Home     Getting Started     To Survive in the Universe    
Inhabited Sky
    News@Sky     Astro Photo     The Collection     Forum     Blog New!     FAQ     Press     Login  

NGC 4151



Upload your image

DSS Images   Other Images

Related articles

An Internet Database of Ultraviolet Continuum Light Curves for Seyfert Galaxies
Using the Multimission Archive at STScI (MAST), we have extractedspectra and determined continuum light curves for 175 Seyfert galaxiesthat have been observed with the International Ultraviolet Explorer andthe Faint Object Spectrograph on the Hubble Space Telescope. To obtainthe light curves as a function of Julian Date, we used fixed bins in theobject's rest frame and measured small regions (between 30 and 60Å) of each spectrum's continuum flux in the range 1150 to 3200Å. We provide access to the UV light curves and other basicinformation about the observations in tabular and graphical form via theInternet at http://www.chara.gsu.edu/PEGA/IUE.

A Survey of O VI, C III, and H I in Highly Ionized High-Velocity Clouds
We present a Far Ultraviolet Spectroscopic Explorer survey of highlyionized high-velocity clouds (HVCs) in 66 extragalactic sight lines with(S/N)1030>8. We search the spectra for high-velocity (100km s-1<|vLSR|<400 km s-1) O VIabsorption and find a total of 63 absorbers, 16 with 21 cm emitting H Icounterparts and 47 ``highly ionized'' absorbers without 21 cm emission.The highly ionized HVC population is characterized by =38+/-10 km s-1 and =13.83+/-0.36, with negative-velocity clouds generally found atl<180deg and positive-velocity clouds found atl>180deg. Eleven of these highly ionized HVCs arepositive-velocity wings (broad O VI features extending asymmetrically tovelocities of up to 300 km s-1). We find that 81% (30 of 37)of highly ionized HVCs have clear accompanying C III absorption, and 76%(29 of 38) have accompanying H I absorption in the Lyman series. Wepresent the first (O VI selected) sample of C III and H I absorptionline HVCs and find =30+/-8 km s-1,logNa(C III) ranges from <12.5 to >14.4, =22+/-5 km s-1, and log Na(H I) ranges from<14.7 to >16.9. The lower average width of the high-velocity H Iabsorbers implies the H I lines arise in a separate, lower temperaturephase than the O VI. The ratio Na(C III)/Na(O VI)is generally constant with velocity in highly ionized HVCs, suggestingthat at least some C III resides in the same gas as the O VI.Collisional ionization equilibrium models with solar abundances canexplain the O VI/C III ratios for temperatures near1.7×105 K; nonequilibrium models with the O VI ``frozenin'' at lower temperatures are also possible. Photoionization models arenot viable since they underpredict O VI by several orders of magnitude.The presence of associated C III and H I strongly suggests the highlyionized HVCs are not formed in the hotter plasma that gives rise to OVII and O VIII X-ray absorption. We find that the shape of the O VIpositive-velocity wing profiles is well reproduced by a radiativelycooling, vertical outflow moving with ballistic dynamics, withT0=106 K, n0~2×10-3cm-3, and v0~250 km s-1. However, theoutflow has to be patchy and out of ionization equilibrium to explainthe sky distribution and the simultaneous presence of O VI, C III, and HI. We found that a spherical outflow can produce high-velocity O VIcomponents (as opposed to the wings), showing that the possible range ofoutflow model results is too broad to conclusively identify whether ornot an outflow has left its signature in the data. An alternative model,supported by the similar multiphase structure and similar O VIproperties of highly ionized and 21 cm HVCs, is one where the highlyionized HVCs represent the low N(H I) tail of the HVC population, withthe O VI formed at the interfaces around the embedded H I cores.Although we cannot rule out the possibility that some highly ionizedHVCs exist in the Local Group or beyond, we favor a Galactic origin.This is based on the recent evidence that both H I HVCs and themillion-degree gas detected in X-ray absorption are Galactic phenomena.Since the highly ionized HVCs appear to trace the interface betweenthese two Galactic phases, it follows that highly ionized HVCs areGalactic themselves. However, the nondetection of high-velocity O VI inhalo star spectra implies that any Galactic high-velocity O VI exists atz distances beyond a few kpc.

