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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.
| Penetrating the Deep Cover of Compton-thick Active Galactic Nuclei
We analyze observations obtained with the Chandra X-Ray Observatory ofbright Compton-thick active galactic nuclei (AGNs), those with columndensities in excess of 1.5×1024 cm-2 alongthe lines of sight. We therefore view the powerful central engines onlyindirectly, even at X-ray energies. Using high spatial resolution andconsidering only galaxies that do not contain circumnuclear starbursts,we reveal the variety of emission AGNs alone may produce. Approximately1% of the continuum's intrinsic flux is detected in reflection in eachcase. The only hard X-ray feature is the prominent Fe Kαfluorescence line, with equivalent width greater than 1 keV in allsources. The Fe line luminosity provides the best X-ray indicator of theunseen intrinsic AGN luminosity. In detail, the morphologies of theextended soft X-ray emission and optical line emission are similar, andline emission dominates the soft X-ray spectra. Thus, we attribute thesoft X-ray emission to material that the central engines photoionize.Because the resulting spectra are complex and do not reveal the AGNsdirectly, crude analysis techniques, such as hardness ratios, wouldmisclassify these galaxies as hosts of intrinsically weak, unabsorbedAGNs and would fail to identify the luminous, absorbed nuclei that arepresent. We demonstrate that a three-band X-ray diagnostic can correctlyclassify Compton-thick AGNs, even when significant soft X-ray lineemission is present. The active nuclei produce most of the galaxies'total observed emission over a broad spectral range, and much of theirlight emerges at far-infrared wavelengths. Stellar contamination of theinfrared emission can be severe, however, making long-wavelength dataalone unreliable indicators of the buried AGN luminosity.
| Discovery of Water Maser Emission in Eight AGNs with 70 m Antennas of NASA's Deep Space Network
We report the discovery of water maser emission in eight active galacticnuclei (AGNs) with the 70 m NASA Deep Space Network (DSN) antennas atTidbinbilla, Australia, and Robledo, Spain. The positions of the newlydiscovered masers, measured with the VLA, are consistent with theoptical positions of the host nuclei to within 1 σ (0.3" radio and1.3" optical) and most likely mark the locations of the embedded centralengines. The spectra of two sources, NGC 3393 and NGC 5495, display thecharacteristic spectral signature of emission from an edge-on accretiondisk, with orbital velocities of ~600 and ~400 km s-1,respectively. In a survey with DSN facilities of 630 AGNs selected fromthe NASA Extragalactic Database, we have discovered a total of 15 watermaser sources. The resulting incidence rate of maser emission amongnearby (vsys<7000 km s-1) Seyfert 1.8-2.0 andLINER systems is ~10% for a typical rms noise level of ~14 mJy over 1.3km s-1 spectral channels. As a result of this work, thenumber of nearby AGNs (vsys<7000 km s-1)observed with <20 mJy rms noise has increased from 130 to 449.
| 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.
| Size and properties of the narrow-line region in Seyfert-2 galaxies from spatially-resolved optical spectroscopy
Context: .While [O III] narrow-band imaging is commonly used to measurethe size of the narrow-line regions (NLRs) in active galactic nuclei(AGNs), it can be contaminated by emission from surrounding starbursts.Recently, we have shown that long-slit spectroscopy provides a valuablealternative approach to probe the size in terms of AGN photoionisation.Moreover, several parameters of the NLR can be directly accessed.Aims: .We here apply the same methods developed and described for theSeyfert-2 galaxy NGC 1386 to study the NLR of five other Seyfert-2galaxies by using high-sensitivity spatially-resolved opticalspectroscopy obtained at the VLT and the NTT. Methods: .We probethe AGN-photoionisation of the NLR and thus, its "real" size usingdiagnostic line-ratio diagrams. We derive physical properties of the NLRsuch as reddening, ionisation parameter, electron density, and velocityas a function of distance from the nucleus. Results: .For NGC5643, the diagnostic diagrams unveil a similar transition between lineratios falling in the AGN regime and those typical for H II regions asfound for NGC 1386, thus determining the size of the NLR. For the otherfour objects, all measured line ratios fall in the AGN regime. In almostall cases, both electron density and ionisation parameter decrease withradius. Deviations from this general behaviour (such as a secondarypeak) seen in both the ionisation parameter and electron density can beinterpreted as signs of shocks from the interaction of a radio jet andthe NLR gas. In several objects, the gaseous velocity distribution ischaracteristic for rotational motion in an (inclined) emission-line diskin the centre. We compare our results to those of NGC 1386 and show thatthe latter can be considered as prototypical also for this largersample. We discuss our findings in detail for each object.
