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Lifetime of nuclear velocity dispersion drops in barred galaxies
We have made hydro/N-body simulations with and without star formation toshed some light on the conditions under which a central kinematicallycold stellar component (characterized by a velocity dispersion drop orσ-drop) could be created in a hot medium (e.g. a bulge) andsurvive enough time to be observed. We found that the time-scale for aσ-drop formation could be short (less than 500 Myr), whereas itslifetime could be long (more than 1 Gyr) provided that the centralregion is continuously or regularly fed by fresh gas which leads to acontinuous star formation activity. Star formation in the centralregion, even at a low rate as 1Msolaryr-1, ismandatory to sustain a permanent σ-drop by replacing heatedparticles by new low-σ ones. We moreover show that as soon as starformation is switched off, the σ-drop begins to disappear.
| 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.
| 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).
| 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.
| Spectral line variability amplitudes in active galactic nuclei
We present the results of a long-term variability campaign of verybroad-line AGNs with line widths broader than FWHM > 5000 kms-1. The main goal of our investigation was to study whetherthe widths of the optical broad emission lines are correlated with theoptical intensity variations on timescales of years. Our AGN sampleconsisted of 10 objects. We detected a significant correlation betweenoptical continuum variability amplitudes and Hβ emission linewidths (FWHM) and, to a lesser degree, between Hβ line intensityvariations and Hβ equivalent widths. We add the spectroscopic dataof variable AGNs from the literature to supplement our sample. The AGNsfrom other optical variability campaigns with different line-widthshelped to improve the statistical significance of our very broad-lineAGN sample. After including the data on 35 additional galaxies, thecorrelation between optical continuum variability amplitudes and Hβemission line widths becomes even more significant and the probabilitythat this is a random correlation drops to 0.7 percent.
| Massive star formation in the central regions of spiral galaxies
Context: . The morphology of massive star formation in the centralregions of galaxies is an important tracer of the dynamical processesthat govern the evolution of disk, bulge, and nuclear activity. Aims. Wepresent optical imaging of the central regions of a sample of 73 spiralgalaxies in the Hα line and in optical broad bands, and deriveinformation on the morphology of massive star formation. Methods. Weobtained images with the William Herschel Telescope, mostly at a spatialresolution of below one second of arc. For most galaxies, no Hαimaging is available in the literature. We outline the observing anddata reduction procedures, list basic properties, and present the I-bandand continuum-subtracted Hα images. We classify the morphology ofthe nuclear and circumnuclear Hα emission and explore trends withhost galaxy parameters. Results. We confirm that late-type galaxies havea patchy circumnuclear appearance in Hα, and that nuclear ringsoccur primarily in spiral types Sa-Sbc. We identify a number ofpreviously unknown nuclear rings, and confirm that nuclear rings arepredominantly hosted by barred galaxies. Conclusions. Other than instimulating nuclear rings, bars do not influence the relative strengthof the nuclear Hα peak, nor the circumnuclear Hα morphology.Even considering that our selection criteria led to an over-abundance ofgalaxies with close massive companions, we do not find any significantinfluence of the presence or absence of a close companion on therelative strength of the nuclear Hα peak, nor on the Hαmorphology around the nucleus.
| 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.
| 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.
| 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.