A FUSE Survey of High-Latitude Galactic Molecular Hydrogen
Measurements of molecular hydrogen (H2) column densities arepresented for the first six rotational levels (J=0-5) for 73extragalactic targets observed with the Far Ultraviolet SpectroscopicExplorer (FUSE). All of these have a final signal-to-noise ratio largerthan 10 and are located at Galactic latitude |b|>20deg.The individual observations were calibrated with the FUSE calibrationpipeline CalFUSE version 2.1 or higher and then carefully aligned invelocity. The final velocity shifts for all the FUSE segments arelisted. H2 column densities or limits are determined for thesix lowest rotational (J) levels for each H I component in the line ofsight, using a curve-of-growth approach at low column densities(<16.5) and Voigt-profile fitting at higher column densities.Detections include 65 measurements of low-velocity H2 in theGalactic disk and lower halo. Eight sight lines yield nondetections forGalactic H2. The measured column densities range fromlogN(H2)=14 to 20. Strong correlations are found betweenlogN(H2) and T01, the excitation temperature ofthe H2, as well as between logN(H2) and the levelpopulation ratios (log[N(J')/N(J)]). The average fraction ofnuclei in molecular hydrogen [f(H2)] in each sight line iscalculated; however, because there are many H I clouds in each sightline, the physics of the transition from H I to H2 cannot bestudied. Detections also include H2 in 16intermediate-velocity clouds in the Galactic halo (out of 35 IVCs).Molecular hydrogen is seen in one high-velocity cloud (the Leading Armof the Magellanic Stream), although 19 high-velocity clouds areintersected; this strongly suggests that dust is rare or absent in theseobjects. Finally, there are five detections of H2 in externalgalaxies.

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.

The Mass of the Central Black Hole in the Seyfert Galaxy NGC 4151
In order to improve the reverberation-mapping-based estimate of the massof the central supermassive black hole in the Seyfert 1 galaxy NGC 4151,we have reanalyzed archival ultraviolet monitoring spectra from twocampaigns undertaken with the International Ultraviolet Explorer. Wemeasure emission-line time delays for four lines, C IV λ1549, HeII λ1640, C III] λ1909, and Mg II λ2798, from bothcampaigns. We combine these measurements with the dispersion of thevariable part of each respective emission line to obtain the mass of thecentral object. Despite the problematic nature of some of the data, weare able to measure a mass of (4.14+/-0.73)×107Msolar, although this, like all reverberation-based masses,is probably systematically uncertain by a factor of 3-4.

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).

Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. II. X-Ray and Ultraviolet Continuum Variability
We report on two Chandra observations, and a simultaneous Hubble SpaceTelescope ultraviolet observation, of the dwarf Seyfert 1 galaxy NGC4395. Each Chandra observation had a duration of ~30 ks, with aseparation of ~50 ks. The spectrum was observed to harden between theseobservations via a scaling down of the soft-band flux. Theinterobservation variability is in a different sense from the observedvariability within each observation and is most likely the result ofincreased absorption. Spectral variations were seen during the firstobservation suggesting that the X-ray emission is produced in more thanone disconnected region. We have also reanalyzed a ~17 ks Chandraobservation conducted in 2000. During the three Chandra observations the2-10 keV flux is about a factor of 2 lower than seen during anXMM-Newton observation conducted in 2003. Moreover, the fractionalvariability amplitude exhibited during the XMM-Newton observation issignificantly softer than seen during the Chandra observations. A powerspectral analysis of the first of the two new Chandra observationsrevealed a peak at 341 s with a formal detection significance of 99%. Asimilar peak was seen previously in the 2000 Chandra data. However, thedetection of this feature is tentative given that it was found inneither the second of our two new Chandra observations nor theXMM-Newton data, and it is much narrower than expected. The Hubble SpaceTelescope observation was conducted during part of the second Chandravisit. A zero-lag correlation between the ultraviolet and X-ray fluxeswas detected with a significance of ~99.5%, consistent with thepredictions of the two-phase model for the X-ray emission from activegalactic nuclei.