| 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.
| Size and properties of the NLR in the Seyfert-2 galaxy NGC 1386
We study the narrow-line region (NLR) of the Seyfert-2 galaxyNGC 1386 by means of long-slit spectroscopy obtainedwith FORS1 at the VLT. We use the galaxy itself for subtracting thestellar template, applying reddening corrections to fit the stellartemplate to the spectra of the NLR. The continuum gets steadily reddertowards the nucleus. The spatial distribution of the reddening derivedfrom the Balmer decrement differs from the continuum reddening,indicating dust within the NLR with a varying column density along theline of sight. Using spatially resolved spectral diagnostics, we find atransition between central line ratios falling into the AGN regime andouter ones in the H II-region regime, occuring at a radius of~6´´ (310 pc) in all three diagnostic diagrams. ApplyingCLOUDY photoionisation models, we show that the observed distinctionbetween H II-like and AGN-like ratios in NGC 1386represents a true difference in ionisation source and cannot beexplained by variations of physical parameters such as ionisationparameter, electron density or metallicity. We interpret it as a realborder between the NLR, i.e. the central AGN-photoionised region andsurrounding H II regions. We derive surface brightness, electrondensity, and ionisation parameter as a function of distance from thenucleus. Both the electron density and the ionisation parameter decreasewith radius. We discuss the consequences of these observations for theinterpretation of the empirical NLR size-luminosity relation. In theouter part of the NLR, we find evidence for shocks, resulting in asecondary peak of the electron-density and ionisation-parameterdistribution north of the nucleus. We compare the NLR velocity curvewith the stellar one and discuss the differences.
| Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research
This review discusses the current status of supermassive black holeresearch, as seen from a purely observational standpoint. Since theearly ‘90s, rapid technological advances, most notably the launchof the Hubble Space Telescope, the commissioning of the VLBA andimprovements in near-infrared speckle imaging techniques, have not onlygiven us incontrovertible proof of the existence of supermassive blackholes, but have unveiled fundamental connections between the mass of thecentral singularity and the global properties of the host galaxy. It isthanks to these observations that we are now, for the first time, in aposition to understand the origin, evolution and cosmic relevance ofthese fascinating objects.
| 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 transmission-dominated to reprocessing-dominated spectral state transitions in Seyfert 2 galaxies
We present Chandra and XMM-Newton observations of a small sample (11objects) of optically selected Seyfert 2 galaxies, for which ASCA andBeppoSAX had suggested Compton-thick obscuration of the active galacticnucleus (AGN). The main goal of this study is to estimate the rate oftransitions between `transmission-dominated' and`reprocessing-dominated' states. We discover one new transition in NGC4939, with a possible additional candidate in NGC 5643. This indicates atypical occurrence rate of at least ~0.02yr-1. Thesetransitions could be due to large changes of the obscuring gas columndensity, or to a transient dimming of the AGN activity, the latterscenario being supported by detailed analysis of the best-studiedevents. Independently of the ultimate mechanism, comparison of theobserved spectral dynamics with Monte Carlo simulations demonstratesthat the obscuring gas is largely inhomogeneous, with multiple absorbingcomponents possibly spread through the whole range of distances from thenucleus between a fraction of parsecs up to several hundred parsecs. Asa by-product of this study, we report the first measurement ever of thecolumn density covering the AGN in NGC 3393 (NH~= 4.4 ×1024cm-2), and the discovery of soft X-rayextended emission, apparently aligned along the host galaxy main axis inNGC 5005. The latter object most likely hosts an historicallymisclassified low-luminosity Compton-thin AGN.