| X-Ray Emission and Optical Polarization of V1432 Aquilae: An Asynchronous Polar
A detailed analysis of X-ray data obtained with ROSAT, ASCA, XMM-Newton,and the Rossi X-ray Timing Explorer (RXTE) for the asynchronous polarV1432 Aquilae is presented. An analysis of Stokes polarimetry dataobtained from the South African Astronomical Observatory (SAAO) is alsopresented. Power spectra from long-baseline ROSAT data show a spinperiod of 12,150 s along with several frequency components related tothe source. However, the second harmonic of the spin period dominatesthe power spectrum in the XMM-Newton data. For the optical circularpolarization, the dominant period corresponds to half the spin period(or its first harmonic). The ROSAT data can be explained as due toaccretion onto two hot spots that are not antipodal. The variations seenin the optical polarization and the ASCA and XMM-Newton X-ray datasuggest the presence of at least three accretion footprints on thesurface of the white dwarf. Two spectral models, a multitemperatureplasma model and a photoionized plasma model, are used to understand thespectral properties of V1432 Aql. The data from the RXTE ProportionalCounter Array (PCA) with its extended high-energy response are used toconstrain the white dwarf mass to 1.2+/-0.1 Msolar using amultitemperature plasma model. The data from the European Photon ImagingCamera (EPIC) on-board XMM-Newton are well fitted by both models. Astrong soft X-ray excess (<0.8 keV) is well modeled by a blackbodycomponent having a temperature of 80-90 eV. The plasma emission linesseen at 6.7 and 7.0 keV are well fitted using the multitemperatureplasma model. However, the fluorescent line at 6.4 keV from cold Ferequires an additional Gaussian component. The multitemperature plasmamodel requires two absorbers: one that covers the source homogeneouslyand another partial absorber covering ~65% of the source. Thephotoionized plasma model, with a range of column densities for the Feions, gives a slightly better overall fit and fits all emission linefeatures. The intensity and spectral modulations due to the rotation ofthe white dwarf at a period of 12,150 s require varying absorberdensities and a varying covering fraction of the absorber for themultitemperature plasma model. The presence of a strong blackbodycomponent, a rotation period of 12,150 s, modulation of the Fefluorescence line flux with 12,150 s period, and a very hard X-raycomponent suggest that V1432 Aql is an unusual polar with X-ray spectralproperties similar to that of a soft intermediate polar.
| BVRI CCD-Photometry of Comparison Stars in the Neighborhoods of Galaxies with Active Nuclei. II
Results are presented from CCD BVRI-observations of 145 comparison starsin the neighborhoods of 14 Seyfert galaxies, 3 quasars, and 5 BL Lacobjects with right ascensions of 12 to 24 hours. The magnitudes of theobserved stars ranged from V=11 to V=17. The typical photometric errorfor stars brighter V=14 is 0m.01. The B, V, Rc, and Ic magnitudes ofmost of these stars were not known previously. 14′ x 14′finding charts are provided. These results can be used for differentialphotometry of the nuclei of active galaxies in the B, V, Rc, and Icbands.
| 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.
| Structural parameters of nearby emission-line galaxies
We present the results of an investigation on the main structuralproperties derived from VRI and Hα surface photometry of galaxieshosting nuclear emission-line regions [including Seyfert 1, Seyfert 2,low-ionization nuclear emission region (LINER) and starburst galaxies]as compared with normal galaxies. Our original sample comprises 22active galaxies, four starbursts and one normal galaxy and has beenextended with several samples obtained from the literature. Bulge anddisc parameters, along with the bulge-to-disc luminosity ratio, havebeen derived applying an iterative procedure. The resulting parametershave been combined with additional data in order to reach astatistically significant sample. We find some differences in the bulgedistribution across the different nuclear types that could implyfamilies of bulges with different physical properties. Bulge and disccharacteristic colours have been defined and derived for our sample andcompared with a control sample of early-type objects. The resultssuggest that bulge and disc stellar populations are comparable in normaland active galaxies.
| 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.
| Relativistic spectral features from X-ray-illuminated spots and the measure of the black hole mass in active galactic nuclei
Narrow spectral features in the 5-6 keV range were recently discoveredin the X-ray spectra of a few active galactic nuclei. We discuss thepossibility that these features are due to localized spots which occuron the surface of an accretion disc following its illumination byflares. We present detailed line profiles as a function of orbital phaseof the spot and its radial distance from a central black hole.Comparison of these computed profiles with observed features can help toestimate parameters of the system. In principle, this method can providea powerful tool to measure the mass of super-massive black holes inactive galactic nuclei. By comparing our simulations with the Chandraand XMM-Newton results, we show, however, that spectra from presentgeneration X-ray satellites are not of good enough quality to exploitthe method fully and determine the black hole mass with sufficientaccuracy. This task has to be deferred to future missions with highthroughput and high energy resolution, such as Constellation-X and Xeus.
