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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.
| 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 XMM-Newton View of the Relativistic Spectral Features in AX J0447-0627
The XMM-Newton observation of the optically type 1 active galacticnucleus (AGN) AX J0447-0627 (z=0.214) unambiguously reveals a complex,bright, and prominent set of lines in the 4-8 keV rest-frame energyrange. Although from a phenomenological point of view, the observedproperties can be described by a simple power-law model plus five narrowGaussian lines (at rest-frame energies of ~4.49, ~5.55, ~6.39, ~7.02,and ~7.85 keV), we find that a model comprising a power law(Γ~2.2), a reflected relativistic continuum, a narrow Fe IKα line from neutral material, as well as a broad Fe Kαrelativistic line from a ionized accretion disk, represents a goodphysical description of the data. The ``double-horned'' profile of therelativistic line implies an inclination of the accretion disk of~45° and an origin in a narrow region of the disk, fromRin~19GM/c2 to Rout~30GM/c2.The narrow Fe I Kα line from neutral material is probably producedfar from the central black hole, most likely in the putative moleculartorus. Although some of these properties have already been found inother type 1 AGNs and discussed in the literature, at odds with theobjects reported so far we measure high equivalent widths of theobserved lines: ~1.4 keV for the double-horned relativistic line and~0.4 keV for the narrow line.
| A Uniform Database of 2.2-16.5 μm Spectra from the ISOCAM CVF Spectrometer
We present all ISOCAM circular variable filter (CVF) spectra that covermore than one-third of the 2.2-16.5 μm spectral range of theinstrument. The 364 spectra have been classified according to theclassification system of Kraemer et al., as modified by Hodge et al. toaccount for the shorter wavelength range. Prior to classification, thespectra were processed and recalibrated to create a uniform database.Aperture photometry was performed at each wavelength centered on thebrightest position in each image field and the various spectral segmentsmerged into a single spectrum. The aperture was the same for all scalesizes of the images. Since this procedure differs fundamentally fromthat used in the initial ISOCAM calibration, a recalibration of thespectral response of the instrument was required for the aperturephotometry. The recalibrated spectra and the software used to createthem are available to the community on-line via the ISO Data Archive.Several new groups were added to the KSPW system to describe spectrawith no counterparts in either the SWS or PHT-S databases: CA, E/SA,UE/SA, and SSA. The zodiacal dust cloud provides the most commonbackground continuum to the spectral features, visible in almost 40% ofthe processed sources. The most characteristic and ubiquitous spectralfeatures observed in the CVF spectral atlas are those of theunidentified infrared bands (UIR), which are typically attributed toultraviolet-excited fluorescence of large molecules containing aromatichydrocarbons. The UIR features commonly occur superimposed on thezodiacal background (18%) but can also appear in conjunction with otherspectral features, such as fine-structure emission lines or silicateabsorption. In at least 13 of the galaxies observed, the pattern of UIRemission features has been noticeably shifted to longer wavelengths.Based on observations with the Infrared Space Observatory, a EuropeanSpace Agency (ESA) project with instruments funded by ESA Member States(especially the Principal Investigator countries: France, Germany, theNetherlands, and the United Kingdom) and with the participation of theInstitute of Space and Astronautical Science (ISAS) and the NationalAeronautics and Space Administration (NASA).
| Spatial Correlation Function of X-Ray-selected Active Galactic Nuclei
We present a detailed description of the first direct measurement of thespatial correlation function of X-ray-selected active galactic nuclei(AGNs). This result is based on an X-ray flux-limited sample of 219 AGNsdiscovered in the contiguous 80.7 deg2 region of the ROSATNorth Ecliptic Pole (NEP) Survey. Clustering is detected at the 4σ level at comoving scales in the interval r=5-60h-1Mpc. Fitting the data with a power law of slope γ=1.8, we find acorrelation length ofr0=7.4+1.8-1.9h-1 Mpc(ΩM=0.3, ΩΛ=0.7). The medianredshift of the AGN contributing to the signal is zξ=0.22.This clustering amplitude implies that X-ray-selected AGNs are spatiallydistributed in a manner similar to that of optically selected AGNs.Furthermore, the ROSAT NEP determination establishes the local behaviorof AGN clustering, a regime that is poorly sampled in general. Combinedwith high-redshift measures from optical studies, the ROSAT NEP resultsargue that the AGN correlation strength essentially does not evolve withredshift, at least out to z~2.2. In the local universe, X-ray-selectedAGNs appear to be unbiased relative to galaxies, and the inferred X-raybias parameter is near unity, bX~1. Hence, X-ray-selectedAGNs closely trace the underlying mass distribution. The ROSAT NEP AGNcatalog, presented here, features complete optical identifications andspectroscopic redshifts. The median redshift, X-ray flux, and X-rayluminosity are z=0.41, fX=1.1×10-13 ergscm-2 s-1, andLX=9.2×1043h-270 ergss-1 (0.5-2.0 keV), respectively. Unobscured, type I AGNs arethe dominant constituents (90%) of this soft X-ray-selected sample ofAGNs.
