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Ýlgili Makaleler
Discovery of counter-rotating gas in the galaxies NGC 1596 and 3203 and the incidence of gas counter-rotation in S0 galaxies
We have identified two new galaxies with gas counter-rotation (NGC 1596and 3203) and have confirmed similar behaviour in another one (NGC 128),this using results from separate studies of the ionized-gas and stellarkinematics of a well-defined sample of 30 edge-on disc galaxies. Gascounter-rotators thus represent 10 +/- 5 per cent of our sample, but thefraction climbs to 21 +/- 11 per cent when only lenticular (S0) galaxiesare considered and to 27 +/- 13 per cent for S0 galaxies with detectedionized gas only. Those fractions are consistent with but slightlyhigher than previous studies. A compilation from well-defined studies ofS0 galaxies in the literature yields fractions of 15 +/- 4 and 23 +/- 5per cent, respectively. Although mainly based on circumstantialevidence, we argue that the counter-rotating gas originates primarilyfrom minor mergers and tidally induced transfer of material from nearbyobjects. Assuming isotropic accretion, twice those fractions of objectsmust have undergone similar processes, underlining the importance of(minor) accretion for galaxy evolution. Applications of gascounter-rotators to barred galaxy dynamics are also discussed.
| On the X-Ray Baldwin Effect for Narrow Fe Kα Emission Lines
Most active galactic nuclei (AGNs) exhibit a narrow Fe Kα line at~6.4 keV in the X-ray spectra, due to the fluorescent emission from coldmaterial far from the inner accretion disk. Using XMM-Newtonobservations, Page et al. found that the equivalent width (EW) of thenarrow Fe Kα line decreases with increasing luminosity(EW~L-0.17+/-0.08), suggesting a decrease in the coveringfactor of the material emitting the line (presumably the torus). Bycombining the archival Chandra HETG observations of 34 type 1 AGNs withXMM observations in the literature, we build a much larger sample with101 AGNs. We find a similar X-ray Baldwin effect in the sample(EW~L-0.2015+/-0.0426) however, we note that theanticorrelation is dominated by the radio-loud AGNs in the sample, whoseX-ray spectra might be contaminated by the relativistic jet. Excludingthe radio-loud AGNs, we find a much weaker anticorrelation(EW~L-0.1019+/-0.0524). We present Monte Carlo simulationsshowing that such a weak anticorrelation can be attributed to therelative short timescale variations of the X-ray continuum.
| 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 Host Galaxies of Narrow-Line Seyfert 1 Galaxies: Nuclear Dust Morphology and Starburst Rings
We present a study of the nuclear morphology of a sample of narrow- andbroad-line Seyfert 1 galaxies (NLS1s and BLS1s, respectively) based onbroadband images in the Hubble Space Telescope archives. In our previousstudy we found that large-scale stellar bars at >1 kpc from thenucleus are more common in NLS1s than BLS1s. In this paper we find thatNLS1s preferentially have grand-design dust spirals within ~1 kpc oftheir centers. We also find that NLS1s have a higher fraction of nuclearstar-forming rings than BLS1s. We find that many of the morphologicaldifferences are due to the presence or absence of a large-scale stellarbar within the spiral host galaxy. In general, barred Seyfert 1 galaxiestend to have grand-design dust spirals at their centers, confirming theresults of other researchers. The high fraction of grand-design nucleardust spirals and stellar nuclear rings observed in NLS1s' host galaxiessuggests a means for efficient fueling of their nuclei to support theirhigh Eddington ratios.
| Extragalactic Science with Tunable Filters
Tunable filters provide unique capabilities to carry out a wide array ofextragalactic projects. The emphasis of this review is on sciencerelating to starburst and active galaxies. Future avenues of researchwith 8-meter class telescopes equipped with tunable filters are alsodiscussed briefly.
| Water-Vapor Maser Survey for Active Galactic Nuclei: A Megamaser in NGC 6926
We made a survey of water-vapor maser emission for 93 AGNs with theNobeyama 45-m and Mopra 22-m telescopes from 1999 to 2002. A megamaserwas detected in a Seyfert 2 galaxy, NGC 6926, at a distance of 80Mpc, in2002 June. [Greenhill et al. (2003a) have also reported a detection ofthe megamaser at the close date.] The peak flux density was 110mJy, andthe total isotropic luminosity was 340 Lȯ. The masershows triply peaked spectrum, suggesting an edge-on disk. A narrow-linefeature of the maser components at VLSR = 6001 kms-1 was strongly variable with a time scale of a few tens ofdays, and the variation should be of intrinsic origin. We also showed apossibility of variability of water-vapor maser emission of a megamaserpreviously detected in a Seyfert/ultraluminous FIR galaxy, NGC 6240.
