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The hot, warm and cold gas in Arp 227 - an evolving poor group
Arp 227 represents a prototypical example of an interacting mixed pairof galaxies located in a low-density environment. We investigate the gasproperties of the pair in the X-ray, Hα, HI and CO bands. We alsodetect two additional members of the group in HI which indicates thatthe pair constitutes the dominant members of a loose group.The HI distribution shows a tail of gas that connects the spiral member,NGC 470, to the lenticular, NGC 474, showing that the two main membersare currently undergoing interaction. The Hα emission reveals thepresence of secondary components at the centre of NGC 470, superposed onthe main component tracing the rotation of the galaxy. This latter mapsa nearly unperturbed velocity field. The dominant, nearly unperturbedtrend of the kinematics is confirmed by CO observations, althoughrestricted to the centre of the galaxy. The X-ray luminosity of NGC 470is comparable with that of a `normal' spiral galaxy. NGC 474 on theother hand is very gas-poor and has not been detected in Hα. ItsX-ray luminosity is consistent with the low end of the expected emissionfrom discrete sources.Arp 227 as a loose group shows several signatures of galaxy-galaxyinteraction. Our observations suggest the presence of signatures ofinteraction in the overall kinematics of the spiral companion. Theongoing interaction is clearly visible only in the outer HI halo of NGC470. While the large shell system of NGC 474 could be associated with anaccretion event, the secondary components in the Hα profile in thecentre of NGC 470 could be due to the interaction with the companion.The low X-ray luminosity of NGC 470 seems to be a characteristic ofdynamically young systems. All the above evidence suggest that Arp 227is an evolving group in the early phase of its evolution and that itsdrivers are the accretion of faint galaxies and the ongoing large-scaleinteraction between NGC 470 and 474.
| The Cool ISM in S0 Galaxies. II. A Survey of Atomic Gas
The place of lenticular galaxies within the range of types of galaxiesremains unclear. We previously reported the mass of molecular hydrogenfor a volume-limited sample of lenticular galaxies, where we saw thatthe amount of gas was less than that predicted by the return of stellarmass to the interstellar medium. Here we report observations of atomichydrogen (H I) for the same sample. Detections in several galaxies makemore compelling the case presented in our earlier paper that the mass ofcool gas in S0 galaxies cuts off at ~10% of what is expected fromcurrent models of gas return from stellar evolution. The molecular andatomic phases of the gas in our sample galaxies appear to be separateand distinct, both spatially and in velocity space. We propose that themolecular gas arises mostly from the stellar mass returned to thegalaxy, while the atomic hydrogen is mainly accumulated from externalsources (infall, captured dwarfs, etc.). While this proposal fits mostof the observations, it makes the presence of the upper mass cutoff evenmore mysterious.
| Black Hole Masses of Active Galaxies with Double-peaked Balmer Emission Lines
We have obtained near-IR spectra of five AGNs that exhibit double-peakedBalmer emission lines (NGC 1097, Pictor A, PKS 0921-213, 1E0450.30-1817, and IRAS 0236.6-3101). The stellar velocity dispersions ofthe host galaxies were measured from the Ca II λλ8494,8542, 8662 absorption lines and were found to range from 140 to 200 kms-1. Using the well-known correlation between the black holemass and the stellar velocity dispersion, the black hole masses in thesegalaxies were estimated to range from 4×107 to1.2×108 Msolar. We supplement theobservations presented here with estimates of the black holes masses forfive additional double-peaked emitters (Arp 102B, 3C 390.3, NGC 4579,NGC 4203, and M81) obtained by other authors using similar methods.Using these black hole masses, we infer the ratio of the bolometricluminosity to the Eddington luminosity,(Lbol/LEdd). We find that two objects (Pictor Aand PKS 0921-213) have Lbol/LEdd~0.2, whereas theother objects have Lbol/LEdd<~10-2(nearby, low-luminosity double-peaked emitters are the most extreme,with Lbol/LEdd<~10-4). The physicaltimescales in the outer regions of the accretion disks (atr~103GM/c2) in these objects were also estimatedand range from a few months for the dynamical timescale to severaldecades for the sound crossing timescale. The profile variability inthese objects is typically an order of magnitude longer than thedynamical time, but we note that variability occurring on the dynamicaltimescale has not been ruled out by the observations.Based on observations carried out at Cerro Tololo Inter-AmericanObservatory, which is operated by AURA, Inc., under a cooperativeagreement with the National Science Foundation.
