Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
3D NIR spectroscopy at subarcsecond resolution
We present a scientific case approached through high quality 3D NIRspectroscopy performed with CIRPASS, attached to the Gemini Southtelescope. A binary mass concentration at the nucleus of the galaxy M 83was suggested by Thatte et al. [A&A 364 (2000) L47] and Mast et al.[BAAA 45 (2002) 98. Astroph#0505264] determined the possible position ofthe hidden secondary mass concentration with 2D H-alpha kinematics. Thepreliminary results of the NIR study presented here are based in almost1500 spectra centered in the wavelength 1.3 μm, with a spectralresolving power of 3200. They allow us to unveil, with 0.36″ (6.4pc) sampling and subarcsecond resolution, the velocity field in a regionof 13″ × 9″ around the optical nucleus. We confirmthat the optical nucleus is not located at the most important center ofsymmetry of the ionized gas velocity field. The largest black hole thatcould fit to the circular motion in this kinematic center should have amass not larger than 3 × 106(sin i)‑1Mȯ solar masses.
| Investigating a fluctuating-accretion model for the spectral-timing properties of accreting black hole systems
The fluctuating-accretion model of Lyubarskii and its extension byKotov, Churazov & Gilfanov seek to explain the spectral-timingproperties of the X-ray variability of accreting black holes in terms ofinward-propagating mass accretion fluctuations produced at a broad rangeof radii. The fluctuations modulate the X-ray emitting region as theymove inwards and can produce temporal-frequency-dependent lags betweenenergy bands, and energy-dependent power spectral densities (PSDs) as aresult of the different emissivity profiles, which may be expected atdifferent X-ray energies. Here, we use a simple numerical implementationto investigate in detail the X-ray spectral-timing properties of themodel and their relation to several physically interesting parameters,namely the emissivity profile in different energy bands, the geometricalthickness and viscosity parameter of the accretion flow, the strength ofdamping on the fluctuations and the temporal coherence (measured by the`quality factor', Q) of the fluctuations introduced at each radius. Wefind that a geometrically thick flow with large viscosity parameter isfavoured, and we confirm that the predicted lags are quite robust tochanges in the emissivity profile and physical parameters of theaccretion flow, which may help to explain the similarity of the lagspectra in the low/hard and high/soft states of Cyg X-1. We alsodemonstrate the model regime where the light curves in different energybands are highly spectrally coherent. We compare model predictionsdirectly to X-ray data from the narrow line Seyfert 1 galaxy NGC 4051and the black hole X-ray binary (BHXRB) Cyg X-1 in its high/soft state,and we show that this general scheme can reproduce simultaneously thetime lags and energy-dependence of the PSD.
| Gas in early-type galaxies: cross-fuelling in late-type-early-type pairs?
We present 12CO (J= 1-0) and 12CO (J= 2-1)observations of eight early-type galaxies, forming part of a sample ofinteracting galaxies, each consisting of one late- and one early-typesystem. All of the early-type galaxies observed are undetected in CO tolow levels, allowing us to place tight constraints on their moleculargas content. Additionally, we present HI absorption data for one system.The implications for possible gas transfer from the late- to theearly-type galaxy during the interaction are discussed.
| On the Lengths, Colors, and Ages of 18 Face-on Bars
Along with a brief analysis we present data obtained from BVRI andKs images of a sample of 19 galaxies (18 barred and 1unbarred), which will be further explored in a future paper. We measuredthe lengths and colors of the bars, created color maps, and estimatedglobal color gradients. Applying a method developed in a companionpaper, we could distinguish for seven galaxies in our sample those whosebars have been recently formed from the ones with already evolved bars.We estimated an average difference in the optical colors between youngand evolved bars that may be translated to an age difference of theorder of 10 Gyr, meaning that bars may be, at least in some cases,long-standing structures. Moreover, our results show that, on average,evolved bars are longer than young bars. This seems to indicate that,during its evolution, a bar grows longer by capturing stars from thedisk, in agreement with recent numerical and analytical results.Although the statistical significance of these results is low, andfurther studies are needed to confirm them, we discuss the implicationsfrom our results on the possibility of bars being a recurrentphenomenon. We also present isophotal contours for all our images aswell as radial profiles of relevant photometric and geometricparameters.
