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|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.
|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.
|Molecular Outflows in Galaxy Merger Simulations with Embedded Active Galactic Nuclei|
We study the effects of feedback from active galactic nuclei (AGNs) onemission from molecular gas in galaxy mergers by combining hydrodynamicsimulations that include black holes with a three-dimensional, non-localthermodynamic equilibrium (LTE) radiative transfer code. We find thatmolecular clouds entrained in AGN winds produce an extended COmorphology with significant off-nuclear emission, which may bedetectable via contour mapping. Furthermore, kinematic signatures ofthese molecular outflows are visible in emission-line profiles when theoutflow has a large line-of-sight velocity. Our results can helpinterpret current and upcoming observations of luminous infraredgalaxies, as well as provide a detailed test of subresolutionprescriptions for supermassive black hole growth in galaxy-scalehydrodynamic simulations.
|The Low-z Intergalactic Medium. II. Lyβ, O VI, and C III Forest|
We present the results of a large survey of H I, O VI, and C IIIabsorption lines in the low-redshift (z<0.3) intergalactic medium(IGM). We begin with 171 strong Lyα absorption lines(Wλ>=80 mÅ) in 31 AGN sight lines studiedwith the Hubble Space Telescope and measure corresponding absorptionfrom higher order Lyman lines with FUSE. Higher order Lyman lines areused to determine NHI and bHI accurately through acurve-of-growth (COG) analysis. We find that the number of H I absorbersper column density bin is a power-law distribution,dNscr/dNHI~N-βHI, withβHI=1.68+/-0.11. We made 40 detections of O VIλλ1032, 1038 and 30 detections of C III λ977 out of129 and 148 potential absorbers, respectively. The column densitydistribution of C III absorbers has βCIII=1.68+/-0.04,similar to βHI but not as steep asβOVI=2.2+/-0.1. From the absorption-line frequency,dNscrCIII/dz=12+3-2 forWλ(C III)>30 mÅ, we calculate a typical IGMabsorber size r0~400 kpc, similar to scales derived by othermeans. The COG-derived b-values show that H I samples material withT<105 K, incompatible with a hot IGM phase. By calculatinga grid of CLOUDY models of IGM absorbers with a range of collisional andphotoionization parameters, we find it difficult to simultaneouslyaccount for the O VI and C III observations with a single phase.Instead, the observations require a multiphase IGM in which H I and CIII arise in photoionized regions, while O VI is produced primarilythrough shocks. From the multiphase ratioNHI/NCIII, we infer the IGM metallicity to beZC=0.12 Zsolar, similar to our previous estimateof ZO=0.09 Zsolar from O VI.
|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.
|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.
|Interstellar polarization and the position-angle orientations of type 1 Seyfert galaxies|
We comment on recent spectropolarimetric studies that compare theobserved polarization position angles (PAs) of type 1 Seyfert galaxiesnear Hα with the observed orientations of their radio-source axeson the sky. For a Seyfert galaxy in which scattering occurs mainly in anequatorial scattering region, the polarization PA is expected to beparallel to the radio axis, while for a case in which light scatterspredominantly in the polar regions, the Hα polarization PA shouldbe perpendicular to the radio axis. In practice, these correlations aredifficult to establish because a Galactic interstellar polarizationcontribution can introduce a significant uncertainty into thepolarization PA determination, even when the magnitude of interstellarpolarization is small. We show how such uncertainties may affect theanalysis of PA alignments, and present spectropolarimetric observationsof a probe star along the line of sight to the type 1 Seyfert galaxy Mrk871 that allow us to assess the intrinsic Hα polarization and PAof Mrk 871. These results suggest that spectropolarimetric observationsof such probe stars should form an integral part of future polarizationstudies of Seyfert galaxies.