| The Nearby Damped Lyα Absorber SBS 1543+593: A Large H I Envelope in a Gas-rich Galaxy Group
We present a Very Large Array H I 21 cm map and optical observations ofthe region around one of the nearest damped Lyα absorbers beyondthe Local Group, SBS 1543+593. Two previously uncataloged galaxies havebeen discovered, and a redshift has been determined for a third. Allthree of these galaxies are at the redshift of SBS 1543+593 and are<~185 kpc from the damped Lyα absorber. We discuss the H I andoptical properties of SBS 1543+593 and its newly identified neighbors.Both SBS 1543+593 and RBTB 154542.8+591132 have baryonic components thatare dominated by neutral gas-unusual for damped Lyα absorbers, forwhich only ~5% of the H I cross section originates in such stronglygas-dominated systems. What remains unknown is whether low-mass gas-richgroups are common surrounding gas-rich galaxies in the local universeand whether the low star formation rate in these systems is indicativeof a young system or a stable, slowly evolving system. We discuss theseevolutionary scenarios and future prospects for answering thesequestions.
| 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.
| The host galaxy/AGN connection in nearby early-type galaxies. A new view of the origin of the radio-quiet/radio-loud dichotomy?
This is the third in a series of three papers exploring the connectionbetween the multiwavelength properties of AGN in nearby early-typegalaxies and the characteristics of their hosts. Starting from aninitial sample of 332 galaxies, we selected 116 AGN candidates requiringthe detection of a radio source with a flux limit of ~1 mJy, as measuredfrom 5 GHz VLA observations. In Paper I we classified the objects withavailable archival HST images into "core" and "power-law" galaxies,discriminating on the basis of the nuclear slope of their brightnessprofiles. We used HST and Chandra data to isolate the nuclear emissionof these galaxies in the optical and X-ray bands, thus enabling us (oncecombined with the radio data) to study the multiwavelength behaviour oftheir nuclei. The properties of the nuclei hosted by the 29 coregalaxies were presented in Paper II Core galaxies invariably host aradio-loud nucleus, with a median radio-loudness of Log R = 3.6 and anX-ray based radio-loudness parameter of Log RX = -1.3. Herewe discuss the properties of the nuclei of the 22 "power-law" galaxies.They show a substantial excess of optical and X-ray emission withrespect to core galaxies at the same level of radio luminosity.Conversely, their radio-loudness parameters, Log R 1.6 and LogRX -3.3, are similar to those measured in Seyfertgalaxies. Thus the radio-loudness of AGN hosted by early-type galaxiesappears to be univocally related to the host's brightness profile:radio-loud AGN are only hosted by core galaxies, while radio-quiet AGNare found only in power-law galaxies. The brightness profile isdetermined by the galaxy's evolution, through its merger history; ourresults suggest that the same process sets the AGN flavour. In thisscenario, the black holes hosted by the merging galaxies rapidly sinktoward the centre of the newly formed object, setting its nuclearconfiguration, described by e.g. the total mass, spin, mass ratio, orseparation of the SMBHs. These parameters are most likely at the originof the different levels of the AGN radio-loudness. This connection mightopen a new path toward understanding the origin of theradio-loud/radio-quiet AGN dichotomy and provide us with a further toolfor exploring the co-evolution of galaxies and supermassive black holes.
| A radio census of nuclear activity in nearby galaxies
In order to determine the incidence of black hole accretion-drivennuclear activity in nearby galaxies, as manifested by their radioemission, we have carried out a high-resolution Multi-ElementRadio-Linked Interferometer Network (MERLIN) survey of LINERs andcomposite LINER/Hii galaxies from a complete magnitude-limited sample ofbright nearby galaxies (Palomar sample) with unknown arcsecond-scaleradio properties. There are fifteen radio detections, of which three arenew subarcsecond-scale radio core detections, all being candidate AGN.The detected galaxies supplement the already known low-luminosity AGN -low-luminosity Seyferts, LINERs and composite LINER/Hii galaxies - inthe Palomar sample. Combining all radio-detected Seyferts, LINERs andcomposite LINER/Hii galaxies (LTS sources), we obtain an overall radiodetection rate of 54% (22% of all bright nearby galaxies) and weestimate that at least ~50% (~20% of all bright nearby galaxies) aretrue AGN. The radio powers of the LTS galaxies allow the construction ofa local radio luminosity function. By comparing the luminosity functionwith those of selected moderate-redshift AGN, selected from the 2dF/NVSSsurvey, we find that LTS sources naturally extend the RLF of powerfulAGN down to powers of about 10 times that of Sgr A*.
| 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.
| How large are the bars in barred galaxies?
