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Dark and Baryonic Matter in Bright Spiral Galaxies. I. Near-Infrared and Optical Broadband Surface Photometry of 30 Galaxies
We present photometrically calibrated images and surface photometry inthe B, V, R, J, H, and K bands of 25, and in the g, r, and K bands offive nearby bright (B0T<12.5 mag) spiralgalaxies with inclinations of 30°-65° spanning the Hubblesequence from Sa to Scd. Data are from The Ohio State University BrightSpiral Galaxy Survey, the Two Micron All Sky Survey, and the SloanDigital Sky Survey Second Data Release. Radial surface brightnessprofiles are extracted, and integrated magnitudes are measured from theprofiles. Axis ratios, position angles, and scale lengths are measuredfrom the near-infrared images. A one-dimensional bulge/diskdecomposition is performed on the near-infrared images of galaxies witha nonnegligible bulge component, and an exponential disk is fit to theradial surface brightness profiles of the remaining galaxies.Based in part on observations obtained at the Cerro TololoInter-American Observatory, operated by the Association of Universitiesfor Research in Astronomy, Inc., under a cooperative agreement with theNational Science Foundation.
| 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.
| Constraining Dark Matter Halo Profiles and Galaxy Formation Models Using Spiral Arm Morphology. I. Method Outline
We investigate the use of spiral arm pitch angles as a probe of diskgalaxy mass profiles. We confirm our previous result that spiral armpitch angles (P) are well correlated with the rate of shear (S) in diskgalaxy rotation curves by using a much larger sample (51 galaxies) thanused previously (17 galaxies). We use this correlation to argue thatimaging data alone can provide a powerful probe of galactic massdistributions out to large look-back times. In contrast to previouswork, we show that observed spiral arm pitch angles are similar whenmeasured in the optical (at 0.4 μm) and the near-infrared (at 2.1μm) with a mean difference of 2.3d+/-2.7d. This is then used tostrengthen the known correlation between P and S using B-band images. Wethen use two example galaxies to demonstrate how an inferred shear ratecoupled with a bulge-disk decomposition model and a Tully-Fisher-derivedvelocity normalization can be used to place constraints on a galaxy'sbaryon fraction and dark matter halo profile. We show that ESO 582-G12,a galaxy with a high shear rate (slightly declining rotation curve) at~10 kpc, favors an adiabatically contracted halo, with high initial NFWconcentration (cvir>16) and a high fraction of halobaryons in the form of stars (~15%-40%). In contrast, IC 2522 has a lowshear rate (rising rotation curve) at ~10 kpc and favorsnonadiabatically contracted models with low NFW concentrations(cvir~=2-8) and a low stellar baryon fraction <10%.
| 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.
| Dark and Baryonic Matter in Bright Spiral Galaxies. II. Radial Distributions for 34 Galaxies
We decompose the rotation curves of 34 bright spiral galaxies intobaryonic and dark matter components. Stellar mass profiles are createdby applying color-M/L relations to near-infrared and optical photometry.We find that the radial profile of the baryonic-to-dark-matter ratio isself-similar for all galaxies, when scaled to the radius at which thecontribution of the baryonic mass to the rotation curve equals that ofthe dark matter (RX). We argue that this is due to thequasi-exponential nature of disks and rotation curves that are nearlyflat after an initial rise. The radius RX is found tocorrelate most strongly with baryonic rotation speed, such that galaxieswith RX measurements that lie further out in their disksrotate faster. This quantity also correlates very strongly with stellarmass, Hubble type, and observed rotation speed; B-band central surfacebrightness is less related to RX than these other galaxyproperties. Most of the galaxies in our sample appear to be close tomaximal disk. For these galaxies, we find that maximum observed rotationspeeds are tightly correlated with maximum rotation speeds predictedfrom the baryon distributions, such that one can create a Tully-Fisherrelation based on surface photometry and redshifts alone. Finally, wecompare our data to the NFW parameterization for dark matter profileswith and without including adiabatic contraction as it is most commonlyimplemented. Fits are generally poor, and all but two galaxies arebetter fit if adiabatic contraction is not performed. In order to havebetter fits, and especially to accommodate adiabatic contraction,baryons would need to contribute very little to the total mass in theinner parts of galaxies, seemingly in contrast with other observationalconstraints.
| 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).
| 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.
| 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.
| 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.
| 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.
| 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.
| 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.