| The X-ray emission properties and the dichotomy in the central stellar cusp shapes of early-type galaxies
The Hubble Space Telescope has revealed a dichotomy in the centralsurface brightness profiles of early-type galaxies, which havesubsequently been grouped into two families: core, boxy, anisotropicsystems; and cuspy (`power-law'), discy, rotating ones. Here weinvestigate whether a dichotomy is also present in the X-ray propertiesof the two families. We consider both their total soft emission(LSX,tot), which is a measure of the galactic hot gascontent, and their nuclear hard emission (LHX,nuc), mostlycoming from Chandra observations, which is a measure of the nuclearactivity. At any optical luminosity, the highest LSX,totvalues are reached by core galaxies; this is explained by their beingthe central dominant galaxies of groups, subclusters or clusters, inmany of the logLSX,tot (ergs-1) >~ 41.5 cases.The highest LHX,nuc values, similar to those of classicalactive galactic nuclei (AGNs), in this sample are hosted only by core orintermediate galaxies; at low luminosity AGN levels, LHX,nucis independent of the central stellar profile shape. The presence ofoptical nuclei (also found by HST) is unrelated to the level ofLHX,nuc, even though the highest LHX,nuc are allassociated with optical nuclei. The implications of these findings forgalaxy evolution and accretion modalities at the present epoch arediscussed.
| Stellar abundance gradients in galactic discs - I. Method and spectral line gradients
We describe the technique of absorption-line imaging of galaxy discsusing the Taurus Tunable Filter on the Anglo-Australian Telescope anddemonstrate its sensitivity to the behaviour of spectral featuresassociated with Mg and Fe. Radial profiles of Mg2 and Fe5270line strengths are presented for a sample of eight face-on spiralgalaxies spanning a range of Hubble types. Signatures of phenomenaincluding merger-induced star formation, HII rings and galactic bars arealso reported. This study demonstrates the capacity of tunable filtersto measure Mg and Fe line strengths across the face of spiral galaxies,which can ultimately reveal clues about the star formation history andchemical evolution.
| Principal components in active galactic nuclei variability data and the estimation of the flux contributions from different components
It has been found that the near-infrared flux variations of Seyfertgalaxies satisfy relations of the formFi~αij+βijFj,where Fi, Fj are the fluxes in filters i and j;and αi,j, βi,j are constants. Theserelations have been used to estimate the constant contributions of thenon-variable underlying galaxies. The paper attempts a formal treatmentof the estimation procedure, allowing for the possible presence of athird component, namely non-variable hot dust. In an analysis of asample of 38 Seyfert galaxies, inclusion of the hot dust componentimproves the model fit in approximately half the cases. All derived dusttemperatures are below 300 K, in the range 540-860 K or above 1300 K. Anoteworthy feature is the estimation of confidence intervals for thecomponent contributions: this is achieved by bootstrapping. It is alsopointed out that the model implies that such data could be fruitfullyanalysed in terms of principal components.
| FeXXV and FeXXVI lines from low-velocity, photoionized gas in the X-ray spectra of active galactic nuclei
We have calculated the equivalent widths of the absorption linesproduced by FeXXV and FeXXVI in a Compton-thin, low-velocityphotoionized material illuminated by the nuclear continuum in activegalactic nuclei. The results, plotted against the ionization parameterand the column density of the gas, are a complement to those presentedby Bianchi & Matt for the emission lines from the same ionicspecies. As an extension to the work by Bianchi & Matt, we alsopresent a qualitative discussion on the different contributions to theHe-like iron emission line complex in the regimes where recombination orresonant scattering dominates, providing a useful diagnostic tool tomeasure the column density of the gas. Future high-resolution missions(e.g. Astro-E2) will allow us to fully take advantage of these plasmadiagnostics. In the meantime, we compare our results with an up-to-datelist of Compton-thick and unobscured (at least at the iron line energy)Seyfert galaxies with emission and/or absorption lines from H- andHe-like iron observed with Chandra and XMM-Newton.
