Contenidos
Imágenes
Subir su imagen
DSS Images Other Images
Artículos relacionados
SN 2004A: another Type II-P supernova with a red supergiant progenitor
We present a monitoring study of SN 2004A and probable discovery of aprogenitor star in pre-explosion Hubble Space Telescope (HST) images.The photometric and spectroscopic monitoring of SN 2004A show that itwas a normal Type II-P which was discovered in NGC 6207 about two weeksafter explosion. We compare SN 2004A to the similar Type II-P SN 1999emand estimate an explosion epoch of 2004 January 6. We also calculatethree new distances to NGC 6207 of 21.0 +/- 4.3,21.4 +/- 3.5 and 25.1+/- 1.7Mpc. The former was calculated using the Standard Candle Method(SCM) for SNeII-P, and the latter two from the brightest supergiantsmethod (BSM). We combine these three distances with existing kinematicdistances, to derive a mean value of 20.3 +/- 3.4Mpc. Using thisdistance, we estimate that the ejected nickel mass in the explosion is0.046+0.031-0.017Msolar. The progenitorof SN 2004A is identified in pre-explosion WFPC2 F814W images with amagnitude of mF814W = 24.3 +/- 0.3, but is below thedetection limit of the F606W images. We show that this was likely a redsupergiant (RSG) with a mass of9+3-2Msolar. The object is detected at4.7σ above the background noise. Even if this detection isspurious, the 5σ upper limit would give a robust upper mass limitof 12Msolar for a RSG progenitor. These initial masses arevery similar to those of two previously identified RSG progenitors ofthe Type II-P SNe 2004gd (8+4-2Msolar)and 2005cs (9+3-2Msolar).
| Forming supermassive black holes by accreting dark and baryon matter
Given a large-scale mixture of self-interacting dark matter (SIDM)particles and baryon matter distributed in the early Universe, weadvance here a two-phase accretion scenario for forming supermassiveblack holes (SMBHs) with masses around ~109Msolarat high redshifts z(>~6). The first phase is conceived to involve arapid quasi-spherical and quasi-steady Bondi accretion of mainly SIDMparticles embedded with baryon matter on to seed black holes (BHs)created at redshifts z<~ 30 by the first generation of massivePopulation III stars; this earlier phase rapidly gives birth tosignificantly enlarged seed BH masses of during z~ 20-15, whereσ0 is the cross-section per unit mass of SIDM particlesand Cs is the velocity dispersion in the SIDM halo referredto as an effective `sound speed'. The second phase of BH mass growth isenvisaged to proceed primarily via baryon accretion, eventually leadingto SMBH masses of MBH~ 109Msolar suchSMBHs may form either by z~ 6 for a sustained accretion at the Eddingtonlimit or later at lower z for sub-Eddington mean accretion rates. Inbetween these two phases, there is a transitional yet sustaineddiffusively limited accretion of SIDM particles which in an eventualsteady state would be much lower than the accretion rates of the twomain phases. We intend to account for the reported detections of a fewSMBHs at early epochs, e.g. Sloan Digital Sky Survey (SDSS) 1148+5251and so forth, without necessarily resorting to either super-Eddingtonbaryon accretion or very frequent BH merging processes. Only extremelymassive dark SIDM haloes associated with rare peaks of densityfluctuations in the early Universe may harbour such early SMBHs orquasars. Observational consequences are discussed. During the finalstage of accumulating a SMBH mass, violent feedback in circumnuclearenvirons of a galactic nucleus leads to the central bulge formation andgives rise to the familiar empirical MBH-σbcorrelation inferred for nearby normal galaxies with σbbeing the stellar velocity dispersion in the galactic bulge; in ourscenario, the central SMBH formation precedes that of the galacticbulge.
| High-Resolution Optical Velocity Fields of 11 Low Surface Brightness Galaxies
We present high-resolution two-dimensional velocity fields from integralfield spectroscopy, along with derived rotation curves for 11 lowsurface brightness galaxies. We fit NFW and pseudoisothermal halo modelsto the new data combined with previous long-slit and H I data. In mostcases, we find the pseudoisothermal halo to better represent the datathan the NFW halo, as the NFW concentrations are often lower thanexpected for a ΛCDM cosmology. We also compare our results toprevious studies and find that including the new two-dimensional opticaldata does not significantly alter the halo parameters but does decreasethe uncertainties by roughly a factor of 2.