| 17 New Identified INTEGRAL Sources
We report the first detection in the hard X-ray band (>20 keV) of 17known sources. The sources have been detected by the IBIS/ISGRIinstrument on board INTEGRAL and found in all-sky mosaics built fromdata of the INTEGRAL core program accumulated between February 27 andOctober 19, 2003.
| A Hard X-ray Survey of the Sagittarius Arm Tangent with the IBIS Telescope of the INTEGRAL Observatory: A Catalog of Sources
We analyze the images of the Sagittarius Arm tangent obtained with theIBIS telescope of the INTEGRAL observatory in the energy range 18-120keV during its observations in the spring of 2003. We detected 28sources at a statistically significant level with fluxes above 1.4 mCrabin the energy range 18-60 keV. Of these sources, 16 were previouslyidentified as binaries of various classes in our Galaxy, 3 wereidentified as extragalactic objects, 2 were identified as pulsars insupernova remnants, and 7 sources were of an unknown nature. Theseobservations revealed three new sources. A statistically significantflux in the energy range 60-120 keV was recorded from 13 sources.
| 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.
| 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.
| A Search for Short-Timescale Microvariability in Active Galactic Nuclei in the Ultraviolet
We observed four active galactic nuclei (AGNs) (the type 1 Seyfertsystems 3C 249.1, NGC 6814, and Mrk 205, and the BL Lac object 3C 371)using the High Speed Photometer on the Hubble Space Telescope (HST) tosearch for short-timescale microvariability in the UV. Continuousobservations of ~3000 s duration were obtained for each system onseveral consecutive HST orbits using a 1 s sample time in a 1400-3000Å bandpass. No photometric variability greater than 0.3% (0.003mag) was detected in any AGN on timescales shorter than 1500 s. Thedistribution of photon arrival times observed from each source wasconsistent with Poisson statistics. Because of HST optical problems, thelimit on photometric variability at longer timescales is less precise.These results restrict the masses of black holes as the central enginesof AGNs and the diskoseismic oscillations of any accretion disk aroundsuch a black hole.
| Stacking Searches for Gamma-Ray Emission above 100 MeV from Radio and Seyfert Galaxies
The EGRET telescope on board Compton Gamma Ray Observatory detected morethan 60 sources of high-energy gamma radiation associated with activegalactic nuclei (AGNs). All but one of those belong to the blazarsubclass; the only exception is the nearby radio galaxy Centaurus A.Since there is no obvious reason other than proximity to expect Cen A tobe the only nonblazar AGN emitting in high-energy gamma rays, we haveutilized the ``stacking'' technique to search for emission above 100 MeVfrom two nonblazar AGN subclasses, radio galaxies and Seyfert galaxies.Maps of gamma-ray counts, exposure, and diffuse background have beencreated, then co-added in varying numbers based on sorts by redshift, 5GHz flux density, and optical brightness, and finally tested forgamma-ray emission. No detection significance greater than 2 σ hasbeen found for any subclass, sorting parameter, or number of objectsco-added. Monte Carlo simulations have also been performed to validatethe technique and estimate the significance of the results.
| Gravitational Bar and Spiral Arm Torques from Ks-band Observations and Implications for the Pattern Speeds
We have obtained deep near-infrared Ks-band William HerschelTelescope observations of a sample of 15 nearby spiral galaxies having arange of Hubble types and apparent bar strengths. The near-infraredlight distributions are converted into gravitational potentials, and themaximum relative gravitational torques due to the bars and the spiralsare estimated. We find that spiral strength, Qs, and barstrength, Qb, correlate well with other measures of spiralarm and bar amplitudes and that spiral and bar strengths also correlatewell with each other. We also find a correlation between the positionangle of the end of the bar and the position angle of the inner spiral.These correlations suggest that the bars and spirals grow together withthe same rates and pattern speeds. We also show that the strongest barstend to have the most open spiral patterns. Because open spirals implyhigh disk-to-halo mass ratios, bars and spirals most likely growtogether as a combined disk instability. They stop growing for differentreasons, however, giving the observed variation in bar-spiralmorphologies. Bar growth stops because of saturation when most of theinner disk is in the bar, and spiral growth stops because of increasedstability as the gas leaves and the outer disk heats up.
