Contingut
Imatges
Carregar la teva Imatge
DSS Images Other Images
Articles Relacionats
Discovery of Water Maser Emission in Five AGNs and a Possible Correlation Between Water Maser and Nuclear 2-10 keV Luminosities
We report the discovery of water maser emission in five active galacticnuclei (AGNs) with the 100 m Green Bank Telescope (GBT). The positionsof the newly discovered masers, measured with the VLA, are consistentwith the optical positions of the host nuclei to within 1 σ (0.3"radio and 1.3" optical) and most likely mark the locations of theembedded central engines. The spectra of three sources, 2MASXJ08362280+3327383, NGC 6264, and UGC 09618 NED02, display thecharacteristic spectral signature of emission from an edge-on accretiondisk with maximum orbital velocity of ~700, ~800, and ~1300 kms-1, respectively. We also present a GBT spectrum of apreviously known source, Mrk 0034, and interpret the narrow Dopplercomponents reported here as indirect evidence that the emissionoriginates in an edge-on accretion disk with orbital velocity of ~500 kms-1. We obtained a detection rate of 12% (5 out of 41) amongSeyfert 2 and LINER systems with 10,000 kms-1
| Discovery of Water Maser Emission in Eight AGNs with 70 m Antennas of NASA's Deep Space Network
We report the discovery of water maser emission in eight active galacticnuclei (AGNs) with the 70 m NASA Deep Space Network (DSN) antennas atTidbinbilla, Australia, and Robledo, Spain. The positions of the newlydiscovered masers, measured with the VLA, are consistent with theoptical positions of the host nuclei to within 1 σ (0.3" radio and1.3" optical) and most likely mark the locations of the embedded centralengines. The spectra of two sources, NGC 3393 and NGC 5495, display thecharacteristic spectral signature of emission from an edge-on accretiondisk, with orbital velocities of ~600 and ~400 km s-1,respectively. In a survey with DSN facilities of 630 AGNs selected fromthe NASA Extragalactic Database, we have discovered a total of 15 watermaser sources. The resulting incidence rate of maser emission amongnearby (vsys<7000 km s-1) Seyfert 1.8-2.0 andLINER systems is ~10% for a typical rms noise level of ~14 mJy over 1.3km s-1 spectral channels. As a result of this work, thenumber of nearby AGNs (vsys<7000 km s-1)observed with <20 mJy rms noise has increased from 130 to 449.
| X-Ray Emission from Megamaser Galaxy IC 2560
An observation of the H2O megamaser galaxy IC 2560 with theChandra X-Ray Observatory reveals a complex spectrum composed of softX-ray emission due to multitemperature thermal plasma and a hardcontinuum with strong emission lines. The continuum is most likely aCompton reflection (reprocessing) of primary emission that is completelyabsorbed at least up to 7 keV. The lines can be identified withfluorescence from Si, S, and Fe in the lowest ionization stages. Theequivalent widths of the Si and S lines are broadly compatible withthose anticipated for reprocessing by optically thick cold plasma ofsolar abundances, while the large equivalent width of the Fe linerequires some overabundance of iron. A contribution to the line from atransmitted component cannot be ruled out, but the limits on thestrength of the Compton shoulder make it less likely. From thebolometric luminosity of the nuclear region, we infer that the sourceradiates at 1%-10% of its Eddington luminosity for an adopted centralmass of 3×106 Msolar. The overall spectrumis consistent with the hypotheses that the central engines powering thedetected megamasers in accretion disks are obscured from direct view bythe associated accretion disk material itself and that there is acorrelation between the occurrence of megamaser emission andCompton-thick absorption columns. For the 11 known galaxies with bothcolumn density measurements and maser emission believed to arise fromaccretion disks, eight AGNs are Compton thick.