| 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.
| The Star-forming Torus and Stellar Dynamical Black Hole Mass in the Seyfert 1 Nucleus of NGC 3227
We report R~4300 VLT SINFONI adaptive optics integral field K-bandspectroscopy of the nucleus of the Seyfert 1 galaxy NGC 3227 at aspatial resolution of 0.085" (7 pc). We present the morphologies andkinematics of emission lines and absorption features and give the firstderivation of a black hole mass in a Seyfert 1 nucleus from stellardynamics (marginally resolving the black hole's sphere of influence). Weshow that the gas in the nucleus has a mean column density of order1024 cm-2 and that it is geometrically thick, inagreement with the standard ``molecular torus'' scenario. We discusspossible heating processes responsible for maintaining the verticalheight of the torus. We also resolve the nuclear stellar distributionand find that within a few parsecs of the AGN there has been an intensestarburst, the most recent episode of which began ~40 Myr ago but hasnow ceased. The current luminosity of stars within 30 pc of the AGN,~3×109 Lsolar, is comparable to that of theAGN. We argue that the star formation has been occurring in theobscuring material. Finally, we apply Schwarzschild orbit superpositionmodels to our full two-dimensional data and derive the mass of the blackhole, paying careful attention to the input parameters, which are oftenuncertain. Our models yield a 1 σ range for the black hole mass ofMBH=7×106-2×107Msolar.Based on observations at the European Southern Observatory VLT(074.B-9012).
| Cosmic Evolution of Black Holes and Spheroids. I. The MBH-σ Relation at z = 0.36
We test the evolution of the correlation between black hole mass andbulge velocity dispersion (MBH-σ), using a carefullyselected sample of 14 Seyfert 1 galaxies at z=0.36+/-0.01. We measurevelocity dispersion from stellar absorption lines around Mg b (5175Å) and Fe (5270 Å) using high-S/N Keck spectra and estimateblack hole mass from the Hβ line width and the optical luminosityat 5100 Å, based on the empirically calibrated photoionizationmethod. We find a significant offset from the local relation, in thesense that velocity dispersions were smaller for given black hole massesat z=0.36 than locally. We investigate various sources of systematicuncertainties and find that those cannot account for the observedoffset. The measured offset isΔlogMBH=0.62+/-0.10+/-0.25 i.e.,Δlogσ=0.15+/-0.03+/-0.06, where the error bars include arandom component and an upper limit to the systematics. At face value,this result implies a substantial growth of bulges in the last 4 Gyr,assuming that the local MBH-σ relation is the universalevolutionary endpoint. Along with two samples of active galaxies withconsistently determined black hole mass and stellar velocity dispersiontaken from the literature, we quantify the observed evolution with thebest-fit linear relation:ΔlogMBH=(1.66+/-0.43)z+(0.04+/-0.09) with respect tothe local relationship of Tremaine and coworkers, andΔlogMBH=(1.55+/-0.46)z+(0.01+/-0.12) with respect tothat of Ferrarese. This result is consistent with the growth of blackholes predating the final growth of bulges at these mass scales(<σ>=170 km s-1).