|The relationship between X-ray variability amplitude and black hole mass in active galactic nuclei|
We have investigated the relationship between the X-ray variabilityamplitude and black hole mass for a sample of 46 radio-quiet activegalactic nuclei observed by ASCA. 33 of the objects in our sampleexhibited significant variability over a time-scale of ~40 ks. Wedetermined the normalized excess variance in the 2-10 keV light curvesof these objects and found a significant anticorrelation between excessvariance and black hole mass. Unlike most previous studies, we havequantified the variability using nearly the same time-scale for allobjects. Moreover, we provide a prescription for estimating theuncertainties in variance which accounts both for measurementuncertainties and for the stochastic nature of the variability. We alsopresent an analytical method to predict the excess variance from a modelpower spectrum accounting for binning, sampling and windowing effects.Using this, we modelled the variance-mass relation assuming all objectshave a universal twice-broken power spectrum, with the position of thebreaks being dependent on mass. This accounts for the general form ofthe variance-mass relationship but is formally a poor fit and there isconsiderable scatter. We investigated this scatter as a function of theX-ray photon index, luminosity and Eddington ratio. After accounting forthe primary dependence of excess variance on mass, we find nosignificant correlation with either luminosity or X-ray spectral slope.We do find an anticorrelation between excess variance and the Eddingtonratio, although this relation might be an artefact owing to theuncertainties in the mass measurements. It remains to be establishedthat enhanced X-ray variability is a property of objects with steepX-ray slopes or large Eddington ratios. Narrow-line Seyfert 1 galaxies,in particular, are consistent with being more variable than theirbroad-line counterparts solely because they tend to have smaller masses.
|Principal components in active galactic nuclei variability data and the estimation of the flux contributions from different components|
It has been found that the near-infrared flux variations of Seyfertgalaxies satisfy relations of the formFi~αij+βijFj,where Fi, Fj are the fluxes in filters i and j;and αi,j, βi,j are constants. Theserelations have been used to estimate the constant contributions of thenon-variable underlying galaxies. The paper attempts a formal treatmentof the estimation procedure, allowing for the possible presence of athird component, namely non-variable hot dust. In an analysis of asample of 38 Seyfert galaxies, inclusion of the hot dust componentimproves the model fit in approximately half the cases. All derived dusttemperatures are below 300 K, in the range 540-860 K or above 1300 K. Anoteworthy feature is the estimation of confidence intervals for thecomponent contributions: this is achieved by bootstrapping. It is alsopointed out that the model implies that such data could be fruitfullyanalysed in terms of principal components.
|Opacity Variations in the Ionized Absorption in NGC 3783: A Compact Absorber|
We show that the Fe VII-Fe XII M-shell unresolved transition array (UTA)in the Chandra HETGS observation of NGC 3783 (900 ks) clearly changes inopacity on a timescale of 31 days, responding to a factor of ~2 changein the ionizing continuum. The opacity variation is observed at a level>10 σ. There is also evidence for variability in the O VI Kedge (at ~3 σ). The observed changes are consistent with the gasproducing these absorption features (i.e., the low-ionization component)being close to photoionization equilibrium. The gas responsible for theFe XVII-Fe XXII L-shell absorption (i.e., the high-ionization component)does not seem to be responding as expected in photoionizationequilibrium. The observed change in opacity for the UTA implies adensity >1×104 cm-3, thus locating thegas within 6 pc of the X-ray source. The scenario in which the gas iscomposed of a continuous radial range of ionization structures is ruledout, as in such scenario, no opacity variations are expected. Rather,the structure of the absorber is likely composed of heavily clumped gas.
|The Ionized Nuclear Environment in NGC 985 as seen by Chandra and BeppoSAX|
We investigate the ionized environment of the Seyfert 1 galaxy NGC 985with a new Chandra HETGS observation and an archival BeppoSAXobservation. Both spectra exhibit strong residuals to a single-power-lawmodel, indicating the presence of an ionized absorber and a soft excess.A detailed model over the Chandra data shows that the 0.6-8 keVintrinsic continuum can be well represented by a power law (Γ~1.6)plus a blackbody component (kT=0.1 keV). Two absorption components areclearly required to fit the absorption features observed in the Chandraspectrum. The components have a difference of 29 in ionization parameterand 3 in column density. The presence of the low-ionization component isevidenced by an Fe M-shell unresolved transition array produced bycharge states VII-XIII. The high-ionization phase is required by thepresence of broad absorption features arising from several blends of FeL-shell transitions (Fe XVII-XXII). A third highly ionized componentmight also be present, but the data do not allow us to constrain itsproperties. Although poorly constrained, the outflow velocities of thecomponents (581+/-206 km s-1 for the high-ionization phaseand 197+/-184 km s-1 for the low-ionization one) areconsistent with each other and with the outflow velocities of theabsorption components observed in the UV. In addition, thelow-ionization component produces significant amounts of O VI, N V, andC IV, which suggests that a single outflow produces the UV and X-rayfeatures. The broadband (0.1-100 keV) continuum in the BeppoSAX data canbe parameterized by a power law (Γ~1.4), a blackbody (kT=0.1 keV),and a high-energy cutoff (Ec~70 keV). An X-ray luminosityvariation by a factor of 2.3 is observed between the BeppoSAX andChandra observations (separated by almost 3 yr). Variability in theopacity of the absorbers is detected in response to the continuumvariation, but while the colder component is consistent with a simplepicture of photoionization equilibrium, the ionization state of thehotter component seems to increase, while the continuum flux drops. Themost striking result in our analysis is that during both the Chandra andthe BeppoSAX observations, the two absorbing components appear to havethe same pressure. Thus, we suggest that the absorption arises from amultiphase wind. Such a scenario can explain the change in the opacityof both absorption components during the observations, but it requiresthat a third, hotter component be pressure-confining the two phases.Hence, our analysis points to a three-phase medium similar to the windfound in NGC 3783, and it further suggests that such a wind might be acommon characteristic in active galactic nuclei. The pressure-confiningscenario requires fragmentation of the confined phases into a largenumber of clouds.