| An 8.4-GHz Long Baseline Array observation of the unusual Seyfert galaxy NGC 7213
We have observed the type 1.5 Seyfert galaxy NGC 7213 with theAustralian Long Baseline Array (LBA) at 8.4 GHz to discover whether thisobject has the high brightness temperature compact core suggested bylow-frequency variability. Confirmation would support the hypothesisthat radio-intermediate Seyfert galaxies have Doppler-boosted radiojets. Our observation confirms the existence of this core but with aflux density of almost a factor of 6 less than observed 12 yr earlier.Though few studies exist on the long-term radio variability of Seyferts,a decline of this magnitude does appear to be rare.
| The X-ray spectrum of NGC 7213 and the Seyfert-LINER connection
We present an XMM-Newton observation of the Seyfert-LINER(low-ionization nuclear emission-line region) galaxy NGC 7213. The RGSsoft X-ray spectrum is well fitted with a power law plus soft X-raycollisionally ionized thermal plasma (kT=0.18+0.03-0.01 keV). We confirm the presence ofFeI, FeXXV and FeXXVI Kα emission in the EPIC spectrum and settighter constraints on their equivalent widths of82+10-13, 24+9-11 and24+10-13 eV, respectively. We compare the observedproperties together with the inferred mass accretion rate of NGC 7213with those of other Seyfert and LINER galaxies. We find that NGC 7213has intermediate X-ray spectral properties lying between those of theweak active galactic nucleus found in the LINER M81 andhigher-luminosity Seyfert galaxies. There appears to be a continuoussequence of X-ray properties from the Galactic Centre through LINERgalaxies to Seyferts, probably determined by the amount of materialavailable for accretion in the central regions.
| 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 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.
| Narrow Iron Kα Lines in Active Galactic Nuclei: Evolving Populations?
We assemble a sample consisting of 66 active galactic nuclei (AGNs) fromthe literature and from the XMM-Newton archive in order to investigatethe origin of the 6.4 keV narrow iron Kα line (NIKAL). The X-rayBaldwin effect of the NIKAL is confirmed in this sample. We find thatthe equivalent width (EW) of the NIKAL is more strongly inverselycorrelated with the Eddington ratio (E) than with the 2-10 keVX-ray luminosity. Our sample favors the dusty torus origin, with theX-ray Baldwin effect being caused by the changing opening angle of thedusty torus. The EW-E relation can be derived from a toy model ofthe dusty torus. If the unification scheme is valid in all AGNs, we canderive the Baldwin effect from the ratio of type II AGNs to the totalpopulation given by Chandra and Hubble Space Telescope deep surveys.Thus, the evolution of populations could be reflected in the NIKAL'sBaldwin effect.
| 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.
| 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.
| The Centers of Early-Type Galaxies with Hubble Space Telescope. V. New WFPC2 Photometry
We present observations of 77 early-type galaxies imaged with the PC1CCD of the Hubble Space Telescope (HST) WFPC2. ``Nuker-law'' parametricfits to the surface brightness profiles are used to classify the centralstructure into ``core'' or ``power-law'' forms. Core galaxies aretypically rounder than power-law galaxies. Nearly all power-law galaxieswith central ellipticities ɛ>=0.3 have stellar disks,implying that disks are present in power-law galaxies withɛ<0.3 but are not visible because of unfavorable geometry. Afew low-luminosity flattened core galaxies also have disks; these may betransition forms from power-law galaxies to more luminous core galaxies,which lack disks. Several core galaxies have strong isophote twistsinterior to their break radii, although power-law galaxies have interiortwists of similar physical significance when the photometricperturbations implied by the twists are evaluated. Central colorgradients are typically consistent with the envelope gradients; coregalaxies have somewhat weaker color gradients than power-law galaxies.Nuclei are found in 29% of the core galaxies and 60% of the power-lawgalaxies. Nuclei are typically bluer than the surrounding galaxy. Whilesome nuclei are associated with active galactic nuclei (AGNs), just asmany are not; conversely, not all galaxies known to have a low-level AGNexhibit detectable nuclei in the broadband filters. NGC 4073 and 4382are found to have central minima in their intrinsic starlightdistributions; NGC 4382 resembles the double nucleus of M31. In general,the peak brightness location is coincident with the photocenter of thecore to a typical physical scale of <1 pc. Five galaxies, however,have centers significantly displaced from their surrounding cores; thesemay be unresolved asymmetric double nuclei. Finally, as noted byprevious authors, central dust is visible in about half of the galaxies.The presence and strength of dust correlates with nuclear emission;thus, dust may outline gas that is falling into the central black hole.The prevalence of dust and its morphology suggest that dust clouds form,settle to the center, and disappear repeatedly on ~108 yrtimescales. We discuss the hypothesis that cores are created by thedecay of a massive black hole binary formed in a merger. Apart fromtheir brightness profiles, there are no strong differences between coregalaxies and power-law galaxies that demand this scenario; however, therounder shapes of core, their lack of disks, and their reduced colorgradients may be consistent with it.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 (AURA), Inc.,under NASA contract NAS 5-26555. These observations are associated withGO and GTO proposals 5236, 5446, 5454, 5512, 5943, 5990, 5999, 6099,6386, 6554, 6587, 6633, 7468, 8683, and 9107.
