysimulations of varying peanut strength, showing that strong peanuts canbe recognized to inclinations i~=30deg, regardless of thestrength of the bar. We also consider compound systems in which a bulgeis present in the initial conditions, as may happen if bulges form athigh redshift through mergers. We show that in this case, because thevertical structure of the bulge is not derived from that of the disk,the signature of a peanut in s4 is weakened. Thus the samekinematic signature of peanuts can be used to explore bulge formationmechanisms. The observational requirements for identifying peanuts withthis method are challenging, but feasible.
| Type 2 Counterparts of Narrow-Line Seyfert 1 Galaxies
Unified models based on viewing angle successfully explain the observeddifferences between type 1 and type 2 Seyfert galaxies. The existence ofa range in accretion rates (m˙~0.001-1) relative to the Eddingtonrate (from broad-line Seyfert 1's to narrow-line Seyfert 1's, or NLS1's)and the unification of Seyfert galaxies imply that there must be type 2counterparts of NLS1's, that is, Seyfert 2's with high accretion ratesor small black hole masses. One such Seyfert 2, NGC 5506, has alreadybeen unmasked based on near-infrared spectroscopy. Here we confirm theabove result and present evidence for two additional type 2 counterpartsof NLS1's based on XMM-Newton observations. The three active galacticnuclei NGC 7314, 7582, and 5506, with type 1.9/2 optical spectra, showextremely rapid variability, by factors of greater than 2.4, ~1.3, and~1.7 over 200, 350, and 300 s, respectively, and steep 2-12 keV spectra(Γ>~2) in their intrinsic X-ray emission, characteristic ofNLS1's. These observations establish the ``obscured NLS1 galaxies'' as asubclass of Seyfert 2 galaxies.
| On the Maximum Luminosity of Galaxies and Their Central Black Holes: Feedback from Momentum-driven Winds
We investigate large-scale galactic winds driven by momentum deposition.Momentum injection is provided by (1) radiation pressure produced by thecontinuum absorption and scattering of photons on dust grains and (2)supernovae (momentum injection by supernovae is important even if thesupernova energy is radiated away). Radiation can be produced by astarburst or active galactic nucleus (AGN) activity. We argue thatmomentum-driven winds are an efficient mechanism for feedback during theformation of galaxies. We show that above a limiting luminosity,momentum deposition from star formation can expel a significant fractionof the gas in a galaxy. The limiting, Eddington-like luminosity isLM~=(4fgc/G)σ4, where σ isthe galaxy velocity dispersion and fg is the gas fraction;the subscript M refers to momentum driving. A starburst that attainsLM moderates its star formation rate and its luminosity doesnot increase significantly further. We argue that elliptical galaxiesattain this limit during their growth at z>~1 and that this is theorigin of the Faber-Jackson relation. We show that Lyman break galaxiesand ultraluminous infrared galaxies have luminosities nearLM. Since these starbursting galaxies account for asignificant fraction of the star formation at z>~1, this supports ourhypothesis that much of the observed stellar mass in early-type galaxieswas formed during Eddington-limited star formation. Star formation isunlikely to efficiently remove gas from very small scales in galacticnuclei, i.e., scales much smaller than that of a nuclear starburst. Thisgas is available to fuel a central black hole (BH). We argue that a BHclears gas out of its galactic nucleus when the luminosity of the BHitself reaches ~LM. This shuts off the fuel supply to the BHand may also terminate star formation in the surrounding galaxy. As aresult, the BH mass is fixed to beMBH~=(fgκes/πG2)σ4,where κes is the electron scattering opacity. Thislimit is in accord with the observed MBH-σ relation.
| 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.
| BeppoSAX/PDS serendipitous detections at high galactic latitudes
At a flux limit of 10-11 erg cm-2 s-1in the 20-100 keV band, the PDS instrument on-board BeppoSAX offers theopportunity to study the extragalactic sky with an unprecedentedsensitivity. In this work we report on the results of a search in theBeppoSAX archive for serendipitous high energy sources at high galacticlatitudes (\vert b \vert≥13°). We have defined a set of twelveregions in which the PDS/MECS cross-calibration constant is higher thanthe nominal value. We attribute this mismatch to the presence of aserendipitous source in the PDS field of view. In four cases the likelyhigh energy emitter is also present in the MECS field of view. In thesecases, we have performed a broad band spectral analysis (1.5-100 keV) tounderstand the source spectral behaviour and compare it with previousBeppoSAX observations when available. In eight cases the identificationof the source likely to provide the PDS spectrum is based on indirectevidence (extrapolation to lower energies and/or comparison withprevious observations). This approach led to the discovery of six newhard X-ray emitting objects (PKS 2356-611, 2MASX J14585116-1652223, NGC1566, NGC 7319, PKS 0101-649 and ESO 025-G002) and to the presentationthe PDS spectrum of NGC 3227 for the first time. In the remaining fivecases we provide extra BeppoSAX observations that can be compared withmeasurements already published and/or in the archive.