| Seyfert Galaxies and the Hard X-Ray Background: Artificial Chandra Observations of z=0.3 Active Galaxies
Deep X-ray surveys have resolved much of the X-ray background radiationbelow 2 keV into discrete sources, but the background above 8 keVremains largely unresolved. The obscured (type 2) active galactic nuclei(AGNs) that are expected to dominate the hard X-ray background have notyet been detected in sufficient numbers to account for the observedbackground flux. However, deep X-ray surveys have revealed large numbersof faint quiescent and starburst galaxies at moderate redshifts. Inhopes of recovering the missing AGN population, it has been suggestedthat the defining optical spectral features of low-luminosity Seyfertnuclei at large distances may be overwhelmed by their host galaxies,causing them to appear optically quiescent in deep surveys. We test thispossibility by artificially redshifting a sample of 23 nearby,well-studied active galaxies to z=0.3, testing them for X-ray AGNsignatures, and comparing them to the objects detected in deep X-raysurveys. We find that these redshifted galaxies have propertiesconsistent with the deep-field normal and optically bright, X-ray-faintgalaxy (OBXF) populations, supporting the hypothesis that the numbers ofAGNs in deep X-ray surveys are being underestimated and suggesting thatOBXFs should not be ruled out as candidate AGN hosts that couldcontribute to the hard X-ray background source population.
| 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.
| Detection of the Velocity Drift of High-Velocity Water Maser Features of a LINER NGC 4258: Evidence of a Spiral Maser Disk
We monitored the velocities of high-velocity features for aH2O megamaser, NGC 4258, using the 45-m telescope of theNobeyama Radio Observatory. The spiral shock model of maser diskspredicts that the red-shifted features of observed maser spectradecelerate and the blue-shifted features accelerate for trailing arms.To confirm this model, we measured the velocity drift rates. Using thedata of 1992-2005, the drift rate was detected to be ā = -0.036± 0.007 km s-1 yr-1 on the average for 10red-shifted features. The drift rate of a blue-shifted feature was a =0.20 ± 0.10 km s-1 yr-1. These results areconsistent with a prediction of the spiral shock model. The pitch anglesof the spirals were obtained to be overline{θp} =2° ± 1° on the average for the red-shifted features andθp = 13° ± 7° for the blue-shiftedfeature from the measured drift rates.
| 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.
| Mega-Masers and Galaxies
In the Galaxy, microwave radiation can be amplified in the interstellarmedium in the immediate neighborhood of young stellar objects, orcircumstellar envelopes around evolved stars, resulting in cosmic maseremission. Cosmic masers exist because, in contrast to terrestrialconditions, the interstellar gas density is very low so that levelpopulation in molecules is typically not in thermal equilibrium, andsometimes inverted. In the nuclear regions of external galaxies, thereexist very powerful OH ( 18 cm) and H2O ( 1.35 cm) cosmicmasers with line luminosities of 102 104Lȯ, 106 times more luminous than typicalGalactic maser sources. These are the "mega-masers," found inhigh-density molecular gas located within parsecs of active galacticnuclei in the case of H2O mega-masers, or within the central100 pc of nuclear star-burst regions in the case of OH mega-masers.H2O mega-masers are most frequently found in galactic nucleiwith Seyfert2 or LINER spectral characteristics, in spiral and someelliptical galaxies. OH mega-masers are found in ultra-luminous IRgalaxies (ULIRG) with the warmest IR colors, and importantly, the OHluminosity is observed to increase with the IR luminosity:LOH L1.2IR. Because of the extremelyhigh-surface brightness, H2O mega-maser emission can bemapped at sub-milli-arc-second resolution by Very Long BaselineInterferometry (VLBI), providing a powerful tool to probe spatial andkinematic distributions of molecular gas in distant galactic nuclei atscales below one parsec. An excellent example is the active galaxy, NGC4258, in which mapping of the H2O mega-maser emission hasprovided the first direct evidence in an active galactic nucleus for theexistence of a thin Keplerian accretion disk with turbulence, as well ashighly compelling evidence for the existence of a massive black hole.The NGC 4258 mega-maser has also provided a geometric distancedetermination of extremely high precision. H2O mega-maseremission is also found to arise from postshocked gas from the impact ofnuclear jets or outflows on the surrounding molecular clouds.High-resolution observations have shown that OH mega-masers originatefrom the molecular gas medium in 100-pc scale nuclear star-burstregions. It is proposed that such extreme star-burst regions, withextensive high-density gas bathed in a very high far-IR radiation field,are conducive to the formation of a very large number of OH masersources that collectively produce the OH mega-maser emission. In theearly Universe, galaxies or mergers could go through a very luminousphase, powered by intensive star-bursts and AGN formation, and couldhave extremely large OH and H2O maser luminosities, possiblyproducing giga-masers. With the increasing sensitivity of new telescopesand receivers, surveys and high-resolution studies of mega-masers andgiga-masers will be very important tracers and high-resolution probes ofactive galactic nuclei, dust embedded star-bursts in the earliestgalaxies and galaxy mergers in the epoch of very active star formationat z 2 and beyond. Distance determination of giga-masers at z 1 2can provide on independent measure of how fast the universe isexpanding.