| Inner-truncated Disks in Galaxies
We present an analysis of the disk brightness profiles of 218 spiral andlenticular galaxies. At least 28% of disk galaxies exhibit innertruncations in these profiles. There are no significant trends oftruncation incidence with Hubble type, but the incidence among barredsystems is 49%, more than 4 times that for nonbarred galaxies. However,not all barred systems have inner truncations, and not allinner-truncated systems are currently barred. Truncations represent areal dearth of disk stars in the inner regions and are not an artifactof our selection or fitting procedures nor the result of obscuration bydust. Disk surface brightness profiles in the outer regions are wellrepresented by simple exponentials for both truncated and nontruncateddisks. However, truncated and nontruncated systems have systematicallydifferent slopes and central surface brightness parameters for theirdisk brightness distributions. Truncation radii do not appear tocorrelate well with the sizes or brightnesses of the bulges. Thissuggests that the low angular momentum material apparently missing fromthe inner disk was not simply consumed in forming the bulge population.Disk parameters and the statistics of bar orientations in our sampleindicate that the missing stars of the inner disk have not simply beenredistributed azimuthally into bar structures. The sharpness of thebrightness truncations and their locations with respect to othergalactic structures suggest that resonances associated with diskkinematics, or tidal interactions with the mass of bulge stars, might beresponsible for this phenomenon.
| RXTE all-sky slew survey. Catalog of X-ray sources at |b|>10o
We report results of a serendipitous hard X-ray (3-20 keV), nearlyall-sky (|b|>10o) survey based on RXTE/PCA observationsperformed during satellite reorientations in 1996-2002. The survey is80% (90%) complete to a 4σ limiting flux of ≈ 1.8 (2.5) ×10-11 erg s-1 cm-2 in the 3-20 keVband. The achieved sensitivity in the 3-8 keV and 8-20 keV subbands issimilar to and an order of magnitude higher than that of the previouslyrecord HEAO-1 A1 and HEAO-1 A4 all-sky surveys, respectively. A combined7× 103 sq deg area of the sky is sampled to flux levelsbelow 10-11 erg s-1 cm-2 (3-20 keV). Intotal 294 sources are detected and localized to better than 1 deg. 236(80%) of these can be confidently associated with a known astrophysicalobject; another 22 likely result from the superposition of 2 or 3closely located known sources. 35 detected sources remain unidentified,although for 12 of these we report a likely soft X-ray counterpart fromthe ROSAT all-sky survey bright source catalog. Of the reliablyidentified sources, 63 have local origin (Milky Way, LMC or SMC), 64 areclusters of galaxies and 100 are active galactic nuclei (AGN). The factthat the unidentified X-ray sources have hard spectra suggests that themajority of them are AGN, including highly obscured ones(NH>1023 cm-2). For the first timewe present a log N-log S diagram for extragalactic sources above4× 10-12 erg s-1 cm-2 at 8-20keV.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/418/927
| Minor-axis velocity gradients in disk galaxies
We present the ionized-gas kinematics and photometry of a sample of 4spiral galaxies which are characterized by a zero-velocity plateau alongthe major axis and a velocity gradient along the minor axis,respectively. By combining these new kinematical data with thoseavailable in the literature for the ionized-gas component of the S0s andspirals listed in the Revised Shapley-Ames Catalog of Bright Galaxies werealized that about 50% of unbarred galaxies show a remarkable gasvelocity gradient along the optical minor axis. This fraction rises toabout 60% if we include unbarred galaxies with an irregular velocityprofile along the minor axis. This phenomenon is observed all along theHubble sequence of disk galaxies, and it is particularly frequent inearly-type spirals. Since minor-axis velocity gradients are unexpectedif the gas is moving onto circular orbits in a disk coplanar to thestellar