| Intergalactic neutral hydrogen gas in the Grus quartet of galaxies
Australia Telescope Compact Array multi-configuration mosaicing of theGrus quartet of galaxies reveals the presence of spectacular tidalstructures. 2.1×109 Mȯ of neutralatomic hydrogen (H I) gas, i.e. 11% of all H I in the group, are foundto be dragged from NGC 7582 into intergalactic space. About1.34×109 Mȯ of H I gas are contained ina tidal tail emanating from the north-western disk of NGC 7582, with aprojected length of about 85 kpc and width of up to 32 kpc and arelative velocity with respect to the centre of NGC 7582 of 130-140 kms-1. 7.7×108 Mȯ of H Ireside in an intergalactic H I cloud 48 kpc West of NGC 7582, whichmight originate from the disk of NGC 7582 as well and has no opticalcounterpart in a red Digital Sky Survey (DSS) image. These observationsprove that tidal stripping is occurring in the Grus quartet and thattidal features in compact groups can be potentially importantcontributors of metal-enriched matter to the intergalactic medium. Thetidal features around NGC 7582 cover an area of about 2000kpc2, almost doubling the group's cross-section forLyman-α absorption of light from background sources compared tothe optical extent of the member galaxies.
| Bar-induced perturbation strengths of the galaxies in the Ohio State University Bright Galaxy Survey - I
Bar-induced perturbation strengths are calculated for a well-definedmagnitude-limited sample of 180 spiral galaxies, based on the Ohio StateUniversity Bright Galaxy Survey. We use a gravitational torque method,the ratio of the maximal tangential force to the mean axisymmetricradial force, as a quantitative measure of the bar strength. Thegravitational potential is inferred from an H-band light distribution byassuming that the M/L ratio is constant throughout the disc. Galaxiesare deprojected using orientation parameters based on B-band images. Inorder to eliminate artificial stretching of the bulge, two-dimensionalbar-bulge-disc decomposition has been used to derive a reliable bulgemodel. This bulge model is subtracted from an image, the disc isdeprojected assuming it is thin, and then the bulge is added back byassuming that its mass distribution is spherically symmetric. We findthat removing the artificial bulge stretch is important especially forgalaxies having bars inside large bulges. We also find that the massesof the bulges can be significantly overestimated if bars are not takeninto account in the decomposition.Bars are identified using Fourier methods by requiring that the phasesof the main modes (m= 2, m= 4) are maintained nearly constant in the barregion. With such methods, bars are found in 65 per cent of the galaxiesin our sample, most of them being classified as SB-type systems in thenear-infrared by Eskridge and co-workers. We also suggest that as muchas ~70 per cent of the galaxies classified as SAB-types in thenear-infrared might actually be non-barred systems, many of them havingcentral ovals. It is also possible that a small fraction of the SAB-typegalaxies have weak non-classical bars with spiral-like morphologies.
| Connecting the cosmic infrared background to the X-ray background
We estimate the contribution of active galactic nuclei (AGN) and oftheir host galaxies to the infrared background. We use the luminosityfunction and evolution of AGN recently determined by the hard X-raysurveys, and new spectral energy distributions connecting the X-ray andthe infrared emission, divided in intervals of absorption. These twoingredients allow us to determine the contribution of AGN to theinfrared background by using mostly observed quantities, with only minorassumptions. We find that AGN emission contributes little to theinfrared background (<5 per cent over most of the infrared bands),implying that the latter is dominated by star formation. However, AGNhost galaxies may contribute significantly to the infrared background,and more specifically 10-20 per cent in the 1-20 μm range and ~5 percent at λ < 60μm. We also give the contribution of AGN andof their host galaxies to the source number counts in various infraredbands, focusing on those which will be observed with Spitzer. We alsoreport a significant discrepancy between the expected contribution ofAGN hosts to the submillimetre background and bright submillimetrenumber counts with the observational constraints. We discuss the causesand implications of this discrepancy and the possible effects on theSpitzer far-infrared bands.
