Ýçindekiler
Görüntüler
Resim Yükleyin
DSS Images Other Images
Ýlgili Makaleler
RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Programs for laser-AO assisted integral-field spectrometers on ionized flows
An AO-assisted integral-field spectrograph is becoming the mostefficient tool with which to explore ionized gas outflows. It maps faintspectral lines that diagnose cloud dust content, gas pressure,excitation mechanism, and chemical abundances. Coupled with recentimprovements in photoionization models, the total mass hence flowenergetics can be estimated. Establishing a consistent dynamicalframework requires linking multi-frequency datasets to track the energyflow through its optimal-contrast emission in the various ISM phases. Ishow HST results on AGN, starburst nuclei, and Galactic Herbig-HaroObjects that need complementary 3D spectra at comparable spatialresolution to come soon from laser-guided AO + integral-fieldspectrographs at the William-Herschel and SOAR telescopes.
| Why the fundamental plane of black hole activity is not simply a distance driven artifact
The fundamental plane of black hole activity is a non-linear correlationamong radio core luminosity, X-ray luminosity and mass of all accretingblack holes, both of stellar mass and supermassive, found by Merloni etal. [Merloni, A., Heinz, S., di Matteo, T. 2003. MNRAS 345, 1057(MHD03)] and, independently, by Falcke et al. [Falcke, H., Körding,E., Markoff, S., 2004. A& A 414, 895]. Here we further examine anumber of statistical issues related to this correlation. In particular,we discuss the issue of sample selection and quantify the biasintroduced by the effect of distance in two of the correlatedquantities. We demonstrate that the fundamental plane relation cannot bea distance artifact, and that its non-linearity must represent anintrinsic characteristic of accreting black holes. We also discusspossible future observational strategies to improve our understanding ofthis correlation.
| Scalar potential model of redshift and discrete redshift
On the galactic scale the universe is inhomogeneous and redshift z isoccasionally less than zero. A scalar potential model (SPM) that linksthe galaxy scale z to the cosmological scale z of the Hubble Law ispostulated. Several differences among galaxy types suggest that spiralgalaxies are Sources and that early type, lenticular, and irregulargalaxies are Sinks of a scalar potential field. The morphology-radiusand the intragalactic medium cluster observations support the movementof matter from Source galaxies to Sink galaxies. A cell structure ofgalaxy groups and clusters is proposed to resolve a paradox concerningthe scalar potential like the Olber’s paradox concerning light.For the sample galaxies, the ratio of the luminosity of Source galaxiesto the luminosity of Sink galaxies approaches 2.7 ± 0.1. Anequation is derived from sample data, which is anisotropic andinhomogeneous, relating z of and the distance D to galaxies. Thecalculated z has a correlation coefficient of 0.88 with the measured zfor a sample of 32 spiral galaxies with D calculated using Cepheidvariable stars. The equation is consistent with z < 0 observations ofclose galaxies. At low cosmological distances, the equation reduces to z≈ exp(KD)‑1 ≈ KD, where K is a constant, positive value. Theequation predicts z from galaxies over 18 Gpc distant approaches aconstant value on the order of 500. The SPM of z provides a physicalbasis for the z of particle photons. Further, the SPM qualitativelysuggests the discrete variations in z, which was reported by Tifft[Tifft, W.G., 1997. Astrophy. J. 485, 465] and confirmed by others, areconsistent with the SPM.
| An ultraluminous X-ray microquasar in NGC5408?
We studied the radio source associated with the ultraluminous X-raysource in NGC5408 (LX ~ 1040ergs-1).The radio spectrum is steep (index ~ -1), consistent with optically thinsynchrotron emission, not with flat-spectrum core emission. Its fluxdensity (~0.28 mJy at 4.8 GHz, at a distance of 4.8 Mpc) was the same inthe March 2000 and December 2004 observations, suggesting steadyemission rather than a transient outburst. However, it is orders ofmagnitude higher than expected from steady jets in stellar-massmicroquasar. Based on its radio flux and spectral index, we suggest thatthe radio source is either an unusually bright supernova remnant, or,more likely, a radio lobe powered by a jet from the black hole (BH).Moreover, there is speculative evidence that the source is marginallyresolved with a radius ~30 pc. A faint HII region of similar sizeappears to coincide with the radio and X-ray sources, but its ionizationmechanism remains unclear. Using a self-similar solution for theexpansion of a jet-powered electron-positron plasma bubble, in theminimum-energy approximation, we show that the observed flux and(speculative) size are consistent with an average jet power ~ 7 ×1038ergs-1 ~ 0.1LX ~0.1LEdd, an age ~105 yr, a current velocity ofexpansion ~80 km s-1. We briefly discuss the importance ofthis source as a key to understand the balance between luminosity andjet power in accreting BHs.