I present a study of the sizes (semimajor axes) of bars in discgalaxies, combining a detailed R-band study of 65 S0-Sb galaxies withthe B-band measurements of 70 Sb-Sd galaxies from Martin (1995). As hasbeen noted before with smaller samples, bars in early-type (S0-Sb)galaxies are clearly larger than bars in late-type (Sc-Sd) galaxies;this is true both for relative sizes (bar length as fraction ofisophotal radius R25 or exponential disc scalelength h) andabsolute sizes (kpc). S0-Sab bars extend to ~1-10 kpc (mean ~ 3.3 kpc),~0.2-0.8R25 (mean ~ 0.38R25) and ~0.5-2.5h (mean ~1.4h). Late-type bars extend to only ~0.5-3.5 kpc,~0.05-0.35R25 and 0.2-1.5h their mean sizes are ~1.5 kpc, ~0.14R25 and ~0.6h. Sb galaxies resemble earlier-type galaxiesin terms of bar size relative to h; their smallerR25-relative sizes may be a side effect of higher starformation, which increases R25 but not h. Sbc galaxies form atransition between the early- and late-type regimes. For S0-Sbcgalaxies, bar size correlates well with disc size (both R25and h); these correlations are stronger than the known correlation withMB. All correlations appear to be weaker or absent forlate-type galaxies; in particular, there seems to be no correlationbetween bar size and either h or MB for Sc-Sd galaxies.Because bar size scales with disc size and galaxy magnitude for mostHubble types, studies of bar evolution with redshift should selectsamples with similar distributions of disc size or magnitude(extrapolated to present-day values); otherwise, bar frequencies andsizes could be mis-estimated. Because early-type galaxies tend to havelarger bars, resolution-limited studies will preferentially find bars inearly-type galaxies (assuming no significant differential evolution inbar sizes). I show that the bars detected in Hubble Space Telescope(HST) near-infrared(IR) images at z~ 1 by Sheth et al. have absolutesizes consistent with those in bright, nearby S0-Sb galaxies. I alsocompare the sizes of real bars with those produced in simulations anddiscuss some possible implications for scenarios of secular evolutionalong the Hubble sequence. Simulations often produce bars as large as(or larger than) those seen in S0-Sb galaxies, but rarely any as smallas those in Sc-Sd galaxies.
| A high-frequency radio survey of low-luminosity active galactic nuclei
We investigate the high-frequency radio spectra of 20 low-luminosityactive galactic nuclei (LLAGNs) with compact radio cores. Our millimetresurvey with the Nobeyama Millimetre Array (NMA) and analyses ofsubmillimetre archival data that had been obtained with theSubmillimetre Common User Bolometer Array (SCUBA) on the James ClerkMaxwell Telescope (JCMT) reveal the following properties. At least halfof the LLAGNs show inverted spectra between 15 and 96 GHz; we use thepublished data at 15 GHz with the Very Large Array (VLA) in a0.15-arcsec resolution and our measurements at 96 GHz with the NMA in a7-arcsec resolution. The inverted spectra are not artificially made dueto their unmatched beam sizes, because of little diffuse contaminationfrom dust, HII regions, or extended jets in these LLAGNs. Suchhigh-frequency inverted spectra are apparently consistent with a`submillimetre bump', which is predicted by an advection-dominatedaccretion flow (ADAF) model. We find a strong correlation between thehigh-frequency spectral index and low-frequency core power measured withvery-long-baseline-interferometry (VLBI) instruments. The invertedspectra are found exclusively in low-core-power sources, while steepspectra are in high-core-power ones with prominent pc-scale jets. Thissuggests that the ADAF and non-thermal jets may coexist. The flux ratiosbetween disc and jet seem to be different from LLAGN to LLAGN; disccomponents can be seen in nuclear radio spectra only if the jets arefaint.
| The X-ray spectrum of NGC 7213 and the Seyfert-LINER connection
We present an XMM-Newton observation of the Seyfert-LINER(low-ionization nuclear emission-line region) galaxy NGC 7213. The RGSsoft X-ray spectrum is well fitted with a power law plus soft X-raycollisionally ionized thermal plasma (kT=0.18+0.03-0.01 keV). We confirm the presence ofFeI, FeXXV and FeXXVI Kα emission in the EPIC spectrum and settighter constraints on their equivalent widths of82+10-13, 24+9-11 and24+10-13 eV, respectively. We compare the observedproperties together with the inferred mass accretion rate of NGC 7213with those of other Seyfert and LINER galaxies. We find that NGC 7213has intermediate X-ray spectral properties lying between those of theweak active galactic nucleus found in the LINER M81 andhigher-luminosity Seyfert galaxies. There appears to be a continuoussequence of X-ray properties from the Galactic Centre through LINERgalaxies to Seyferts, probably determined by the amount of materialavailable for accretion in the central regions.