| The ISOPHOT 170 μm Serendipity Survey II. The catalog of optically identified galaxies%
The ISOPHOT Serendipity Sky Survey strip-scanning measurements covering≈15% of the far-infrared (FIR) sky at 170 μm were searched forcompact sources associated with optically identified galaxies. CompactSerendipity Survey sources with a high signal-to-noise ratio in at leasttwo ISOPHOT C200 detector pixels were selected that have a positionalassociation with a galaxy identification in the NED and/or Simbaddatabases and a galaxy counterpart visible on the Digitized Sky Surveyplates. A catalog with 170 μm fluxes for more than 1900 galaxies hasbeen established, 200 of which were measured several times. The faintest170 μm fluxes reach values just below 0.5 Jy, while the brightest,already somewhat extended galaxies have fluxes up to ≈600 Jy. For thevast majority of listed galaxies, the 170 μm fluxes were measured forthe first time. While most of the galaxies are spirals, about 70 of thesources are classified as ellipticals or lenticulars. This is the onlycurrently available large-scale galaxy catalog containing a sufficientnumber of sources with 170 μm fluxes to allow further statisticalstudies of various FIR properties.Based on observations with ISO, an ESA project with instruments fundedby ESA Member States (especially the PI countries: France, Germany, TheNetherlands and the UK) and with the participation of ISAS and NASA.Members of the Consortium on the ISOPHOT Serendipity Survey (CISS) areMPIA Heidelberg, ESA ISO SOC Villafranca, AIP Potsdam, IPAC Pasadena,Imperial College London.Full Table 4 and Table 6 are only available in electronic form at theCDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/422/39
| The ROSAT North Ecliptic Pole Survey: the Optical Identifications
The X-ray data around the North Ecliptic Pole (NEP) of the ROSAT All SkySurvey have been used to construct a contiguous area survey consistingof a sample of 445 individual X-ray sources above a flux of~2×10-14 ergs cm-2 s-1 in the0.5-2.0 keV energy band. The NEP survey is centered atα2000=18h00m,δ2000=+66deg33' and covers aregion of 80.7 deg2 at a moderate Galactic latitude ofb=29.8d. Hence, the NEP survey is as deep and covers a comparable solidangle to the ROSAT serendipitous surveys but is also contiguous. We haveidentified 99.6% of the sources and determined redshifts for theextragalactic objects. In this paper we present the opticalidentifications of the NEP catalog of X-ray sources including basicX-ray data and properties of the sources. We also describe with somedetail the optical identification procedure. The classification of theoptical counterparts to the NEP sources is very similar to that ofprevious surveys, in particular the Einstein Extended Medium SensitivitySurvey (EMSS). The main constituents of the catalog are active galacticnuclei (AGNs) (~49%), either type 1 or type 2 according to the broadnessof their permitted emission lines. Stellar counterparts are the secondmost common identification class (~34%). Clusters and groups of galaxiescomprise 14%, and BL Lacertae objects 2%. One non-AGN galaxy and oneplanetary nebula have also been found. The NEP catalog of X-ray sourcesis a homogeneous sample of astronomical objects featuring completeoptical identification.
| The Lack of Broad-Line Regions in Low Accretion Rate Active Galactic Nuclei as Evidence of Their Origin in the Accretion Disk
In this Letter, we present evidence suggesting that the absence orpresence of hidden broad-line regions (HBLRs) in Seyfert 2 galaxies isregulated by the rate at which matter accretes onto a centralsupermassive black hole, in units of the Eddington rate. Evidence isbased on data from a subsample of type 2 active galactic nucleiextracted from the Tran spectropolarimetric sample and made up of allthose sources that also have good-quality X-ray spectra available andfor which a bulge luminosity can be estimated. We use the intrinsic(i.e., unabsorbed) X-ray luminosities of these sources and their blackhole masses (estimated by using the well-known relationship betweennuclear mass and bulge luminosity in galaxies) to derive the nuclearaccretion rate in Eddington units. We find that virtually all HBLRsources have accretion rates larger than a threshold value ofmthres~=10-3 (in Eddington units), while non-HBLRsources lie at m<~mthres. These data nicely fitpredictions from a model proposed by Nicastro in which the broad-lineregions (BLRs) are formed by accretion disk instabilities occurring inproximity of the critical radius at which the disk changes from gaspressure dominated to radiation pressure dominated. This radiusdiminishes with decreasing m for low enough accretion rates (andtherefore luminosities), the critical radius becomes smaller than theinnermost stable orbit and BLRs cannot form.