| 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 Link between Star Formation and Accretion in LINERs: A Comparison with Other Active Galactic Nucleus Subclasses
We present archival high-resolution X-ray imaging observations of 25nearby LINERs observed by ACIS on board Chandra. This sample builds onour previously published proprietary and archival X-ray observations andincludes the complete set of LINERs with published black hole masses andFIR luminosities that have been observed by Chandra. Of the 82 LINERsobserved by Chandra, 41 (50%) display hard nuclear cores consistent withan AGN. The nuclear 2-10 keV luminosities of these AGN-LINERs range from~2×1038 to ~1×1044 ergss-1. Reinforcing our previous work, we find a significantcorrelation between the Eddington ratio,Lbol/LEdd, and the FIR luminosity,LFIR, as well as the IR brightness ratio,LFIR/LB, in the host galaxy of AGN-LINERs thatextends over 7 orders of magnitude in Lbol/LEdd.Combining our AGN-LINER sample with galaxies from other AGN subclasses,we find that this correlation is reinforced in a sample of 129 AGNs,extending over almost 9 orders of magnitude inLbol/LEdd. Using archival and previously publishedobservations of the 6.2 μm PAH feature from ISO, we find that it isunlikely that dust heating by the AGN dominates the FIR luminosity inour sample of AGNs. Our results may therefore imply a fundamental linkbetween the mass accretion rate (M˙), as measured by the Eddingtonratio, and the star formation rate (SFR), as measured by the FIRluminosity. Apart from the overall correlation, we find that thedifferent AGN subclasses occupy distinct regions in the LFIRand Lbol/LEdd plane. Assuming a constant radiativeefficiency for accretion, our results may imply a variation in theSFR/M˙ ratio as a function of AGN activity level, a result that mayhave significant consequences for our understanding of galaxy formationand black hole growth.
| 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 Connection between the Narrow-Line Region and the UV Absorbers in Seyfert Galaxies
We present evidence that the outflowing UV absorbers in Seyfert 1galaxies arise primarily in their inner narrow- (emission-) line regions(NLRs), based on similarities in their locations, kinematics, andphysical conditions. Hubble Space Telescope observations show thatnearly all Seyfert galaxies have bright, central knots of [O III]emission in their NLRs with radii of tens of parsecs. These sizes areconsistent with most previous estimates of the distances of UV (andX-ray) absorbers from their central continuum sources and a recentlyobtained reliable distance of ~25 pc for a UV absorber in the Seyfert 1galaxy NGC 3783. The nuclear emission-line knots in a sample of 10Seyfert galaxies have velocity widths of 300-1100 km s-1(half-width at zero intensity), similar to the radial velocities of mostUV absorbers. The highest radial velocity for a Seyfert UV absorber todate is only -2100 km s-1, which is much lower than typicalbroad-line region (BLR) velocities. There is also mounting evidence thatthe NLR clouds are outflowing from the nucleus, like the UV absorbers.If our hypothesis is correct, then the NLR should have a component witha high global covering factor (Cg) of the continuum sourceand a BLR to match that found from previous surveys of UV absorbers(Cg=0.5-1.0). Using Space Telescope Imaging Spectrographspectra of NGC 4151, obtained when the continuum and BLR fluxes werelow, we find evidence for optically thin gas in its nuclearemission-line knot. We are able to match the line ratios from this gaswith photoionization models that include a component withCg~1 and an ionization parameter and hydrogen column densitythat are typical of UV absorbers.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555.
| Size and properties of AGN narrow-line regions from emission-line diagnostics.