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.

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.

Spatially Resolved Mid-Infrared Spectroscopy of NGC 1068: The Nature and Distribution of the Nuclear Material
We present spatially resolved, near-diffraction-limited 10 μm spectraof the nucleus of the Seyfert 2 galaxy NGC 1068, obtained with Michelle,the mid-IR imager and spectrometer on the 8.1 m Gemini North Telescope.The spectra cover the nucleus and the central 6.0"×0.4" of theionization cones at a spatial resolution of approximately 0.4" (~30 pc).The spectra extracted in 0.4" steps along the slit reveal strikingvariations in continuum slope, silicate feature profile and depth, andfine-structure line fluxes on subarcsecond scales, illustrating inunprecedented detail the complexity of the circumnuclear regions of NGC1068 at mid-IR wavelengths. A comparison of photometry in variousapertures reveals two distinct components: a compact (radius<15 pc),bright source within the central 0.4"×0.4" and extended, lowerbrightness emission. We identify the compact source with theAGN-obscuring torus, and the diffuse component with dust in theionization cones. While the torus emission dominates the flux observedin the near-IR, the mid-IR flux measured with apertures larger thanabout 1" is dominated instead by emission from the ionization cones;despite its higher brightness, the torus contributes <30% of the 11.6μm flux in the central 1.2" region. Many previous attempts todetermine the torus spectral energy distribution are thus likely to besignificantly affected by contamination from the extended emission. Theobserved spectrum of the compact source is compared with clumpy torusmodels. The models require most of the clouds to be located within a fewparsecs of the central engine, in agreement with recent mid-IRinterferometric observations. We also present a UKIRT/CGS4 5 μmspectrum covering the R(0)-R(4) lines of the fundamentalvibration-rotation band of 12CO. None of these lines wasdetected, and we discuss these nondetections in terms of the fillingfactor and composition of the nuclear clouds.

Accretion and Nuclear Activity of Quiescent Supermassive Black Holes. I. X-Ray Study
We have studied the nuclear activity in a sample of six quiescentearly-type galaxies, with new Chandra data and archival HST opticalimages. Their nuclear sources have X-ray luminosities~1038-1039 ergs s-1(LX/LEdd~10-8 to 10-7) andcolors or spectra consistent with accreting supermassive black holes(SMBHs), except for the nucleus of NGC 4486B, which is softer thantypical AGN spectra. In a few cases, the X-ray morphology of the nuclearsources shows hints of marginally extended structures, in addition tothe surrounding diffuse thermal emission from hot gas, which isdetectable on scales >~1 kpc. In one case (NGC 5845), a dusty diskmay partially obstruct our direct view of the SMBH. We have estimatedthe temperature and density of the hot interstellar medium, which is onemajor source of fuel for the accreting SMBH; typical central densitiesare ne~(0.02+/-0.01) cm-3. Assuming that the hotgas is captured by the SMBH at the Bondi rate, we show that the observedX-ray luminosities are too faint to be consistent with standard diskaccretion, but brighter than predicted by radiatively inefficientsolutions (e.g., advection-dominated accretion flows [ADAFs]). In total,there are ~20 galaxies for which SMBH mass, hot gas density, and nuclearX-ray luminosity are simultaneously known. In some cases, the nuclearsources are brighter than predicted by the ADAF model; in other cases,they are consistent or fainter. We discuss the apparent lack ofcorrelations between Bondi rate and X-ray luminosity and suggest that,in order to understand the observed distribution, we need to know twoadditional parameters: the amount of gas supplied by the stellarpopulation inside the accretion radius, and the fraction (possibly<<1) of the total gas available that is accreted by the SMBH. Weleave a detailed study of these issues to a subsequent paper.

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).