| The evolution of misaligned accretion discs and spinning black holes
In this paper, we consider the process of alignment of a spinning blackhole and a surrounding misaligned accretion disc. We use a simplifiedset of equations, that describe the evolution of the system in the casewhere the propagation of warping disturbances in the accretion discoccurs diffusively, a situation likely to be common in the thin discs inactive galactic nuclei (AGN). We also allow the direction of the holespin to move under the action of the disc torques. In such a way, theevolution of the hole-disc system is computed self-consistently. Weconsider a number of different situations and we explore the relevantparameter range, by varying the location of the warp radiusRw and the propagation speed of the warp. We find that thedissipation associated with the twisting of the disc results in a largeincrease in the accretion rate through the disc, so that AGN accretingfrom a misaligned disc are likely to be significantly more luminous thanthose accreting from a flat disc. We compute explicitly the time-scalesfor the warping of the disc and for the alignment process and compareour results with earlier estimates based on simplified steady-statesolutions. We also confirm earlier predictions that, under appropriatecircumstances, accretion can proceed in a counter-aligned fashion, sothat the accreted material will spin-down the hole, rather than spinningit up. Our results have implication in a number of differentobservational features of AGN such as the orientation and shape of jets,the shape of X-ray iron lines and the possibility of obscuration andabsorption of X-ray by the outer disc as well as the general issue ofthe spin history of black holes during their growth.
| Constraints on alternatives to supermassive black holes
Observations of the centres of galaxies continue to evolve, and it isuseful to take a fresh look at the constraints that exist onalternatives to supermassive black holes at their centres. We discussconstraints complementary to those of Maoz and demonstrate that anextremely wide range of other possibilities can be excluded. Inparticular, we present the new argument that for the velocitydispersions inferred for many galactic nuclei, even binaries made ofpoint masses cannot stave off core collapse because hard binaries are sotight that they merge via emission of gravitational radiation beforethey can engage in three-body or four-body interactions. We also showthat under these conditions core collapse leads inevitably to runawaygrowth of a central black hole with a significant fraction of theinitial mass, regardless of the masses of the individual stars. Forclusters of non-interacting low-mass objects (from low-mass stars toelementary particles), the relaxation of stars and compact objects thatpass inside the dark region will be accelerated by interactions with thedark mass. If the dark region is instead a self-supported object, suchas a fermion ball, then if stellar-mass black holes exist they willcollide with the object, settle, and consume it. The net result is thatthe keyhole through which alternatives to supermassive black holes mustpass is substantially smaller and more contrived than it was even a fewyears ago.
| Non-linear bending waves in Keplerian accretion discs
The non-linear dynamics of a warped accretion disc is investigated inthe important case of a thin Keplerian disc with negligible viscosityand self-gravity. A one-dimensional evolutionary equation is formallyderived that describes the primary non-linear and dispersive effects onpropagating bending waves other than parametric instabilities. It hasthe form of a derivative non-linear Schrödinger (DNLS) equationwith coefficients that are obtained explicitly for a particular model ofa disc. The properties of this equation are analysed in some detail andillustrative numerical solutions are presented. The non-linear anddispersive effects both depend on the compressibility of the gas throughits adiabatic index Γ. In the physically realistic case Γ< 3, non-linearity does not lead to the steepening of bending wavesbut instead enhances their linear dispersion. In the opposite caseΓ > 3, non-linearity leads to wave steepening and solitarywaves are supported. The effects of a small effective viscosity, whichmay suppress parametric instabilities, are also considered. Thisanalysis may provide a useful point of comparison between theory andnumerical simulations of warped accretion discs.
