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Can the unresolved X-ray background be explained by the emission from the optically-detected faint galaxies of the GOODS project?
The emission from individual X-ray sources in the Chandra Deep Fieldsand XMM-Newton Lockman Hole shows that almost half of the hard X-raybackground above 6keV is unresolved and implies the existence of amissing population of heavily obscured active galactic nuclei (AGN). Wehave stacked the 0.5-8keV X-ray emission from optical sources in theGreat Observatories Origins Deep Survey (GOODS; which covers the ChandraDeep Fields) to determine whether these galaxies, which are individuallyundetected in X-rays, are hosting the hypothesized missing AGN. In the0.5-6keV energy range, the stacked-source emission corresponds to theremaining 10-20 per cent of the total background - the fraction that hasnot been resolved by Chandra. The spectrum of the stacked emission isconsistent with starburst activity or weak AGN emission. In the 6-8keVband, we find that upper limits to the stacked X-ray intensity from theGOODS galaxies are consistent with the ~40 per cent of the totalbackground that remains unresolved, but further selection refinement isrequired to identify the X-ray sources and confirm their contribution.
| Soft gamma repeaters outside the Local Group
We propose that the best sites to search for soft gamma repeaters (SGRs)outside the Local Group are galaxies with active massive-star formation.Different possibilities to observe SGR activity from these sites arediscussed. In particular, we have searched for giant flares from thenearby galaxies (~2-4 Mpc away) M82, M83, NGC 253 and 4945 in the Burstand Transient Source Experiment (BATSE) data. No candidate giant SGRflares were found. The absence of such detections implies that the rateof giant flares with energy release in the initial spike above 0.5× 1044 erg is less than 1/30 yr-1 in ourGalaxy. However, hyperflares similar to that of 2004 December 27 can beobserved from larger distances. Nevertheless, we do not see anysignificant excess of short GRBs from the Virgo galaxy cluster or fromthe galaxies Arp 299 and NGC 3256 (both with extremely high starformation rates). This implies that the Galactic rate of hyperflareswith energy release ~1046 erg is less than ~10-3yr-1. With this constraint the fraction of possibleextragalactic SGR hyperflares among BATSE's short GRBs should not exceeda few per cent. We present the list of short GRBs coincident with thegalaxies mentioned above, and discuss the possibility that some of themare SGR giant flares. We propose that the best target for theobservations of extragalactic SGR flares with Swift is the Virgocluster.
| Optical Counterparts of Ultraluminous X-Ray Sources Identified from Archival HST WFPC2 Images
We present a systematic analysis of archival HST WFPC2 ``Association''data sets that correlate with the Chandra positions of a set of 44ultraluminous X-ray sources (ULXs) of nearby galaxies. The mainmotivation is to address the nature of ULXs by searching for opticalcounterparts. Sixteen of the ULXs are found in early-type galaxies (RC3Hubble type <3). We have improved the Chandra/HST relative astrometrywhenever possible, resulting in errors circles of 0.3"-1.7" in size.Disparate numbers of potential ULX counterparts are found, and in somecases none are found. The lack of or low number of counterparts in somecases may be due to insufficient depth in the WFPC2 images. Particularlyin late-type galaxies, the HST image in the ULX region was often complexor crowded, requiring source detection to be performed manually. Wetherefore address various scenarios for the nature of the ULX since itis not known which, if any, of the sources found are true counterparts.The optical luminosities of the sources are typically in the range104-106 Lsolar, with (effective) Vmagnitudes typically in the range 22-24. In several cases colorinformation is available, with the colors roughly tending to be more redin early-type galaxies. This suggests that, in general, the (potential)counterparts found in early-type galaxies are likely to be older stellarpopulations and are probably globular clusters. Several early-typegalaxy counterparts have blue colors, which may be due to youngerstellar populations in the host galaxies, however, these could also bebackground sources. In spiral galaxies the sources may also be due tolocalized structure in the disks rather than bound stellar systems.Alternatively, some of the counterparts in late-type galaxies may beisolated supergiant stars. The observed X-ray/optical flux ratio isdiluted by the optical emission of the cluster in cases where the systemis an X-ray binary in a cluster, particularly in the case of a low-massX-ray binaries in an old cluster. If any of the counterparts are boundsystems with ~104-106 stars and are the truecounterparts to the ULX sources, then the X-ray luminosities of the ULXare generally well below the Eddington limit for a black hole with mass~0.1% of the cluster mass. Finally, we find that the optical flux of thecounterparts is consistent with being dominated by emission from anaccretion disk around an intermediate-mass black hole if the black holehappens to have a mass >~102 Msolar and isaccreting at close to the Eddington rate, unless the accretion disk isirradiated (which would result in high optical disk luminosities atlower black hole masses).Based on observations made with the NASA/ESA Hubble Space Telescope,obtained from the Data Archive at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS 5-26555. This project isassociated with Archival proposal 9545.