| The Unified Model and Evolution of Active Galaxies: Implications from a Spectropolarimetric Study
We extend the analysis presented in Paper I of a spectropolarimetricsurvey of the CfA and 12 μm samples of Seyfert 2 galaxies (S2s). Weconfirm that polarized (hidden) broad-line region (HBLR) S2s tend tohave hotter circumnuclear dust temperatures, show mid-IR spectra morecharacteristic of Seyfert 1 galaxies (S1s), and are intrinsically moreluminous than non-HBLR S2s. The levels of obscuration and circumnuclearstar formation, however, appear to be similar between HBLR and non-HBLRS2 galaxies, based on an examination of various observationalindicators. HBLR S2s, on average, share many similar large-scale,presumably isotropic, characteristics with S1s, as would be expected ifthe unified model is correct, while non-HBLR S2s generally do not. Theactive nuclear engines of non-HBLR S2s, then, appear to be truly weakerthan HBLR S2s, which in turn are fully consistent with being S1s viewedfrom another direction. There is also evidence that the fraction ofdetected HBLRs increases with the radio power of the active galacticnucleus. Thus, all S2 galaxies may not be intrinsically similar innature, and we speculate that evolutionary processes may be at work.
| Fe Kα emission from photoionized slabs: the impact of the iron abundance
Iron Kα emission from photoionized and optically thick material isobserved in a variety of astrophysical environments including X-raybinaries, active galactic nuclei and possibly gamma-ray bursts. Thispaper presents calculations showing how the equivalent width (EW) of theFe Kα line depends on the iron abundance of the illuminated gasand its ionization state - two variables subject to significant cosmicscatter. Reflection spectra from a constant density slab which isilluminated with a power-law spectrum with photon-index Γ arecomputed using the code of Ross & Fabian. When the Fe Kα EW ismeasured from the reflection spectra alone, we find that it can reachvalues greater than 6keV if the Fe abundance is about 10 times the solarvalue and the illuminated gas is neutral. EWs of about 1keV are obtainedwhen the gas is ionized. In contrast, when the EW is measured from theincident+reflected spectrum, the largest EWs are ~800keV and are foundwhen the gas is ionized. When Γ is increased, the Fe Kα linegenerally weakens, but significant emission can persist to largerionization parameters. The iron abundance has its greatest impact on theEW when it is less than 5 times the solar value. When the abundance isfurther increased, the line strengthens only marginally. Therefore, weconclude that Fe Kα lines with EWs much greater than 800eV areunlikely to be produced by gas with a supersolar Fe abundance. Theseresults should be useful in interpreting Fe Kα emission wheneverit arises from optically thick fluorescence.
| Comparisons of Infrared Colors and Emission-line Intensities between Two types of Seyfert 2 Galaxies
We study the relation between the infrared colors, [OIII] emissionlines, gaseous absorbing column density (NH),and thedetectability of the polarized (hidden) broad-line region (HBLR) in alarge sample of 75 Seyfert 2 galaxies (Sy2s). From the indicators ofstar-formation activity, f60/f100 andLFIR/LB, we find some evidence that the Sy2swithout HBLR show higher star-formation activities than those with HBLR,in agreement with previous prediction. Also, we confirm that the HBLRSy2s tend to have a larger luminosity ratio of the core to the hostgalaxy, suggesting that the HBLR Sy2s display more powerful AGNactivity. However, the level of obscuration found in previous papers isnearly indistinguishable between the two types of Sy2s. The resultssupport the statement that the non-HBLR Sy2s, with a weaker corecomponent and a stronger star-formation activity component, areintrinsically different from the HBLR Sy2s, which are Sy1 systems with ahidden powerful AGN core and a low star-formation activity. Theindications are that the non-HBLR Sy2s might be at an earlierevolutionary phase than the HBLR Sy2s.
| Seyfert 2 Galaxies with Spectropolarimetric Observations
We present a compilation of radio, infrared, optical, and hard X-ray(2-10 keV) data for a sample of 90 Seyfert 2 galaxies (Sy2s) withspectropolarimetric observations (41 Sy2s with detection of polarizedbroad lines [PBLs] and 49 without PBLs). Compared to Sy2s without PBLs,Sy2s with PBLs tend to be earlier type spirals and show warmermidinfrared color and significant excess of emissions (including thehard X-ray [2-10 keV], [O III] λ5007, infrared [25 μm], andradio). Our analyses indicate that the majority of Sy2s without PBLs arethose sources having less powerful active galactic nucleus (AGN)activities, most likely caused by a low accretion rate. It implies thatthe detectability of the polarized broad emission lines in Sy2s maydepend on their central AGN activities in most cases. Based on theavailable data, we find no compelling evidence for the presence of twotypes of Sy2s; one of which has been proposed to be intrinsicallydifferent from Sy2s claimed in the unification model.