one, we conclude that non-circular and off-plane gas motions arenot rare in the inner regions of disk galaxies.Based on observations carried out at the European Southern Observatoryin La Silla (Chile) (ESO 69.B-0706 and 70.B-0338), with the MultipleMirror Telescope which is a joint facility of the SmithsonianInstitution and the University of Arizona, and with the ItalianTelescopio Nazionale Galileo (AOT-5, 3-18) at the Observatorio del Roquede los Muchachos in La Palma (Spain).Table 1 is only available in electronic form athttp://www.edpsciences.org. Table 5 is only available in electronic format the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) orvia http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/507
| Long slit spectroscopy of a sample of isolated spirals with and without an AGN
We present the kinematical data obtained for a sample of active(Seyfert) and non active isolated spiral galaxies, based on long slitspectra along several position angles in the Hα line region and,in some cases, in the Ca triplet region as well. Gas velocitydistributions are presented, together with a simple circular rotationmodel that allows us to determine the kinematical major axes. Stellarvelocity distributions are also shown. The main result is that activeand control galaxies seem to be equivalent in all kinematical aspects.For both subsamples, the departure from pure circular rotation in somegalaxies can be explained by the presence of a bar and/or of a spiralarm. They also present the same kind of peculiarities, in particular,S-shape structures are quite common near the nuclear regions. Theydefine very similar Tully-Fisher relations. Emission line ratios aregiven for all the detected HII regions; the analysis of the[NII]/Hα metallicity indicator shows that active and non-activegalaxies have indistinguishable disk metallicities. These results arguein favour of active and non-active isolated spiral galaxies havingessentially the same properties, in agreement with our previous resultsbased on the analysis of near infrared images. It appears now necessaryto confirm these results on a larger sample.Based on observations made with WHT operated on the island of La Palmaby ING in the Spanish Observatorio del Roque de Los Muchachos of theInstituto de Astrofísica de Canarias, the European SouthernObservatory (La Silla), Calar Alto Observatory (Almería, Spain)and Las Campanas Observatories (Chile).Table 3 and Figs. \ref{res_cen_u1395}, 5, 7, 9, 11, 13, 15, 17, 19, 21,23, 25, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50 and 52 are onlyavailable in electronic form at http://www.edpsciences.orgTable 5 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/416/475
| 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.
| Black Hole-Bulge Relation for Narrow-Line Objects
It has been thought that narrow-line Seyfert 1 galaxies are likely to bein the early stages of the evolution of active galaxies. To test thissuggestion, the ratios of the central massive black hole (MBH) mass tothe bulge mass (Mbh/Mbulge) were estimated for 22Narrow Line AGNs (NL AGNs). It is found that NL AGNs appear to havegenuinely lower MBH/Bulge mass ratio (Mbh/Mbulge).The mean log (Mbh/Mbulge) for 22 NL AGNs is -3.9± 0.07, which is an order of magnitude lower than that for BroadLine AGNs and quiescent galaxies. We suggest a nonlinear MBH/Bulgerelation and find there exists a relation between the Mbh /Mbulge and the velocity dispersion, σ, derived from the[O III ] width. A scenario of MBH growth for NLAGNs is one of our interpretations of the nonline ar MBH/Bulge relation.The MBH growth timescales for 22 NL AGNs were calculated, with a meanvalue (1.29 ± 0.24) × 108 yr. Another plausibleinterpretation is also possible: that NL AGNs occur inlow-Mbulge galaxies and that in such galaxiesMbh/Mbulge is lower than that in galaxies with ahigher Mbulge, if we consider that NL AGNs already have their``final'' Mbh/Mbulge. More information of thebulge in NL AGNs is needed to clarify the black hole-bulge relation.
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