| Radio Emission on Subparsec Scales from the Intermediate-Mass Black Hole in NGC 4395
The Seyfert 1 nucleus of NGC 4395 is energized by a black hole of mass3.6×105 Msolar, making it one of only twonuclear black holes of intermediate mass, 103-106Msolar, detected in the radio regime. Building on UV andX-ray evidence for outflows from this Seyfert nucleus, the VLBI HighSensitivity Array was used at 1.4 GHz to search for extended structureon scales greater than 5 mas (0.1 pc). Elongated emission wasdiscovered, extending over 15 mas (0.3 pc) and suggesting an outflow onsubparsec scales from this intermediate-mass black hole. The Seyfertnucleus is located at the center of an elliptical star cluster, and theelongation position angle of the subparsec radio structure is only19° from the star cluster's minor axis.
| Core-Collapse Very Massive Stars: Evolution, Explosion, and Nucleosynthesis of Population III 500-1000 Msolar Stars
We calculate evolution, collapse, explosion, and nucleosynthesis ofPopulation III very massive stars with 500 and 1000 Msolar.Presupernova evolution is calculated in spherical symmetry. Collapse andexplosion are calculated by a two-dimensional code, based on the bipolarjet models. We compare the results of nucleosynthesis with the abundancepatterns of intracluster matter, hot gases in M82, and extremelymetal-poor stars in the Galactic halo. It was found that both 500 and1000 Msolar models enter the region of pair instability butcontinue to undergo core collapse. In the presupernova stage,silicon-burning regions occupy a large fraction, more than 20% of thetotal mass. For moderately aspherical explosions, the patterns ofnucleosynthesis match the observational data of both the intraclustermedium and M82. Our results suggest that explosions of Population IIIcore-collapse very massive stars contribute significantly to thechemical evolution of gases in clusters of galaxies. For Galactic halostars our [O/Fe] ratios are smaller than the observational abundances.However, our proposed scenario is naturally consistent with thisoutcome. The final black hole masses are ~230 and ~500 Msolarfor the 500 and 1000 Msolar models, respectively. This resultmay support the view that Population III very massive stars areresponsible for the origin of intermediate-mass black holes, which wererecently reported to be discovered.
| Multiwavelength Monitoring of the Dwarf Seyfert 1 Galaxy NGC 4395. II. X-Ray and Ultraviolet Continuum Variability
We report on two Chandra observations, and a simultaneous Hubble SpaceTelescope ultraviolet observation, of the dwarf Seyfert 1 galaxy NGC4395. Each Chandra observation had a duration of ~30 ks, with aseparation of ~50 ks. The spectrum was observed to harden between theseobservations via a scaling down of the soft-band flux. Theinterobservation variability is in a different sense from the observedvariability within each observation and is most likely the result ofincreased absorption. Spectral variations were seen during the firstobservation suggesting that the X-ray emission is produced in more thanone disconnected region. We have also reanalyzed a ~17 ks Chandraobservation conducted in 2000. During the three Chandra observations the2-10 keV flux is about a factor of 2 lower than seen during anXMM-Newton observation conducted in 2003. Moreover, the fractionalvariability amplitude exhibited during the XMM-Newton observation issignificantly softer than seen during the Chandra observations. A powerspectral analysis of the first of the two new Chandra observationsrevealed a peak at 341 s with a formal detection significance of 99%. Asimilar peak was seen previously in the 2000 Chandra data. However, thedetection of this feature is tentative given that it was found inneither the second of our two new Chandra observations nor theXMM-Newton data, and it is much narrower than expected. The Hubble SpaceTelescope observation was conducted during part of the second Chandravisit. A zero-lag correlation between the ultraviolet and X-ray fluxeswas detected with a significance of ~99.5%, consistent with thepredictions of the two-phase model for the X-ray emission from activegalactic nuclei.