| A 2 Millimeter Spectral Line Survey of the Starburst Galaxy NGC 253
We present the first unbiased molecular line survey toward anextragalactic source, namely the nuclear region of the starburst galaxyNGC 253. The scan covers the frequency band from 129.1 to 175.2 GHz,i.e., most of the 2 mm atmospheric window. We identify 111 spectralfeatures as transitions from 25 different molecular species. Eight ofwhich (three tentatively) are detected for the first time in theextragalactic interstellar medium. Among these newly detected species,we detected the rare isotopomers 34SO andHC18O+. Tentative detections of two deuteratedspecies, DNC and N2D+, are reported for the firsttime from a target beyond the Magellanic Clouds. In addition, threehydrogen recombination lines are identified, while no organic moleculeslarger than methanol are detected. Column densities and rotationtemperatures are calculated for all the species, including an upperlimit to the ethanol abundance. A comparison of the chemical compositionof the nuclear environment of NGC 253 with those of selected nearbygalaxies demonstrates the chemical resemblance of IC 342 and NGC 4945 tothat of NGC 253. On the other hand, the chemistries characterizing NGC253 and M82 are clearly different. We also present a comparison of thechemical composition of NGC 253 with those observed in Galacticprototypical sources. The chemistry of NGC 253 shows a strikingsimilarity with the chemistry observed toward the Galactic centermolecular clouds, which are thought to be dominated by low-velocityshocks. This resemblance strongly suggests that the heating in thenuclear environment of NGC 253 is dominated by the same mechanism asthat in the central region of the Milky Way.
| On the Fraction of X-Ray-obscured Quasars in the Local Universe
Recent wide-area hard X-ray and soft gamma-ray surveys have shown thatthe fraction of X-ray-obscured active galactic nuclei (AGNs) in thelocal universe significantly decreases with intrinsic luminosity. Inthis Letter we point out that two corrections have to be made to thesamples: (1) radio-loud AGNs have to be excluded, since their X-rayemission might be dominated by the jet component, and (2) Compton-thicksources have to be excluded too, since their hard X-ray and softgamma-ray emission are also strongly attenuated by Compton scattering.The soft gamma-ray-selected AGN samples obtained by Swift and INTEGRALprovide the best opportunity to study the fraction of obscured AGNs inthe local universe in the least biased way. We choose these samples tocheck if the corrections could alter the above result on the fraction ofobscured AGNs. We find that before the corrections both samples showsignificant anticorrelation between LX and NH,indicating an obvious decrease in the fraction of obscured AGNs withluminosity. However, after the corrections, we find only marginalevidence of anticorrelation (at the 98% confidence level) in the Swiftsample and no evidence at all in the INTEGRAL sample, which consists ofa comparable number of objects. We conclude that current samples onlyshow a marginal decrease in the fraction of obscured AGNs in the localuniverse and that much larger samples are required in order to reach amore robust conclusion.