| Supermassive Black Holes: Relation to Dark Halos
Estimates of the masses of supermassive black holes (M bh ) in thenuclei of disk galaxies with known rotation curves are compared withestimates of the rotational velocities V m and the“indicative” masses of the galaxies M i . Although there isa correlation between M bh and V m or M i , it is appreciably weakerthan the correlation with the central velocity dispersion. The values ofM bh for early-type galaxies (S0-Sab), which have more massive bulges,are, on average, higher than the values for late-type galaxies with thesame rotational velocities. We conclude that the black-hole masses aredetermined primarily by the properties of the bulge and not therotational velocity or the mass of the galaxy.
| Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data Analysis
X-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources.
| The Link between Star Formation and Accretion in LINERs: A Comparison with Other Active Galactic Nucleus Subclasses
We present archival high-resolution X-ray imaging observations of 25nearby LINERs observed by ACIS on board Chandra. This sample builds onour previously published proprietary and archival X-ray observations andincludes the complete set of LINERs with published black hole masses andFIR luminosities that have been observed by Chandra. Of the 82 LINERsobserved by Chandra, 41 (50%) display hard nuclear cores consistent withan AGN. The nuclear 2-10 keV luminosities of these AGN-LINERs range from~2×1038 to ~1×1044 ergss-1. Reinforcing our previous work, we find a significantcorrelation between the Eddington ratio,Lbol/LEdd, and the FIR luminosity,LFIR, as well as the IR brightness ratio,LFIR/LB, in the host galaxy of AGN-LINERs thatextends over 7 orders of magnitude in Lbol/LEdd.Combining our AGN-LINER sample with galaxies from other AGN subclasses,we find that this correlation is reinforced in a sample of 129 AGNs,extending over almost 9 orders of magnitude inLbol/LEdd. Using archival and previously publishedobservations of the 6.2 μm PAH feature from ISO, we find that it isunlikely that dust heating by the AGN dominates the FIR luminosity inour sample of AGNs. Our results may therefore imply a fundamental linkbetween the mass accretion rate (M˙), as measured by the Eddingtonratio, and the star formation rate (SFR), as measured by the FIRluminosity. Apart from the overall correlation, we find that thedifferent AGN subclasses occupy distinct regions in the LFIRand Lbol/LEdd plane. Assuming a constant radiativeefficiency for accretion, our results may imply a variation in theSFR/M˙ ratio as a function of AGN activity level, a result that mayhave significant consequences for our understanding of galaxy formationand black hole growth.
| The Stellar Populations in the Central Parsecs of Galactic Bulges
We present Hubble Space Telescope blue spectra at intermediate spectralresolution for the nuclei of 23 nearby disk galaxies. These objects wereselected to have nebular emission in their nuclei and span a range ofemission-line classifications, as well as Hubble types. In this paper wefocus on the stellar population as revealed by the continuum spectralenergy distribution measured within the central 0.13" (~8 pc) of thesegalaxies. The data were modeled with linear combinations of single-agestellar population synthesis models. The large majority (~80%) of thesurveyed nuclei have spectra whose features are consistent with apredominantly old (>~5×109 yr) stellar population.Approximately 25% of these nuclei show evidence of a component with ageyounger than 1 Gyr, with the incidence of these stars related to thenebular classification. Successful model fits imply an average reddeningcorresponding to AV~0.4 mag and a stellar metallicity of1-2.5 Zsolar. We discuss the implications of these resultsfor understanding the star formation history in the environment ofquiescent and active supermassive black holes. Our findings reinforcethe picture wherein Seyfert nuclei and the majority of low-ionizationnuclear emission-line regions are predominantly accretion-powered andsuggest that much of the central star formation in H II nuclei isactually circumnuclear.Based on observations obtained with the Hubble Space Telescope, which isoperated by AURA, Inc., under NASA contract NAS5-26555.
| The Size of the Radio-Emitting Region in Low-Luminosity Active Galactic Nuclei
We have used the VLA to study radio variability among a sample of 18low-luminosity active galactic nuclei (LLAGNs) on timescales of a fewhours to 10 days. The goal was to measure or limit the sizes of theLLAGN radio-emitting regions in order to use the size measurements asinput to models of the radio emission mechanisms in LLAGNs. We detectvariability on typical timescales of a few days at a confidence level of99% in half the target galaxies. Either variability that is intrinsic tothe radio-emitting regions or that is caused by scintillation in theGalactic interstellar medium is consistent with the data. For eitherinterpretation, the brightness temperature of the emission is below theinverse Compton limit for all our LLAGNs and has a mean value of about1010 K. The variability measurements plus VLBI upper limitsimply that the typical angular size of the LLAGN radio cores at 8.5 GHzis 0.2 mas, plus or minus a factor of 2. The ~1010 Kbrightness temperature strongly suggests that a population ofhigh-energy nonthermal electrons must be present, in addition to ahypothesized thermal population in an accretion flow, in order toproduce the observed radio emission.
| The Hubble Space Telescope View of LINER Nuclei: Evidence for a Dual Population?