| Lens-Aided Multi-Angle Spectroscopy (LAMAS) Reveals Small-Scale Outflow Structure in Quasars
Spectral differences between lensed quasar image components are common.Since lensing is intrinsically achromatic, these differences aretypically explained as the effect of either microlensing, or as lightpath time delays sampling intrinsic quasar spectral variability. Here weadvance a novel third hypothesis: some spectral differences are due tosmall line-of-sight differences through quasar disk wind outflows. Inparticular, we propose that variable spectral differences seen only incomponent A of the widest separation lens SDSS J1004+4112 are due todifferential absorption along the sight lines. The absorber propertiesrequired by this hypothesis are akin to known broad absorption line(BAL) outflows but must have a broader, smoother velocity profile. Weinterpret the observed C IV emission-line variability as furtherevidence for spatial fine structure transverse to the line of sight.Since outflows are likely to be rotating, such absorber fine structurecan consistently explain some of the UV and X-ray variability seen inAGNs. The implications are many: (1) Spectroscopic differences in otherlensed objects may be due to this ``lens-aided multi-anglespectroscopy'' (LAMAS). (2) Outflows have fine structure on size scalesof arcseconds, as seen from the nucleus. (3) Assuming either broadabsorption line region sizes proposed in recent wind models, ortypically assumed continuum emission region sizes, LAMAS and/orvariability provide broadly consistent absorber size scale estimates of~1015 cm. (4) Very broad smooth absorption may be ubiquitousin quasar spectra, even when no obvious troughs are seen.
| 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.
| A Galactic Origin for the Local Ionized X-Ray Absorbers
Recent Chandra and XMM-Newton observations of distant quasars have shownstrong local (z~0) X-ray absorption lines from highly ionized gas,primarily He-like oxygen. The nature of these X-ray absorbers, i.e.,whether they are part of the hot gas associated with the Milky Way orpart of the intragroup medium in the Local Group, remains a puzzle dueto the uncertainties in the distance. We present in this paper a surveyof 20 AGNs with Chandra and XMM-Newton archival data. About 40% of thetargets show local O VII He α absorption with column densitiesaround 1016 cm-2; in particular, O VII absorptionis present in all the high-quality spectra. We estimate that the skycovering fraction of this O VII-absorbing gas is at least 63%, at 90%confidence, and likely to be unity given enough high-quality spectra. Onthe basis of (1) the expected number of absorbers along sight linestoward distant AGNs, (2) joint analysis with X-ray emissionmeasurements, and (3) mass estimation, we argue that the observed X-rayabsorbers are part of the hot gas associated with our Galaxy. Futureobservations will significantly improve our understanding of thecovering fraction and provide robust tests of this result.
| The Radius-Luminosity Relationship for Active Galactic Nuclei: The Effect of Host-Galaxy Starlight on Luminosity Measurements
We have obtained high-resolution images of the central regions of 14reverberation-mapped active galactic nuclei (AGNs) using the HubbleSpace Telescope Advanced Camera for Surveys High Resolution Camera toaccount for host-galaxy starlight contamination of measured AGNluminosities. We measure the host-galaxy starlight contribution to thecontinuum luminosity at 5100 Å through the typical ground-basedslit position and geometry used in the reverberation-mapping campaigns.We find that removing the starlight contribution results in asignificant correction to the luminosity of each AGN both for lowerluminosity sources, as expected, but also for the higher luminositysources such as the PG quasars. After accounting for the host galaxystarlight, we revisit the well-known broad-line region radius-luminosityrelationship for nearby AGNs. We find the power-law slope of therelationship for the Hβ line to be 0.518+/-0.039, shallower thanwhat was previously reported and consistent with the slope of 0.5expected from the naive theoretical assumption that all AGNs have, onaverage, the same ionizing spectrum and the same ionization parameterand gas density in the Hβ line-emitting region.
| The MBH-σ* Relation in Local Active Galaxies
We examine whether active galaxies obey the same relation between blackhole mass and stellar velocity dispersion as inactive systems, using thelargest published sample of velocity dispersions for active nuclei todate. The combination of 56 original measurements with objects from theliterature not only increases the sample from the 15 consideredpreviously to 88 objects but allows us to cover an unprecedented rangein both stellar velocity dispersion (30-268 km s-1) and blackhole mass (105-108.6 Msolar). In theMBH-σ* relation of active galaxies, we finda lower zero point than the best-fit relation of Tremaine et al. forinactive galaxies, and an upper limit on the intrinsic scatter of 0.4dex. There is also evidence of a flatter slope at low black hole masses.We discuss potential contributors to the observed offsets, includingvariations in the geometry of the broad-line region, evolution in theMBH-σ* relation, and differential growthbetween black holes and galaxy bulges.
