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Lifetime of nuclear velocity dispersion drops in barred galaxies
We have made hydro/N-body simulations with and without star formation toshed some light on the conditions under which a central kinematicallycold stellar component (characterized by a velocity dispersion drop orσ-drop) could be created in a hot medium (e.g. a bulge) andsurvive enough time to be observed. We found that the time-scale for aσ-drop formation could be short (less than 500 Myr), whereas itslifetime could be long (more than 1 Gyr) provided that the centralregion is continuously or regularly fed by fresh gas which leads to acontinuous star formation activity. Star formation in the centralregion, even at a low rate as 1Msolaryr-1, ismandatory to sustain a permanent σ-drop by replacing heatedparticles by new low-σ ones. We moreover show that as soon as starformation is switched off, the σ-drop begins to disappear.
| Hαkinematics of the SINGS nearby galaxies survey - I*
This is the first part of an Hαkinematics follow-up survey of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample. The data for28galaxies are presented. The observations were done on three differenttelescopes with Fabry-Perot of New Technology for the Observatoire dumont Megantic (FaNTOmM), an integral field photon-counting spectrometer,installed in the respective focal reducer of each telescope. The datareduction was done through a newly built pipeline with the aim ofproducing the most homogenous data set possible. Adaptive spatialbinning was applied to the data cubes in order to get a constantsignal-to-noise ratio across the field of view. Radial velocity andmonochromatic maps were generated using a new algorithm, and thekinematical parameters were derived using tilted-ring models.
| A Virgo high-resolution Hα kinematical survey - II. The Atlas
A catalogue of ionized gas velocity fields for a sample of 30 spiral andirregular galaxies of the Virgo cluster has been obtained by using 3Doptical data. The aim of this survey is to study the influence ofhigh-density environments on the gaseous kinematics of local clustergalaxies. Observations of the Hα line by means of Fabry-Perotinterferometry have been performed at the Canada-France-HawaiiTelescope, European Southern Observatory 3.6-m telescope, Observatoirede Haute-Provence 1.93-m telescope and Observatoire du montMégantic telescope at angular and spectral samplings from 0.4 to1.6arcsec and 7 to 16kms-1. A recently developed, automaticand adaptive spatial binning technique is used to reach a nearlyconstant signal-to-noise ratio (S/N) over the whole field of view,allowing us to keep a high spatial resolution in high-S/N regions andextend the detection of signal in low-S/N regions. This paper is part ofa series and presents the integrated emission-line and velocity maps ofthe galaxies. Both Hα morphologies and kinematics exhibit signs ofperturbations in the form of, for example, external filaments, inner andnuclear spiral- and ring-like structures, inner kinematical twists,kinematical decoupling of a nuclear spiral, streaming motions alongspiral arms and misalignment between kinematical and photometricorientation axes.
| On the Lengths, Colors, and Ages of 18 Face-on Bars
Along with a brief analysis we present data obtained from BVRI andKs images of a sample of 19 galaxies (18 barred and 1unbarred), which will be further explored in a future paper. We measuredthe lengths and colors of the bars, created color maps, and estimatedglobal color gradients. Applying a method developed in a companionpaper, we could distinguish for seven galaxies in our sample those whosebars have been recently formed from the ones with already evolved bars.We estimated an average difference in the optical colors between youngand evolved bars that may be translated to an age difference of theorder of 10 Gyr, meaning that bars may be, at least in some cases,long-standing structures. Moreover, our results show that, on average,evolved bars are longer than young bars. This seems to indicate that,during its evolution, a bar grows longer by capturing stars from thedisk, in agreement with recent numerical and analytical results.Although the statistical significance of these results is low, andfurther studies are needed to confirm them, we discuss the implicationsfrom our results on the possibility of bars being a recurrentphenomenon. We also present isophotal contours for all our images aswell as radial profiles of relevant photometric and geometricparameters.