We study a complete, distance-limited sample of 25 LINERs, 21 of whichhave been imaged with the Hubble Space Telescope. In nine objects wedetect an unresolved nucleus. To study their physical properties, wecompare the radio and optical properties of the nuclei of LINERs withthose of other samples of local active galactic nuclei (AGNs), namely,Seyfert galaxies and low-luminosity radio galaxies (LLRGs). Our resultsshow that the LINER population is not homogeneous, as there are twosubclasses: (1) the first class is similar to the LLRG class, as itextends the population of radio-loud nuclei to lower luminosities; (2)the second is similar to Seyfert galaxies and extends the properties ofradio-quiet nuclei toward the lowest luminosities. The objects areoptimally discriminated in the plane formed by the black hole massversus nuclear radio loudness: all radio-loud LINERs haveMBH>~108Msolar, while Seyfertgalaxies and radio-quiet LINERs haveMBH<~108Msolar. The different natureof the various classes of local AGNs are best understood when thefraction of the Eddington luminosity they irradiate,Lo/LEdd, is plotted against the nuclearradio-loudness parameter: Seyfert galaxies are associated withrelatively high radiative efficienciesLo/LEdd>~10-4 (and high accretionrates onto low-mass black holes); LLRGs are associated with lowradiative efficiencies (and low accretion rates onto high-mass blackholes); all LINERs have low radiative efficiency (and accretion rates)and can be radio-loud or radio-quiet, depending on their black holemass.Based on observations obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.
| The Murmur of the Sleeping Black Hole: Detection of Nuclear Ultraviolet Variability in LINER Galaxies
LINER nuclei, which are present in many nearby galactic bulges, may bethe manifestation of low-rate or low-radiative-efficiency accretion ontosupermassive central black holes. However, it has been unclear whetherthe compact UV nuclear sources present in many LINERs are clusters ofmassive stars, rather than being directly related to the accretionprocess. We have used the Hubble Space Telescope to monitor the UVvariability of a sample of 17 galaxies with LINER nuclei and compactnuclear UV sources. Fifteen of the 17 galaxies were observed more thanonce, with two to five epochs per galaxy, spanning up to a year. Wedetect significant variability in most of the sample, with peak-to-peakamplitudes from a few percent to 50%. In most cases, correlatedvariations are seen in two independent bands (F250W and F330W).Comparison to previous UV measurements indicates, for many objects,long-term variations by factors of a few over decade timescales.Variability is detected in LINERs with and without detected compactradio cores, in LINERs that have broad Hα wings detected in theiroptical spectra (``LINER 1s''), and in those that do not (``LINER 2s'').This variability demonstrates the existence of a nonstellar component inthe UV continuum of all types and sets a lower limit to the luminosityof this component. Interestingly, all the LINERs that have detectedradio cores have variable UV nuclei, as one would expect from bona fideactive galactic nuclei. We note a trend in the UV color (F250W/F330W)with spectral type-LINER 1s tend to be bluer than LINER 2s. This trendmay indicate a link between the shape of the nonstellar continuum andthe presence or the visibility of a broad-line region. In one target,the poststarburst galaxy NGC 4736, we detect variability in a previouslynoted UV source that is offset by 2.5" (~60 pc in projection) from thenucleus. This may be the nearest example of a binary active nucleus andof the process leading to black hole merging.Based on observations with the Hubble Space Telescope, which is operatedby AURA, Inc., under NASA contract NAS 5-26555.
| The Nature of Ultraluminous X-Ray Sources
We present spectroscopic observations of six optical counterparts ofultraluminous X-ray sources (ULXs) around nearby galaxies. The spectraof the six objects show the presence of broad emission features. Theidentification of these allow us to classify all of the objects asquasars at higher redshift than their assigned parent galaxy. This isone of the first and largest identifications of such objects usingunambigous optical spectral features. These results, in conjuction withprevious similar identifications of other sources, indicate thathigh-redshift quasars represent an important fraction of cataloged ULXsources. We estimate the density of such sources and compare this withexpectations for a population of randomly distributed backgroundquasars.