| Scale Heights of Non-Edge-on Spiral Galaxies
We present a method of calculating the scale height of non-edge-onspiral galaxies, together with a formula for errors. The method is basedon solving Poisson's equation for a logarithmic disturbance of matterdensity in spiral galaxies. We show that the spiral arms can not extendto inside the ``forbidden radius'' r0, due to the effect ofthe finite thickness of the disk. The method is tested by re-calculatingthe scale heights of 71 northern spiral galaxies previously calculatedby Ma, Peng & Gu. Our results differ from theirs by less than 9%. Wealso present the scale heights of a further 23 non-edge-on spiralgalaxies.
| Cepheid Distances to SNe Ia Host Galaxies Based on a Revised Photometric Zero Point of the HST WFPC2 and New PL Relations and Metallicity Corrections
With this paper we continue the preparation for a forthcoming summaryreport of our experiment with the HST to determine the Hubble constantusing Type Ia supernovae as standard candles. Two problems areaddressed. (1) We examine the need for, and determine the value of, thecorrections to the apparent magnitudes of our program Cepheids in the 11previous calibration papers due to sensitivity drifts and chargetransfer effects of the HST WFPC2 camera over the life time of theexperiment from 1992 to 2001. (2) The corrected apparent magnitudes areapplied to all our previous photometric data from which revised distancemoduli are calculated for the eight program galaxies that are parents tothe calibrator Ia supernovae. Two different Cepheid P-L relations areused; one for the Galaxy and one for the LMC. These differ both in slopeand zero point at a fixed period. The procedures for determining theabsorption and reddening corrections for each Cepheid are discussed.Corrections for the effects of metallicity differences between theprogram galaxies and the two adopted P-L relations are derived andapplied. The distance moduli derived here for the eight supernovaeprogram galaxies, and for 29 others, average 0.20 mag fainter (moredistant) than those derived by Gibson et al. and Freedman et al. intheir 2000 and 2001 summary papers for reasons discussed in this paper.The effect on the Hubble constant is the subject of our forthcomingsummary paper.
| The Star-forming Torus and Stellar Dynamical Black Hole Mass in the Seyfert 1 Nucleus of NGC 3227
We report R~4300 VLT SINFONI adaptive optics integral field K-bandspectroscopy of the nucleus of the Seyfert 1 galaxy NGC 3227 at aspatial resolution of 0.085" (7 pc). We present the morphologies andkinematics of emission lines and absorption features and give the firstderivation of a black hole mass in a Seyfert 1 nucleus from stellardynamics (marginally resolving the black hole's sphere of influence). Weshow that the gas in the nucleus has a mean column density of order1024 cm-2 and that it is geometrically thick, inagreement with the standard ``molecular torus'' scenario. We discusspossible heating processes responsible for maintaining the verticalheight of the torus. We also resolve the nuclear stellar distributionand find that within a few parsecs of the AGN there has been an intensestarburst, the most recent episode of which began ~40 Myr ago but hasnow ceased. The current luminosity of stars within 30 pc of the AGN,~3×109 Lsolar, is comparable to that of theAGN. We argue that the star formation has been occurring in theobscuring material. Finally, we apply Schwarzschild orbit superpositionmodels to our full two-dimensional data and derive the mass of the blackhole, paying careful attention to the input parameters, which are oftenuncertain. Our models yield a 1 σ range for the black hole mass ofMBH=7×106-2×107Msolar.Based on observations at the European Southern Observatory VLT(074.B-9012).
| Cosmic Evolution of Black Holes and Spheroids. I. The MBH-σ Relation at z = 0.36
We test the evolution of the correlation between black hole mass andbulge velocity dispersion (MBH-σ), using a carefullyselected sample of 14 Seyfert 1 galaxies at z=0.36+/-0.01. We measurevelocity dispersion from stellar absorption lines around Mg b (5175Å) and Fe (5270 Å) using high-S/N Keck spectra and estimateblack hole mass from the Hβ line width and the optical luminosityat 5100 Å, based on the empirically calibrated photoionizationmethod. We find a significant offset from the local relation, in thesense that velocity dispersions were smaller for given black hole massesat z=0.36 than locally. We investigate various sources of systematicuncertainties and find that those cannot account for the observedoffset. The measured offset isΔlogMBH=0.62+/-0.10+/-0.25 i.e.,Δlogσ=0.15+/-0.03+/-0.06, where the error bars include arandom component and an upper limit to the systematics. At face value,this result implies a substantial growth of bulges in the last 4 Gyr,assuming that the local MBH-σ relation is the universalevolutionary endpoint. Along with two samples of active galaxies withconsistently determined black hole mass and stellar velocity dispersiontaken from the literature, we quantify the observed evolution with thebest-fit linear relation:ΔlogMBH=(1.66+/-0.43)z+(0.04+/-0.09) with respect tothe local relationship of Tremaine and coworkers, andΔlogMBH=(1.55+/-0.46)z+(0.01+/-0.12) with respect tothat of Ferrarese. This result is consistent with the growth of blackholes predating the final growth of bulges at these mass scales(<σ>=170 km s-1).
| Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
We estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion.
| A Galactic Origin for the Local Ionized X-Ray Absorbers
Recent Chandra and XMM-Newton observations of distant quasars have shownstrong local (z~0) X-ray absorption lines from highly ionized gas,primarily He-like oxygen. The nature of these X-ray absorbers, i.e.,whether they are part of the hot gas associated with the Milky Way orpart of the intragroup medium in the Local Group, remains a puzzle dueto the uncertainties in the distance. We present in this paper a surveyof 20 AGNs with Chandra and XMM-Newton archival data. About 40% of thetargets show local O VII He α absorption with column densitiesaround 1016 cm-2; in particular, O VII absorptionis present in all the high-quality spectra. We estimate that the skycovering fraction of this O VII-absorbing gas is at least 63%, at 90%confidence, and likely to be unity given enough high-quality spectra. Onthe basis of (1) the expected number of absorbers along sight linestoward distant AGNs, (2) joint analysis with X-ray emissionmeasurements, and (3) mass estimation, we argue that the observed X-rayabsorbers are part of the hot gas associated with our Galaxy. Futureobservations will significantly improve our understanding of thecovering fraction and provide robust tests of this result.
| Radiatively Inefficient Accretion Flow in the Nucleus of NGC 1097
We present a model for the accretion flow around the supermassive blackhole in the LINER nucleus of NGC 1097 that fits the optical to X-rayspectral energy distribution (SED). The X-ray segment of the SED isbased on observations with the Chandra X-Ray Observatory, which arereported here for the first time. The inner part of the flow is modeledas a radiatively inefficient accretion flow (RIAF), and the outer partas a standard thin disk. The value of the transition radius(rtr~225RS, where RS=2GM/c2)between the RIAF and the outer thin disk was obtained from our previousfitting of the double-peaked Balmer emission line profile, whichoriginates in the thin disk. The black hole mass was inferred frommeasurements of the stellar velocity dispersion in the host galaxy. Whenthese parameters are used in the accretion flow model, the SED can besuccessfully reproduced, which shows that the line profile model and theaccretion flow model are consistent with each other. A small remainingexcess in the near-UV is accounted for by the contribution of anobscured starburst located within 9 pc from the nucleus, as we reportedin an earlier paper. The radio flux is consistent with synchrotronemission of a relativistic jet modeled by means of the internal shockscenario. In an appendix we also analyze the Chandra X-ray observationsof the ~1 kpc circumnuclear star-forming ring and of an ultraluminouscompact X-ray source located outside the ring.
| How Much Mass Do Supermassive Black Holes Eat in Their Old Age?
We consider the distribution of local supermassive black hole Eddingtonratios and accretion rates, accounting for the dependence of radiativeefficiency and bolometric corrections on the accretion rate. We findthat black hole mass growth, both of the integrated mass density and themasses of most individual objects, must be dominated by an earlier,radiatively efficient, high accretion rate stage, and not by theradiatively inefficient low accretion rate phase in which most localsupermassive black holes are currently observed. This conclusion isparticularly true of supermassive black holes in elliptical hostgalaxies, as expected if they have undergone merger activity in the pastthat would fuel quasar activity and rapid growth. We discuss models ofthe time evolution of accretion rates and show that they all predictsignificant mass growth in a prior radiatively efficient state. The onlyway to avoid this conclusion is through careful fine-tuning of theaccretion/quasar timescale to a value that is inconsistent withobservations. Our results agree with a wide range of observationalinferences drawn from the quasar luminosity function and X-raybackground synthesis models, but our approach has the virtue of beingindependent of the modeling of source populations. Models in which blackholes spend the great majority of their time in low accretion ratephases are thus completely consistent both with observations implyingmass gain in relatively short, high accretion rate phases and with thelocal distribution of accretion rates.
| 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.