Not Available
| Probing the Kinematics of the Narrow-Line Region in Seyfert Galaxies with Slitless Spectroscopy: Observational Results
We present slitless spectra of 10 Seyfert galaxies observed with theSpace Telescope Imaging Spectrograph on the Hubble Space Telescope(HST). The spectra cover the [O III] λλ4959, 5007 emissionlines at a spectral resolving power of λ/Δλ~9000 anda spatial resolution of ~0.1". We compare the slitless spectra withprevious HST narrowband images to determine the velocity shifts anddispersions of the bright emission-line knots in the narrow-line regions(NLRs) of these Seyfert galaxies, which extend out to at least severalhundred parsecs from their nuclei. Many knots are spatially resolvedwith sizes of tenths of arcseconds, corresponding to tens of parsecs,and yet they appear to move coherently with radial velocities betweenzero and +/-1200 km s-1 with respect to the systemicvelocities of their host galaxies. The knots also show a broad range invelocity dispersion, ranging from ~30 km s-1 (the velocityresolution) to ~1000 km s-1 FWHM. Most of the Seyfertgalaxies in this sample show an organized flow pattern, with radialvelocities near zero at the nucleus (defined by the optical continuumpeak) and increasing to maximum blueshifts and redshifts within ~1" ofthe nucleus, followed by a decline to the systemic velocity. However,there are large local variations around this pattern, and in one case(NGC 7212), the radial velocities are nearly chaotic. The emission-lineknots also follow a general trend of decreasing velocity dispersion withincreasing distance from the nucleus. In the Seyfert 2 galaxies, thepresence of blueshifts and redshifts on either side of the nucleusindicates that rotation alone cannot explain the observed radialvelocities. The most straightforward interpretation is that radialoutflow plays an important role in the NLR kinematics. Each of theSeyfert galaxies in this sample (with the exception of Mrk 3) shows abright, compact (FWHM<=0.5") [O III] knot at the position of itsoptical nucleus. These nuclear emission-line knots have radial velocitycentroids near zero, but they typically have the highest velocitydispersions. Their similar properties suggest that they may be a common,distinct component of the NLR.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS5-26555. These observations areassociated with proposal GO-8340.
| New H2O masers in Seyfert and FIR bright galaxies
Using the Effelsberg 100-m telescope, detections of four extragalacticwater vapor masers are reported. Isotropic luminosities are ~50, 1000, 1and 230 Lȯ for Mrk 1066 (UGC 2456), Mrk 34, NGC 3556 andArp 299, respectively. Mrk 34 contains by far the most distant and oneof the most luminous water vapor megamasers so far reported in a Seyfertgalaxy. The interacting system Arp 299 appears to show two maserhotspots separated by approximately 20´´. With these newresults and even more recent data from Braatz et al. (2004, ApJ, 617,L29), the detection rate in our sample of Seyferts with known jet-NarrowLine Region interactions becomes 50% (7/14), while in star forminggalaxies with high (S100~μ m>50 Jy) far infrared fluxesthe detection rate is 22% (10/45). The jet-NLR interaction sample maynot only contain “jet-masers” but also a significant numberof accretion “disk-masers” like those seen in NGC 4258. Astatistical analysis of 53 extragalactic H2O sources (excluding theGalaxy and the Magellanic Clouds) indicates (1) that the correlationbetween IRAS Point Source and H2O luminosities, established forindividual star forming regions in the galactic disk, also holds forAGN-dominated megamaser galaxies; (2) that maser luminosities are notcorrelated with 60 μm/100 μm color temperatures; and (3) that onlya small fraction of the luminous megamasers (L_H_2O > 100Lȯ) detectable with 100-m sized telescopes have so farbeen identified. The H2O luminosity function (LF) suggests that thenumber of galaxies with 1 Lȯ < L_H_2O < 10Lȯ, the transition range between“kilomasers” (mostly star formation) and“megamasers” (active galactic nuclei), is small. The overallslope of the LF, ~-1.5, indicates that the number of detectable masersis almost independent of their luminosity. If the LF is not steepeningat very high maser luminosities and if it is possible to find suitablecandidate sources, H2O megamasers at significant redshifts should bedetectable even with present day state-of-the-art facilities.