| An observational study of central engines in active galactic nuclei
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| 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.
| Long-term infrared photometry of Seyferts
Long-term (up to 10 000 d) monitoring has been undertaken for 41Seyferts in the near-infrared (1.25-3.45 μm). All but two showedvariability, with amplitudes at K in the range <0.1 to >1.1 mag.The time-scale for detectable change is from about one week to a fewyears.Where contemporary observations of variability in X-rays, ultraviolet(UV) or visible light exist, it is found that the near-infrared variesin a similar way, though in some cases the shorter-wavelength infrared(IR) bands are diluted by underlying galaxy radiation.A simple cross-correlation study indicates that there is evidence fordelays of up to several hundred d between the variations seen at theshortest wavelengths (U or J) and the longest (L) in many galaxies. Inparticular, the data for Fairall 9 now extend to twice the intervalcovered in earlier publications and the delay between its UV and IRoutputs is seen to persist.An analysis of the fluxes shows that, for any given galaxy, the coloursof the variable component of its nucleus are usually independent of thelevel of activity. The state of activity of the galaxy can beparameterized.Taken over the whole sample, the colours of the variable components fallwithin moderately narrow ranges. In particular, the H-K colour isappropriate to a blackbody of temperature 1600 K. The H-K excess for aheavily reddened nucleus can be determined and used to findEB-V, which can be compared to the values found from thevisible region broad line ratios.Using flux-flux diagrams, the flux within the aperture from theunderlying galaxies can often be determined without the need for modelsurface brightness profiles. In many galaxies it is apparent that theremust be an additional constant contribution from warm dust.
| Seyferts on the edge: polar scattering and orientation-dependent polarization in Seyfert 1 nuclei
We have identified 12 Seyfert 1 galaxies that exhibit opticalpolarization spectra similar to those of Seyfert 2 galaxies in whichpolarized broad lines are detected. We present new spectropolarimetricobservations of three of them: Was 45, Mrk 231 and NGC 3227. Theseobjects appear to be polarized as a result of far-field scattering inthe polar illumination cones of the circumnuclear torus. We estimatethat they represent between 10 and 30 per cent of the Seyfert 1population; they are found amongst all the main spectroscopic subtypes,including narrow-line Seyfert 1 galaxies. We have shown elsewhere thatSeyfert 1 nuclei more commonly have polarization characteristics thatcan be attributed to scattering by a compact `equatorial' scatteringregion located inside the torus. We propose that both equatorial andpolar scattering regions are present in all Seyfert galaxies and arguethat the observed range of polarization properties can be broadlyunderstood as an orientation effect. In this scheme, polar-scatteredSeyfert 1 galaxies represent the transition between unobscured (themajority of type 1) and obscured (type 2) Seyferts. They are viewedthrough the upper layers of the torus and are thus subject to moderateextinction (AV~ 1-4 mag) sufficient to suppress polarizedlight from the equatorial scattering region, but not the broad wings ofthe Balmer lines. The orientation of the polarization position anglerelative to the radio source is broadly consistent with thetwo-component scattering model. More generally, we find that amongstSeyfert 1 galaxies, parallel, perpendicular and intermediateorientations of the polarization position angle relative to the radioaxis occur roughly in the proportions 2:1:1.
| 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).