Discovery of Water Maser Emission in Eight AGNs with 70 m Antennas of NASA's Deep Space Network
We report the discovery of water maser emission in eight active galacticnuclei (AGNs) with the 70 m NASA Deep Space Network (DSN) antennas atTidbinbilla, Australia, and Robledo, Spain. The positions of the newlydiscovered masers, measured with the VLA, are consistent with theoptical positions of the host nuclei to within 1 σ (0.3" radio and1.3" optical) and most likely mark the locations of the embedded centralengines. The spectra of two sources, NGC 3393 and NGC 5495, display thecharacteristic spectral signature of emission from an edge-on accretiondisk, with orbital velocities of ~600 and ~400 km s-1,respectively. In a survey with DSN facilities of 630 AGNs selected fromthe NASA Extragalactic Database, we have discovered a total of 15 watermaser sources. The resulting incidence rate of maser emission amongnearby (vsys<7000 km s-1) Seyfert 1.8-2.0 andLINER systems is ~10% for a typical rms noise level of ~14 mJy over 1.3km s-1 spectral channels. As a result of this work, thenumber of nearby AGNs (vsys<7000 km s-1)observed with <20 mJy rms noise has increased from 130 to 449.

INTEGRAL IBIS Extragalactic Survey: Active Galactic Nuclei Selected at 20-100 keV
Analysis of International Gamma-Ray Astrophysics Laboratory (INTEGRAL)Core Programme and public open-time observations performed up to 2005April provides a sample of 62 active galactic nuclei in the 20-100 keVband above a flux limit of ~1.5×10-11 ergscm-2 s-1. Most (42) of the sources in the sampleare Seyfert galaxies, almost equally divided between type 1 and type 2objects; six are blazars, and 14 are still unclassified. Excluding theblazars, the average redshift of our sample is 0.021, while the meanluminosity is logL=43.45. We find that absorption is present in 65% ofthe objects, with 14% of the total sample due to Compton-thick activegalaxies. In agreement with both Swift BAT team results and 2-10 keVstudies, the fraction of absorbed objects decreases with the 20-100 keVluminosity. All Seyfert 2's in our sample are absorbed, as are 33% ofSeyfert 1's. The present data highlight the capability of INTEGRAL toprobe the extragalactic gamma-ray sky and to find new and/or absorbedactive galaxies.Based on observations obtained with INTEGRAL, an ESA project withinstruments and science data center funded by ESA member states(especially the PI countries: Denmark, France, Germany, Italy,Switzerland, Spain), the Czech Republic, and Poland and with theparticipation of Russia and the US.

A FUSE Survey of Interstellar Molecular Hydrogen toward High-Latitude AGNs
We report results from a Far Ultraviolet Spectroscopic Explorer (FUSE)survey of interstellar molecular hydrogen (H2) along 45 sightlines to AGNs at high Galactic latitudes (b>20deg). Most(39 out of 45) of the sight lines show detectable Galactic H2absorption from Lyman and Werner bands between 1000 and 1126 Å,with column densities ranging fromNH2=1014.17 to 1019.82cm-2. In the northern Galactic hemisphere, we identify manyregions of low NH2 (<=1015cm-2) between l=60deg and 180° and atb>54deg. These ``H2 holes'' provide valuable,uncontaminated sight lines for extragalactic UV spectroscopy, and a fewmay be related to the ``Northern Chimney'' (low Na I absorption) and the``Lockman Hole'' (low NHI). A comparison of high-latitudeH2 with 139 OB star sight lines surveyed in the Galactic disksuggests that high-latitude and disk H2 clouds may havedifferent rates of heating, cooling, and UV excitation. For rotationalstates J=0 and 1, the mean excitation temperature at high latitude,=124+/-8 K, is somewhat higher thanthat in the Galactic disk, =86+/-20K. For J>=2, the mean =498+/-28 K, and thecolumn-density ratios, N(3)/N(1), N(4)/N(0), and N(4)/N(2), indicate acomparable degree of UV excitation in the disk and low halo for sightlines with NH2>=1018cm-2. The distribution of molecular fractions at highlatitude shows a transition at lower total hydrogen column density(logNhlH~20.38+/-0.13) than in the Galactic disk(logNdiskH~20.7). If the UV radiation fields aresimilar in disk and low halo, this suggests an enhanced H2(dust-catalyzed) formation rate in higher density, compressed clouds,which could be detectable as high-latitude, sheetlike infrared cirrus.

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.