| Disks, tori, and cocoons: emission and absorption diagnostics of AGN environments
One of the most important problems in the study of active galaxies isunderstanding the detailed geometry, physics, and evolution of thecentral engines and their environments. The leading models involve anaccretion disk and torus structure around a central dense object,thought to be a supermassive black hole. Gas found in the environment ofactive galactic nuclei (AGN) is associated with different structures:molecular accretion disks, larger scale atomic tori, ionized and neutral“cocoons” in which the nuclear regions can be embedded. Allof them can be studied at radio wavelengths by various means. Here, wesummarize the work that has been done to date in the radio band tocharacterize these structures. Much has been learned about the centralfew parsecs of AGN in the last few decades with contemporary instrumentsbut the picture remains incomplete. In order to be able to define a moreaccurate model of this region, significant advances in sensitivity,spectral and angular resolution, and bandpass stability are required.The necessary advances will only be provided by the Square KilometerArray and we discuss the possibilities that these dramatic improvementswill open for the study of the gas in the central region of AGN.
| 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 GBT Atlas of H2O Masers in Galactic Nuclei
Water vapor masers have been detected toward the nuclei of some 63galaxies, nearly all AGNs. We are using the Green Bank Telescope (GBT)to obtain sensitive spectra of most of the masers available in thenorthern sky. The spectra cover 380 MHz (roughly 5100 km/s) with achannel spacing of 24 kHz (0.3 km/s) at sensitivities ranging from 2 to5 mJy per 24 kHz channel. This atlas serves to advance studies of maserphenomenology, to reveal new maser features, to identify galaxies withweak, broad emission components, and to provide a base for long-termvariability studies. Several maser systems include Doppler componentswhich are symmetrically red- and blue-shifted from the systemicrecession velocity, a signature of a maser in an accretion disk. We havebeen monitoring the disk masers in the galaxies NGC 1386, IC 2560, Mrk1419, and NGC 6323 in order to track velocity drifts among individualfeatures, and thereby measure the centripetal acceleration of gas in thedisk. We use these measurements to obtain an estimate of the centralblack hole mass in each source.
| Molecular hydrogen and [Fe II] in Active Galactic Nuclei
Near-infrared spectroscopy is used to study the kinematics andexcitation mechanisms of H2 and [Fe II] lines in a sample ofmostly Seyfert 1 galaxies. The spectral coverage allows simultaneousobservation of the JHK bands, thus eliminating the aperture and seeingeffects that have usually plagued previous works. The H2lines are unresolved in all objects in which they were detected whilethe [Fe II] lines have widths implying gas velocities of up to 650 ~kms-1. This suggests that, very likely, the H2 and[Fe II] emission does not originate from the same parcel of gas.Molecular H2 lines were detected in 90% of the sample,including PG objects, indicating detectable amounts of molecularmaterial even in objects with low levels of circumnuclear starburstactivity. Analysis of the observations favors thermal excitationmechanisms for the H2 lines. Indeed, in NGC 3227, Mrk 766,NGC 4051 and NGC 4151, the molecular emission is found to be purelythermal but with heating processes that vary between the objects.Thermal excitation is also confirmed by the rather similar vibrationaland rotational temperatures in the objects for which data wereavailable. [Fe II] lines are detected in all of the sample AGN. The [FeII] 1.254 μm/Paβ ratio is compatible with excitation of the [FeII] lines by the active nucleus in most Seyfert 1 galaxies, but in Mrk766 the ratio implies a stellar origin. A correlation betweenH2/Brγ and [Fe II]/Paβ is found for our sampleobjects supplemented by data from the literature. The correlation ofthese line ratios is a useful diagnostic tool in the NIR to separateemitting line objects by their level of nuclear activity. X-rayexcitation models are able to explain the observed H2 andpart of the [Fe II] emission but fail to explain the observations inSeyfert 2 galaxies. Most likely, a combination of X-ray heating, shocksdriven by the radio jet and circumnuclear star formation contributes, indifferent proportions, to the H2 and [Fe II] lines observed.In most of our sample objects, the [Fe II] 1.257 μm/1.644 μm ratiois found to be 30% lower than the intrinsic value based on currentatomic data. This implies either that the extinction towards the [FeII]-emitting clouds is very similar in most objects or there arepossible inaccuracies in the A-values in the Fe II transitions.Based on observations taken at the Infrared Telescope Facility, which isoperated by the University of Hawaii under Cooperative Agreement no. NCC5-538 with the National Aeronautics and Space Administration, Office ofSpace Science, Planetary Astronomy Program.