| A New Water Vapor Megamaser
We report on the detection of a new megamaser, the31,3-22,0 H2O line(ν0=183.310 GHz) in Arp 220, using the Institut deRadioastronomie Millimétrique (IRAM) 30 m telescope. The line isabout 350 km s-1 wide with a total luminosity of ~2.5 ×108 K km s-1 pc2. Although OHmegamasers were first discovered in this source, no emission is seen inthe 61,6-52,3 H2O transition(ν0=22.235 GHz), a line otherwise detected as a megamaserin about 50 sources to date. This fact puts interesting constraints onthe physical conditions of the central region of Arp 220 that arefurther strengthened by the HCN and HNC J=3-2 and J=1-0 luminosities [inthe range (1.5-10) × 108 K km s-1pc2]. A scenario with ~106 star-forming coressimilar to those found in Sgr B2 in the central kiloparsec of Arp 220would be compatible with these data and would explain the lack of 22 GHzH2O emission. This result opens up the possibility of usingthe 183 GHz H2O line as an additional tool to explore thephysical conditions in luminous and ultraluminous infrared galaxies(LIRGs and ULIRGs, respectively) and their starburst or active galacticnucleus (AGN) nature, with a potential interest for high angularresolution observations with the Atacama Large Millimeter Array (ALMA).
| A Comprehensive Search for Gamma-Ray Lines in the First Year of Data from the INTEGRAL Spectrometer
We have carried out an extensive search for gamma-ray lines in the firstyear of public data from the spectrometer (SPI) on the INTEGRAL mission.INTEGRAL has spent a large fraction of its observing time in theGalactic plane with particular concentration in the Galactic center (GC)region (~3 Ms in the first year). Hence the most sensitive searchregions are in the Galactic plane and center. The phase space of thesearch spans the energy range 20-8000 keV and line widths from 0 to 1000keV (FWHM). It includes both diffuse and pointlike emission. We havesearched for variable emission on timescales down to ~1000 s. Diffuseemission has been searched for on a range of different spatial scalesfrom ~20° (the approximate field of view of the spectrometer) up tothe entire Galactic plane. Our search procedures were verified by therecovery of the known gamma-ray lines at 511 and 1809 keV at theappropriate intensities and significances. We find no evidence for anypreviously unknown gamma-ray lines. The upper limits range from afew×10-5 to a few×10-2 cm-2s-1 depending on line width, energy, and exposure; regions ofstrong instrumental background lines were excluded from the search.Comparison is made between our results and various prior predictions ofastrophysical lines.
| 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.
| Chandra and Spitzer Unveil Heavily Obscured Quasars in the Chandra/SWIRE Survey
Using the large multiwavelength data set in the Chandra/SWIRE Survey(0.6 deg2 in the Lockman Hole), we show evidence for theexistence of highly obscured (Compton-thick) AGNs, estimate a lowerlimit to their surface density, and characterize their multiwavelengthproperties. Two independent selection methods based on the X-ray andinfrared spectral properties are presented. The two selected samplescontain (1) five X-ray sources with hard X-ray spectra and columndensities >~1024 cm-2 and (2) 120 infraredsources with red and AGN-dominated infrared SEDs. We estimate a surfacedensity of at least 25 Compton-thick AGNs deg-2 detected inthe infrared in the Chandra/SWIRE field, of which ~40% show distinct AGNsignatures in their optical/near-infrared SEDs, the remaining beingdominated by the host galaxy emission. Only ~33% of all Compton-thickAGNs are detected in the X-rays at our depth [F(0.3-8keV)>10-15 ergs cm-2 s-1]. We reportthe discovery of two sources in our sample of Compton-thick AGNs, SWIREJ104409.95+585224.8 (z=2.54) and SWIRE J104406.30+583954.1 (z=2.43),which are the most luminous Compton-thick AGNs at high z currentlyknown. The properties of these two sources are discussed in detail withan analysis of their spectra, SEDs, luminosities, and black hole masses.Some of the data presented herein were obtained at the W. M. KeckObservatory, which is operated as a scientific partnership among theCalifornia Institute of Technology, the University of California, andthe National Aeronautics and Space Administration. The Observatory wasmade possible by the generous financial support of the W. M. KeckFoundation. Based on observations at the Kitt Peak National Observatory,National Optical Astronomy Observatory, which is operated by theAssociation of Universities for Research in Astronomy, Inc., undercooperative agreement with the National Science Foundation. The NationalRadio Astronomy Observatory is a facility of the National ScienceFoundation operated under a cooperative agreement by AssociatedUniversities, Inc.