| First Detection of Near-Infrared Intraday Variations in the Seyfert 1 Nucleus NGC 4395
We carried out a one-night optical V and near-infrared JHK monitoringobservation of the least luminous Seyfert 1 galaxy, NGC 4395, on 2004May 1, and detected for the first time the intraday flux variations inthe J and H bands, while such variation was not clearly seen for the Kband. The detected J and H variations are synchronized with the fluxvariation in the V band, which indicates that the intraday-variablecomponent of near-infrared continuum emission of the NGC 4395 nucleus isan extension of power-law continuum emission to the near-infrared andoriginates in an outer region of the central accretion disk. On theother hand, from our regular program of long-term optical BVI andnear-infrared JHK monitoring observation of NGC 4395 from 2004 February12 until 2005 January 22, we found large flux variations in all thebands on timescales of days to months. The optical BVI variations arealmost synchronized with each other but not completely with thenear-infrared JHK variations. The color temperature of the near-infraredvariable component is estimated to be T=1320-1710 K, in agreement withthermal emission from hot dust tori in active galactic nuclei (AGNs). Wetherefore conclude that the near-infrared variation consists of twocomponents having different timescales, so that a small K-flux variationon a timescale of a few hours would possibly be veiled by largevariation of thermal dust emission on a timescale of days.
| Evidence for Line Broadening by Electron Scattering in the Broad-Line Region of NGC 4395
A high-quality Keck spectrum of the Hα line in NGC 4395 revealssymmetric exponential wings, fv~e-v/σ, withσ~=500 km s-1. The wings extend out to >~2500 kms-1 from the line core, and down to a flux density of<~10-3 of the peak flux density. Numerical and analyticcalculations indicate that exponential wings are expected for opticallythin, isotropic, thermal electron scattering. Such scattering producesexponential wings withσ~=1.1σe(lnτ-1e)0.45,where σe is the electron velocity dispersion, andτe is the electron-scattering optical depth. The Hαwings in NGC 4395 are well fit by an electron-scattering model withτe=0.34 and an electron temperatureTe=1.1×104 K. Such conditions are producedin photoionized gas with an ionization parameter U~=0.3, as expected inthe broad-line region (BLR). Similar analysis of the [O III]λ5007 line yields τe<0.01, consistent with thelower ionization in the narrow-line region. If the electron-scatteringinterpretation is correct, there should be a tight correlation betweenτe and the ionizing flux on timescales shorter than theBLR dynamical time, or ~1 week for NGC 4395. In contrast, the value ofσ should remain nearly constant on these timescales. Such wingsmay be discernible in other objects with unusually narrow Balmer lines,and they can provide a useful direct probe of Te andτe in the BLR.
| The MBH-σ* Relation in Local Active Galaxies
We examine whether active galaxies obey the same relation between blackhole mass and stellar velocity dispersion as inactive systems, using thelargest published sample of velocity dispersions for active nuclei todate. The combination of 56 original measurements with objects from theliterature not only increases the sample from the 15 consideredpreviously to 88 objects but allows us to cover an unprecedented rangein both stellar velocity dispersion (30-268 km s-1) and blackhole mass (105-108.6 Msolar). In theMBH-σ* relation of active galaxies, we finda lower zero point than the best-fit relation of Tremaine et al. forinactive galaxies, and an upper limit on the intrinsic scatter of 0.4dex. There is also evidence of a flatter slope at low black hole masses.We discuss potential contributors to the observed offsets, includingvariations in the geometry of the broad-line region, evolution in theMBH-σ* relation, and differential growthbetween black holes and galaxy bulges.