| High-resolution radio observations of Seyfert galaxies in the extended 12-μm sample - II. The properties of compact radio components
We discuss the properties of compact nuclear radio components in Seyfertgalaxies from the extended 12-μm AGN sample of Rush et al. Our mainresults can be summarized as follows. Type 1 and type 2 Seyferts producecompact radio components which are indistinguishable in strength andaspect, indicating that their central engines are alike, as proposed bythe unification model. Infrared IRAS fluxes are more closely correlatedwith low-resolution radio fluxes than high-resolution radio fluxes,suggesting that they are dominated by kiloparsec-scale, extranuclearemission regions; extranuclear emission may be stronger in type 2Seyferts. Early-type Seyfert galaxies tend to have stronger nuclearradio emission than late-type Seyfert galaxies. V-shaped extendedemission-line regions, indicative of `ionization cones', are usuallyfound in sources with large, collimated radio outflows. Hidden broadlines are most likely to be found in sources with powerful nuclear radiosources. Type 1 and type 2 Seyferts selected by their IRAS 12-μm fluxdensities have well-matched properties.
| The ASCA HSS: looking for type 2 AGN
We will briefly discuss the X-ray spectral properties of the objects inthe ASCA Hard Serendipitous Survey (HSS) sample and its present AGNcontents. We also present X-ray spectroscopy of two Type 2 AGN belongingto the sample.
| The ASCA Medium Sensitivity Survey (the GIS Catalog Project): Source Catalog
We present the first X-ray source catalog of the ASCA Medium SensitivitySurvey (AMSS, or the GIS catalog project), constructed from data atGalactic latitudes b>10deg observed between 1993 May and 1996December. The catalog utilizes 368 combined fields and contains 1343sources with the detection significance above 5 σ either in thesurvey bands of 0.7-7 keV, 2-10 keV, or 0.7-2 keV, including targetsources. For each source, the ASCA source name, position, a 90% errorradius, count rates in the three bands, detection significances, fluxes,and a hardness ratio are provided. With extensive simulations, wecarefully evaluate the data quality of the catalog. Results fromcross-correlation with other existing catalogs are briefly summarized.
| Hidden Broad-Line Seyfert 2 Galaxies in the CFA and 12 μM Samples
We report the results of a spectropolarimetric survey of the CfA and 12μm samples of Seyfert 2 (S2) galaxies. Polarized (hidden) broad-lineregions (HBLRs) are confirmed in a number of galaxies, and several newcases (F02581-1136, MCG -3-58-7, NGC 5995, NGC 6552, NGC 7682) arereported. The 12 μm S2 galaxy sample shows a significantly higherincidence of HBLRs (50%) than its CfA counterpart (30%), suggesting thatthe latter may be incomplete in hidden active galactic nuclei. Comparedto the non-HBLR S2 galaxies, the HBLR S2 galaxies display distinctlyhigher radio power relative to their far-infrared output and hotter dusttemperature as indicated by the f25/f60 color.However, the level of obscuration is indistinguishable between the twotypes of S2 galaxies. These results strongly support the existence oftwo intrinsically different populations of S2 galaxies: one harboring anenergetic, hidden S1 nucleus with a broad-line region and the other a``pure'' S2 galaxy, with a weak or absent S1 nucleus and a strong,perhaps dominating starburst component. Thus, the simple purelyorientation-based unification model is not applicable to all Seyfertgalaxies.
| Excess hard X-ray emission from the obscured low luminosity AGN in the nearby galaxy M 51 (NGC 5194)
We observed the nearby galaxy M 51 (NGC 5194) with BeppoSAX. The X-rayproperties of the nucleus below 10 keV are almost the same as the ASCAresults regarding the hard component and the neutral Fe Kα line,but the intensity is about half of the ASCA 1993 data. Beyond this, inthe BeppoSAX PDS data, we detected a bright hard X-ray emissioncomponent which dominates above 10 keV. The 10-100 keV flux andluminosity of this component are respectively 2×10-11erg s-1 cm-2 and 2×1041 ergs-1. These are about 10 times higher than the extrapolationfrom the soft X-ray band, and similar to the flux observed with Ginga,which found a bright power law component in 2-20 keV band. Consideringother wavelength properties and the X-ray luminosity, together withstrong neutral Fe K line, the hard X-ray emission most likely arisesfrom a low luminosity active nucleus, which is obscured with a columndensity of ~1024 cm-2. This suggests that hiddenlow luminosity AGNs may well be present in other nearby galaxies. Weinterpret the discrepancy between Ginga and other X-ray satellites to bedue to a large variability of absorption column density toward the lineof sight over several years, suggesting that the Compton thickabsorption material may be present on a spatial scale of a parsec. Apartfrom the nucleus, several ultra-luminous off-nuclear X-ray sourcesdetected in M 51 exhibit long-term time variability, suggesting thestate transition similar to that observed in Galactic black holecandidates.