| Mid-Infrared Spectral Diagnostics of Nuclear and Extranuclear Regions in Nearby Galaxies
Mid-infrared diagnostics are presented for a large portion of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archivaldata from ISO and Spitzer. The SINGS data set includes low- andhigh-resolution spectral maps and broadband imaging in the infrared forover 160 nuclear and extranuclear regions within 75 nearby galaxiesspanning a wide range of morphologies, metallicities, luminosities, andstar formation rates. Our main result is that these mid-infrareddiagnostics effectively constrain a target's dominant power source. Thecombination of a high-ionization line index and PAH strength serves asan efficient discriminant between AGNs and star-forming nuclei,confirming progress made with ISO spectroscopy on starbursting andultraluminous infrared galaxies. The sensitivity of Spitzer allows us toprobe fainter nuclear and star-forming regions within galaxy disks. Wefind that both star-forming nuclei and extranuclear regions stand apartfrom nuclei that are powered by Seyfert or LINER activity. In fact, weidentify areas within four diagnostic diagrams containing >90%Seyfert/LINER nuclei or >90% H II regions/H II nuclei. We also findthat, compared to starbursting nuclei, extranuclear regions typicallyseparate even further from AGNs, especially for low-metallicityextranuclear environments. In addition, instead of the traditionalmid-infrared approach to differentiating between AGNs and star-formingsources that utilizes relatively weak high-ionization lines, we showthat strong low-ionization cooling lines of X-ray-dominated regions like[Si II] 34.82 μm can alternatively be used as excellentdiscriminants. Finally, the typical target in this sample showsrelatively modest interstellar electron density (~400 cm-3)and obscuration (AV~1.0 mag for a foreground screen),consistent with a lack of dense clumps of highly obscured gas and dustresiding in the emitting regions.
| Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation
We estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion.
| Radiatively Inefficient Accretion Flow in the Nucleus of NGC 1097
We present a model for the accretion flow around the supermassive blackhole in the LINER nucleus of NGC 1097 that fits the optical to X-rayspectral energy distribution (SED). The X-ray segment of the SED isbased on observations with the Chandra X-Ray Observatory, which arereported here for the first time. The inner part of the flow is modeledas a radiatively inefficient accretion flow (RIAF), and the outer partas a standard thin disk. The value of the transition radius(rtr~225RS, where RS=2GM/c2)between the RIAF and the outer thin disk was obtained from our previousfitting of the double-peaked Balmer emission line profile, whichoriginates in the thin disk. The black hole mass was inferred frommeasurements of the stellar velocity dispersion in the host galaxy. Whenthese parameters are used in the accretion flow model, the SED can besuccessfully reproduced, which shows that the line profile model and theaccretion flow model are consistent with each other. A small remainingexcess in the near-UV is accounted for by the contribution of anobscured starburst located within 9 pc from the nucleus, as we reportedin an earlier paper. The radio flux is consistent with synchrotronemission of a relativistic jet modeled by means of the internal shockscenario. In an appendix we also analyze the Chandra X-ray observationsof the ~1 kpc circumnuclear star-forming ring and of an ultraluminouscompact X-ray source located outside the ring.
| 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.
| Spatially Resolved Narrow-Line Region Kinematics in Active Galactic Nuclei
We have analyzed Hubble Space Telescope spectroscopy of 24 nearby activegalactic nuclei (AGNs) to investigate spatially resolved gas kinematicsin the narrow-line region (NLR). These observations effectively isolatethe nuclear line profiles on less than 100 pc scales and are used toinvestigate the origin of the substantial scatter between the widths ofstrong NLR lines and the stellar velocity dispersion σ*of the host galaxy, a quantity that relates with substantially lessscatter to the mass of the central, supermassive black hole and moregenerally characterize variations in the NLR velocity field with radius.We find that line widths measured with STIS at a range of spatial scalessystematically underestimate both σ* and the line widthmeasured from ground-based observations, although they do havecomparably large scatter to the relation between ground-based NLR linewidth and σ*. There are no obvious trends in theresiduals when compared with a range of host galaxy and nuclearproperties. The widths and asymmetries of [O III] λ5007 and [SII] λλ6716, 6731 as a function of radius exhibit a widerange of behavior. Some of the most common phenomena are substantialwidth increases from the STIS to the large-scale, ground-based apertureand almost no change in line profile between the unresolved nuclearspectrum and ground-based measurements. We identify asymmetries in asurprisingly large fraction of low-ionization [S II] line profiles andseveral examples of substantial red asymmetries in both [O III] and [SII]. These results underscore the complexity of the circumnuclearmaterial that constitutes the NLR and suggest that the scatter in theNLR width and σ* correlation cannot be substantiallyreduced with a simple set of empirical relations.