| Determining Central Black Hole Masses in Distant Active Galaxies and Quasars. II. Improved Optical and UV Scaling Relationships
We present four improved empirical relationships useful for estimatingthe central black hole mass in nearby AGNs and distant luminous quasarsalike using either optical or UV single-epoch spectroscopy. These massscaling relationships between line widths and luminosity are based onrecently improved empirical relationships between the broad-line regionsize and luminosities in various energy bands and are calibrated to theimproved mass measurements of nearby AGNs based on emission-linereverberation mapping. The mass scaling relationship based on theHβ line luminosity allows mass estimates for low-redshift sourceswith strong contamination of the optical continuum luminosity by stellaror nonthermal emission, while that based on the C IV λ1549 linedispersion allows mass estimates in cases where only the line dispersion(as opposed to the FWHM) can be reliably determined. We estimate thatthe absolute uncertainties in masses given by these mass scalingrelationships are typically around a factor of 4. We include in anappendix mass estimates for all of the Bright Quasar Survey (PG) quasarsfor which direct reverberation-based mass measurements are notavailable.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained from the Data Archive at the Space Telescope ScienceInstitute, which is operated by the Association of Universities forResearch in Astronomy, Inc., under NASA contract NAS 5-26555.
| Measuring Stellar Velocity Dispersions in Active Galaxies
We present stellar velocity dispersion (σ*)measurements for a significant sample of 40 broad-line (type 1) activegalaxies for use in testing the well-known relation black hole mass andstellar velocity dispersion. The objects are selected to contain Ca IItriplet, Mg I b triplet, and Ca H+K stellar absorption features in theiroptical spectra so that we may use them to perform extensive tests ofthe systematic biases introduced by both template mismatch andcontamination from the active galactic nucleus (AGN). We use the Ca IItriplet as a benchmark to evaluate the utility of the other spectralregions in the presence of AGN contamination. Broad Fe II emission,extending from ~5050 to 5520 Å, in combination with narrow coronalemission lines, can seriously bias σ* measurements fromthe Mg I b region, highlighting the need for extreme caution in its use.However, we argue that at luminosities constituting a moderate fractionof the Eddington limit, when the Fe II lines are both weak and smoothrelative to the stellar lines, it is possible to derive meaningfulmeasurements with careful selection of the fitting region. Inparticular, to avoid the contamination of coronal lines, we advocate theuse of the region 5250-5820 Å, which is rich in Fe absorptionfeatures. At higher AGN contaminations, the Ca H+K region may providethe only recourse for estimating σ*. These features arenotoriously unreliable, due to a strong dependence on spectral type, asteep local continuum, and large intrinsic broadening. Indeed, we find astrong systematic trend in comparisons of Ca H+K with other spectralregions. Luckily the offset is well described by a simple linear fit asa function of σ*, which enables us to remove the biasand thus extract unbiased σ* measurements from thisregion. We lay the groundwork for an extensive comparison between blackhole mass and bulge velocity dispersion in active galaxies, as describedin a companion paper by Greene & Ho.
| The First INTEGRAL AGN Catalog
We present the first INTEGRAL AGN catalog, based on observationsperformed from launch of the mission in 2002 October until 2004 January.The catalog includes 42 AGNs, of which 10 are Seyfert 1, 17 are Seyfert2, and 9 are intermediate Seyfert 1.5. The fraction of blazars is rathersmall, with five detected objects, and only one galaxy cluster and nostarburst galaxies have been detected so far. A complete subset consistsof 32 AGNs with a significance limit of 7 σ in the INTEGRAL ISGRI20-40 keV data. Although the sample is not flux limited, thedistribution of sources shows a ratio of obscured to unobscured AGNs of1.5-2.0, consistent with luminosity-dependent unified models for AGNs.Only four Compton-thick AGNs are found in the sample. Based on theINTEGRAL data presented here, the Seyfert 2 spectra are slightly harder(Γ=1.95+/-0.01) than Seyfert 1.5 (Γ=2.10+/-0.02) and Seyfert1 (Γ=2.11+/-0.05).