| Origin of Radio Emission from Nearby Low-Luminosity Active Galactic Nuclei
We use the observational data in radio, optical, and X-ray wave bandsfor a sample of active galactic nuclei (AGNs) with measured black holemasses to explore the origin of radio emission from nearbylow-luminosity active galactic nuclei (LLAGNs). The maximal luminosityof an advection-dominated accretion flow (ADAF) can be calculated for agiven black hole mass, as there is a critical accretion rate above whichthe ADAF is no longer present. We find that the radio luminosities arehigher than the maximal luminosities expected from the ADAF model formost sources in this sample. This implies that the radio emission ispredominantly from the jets in these sources. The radio emission from asmall fraction of the sources (15/60; referred to as radio-weak sources)in this sample can be explained by the ADAF model. However, comparingthe observed multiband emission data with the spectra calculated for theADAF or adiabatic inflow-outflow solution (ADIOS) cases, we find thatneither ADAF nor ADIOS models can reproduce the observed multibandemission simultaneously, with reasonable magnetic field strengths, forthese radio-weak sources. A variety of other possibilities arediscussed, and we suggest that the radio emission is probably dominatedby jet emission even in these radio-weak LLAGNs.
| A Chandra Snapshot Survey of Infrared-bright LINERs: A Possible Link Between Star Formation, Active Galactic Nucleus Fueling, and Mass Accretion
We present results from a high-resolution X-ray imaging study of nearbyLINERs observed by ACIS on board Chandra. This study complements andextends previous X-ray studies of LINERs, focusing on the underexploredpopulation of nearby dust-enshrouded infrared-bright LINERs. The sampleconsists of 15 IR-bright LINERs (LFIR/LB>3),with distances that range from 11 to 26 Mpc. Combining our sample withprevious Chandra studies, we find that ~51% (28/55) of the LINERsdisplay compact hard X-ray cores. The nuclear 2-10 keV luminosities ofthe galaxies in this expanded sample range from ~2×1038to ~2×1044 ergs s-1. We find that the mostextreme IR-faint LINERs are exclusively active galactic nuclei (AGNs).The fraction of LINERs containing AGNs appears to decrease with IRbrightness and increase again at the highest values ofLFIR/LB. We find that of the 24 LINERs showingcompact nuclear hard X-ray cores in the expanded sample that wereobserved at Hα wavelengths, only eight actually show evidence of abroad line. Similarly, of the 14 LINERs showing compact nuclear hardX-ray cores with corresponding radio observations, only eight display acompact flat spectrum radio core. These findings emphasize the need forhigh-resolution X-ray imaging observations in the study of IR-brightLINERs. Finally, we find an intriguing trend in the Eddington ratioversus LFIR and LFIR/LB for theAGN-LINERs in the expanded sample that extends over 7 orders ofmagnitude in L/LEdd. This correlation may imply a linkbetween black hole growth, as measured by the Eddington ratio, and thestar formation rate, as measured by the far-IR luminosity andIR-brightness ratio. If the far-IR luminosity is an indicator of themolecular gas content in our sample of LINERs, our results may furtherindicate that the mass accretion rate scales with the host galaxy's fuelsupply. We discuss the potential implications of our results in theframework of black hole growth and AGN fueling in low-luminosity AGNs.
| The Westerbork HI survey of spiral and irregular galaxies. III. HI observations of early-type disk galaxies
We present Hi observations of 68 early-type disk galaxies from the WHISPsurvey. They have morphological types between S0 and Sab and absoluteB-band magnitudes between -14 and -22. These galaxies form the massive,high surface-brightness extreme of the disk galaxy population, few ofwhich have been imaged in Hi before. The Hi properties of the galaxiesin our sample span a large range; the average values of MHI/LB and DH I/D25 are comparableto the ones found in later-type spirals, but the dispersions around themean are larger. No significant differences are found between the S0/S0aand the Sa/Sab galaxies. Our early-type disk galaxies follow the same Himass-diameter relation as later-type spiral galaxies, but theireffective Hi surface densities are slightly lower than those found inlater-type systems. In some galaxies, distinct rings of Hi emissioncoincide with regions of enhanced star formation, even though theaverage gas densities are far below the threshold of star formationderived by Kennicutt (1989, ApJ, 344, 685). Apparently, additionalmechanisms, as yet unknown, regulate star formation at low surfacedensities. Many of the galaxies in our sample have lopsided gasmorphologies; in most cases this can be linked to recent or ongoinginteractions or merger events. Asymmetries are rare in quiescentgalaxies. Kinematic lopsidedness is rare, both in interacting andisolated systems. In the appendix, we present an atlas of the Hiobservations: for all galaxies we show Hi surface density maps, globalprofiles, velocity fields and radial surface density profiles.