| New H2O masers in Seyfert and FIR bright galaxies
Using the Effelsberg 100-m telescope, detections of four extragalacticwater vapor masers are reported. Isotropic luminosities are ~50, 1000, 1and 230 Lȯ for Mrk 1066 (UGC 2456), Mrk 34, NGC 3556 andArp 299, respectively. Mrk 34 contains by far the most distant and oneof the most luminous water vapor megamasers so far reported in a Seyfertgalaxy. The interacting system Arp 299 appears to show two maserhotspots separated by approximately 20´´. With these newresults and even more recent data from Braatz et al. (2004, ApJ, 617,L29), the detection rate in our sample of Seyferts with known jet-NarrowLine Region interactions becomes 50% (7/14), while in star forminggalaxies with high (S100~μ m>50 Jy) far infrared fluxesthe detection rate is 22% (10/45). The jet-NLR interaction sample maynot only contain “jet-masers” but also a significant numberof accretion “disk-masers” like those seen in NGC 4258. Astatistical analysis of 53 extragalactic H2O sources (excluding theGalaxy and the Magellanic Clouds) indicates (1) that the correlationbetween IRAS Point Source and H2O luminosities, established forindividual star forming regions in the galactic disk, also holds forAGN-dominated megamaser galaxies; (2) that maser luminosities are notcorrelated with 60 μm/100 μm color temperatures; and (3) that onlya small fraction of the luminous megamasers (L_H_2O > 100Lȯ) detectable with 100-m sized telescopes have so farbeen identified. The H2O luminosity function (LF) suggests that thenumber of galaxies with 1 Lȯ < L_H_2O < 10Lȯ, the transition range between“kilomasers” (mostly star formation) and“megamasers” (active galactic nuclei), is small. The overallslope of the LF, ~-1.5, indicates that the number of detectable masersis almost independent of their luminosity. If the LF is not steepeningat very high maser luminosities and if it is possible to find suitablecandidate sources, H2O megamasers at significant redshifts should bedetectable even with present day state-of-the-art facilities.
| Disks, tori, and cocoons: emission and absorption diagnostics of AGN environments
One of the most important problems in the study of active galaxies isunderstanding the detailed geometry, physics, and evolution of thecentral engines and their environments. The leading models involve anaccretion disk and torus structure around a central dense object,thought to be a supermassive black hole. Gas found in the environment ofactive galactic nuclei (AGN) is associated with different structures:molecular accretion disks, larger scale atomic tori, ionized and neutral“cocoons” in which the nuclear regions can be embedded. Allof them can be studied at radio wavelengths by various means. Here, wesummarize the work that has been done to date in the radio band tocharacterize these structures. Much has been learned about the centralfew parsecs of AGN in the last few decades with contemporary instrumentsbut the picture remains incomplete. In order to be able to define a moreaccurate model of this region, significant advances in sensitivity,spectral and angular resolution, and bandpass stability are required.The necessary advances will only be provided by the Square KilometerArray and we discuss the possibilities that these dramatic improvementswill open for the study of the gas in the central region of AGN.
| 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.
| 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.
| Morphological Type Dependence in the Tully-Fisher Relationship
The Tully-Fisher relationship is subject to morphological typedependence such that galaxies of morphology similar to Sc I galaxies andSeyfert galaxies are more luminous at a given rotational velocity thangalaxies of other morphological classification. This effect is mostprevalent in the B band. It is shown that the type effect is not simplyan artifact of the calibrator sample but is also present in clustersamples. The type effect is corrected by creating type-dependentTully-Fisher relations for Sc I group galaxies and Sb/Sc III groupgalaxies. It is shown that with single calibrations, the distances to ScI group galaxies are systematically underestimated, while the distancesto Sb/Sc III group galaxies are systematically overestimated.Tully-Fisher slope and scatter are also considered in the context oftype-dependent Tully-Fisher relations. It is concluded that the use oftype-dependent Tully-Fisher relations provides significant improvementin the distances to individual galaxies and the refined distances toclusters of galaxies.