|Probing Halos of Galaxies at Very Large Radii Using Background QSOs|
Gaseous halos of nine nearby galaxies (with redshifts cz<6000 kms-1) were probed at large galactocentric radii usingbackground quasars observed with the Hubble Space Telescope Goddard HighResolution Spectrograph and the Space Telescope Imaging Spectrograph.The projected quasar-galaxy separations range from 55 to 387h-175 kpc. Lyα absorption lines weresuccessfully detected in the spectra of five quasars, at impactparameters of up to ~170 h-175 kpc from the centerof the nearby galaxy, and in each case at wavelengths consistent withthe galaxy's redshift. Our observations include the lowest redshiftLyα lines detected to date. H I velocity fields were obtained atthe Very Large Array for three of the galaxies in our sample (in onecase the velocity field was available from the literature) to derivetheir rotation curves. When comparing the inner rotation curves of thegalaxies with the velocity at large radius provided by the Lyαline, it is apparent that it is very difficult to explain the observedLyα velocity as due to gas in an extended rotating disk. In mostcases, one would need to invoke large warps in the outer gas disks andalso thick gas disks to reconcile the observed velocities with thepredicted ones. Indeed, in one case, the Lyα line velocityindicates, in fact, counterrotation with respect to the inner diskrotation. In light of these results, we conclude that in a typicalgalaxy there is no longer detectable atomic gas corotating in anextended disk at radii greater than 35α-1, whereα-1 is the stellar disk exponential scale length. Thecosmic web is the most likely origin for the detected Lyα lines.Our observations confirm the recent Bowen et al. correlation ofequivalent widths with the local volume density of galaxies around thesight line, and the observed equivalent widths of the lines areconsistent with expectations of the cosmic web.Based on observations with the NASA ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS5-26555.
|Long-term infrared photometry of Seyferts|
Long-term (up to 10 000 d) monitoring has been undertaken for 41Seyferts in the near-infrared (1.25-3.45 μm). All but two showedvariability, with amplitudes at K in the range <0.1 to >1.1 mag.The time-scale for detectable change is from about one week to a fewyears.Where contemporary observations of variability in X-rays, ultraviolet(UV) or visible light exist, it is found that the near-infrared variesin a similar way, though in some cases the shorter-wavelength infrared(IR) bands are diluted by underlying galaxy radiation.A simple cross-correlation study indicates that there is evidence fordelays of up to several hundred d between the variations seen at theshortest wavelengths (U or J) and the longest (L) in many galaxies. Inparticular, the data for Fairall 9 now extend to twice the intervalcovered in earlier publications and the delay between its UV and IRoutputs is seen to persist.An analysis of the fluxes shows that, for any given galaxy, the coloursof the variable component of its nucleus are usually independent of thelevel of activity. The state of activity of the galaxy can beparameterized.Taken over the whole sample, the colours of the variable components fallwithin moderately narrow ranges. In particular, the H-K colour isappropriate to a blackbody of temperature 1600 K. The H-K excess for aheavily reddened nucleus can be determined and used to findEB-V, which can be compared to the values found from thevisible region broad line ratios.Using flux-flux diagrams, the flux within the aperture from theunderlying galaxies can often be determined without the need for modelsurface brightness profiles. In many galaxies it is apparent that theremust be an additional constant contribution from warm dust.