| Hard X-ray emission from elliptical galaxies
We report the detection of hard X-ray emission components in the spectraof six nearby, giant elliptical galaxies observed with the ASCAsatellite. The systems studied, which exhibit strong dynamical evidencefor supermassive black holes in their nuclei, are M87, NGC 1399 and NGC4696 (the dominant galaxies of the Virgo, Fornax and Centaurus clusters,respectively) and NGC 4472, 4636 and 4649 (three further giantellipticals in the Virgo cluster). The ASCA data for all six sourcesprovide clear evidence for hard emission components, which can beparametrized by power-law models with photon indices in the rangeΓ=0.6-1.5 (mean value 1.2) and intrinsic 1-10keV luminosities of2x1040-2x1042ergs-1. Our results implythe identification of a new class of accreting X-ray source, with X-rayspectra significantly harder than those of binary X-ray sources, Seyfertnuclei or low-luminosity active galactic nuclei, and bolometricluminosities relatively dominated by their X-ray emission. We discussvarious possible origins for the hard X-ray emission and argue that itis most likely to be due to accretion on to the central supermassiveblack holes, via low radiative efficiency accretion flows coupled withstrong outflows. In the case of M87, our detected power-law flux is ingood agreement with a previously reported measurement from ROSAT HighResolution Imager observations, which were able to resolve the jet fromthe nuclear X-ray emission components. We confirm previous resultsshowing that the use of multiphase models in the analysis of the ASCAdata leads to determinations of approximately solar emission-weightedmetallicities for the X-ray gas in the galaxies. We also present resultson the individual element abundances in NGC 4636.
| ISOCAM observations of the very deep IRAS 60 micron sample in the NEP region. I. The data
We present the results of ISO observations with the CAM LW3 filter,centered at lambda_eff = 14.3 mu m, of 94 out of the 98 galaxiescomprising the complete 60 mu m IRAS deep survey (IDS) sample in thenorth ecliptic polar region. In addition, we observed a source detectedby IRAS at 25 mu m and found to have particularly interestingproperties. Altogether, 106 sources were detected with a signal to noiseratio >= 3 and 69 with S/N>= 5 in the 3.2'x 3.2' fields centeredon the nominal positions of IRAS sources. Sixty-five >= 3sigmadetections (49 of which at >= 5sigma ) are likely identifications ofIRAS sources. Ten additional IRAS sources have possible >= 3sigmaISOCAM counterparts. In 6 further cases, signals at the 2-3sigma levelwere detected close to the IRAS position. Indications that IRAS sourcesmight actually be multiple (source confusion) were found in 4 IDSfields. On the whole, we confirm the reality of 69 to 90% of IDSsources. Appropriate statistical corrections for the bias affectingfaint flux estimates were applied to ISOCAM data. Ten serendipitoussources were detected at >= 5sigma , with S(14.3 mum ) >= 3.5 mJy.The corresponding areal density is consistent with that found inprevious surveys. Finding charts for all observed fields are given.Based on observations with the Infrared Space Observatory (ISO). ISO isan ESA project with instruments funded by ESA Member States (especiallyby the PI countries: France, Germany, The Netherlands and the UnitedKingdom) and with participation of ISAS and NASA.
| Deep far infrared ISOPHOT survey in ``Selected Area 57". I. Observations and source counts
We present here the results of a deep survey in a 0.4 deg2blank field in Selected Area 57 conducted with the ISOPHOT instrumentaboard ESAs Infrared Space Observatory (ISO at both 60 mu m and 90 mu m.The resulting sky maps have a spatial resolution of 15 x 23arcsec2 per pixel which is much higher than the 90 x 90arcsec2 pixels of the IRAS All Sky Survey. We describe themain instrumental effects encountered in our data, outline our datareduction and analysis scheme and present astrometry and photometry ofthe detected point sources. With a formal signal to noise ratio of 6.75we have source detection limits of 90 mJy at 60 μm and 50 mJy at 90μm. To these limits we find cumulated number densities of 5+/- 3.5deg-2 at 60 μm and 14.8+/- 5.0 deg-2 at 90μm. These number densities of sources are found to be lower thanpreviously reported results from ISO but the data do not allow us todiscriminate between no-evolution scenarios and various evolutionarymodels. Based on observations with ISO, an ESA project with instrumentsfunded by ESA member states (especially the PI countries: France,Germany, the Netherlands, and the United Kingdom) and with theparticipation of ISAS and NASA.