| Spectral characteristics of water megamaser galaxies. II. ESO 103-G035, TXS 2226-184, and IC 1481
Long-slit optical emission-line spectra of the H2O megamasergalaxies ESO 103-G035, TXS2226-184, and IC 1481 are evaluated inorder to look for characteristics typical for water-megamaser galaxies.We present rotation curves, line ratios, electron densities,temperatures, and Hβ luminosities. The successful line-profiledecompositions rest on d-Lorentzians with an additional parameter d toadjust the wings, rather than Gaussians or Lorentzians as basicfunctions. No significant velocity gradient is found along the majoraxis in the innermost 2 kpc of TXS 2226-184.IC 1481 reveals a spectrum suggestive of a vigorousstarburst in the central kiloparsec 108 years ago. None ofthe three galaxies shows any hints for outflows nor special featureswhich could give clues to the presence of H2O megamaseremission. The galaxies are of normal Seyfert-2 (ESO103-G035) or LINER (TXS 2226-184,IC 1481) type.Based on observations collected at the European Southern Observatory,Chile.
| The relation between AGN hard X-ray emission and mid-infrared continuum from ISO spectra: Scatter and unification aspects
We use mid-infrared spectral decomposition to separate the 6 μmmid-infrared AGN continuum from the host emission in the ISO lowresolution spectra of 71 active galaxies and compare the results toobserved and intrinsic 2-10 keV hard X-ray fluxes from the literature.We find a correlation between mid-infrared luminosity and absorptioncorrected hard X-ray luminosity, but the scatter is about an order ofmagnitude, significantly larger than previously found with smallerstatistics. Main contributors to this scatter are likely variations inthe geometry of absorbing dust, and AGN variability in combination withnon-simultaneous observations. There is no significant differencebetween type 1 and type 2 objects in the average ratio of mid-infraredand hard X-ray emission, a result which is not consistent with the mostsimple version of a unified scheme in which an optically andgeometrically thick torus dominates the mid-infrared AGN continuum. Mostprobably, significant non-torus contributions to the AGN mid-IRcontinuum are masking the expected difference between the two types ofAGN.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/418/465Based on observations with ISO, an ESA project with instruments fundedby ESA member states (especially the PI countries: France, Germany, TheNetherlands, and the UK) with the participation of ISAS and NASA.