| The host galaxy/AGN connection in nearby early-type galaxies. Sample selection and hosts brightness profiles
This is the first of a series of three papers exploring the connectionbetween the multiwavelength properties of AGNs in nearby early-typegalaxies and the characteristics of their hosts. We selected twosamples, both with high resolution 5 GHz VLA observations available andproviding measurements down to 1 mJy level, reaching radio-luminositiesas low as 1019 W Hz-1. We focus on the 116radio-detected galaxies as to boost the fraction of AGN with respect toa purely optically selected sample. Here we present the analysis of theoptical brightness profiles based on archival HST images, available for65 objects. We separate early-type galaxies on the basis of the slope oftheir nuclear brightness profiles, into core and power-law galaxiesfollowing the Nuker's scheme, rather than on the traditionalmorphological classification (i.e. into E and S0 galaxies). Our sampleof AGN candidates is indistinguishable, when their brightness profilesare concerned, from galaxies of similar optical luminosity but hostingweaker (or no) radio-sources. We confirm previous findings thatrelatively bright radio-sources (Lr > 1021.5 WHz-1) are uniquely associated to core galaxies. However,below this threshold in radio-luminosity core and power-law galaxiescoexist and they do not show any apparent difference in theirradio-properties. Not surprisingly, since our sample is deliberatelybiased to favour the inclusion of active galaxies, we found a higherfraction of optically nucleated galaxies. Addressing the multiwavelengthproperties of these nuclei will be the aim of the two forthcomingpapers.
| Investigation of flat spectrum radio sources by the interplanetary scintillation method at 111 MHz
Interplanetary scintillation observations of 48 of the 55 Augusto et al.(1998) flat spectrum radio sources were carried out at 111 MHz using theinterplanetary scintillation method on the Large Phased Array (LPA) inRussia. Due to the large size of the LPA beam (1 ° × 0.5°) a careful inspection of all possible confusion sources was madeusing extant large radio surveys: 37 of the 48 sources are not confused.We were able to estimate the scintillating flux densities of 13 sources,getting upper limits for the remaining 35. Gathering more or improvingextant VLBI data on these sources might significantly improve ourresults. This proof-of-concept project tells us that compact (<1'')flat spectrum radio sources show strong enough scintillations at 111 MHzto establish/constrain their spectra (low-frequency end).
| Radio sources in low-luminosity active galactic nuclei. IV. Radio luminosity function, importance of jet power, and radio properties of the complete Palomar sample
We present the completed results of a high resolution radio imagingsurvey of all ( 200) low-luminosity active galactic nuclei (LLAGNs) andAGNs in the Palomar Spectroscopic Sample of all ( 488) bright northerngalaxies. The high incidences of pc-scale radio nuclei, with impliedbrightness temperatures ≳107 K, and sub-parsec jetsargue for accreting black holes in ≳50% of all LINERs andlow-luminosity Seyferts; there is no evidence against all LLAGNs beingmini-AGNs. The detected parsec-scale radio nuclei are preferentiallyfound in massive ellipticals and in type 1 nuclei (i.e. nuclei withbroad Hα emission). The radio luminosity function (RLF) of PalomarSample LLAGNs and AGNs extends three orders of magnitude below, and iscontinuous with, that of “classical” AGNs. We find marginalevidence for a low-luminosity turnover in the RLF; nevertheless LLAGNsare responsible for a significant fraction of present day massaccretion. Adopting a model of a relativistic jet from Falcke &Biermann, we show that the accretion power output in LLAGNs is dominatedby the kinetic power in the observed jets rather than the radiatedbolometric luminosity. The Palomar LLAGNs and AGNs follow the samescaling between jet kinetic power and narrow line region (NLR)luminosity as the parsec to kilo-parsec jets in powerful radio galaxies.Eddington ratios {l_Edd} (=L_Emitted/L_Eddington) of≤10-1{-}10-5 are implied in jet models of theradio emission. We find evidence that, in analogy to Galactic black holecandidates, LINERs are in a “low/hard” state (gas poornuclei, low Eddington ratio, ability to launch collimated jets) whilelow-luminosity Seyferts are in a “high” state (gas richnuclei, higher Eddington ratio, less likely to launch collimated jets).In addition to dominating the radiated bolometric luminosity of thenucleus, the radio jets are energetically more significant thansupernovae in the host galaxies, and are potentially able to depositsufficient energy into the innermost parsecs to significantly slow thegas supply to the accretion disk.