| Determining Central Black Hole Masses in Distant Active Galaxies and Quasars. II. Improved Optical and UV Scaling Relationships
We present four improved empirical relationships useful for estimatingthe central black hole mass in nearby AGNs and distant luminous quasarsalike using either optical or UV single-epoch spectroscopy. These massscaling relationships between line widths and luminosity are based onrecently improved empirical relationships between the broad-line regionsize and luminosities in various energy bands and are calibrated to theimproved mass measurements of nearby AGNs based on emission-linereverberation mapping. The mass scaling relationship based on theHβ line luminosity allows mass estimates for low-redshift sourceswith strong contamination of the optical continuum luminosity by stellaror nonthermal emission, while that based on the C IV λ1549 linedispersion allows mass estimates in cases where only the line dispersion(as opposed to the FWHM) can be reliably determined. We estimate thatthe absolute uncertainties in masses given by these mass scalingrelationships are typically around a factor of 4. We include in anappendix mass estimates for all of the Bright Quasar Survey (PG) quasarsfor which direct reverberation-based mass measurements are notavailable.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained from the Data Archive at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| Reverberation Measurements of the Inner Radius of the Dust Torus in Nearby Seyfert 1 Galaxies
The most intense monitoring observations yet made in the optical andnear-infrared wave bands were carried out for Seyfert 1 galaxies NGC5548, NGC 4051, NGC 3227, and NGC 7469 by the MAGNUM telescope, andclear time-delayed responses of the K-band flux variations to the V-bandflux variations were found for all of these galaxies. Their H-K colortemperatures of 1500-1800 K, estimated from their observed fluxvariation gradients, support a view that the bulk of the K flux shouldoriginate in the thermal radiation of hot dust surrounding the centralengine and that the lag time should correspond to light-travel distancebetween them. Cross-correlation analysis measures their lag times to be47-53 (NGC 5548), 11-18 (NGC 4051), about 20 (NGC 3227), and 65-87 (NGC7469) days. The lag times are tightly correlated with the opticalluminosities, as expected from dust reverberation(Δt~L0.5), while weakly with the central virial masses,which suggests that the inner radii of the dust tori around activenuclei have one-to-one correspondences with their central luminosities.In the lag time versus central luminosity diagram, the K-band lag timesplace an upper boundary on the similar lag times of broad emission linesin the literature, which not only supports the unified scheme of AGNsbut also implies a physical transition from the BLR out to the dusttorus that encircles the BLR. Correlated short-term V-band and X-rayflux variations in NGC 5548 are also found with a delay of 1 or 2 days,indicating the thermal reprocessing of X-ray emission by the centralaccretion flow.
| 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).
| A Sample of IRAS Infrared-selected Seyfert 1.5 Galaxies: Infrared Color α(60, 25)-dominated Eigenvector 1
The well-documented E1 relationships are first extended to infraredcolor α(60, 25) and flux ratio [O III]/Hβn bycomparing emission-line properties to continuum properties in infraredwavelengths. Both direct correlations and a principal component analysisare used in a sample of 50 IRAS IR-selected Seyfert 1.5 galaxies. Inaddition, to confirm the correlations of E1 in Boroson & Green, oureigenvector 1 turns out to be dominated by the mid-infrared colorα(60, 25) and most strongly affected by RFe, [OIII]/Hβn, and EW(Hβb). Our analysisindicates that the objects with large E1 tend to coexist with relativelyyoung nuclear stellar populations, which implies that E1 is related tothe nuclear star formation history. The IR-dominated eigenvector 1 cantherefore be inferred to be interpreted as the ``age'' of an AGN. Inconfirmation of the work of Xu and coworkers, it is clear that theextreme Seyfert galaxies with both large RFe and large [OIII]/Hβn are rare in our universe.