| Discovery of Water Maser Emission in Eight AGNs with 70 m Antennas of NASA's Deep Space Network
We report the discovery of water maser emission in eight active galacticnuclei (AGNs) with the 70 m NASA Deep Space Network (DSN) antennas atTidbinbilla, Australia, and Robledo, Spain. The positions of the newlydiscovered masers, measured with the VLA, are consistent with theoptical positions of the host nuclei to within 1 σ (0.3" radio and1.3" optical) and most likely mark the locations of the embedded centralengines. The spectra of two sources, NGC 3393 and NGC 5495, display thecharacteristic spectral signature of emission from an edge-on accretiondisk, with orbital velocities of ~600 and ~400 km s-1,respectively. In a survey with DSN facilities of 630 AGNs selected fromthe NASA Extragalactic Database, we have discovered a total of 15 watermaser sources. The resulting incidence rate of maser emission amongnearby (vsys<7000 km s-1) Seyfert 1.8-2.0 andLINER systems is ~10% for a typical rms noise level of ~14 mJy over 1.3km s-1 spectral channels. As a result of this work, thenumber of nearby AGNs (vsys<7000 km s-1)observed with <20 mJy rms noise has increased from 130 to 449.
| X-Ray Emission from Megamaser Galaxy IC 2560
An observation of the H2O megamaser galaxy IC 2560 with theChandra X-Ray Observatory reveals a complex spectrum composed of softX-ray emission due to multitemperature thermal plasma and a hardcontinuum with strong emission lines. The continuum is most likely aCompton reflection (reprocessing) of primary emission that is completelyabsorbed at least up to 7 keV. The lines can be identified withfluorescence from Si, S, and Fe in the lowest ionization stages. Theequivalent widths of the Si and S lines are broadly compatible withthose anticipated for reprocessing by optically thick cold plasma ofsolar abundances, while the large equivalent width of the Fe linerequires some overabundance of iron. A contribution to the line from atransmitted component cannot be ruled out, but the limits on thestrength of the Compton shoulder make it less likely. From thebolometric luminosity of the nuclear region, we infer that the sourceradiates at 1%-10% of its Eddington luminosity for an adopted centralmass of 3×106 Msolar. The overall spectrumis consistent with the hypotheses that the central engines powering thedetected megamasers in accretion disks are obscured from direct view bythe associated accretion disk material itself and that there is acorrelation between the occurrence of megamaser emission andCompton-thick absorption columns. For the 11 known galaxies with bothcolumn density measurements and maser emission believed to arise fromaccretion disks, eight AGNs are Compton thick.
| Objective Classification of Spiral Galaxies Having Extended Rotation Curves Beyond the Optical Radius
We carry out an objective classification of four samples of spiralgalaxies having extended rotation curves beyond the optical radius. Amultivariate statistical analysis (viz., principal component analysis[PCA]) shows that about 96% of the total variation is due to twocomponents, one being the combination of absolute blue magnitude andmaximum rotational velocity beyond the optical region and the otherbeing the central density of the halo. On the basis of PCA a fundamentalplane has been constructed that reduces the scatter in the Tully-Fisherrelation up to a maximum of 16%. A multiple stepwise regression analysisof the variation of the overall shape of the rotation curves shows thatit is mainly determined by the central surface brightness, while theshape purely in the outer part of the galaxy (beyond the optical radius)is mainly determined by the size of the galactic disk.