| Mid-infrared emission of galactic nuclei. TIMMI2 versus ISO observations and models
We investigate the mid-infrared radiation of galaxies that are poweredby a starburst or by an AGN. For this end, we compare the spectraobtained at different spatial scales in a sample of infrared brightgalaxies. ISO observations which include emission of the nucleus as wellas most of the host galaxy are compared with TIMMI2 spectra of thenuclear region. We find that ISO spectra are generally dominated bystrong PAH bands. However, this is no longer true when inspecting themid-infrared emission of the pure nucleus. Here PAH emission is detectedin starbursts whereas it is significantly reduced or completely absentin AGNs. A physical explanation of these new observational results ispresented by examining the temperature fluctuation of a PAH afterinteraction with a photon. It turns out that the hardness of theradiation field is a key parameter for quantifying the photo-destructionof small grains. Our theoretical study predicts PAH evaporation in softX-ray environments. Radiative transfer calculations of clumpy starburstsand AGN corroborate the observational fact that PAH emission isconnected to starburst activity whereas PAHs are destroyed near an AGN.The radiative transfer models predict for starbursts a much largermid-infrared size than for AGN. This is confirmed by our TIMMI2acquisition images: We find that the mid-infrared emission of Seyfertsis dominated by a compact core while most of the starbursts arespatially resolved.Based on ESO: 68.B-0066(A) and observations with ISO, an ESA projectwith instruments funded by ESA Member States (especially the PIcountries: France, Germany, The Netherlands and the UK) with theparticipation of ISAS and NASA.}
| A Possible Signature of Connection between Blazars and Seyfert Galaxies
The accretion rates (dot{M}) and their correlation with cosmologicalredshifts for a sample of blazars and Seyfert galaxies are presented.The sample includes 77 blazars (28 FSRQs, 26 LBLs, and 23 HBLs) and 60Seyfert galaxies, of which the extended spectral energy distributioninformation and redshifts are available. Within the framework ofaccreting black holes, the accretion rates for these sources wereestimated based on their bolometric luminosities. The result shows thatthe accretion rates are significantly different for each subclass of theblazars and Seyfert galaxies. Their averages are, respectively, 50.2,17.0, 1.0, 0.1Modot yr-1 for the FSRQs, LBLs, HBLs, and theSeyfert galaxies, exhibiting a well descending sequence ofFSRQs-LBLs-HBLs-Seyfert galaxies. They are strongly correlated with theredshifts for both blazars and Seyfert galaxies. The linear correlationcoefficients are 0.81 and 0.68 with a chance probab ility of p <0.0001, respectively. A plot of dot{M} - z shows that the blazars andthe Seyfert galaxies distribute in a distinguishable regions with aconnection at z 0.7 and almost all the sources lie in a narrowregion of z1.40 ≤ dot{M} ≤ 250 z1.40,illustrating a strong correlation between the two quantities for thewhole sample. The regression line is dot{M} = (14.5 ± 1.2)z1.40±0.06 Modot yr-1 with a linearcoefficient of 0.93 and a chance probability of p < 0.0001,suggesting a connection between blazars and Seyfert galaxies. Thisconnection might imply that the two classes are on the same evolutionarysequence. Although the correlations of the data are formally solid, theconclusion may be affected by one source of considerable uncertainty atthe data level, which is also discussed.
| A Fundamental Plane of black hole activity
We examine the disc-jet connection in stellar mass and supermassiveblack holes by investigating the properties of their compact emission inthe X-ray and radio bands. We compile a sample of ~100 active galacticnuclei with measured masses, 5-GHz core emission, and 2-10 keVluminosities, together with eight galactic black holes with a total of~50 simultaneous observations in the radio and X-ray bands. Using thissample, we study the correlations between the radio (LR) andthe X-ray (LX) luminosity and the black hole mass (M). Wefind that the radio luminosity is correlated with bothM andLX, at a highly significant level. In particular, we showthat the sources define a `Fundamental Plane' in the three-dimensional(logLR, logLX, logM) space, given bylogLR= (0.60+0.11-0.11)logLX+ (0.78+0.11-0.09) logM+7.33+4.05-4.07, with a substantial scatter ofσR= 0.88. We compare our results to the theoreticalrelations between radio flux, black hole mass, and accretion ratederived by Heinz & Sunyaev. Such relations depend only on theassumed accretion model and on the observed radio spectral index.Therefore, we are able to show that the X-ray emission from black holesaccreting at less than a few per cent of the Eddington rate is unlikelyto be produced by radiatively efficient accretion, and is marginallyconsistent with optically thin synchrotron emission from the jet. On theother hand, models for radiatively inefficient accretion flows seem toagree well with the data.