| Spatially Resolved Narrow-Line Region Kinematics in Active Galactic Nuclei
We have analyzed Hubble Space Telescope spectroscopy of 24 nearby activegalactic nuclei (AGNs) to investigate spatially resolved gas kinematicsin the narrow-line region (NLR). These observations effectively isolatethe nuclear line profiles on less than 100 pc scales and are used toinvestigate the origin of the substantial scatter between the widths ofstrong NLR lines and the stellar velocity dispersion σ*of the host galaxy, a quantity that relates with substantially lessscatter to the mass of the central, supermassive black hole and moregenerally characterize variations in the NLR velocity field with radius.We find that line widths measured with STIS at a range of spatial scalessystematically underestimate both σ* and the line widthmeasured from ground-based observations, although they do havecomparably large scatter to the relation between ground-based NLR linewidth and σ*. There are no obvious trends in theresiduals when compared with a range of host galaxy and nuclearproperties. The widths and asymmetries of [O III] λ5007 and [SII] λλ6716, 6731 as a function of radius exhibit a widerange of behavior. Some of the most common phenomena are substantialwidth increases from the STIS to the large-scale, ground-based apertureand almost no change in line profile between the unresolved nuclearspectrum and ground-based measurements. We identify asymmetries in asurprisingly large fraction of low-ionization [S II] line profiles andseveral examples of substantial red asymmetries in both [O III] and [SII]. These results underscore the complexity of the circumnuclearmaterial that constitutes the NLR and suggest that the scatter in theNLR width and σ* correlation cannot be substantiallyreduced with a simple set of empirical relations.
| Swift Observations of the Highly X-Ray Variable Narrow-Line Seyfert 1 Galaxy RX J0148.3-2758
We report on Swift observations of the narrow-line Seyfert 1 galaxy RXJ0148.3-2758. It was observed for 41.6 ks in 2005 May and for 15.8 ks in2005 December. On short as well as on long timescales, RX J0148.3-2758is a highly variable source. It doubles its X-ray flux within 18-25 ks.The observation of 2005 December 9, which had a flux 4 times lower thanduring the 2005 May observations, shows a significant hardening of theX-ray hardness ratio compared with the 2005 May and December 20 and 21observations. A detailed analysis of the X-ray spectra shows that weactually observe two spectral changes in RX J0148.3-2758: first, adecrease of the soft X-ray component between 2005 May and December 9,which is most likely due to an increase of the intrinsic absorbercolumn, and second, a decrease of the hard X-ray flux in the December 20and 21 observations. The soft X-ray spectral slopeαX,soft=2.58+0.15-0.12 during thehigh state in 2005 May agrees well with that measured by ROSAT(αX,soft=2.54+/-0.82). This soft X-ray spectrum issuperposed on a hard X-ray component withαX,hard=0.96+0.16-0.14, which isconsistent with the hard X-ray spectral slopeαX,hard=1.11+0.16-0.19 found byASCA. The soft X-ray slopeαX,soft=1.93+0.58-0.42 measuredduring the December 9 observation agrees well withαX,soft=2.03+0.23-0.20 measuredfrom the ASCA observation when RX J0148.3-2758 was also in a low state.In contrast to the strong X-ray variability, the analysis of the SwiftUV-Optical Telescope (UVOT) photometry from 2005 December of RXJ0148.3-2758 shows no significant variability in any of the six UVOTfilters. From the simultaneous X-ray and UV observations in 2005December we measured the X-ray loudness αox and foundit to vary between αox=1.5 and 1.8. Our Swiftobservations of RX J0148.3-2758 demonstrate the great potential that themultiwavelength observatory Swift has for active galactic nucleusscience.