| INTEGRAL and RXTE Observations of Centaurus A
INTEGRAL and RXTE performed three simultaneous observations of thenearby radio galaxy Centaurus A in 2003 March, 2004 January, and 2004February with the goals of investigating the geometry and emissionprocesses via the spectral/temporal variability of the X-ray/low-energygamma-ray flux, and intercalibration of the INTEGRAL instruments withrespect to those on RXTE. Cen A was detected by both sets of instrumentsfrom 3 to 240 keV. When combined with earlier archival RXTE results, wefind the power-law continuum flux and the line-of-sight column depthvaried independently by 60% between 2000 January and 2003 March.Including the three archival RXTE observations, the iron-line flux wasessentially unchanging, and from this we conclude that theiron-line-emitting material is distant from the site of the continuumemission, and that the origin of the iron-line flux is still an openquestion. Taking X-ray spectral measurements from satellite missionssince 1970 into account, we discover a variability in the column depthbetween 1.0×1023 and 1.5×1023cm-2 separated by approximately 20 yr, and suggest thatvariations in the edge of a warped accretion disk viewed nearly edge-onmight be the cause. The INTEGRAL OSA 4.2 calibration of JEM-X, ISGRI,and SPI yields power-law indices consistent with the RXTE PCA and HEXTEvalues, but the indices derived from ISGRI alone are about 0.2 greater.Significant systematics are the limiting factor for INTEGRAL spectralparameter determination.
| 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).
| Extended X-Ray Emission from QSOs
We report Chandra ACIS observations of the fields of four QSOs showingstrong extended optical emission-line regions. Two of these show noevidence for significant extended X-ray emission. The remaining twofields, those of 3C 249.1 and 4C 37.43, show discrete (but resolved)X-ray sources at distances ranging from ~10 to ~40 kpc from the nucleus.In addition, 4C 37.43 also may show a region of diffuse X-ray emissionextending out to ~65 kpc and centered on the QSO. It has been suggestedthat extended emission-line regions such as these may originate in thecooling of a hot intragroup medium. We do not detect a general extendedmedium in any of our fields, and the upper limits we can place on itspresence indicate cooling times of at least a few 109 yr. Thediscrete X-ray emission sources we detect cannot be explained as theX-ray jets frequently seen associated with radio-loud quasars, nor canthey be due to electron scattering of nuclear emission. The mostplausible explanation is that they result from high-speed shocks fromgalactic superwinds resulting either from a starburst in the QSO hostgalaxy or from the activation of the QSO itself. Evidence from thedensities and velocities found in studies of the extended opticalemission around QSOs also supports this interpretation.Based in part on data obtained with the Chandra X-Ray Observatory, whichis operated for the National Aeronautics and Space Administration by theSmithsonian Astrophysical Observatory. Also based in part onobservations made with the NASA/ESA Hubble Space Telescope, obtainedfrom the Data Archive at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research 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.