| A Variability Study of the Seyfert 2 Galaxy NGC 6300 with XMM-Newton
We present the results of timing analysis of the XMM-Newton observationof the Seyfert 2 galaxy NGC 6300. The hard X-ray spectrum above 2 keVconsists of a Compton-thin absorbed power law, as is often seen inSeyfert 2 galaxies. We clearly detected rapid time variability on atimescale of about 1000 s from the light curve above 2 keV. The excessvariance of the time variability (σ2rms) iscalculated to be ~0.12, and the periodogram of the light curve is wellrepresented by a power-law function with a slope of 1.75. In contrast toprevious results from Seyfert 2 nuclei, these variabilitycharacteristics are consistent with those of Seyfert 1 galaxies. Thisconsistency suggests that NGC 6300 has a black hole mass and accretionproperties similar to Seyfert 1 galaxies. Using the relation betweentime variability and central black hole mass by Hayashida et al., theblack hole mass of NGC 6300 is estimated to be ~2.8×105Msolar. Taking the uncertainty of this method into account,the black hole mass is less than 107 Msolar.Taking the bolometric luminosity of 3.3×1043 ergss-1 into consideration, this yields an accretion rate of>0.03 of the Eddington value, comparable to estimates from Seyfert 1galaxies using this method. The time variability analysis suggests thatNGC 6300 actually has a Seyfert 1 nucleus obscured by thick matter andmore generally provides a new pillar of support for the unified model ofSeyfert galaxies based on obscuration.
| Star Formation in H I-selected Galaxies. II. H II Region Properties
A sample of 69 galaxies with radial velocities less than 2500 kms-1 was selected from the H I Parkes All Sky Survey (HIPASS)to deduce details about star formation in nearby disk galaxies selectedwith no bias to optical surface brightness selection effects. Broadband(B and R) and narrowband (Hα) images were obtained for all ofthese objects. More than half of the sample galaxies are late-type,dwarf disks (mostly Sc and Sm galaxies). We have measured the propertiesof the H II regions on Hα continuum-subtracted images, using theHIIphot package developed by Thilker et al. All but one of the galaxiescontained at least one detectable H II region. Examination of theproperties of the H II regions in each galaxy revealed that thebrightest regions in higher surface brightness galaxies tend to be moreluminous than those in lower surface brightness galaxies. A higherfraction (referred to as the diffuse fraction) of the Hα emissionfrom lower surface brightness galaxies comes from diffuse ionized gas. HII region luminosity functions (LFs) co-added according to surfacebrightness show that the shapes of the LFs for the lowest surfacebrightness galaxies are different from those for typical spiralgalaxies. This discrepancy could be caused by the lowest surfacebrightness galaxies having somewhat episodic star formation or by themforming a relatively larger fraction of their stars outside of dense,massive molecular clouds. In general, the results imply that theconditions under which star formation occurs in lower surface brightnessgalaxies are different than in more typical, higher surface brightnessspiral galaxies.
| Rapid Compton-thick/Compton-thin Transitions in the Seyfert 2 Galaxy NGC 1365
We present multiple Chandra and XMM-Newton observations of the type 1.8Seyfert galaxy NGC 1365, which shows the most dramatic X-ray spectralchanges observed so far in an active galactic nucleus: the sourceswitched from reflection-dominated to transmission-dominated and back injust 6 weeks. During this time the soft thermal component, arising froman ~1 kpc region around the center, remained constant. The reflectioncomponent is constant at all timescales, and its high flux relative tothe primary component implies the presence of thick gas covering a largefraction of the solid angle. The presence of this gas, and the fastvariability timescale, suggest that the Compton-thick to Compton-thinchange is due to variation in the line-of-sight absorber rather than toextreme intrinsic emission variability. We discuss a structure of thecircumnuclear absorber/reflector that can explain the observed X-rayspectral and temporal properties.