|Seyferts on the edge: polar scattering and orientation-dependent polarization in Seyfert 1 nuclei|
We have identified 12 Seyfert 1 galaxies that exhibit opticalpolarization spectra similar to those of Seyfert 2 galaxies in whichpolarized broad lines are detected. We present new spectropolarimetricobservations of three of them: Was 45, Mrk 231 and NGC 3227. Theseobjects appear to be polarized as a result of far-field scattering inthe polar illumination cones of the circumnuclear torus. We estimatethat they represent between 10 and 30 per cent of the Seyfert 1population; they are found amongst all the main spectroscopic subtypes,including narrow-line Seyfert 1 galaxies. We have shown elsewhere thatSeyfert 1 nuclei more commonly have polarization characteristics thatcan be attributed to scattering by a compact `equatorial' scatteringregion located inside the torus. We propose that both equatorial andpolar scattering regions are present in all Seyfert galaxies and arguethat the observed range of polarization properties can be broadlyunderstood as an orientation effect. In this scheme, polar-scatteredSeyfert 1 galaxies represent the transition between unobscured (themajority of type 1) and obscured (type 2) Seyferts. They are viewedthrough the upper layers of the torus and are thus subject to moderateextinction (AV~ 1-4 mag) sufficient to suppress polarizedlight from the equatorial scattering region, but not the broad wings ofthe Balmer lines. The orientation of the polarization position anglerelative to the radio source is broadly consistent with thetwo-component scattering model. More generally, we find that amongstSeyfert 1 galaxies, parallel, perpendicular and intermediateorientations of the polarization position angle relative to the radioaxis occur roughly in the proportions 2:1:1.
|The Local Lyα Forest. IV. Space Telescope Imaging Spectrograph G140M Spectra and Results on the Distribution and Baryon Content of H I Absorbers|
We present Hubble Space Telescope (HST) Space Telescope ImagingSpectrograph (STIS) G140M spectra of 15 extragalactic targets, which wecombine with GHRS/G160M data to examine the statistical properties ofthe low-z Lyα forest. With STIS, we detect 109 Lyα absorbersat significance level (SL) of >=4 σ over 0.002=4σ overΔz=1.157. We evaluate the physical properties of these Lyαabsorbers and compare them to their high-z counterparts. Using twodifferent models for Lyα forest absorbers, we determine that thewarm, photoionized intergalactic medium contains 29%+/-4% of the totalbaryon inventory at z=0 (assumingJ0=1.3×10-23 ergs cm-2s-1 Hz-1 sr-1). We derive thedistribution in column density, N-1.65+/-0.07HIfor 12.5<=log[NHI(cm-2)]<=14.5, breaking toa flatter slope above log[NHI]~14.5. As with the highequivalent width (W>240mÅ) absorbers, the number densityof low-W absorbers at z=0 is well above the extrapolation ofdN/dz from z>2. However,log[(dN/dz)z=0]=1.40+/-0.08 for W>240mÅ is25% below the value obtained by the HST QSO Key Project, a differencethat may arise from line blending. The slowing of the number densityevolution of high-W Lyα clouds is not as great as previouslymeasured, and the break to slower evolution may occur later thanpreviously suggested (z~1.0 rather than 1.6). We find a 7.2 σexcess in the two-point correlation function (TPCF) of Lyαabsorbers for velocity separations Δv<=260kms-1,which is exclusively due to the higher column density clouds. From ourprevious result that higher column density Lyα clouds cluster morestrongly with galaxies, this TPCF suggests a physical difference betweenthe higher and lower column density clouds in our sample. The systematicerror produced by cosmic variance on these results increases the totalerrors on derived quantities by ~sqrt(2).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.
|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.