| The surface brightness and colour-magnitude relations for Fornax cluster galaxies
We present BVI photometry of 190 galaxies in the central 4 ×3deg2 region of the Fornax cluster observed with the MichiganCurtis Schmidt Telescope. Results from the Fornax Cluster SpectroscopicSurvey (FCSS) and the Flair-II Fornax Surveys have been used to confirmthe membership status of galaxies in the Fornax Cluster Catalogue (FCC).In our catalogue of 213 member galaxies, 92 (43 per cent) have confirmedradial velocities.In this paper, we investigate the surface brightness-magnitude relationfor Fornax cluster galaxies. Particular attention is given to the sampleof cluster dwarfs and the newly discovered ultracompact dwarf galaxies(UCDs) from the FCSS. We examine the reliability of the surfacebrightness-magnitude relation as a method for determining clustermembership and find that at surface brightnesses fainter than 22 magarcsec-2, it fails in its ability to distinguish betweencluster members and barely resolved background galaxies. Cluster membersexhibit a strong surface brightness-magnitude relation. Both elliptical(E) galaxies and dwarf elliptical (dE) galaxies increase in surfacebrightness as luminosity decreases. The UCDs lie off the locus of therelation.B-V and V-I colours are determined for a sample of 113 cluster galaxiesand the colour-magnitude relation is explored for each morphologicaltype. The UCDs lie off the locus of the colour-magnitude relation. Theirmean V-I colours (~1.09) are similar to those of globular clustersassociated with NGC 1399. The location of the UCDs on both surfacebrightness and colour-magnitude plots supports the `galaxy threshing'model for infalling nucleated dwarf elliptical (dE, N) galaxies.
| Near-infrared spectroscopy of nearby Seyfert galaxies - II. Molecular content and coronal emission
We present subarcsec near-infrared 1.5-2.5 μm moderate resolutionlong-slit spectra of eight nearby Seyfert galaxies (z < 0.01), bothparallel to the ionization cone and perpendicular to it. These spectracomplement similar data on six Seyferts, presented in Reunanen,Kotilainen & Prieto, and are used to study the spatial extent of theline emission, the integrated masses of excited H2 and theexcitation mechanisms of interstellar gas.Large concentrations of molecular gas (H2) are present in thenucleus regardless of the Seyfert type. The spatial extent of theH2 emission is larger perpendicular to the cone than parallelto it in 6/8 (75 per cent) galaxies, in agreement with the unifiedmodels of active galactic nuclei. The full width at half maximum (FWHM)sizes of the nuclear H2 emission range from <20 to ~300pc, and are larger than the predicted sizes for molecular torus (1-100pc). Thus the emission probably arises from the material surrounding thetorus rather than directly from the torus.Broad Brγ was detected in nearly half of the optically classifiedSeyfert 2 galaxies, including two objects with no evidence for a hiddenpolarized broad line region. This high detection rate stresses theimportance of extinction effects as the main cause for the Seyfertdichotomy.Brγ and [Fe II] correlate both spatially and kinematically.Nuclear [Fe II] emission is generally blueshifted which, together withthe high Brγ/[Fe II] ratios, suggests shocks as the dominantexcitation mechanism in Seyfert galaxies.Bright coronal emission lines [SiVI] and [Si VII] are common inSeyferts, as they are detected in ~60 per cent of the galaxies. In threegalaxies the coronal lines are extended only in the direction parallelto the cone. This could be explained by a strongly collimated radiationfield or, most plausibly, by shock excitation due to the jet orsuperwind interacting with the interstellar medium.
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