Is the Broad-Line Region Clumped or Smooth? Constraints from the Hα Profile in NGC 4395, the Least Luminous Seyfert 1 Galaxy
The origin and configuration of the gas that emits broad lines in Type Iactive galactic nuclei is not established yet. The lack of small-scalestructure in the broad emission-line profiles is consistent with eithera smooth gas flow or a clumped flow with many small clouds. Anattractive possibility for the origin of many small clouds is theatmospheres of bloated stars, an origin that also provides a naturalmechanism for the cloud confinement. Earlier studies of the broad-lineprofiles have already put strong lower limits on the minimum number ofsuch stars, but these limits are sensitive to the assumed width of thelines produced by each cloud. Here we revisit this problem usinghigh-resolution Keck spectra of the Hα line in NGC 4395, which hasthe smallest known broad-line region (~1014 cm). Only ahandful of the required bloated stars (each havingr*~1014 cm) could fit into the broad-line regionof NGC 4395, yet the observed smoothness of the Hα line implies alower limit of ~104-105 on the number of discreteclouds. This conclusively rules out the bloated-stars scenario,regardless of any plausible line-broadening mechanisms. The upper limiton the size of the clouds is ~1012 cm, which is comparable tothe size implied by photoionization models. This strongly suggests thatgas in the broad-line region is structured as a smooth rather than aclumped flow, most likely in a rotationally dominated thick disklikeconfiguration. However, it remains to be clarified why such a smooth,gravity-dominated flow generates double-peaked emission lines only in asmall fraction of active galactic nuclei.

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.

Kinematics of the Narrow-Line Region in the Seyfert 2 Galaxy NGC 1068: Dynamical Effects of the Radio Jet
We present a study of high-resolution long-slit spectra of thenarrow-line region (NLR) in NGC 1068 obtained with the Space TelescopeImaging Spectrograph aboard the Hubble Space Telescope (HST). Thespectra were retrieved from the Multimission Archive at the SpaceTelescope Science Institute obtained from two visits and seven orbits ofHST time. We also obtained MERLIN radio maps of the center of NGC 1068to examine the dependence of the NLR cloud velocities on the radiostructure. The radial velocities and velocity dispersions of the brightNLR clouds appear to be unaffected by the radio knots, indicating thatthe radio jet is not the principal driving force on the outflowing NLRclouds. However, the velocities of the fainter NLR clouds are split nearknots in the jet, indicating a possible interaction. Biconical outflowmodels were generated to match the data and for comparison to previousmodels done with lower dispersion observations. The general trend is anincrease in radial velocity roughly proportional to distance from thenucleus followed by a linear decrease after roughly 100 pc similar tothat seen in other Seyfert galaxies, indicating common acceleration anddeceleration mechanisms.Based on observations made with the NASA/ESA Hubble Space Telescope. TheSpace Telescope Science Institute is operated by the Association ofUniversities for Research in Astronomy, Inc., under NASA contractNAS5-26555.

Star Formation in Satellite Galaxies
We present narrowband observations of the Hα emission in a sampleof 31 satellites orbiting isolated giant spiral galaxies. The samplestudied spans the range -19 mag

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.