| On the Relationship between the Optical Emission-Line and X-Ray Luminosities in Seyfert 1 Galaxies
We have explored the relationship between the [O III] λ5007 andthe 2-10 keV luminosities for a sample of broad- and narrow-line Seyfert1 galaxies (BLSy1s and NLSy1s, respectively). We find that both types ofSeyfert galaxies span the same range in luminosity and possess similar[O III]/X-ray ratios. The NLSy1s are more luminous than BLSy1s whennormalized to their central black hole masses, a fact attributed tohigher mass accretion rates. However, we find no evidence for elevated[O III]/X-ray ratios in NLSy1s, which would have been expected if theyhad excess extreme-ultraviolet (EUV) continuum emission compared toBLSy1s. Also, other studies suggest that the gas in narrow-line regions(NLRs) of NLSy1s and BLSy1s spans a similar range in ionization,contrary to what is expected if those of the former are exposed to astronger flux of EUV radiation. The simplest interpretation is that,like BLSy1s, a large EUV bump is not present in NLSy1s. However, we showthat the [O III]/X-ray ratio can be lowered as a result of absorption ofthe ionizing continuum by gas close to the central source, althoughthere is no evidence that intrinsic line-of-sight absorption is morecommon among NLSy1s, as would be expected if there were a larger amountof circumnuclear gas. Other possible explanations include (1)anisotropic emission of the ionizing radiation; (2) higher gas densitiesin the NLRs of NLSy1s, resulting in lower average ionization; or (3) thepresence of strong winds in the nuclei of NLSy1s that may drive off muchof the gas in the NLR, resulting in lower cover fraction and weaker [OIII] emission.
| The Distribution of Maximum Relative Gravitational Torques in Disk Galaxies
The maximum value of the ratio of the tangential force to the meanbackground radial force is a useful quantitative measure of the strengthof nonaxisymmetric perturbations in disk galaxies. Here we consider thedistribution of this ratio, called Qg, for a statisticallywell-defined sample of 180 spiral galaxies from the Ohio StateUniversity Bright Galaxy Survey and the Two Micron All Sky Survey. Theratio Qg can be interpreted as the maximum gravitationaltorque per unit mass per unit square of the circular speed and isderived from gravitational potentials inferred from near-infrared imagesunder the assumptions of a constant mass-to-light ratio and anexponential vertical density law. In order to derive the most reliablemaximum relative torques, orientation parameters based on blue-lightisophotes are used to deproject the galaxies, and the more sphericalshapes of bulges are taken into account using two-dimensionaldecompositions that allow for analytical fits to bulges, disks, andbars. Also, vertical scale heights hz are derived by scalingthe radial scale lengths hR from the two-dimensionaldecompositions, allowing for the type dependence ofhR/hz indicated by optical and near-infraredstudies of edge-on spiral galaxies. The impact of dark matter isassessed using a ``universal rotation curve'' parameterization and isfound to be relatively insignificant for our sample. In agreement with aprevious study by Block et al., the distribution of maximum relativegravitational torques is asymmetric toward large values and shows adeficiency of low-Qg galaxies. However, because of the aboverefinements, our distribution shows more low-Qg galaxies thanthat of Block et al. We also find a significant type dependence inmaximum relative gravitational torques, in the sense that Qgis lower on average in early-type spirals than in late-type spirals. Theeffect persists even when the sample is separated into bar-dominated andspiral-dominated subsamples and also when near-infrared types are used,as opposed to optical types.
| Gas near active galactic nuclei: A search for the 4.7 μm CO band
In order to constrain the properties of dense and warm gas around activegalactic nuclei, we have searched Infrared Space Observatory spectra oflocal active galactic nuclei (AGN) for the signature of the 4.7 μmfundamental ro-vibrational band of carbon monoxide. Low resolutionspectra of 31 AGN put upper limits on the presence of wide absorptionbands corresponding to absorption by large columns of warm and dense gasagainst the nuclear dust continuum. High resolution (R2500) spectraof NGC 1068 detect no significant absorption oremission in individual lines, to a 3σ limit of 7% of thecontinuum. The limits set on CO absorption in local AGN are much lowerthan the recent Spitzer Space Telescope detection of strong COabsorption by dense and warm gas in the obscured ultraluminous infraredgalaxy IRAS F00183-7111, despite evidence for dense material on parsecscales near an AGN in both types of objects. This suggests that suchdeep absorptions are not intimately related to the obscuring ``torus''material invoked in local AGN, but rather are a signature of thepeculiar conditions in the circumnuclear region of highly obscuredinfrared galaxies like IRAS F00183-7111. They may reflect fully coveredrather than torus geometries.Based 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.