| The hard X-ray emission of luminous infrared galaxies
We present a study of the hard X-ray properties of a sample thatincludes all the Luminous Infrared Galaxies (LIGs, LIR >1011 L_sun) observed in the 2-10 keV energy band (new andarchival data). We find that a significant fraction of the sourcesoptically classified as AGNs do not show any indication of nuclearactivity in the X rays, thus suggesting heavy absorption along our lineof sight. The absence of strong emission in the 20-200 keV band in asubsample of LIGs observed with BeppoSAX suggests that in many casesthese sources are completely Compton thick (N_H > 1025cm-2). From a comparison between the infrared and the X-rayemission we deduce that the mid-IR emission is absorbed by a lowercolumn density than the X-ray emission or, alternatively, that thedust-to-gas ratio is lower than Galactic. We describe a simple modelthat reproduces the IR-X correlation by means of mixed AGN and starburstcontributions and we compare the predictions of this model with theobservational data at X-ray and optical wavelengths. Finally, we discussthe biases that affect the currently available samples of LIGs andbriefly analyze a small unbiased sample, finding that at least 50% ofthe sources host a (weak) AGN.
| Arcsecond Positions of UGC Galaxies
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
| A Three-dimensional Diagnostic Diagram for Seyfert 2 Galaxies: Probing X-Ray Absorption and Compton Thickness
We present and discuss a three-dimensional diagnostic diagram forSeyfert 2 galaxies obtained using X-ray and [O III] data on a largesample of objects (reported in the Appendix). The diagram shows theKalpha iron line equivalent width as a function of both the columndensity derived from the photoelectric cutoff and the 2-10 keV fluxnormalized to the [O III] optical-line flux (the latter corrected forextinction and assumed to be a true indicator of the source intrinsicluminosity). We find that the hard X-ray properties of type 2 objectsdepend on a single parameter, the absorbing column density along theline of sight, in accordance with the unified model. The diagram can beused to identify Compton-thick sources and to isolate and study peculiarobjects. From this analysis we have obtained a column densitydistribution of Seyfert 2 galaxies that is thought to be a goodapproximation of the real distribution. A large population of heavilyabsorbed objects is discovered, including many Compton-thick candidates.Our results indicate that the mean log N_H cm^-2 in type 2 Seyfertgalaxies is 23.5 and that as many as 23%-30% of sources haveN_H>=10^24 cm^-2.
| A Medium Survey of the Hard X-Ray Sky with ASCA. II. The Source's Broadband X-Ray Spectral Properties
A complete sample of 60 serendipitous hard X-ray sources with flux inthe range ~1x10^-13 ergs cm^-2 s^-1 to ~4x10^-12 ergs cm^-2 s^-1 (2-10keV), detected in 87 ASCA GIS2 images, was recently presented in theliterature. Using this sample it was possible to extend the descriptionof the 2-10 keV logN(>S)-logS down to a flux limit of ~6x10^-14 ergscm^-2 s^-1 (the faintest detectable flux), resolving about a quarter ofthe cosmic X-ray background (CXB). In this paper we have combined theASCA GIS2 and GIS3 data of these sources to investigate their X-rayspectral properties using the hardness ratios and the stacked-spectramethod. Because of the sample statistical representativeness, theresults presented here, which refer to the faintest hard X-ray sourcesthat can be studied with the current instrumentation, are relevant tothe understanding of the CXB and of the active galactic nucleus (AGN)unification scheme. The ``stacked'' spectra show that the averagesource's spectrum hardens toward fainter fluxes; it changes from anenergy spectral index <α_E>=0.87+/-0.08 for the 20 brightestsources (2-10 keV count rate >=3.9x10^-3 counts s^-1, the ``bright''sample) to <α_E>=0.36+/-0.14 for the remaining 40 faintersources (the ``faint'' sample). The dividing line of 3.9x10^-3 countss^-1 corresponds to unabsorbed 2-10 keV fluxes in the range ~5.4x10^-13to ~3.1x10^-13 ergs cm^-2 s^-1 for a source described by a power-lawmodel with energy spectral index between 0.0 and 2.0. It thus seems thatwe are now beginning to detect those sources that have the ``correct''spectral shape to be responsible for the 2-10 keV CXB. Thehardness-ratio analyses indicate that this flattening is due to apopulation of sources with very hard spectra showing up in the faintsample; about half of the sources in this sample requireα_E<~0.5, while only ~10% of the sources in the bright sampleare consistent with an energy spectral index so flat. A number ofsources (~30%) in the faint sample seem to be characterized by anapparently ``inverted'' X-ray spectrum (i.e., α_E<~0.0). Theseobjects are probably extremely absorbed sources, as expected from theCXB synthesis models based on the AGN unification scheme, if not a newpopulation of very hard serendipitous sources. The broadband (0.7-10keV) spectral properties of the selected sources, as inferred from thehardness-ratios diagram, seem to be more complex than is expected from asimple absorbed power-law model. We have thus investigated more complexmodels, in line with the AGN unification scheme, and we find that thesemodels seem to be able to explain the overall spectral properties of thepresent sample; this result also seems to be suggested by a comparisonof the hardness-ratio diagram of the serendipitous ASCA sources withthat obtained using a sample of nearby and well-known Seyfert 1 andSeyfert 2 galaxies observed with ASCA.