| Oxygen and Nitrogen in Leo A and GR 8
We present elemental abundances for multiple H II regions in Leo A andGR 8 obtained from long-slit optical spectroscopy of these two nearbylow-luminosity dwarf irregular galaxies. As expected from theirluminosities, and in agreement with previous observations, the derivedoxygen abundances are extremely low in both galaxies. Highsignal-to-noise ratio (S/N) observations of a planetary nebula in Leo Ayield 12+log(O/H)=7.30+/-0.05 semiempirical calculations of the oxygenabundance in four H II regions in Leo A indicate12+log(O/H)=7.38+/-0.10. These results confirm that Leo A has one of thelowest ISM metal abundances of known nearby galaxies. Based on resultsfrom two H II regions with high S/N measurements of the weak [O III]λ4363 line, the mean oxygen abundance of GR 8 is12+log(O/H)=7.65+/-0.06 using ``empirical'' and ``semiempirical''methods, similar abundances are derived for six other GR 8 H II regions.Similar to previous results in other low-metallicity galaxies, the meanlog(N/O)=-1.53+/-0.09 for Leo A and -1.51+/-0.07 for GR 8. There is noevidence of significant variations in either O/H or N/O in the H IIregions. The metallicity-luminosity relation for nearby (D<5 Mpc)dwarf irregular galaxies with measured oxygen abundances has a meancorrelation of 12+log(O/H)=5.67MB-0.151MB, with adispersion in oxygen about the relationship of σ=0.21. Theseobservations confirm that gas-rich, low-luminosity galaxies haveextremely low elemental abundances in the ionized gas phase of theirinterstellar media. Although Leo A has one of the lowest metalabundances of known nearby galaxies, detection of tracers of an olderstellar population (RR Lyrae variable stars, horizontal branch stars,and a well-populated red giant branch) indicate that it is not a newlyformed galaxy, as has been proposed for some other similarlow-metallicity star-forming galaxies.
| Is the Broad-Line Region Clumped or Smooth? Constraints from the Hα Profile in NGC 4395, the Least Luminous Seyfert 1 Galaxy
The origin and configuration of the gas that emits broad lines in Type Iactive galactic nuclei is not established yet. The lack of small-scalestructure in the broad emission-line profiles is consistent with eithera smooth gas flow or a clumped flow with many small clouds. Anattractive possibility for the origin of many small clouds is theatmospheres of bloated stars, an origin that also provides a naturalmechanism for the cloud confinement. Earlier studies of the broad-lineprofiles have already put strong lower limits on the minimum number ofsuch stars, but these limits are sensitive to the assumed width of thelines produced by each cloud. Here we revisit this problem usinghigh-resolution Keck spectra of the Hα line in NGC 4395, which hasthe smallest known broad-line region (~1014 cm). Only ahandful of the required bloated stars (each havingr*~1014 cm) could fit into the broad-line regionof NGC 4395, yet the observed smoothness of the Hα line implies alower limit of ~104-105 on the number of discreteclouds. This conclusively rules out the bloated-stars scenario,regardless of any plausible line-broadening mechanisms. The upper limiton the size of the clouds is ~1012 cm, which is comparable tothe size implied by photoionization models. This strongly suggests thatgas in the broad-line region is structured as a smooth rather than aclumped flow, most likely in a rotationally dominated thick disklikeconfiguration. However, it remains to be clarified why such a smooth,gravity-dominated flow generates double-peaked emission lines only in asmall fraction of active galactic nuclei.
| The Radio Quiescence of Active Galaxies with High Accretion Rates
We present 6 cm Very Large Array observations of the Greene & Hosample of 19 low-mass active galaxies with high accretion rates. This isone of the only studies of a uniform sample of narrow-line Seyfert 1(NLS1) galaxies with such high sensitivity and resolution. Although wedetect only one source, the entire sample is very radio quiet down tostrong limits. GH 10 was found to have a radio power of8.5×1021 W Hz-1 and a ratioR≡f6cm/f4400Å of 2.8. The 3 σupper limits for the remaining nondetections correspond to radio powersfrom 3×1020 to 8×1021 WHz-1 and 0.47
| Masses of the local group and of the M81 group estimated from distortions in the local velocity field
Based on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies.
| 1E 1207.4-5209: Are the optical and X-ray spectra blueshifted?