| The 1000 Brightest HIPASS Galaxies: H I Properties
We present the HIPASS Bright Galaxy Catalog (BGC), which contains the1000 H I brightest galaxies in the southern sky as obtained from the H IParkes All-Sky Survey (HIPASS). The selection of the brightest sourcesis based on their H I peak flux density (Speak>~116 mJy)as measured from the spatially integrated HIPASS spectrum. The derived HI masses range from ~107 to 4×1010Msolar. While the BGC (z<0.03) is complete inSpeak, only a subset of ~500 sources can be consideredcomplete in integrated H I flux density (FHI>~25 Jy kms-1). The HIPASS BGC contains a total of 158 new redshifts.These belong to 91 new sources for which no optical or infraredcounterparts have previously been cataloged, an additional 51 galaxiesfor which no redshifts were previously known, and 16 galaxies for whichthe cataloged optical velocities disagree. Of the 91 newly cataloged BGCsources, only four are definite H I clouds: while three are likelyMagellanic debris with velocities around 400 km s-1, one is atidal cloud associated with the NGC 2442 galaxy group. The remaining 87new BGC sources, the majority of which lie in the zone of avoidance,appear to be galaxies. We identified optical counterparts to all but oneof the 30 new galaxies at Galactic latitudes |b|>10deg.Therefore, the BGC yields no evidence for a population of``free-floating'' intergalactic H I clouds without associated opticalcounterparts. HIPASS provides a clear view of the local large-scalestructure. The dominant features in the sky distribution of the BGC arethe Supergalactic Plane and the Local Void. In addition, one can clearlysee the Centaurus Wall, which connects via the Hydra and Antlia Clustersto the Puppis Filament. Some previously hardly noticable galaxy groupsstand out quite distinctly in the H I sky distribution. Several newstructures, including some not behind the Milky Way, are seen for thefirst time.
| A GBT Atlas of H2O Masers in Galactic Nuclei
Water vapor masers have been detected toward the nuclei of some 63galaxies, nearly all AGNs. We are using the Green Bank Telescope (GBT)to obtain sensitive spectra of most of the masers available in thenorthern sky. The spectra cover 380 MHz (roughly 5100 km/s) with achannel spacing of 24 kHz (0.3 km/s) at sensitivities ranging from 2 to5 mJy per 24 kHz channel. This atlas serves to advance studies of maserphenomenology, to reveal new maser features, to identify galaxies withweak, broad emission components, and to provide a base for long-termvariability studies. Several maser systems include Doppler componentswhich are symmetrically red- and blue-shifted from the systemicrecession velocity, a signature of a maser in an accretion disk. We havebeen monitoring the disk masers in the galaxies NGC 1386, IC 2560, Mrk1419, and NGC 6323 in order to track velocity drifts among individualfeatures, and thereby measure the centripetal acceleration of gas in thedisk. We use these measurements to obtain an estimate of the centralblack hole mass in each source.
| Spectral characteristics of water megamaser galaxies. II. ESO 103-G035, TXS 2226-184, and IC 1481
Long-slit optical emission-line spectra of the H2O megamasergalaxies ESO 103-G035, TXS2226-184, and IC 1481 are evaluated inorder to look for characteristics typical for water-megamaser galaxies.We present rotation curves, line ratios, electron densities,temperatures, and Hβ luminosities. The successful line-profiledecompositions rest on d-Lorentzians with an additional parameter d toadjust the wings, rather than Gaussians or Lorentzians as basicfunctions. No significant velocity gradient is found along the majoraxis in the innermost 2 kpc of TXS 2226-184.IC 1481 reveals a spectrum suggestive of a vigorousstarburst in the central kiloparsec 108 years ago. None ofthe three galaxies shows any hints for outflows nor special featureswhich could give clues to the presence of H2O megamaseremission. The galaxies are of normal Seyfert-2 (ESO103-G035) or LINER (TXS 2226-184,IC 1481) type.Based on observations collected at the European Southern Observatory,Chile.