| Spiral galaxies observed in the near-infrared K band. I. Data analysis and structural parameters
Deep surface photometry in the K band was obtained for 54 normal spiralgalaxies, with the aim of quantifying the percentage of faint bars andstudying the morphology of spiral arms. The sample was chosen to cover awider range of morphological types while inclination angles anddistances were limited to allow a detailed investigation of the internalstructure of their disks and future observations and studies of the diskkinematics. An additional constraint for a well defined subsample wasthat no bar structure was seen on images in the visual bands. Accuratesky projection parameters were determined from the K maps comparingseveral different methods. The surface brightness distribution wasdecomposed into axisymmetric components while bars and spiral structureswere analyzed using Fourier techniques.Bulges were best represented by a Sérsic r1/n law withan index in the typical range of 1-2. The central surface brightness ofthe exponential disk and bulge-to-disk ratio only showed weakcorrelation with Hubble type. Indications of a central point source werefound in many of the galaxies. An additional central, steep, exponentialdisk improved the fit for more than 80% of the galaxies suggesting thatmany of the bulges are oblate.Bars down to the detection level at a relative amplitude of 3% weredetected in 26 of 30 galaxies in a subsample classified as ordinary SAspirals. This would correspond to only 5% of all spiral galaxies beingnon-barred at this level. In several cases, bars are significantlyoffset compared to the starting points of the main spiral pattern whichindicates that bar and spiral have different pattern speeds. A smallfraction (10%) of the sample has complex central structuresconsisting of several sets of bars, arcs or spirals.A majority of the galaxies (60%) displays a two-armed, grand-designspiral pattern in their inner parts which often breaks up into multiplearms in their outer regions. Phase shifts between the inner and outerpatterns suggest in some cases that they belong to different spiralmodes. The pitch angles of the main two-armed symmetric spiral patternin the galaxies have a typical range of 5-30 °. The sample shows alack of strong, tight spirals which could indicate that such patternsare damped by non-linear, dynamical effects due to their high radialforce perturbations.Based on observations collected at the European Southern Observatory, LaSilla, Chile; programs: ESO 63.N-0343, 65.N-0287, 66.N-0257.Table 2 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/423/849Appendix A is only available in electronic form athttp://www.edpsciences.org
| Statistical properties of local active galactic nuclei inferred from the RXTE 3-20 keV all-sky survey
We compiled a sample of 95 AGNs serendipitously detected in the 3-20 keVband at Galactic latitude |b|>10o during the RXTE slewsurvey (XSS, Revnivtsev et al. 2004), and utilize it to study thestatistical properties of the local population of AGNs, including theX-ray luminosity function and absorption distribution. We find thatamong low X-ray luminosity (L3-20< 1043.5 ergs-1) AGNs, the ratio of absorbed (characterized by intrinsicabsorption in the range 1022cm-21041 erg s-1 estimated here issmaller than the earlier estimated total X-ray volume emissivity in thelocal Universe, suggesting that a comparable X-ray flux may be producedtogether by lower luminosity AGNs, non-active galaxies and clusters ofgalaxies. Finally, we present a sample of 35 AGN candidates, composed ofunidentified XSS sources.Tables 1 and 2 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/423/469
| X-ray reprocessing in Seyfert galaxies: Simultaneous XMM-Newton/BeppoSAX observations
We selected a sample of eight bright unobscured (at least at the ironline energy) Seyfert galaxies observed simultaneously by XMM-Newton andBeppoSAX, taking advantage of the complementary characteristics of thetwo missions. The main results of our analysis can be summarized asfollows: narrow neutral iron lines are confirmed to be an ubiquitouscomponent in Seyfert spectra; none of the analyzed sources showsunambiguously a broad relativistic iron line; all the sources of oursample (with a single exception) show the presence of a Comptonreflection component; emission lines from ionized iron are observed insome sources; peculiar weak features around 5-6 keV (possibly arisingfrom rotating spots on the accretion disk) are detected in two sources.The scenario emerging from these results strongly requires somecorrections for the classical model of reprocessing from the accretiondisk. As for materials farther away from the Black Hole, our resultsrepresent a positive test for the Unification Model, suggesting thepresence of the torus in (almost) all sources, even if unobscured.
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