1E 1207.4-5209 is an X-ray source located at the centre of the supernovaremnant (SNR) PKS 1209-52 (G296.5+10.0) and is thought to be an isolatedneutron star (INS) associated with the SNR. Its optical spectrum showsseveral absorption lines and the X-ray spectrum exhibits three variableabsorption features at what appears to be harmonically relatedwavelengths, the latter being interpreted as due to cyclotron resonance.However, there are several serious problems, uncertainties anddifficulties in the above association (SNR/INS) and in theinterpretation of the spectra. In view of these, we suggest analternative explanation of the observed spectra in terms of blueshifts.This implies that the optical and X-ray absorption spectra are due tothe central object of the SNR in association with two separateabsorption clouds ejected at successively increasing speeds. The cloudshave their origins in jets resulting from the merger of two very massivecompact stars.
| The K Luminosity-Metallicity Relation for Dwarf Galaxies and the Tidal Dwarf Galaxies in the Tails of HCG 31
We determine a K-band luminosity-metallicity (L-Z) relation for dwarfirregular galaxies over a large range of magnitudes,-20.5
| 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.
| Neutral Hydrogen in the Polar Ring Galaxy UGC 9796
We have used the Very Large Array to observe the polar ring galaxy UGC9796 (II Zw 73; PRC A-06) in the 21 cm line of neutral hydrogen. All theneutral gas in this galaxy is associated with the polar ring. Ourobservations show that ~=5×109 Msolar of H Irotates in a plane projected about 25° from perpendicular to theequatorial plane of the early-type host galaxy, consistent with earlierobservations in the Hα line. The outermost gas appears to bendaway from the minor axis of the host system, in the same sense as thestellar ring. UGC 9796 is in a very gas-rich environment: our 21 cmimages show five gas-rich companion galaxies in the same field within100 km s-1, with a total of ~=2×1010Msolar between them. However, we see no H I streamersconnecting the systems or any other evidence for an ongoing interactionbetween UGC 9796 and its companions.
| A Comparison of Hα and Stellar Scale Lengths in Virgo and Field Spirals
The scale lengths of the old stars and ionized gas distributions arecompared for similar samples of Virgo Cluster members and field spiralgalaxies via Hα and broad R-band surface photometry. While theR-band and Hα scale lengths are, on average, comparable for thecombined sample, we find significant differences between the field andcluster samples. While the Hα scale lengths of the field galaxiesare a factor of 1.14+/-0.07 longer, on average, than their R-band scalelengths, the Hα scale lengths of Virgo Cluster members are, onaverage, 20% smaller than their R-band scale lengths. Furthermore, inVirgo, the scale length ratios are correlated with the size of thestar-forming disk: galaxies with smaller overall Hα extents alsoshow steeper radial falloff of star formation activity. At the sametime, we find no strong trends in scale length ratio as a function ofother galaxy properties, including galaxy luminosity, inclination,morphological type, central R-band light concentration, or bar type. Ourresults for Hα emission are similar to other results for dustemission, suggesting that Hα and dust have similar distributions.The environmental dependence of the Hα scale length placesadditional constraints on the evolutionary process(es) that cause gasdepletion and a suppression of the star formation rate in clusters ofgalaxies.
| 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.
| Toward a clean sample of ultra-luminous X-ray sources
Context: .Observational follow-up programmes for the characterization ofultra-luminous X-ray sources (ULXs) require the construction of cleansamples of such sources in which the contamination byforeground/background sources is minimum. Aims: .We calculate thedegree of foreground/background contaminants among the ULX samplecandidates in a published catalogue and compare these computations withavailable spectroscopic identifications. Methods: .We usestatistics based on known densities of X-ray sources and AGN/QSOsselected in the optical. The analysis is done individually for eachparent galaxy. The existing identifications of the optical counterpartsare compiled from the literature. Results: .More than a half ofthe ULXs, within twice the distance of the major axis of the 25mag/arcsec2 isophote from RC3 nearby galaxies and with X-rayluminosities L_X[ 2-10 keV] ≥ 1039 erg/s, are expected tobe high redshift background QSOs. A list of 25 objects (clean sample)confirmed to be real ULXs or to have a low probability of beingcontaminant foreground/background objects is provided.