| The Double Active Galactic Nucleus in NGC 6240 Revealed through 3-5 μm Spectroscopy
We present 3-5 μm spectroscopy of the interacting system NGC 6240,revealing the presence of two active galactic nuclei (AGNs). Thebrightest (southern) nucleus shows up with a starburst-like emission,with a prominent 3.3 μm emission feature. However, the presence of anAGN is revealed by the detection of a broad Brα emission line,with a width of ~1800 km s-1. The spectrum of the faintest(northern) nucleus shows typical AGN features, such as a steep continuumand broad absorption features in the M band. We discuss the physicalproperties of the dusty absorbers/emitters, and we show that in bothnuclei, the AGN is dominant in the 3-5 μm band but that itscontribution to the total luminosity is small (a few percent of thestarburst emission).Based on observations collected at the European Southern Observatory,Chile (proposal 73.B-0574).
| INTEGRAL IBIS Extragalactic Survey: Active Galactic Nuclei Selected at 20-100 keV
Analysis of International Gamma-Ray Astrophysics Laboratory (INTEGRAL)Core Programme and public open-time observations performed up to 2005April provides a sample of 62 active galactic nuclei in the 20-100 keVband above a flux limit of ~1.5×10-11 ergscm-2 s-1. Most (42) of the sources in the sampleare Seyfert galaxies, almost equally divided between type 1 and type 2objects; six are blazars, and 14 are still unclassified. Excluding theblazars, the average redshift of our sample is 0.021, while the meanluminosity is logL=43.45. We find that absorption is present in 65% ofthe objects, with 14% of the total sample due to Compton-thick activegalaxies. In agreement with both Swift BAT team results and 2-10 keVstudies, the fraction of absorbed objects decreases with the 20-100 keVluminosity. All Seyfert 2's in our sample are absorbed, as are 33% ofSeyfert 1's. The present data highlight the capability of INTEGRAL toprobe the extragalactic gamma-ray sky and to find new and/or absorbedactive galaxies.Based on observations obtained with INTEGRAL, an ESA project withinstruments and science data center funded by ESA member states(especially the PI countries: Denmark, France, Germany, Italy,Switzerland, Spain), the Czech Republic, and Poland and with theparticipation of Russia and the US.
| 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.
| Infrared 3-4 μm Spectroscopy of Infrared Luminous Galaxies with Possible Signatures of Obscured Active Galactic Nuclei
We present the results of infrared 2.8-4.1 μm (L-band) spectroscopyof nearby infrared luminous galaxies with possible signatures ofdust-obscured active galactic nuclei (AGNs) in data at otherwavelengths. The samples are chosen to include sources with a radioexcess relative to far-infrared emission, strong absorption features inmid-infrared 5-11.5 μm spectra, unusually weak [C II] 158 μmemission relative to the far-infrared continuum, and radio galaxiesclassified optically as narrow-line objects. Our aim is to investigatewhether the signatures of possible obscured AGNs can be detected in ourL-band spectra based on the strengths of emission and absorptionfeatures. Six of nine observed sources clearly show 3.3 μm polycyclicaromatic hydrocarbon emission features, a good starburst indicator. Anabsorption feature at 3.1 μm due to ice-covered dust is detected inIRAS 04154+1755 and IRAS 17208-0014. The signature of a barecarbonaceous dust absorption feature at 3.4 μm is seen in NGC 1377.Our L-band spectra reveal strong signatures of obscured AGNs in allthree optical Seyfert 2 galaxies (IRAS 04154+1755, Cygnus A, and 3C 234)and two galaxies classified optically as non-Seyfert galaxies (NGC 828and NGC 1377). Among the remaining optical non-Seyfert galaxies, IRAS17208-0014 might also show a buried AGN signature, whereas no explicitAGN evidence is seen in the L-band spectra of the mid-infraredabsorption feature source IRAS 15250+3609 and two weak [C II] emitters,IC 860 and CGCG 1510.8+0725.Based in part on data collected at the Subaru Telescope, which isoperated by the National Astronomical Observatory of Japan.