| The Distribution of Bar and Spiral Arm Strengths in Disk Galaxies
The distribution of bar strengths in disk galaxies is a fundamentalproperty of the galaxy population that has only begun to be explored. Wehave applied the bar-spiral separation method of Buta and coworkers toderive the distribution of maximum relative gravitational bar torques,Qb, for 147 spiral galaxies in the statistically well-definedOhio State University Bright Galaxy Survey (OSUBGS) sample. Our goal isto examine the properties of bars as independently as possible of theirassociated spirals. We find that the distribution of bar strengthdeclines smoothly with increasing Qb, with more than 40% ofthe sample having Qb<=0.1. In the context of recurrent barformation, this suggests that strongly barred states are relativelyshort-lived compared to weakly barred or nonbarred states. We do notfind compelling evidence for a bimodal distribution of bar strengths.Instead, the distribution is fairly smooth in the range0.0<=Qb<0.8. Our analysis also provides a first look atspiral strengths Qs in the OSUBGS sample, based on the sametorque indicator. We are able to verify a possible weak correlationbetween Qs and Qb, in the sense that galaxies withthe strongest bars tend to also have strong spirals.
| INTEGRAL observations of six AGN in the Galactic Plane
We present results on approximately one year of INTEGRAL observations ofsix AGN detected during the regular scans of the Galactic Plane. Thesample is composed by five Seyfert 2 objects (MCG -05-23-16, NGC 4945,the Circinus galaxy, NGC 6300, ESO 103-G35) and the radio galaxyCentaurus A. The continuum emission of each of these sources is wellrepresented by a highly absorbed (N_H>1022cm-2) power law, with average spectral index Γ = 1.9± 0.3. A high energy exponential cut-off at Ec 50 ~keV is required to fit the spectrum of the Circinus galaxy, whereasa lower limit of 130 keV has been found for NGC 4945 and no cut-off hasbeen detected for NGC 6300 in the energy range covered by these INTEGRALdata. The flux of Centaurus A was found to vary by a factor of ~2 in 10months, showing a spectral change between the high and low state, whichcan be modelled equally well by a change in the absorption(NH from 17 to 33 × 1022 cm-2) orby the presence of a cut-off at 120 keV in the low state spectrum.A comparison with recently reprocessed BeppoSAX/PDS data shows a generalagreement with INTEGRAL results. The high energy cut-off in the hardX-ray spectra appears to be a common but not universal characteristic ofSeyfert 2 and to span a wide range of energies.
| A search for changing-look AGN in the Grossan catalog
We observed with XMM-Newton 4 objects selected from the Grossan (1992,Ph.D. Thesis) catalog, with the aim to search for new "changing-look"AGN. The sample includes all the sources which showed in subsequentobservations a flux much lower than the one measured with HEAO A-1: NGC7674, NGC 4968, IRAS 13218+0552 and NGC 1667. None of the sources wascaught in a high flux state during the XMM-Newton observations, whoseanalysis reveal they are all likely Compton-thick objects. We suggestthat, for all the sources, potential problems with the HEAO A-1 sourceidentification and flux measurement prevent us from being certain thatthe HEAO A-1 data represent a putative "high" state for these objects.Nonetheless, based on the high flux state and Compton-thin spectrum ofits GINGA observation, NGC 7674 represents probably the sixth known caseof a "changing-look" Seyfert 2 galaxy. From the X-ray variabilitypattern, we can estimate a likely lower limit of a few parsec to thedistance of the inner walls of the torus in this object. Remarkably,IRAS 13218+0552 was not detected by XMM-Newton, despite being currentlyclassified as a Seyfert 1 with a large [OIII] flux. However, theoriginal classification was likely to be affected by an extreme velocityoutflow component in the emission lines. The object likely harbors anhighly obscured AGN and should be re-classified as a type 2 source.
| 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.
| Reflection Component in the Hard X-Ray Emission from the Seyfert 2 Galaxy Mrk 1210
The Seyfert 2 galaxy Mrk 1210 was found to exhibit a flat hard X-raycomponent by ASCA, although ASCA could not distinguish whether it is anabsorbed direct component or a reflected one. We then observed Mrk 1210with BeppoSAX, and found that the X-ray spectral properties are quitedifferent from those of ASCA, as have been confirmed with XMM-Newton;the flux is significantly higher than that in the ASCA observation, anda clear absorption cut-off appears below 5keV. A bright hard X-rayemission is detected up to 100keV. The reflection component is necessaryto describe the BeppoSAX PDS spectrum, and represents the ASCA hardcomponent very well. Therefore, the hard component in the ASCA spectrumis a reflected one, whose intensity is almost constant over 6 yr. Thisindicates that a dramatic spectral variability is attributed to a largechange of the absorption column density by a factor of >5. ratherthan the variability of the nuclear emission. The change in theabsorption column density means that the torus is not homogeneous, buthas a blobby structure with a typical blob size of < 0.001 pc.
| 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.