|C II Radiative Cooling of the Diffuse Gas in the Milky Way|
The heating and cooling of the interstellar medium (ISM) allow the gasin the ISM to coexist at very different temperatures in thermal pressureequilibrium. The rate at which the gas cools or heats is therefore afundamental ingredient for any theory of the ISM. The heating cannot bedirectly determined, but the cooling can be inferred from observationsof .CII*, which is an important coolant in differentenvironments. The amount of cooling can be measured through either theintensity of the 157.7 μm [C II] emission line or the CII*absorption lines at 1037.018 and 1335.708 Å, observable with theFar Ultraviolet Spectroscopic Explorer and the Space Telescope ImagingSpectrograph on board the Hubble Space Telescope, respectively. Wepresent the results of a survey of these far-UV absorption lines in 43objects situated at |b|>~30deg. Measured column densitiesof CII*, S II, P II, and Fe II are combined with H I 21 cmemission measurements to derive the cooling rates (per H atom using H Iand per nucleon using S II) and to analyze the ionization structure,depletion, and metallicity content of the low-, intermediate-, andhigh-velocity clouds (LVCs, IVCs, and HVCs) along the different sightlines. Based on the depletion and the ionization structure, the LVCs,IVCs, and HVCs consist mostly of warm neutral and ionized clouds. Forthe LVCs, the mean cooling rate in ergs s-1 per H atom is-25.70+0.19-0.36 dex (1 σ dispersion). Witha smaller sample and a bias toward high H I column density, the coolingrate per nucleon is similar. The corresponding total Galactic C IIluminosity in the 157.7 μm emission line isL~2.6×107 Lsolar. CombiningN(CII*) with the intensity of Hα emission, we derivethat ~50% of the CII* radiative cooling comes from the warmionized medium (WIM). The large dispersion in the cooling rates iscertainly due to a combination of differences in the ionizationfraction, in the dust-to-gas fraction, and physical conditions betweensight lines. For the IVC Intermediate-Velocity (IV) Arch at z~1 kpc wefind that on average the cooling is a factor of 2 lower than in the LVCsthat probe gas at lower z. For an HVC (complex C, at z>6 kpc) we findthe much lower rate of -26.99+0.21-0.53 dex,similar to the rates observed in a sample of damped Lyα absorbersystems (DLAs). The fact that in the Milky Way a substantial fraction ofthe C II cooling comes from the WIM implies that this is probably alsotrue in the DLAs. We also derive the electron density, assuming atypical temperature of the warm gas of 6000 K: for the LVCs,=0.08+/-0.04 cm-3, and for the IV Arch,=0.03+/-0.01 cm-3 (1 σdispersion). Finally, we measured the column densities N(S II) and N(PII) in many sight lines and confirm that sulphur appears undepleted inthe ISM. Phosphorus becomes progressively more deficient whenlogN(HI)>19.7 dex, which can mean that either P becomes more depletedinto dust as more neutral gas is present or P is always depleted byabout -0.3 dex, but the higher value of P II at lower H I column densityindicates the need for an ionization correction.
|A Composite Extreme-Ultraviolet QSO Spectrum from FUSE|
The Far Ultraviolet Spectroscopic Explorer (FUSE) has surveyed a largesample (>100) of active galactic nuclei (AGNs) in the low-redshiftuniverse (z<1). Its response at short wavelengths makes it possibleto measure directly the far-ultraviolet spectral properties ofquasi-stellar objects (QSOs) and Seyfert 1 galaxies at z<0.3. Usingarchival FUSE spectra, we form a composite extreme-ultraviolet (EUV)spectrum of QSOs at z<=0.67. After consideration of many possiblesources of systematic error in our analysis, we find that the spectralslope of the FUSE composite spectrum,α=-0.56+0.38-0.28 forFν~να, is significantly harder thanthe EUV (λ<~1200 Å) portion of the composite spectrum ofQSOs with z>0.33 formed from archival Hubble Space Telescope (HST)spectra, α=-1.76+/-0.12. We identify several prominent emissionlines in the FUSE composite and find that the high-ionization O VI andNe VIII emission lines are enhanced relative to the HST composite.Power-law continuum fits to the individual FUSE AGN spectra reveal acorrelation between EUV spectral slope and AGN luminosity in the FUSEand FUSE+HST samples, in the sense that lower luminosity AGNs showharder spectral slopes. We find an anticorrelation between the hardnessof the EUV spectral slope and AGN black hole mass, using estimates ofthis quantity found in the literature. We interpret these results in thecontext of the well-known anticorrelation between AGN luminosity andemission-line strength, the Baldwin effect, given that the medianluminosity of the FUSE AGN sample is an order of magnitude lower thanthat of the HST sample.