| 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.
| Modeling the warm absorber in active galactic nuclei
We present a wide grid of models for the structure and transmissionproperties of warm absorbers in active galactic nuclei (AGN). Contraryto commonly used constant density models, our absorbing cloud is assumedto be under constant total (gas plus radiation) pressure. Thisassumption implies the coexistence of material at different temperaturesand ionization states, which is a natural consequence of pressure andthermal equilibrium. Our photoionization code allows us to compute theprofiles of the density, the temperature, the gas pressure, theradiation pressure and the ionization state across the cloud, and tocalculate the radiative transfer of continuum and lines includingCompton scattering. Therefore, equivalent widths of both saturated andunsaturated lines are properly modeled. For each pair of the incidentspectrum slope and the ionization parameter at the cloud surface thereis a natural upper limit to the total column densities of the cloud dueto thermal instabilities. These maximum values are comparable to theobservational constraints on the column density of warm absorbers whichmay give support to constant total pressure models. In all models wenote considerable absorption around 6.4 keV which modifies the intrinsicrelativistically broadened iron line profile originating in an accretiondisk illuminated atmosphere. Our models can be applied to fitting thespectroscopic data from the XMM-Newton and Chandra satellites.
| X-ray spectral survey with XMM-Newton of a complete sample of nearby Seyfert galaxies
Results obtained from an X-ray spectral survey of nearby Seyfertgalaxies using XMM-Newton are reported. The sample was opticallyselected, well defined, complete in B magnitude, and distance limited:it consists of the nearest (D 22 Mpc) 27 Seyfert galaxies (9 oftype 1, 18 of type 2) taken from the Ho et al. (1997a, ApJS, 112, 315)sample. This is one of the largest atlases of hard X-ray spectra oflow-luminosity active galaxies ever assembled. All nuclear sourcesexcept two Seyfert 2s are detected between 2 and 10 keV, half for thefirst time ever, and average spectra are obtained for all of them.Nuclear luminosities reach values down to 1038 ergs-1. The shape of the distribution of X-ray parameters isaffected by the presence of Compton-thick objects (30% among type2s). The latter have been identified either directly from their intenseFeK line and flat X-ray spectra, or indirectly with flux diagnosticdiagrams which use isotropic indicators. After taking into account thesehighly absorbed sources, we find that (i) the intrinsic X-ray spectralproperties (i.e., spectral shapes and luminosities above 2 keV) areconsistent between type 1 and type 2 Seyferts, as expected from "unifiedmodels"; (ii) Seyfert galaxies as a whole are distributed fairlycontinuously over the entire range of N_H, between 1020 and1025 cm-2; and (iii) while Seyfert 1s tend to havelower NH and Seyfert 2s tend to have the highest, we find 30%and 10% exceptions, respectively. Overall the sample is of sufficientquality to well represent the average intrinsic X-ray spectralproperties of nearby active galactic nuclei, including a proper estimateof the distribution of their absorbing columns. Finally, we concludethat, with the exception of a few cases, the present study agrees withpredictions of unified models of Seyfert galaxies, and extends theirvalidity down to very low luminosities.