| The star formation history of Seyfert 2 nuclei
We present a study of the stellar populations in the central ~200 pc ofa large and homogeneous sample comprising 79 nearby galaxies, most ofwhich are Seyfert 2s. The star formation history of these nuclei isreconstructed by means of state-of-the-art population synthesismodelling of their spectra in the 3500-5200 Åinterval. Aquasar-like featureless continuum (FC) is added to the models to accountfor possible scattered light from a hidden active galactic nucleus(AGN).We find the following. (1) The star formation history of Seyfert 2nuclei is remarkably heterogeneous: young starbursts, intermediate-ageand old stellar populations all appear in significant and widely varyingproportions. (2) A significant fraction of the nuclei show a strong FCcomponent, but this FC is not always an indication of a hidden AGN: itcan also betray the presence of a young, dusty starburst. (3) We detectweak broad Hβ emission in several Seyfert 2s after cleaning theobserved spectrum by subtracting the synthesis model. These are mostlikely the weak scattered lines from the hidden broad-line regionenvisaged in the unified model, given that in most of these casesindependent spectropolarimetry data find a hidden Seyfert 1. (4) The FCstrengths obtained by the spectral decomposition are substantiallylarger for the Seyfert 2s which present evidence of broad lines,implying that the scattered non-stellar continuum is also detected. (5)There is no correlation between the star formation in the nucleus andeither the central or overall morphology of the parent galaxies.
| The impact of broad mid-infrared emission bands on extragalactic surveys - a case study: the unidentified 22-μm feature
The effect of unidentified dust emission features in the mid-infrared(MIR) spectral energy distributions of dusty star-forming galaxies isinvestigated. As a case study, we discuss the broad emission feature at2 μm of Chan & Onaka. By modelling a distribution of featurestrengths using a starburst galaxy template, we speculativelyinvestigate the influence of the feature on galaxy fluxes, colours anddetected numbers. We find that the fluxes and colours of galaxies aresystematically affected by the feature from redshifts 0-3, atwavelengths ranging from 20 to 70 μm by up to factors of ~2,resulting in bluer MIR/far-infrared (FIR) and FIR/MIR colours at localand high redshifts, respectively, as the feature is redshifted in andout of the various observation passbands. Furthermore, we find that ifthe feature is prolific, it will manifest itself in the same manner asevolution in the infrared (IR) galaxy population in a similar way to theunidentified IR bands at shorter wavelengths. We also consider theeffect of the feature on the future IR SIRTF (Spitzer) and ASTRO-F spacemissions. We conclude that although the bulk effects of the feature maybe small, careful consideration will have to be made when discussing thecolours of sources in the relevant bands and redshift ranges affected bythe feature emission.
| Chandra observations of the flat spectrum Seyfert-2 galaxies NGC 2110 and NGC 7582.
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| 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).
| Circumnuclear Structure and Black Hole Fueling: Hubble Space Telescope NICMOS Imaging of 250 Active and Normal Galaxies
Why are the nuclei of some galaxies more active than others? If mostgalaxies harbor a central massive black hole, the main difference isprobably in how well it is fueled by its surroundings. We investigatethe hypothesis that such a difference can be seen in the detailedcircumnuclear morphologies of galaxies using several quantitativelydefined features, including bars, isophotal twists, boxy and diskyisophotes, and strong nonaxisymmetric features in unsharp-masked images.These diagnostics are applied to 250 high-resolution images of galaxycenters obtained in the near-infrared with NICMOS on the Hubble SpaceTelescope. To guard against the influence of possible biases andselection effects, we have carefully matched samples of Seyfert 1,Seyfert 2, LINER, starburst, and normal galaxies in their basicproperties, taking particular care to ensure that each was observed witha similar average scale (10-15 pc pixel-1). Severalmorphological differences among our five different spectroscopicclassifications emerge from the analysis. The H II/starburst galaxiesshow the strongest deviations from smooth elliptical isophotes, whilethe normal galaxies and LINERs have the least disturbed morphology. TheSeyfert 2s have significantly more twisted isophotes than any othercategory, and the early-type Seyfert 2s are significantly more disturbedthan the early-type Seyfert 1s. The morphological differences betweenSeyfert 1s and Seyfert 2s suggest that more is at work than simply theviewing angle of the central engine. They may correspond to differentevolutionary stages.
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