| Unveiling the nature of the highly obscured active galactic nucleus in NGC 5643 with XMM-Newton
We present results from an XMM-Newton observation of the nearby Seyfert2 galaxy NGC 5643. The nucleus exhibits a very flat X-ray continuumabove 2 keV, together with a prominent Kα fluorescent iron line.This indicates heavy obscuration. We measure an absorbing column densityNH in the range 6-10 × 1023 cm-2,either directly covering the nuclear emission, or covering its Comptonreflection. In the latter case, we might be observing a rather unusualgeometry for the absorber, whereby reflection from the inner far side ofa torus is in turn obscured by its near side outer atmosphere. Thenuclear emission might be then either covered by a Compton-thickabsorber, or undergoing a transient state of low activity. A secondsource (christened `X-1' in this paper) at the outskirts of the NGC 5643optical surface outshines the nucleus in X-rays. If belonging to NGC5643, it is the third brightest (LX~ 4 ×1040 erg s-1) known ultraluminous X-ray source.Comparison with past large aperture spectra of NGC 5643 unveils dramaticX-ray spectral changes above 1 keV. We interpret these as due tovariability of the active nucleus and of source X-1 intrinsic X-raypowers by factors of >=10 and 5, respectively.
| 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.
| Spectropolarimetry of Compton-thin Seyfert 2 galaxies
We present new spectropolarimetry of a sample of nearby Compton-thinSeyfert 2 galaxies (i.e. those with NH < 1023cm-2). We show that the detection rate of scattered broadHα in this sample is considerably higher than in Seyfert 2galaxies as a whole. Our results also show that in this low obscurationset it is possible to find scattered broad Hα even when the globalproperties of the galaxy are largely dominated by the host galaxy andnot the active galactic nucleus. These results argue against theexistence of a population of `pure' Seyfert 2 galaxies.
| Neutral hydrogen gas in interacting galaxies: the NGC 6221/6215 galaxy group
Neutral hydrogen observations of the spiral galaxies NGC 6221 and 6215with the Australia Telescope Compact Array (ATCA) reveal a wide,two-stranded bridge of at least 3 × 108Msolar which can be traced between the two galaxies over aprojected distance of 100 kpc. The velocity gradient of the HI bridgeprovides a rough estimate for the time since the encounter of 500 Myr.For NGC 6221, the brightest and most massive galaxy of the group, wederive a dynamical mass of Mtot= 8 × 1010Msolar, while its companion NGC 6215 has a mass of onlyMtot~ 2 × 109 Msolar. Further, wefind three low-surface-brightness dwarf galaxies (Dwarfs 1, 2 and 3) inthe neighbourhood of NGC 6221/15 with HI masses of 3.3, 0.6 and 0.3× 108 Msolar, respectively. The smallest,previously uncatalogued galaxy, Dwarf 3, lies between NGC 6221 and 6215,and may have formed out of bridge material.The brightest part of the HI bridge lies roughly halfway between theinteracting galaxies, indicating that bridge gas close to NGC 6221 and6215 may have fallen back to the galaxies. The asymmetric extensions tothe HI envelope of NGC 6221 are likely to be reaccreted gas, stillsettling in. Also, the peculiar velocity field of NGC 6215 may beexplained by accreted bridge material settling into a plane offset fromthe old disc.
| An XMM-Newton Observation of the Seyfert 2 Galaxy NGC 6300. I. The Nucleus
We present results from a half-day observation by XMM-Newton of thenucleus of the nearby Seyfert 2 galaxy NGC 6300. The X-ray spectrum ofthe nucleus consists of a heavily absorbed hard component dominating the3-10 keV band and a soft component seen in the 0.2-2 keV band. In thehard band, the spectrum is well fitted by a power-law model with photonindex of 1.83+/-0.08 attenuated by a Compton-thin absorber(NH~=2.2×1023 cm-2). A narrowiron line is detected at 6.43+0.01-0.02 keV withan equivalent width of ~150 eV; the line velocity width is marginallyresolved to be σ~60 eV. The soft emission can be modeled as apower law and may be emission scattered by surrounding plasma. Rapid andhigh-amplitude variability is observed in the hard X-ray band, whereasboth the iron line and the soft emission show no significantvariability. It is suggested that the nucleus has experienced an overalllong-term trend of decreasing hard X-ray intensity on a timescale ofyears. We discuss the origins of the spectral components.
| 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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