| The Molonglo Southern 4 Jy Sample (MS4). II. ATCA Imaging and Optical Identification
Of the 228 sources in the Molonglo Southern 4 Jy sample (MS4), the 133with angular sizes <35" have been imaged at 5 GHz at 2"-4" resolutionwith the Australia Telescope Compact Array. More than 90% of the samplehas been reliably optically identified, either on the plates of the UKSchmidt Southern Sky Survey or on R-band CCD images made with theAnglo-Australian Telescope. A subsample of 137 sources, the SMS4,defined to be a close southern equivalent of the northern 3CRR sample,was found to have global properties mostly consistent with the northernsample. Linear sizes of MS4 galaxies and quasars were found to beconsistent with galaxy-quasar unification models of orientation andevolution.
| 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.
| Unveiling the nature of INTEGRAL objects through optical spectroscopy. IV. A study of six new hard X-ray sources
We present further results from our onging optical spectrophotometriccampaign at the Astronomical Observatory of Bologna in Loiano (Italy) onunidentified hard X-ray sources detected by INTEGRAL. We observedspectroscopically the putative optical counterparts of the INTEGRALsources IGR J00234+6141, IGR J01583+6713, IGR J06074+2205, IGRJ13091+1137 and IGR J20286+2544. We find that the first two are Galacticobjects, namely a Cataclysmic Variable at a distance d 300 pc anda Be/X transient High-Mass X-ray Binary (HMXB) located at ~6.4 kpc,respectively, whereas the last one is identified with MCG +04-48-002, aStarburst/H ii galaxy at redshift z = 0.013 hiding a Seyfert 2 nucleus.We identify IGR J13091+1137 as the (likely Seyfert 2 type) activenucleus of galaxy NGC 4992, which we classify as an X-ray Bright,Optically Normal Galaxy; this is the first example of this type ofobject to be detected by INTEGRAL, and one of the closest of this class.We moreover confirm the possible Be/X nature of IGR J06074+2205, and weestimate it to be at a distance of ~1 kpc. We also reexamine thespectrum of the z = 0.087 elliptical radio galaxy PKS 0352-686, thepossible counterpart of the INTEGRAL source IGR J03532-6829, and we findthat it is a BL Lac. Physical parameters for these sources are alsoevaluated by discussing our findings in the context of the availablemultiwavelength information. These identifications further stress theimportance of INTEGRAL in the study of the hard X-ray spectrum of ActiveGalactic Nuclei, HMXBs and Cataclysmic Variables.
| A 0.8 mm heterodyne facility receiver for the APEX telescope
Aims.The new APEX telescope, located on Llano Chajnantor in NorthernChile, will have high resolution spectroscopic instruments covering thewavelength region from 0.20 to 1.30 mm (210-1500 GHz). Methods:.In May 2005, the first facility receiver for the band 0.79-1.07 mm(279-381 GHz) was installed together with backends providing down to 60kHz spectral resolution. This instrument that operates in doublesideband mode uses superconducting tunnel junctions (SIS) as mixingelements operating at 4 K to achieve close to quantum-limited noiseperformances. The receiver is cooled by a closed-cycle cooling machinethat allows continuous operation. The receiver design minimizes movingparts and is fully operated by remote to improve its reliability and theease of use. Results: .The double sideband (DSB) receivertemperatures are in the range 50-70 K, which typically results in a DSBsystem noise temperature of about 100 K in excellent weather conditionsand between 100-200 K in good weather conditions.
| Optical polarimetric monitoring of the type II-plateau SN 2005af
Aims.Core-collapse supernovae may show significant polarization thatimplies non-spherically symmetric explosions. We observed the typeII-plateau SN 2005af using optical polarimetry inorder to verify whether any asphericity is present in the supernovatemporal evolution. Methods: .We used the IAGPOL imagingpolarimeter to obtain optical linear polarization measurements in {R}(five epochs) and {V} (one epoch) broadbands. Interstellar polarizationwas estimated from the field stars in the CCD frames. The opticalpolarimetric monitoring began around one month after the explosion andlasted 30 days, between the plateau and the early nebular phase.Results: .The weighted mean observed polarization in {R} band was [ 1.89± 0.03] % at position angle (PA) 54.1 °. After foregroundsubtraction, the level of the average intrinsic polarization forSN 2005af was 0.5% with a slight enhancement duringthe plateau phase and a decline at early nebular phase. A rotation in PAon a time scale of days was also observed. The polarimetric evolution ofSN 2005af in the observed epochs is consistent withan overall asphericity of 20% and an inclination of 30°. Evidencefor a more complex, evolving asphericity, possibly involving clumps inthe SN 2005af envelope, is found.