| Infrared Properties of Star-forming Dwarf Galaxies. II. Blue Compact Dwarf Galaxies in the Virgo Cluster
A sample of 16 blue compact dwarf galaxies (BCDs) in the Virgo Clusterhas been imaged in the near-infrared (NIR) in J and Ks on the2.1 m telescope at the Observatorio Astronómico Nacional in theSierra San Pedro Mártir in Mexico. Isophotes as faint asμJ=24 mag arcsec-2 andμKs=23 mag arcsec-2 have beenreached in most of the targets. Surface brightness profiles can befitted across the whole range of radii by the sum of two components: ahyperbolic secant (sech) function, which is known to fit the lightprofiles of dwarf irregular galaxies (dIs), and a Gaussian component,which quantifies the starburst near the center. Isophotal and totalfitted NIR magnitudes have been calculated, along with semimajor axes atμJ=23 mag arcsec-2 andμKs=22 mag arcsec-2. The diffuseunderlying component and the young starburst have been quantified usingthe profile fitting. Most color profiles show a constant color, betweenJ-Ks=0.7 and 0.9 mag. The diffuse component represents theoverwhelming majority of the NIR light for most BCDs, with the starburstenhancing the flux by less than about 0.3 mag. Linear correlations werefound between the sech scale length and the sech magnitude and betweenthe sech semimajor axis and the sech magnitude. Overall, galaxies withmore luminous diffuse components are larger and brighter in the center.The central burst correlates with the diffuse component, with brighterBCDs having stronger starbursts, suggesting that more massive objectsare forming stars more efficiently. BCDs lie on the ``fundamentalplane'' defined by dIs in Paper I, following the same relation betweensech absolute magnitude, sech central surface brightness, and thehydrogen line width W20, although the scatter is larger thanfor the dIs. On the other hand, correlations between the sech absolutemagnitude and the sech central surface brightness in Ks forBCDs and dIs are equally good, indicating that BCD line widths may beenhanced by turbulence or winds.These data were acquired at the Observatorio Astronómico Nacionalin the Sierra San Pedro Mártir, Mexico.
| Examining the Seyfert-Starburst Connection with Arcsecond-Resolution Radio Continuum Observations
We compare the arcsecond-scale circumnuclear radio continuum propertiesof five Seyfert and five starburst galaxies, concentrating on the searchfor any structures that could imply a spatial or causal connectionbetween the nuclear activity and a circumnuclear starburst ring. Noevidence is found in the radio emission for a link between thetriggering or feeding of nuclear activity and the properties ofcircumnuclear star formation. Conversely, there is no clear evidence ofnuclear outflows or jets triggering activity in the circumnuclear ringsof star formation. Interestingly, the difference in the angle betweenthe apparent orientation of the most elongated radio emission and theorientation of the major axis of the galaxy is on average larger inSeyfert galaxies than in starburst galaxies, and Seyfert galaxies appearto have a larger physical size scale of the circumnuclear radiocontinuum emission. The concentration, asymmetry, and clumpinessparameters of radio continuum emission in Seyfert galaxies andstarbursts are comparable, as are the radial profiles of radio continuumand near-infrared line emission. The circumnuclear star formation andsupernova rates do not depend on the level of nuclear activity. Theradio emission usually traces the near-infrared Brγ andH2 1-0 S(1) line emission on large spatial scales, butlocally their distributions are different, most likely because of theeffects of varying local magnetic fields and dust absorption andscattering.
| Seyfert Galaxies and the Hard X-Ray Background: Artificial Chandra Observations of z=0.3 Active Galaxies
Deep X-ray surveys have resolved much of the X-ray background radiationbelow 2 keV into discrete sources, but the background above 8 keVremains largely unresolved. The obscured (type 2) active galactic nuclei(AGNs) that are expected to dominate the hard X-ray background have notyet been detected in sufficient numbers to account for the observedbackground flux. However, deep X-ray surveys have revealed large numbersof faint quiescent and starburst galaxies at moderate redshifts. Inhopes of recovering the missing AGN population, it has been suggestedthat the defining optical spectral features of low-luminosity Seyfertnuclei at large distances may be overwhelmed by their host galaxies,causing them to appear optically quiescent in deep surveys. We test thispossibility by artificially redshifting a sample of 23 nearby,well-studied active galaxies to z=0.3, testing them for X-ray AGNsignatures, and comparing them to the objects detected in deep X-raysurveys. We find that these redshifted galaxies have propertiesconsistent with the deep-field normal and optically bright, X-ray-faintgalaxy (OBXF) populations, supporting the hypothesis that the numbers ofAGNs in deep X-ray surveys are being underestimated and suggesting thatOBXFs should not be ruled out as candidate AGN hosts that couldcontribute to the hard X-ray background source population.