Spitzer IRS Spectra of a Large Sample of Seyfert Galaxies: A Variety of Infrared Spectral Energy Distributions in the Local Active Galactic Nucleus Population
We are conducting a large observing program with the Spitzer SpaceTelescope to determine the mid- to far-IR spectral energy distributionsof a well-defined sample of 87 nearby, 12 μm-selected Seyfertgalaxies. In this paper we present the results of Spitzer IRSlow-resolution spectroscopy of a statistically representative subsampleof 51 of the galaxies (59%), with an analysis of the continuum shapesand a comparison of the Seyfert types. We find that the spectra clearlydivide into groups based on their continuum shapes and spectralfeatures. The largest group (47% of the sample of 51) shows a very redcontinuum suggestive of cool dust and strong emission featuresattributed to polycyclic aromatic hydrocarbons. Sixteen objects (31%)have a power-law continuum with spectral indices of α5-20μm=-2.3 to -0.9 that flatten to α20-35μm=-1.1 to 0.0 at ~20 μm. Clear silicate emission featuresat 10 and 18 μm are found in two of these objects (Mrk 6 and Mrk335). A further 16% of the sample show power-law continua withunchanging slopes of α5-35 μm=-1.7 to -1.1. Twoobjects are dominated by a broad silicate absorption feature. One objectin the sample shows an unusual spectrum dominated by emission features,which is unlike any of the other spectra. Some spectral features areclearly related to a starburst contribution to the IR spectrum, whilethe mechanisms producing observed power-law continuum shapes, attributedto an active galactic nucleus (AGN) component, may be dust or nonthermalemission. The IR spectral types appear to be related to the Seyferttypes. Principal component analysis results suggest that the relativecontribution of starburst emission may be the dominant cause of variancein the observed spectra. The derived starburst component of eachspectrum, however, contributes <40% of the total flux density. Wecompare the IR emission with the optically thin radio emissionassociated with the AGN and find that Seyfert 1 galaxies have higherratios of IR to radio emission than Seyfert 2 galaxies, as predicted bythe unified model if the torus is optically thick in the mid-IR.However, smooth-density torus models predict a much larger differencebetween Seyfert types 1 and 2 than the factor of 2 difference observedin our sample; the observed factor of ~2 difference between the type 1and type 2 galaxies in their IR-to-radio ratios above 15 μm requiresthe standard smooth-density torus models to be optically thin at thesewavelengths. However, the resulting low torus opacity requires that thehigh observed columns detected in X-ray absorption be produced in gaswith a very low dust-to-gas ratio (perhaps within the dust sublimationregion). On the other hand, our observations may be consistent withclumpy torus models containing a steep radial distribution of opticallythick dense clumps. The selection of our sample at 12 μm, where thetorus may be optically thick, implies that there may beorientation-dependent biases in the sample; however, we do not find thatthe sample is biased toward Seyfert 2 galaxies with more luminouscentral engines, as would be expected. We find that the Seyfert 2galaxies typically show stronger starburst contributions than theSeyfert 1 galaxies in the sample, contrary to what is expected based onthe unified scheme for AGNs. This may be due to the selection effectthat only those Seyfert 2 galaxies with strong starburst contributionshad high enough integrated 12 μm flux densities to fall above theflux limit of the sample.

Spectral Statistics and Local Luminosity Function of a Complete Hard X-Ray Sample of the Brightest Active Galactic Nuclei
We have measured the X-ray spectral properties of a completeflux-limited sample of bright active galactic nuclei (AGNs) from theHEAO-1 all-sky catalogs to investigate their statistics and providegreater constraints on the bright end of the hard X-ray luminosityfunction (HXLF) and the AGN population synthesis model of the X-raybackground. Spectral studies using data from ASCA, XMM-Newton, and/orBeppoSAX observations have been made for almost all AGNs in this sample.The spectral measurements enable us to construct the neutral absorbingcolumn density (logNH) distribution and separate HXLFs forabsorbed (logNH[cm-2]>21.5) and unabsorbed AGNsin the local universe. Our results show evidence of a difference in theshapes of the HXLFs of absorbed and unabsorbed AGNs in that the absorbedAGN HXLF drops more rapidly at higher luminosities than that ofunabsorbed AGNs, similar to what has been previously reported. In theLX-NH plot we find no AGNs in the high-luminosity,high-intrinsic-absorption regime (logLX[ergss-1]>44.5 and logNH[cm-2]>21.5)in our sample, in which we expect approximately five AGNs if we assumethat the absorbed and unabsorbed AGNs have identical HXLF shapes. Wealso find that the fluxes observed with ASCA or XMM-Newton are smallerthan that observed with HEAO-1 by a factor of 0.29 on average, which isexpected for reobservation of sources with a factor of ~2.5 variabilityamplitude scale.

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.

Submit a new article

Related links

  • - No Links Found -
Submit a new link

Member of following groups:

Observation and Astrometry data

Constellation:Canes Venatici
Right ascension:12h10m32.70s
Aparent dimensions:6.166′ × 4.786′

Catalogs and designations:
Proper Names   (Edit)
NGC 2000.0NGC 4151

→ Request more catalogs and designations from VizieR