| A Survey of Kiloparsec-Scale Radio Outflows in Radio-Quiet Active Galactic Nuclei
Seyfert galaxies commonly host compact jets spanning 10-100 pc scales,but larger structures are resolved out in long-baseline aperturesynthesis surveys. Previous, targeted studies showed thatkiloparsec-scale radio structures (KSRs) may be a common feature ofSeyfert and LINER galaxies, and the origin of KSRs may be starbursts oractive galactic nuclei (AGNs). We report a new Very Large Array surveyof a complete sample of Seyfert and LINER galaxies. Out of all of thesurveyed radio-quiet sources, we find that 44% (19 out of 43) showextended radio structures at least 1 kpc in total extent that do notmatch the morphology of the disk or its associated star-forming regions.The detection rate is a lower limit owing to the combined effects ofprojection and resolution. The infrared colors of the KSR host galaxiesare unremarkable compared to other Seyfert galaxies, and the large-scaleoutflows orient randomly with respect to the host galaxy axes. The KSRSeyfert galaxies instead stand out by deviating significantly from thefar-infrared-radio correlation for star-forming galaxies, with tendencytoward radio excess, and they are more likely to have a relativelyluminous, compact radio source in the nucleus; these results argue thatKSRs are powered by the AGNs rather than starbursts. The high detectionrate indicates that Seyfert galaxies generate radio outflows over asignificant fraction of their lifetime, which is much longer than thedynamical timescale of an AGN-powered jet but is comparable instead tothe buoyancy timescale. The likely explanation is that the KSRsoriginate from jet plasma that has been decelerated by interaction withthe nuclear interstellar medium (ISM). Based on a simple ram pressureargument, the kinetic power of the jet on kiloparsec scales is about 3orders of magnitude weaker than the power of the jet on 10-100 pcscales. This result is consistent with the interaction model, in whichcase virtually all of the jet power must be lost to the ISM within theinner kiloparsec.
| 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.
| Double Nucleus in M83
M83 is one of the nearest galaxies with enhanced nuclear star formation,and it presents one of the best opportunities to study the kinematicsand physical properties of a circumnuclear starburst. Ourthree-dimensional spectroscopy data in the R band confirm the presenceof a secondary nucleus or mass concentration (previously suggested byThatte and coworkers). We determine the position of this hidden nucleus,which would be more massive than the visible one and was not detected inthe optical Hubble Space Telescope images due, probably, to the strongdust extinction. Using a Keplerian approximation, we estimated for theoptical nucleus a mass of (5.0+/-0.8)×106Msolar/sini (r<1.5"), and for the hidden nucleus, located4''+/-1'' to the northwest (position angle of271deg+/-15deg) of the optical nucleus, a mass of(1.00+/-0.08)×107 Msolar/sini (r<1.5").The emission-line ratio map also unveils the presence of a secondcircumnuclear ring structure, previously discovered by IR imaging(Elmegreen and coworkers). The data allow us to resolve the behavior ofthe interstellar medium inside the circumnuclear ring and around thebinary mass concentration.
| Low-Luminosity Active Galaxies and Their Central Black Holes
Central black hole masses for 117 spiral galaxies representingmorphological stages S0/a through Sc and taken from the largespectroscopic survey of Ho et al. are derived using Ks-banddata from the Two Micron All Sky Survey. Black hole masses are foundusing a calibrated black hole-Ks bulge luminosity relation,while bulge luminosities are measured by means of a two-dimensionalbulge-disk decomposition routine. The black hole masses are correlatedagainst a variety of parameters representing properties of the nucleusand host galaxy. Nuclear properties such as line width (FWHM [N II]), aswell as emission-line ratios (e.g., [O III]/Hβ, [O I]/Hα, [NII]/Hα, and [S II]/Hα), show a very high degree ofcorrelation with black hole mass. The excellent correlation with linewidth supports the view that the emission-line gas is in virialequilibrium with either the black hole or bulge potential. The very goodemission-line ratio correlations may indicate a change in ionizingcontinuum shape with black hole mass in the sense that more massiveblack holes generate harder spectra. Apart from theinclination-corrected rotational velocity, no excellent correlations arefound between black hole mass and host galaxy properties. Significantdifferences are found between the distributions of black hole masses inearly-, mid-, and late-type spiral galaxies (subsamples A, B, and C) inthe sense that early-type galaxies have preferentially larger centralblack holes, consistent with observations that Seyfert galaxies arefound preferentially in early-type systems. The line width distributionsshow a marked difference among subsamples A, B, and C in the sense thatearlier type galaxies have larger line widths. There are also cleardifferences in line ratios between subsamples A+B and C that likely arerelated to the level of ionization in the gas. Finally, aKs-band Simien & de Vaucouleurs diagram shows excellentagreement with the original B-band relation, although there is a largedispersion at a given morphological stage.