| On the X-ray, optical emission line and black hole mass properties of local Seyfert galaxies
We investigate the relation between X-ray nuclear emission, opticalemission line luminosities and black hole masses for a sample of 47Seyfert galaxies. The sample, which has been selected from the Palomaroptical spectroscopic survey of nearby galaxies (Ho et al. 1997a, ApJS,112, 315), covers a wide range of nuclear powers, from L2-10keV ~ 1043 erg/s down to very low luminosities(L2-10 keV ~ 1038 erg/s). Best available data fromChandra, XMM-Newton and, in a few cases, ASCA observations have beenconsidered. Thanks to the good spatial resolution available from theseobservations and a proper modeling of the various spectral components,it has been possible to obtain accurate nuclear X-ray luminosities notcontaminated by off-nuclear sources and/or diffuse emission. X-rayluminosities have then been corrected taking into account the likelycandidate Compton thick sources, which are a high fraction (>30%)among type 2 Seyferts in our sample. The main result of this study isthat we confirm strong linear correlations between 2-10 keV,[OIII]λ5007, Hα luminosities which show the same slope asquasars and luminous Seyfert galaxies, independent of the level ofnuclear activity displayed. Moreover, despite the wide range ofEddington ratios (L/L_Edd) tested here (six orders of magnitude, from0.1 down to ~10-7), no correlation is found between the X-rayor optical emission line luminosities and the black hole mass. Ourresults suggest that Seyfert nuclei in our sample are consistent withbeing a scaled-down version of more luminous AGN.
| A catalogue of ultra-luminous X-ray source coincidences with FIRST radio sources
Aims.We search for ultra luminous X-ray source (ULXs) radio counterpartslocated in nearby galaxies in order to constrain their physicalnature. Methods: .Our work is based on a systematiccross-identification of the most recent and extensive available ULXcatalogues and archival radio data. Results: .A catalogue of 70positional coincidences is reported. Most of them are located within thegalaxy nucleus. Among them, we find 11 new cases of non-nuclear ULXsources with possibly associated radio emission.
| The extragalactic Cepheid bias: a new test using the period-luminosity-color relation
We use the Period-Luminosity-Color relation (PLC) for Cepheids to testfor the existence of a bias in extragalactic distances derived from theclassical Period-Luminosity (PL) relation. We calculate the parametersof the PLC using several galaxies observed with the Hubble SpaceTelescope and show that this calculation must be conducted with a PLCwritten in a form where the parameters are independent. The coefficientsthus obtained are similar to those derived from theoretical models.Calibrating with a few unbiased galaxies, we apply this PLC to allgalaxies of the Hubble Space Telescope Key Program (HSTKP) and comparethe distance moduli with those published by the HSTKP team. The newdistance moduli are larger (more exactly, the larger the distance thelarger the difference), consistent with a bias. Further, the bias trendthat is observed is the same previously obtained from two independentmethods based either on the local Hubble law or on a theoretical modelof the bias. The results are quite stable but when we force the PLCrelation closer to the classical PL relation by using unrealisticparameters, the agreement with HSTKP distance moduli is retrieved. Thisalso suggests that the PL relation leads to biased distance moduli. Thenew distance moduli reduce the scatter in the calibration of theabsolute magnitude of supernovae SNIa at their maximum. This may alsosuggest that the relation between the amplitude at maximum and the decayof the light curve Δ m15 may not be as strong asbelieved.
| Extragalactic H_2O masers and X-ray absorbing column densities
Having conducted a search for the λ 1.3 cm (22 GHz) water vaporline towards galaxies with nuclear activity, large nuclear columndensities or high infrared luminosities, we present H2O spectra for NGC2273, UGC 5101, and NGC 3393 with isotropic luminosities of 7, 1500, and400 Lȯ. The H2O maser in UGC 5101 is by far the mostluminous yet found in an ultraluminous infrared galaxy. NGC 3393 revealsthe classic spectrum of a "disk maser", represented by three distinctgroups of Doppler components. As in all other known cases except NGC4258, the rotation velocity of the putative masing disk is well below1000 km s-1. Based on the literature and archive data, X-rayabsorbing column densities are compiled for the 64 galaxies withreported maser sources beyond the Magellanic Clouds. For NGC 2782 andNGC 5728, we present Chandra archive data that indicate the presence ofan active galactic nucleus in both galaxies. Modeling the hard nuclearX-ray emission, NGC 2782 is best fit by a high energy reflectionspectrum with NH 1024 cm-2. ForNGC 5728, partial absorption with a power law spectrum indicatesNH 8 × 1023 cm-2. Thecorrelation between absorbing column and H2O emission is analyzed. Thereis a striking difference between kilo- and megamasers with megamasersbeing associated with higher column densities. All kilomasers (L_H_2O< 10 Lȯ) except NGC 2273 and NGC 5194 areCompton-thin, i.e. their absorbing columns are <1024cm-2. Among the H{2}O megamasers, 50% arise fromCompton-thick and 85% from heavily obscured (>1023cm-2) active galactic nuclei. These values are not larger butconsistent with those from samples of Seyfert 2 galaxies not selected onthe basis of maser emission. The similarity in column densities can beexplained by small deviations in position between maser spots andnuclear X-ray source and a high degree of clumpiness in thecircumnuclear interstellar medium.