|O VI, N V, and C IV in the Galactic Halo. II. Velocity-Resolved Observations with the Hubble Space Telescope and Far Ultraviolet Spectroscopic Explorer|
We present a survey of N V and O VI (and, where available, C IV) in theGalactic halo, using data from the Far Ultraviolet SpectroscopicExplorer (FUSE) and the Hubble Space Telescope (HST) along 34 sightlines. These ions are usually produced in nonequilibrium processes suchas shocks, evaporative interfaces, or rapidly cooling gas, and thustrace the dynamics of the interstellar medium. Searching for globaltrends in integrated and velocity-resolved column density ratios, wefind large variations in most measures, with some evidence for asystematic trend of higher ionization (lower N V/O VI column densityratio) at larger positive line-of-sight velocities. The slopes oflog[N(N V)/N(O VI)] per unit velocity range from -0.015 to +0.005, witha mean of -0.0032+/-0.0022(r)+/-0.0014(sys) dex (kms-1)-1. We compare this data set with models ofvelocity-resolved high-ion signatures of several common physicalstructures. The dispersion of the ratios, O VI/N V/C IV, supports thegrowing belief that no single model can account for hot halo gas, and infact some models predict much stronger trends than are observed. It isimportant to understand the signatures of different physical structuresto interpret specific lines of sight and future global surveys.
|A Complete Sample of Soft X-Ray-Selected AGNs. I. The Data|
We present the optical spectra and simple statistical analysis for acomplete sample of 110 soft X-ray-selected AGNs. About half of thesources are narrow-line Seyfert 1 galaxies (NLS1's), which have thesteepest X-ray spectra, the strongest Fe II emission, and slightlyweaker [O III] λ5007 emission than broad-line Seyfert 1's(BLS1's). Kolmogorov-Smirnov tests show that NLS1's and BLS1's haveclearly different distributions of the X-ray spectral slopeαX, X-ray short-term variability, and Fe II equivalentwidths and luminosity and Fe II/Hβ ratios. The differences in the[O III]/Hβ and [O III] equivalent widths are only marginal. Wefound no significant differences between NLS1's and BLS1's in theirrest-frame 0.2-2.0 X-ray luminosities, rest-frame 5100 monochromaticluminosities, bolometric luminosities, redshifts, and their Hβequivalent widths.Based in part on observations at the European Southern Observatory, LaSilla (Chile), with the 2.2 m telescope of the Max Planck Society duringMPI and ESO time, and the ESO 1.52 m telescope during ESO time in 1995September and 1999 September.
|Radio emission from AGN detected by the VLA FIRST survey|
Using the most recent (April 2003) version of the VLA FIRST survey radiocatalog, we have searched for radio emission from >2800 AGN takenfrom the most recent (2001) version of the Veron-Cetty and Veron AGNcatalog. These AGN lie in the 9033 square degrees of sky alreadycovered by the VLA FIRST survey. Our work has resulted in positivedetection of radio emission from 775 AGN of which 214 are new detectionsat radio wavelengths.Tables 3 and 4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/35
|Mass Loss from the Nuclei of Active Galaxies|
Blueshifted absorption lines in the UV and X-ray spectra of activegalaxies reveal the presence of massive outflows of ionized gas fromtheir nuclei. The "intrinsic" UV and X-ray absorbers show large globalcovering factors of the central continuum source, and the inferred massloss rates are comparable to the mass accretion rates. Many absorbersshow variable ionic column densities, which are attributed to acombination of variable ionizing flux and motion of gas into and out ofthe line of sight. Detailed studies of the intrinsic absorbers, with theassistance of monitoring observations and photoionization models,provide constraints on their kinematics, physical conditions, andlocations relative to the central continuum source, which range from theinner nucleus (0.01 pc) tothe galactic disk or halo (10 kpc).Dynamical models that make use of thermal winds, radiation pressure,and/or hydromagnetic flows have reached a level of sophistication thatpermits comparisons with the observational constraints.
|An Optical Spectroscopic Atlas of Low-Redshift Active Galactic Nuclei|
We present a spectral atlas of the Hβ region for 215 type 1 AGNs(luminous Seyfert 1/radio galaxy nuclei and low-z quasars) up to z~0.8.Line profiles and measures were derived from the database ofintermediate resolution spectra (R>~1000) with average continuumlevel S/N ratio ~30. Parameters including rest frame equivalent widthand FWHM are provided for the Fe IIopt blend at λ4570,Hβ, He II λ4686, and the [O III] λλ4959, 5007emission lines. We extract clean broad component Hβ profiles andprovide wavelength measurements at 0, 1/4, 1/2, 3/4, and 0.9 peakintensity levels in order to permit a quantitative definition of theHβ broad component for statistical studies. We also discuss sourcesof uncertainty, selection effects, and biases in our sample. The dataare especially important for tests of the eigenvector 1 parameter spaceoccupation and correlation. We show that the I Zw 1 template FeIIopt spectrum reproduces well the observed FeIIopt emission for a wide range of line width and strength. Adetailed analysis of the data within the eigenvector 1 context isdeferred to a companion paper.Based in part on data collected at ESO La Silla.