| Circumnuclear Dust in Nearby Active and Inactive Galaxies. I. Data
The detailed morphology of the interstellar medium (ISM) in the centralkiloparsec of galaxies is controlled by pressure and gravitation. Thecombination of these forces shapes both circumnuclear star formation andthe growth of the central, supermassive black hole. We present visibleand near-infrared Hubble Space Telescope images and color maps of 123nearby galaxies that show the distribution of the cold ISM, as traced bydust, with excellent spatial resolution. These observations reveal thatnuclear dust spirals are found in the majority of active and inactivegalaxies and they possess a wide range in coherence, symmetry, and pitchangle. We have used this large sample to develop a classification systemfor circumnuclear dust structures. In spite of the heterogeneous natureof the complete sample, we only find symmetric, two-arm nuclear dustspirals in galaxies with large-scale bars, and these dust lanes clearlyconnect to dust lanes along the leading edges of the large-scale bars.Not all dust lanes along large-scale bars form two-arm spirals, however,and several instead end in nuclear rings. We find that tightly wound, orlow pitch angle, nuclear dust spirals are more common in unbarredgalaxies than barred galaxies. Finally, the extended narrow-line regionin several of the active galaxies is well resolved. The connectionbetween the ionized gas and circumnuclear dust lanes in four of thesegalaxies provides additional evidence that a significant fraction oftheir extended narrow-line region is ambient gas photoionized in situ bythe active nucleus. In a future paper we will use our classificationsystem for circumnuclear dust to identify differences between active andinactive galaxies, as well as barred and unbarred galaxies, inwell-matched subsamples of these data.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS 5-26555.
| A Survey for H2O Megamasers. III. Monitoring Water Vapor Masers in Active Galaxies
We present single-dish monitoring of the spectra of 13 extragalacticwater megamasers taken over a period of 9 years and a single epoch ofsensitive spectra for seven others. The primary motivation is a searchfor drifting line velocities analogous to those of the systemic featuresin NGC 4258, which are known to result from centripetal acceleration ofgas in an edge-on, subparsec molecular disk. We detect a velocity driftanalogous to that in NGC 4258 in only one source, NGC 2639. Another, themaser source in NGC 1052, exhibits erratic changes in its broad maserprofile over time. Narrow maser features in all of the other diskgalaxies discussed here either remain essentially constant in velocityover the monitoring period or are sufficiently weak or variable inintensity that individual features cannot be traced reliably from oneepoch to the next. In the context of a circumnuclear, molecular diskmodel, our results suggest that either (a) the maser lines seen aresystemic features subject to a much smaller acceleration than present inNGC 4258, presumably because the gas is farther from the nuclear blackhole, or (b) we are detecting ``satellite'' lines for which theacceleration is in the plane of the sky.Our data include the first K-band science observations taken with thenew 100 m Green Bank Telescope (GBT). The GBT data were taken duringtesting and commissioning of several new components and so are subjectto some limitations; nevertheless, they are in most cases the mostsensitive H2O spectra ever taken for each source and cover800 MHz (~=10,800 km s-1) of bandwidth. Many new maserfeatures are detected in these observations. Our data also include atentative and a clear detection of the megamaser in NGC 6240 at epochs ayear and a few months, respectively, prior to the detections reported byHagiwara et al. and Nakai et al.We also report a search for water vapor masers toward the nuclei of 58highly inclined (i>80deg), nearby galaxies. These sourceswere selected to investigate the tendency that H2O megamasersfavor inclined galaxies. None were detected, confirming that megamasersare associated exclusively with active galactic nuclei.