| The supermassive black hole in Centaurus A: a benchmark for gas kinematical measurements
We present new HST Space Telescope Imaging Spectrograph observations ofthe nearby radio galaxy NGC 5128 (Centaurus A). The bright emission linewith longest wavelength accessible from HST, [S III]λ 9533Å, was used to study the kinematics of the ionized gas in thenuclear region with a 0.1 arcsec spatial resolution. The STIS data wereanalized in conjunction with the ground-based near-infrared Very LargeTelescope ISAAC spectra used by Marconi et al. (2001, ApJ, 549, 915) toinfer the presence of a supermassive black hole and measure its mass.The two sets of data have spatial resolutions differing by almost afactor of five but provide independent and consistent measures of the BHmass, which are in agreement with our previous estimate based on theISAAC data alone. The gas kinematical analysis provides a mass ofMBH(1.1±0.1)× 108{M}ȯ for an assumed disk inclination of i=25 deg orMBH= (6.5±0.7)× 107{M}ȯ for i=35 deg, the largest i value allowed by thedata. We performed a detailed analysis of the effects on MBHof the intrinsic surface brightness distribution of the emission line, acrucial ingredient in the gas kinematical analysis. We estimate that theassociated systematic errors are no larger than 0.08 in logMBH, comparable with statistical errors and indicating thatthe method is robust. However, the intrinsic surface brightnessdistribution has a large impact on the value of the gas velocitydispersion. A mismatch between the observed and model velocitydispersion is not necessarily an indication of non-circular motions orkinematically hot gas, but is as easily due to an inaccurate computationarising from too course a model grid, or the adoption of an intrinsicbrightness distribution which is too smooth. The observed velocitydispersion in our spectra can be matched with a circularly rotating diskand also the observed line profiles and the higher order moments in theHermite expansion of the line profiles, h3 and h4,are consistent with emission from such a disk. To our knowledge,Centaurus A is the first external galaxy for which reliable BH massmeasurements from gas and stellar dynamics are available and, as in thecase of the Galactic Center, the MBH gas kinematical estimateis in good agreement with that from stellar dynamics. The BH mass inCentaurus A is in excellent agreement with the correlation with infraredluminosity and mass of the host spheroid but is a factor 2-4 abovethe one with the stellar velocity dispersion. But this disagreement isnot large if one takes into account the intrinsic scatter of theMBH-σe correlation. Finally, the high HSTspatial resolution allows us to constrain the size of any cluster ofdark objects alternative to a BH to r•<0.035 arcsec(≃0.6 pc). Thus Centaurus A ranks among the best cases forsupermassive Black Holes in galactic nuclei.
| The magnetic field in the star-forming region Cepheus A. from H2O maser polarization observations
We present linear and circular polarization observations of the H2Omasers in 4 distinct regions spread over 1 × 2 arcsec around theHW2 high-mass young stellar object in the Cepheus A star-forming region.We find magnetic fields between 100-600 mG in the central maser region,which has been argued to trace a circumstellar disk. The masers furtherfrom HW2 have field strengths between 30-100 mG. In all cases themagnetic field pressure is found to be similar to the dynamic pressure,indicating that the magnetic field is capable of controlling the outflowdynamics around HW2. In addition to several H2O maser complexes observedbefore, we also detect a new maser filament, ≃1 arcsec (≃690AU) East of HW2, which we interpret as a shocked region between the HW2outflow and the surrounding medium. We detect a linear polarizationgradient along the filament as well as a reversal of the magnetic fielddirection. This is thought to mark the transition between the magneticfield associated with the outflow and that found in the surroundingmolecular cloud. In addition to the magnetic field we determine severalother physical properties of the maser region, including density andtemperatures as well as the maser beaming angles.
|
Yeni bir Makale Öner
Ýlgili Baðlantýlar
Yeni Bir Baðlantý Öner
sonraki gruplarýn üyesi:
|
Gözlemler ve gökölçümü verileri
Kataloglar ve belirtme:
|