| A Search for Candidate Radio Supernova Remnants in the Nearby Irregular Starburst Galaxies NGC 4214 and NGC 4395
We present the results of a search for new candidate radio supernovaremnants (SNRs) in the nearby starburst irregular galaxies NGC 4214 andNGC 4395 using archived radio observations made with the Very LargeArray (VLA) at the wavelengths of 3.5 cm, 6 cm and 20 cm for NGC 4214and 6 cm and 20 cm for NGC 4395. These observations were analyzed aspart of our ongoing search for candidate radio SNRs in nearby galaxies:the goal of this search is to prepare a large sample of candidate radioSNRs for the purpose of a robust statistical study of the properties ofthese sources. Based on our analysis, we have confirmed the non-thermalnature of the discrete radio sources alpha and beta in NGC 4214 andclassify these sources as candidate radio SNRs based on their positionalcoincidences with HII regions in that galaxy. We have measured the fluxdensities of the two candidate radio SNRs at each wavelength andcalculated corresponding spectral indices: we have also measured fluxdensities of two other discrete radio sources in these galaxies - rho inNGC 4214 and #3 in NGC 4395 - which we suspect to be additionalcandidate radio SNRs based on their positional coincidences with otherHII regions in these galaxies. However, the radio data presentlyavailable for these sources cannot confirm such a classification andadditional observations are needed. We have also calculated the radioluminosities Lradio at the wavelength of 20 cm for these twocandidate radio SNRs as well as the corresponding values for the minimumtotal energy Emin required to power these radio sources viasynchrotron emission and the corresponding magnetic field strengthBmin. We have compared our mean calculated values for theseproperties with the mean values for populations of candidate radio SNRsin other starburst galaxies: while the values for Lradio andBmin are roughly comparable to the values seen in otherstarburst galaxies, the mean value for Emin is higher thanthe mean value of any other starburst galaxy. Finally, we include thesetwo candidate radio SNRs in a discussion of the Sigma-D relation forextragalactic candidate radio SNRs and find that these sources arelocated on the shallower end of the master Sigma-D relation for allextragalactic SNRs as derived by Urosevic et al. (2005).
| Astrophysics in 2004
In this 14th edition of ApXX,1 we bring you the Sun (§ 2) and Stars(§ 4), the Moon and Planets (§ 3), a truly binary pulsar(§ 5), a kinematic apology (§ 6), the whole universe(§§ 7 and 8), reconsideration of old settled (§ 9) andunsettled (§ 10) issues, and some things that happen only on Earth,some indeed only in these reviews (§§ 10 and 11).
| Reconciling the local galaxy population with damped Lyman α cross-sections and metal abundances
A comprehensive analysis of 355 high-quality Westerbork Synthesis RadioTelescope (WSRT) HI 21-cm line maps of nearby galaxies shows that theproperties and incident rate of damped Lyman α absorption systems(DLAs) observed in the spectra of high-redshift QSOs are in goodagreement with DLAs originating in gas discs of galaxies like those inthe z~ 0 population. Comparison of low-z DLA statistics with the HIincidence rate and column density distribution f(NHI) for thelocal galaxy sample shows no evidence for evolution in the integral`cross-section density'=l-1 (l= mean freepath between absorbers) below z~ 1.5, implying that there is no need fora hidden population of galaxies or HI clouds to contribute significantlyto the DLA cross-section. Compared with z~ 4, our data indicateevolution of a factor of 2 in the comoving density along a line ofsight. We find that dN/dz(z= 0) = 0.045 +/- 0.006. The idea that thelocal galaxy population can explain the DLAs is further strengthened bycomparing the properties of DLAs and DLA galaxies with the expectationsbased on our analysis of local galaxies. The distribution ofluminosities of DLA host galaxies, and of impact parameters between QSOsand the centres of DLA galaxies, is in good agreement with what isexpected from local galaxies. Approximately 87 per cent of low-z DLAgalaxies are expected to be fainter than L*, and 37 per centhave impact parameters less than 1 arcsec at z= 0.5. The analysis showsthat some host galaxies with very low impact parameters and lowluminosities are expected to be missed in optical follow-up surveys. Thewell-known metallicity-luminosity relation in galaxies, in combinationwith metallicity gradients in galaxy discs, causes the expected medianmetallicity of low-z DLAs to be low (~1/7 solar), which is also in goodagreement with observations of low-z DLAs. We find thatf(NHI) can be fitted satisfactorily with a gammadistribution, a single power law is not a good fit at the highest columndensities NHI > 1021cm-2. The vastmajority (~81 per cent) of the HI gas in the local Universe resides incolumn densities above the classical DLA limit (NHI > 2× 1020cm-2), with NHI~1021cm-2 dominating the cosmic HI mass density.
|
Enviar un nuevo artículo
Enlaces relacionados
- - No se han encontrado enlaces -
En viar un nuevo enlace
Miembro de los siguientes grupos:
|
Datos observacionales y astrométricos
Catálogos y designaciones:
|