| Circumnuclear Dust in Nearby Active and Inactive Galaxies. II. Bars, Nuclear Spirals, and the Fueling of Active Galactic Nuclei
We present a detailed study of the relation between circumnuclear dustmorphology, host-galaxy properties, and nuclear activity in nearbygalaxies. We use our sample of 123 nearby galaxies withvisible-near-infrared color maps from the Hubble Space Telescope tocreate well-matched, ``paired'' samples of 28 active and 28 inactivegalaxies, as well as 19 barred and 19 unbarred galaxies, that have thesame host-galaxy properties. Comparison of the barred and unbarredgalaxies shows that grand-design nuclear dust spirals are found only ingalaxies with a large-scale bar. These nuclear dust spirals, which arepresent in approximately one-third of all barred galaxies, also appearto be connected to the dust lanes along the leading edges of thelarge-scale bars. Grand-design nuclear spirals are more common thaninner rings, which are present in only a small minority of the barredgalaxies. Tightly wound nuclear dust spirals, in contrast, show a strongtendency to avoid galaxies with large-scale bars. Comparison of theactive galactic nuclei (AGNs)and inactive samples shows that nucleardust spirals, which may trace shocks and angular momentum dissipation inthe interstellar medium, occur with comparable frequency in both activeand inactive galaxies. The only difference between the active andinactive galaxies is that several inactive galaxies appear to completelylack dust structure in their circumnuclear region, while none of theAGNs lack this structure. The comparable frequency of nuclear spirals inactive and inactive galaxies, combined with previous work that finds nosignificant difference in the frequency of bars or interactions betweenwell-matched active and inactive galaxies, suggests that no universalfueling mechanism for low-luminosity AGNs operates at spatial scalesgreater than a ~100 pc radius from the galactic nuclei. The similaritiesof the circumnuclear environments of active and inactive galaxiessuggest that the lifetime of nuclear activity is less than thecharacteristic inflow time from these spatial scales. Anorder-of-magnitude estimate of this inflow time is the dynamicaltimescale. This sets an upper limit of several million years to thelifetime of an individual episode of nuclear activity.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy, Inc., under NASAcontract NAS5-26555.
| The Discovery of H2O Maser Emission in Seven Active Galactic Nuclei and at High Velocities in the Circinus Galaxy
We report the discovery of H2O maser emission at 1.35 cmwavelength in seven active galactic nuclei (at distances of up to 80Mpc) during a survey conducted at the 70 m diameter antenna of the NASADeep Space Network near Canberra, Australia. The detection rate was ~4%.Two of the maser sources are particularly interesting because theydisplay satellite high-velocity emission lines, which are a signature ofemission from the accretion disks of supermassive black holes when seenedge-on. Three of the masers are coincident, to within uncertainties of0.2", with continuum emission sources that we observed at aboutλ=1.3 cm. We also report the discovery of new spectral featuresin the Circinus galaxy H2O maser that broaden the knownvelocity range of emission therein by a factor of ~1.7. If the newspectral features originate in the Circinus accretion disk, thenmolecular material must survive at radii ~3 times smaller than had beenbelieved previously (~0.03 pc or ~2×105 Schwarzschildradii).
| High-energy sources before INTEGRAL. INTEGRAL reference catalog
We describe the INTEGRAL reference catalog which classifies previouslyknown bright X-ray and gamma-ray sources before the launch of INTEGRAL.These sources are, or have been at least once, brighter than ~ 1 mCrababove 3 keV, and are expected to be detected by INTEGRAL. This catalogis being used in the INTEGRAL Quick Look Analysis to discover newsources or significantly variable sources. We compiled several publishedX-ray and gamma-ray catalogs, and surveyed recent publications for newsources. Consequently, there are 1122 sources in our INTEGRAL referencecatalog. In addition to the source positions, we show an approximatespectral model and expected flux for each source, based on which wederive expected INTEGRAL counting rates. Assuming the default instrumentperformances and at least ~ 105 s exposure time for anypart of the sky, we expect that INTEGRAL will detect at least ~ 700sources below 10 keV and ~ 400 sources above 20 keV over the missionlife.The Catalog is available in electronic form at the CDS via anonymous ftpto cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/411/L59
| Do bulges of early- and late-type spirals have different morphology?