| 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.
| On the X-ray, optical emission line and black hole mass properties of local Seyfert galaxies
We investigate the relation between X-ray nuclear emission, opticalemission line luminosities and black hole masses for a sample of 47Seyfert galaxies. The sample, which has been selected from the Palomaroptical spectroscopic survey of nearby galaxies (Ho et al. 1997a, ApJS,112, 315), covers a wide range of nuclear powers, from L2-10keV ~ 1043 erg/s down to very low luminosities(L2-10 keV ~ 1038 erg/s). Best available data fromChandra, XMM-Newton and, in a few cases, ASCA observations have beenconsidered. Thanks to the good spatial resolution available from theseobservations and a proper modeling of the various spectral components,it has been possible to obtain accurate nuclear X-ray luminosities notcontaminated by off-nuclear sources and/or diffuse emission. X-rayluminosities have then been corrected taking into account the likelycandidate Compton thick sources, which are a high fraction (>30%)among type 2 Seyferts in our sample. The main result of this study isthat we confirm strong linear correlations between 2-10 keV,[OIII]λ5007, Hα luminosities which show the same slope asquasars and luminous Seyfert galaxies, independent of the level ofnuclear activity displayed. Moreover, despite the wide range ofEddington ratios (L/L_Edd) tested here (six orders of magnitude, from0.1 down to ~10-7), no correlation is found between the X-rayor optical emission line luminosities and the black hole mass. Ourresults suggest that Seyfert nuclei in our sample are consistent withbeing a scaled-down version of more luminous AGN.
| 4U 1344-60: a bright intermediate Seyfert galaxy at z = 0.012 with a relativistic Fe Kα emission line
We present analysis of the optical and X-ray spectra of the low Galacticlatitude bright (F_2-10 = 3.6 × 10-11 ergcm-2 s-1) source 4U 1344-60. On the basis of theoptical data we propose to classify 4U 1344-60 as an intermediate typeSeyfert galaxy and we measure a value of z = 0.012±0.001 for itsredshift. From the XMM-Newton observation we find that the overall X-rayspectral shape of 4U 1344-60 is complex and can be described by apower-law continuum (Γ≈ 1.55) obscured by two neutralabsorption components (N_Hf ~ 1022 cm-2and N_Hp ~ 4 × 1022 cm-2), thelatter covering only the ~50% of the primary X-ray source. The X-raydata therefore lend support to our classification of 4U 1344-60. Itexhibits a broad and skewed Fe Kα line at ~6.4 keV, which suggeststhe existence of an accretion disk that is able to reprocess the primarycontinuum down to a few gravitational radii. Such a line represents oneof the clearest examples of a relativistic line observed by XMM-Newtonso far. Our analysis has also revealed the marginal presence of twonarrow line-like emission features at ~4.9 and ~5.2 keV.
| A radio census of nuclear activity in nearby galaxies
In order to determine the incidence of black hole accretion-drivennuclear activity in nearby galaxies, as manifested by their radioemission, we have carried out a high-resolution Multi-ElementRadio-Linked Interferometer Network (MERLIN) survey of LINERs andcomposite LINER/Hii galaxies from a complete magnitude-limited sample ofbright nearby galaxies (Palomar sample) with unknown arcsecond-scaleradio properties. There are fifteen radio detections, of which three arenew subarcsecond-scale radio core detections, all being candidate AGN.The detected galaxies supplement the already known low-luminosity AGN -low-luminosity Seyferts, LINERs and composite LINER/Hii galaxies - inthe Palomar sample. Combining all radio-detected Seyferts, LINERs andcomposite LINER/Hii galaxies (LTS sources), we obtain an overall radiodetection rate of 54% (22% of all bright nearby galaxies) and weestimate that at least ~50% (~20% of all bright nearby galaxies) aretrue AGN. The radio powers of the LTS galaxies allow the construction ofa local radio luminosity function. By comparing the luminosity functionwith those of selected moderate-redshift AGN, selected from the 2dF/NVSSsurvey, we find that LTS sources naturally extend the RLF of powerfulAGN down to powers of about 10 times that of Sgr A*.