|The Seyfert Population in the Local Universe|
The magnitude-limited catalog of the Southern Sky Redshift Survey(SSRS2) is used to characterize the properties of galaxies hostingactive galactic nuclei (AGNs). Using emission-line ratios, we identify atotal of 162 (3%) Seyfert galaxies out of the parent sample with 5399galaxies. The sample contains 121 Seyfert 2 galaxies and 41 Seyfert 1galaxies. The SSRS2 Seyfert galaxies are predominantly in spirals oftypes Sb and earlier or in galaxies with perturbed appearance as theresult of strong interactions or mergers. Seyfert galaxies in thissample are twice as common in barred hosts as the non-Seyfert galaxies.By assigning galaxies to groups using a percolation algorithm, we findthat the Seyfert galaxies in the SSRS2 are more likely to be found inbinary systems when compared with galaxies in the SSRS2 parent sample.However, there is no statistically significant difference between theSeyfert and SSRS2 parent sample when systems with more than two galaxiesare considered. The analysis of the present sample suggests that thereis a stronger correlation between the presence of the AGN phenomenonwith internal properties of galaxies (morphology, presence of bar,luminosity) than with environmental effects (local galaxy density, groupvelocity dispersion, nearest neighbor distance).Partly based on observations at European Southern Observatory (ESO),under the ESO-ON agreement to operate the 1.52 m telescope.
|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 (188.8.131.52) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?/A+A/411/L59
|The intrinsic emission of Seyfert galaxies observed with BeppoSAX/PDS. I. Comparison of the average spectra of the three classes of Seyfert galaxies|
We present a study of the hard X-ray spectrum (>15 keV) of differentclasses of Seyfert galaxies observed with BeppoSAX/PDS. Using hard X-raydata, we avoid absorption effects modifying the Seyfert emission andhave direct access to the central engine of these sources. The aim ofthis study is first to characterize the general properties of the hardX-ray spectrum of Seyfert 1, 1.5 and 2 galaxies and secondly to comparetheir intrinsic emission to test unified models according to which allthe classes have the same nucleus.\ We compute the average spectrum of14 Sy 1, 9 Sy 1.5 and 22 Sy 2 galaxies observed by the PDS (15-136 keV).The average spectrum of Sy 1 differs from that of Sy 2, the firstrequiring the presence of a high energy cutoff which is absent in thesecond. We also show that the reflection component is possibly moreimportant in the Sy 2 emission. The nature of Sy 1.5 galaxies isambiguous as they have a negligible reflection component (like Sy 1) anddo not require a cutoff (like Sy 2).
|Hubble Space Telescope Imaging of the Poststarburst Quasar UN J1025-0040: Evidence for Recent Star Formation|
We present new Hubble Space Telescope (HST) Wide Field Planetary Cameraimages of the poststarburst quasar UN J1025-0040, which contains both anactive galactic nucleus (AGN) and a 400 Myr old nuclear starburst ofsimilar bolometric luminosity (~1011.6 Lsolar).The F450W and F814W images resolve the AGN from the starburst and showthat the bulk of the starlight (~6×1010Msolar) is contained within a central radius of about 600 pcand lacks clear morphological structures at this scale. Equating thepoint-source light in each image with the AGN contribution, wedetermined the ratio of AGN-to-starburst light. This ratio is 69% in thered F814W image, consistent with our previous spectral analysis, but<=50% in the blue F450W image, whereas we had predicted 76%. The HSTimages are consistent with previous photometry, ruling out variability(a fading AGN) as a cause for this result. We can explain the new dataif there is a previously unknown young stellar population present, 40Myr or younger, with as much as 10% of the mass of the dominant 400 Myrold population. This younger starburst may represent the trigger for thecurrent nuclear activity. The multiple starburst ages seen in UNJ1025-0040 and its companion galaxy indicate a complex interaction andstar formation history.