We study HST/NICMOS H-band images of bulges of two equal-sized samplesof early- (TRC3 <= 3) and late-type spiral (mainly Sbc-Sc)galaxies matched in outer disk axis ratio. We find that bulges oflate-type spirals are more elongated than their counterparts inearly-type spirals. Using a KS-test we find that the two distributionsare different at the 98.4% confidence level. We conclude that the twodata sets are different, i.e. late-type galaxies have a broaderellipticity distribution and contain more elongated features in theinner regions. We discuss the possibility that these would correspond tobars at a later evolutionary stage, i.e. secularly evolved bars.Consequent implications are raised, and we discuss relevant questionsregarding the formation and structure of bulges. Are bulges ofearly-type and late-type spirals different? Are their formationscenarios different? Can we talk about bulges in the same way fordifferent types of galaxies?
| Rotation and outflow in the central kiloparsec of the water-megamaser galaxies IC 2560, NGC 1386, NGC 1052, and Mrk 1210
Optical emission-line profiles were evaluated in order to explore thestructure of galactic nuclei containing H2O megamasersources. Long-slit spectra of IC 2560, NGC 1386, NGC 1052 and Mrk 1210were obtained at ~ 100 km s-1 spectral and ~ 2arcsec x 2arcsec spatial resolution. The following individual properties of theobjects were found: The active nucleus of IC 2560 (innermost +/- 2arcsec) emits lines typical for a high-ionization Seyfert-2 spectrum albeitwith comparatively narrow profiles (FWHM ~ 200 km s-1).Line wings are stronger on the blue side than on the red side,suggesting outflow. The central velocity gradient fits into the generalvelocity curve of the galaxy. Attributing it to a rotating disk coplanarwith the galaxy leads to a Keplerian mass of ~ 107Msun inside a radius of 100 pc. - The central 6 arcsec sizedstructure seen on HST Hα and [OIII] images of NGC 1386 appears tobe the inner part of a near-edge-on warped rotating spiral disk that istraced in Hα within a diameter of 17 arcsec along PA 23 degr.This interpretation is based on observed kinematic continuity and atypical S-shaped dust lane crossing the kinematical center. The centralvelocity gradient yields a Keplerian mass estimate of ~ 108Msun inside R=100 pc and 5x 109 Msuninside 0.8 kpc. The total mass of the ionized gas of 105-6Msun is small compared to the dynamical mass of the spiraldisk. - The kinematical gradient of the rotating gas disk in the centerof the elliptical galaxy NGC 1052 yields a mass of ~ 6x 108Msun inside 166 pc. Two components are found: component cal Aarises in a rotating disk, component cal B is blueshifted by ~ 400 kms-1 relative to the disk. cal B likely originates fromoutflowing gas distributed within a wide cone. There is no need for abroad Hα component in unpolarized flux. The peculiar polarizationseen in [OI]lambda 6300 by Barth et al. (\cite{Barth99}) may be relatedto the outflow component. - In Mrk 1210, a redshifted component cal Rcontributes to the exceptional width of [OI]lambda 6300. An additionalblueshifted component cal B strong in [OIII] allows to fit the Hα+ [NII] blend without a broad component of Hα . cal B and cal Rare likely to be outflow components. The location of the brightnessmaximum BM is slightly ( ~ 1arcsec ) shifted relative to the kinematicalcenter KC. Locating the true obscured nucleus in KC makes itunderstandable that BM displays faint broad scattered Hα inpolarized light. - Galactic rotation and outflow of narrow-line gas arecommon features of this sample of water-megamaser galaxies. Alldecomposed line-systems exhibit AGN typical line ratios. Recentdetections of H2O megamasers in starburst galaxies and theapparent asssociation of one megamaser with a Seyfert 1 AGN suggest thatmegamasers can possibly be triggered by optically detectable outflows.The frequently encountered edge-on geometry favoring large molecularcolumn densities appears to be verified for NGC 1386 and IC 2560. ForNGC 1052 and Mrk 1210, maser emission triggered by the opticallydetected outflow components cannot be ruled out.Based on observations collected at the La Silla outstation of ESO. Thedata have been evaluated with the MIDAS data analysis system (versionNov. 96) provided by ESO.
| Detection of Water Maser Flare in the Seyfert/LINER, NGC 6240
We report on a flare-up of water-vapor maser emission in a prototypicalultraluminous FIR galaxy with an AGN, NGC6240. The flux density of thedetected maser line was ≈ 0.1 Jy with FWHM ≈ 5 kms-1, indicating an apparent isotropic luminosity of ≈100 Lsolar at a distance of 100 Mpc, which is typical forwater megamasers. The maser line was redshifted by ≈ 200 kms-1 relative to the systemic velocity of the galaxy, while nosystemic-velocity feature was detected. The previous possible detectionor observed upper limits were at a level lower than the flux density inthe present paper, indicating that the water maser in NGC6240 flared upby more than a factor of 5 in 2001.