| Methanol detection in M 82
Context: .The nuclear starburst region in M 82 shows systematical lowabundances of some complex molecules when compared with other starburstgalaxies. This is likely related to a presumably photodissociationdominated environment. In particular, methanol is known to showrelatively low abundance because it is easily photodissociated.Aims: .We present a multilevel study of the emission of methanol,detected for the first time in this galaxy, and discuss the origin ofits emission. Methods: .Observations of three transitions of CH3OHtowards the center and two positions around the nucleus of M 82 arepresented. Two different components are found, one with high excitation(n(H_2) 10^6 cm-3, T_rot 20 K) and the other withlow excitation (n(H_2) 10^4 cm-3, T_rot 5 K).Results: .The high observed methanol abundance of a few 10-9can only be explained if injection of methanol from dust grains is takeninto account. While the overall [ CH3OH] /[ NH3] ratio is much largerthan observed towards other starbursts, the dense high excitationcomponent shows a similar value to that found in NGC 253 and Maffei 2. Conclusions: .Our observations suggest the molecular material inM 82 to be formed by dense warm cores, shielded from the UV radiationand similar to the molecular clouds in other starbursts, surrounded by aless dense photodissociated halo. The dense warm cores are likely thelocation of recent and future star formation within M 82.
| 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.
| Dwarf elliptical galaxies in Centaurus A group: stellar populations in AM 1339-445 and AM 1343-452
We study the red giant populations of two dE galaxies, AM 1339-445 andAM 1343-452, with the aim of investigating the number and luminosity ofany upper asymptotic giant branch (AGB) stars present. The galaxies aremembers of the Centaurus A group (D ≈ 3.8 Mpc) and are classified asoutlying (R ≈ 350 kpc) satellites of Cen A. The analysis is based onnear-IR photometry for individual red giant stars, derived from imagesobtained with ISAAC on the VLT. The photometry, along with optical dataderived from WFPC2 images retrieved from the HST science archive, enableus to investigate the stellar populations of the dEs in the vicinity ofthe red giant branch (RGB) tip. In both systems we find stars above theRGB tip, which we interpret as intermediate-age upper-AGB stars. Thepresence of such stars is indicative of extended star formation in thesedEs similar to that seen in many, but not all, dEs in the Local Group.For AM 1339-445, the brightest of the upper-AGB stars haveMbol ≈-4.5 while those in AM 1343-452 have Mbol≈ -4.8 mag. These luminosities suggest ages of approximately 6.5± 1 and 4 ± 1 Gyr as estimates for the epoch of the lastepisode of significant star formation in these systems. In both casesthe number of upper-AGB stars suggests that ~15% of the total stellarpopulation is in the form of intermediate-age stars, considerably lessthan is the case for outlying dE satellites of the Milky Way such asFornax and Leo I.
| NGC 7679: an anomalous, composite Seyfert 1 galaxy whose X-ray luminous AGN vanishes at optical wavelengths
Morphological disturbances and gas kinematics of the SB0 galaxy NGC 7679= Arp 216 are investigated to understand the history of this highlycomposite object, where AGN and starburst signatures dominate in theX-ray and optical/IR regime, respectively. Perturbations of the ionizedgas velocity field appear quite mild within 15'' (~5 kpc) of the center,so it can be straightforwardly modeled as a circularly rotating disk.Outside that radius, significant disturbances are seen. In particular,the eastern distorted arm as well as the huge neutral hydrogen bridgeconnecting NGC 7679 to the nearby Seyfert spiral NGC 7682 unambiguouslyrepresent the vestige of a close encounter of the two objects ~500 Myrago. The relationship of such a past event with the much more recent,centrally located starburst (not older than 20 Myr) cannot be easilyestablished. Together, the classification of NGC 7679 is less extremethan that proposed in the past, being simply a (disturbed) galaxy wherestarburst and AGN activity coexist with a starburst dominating thebolometric luminosity.