|JHK' Imaging Photometry of Seyfert 1 Active Galactic Nuclei and Quasars. I. Multiaperture Photometry|
Near-infrared JHK' imaging photometry was obtained of 331 AGNsconsisting mainly of Seyfert 1 active galactic nuclei (AGNs) and quasars(QSOs). This sample was selected to cover a range of radio emissionstrength, redshift from z=0 to 1, and absolute B magnitude fromMB=-29 mag to -18 mag. Among low-z AGNs with z<0.3,Seyfert 1-1.5 AGNs are distributed over a region from a location typicalof ``galaxies'' to a location typical of ``QSOs'' in the two-color J-Hto H-K' diagram, but Seyfert 1.8-2 AGNs are distributed around thelocation of ``galaxies.'' Moreover, bright AGNs with respect to absoluteB magnitude are distributed near the location of ``QSOs,'' while faintAGNs are near the location of ``galaxies.'' The distribution of suchlow-z AGNs in this diagram was found to have little dependence on their6 cm radio flux. The near-infrared colors of the AGNs observed with anaperture of 7 pixels (7.49") are more QSO-like than those observed withlarger apertures up to 15 pixels (16.1"). This aperture effect may beexplained by contamination from the light of host galaxies within largerapertures. This effect is more prominent for less luminous AGNs.
|Lyα Absorption around Nearby Galaxies|
We have used the Space Telescope Imaging Spectrograph (STIS) aboard theHubble Space Telescope to search for Lyα absorption lines in theouter regions of eight nearby galaxies using background quasars and AGNsas probes. Lyα lines are detected within a few hundred kilometersper second of the systemic velocity of the galaxy in all cases. Weconclude that a background line of sight that passes within 26-200h-1 kpc of a foreground galaxy is likely to intercept lowcolumn density neutral hydrogen with logN(HI)>~13.0. The ubiquity ofdetections implies a covering factor of ~=100% for low N(H I) gas aroundgalaxies within 200 h-1 kpc. We find, however, that theLyα lines are usually composed of individual components spread outin velocity over ranges of 300-900 km s-1. Two sight linesshow components that are unusually broad for low-redshift Lyαsystems, with Doppler parameters ~150 km s-1. These may arisein intragroup gas at temperatures of (1-2)×106 K. Wediscuss the difficulty in trying to associate individual absorptioncomponents with the selected galaxies and their neighbors but show thatby degrading our STIS data to lower resolutions, we are able toreproduce the anticorrelation of Lyα equivalent width and impactparameter found at higher redshift. The anticorrelation does not improveby correcting for the absolute magnitude of a galaxy in the same way asfound at higher z. We also show that the equivalent width and columndensity of Lyα complexes (when individual components are summedover ~1000 km s-1) correlate well with a simple estimate ofthe volume density of galaxies brighter than MB=-17.5 at thesame redshift as a Lyα complex. We do not reject the hypothesisthat the selected galaxies are directly responsible for the observedLyα lines, but our analysis indicates that absorption by clumpyintragroup gas is an equally likely explanation.
|X-Ray versus Optical Observations of Active Galactic Nuclei: Evidence for Large Grains?|
Recently, Maiolino et al. constructed a sample of active galactic nucleifor which both the reddening E(B-V) and the column density NHto the nucleus could be determined. For most of the galaxies in theirsample, they found that E(B-V)/NH is substantially smallerthan for the diffuse interstellar medium of our Galaxy. They assertedthat either the dust-to-gas ratio is lower than in the Galaxy or thegrains are so large that they do not extinct or redden efficiently inthe optical. We show that there is no systematic increase in E(B-V) withNH for the Maiolino et al. galaxies, which suggests that theX-ray absorption and optical extinction occur in distinct media. In alater paper, Maiolino et al. suggested that the observed lines of sightfor the previous Maiolino et al. galaxies pass through the ``torus''that obscures the broad-line region and nuclear continuum in Seyfert 2galaxies and argued that the torus grains are larger than Galacticgrains. There is no reason to believe that the lines of sight for thesegalaxies pass through the torus, since the observed column densities arelower than those typically observed in Seyfert 2 galaxies. We suggestinstead that the X-ray absorption occurs in material located off thetorus and/or accretion disk, while the optical extinction occurs inmaterial located beyond the torus. The X-ray absorbing material couldeither be dust-free or contain large grains that do not extinctefficiently in the optical. There is no conclusive evidence that thegrains in active galactic nuclei are systematically larger than those inthe diffuse interstellar medium of our Galaxy. We discuss an alternativeway to probe the properties of dust in Seyfert tori but find thatobservations of Seyfert 2 nuclei with higher resolution than currentlyavailable will be needed in order to place stringent limits on the dust.
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