| X-ray spectral survey with XMM-Newton of a complete sample of nearby Seyfert galaxies
Results obtained from an X-ray spectral survey of nearby Seyfertgalaxies using XMM-Newton are reported. The sample was opticallyselected, well defined, complete in B magnitude, and distance limited:it consists of the nearest (D 22 Mpc) 27 Seyfert galaxies (9 oftype 1, 18 of type 2) taken from the Ho et al. (1997a, ApJS, 112, 315)sample. This is one of the largest atlases of hard X-ray spectra oflow-luminosity active galaxies ever assembled. All nuclear sourcesexcept two Seyfert 2s are detected between 2 and 10 keV, half for thefirst time ever, and average spectra are obtained for all of them.Nuclear luminosities reach values down to 1038 ergs-1. The shape of the distribution of X-ray parameters isaffected by the presence of Compton-thick objects (30% among type2s). The latter have been identified either directly from their intenseFeK line and flat X-ray spectra, or indirectly with flux diagnosticdiagrams which use isotropic indicators. After taking into account thesehighly absorbed sources, we find that (i) the intrinsic X-ray spectralproperties (i.e., spectral shapes and luminosities above 2 keV) areconsistent between type 1 and type 2 Seyferts, as expected from "unifiedmodels"; (ii) Seyfert galaxies as a whole are distributed fairlycontinuously over the entire range of N_H, between 1020 and1025 cm-2; and (iii) while Seyfert 1s tend to havelower NH and Seyfert 2s tend to have the highest, we find 30%and 10% exceptions, respectively. Overall the sample is of sufficientquality to well represent the average intrinsic X-ray spectralproperties of nearby active galactic nuclei, including a proper estimateof the distribution of their absorbing columns. Finally, we concludethat, with the exception of a few cases, the present study agrees withpredictions of unified models of Seyfert galaxies, and extends theirvalidity down to very low luminosities.
| 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.
| First science with SINFONI
Not Available
| Molecular hydrogen and [FeII] in active galactic nuclei - II. Results for Seyfert 2 galaxies
Near-infrared spectroscopy is used to study the kinematics andexcitation mechanisms of H2 and [FeII] lines in a sampledominated by Seyfert 2 galaxies. The spectra simultaneously cover theJHK bands, allowing us to compare line fluxes emitted in the interval0.8-2.4 μm and avoiding aperture and seeing effects. TheH2 lines are systematically narrower than the narrow-lineregion lines, suggesting that, very likely, the H2 does notoriginate from the same parcel of gas that forms the narrow-line region.Emission-line ratios between H2 lines favour thermalexcitation mechanisms for the molecular gas in active galactic nuclei.It was found that non-thermal excitation contributes, at most, 30 percent of the observed H2. Thermal excitation is also confirmedby the rather similar vibrational and rotational temperatures in theobjects (~2000 K). The mass of hot H2 ranges from102 to 103Msolar, with nearly half ofobjects showing values of <500 Msolar. It shows that thefraction of molecular mass present in the nuclear region and emitting inthe near-infrared is a very small fraction of the warm molecular masspresent in the centre. A diagnostic diagram composed of the line ratiosH2/Brγ and [FeII]/Paβ proves to be a useful toolin the near-infrared for separating emission-line objects by theirdegree of nuclear activity. We found that active galactic nuclei arecharacterized by H2 2.121 μm/Brγ and [FeII] 1.257μm/Paβ flux ratios between 0.6 and 2. Starburst/HII galaxiesdisplay line ratios <0.6 while low-ionization nuclear emission-lineregions are characterized by values larger than 2 in either ratio.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
Catálogos y designaciones:
|