| CI and CO in the center of M 51
We present J=2{-}1, J=3{-}2, J=4{-}3 12CO maps as well asJ=2{-}1, J=3{-}2 13CO and 492 GHz [CI] measurements of thecentral region in M 51. The distribution of CO is strongly concentratedtowards the spiral arms. The center itself is poor in, though not devoidof, CO emission. The observed line intensities require modelling with amulti-component molecular gas. A dense component must be present(n(H2) ≈ 103) with kinetic temperature T_kin≈ 100 K, combined with either a less dense (≈ 102cm-3) component of the same temperature, or a more dense(n(H2) ≈ 3 × 103 cm-3) andmuch cooler (T_kin = 10-30 K) component. Atomic carbon amounts arebetween 5 and 10 times those of CO. Much of the molecular gas mass isassociated with the hot PDR phase. The center of M 51 has a face-on gasmass density of about 40±20 Mȯ pc-2,and a well-established CO-to-H{2} conversion ratio X four to five timeslower than the standard Galactic value.
| Supermassive Black Holes in Galactic Nuclei: Past, Present and Future Research
This review discusses the current status of supermassive black holeresearch, as seen from a purely observational standpoint. Since theearly ‘90s, rapid technological advances, most notably the launchof the Hubble Space Telescope, the commissioning of the VLBA andimprovements in near-infrared speckle imaging techniques, have not onlygiven us incontrovertible proof of the existence of supermassive blackholes, but have unveiled fundamental connections between the mass of thecentral singularity and the global properties of the host galaxy. It isthanks to these observations that we are now, for the first time, in aposition to understand the origin, evolution and cosmic relevance ofthese fascinating objects.
| Detection of Neutrinos from Supernovae in Nearby Galaxies
While existing detectors would see a burst of many neutrinos from aMilky Way supernova, the supernova rate is only a few per century. As analternative, we propose the detection of 1 neutrino per supernovafrom galaxies within 10 Mpc, in which there were at least 9core-collapse supernovae since 2002. With a future 1 Mton scaledetector, this could be a faster method for measuring the supernovaneutrino spectrum, which is essential for calibrating numerical modelsand predicting the redshifted diffuse spectrum from distant supernovae.It would also allow a ≳104 times more precise triggertime than optical data alone for high-energy neutrinos and gravitationalwaves.
| Secular Evolution of Barred Galaxies with Massive Central Black Holes
The influence of central black holes on the dynamical evolution of barsin disk galaxies was examined. Once a bar formed by a dynamicalinstability in an infinitesimally thin stellar disk was fully developed,a black hole (BH) was adiabatically added at the center of the disk. Ourresults indicate that a bar can be completely destroyed, in a practicalsense, in a time much smaller than the Hubble time if the central BHmass exceeds about 0.5% of the disk mass. Since this implied minimum BHmass for bar destruction is on the order of 108.5Modot for a typical disk galaxy, this process could occur inthe real Universe. The bar amplitude decreases gradually with time afterthe BH has grown up to its full mass. Surface-of-section plots indicatethat the bar dissolution originates from the chaotic behavior ofbar-supporting orbits. In addition, the scale length and the radialvelocity dispersion of the disk after bar dissolution become much largerthan those of the initial axisymmetric disk. This finding suggests thatit is possible to discriminate between genuine non-barred galaxies andbar-dissolved galaxies induced by massive central BHs from the viewpointof structural properties.
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