| The X-Ray Spectra and Spectral Variability of Intermediate-Type Seyfert Galaxies: ASCA Observations of NGC 4388 and ESO 103-G35
The X-ray spectra of two intermediate-type Seyfert galaxies areinvestigated using ASCA observations separated by more than 1 yr. BothNGC 4388 and ESO 103-G35 exhibit strong, narrow Fe Kalpha line emissionand absorption by cold neutral gas with a column density ~10^23 cm^-2,characteristic of the X-ray spectra of type 2 Seyfert galaxies. Thepower-law continuum flux has changed by a factor of 2 over a timescaleof ~2 yr for both objects, declining in the case of NGC 4388 and risingin ESO 103-G35. No variation was observed in the equivalent width of theFe Kalpha line in the spectra of NGC 4388, implying that the line fluxdeclined with the continuum. We find that the strength of the linecannot be accounted for by fluorescence in line-of-sight material withthe measured column density unless a ``leaky-absorber'' model of thetype favored for IRAS 04575-7537 is employed. The equivalent width ofthe Fe Kalpha emission line is seen to decrease between the observationsof ESO 103-G35 while the continuum flux increased. The 1996 observationof ESO 103-G35 can also be fitted with an absorption edge at 7.4+/-0.2keV due to partially ionized iron, and when an ionized absorber model isfitted to the data it is found that the equivalent column of neutralhydrogen rises to 3.5x10^23 cm^-2. The Fe Kalpha line flux can beaccounted by fluorescence in this material alone, and this model is alsoa good representation of the 1988 and 1991 Ginga observations. There isthen no requirement for a reflection component in the ASCA spectra ofESO 103-G35 or NGC 4388.
| Radio-loud ROSAT sources near the North Ecliptic Pole
A deep and large-area survey of the North Ecliptic Pole region was madewith the ROSAT All-Sky Survey and the VLA to elucidate the population ofradio-loud extragalactic objects. A region of 29.3 square degrees wassurveyed with sensitivities around 5 \ 10(-14) erg s(-1) cm(-2) in thesoft X-ray band and 1 mJy at 1.5 GHz. Optical counterparts were soughton digitized Schmidt plates from POSS-I and II. Seventy-four reliableRASS-VLA sources were found. The sample is a heterogeneous mixture ofSeyfert galaxies, quasars, BL Lac objects, galaxy clusters and groupscontaining a radio galaxy. Optical magnitudes range from B =~ 14 to B> 22. Three results are noteworthy: (1) the bimodality in the ratioof radio to optical emission seen in optically- and X-ray-selected AGNsamples is also evident in the ROSAT-VLA objects; (2) X-ray and radioselection is an effective method for locating poor galaxy clusters andgroups; and (3) a considerable population of optically faint butX-ray/radio-bright objects is present. This last group may be eitherdistant clusters with radio galaxies or ``red quasars".
| The Complex X-Ray Spectrum of 3C 273: ASCA Observations
The results obtained from 9 X-ray observations of 3C 273 performed byASCA are presented (total exposure time ~ 160000 s). The analysis andinterpretation of the data was complicated by the fact that 4 of theseobservations were used for the on-board calibration of the CCDs spectralresponse. The present standard analysis shows that, in agreement withofficial recommendations, a conservative systematic error (at lowenergies) of 2--3 *E(20) cm(-2) must be assumed when analyzing ASCA SISdata. A soft-excess, with variable flux and/or shape, has been clearlydetected as well as flux and spectral variability. An anti-correlationhas been found between the spectral index and the flux in the 2--10 keVenergy range. Fitting the data with the latest available calibrationmatrices, we also detect an emission line at 5.4--5.7 keV (6.3--6.6 keVin the quasar frame) in only the two observations with lowest fluxeswhere it is weak (EW ~ 20--30 eV), narrow and consistent with beingproduced by Fe Kalpha emission from neutral matter. Overall, theobservations are qualitatively consistent with a variable, non-thermalX-ray continuum emission (a power law with Gamma ~ 1.6, possiblyproduced in the innermost regions of the radio-optical jet), plusunderlying ``Seyfert-like'' features (a soft-excess and Fe Kalpha lineemission). When the continuum (jet) emission is in a low state, theSeyfert-like spectral features (soft-excess, iron line, and possibly asteep power law plus reflection continuum) are more easily seen.