| A Survey of Kiloparsec-Scale Radio Outflows in Radio-Quiet Active Galactic Nuclei
Seyfert galaxies commonly host compact jets spanning 10-100 pc scales,but larger structures are resolved out in long-baseline aperturesynthesis surveys. Previous, targeted studies showed thatkiloparsec-scale radio structures (KSRs) may be a common feature ofSeyfert and LINER galaxies, and the origin of KSRs may be starbursts oractive galactic nuclei (AGNs). We report a new Very Large Array surveyof a complete sample of Seyfert and LINER galaxies. Out of all of thesurveyed radio-quiet sources, we find that 44% (19 out of 43) showextended radio structures at least 1 kpc in total extent that do notmatch the morphology of the disk or its associated star-forming regions.The detection rate is a lower limit owing to the combined effects ofprojection and resolution. The infrared colors of the KSR host galaxiesare unremarkable compared to other Seyfert galaxies, and the large-scaleoutflows orient randomly with respect to the host galaxy axes. The KSRSeyfert galaxies instead stand out by deviating significantly from thefar-infrared-radio correlation for star-forming galaxies, with tendencytoward radio excess, and they are more likely to have a relativelyluminous, compact radio source in the nucleus; these results argue thatKSRs are powered by the AGNs rather than starbursts. The high detectionrate indicates that Seyfert galaxies generate radio outflows over asignificant fraction of their lifetime, which is much longer than thedynamical timescale of an AGN-powered jet but is comparable instead tothe buoyancy timescale. The likely explanation is that the KSRsoriginate from jet plasma that has been decelerated by interaction withthe nuclear interstellar medium (ISM). Based on a simple ram pressureargument, the kinetic power of the jet on kiloparsec scales is about 3orders of magnitude weaker than the power of the jet on 10-100 pcscales. This result is consistent with the interaction model, in whichcase virtually all of the jet power must be lost to the ISM within theinner kiloparsec.
| A Comparison of Hα and Stellar Scale Lengths in Virgo and Field Spirals
The scale lengths of the old stars and ionized gas distributions arecompared for similar samples of Virgo Cluster members and field spiralgalaxies via Hα and broad R-band surface photometry. While theR-band and Hα scale lengths are, on average, comparable for thecombined sample, we find significant differences between the field andcluster samples. While the Hα scale lengths of the field galaxiesare a factor of 1.14+/-0.07 longer, on average, than their R-band scalelengths, the Hα scale lengths of Virgo Cluster members are, onaverage, 20% smaller than their R-band scale lengths. Furthermore, inVirgo, the scale length ratios are correlated with the size of thestar-forming disk: galaxies with smaller overall Hα extents alsoshow steeper radial falloff of star formation activity. At the sametime, we find no strong trends in scale length ratio as a function ofother galaxy properties, including galaxy luminosity, inclination,morphological type, central R-band light concentration, or bar type. Ourresults for Hα emission are similar to other results for dustemission, suggesting that Hα and dust have similar distributions.The environmental dependence of the Hα scale length placesadditional constraints on the evolutionary process(es) that cause gasdepletion and a suppression of the star formation rate in clusters ofgalaxies.
| The small dispersion of the mid IR - hard X-ray correlation in active galactic nuclei
Context: .We investigate mid-infrared and X-ray properties of the dustytorus in unification scenarios for active galactic nuclei. Aims:.We use the relation between mid IR and hard X-ray luminosities toconstrain AGN unification scenarios. Methods: .With VISIR at theVLT, we have obtained the currently highest angular resolution (0.35arcsec FWHM) narrow-band mid infrared images of the nuclei of 8 nearbySeyfert galaxies. Combining these observations with X-ray data from theliterature we study the correlation between their mid IR and hard X-rayluminosities. Results: .We find that the rest frame 12.3 μm(LMIR) and 2-10 keV (LX) luminosities arecorrelated at a highly significant level. The best fit power-law to ourdata is log{LMIR} ∝ (1.60 ± 0.22 )log{LX}, showing a much smaller dispersion than earlierstudies. Conclusions: .The similarity in thelog{LMIR}/log{LX} ratio between Sy1s and Sy2s evenusing high angular resolution MIR data implies that the similarity isintrinsic to AGN and not caused by contamination from extra-nuclearemission. This supports clumpy torus models. The exponent of thecorrelation constrains the inner geometry of the torus.
| 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.
| GMRT observations of the field of INTEGRAL X-ray sources - I
Since its launch the INTEGRAL observatory has discovered a variety ofhard X-ray sources in the Galactic plane. Using GMRT, we have maderepeated observations of these sources to search for the radiocounterparts of seventeen of them at low frequencies. The sourcepositions were taken from the various ATEL and IAUC announcementsreporting their discovery. Possible radio counterparts for seven ofthese sources, IGR J06074+2205, IGR J15479-4529, IGR J16479-4514, IGRJ17091-3624, IGR J18027-1455, IGR J18539+0727 and IGR J21247+5058 weredetected within 3σ of the positional uncertainty derived from theINTEGRAL observations. The offset in the radio position was calculatedusing the positions mentioned in the ATEL. We have also analyzed theavailable NVSS images for some of these fields at 1.4 GHz along with ourobservations. In this paper we present the radio images and the best fitpositions for the positive detections. The X-ray variability for some ofthe sources within the time scales of 100 s to 1 ks as seen in theRXTE/PCA light curves suggests their Galactic origin and possible binarynature. We discuss briefly the characteristics of these sources from theavailable information at different wave bands.