| A catalogue of ultraluminous X-ray sources in external galaxies
We present a catalogue of ultraluminous X-ray sources (ULXs) in externalgalaxies. The aim of this catalogue is to provide easy access to theproperties of ULXs, their possible counterparts at other wavelengths(optical, IR, and radio), and their host galaxies. The cataloguecontains 229 ULXs reported in the literature until April 2004. Most ULXsare stellar-mass-black hole X-ray binaries, but it is not excluded thatsome ULXs could be intermediate-mass black holes. A small fraction ofthe candidate ULXs may be background Active Galactic Nuclei (AGN) andSupernova Remnants (SNRs). ULXs with luminosity above 1040ergs s-1 are found in both starburst galaxies and in thehalos of early-type galaxies.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/429/1125
| The complex structure of low-luminosity active galactic nuclei: NGC 4579
We have modelled the low-luminosity active galactic nucleus (AGN) NGC4579 by explaining both the continuum and the line spectra observed withdifferent apertures. It was found that the nuclear emission is dominatedby an AGN such that the flux from the active centre (AC) is relativelylow compared with that of the narrow emission-line region (NLR) ofSeyfert galaxies. However, the contribution of a young starburst cannotbe neglected, as well as that of shock-dominated clouds with velocitiesof 100, 300 and 500kms-1. A small contribution from an olderstarburst with an age of 4.5 Myr, probably located in the externalnuclear region, is also found. HII regions appear in the extendedregions (~1 kpc), where radiation and shock-dominated clouds withVs= 100kms-1 prevail. The continuum SED of NGC4579 is characterized by the strong flux from an old stellar population.Emissions in the radio range show synchrotron radiation from the base ofthe jet outflowing from the accretion disc within 0.1 pc from the activecentre. Radio emission within intermediate distances (10-20 pc) isexplained by the bremsstrahlung from gas downstream of low-velocityshocks (Vs= 100kms-1) reached by a relatively lowradiation flux from the AC. In extended regions (>100 pc) the radioemission is synchrotron radiation created by the Fermi mechanism at theshock front. The shocks are created by collision of clouds with the jet.All types of emissions observed at different radius from the centre canbe reconciled with the presence of the jet.
| Stellar Velocity Dispersion and Mass Estimation for Galactic Disks
Available velocity dispersion estimates for the old stellar populationof galactic disks at galactocentric distances r=2L (where L is thephotometric radial scale length of the disk) are used to determine thethreshold local surface density of disks that are stable againstgravitational perturbations. The mass of the disk Mdcalculated under the assumption of its marginal stability is comparedwith the total mass Mt and luminosity LB of thegalaxy within r=4L. We corroborate the conclusion that a substantialfraction of the mass in galaxies is probably located in their darkhalos. The ratio of the radial velocity dispersion to the circularvelocity increases along the sequence of galactic color indices anddecreases from the early to late morphological types. For most of thegalaxies with large color indices (B-V)0 > 0.75, whichmainly belong to the S0 type, the velocity dispersion exceedssignificantly the threshold value required for the disk to be stable.The reverse situation is true for spiral galaxies: the ratiosMd/LB for these agree well with those expected forevolving stellar systems with the observed color indices. This suggeststhat the disks of spiral galaxies underwent no significant dynamicalheating after they reached a quasi-equilibrium stable state.
| Mid-Infrared Galaxy Morphology along the Hubble Sequence
The mid-infrared emission from 18 nearby galaxies were imaged with theInfrared Array Camera (IRAC) on the Spitzer Space Telescope, samplingthe spatial distributions of the reddening-free stellar photosphericemission and the warm dust in the interstellar medium. These twocomponents provide a new framework for galaxy morphology classificationin which the presence of spiral arms and their emission strengthrelative to the starlight can be measured directly and with highcontrast. Four mid-infrared classification methods are explored, threeof which are based on quantitative global parameters (colors andbulge-to-disk ratio) that are similar to those used in the past foroptical studies; in this limited sample, all correlate well withtraditional B-band classification. We suggest reasons why infraredclassification may be superior to optical classification.
| Spatial Distribution of Warm Dust in Early-Type Galaxies
Images taken with the Infrared Array Camera on the Spitzer SpaceTelescope show that the spatial distribution of warm dust emission inlenticular galaxies is often organized into dynamically stablestructures strongly resembling spiral arms. These galaxies havebulge-to-disk ratios and colors for their stellar content that areappropriate for their morphological classification. Two of the threegalaxies with warm dust detected at 8.0 μm also show far-IR emissionexpected from that dust. More importantly, the [5.8]-[8.0] color of thedust emission matches the colors found for late-type, star-forminggalaxies, as well as theoretical predictions for polycyclic aromatichydrocarbon emission from dust grains. The spatially resolved duststructures may be powerful indicators of the evolutionary history of thelenticular class of galaxies, either as a tracer of ongoing quiescentstar formation or as a fossil record of a previous episode of moreactive star formation.
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