| Optical and X-Ray Spectroscopy of 1E 0449.4-1823: Demise of the Original Type 2 QSO
New optical spectra of the original narrow-line quasar 1E 0449.4-1823show that it now has broad emission lines of considerable strength,eliminating it as a "type 2 QSO" candidate. Although broad emission linecomponents were probably weakly present in 1981 and 1984, they havecertainly increased in strength and are accompanied by Balmer continuumemission that makes the spectrum bluer than it was previously. Wesuggest that the behavior of 1E 0449.4-1823 is the same as that of someSeyfert 1.8 and 1.9 galaxies, in which Goodrich attributed long-termvariations of their broad Balmer lines to dynamical motions of obscuringmaterial located in or around the broad-line region. The opticalcontinuum and broad emission line regions of 1E 0449.4-1823 may still bepartly covered in our line of sight, which would explain its largeforbidden-line equivalent widths and flat alpha ox relative to otherlow-redshift QSOs. Also present are apparent absorption features in thebroad Balmer lines and in Mg II, which may be related to the pastobscuration and current emergence of the broad-line region. However, itis difficult to distinguish absorption from broad emission line peaksthat are displaced in velocity; we consider the latter a plausiblecompeting interpretation of these peculiar line profiles. An ASCA X-rayspectrum of 1E 0449.4-1823 can be fitted with a power law of Gamma=1.63^{+0.12}_{-0.09} , intrinsic NH < 9 x 1020 cm-2, and no Fe Kalpha line emission. Its 2-10 keV luminosity is 6.7 x 1044 ergs s-1.Thus, there is no evidence for Seyfert 2 properties in the X-rayemission from 1E 0449.4-1823, which resembles that of an ordinary QSO.With regard to the still hypothetical type 2 QSOs, we argue that thereis little evidence for the existence of any among X-ray-selectedsamples.
| On the Origin of Broad Fe K alpha and H i H alpha Lines in Active Galactic Nuclei
We examine the properties of the Fe emission lines that arise near 6.4keV in the ASCA spectra of AGNs. Our emphasis is on the Seyfert 1galaxies where broad and apparently complex Fe K alpha emission isobserved. We consider various origins for the line but focus on the prosand cons for line-emitting accretion disk models. We develop a simplemodel of an illuminated disk capable of producing both X-ray and opticallines. The model is able to reproduce the observed Fe K alpha FWHM ratioas well as the radii of maximum emissivity implied by the profileredshifts. The overall profile shapes, however, do not fit well thepredictions of our disk illumination model nor do we derive alwaysconsistent disk inclinations for the two lines. We conclude that theevidence for and against an accretion disk origin for the Fe K alphaemission is equivocal. The bulk of the data requires a very disparateset of line fits, shedding little light on a coherent physical model. Webriefly consider alternatives to disk-emission and show that a simplebicone model can reproduce the Fe line profiles equally well.
| An ASCA Observation of M51 (NGC 5194): Iron K Emission from an Obscured Active Galactic Nucleus
We present an ASCA observation of the nearby spiral galaxy M51 (NGC5194). We detected hard X-ray emission with a photon index of ~1.4 and aluminosity of LX ~ 1.1 x 1040 ergs s-1 in the 2-10 keV band (assuming adistance of 9.6 Mpc). A strong fluorescent iron K line (equivalent width~900 eV) was detected at 6.4 keV in the X-ray spectra. Such an intenseiron line is characteristic of Seyfert 2 galaxies and strongly suggeststhe presence of a heavily obscured active galactic nucleus (AGN).However, the X-ray image is extended even at energies above 2 keV. Fromthe strong iron line and the extended hard X-ray image we speculate thatthe AGN is obscured by matter with a hydrogen column density more thanseveral times 1023 cm-2 and that the observed 2-10 keV X-ray flux is notdominated by emission from the AGN but rather by other components, suchas low-mass X-ray binaries, which typically dominate the X-ray emissionof normal spiral galaxies. Emission lines from O K, Ne K, Fe L, Mg K andSi K were detected in the soft energy spectra, which indicate thepresence of hot gas. The soft component is well represented by aRaymond-Smith thermal plasma model (with kT ~ 0.4 keV), which suggests alower iron abundance (<0.1 solar) than other elements (~0.1-0.4solar), or by a two temperature (kT ~ 0.3 keV and kT ~ 0.8 keV) modelwith ~0.1 solar abundance, which is reminiscent of the X-ray-emittinggas in starburst galaxies.
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