| Study of the X-Ray Background Spectrum and Its Large-Scale Fluctuation with ASCA
We studied the energy spectrum and the large-scale fluctuation of theX-ray background with the ASCA GIS instrument based on the ASCA MediumSensitivity Survey and Large Sky Survey observations. A total of 91fields with Galactic latitude |b| > 10° were selected with a skycoverage of 50 deg2 and 4.2 Ms of exposure. For each field,non-X-ray events were carefully subtracted and sources brighter than ~ 2× 1013 erg cm-2 s-1 (2-10keV)were eliminated. Spectral fits with a single power-law model for theindividual 0.7-10 keV spectra showed a significant excess below ~ 2keV,which could be expressed by an additional thermal model with kT ~= 0.4keV or a steep power-law model with a photon index ofΓsoft ~= 6. The 0.5-2keV intensities of the softthermal component varied significantly from field to field by 1 σ= 52 +4-5%, and showed a maximum toward theGalactic Center. This component is considered to be entirely Galactic.As for the hard power-law component, an average photon index of 91fields was obtained to be Γhard = 1.412 +/- 0.007 +/-0.025 and the average 2-10keV intensity was calculated asFhardX = (6.38 +/- 0.04 +/- 0.64) ×10-8erg cm-2 s-1 sr-1 (1σ statistical and systematic errors). The Galactic component ismarginally detected in the hard band. The 2-10keV intensities show a 1σ deviation of 6.49+0.56-0.61%, whiledeviation due to the reproducibility of the particle background is 3.2%.The observed deviation can be explained by the Poisson noise of thesource count in the f.o.v. (~ 0.5 deg2), even assuming asingle N-logS relation on the whole s ky. Based on the observedfluctuation and the absolute intensity, an acceptable region of theN-logS relation was derived, showing a consistent feature with therecent Chandra and XMM-Newton results. The fluctuation of the spectralindex was also examined; it implied a large amount of hard sources and asubstantial variation in the intrinsic source spectra(Γs ~= 1.1 +/- 1.0).
| Powerful Water Masers in Active Galactic Nuclei
Luminous water maser emission in the 616-523 lineat 22GHz has been detected from two dozen galaxies. In all cases theemission is confined to the nucleus and has been found only in AGN, inparticular, in Type 2 Seyferts and LINERs. I argue that most of theobserved megamaser sources are powered by X-ray irradiation of dense gasby the central engine. After briefly reviewing the physics of theseX-Ray Dissociation Regions, I discuss in detail the observations of themaser disk in NGC 4258, its implications, and compare alternative modelsfor the maser emission. I then discuss the observations of the othersources that have been imaged with VLBI to date, and how they do or donot fit into the framework of a thin, rotating disk, as in NGC 4258.Finally, I briefly discuss future prospects, especially the possibilityof detecting other water maser transitions.
| A Chandra observation of the H2O megamaser IC 2560
A short Chandra ACIS-S observation of the Seyfert 2 galaxy IC 2560,which hosts a luminous nuclear water megamaser, shows (1) that the X-rayemission is extended; (2) that the X-ray spectrum displays emissionfeatures in the soft (E < 2 keV) X-ray band (this is the majorcomponent of the extended emission); and (3) a very strong (EW ~ 3.6keV) iron Kα line at 6.4 keV on a flat continuum. This lastfeature clearly indicates that the X-ray source is hidden behindCompton-thick obscuration, so that the intrinsic hard X-ray luminositymust be much higher than that observed, probably close to ~3 ×1042 erg s-1. We briefly discuss the implicationsfor powering of the maser emission and the central source.
|
Enviar un nou article
Enllaços Relacionats
- - No s'ha trobat enllaços -
Enviar un nou enllaç
Membre dels grups següents:
|
Dades d'Observació i Astrometria
Catàlegs i designacions:
|
NEWS@SKY (Science&Space News)