| X-ray spectral survey with XMM-Newton of a complete sample of nearby Seyfert galaxies
Results obtained from an X-ray spectral survey of nearby Seyfertgalaxies using XMM-Newton are reported. The sample was opticallyselected, well defined, complete in B magnitude, and distance limited:it consists of the nearest (D 22 Mpc) 27 Seyfert galaxies (9 oftype 1, 18 of type 2) taken from the Ho et al. (1997a, ApJS, 112, 315)sample. This is one of the largest atlases of hard X-ray spectra oflow-luminosity active galaxies ever assembled. All nuclear sourcesexcept two Seyfert 2s are detected between 2 and 10 keV, half for thefirst time ever, and average spectra are obtained for all of them.Nuclear luminosities reach values down to 1038 ergs-1. The shape of the distribution of X-ray parameters isaffected by the presence of Compton-thick objects (30% among type2s). The latter have been identified either directly from their intenseFeK line and flat X-ray spectra, or indirectly with flux diagnosticdiagrams which use isotropic indicators. After taking into account thesehighly absorbed sources, we find that (i) the intrinsic X-ray spectralproperties (i.e., spectral shapes and luminosities above 2 keV) areconsistent between type 1 and type 2 Seyferts, as expected from "unifiedmodels"; (ii) Seyfert galaxies as a whole are distributed fairlycontinuously over the entire range of N_H, between 1020 and1025 cm-2; and (iii) while Seyfert 1s tend to havelower NH and Seyfert 2s tend to have the highest, we find 30%and 10% exceptions, respectively. Overall the sample is of sufficientquality to well represent the average intrinsic X-ray spectralproperties of nearby active galactic nuclei, including a proper estimateof the distribution of their absorbing columns. Finally, we concludethat, with the exception of a few cases, the present study agrees withpredictions of unified models of Seyfert galaxies, and extends theirvalidity down to very low luminosities.
| The Schmidt Law at High Molecular Densities
We combined Hα and recent high-resolution12CO(J=1‑0) data to consider the quantitative relationbetween the gas mass and the star-formation rate, or the so-calledSchmidt law in nearby spiral galaxies at regions of high moleculardensity. The relation between the gas quantity and the star-formationrate has not been previously studied for high-density regions, but usinghigh-resolution CO data obtained at the Nobeyama Millimeter Array, wefound that the Schmidt law is valid at densities as high as 103Modotpc-2 for sample spiral galaxies, which is anorder of magnitude denser than what has been known to be the maximumdensity at which the empirical law holds for non-starburst galaxies.Furthermore, we obtained a Schmidt law index of N = 1.33 ± 0.09and a roughly constant star-formation efficiency over the entire disk,even within several hundred parsecs of the nucleus. These results implythat the physics of star formation does not change in the centralregions of spiral galaxies. Comparisons with starburst galaxies are alsogiven. We find a possible discontinuity in the Schmidt law betweennormal and starburst 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.
| A further argument for the iron lines in active galactic nuclei arising from cerenkov line-like radiation - Calculation for an optically thin case
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| 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.
| UV/Optical Nuclear Activity in the gE Galaxy NGC 1399
Using HST STIS, we have detected far-ultraviolet nuclear activity in thegiant elliptical galaxy NGC 1399, the central and brightest galaxy inthe Fornax I cluster. The source reached a maximum observed far-UVluminosity of ~1.2×1039 ergs s-1 in 1999January. It was detectable in earlier HST archival images in 1996 (Bband) but not in 1991 (V band) or 1993 (UV). It faded by a factor of ~4by mid-2000. The source is almost certainly associated with thelow-luminosity AGN responsible for the radio emission in NGC 1399. Theproperties of the outburst are remarkably similar to the UV-brightnuclear transient discovered earlier in NGC 4552 by Renzini andcoworkers. The source is much fainter than expected from its Bondiaccretion rate (estimated from Chandra high-resolution X-ray images),even in the context of ``radiatively inefficient accretion flow''models, and its variability also appears inconsistent with such models.High spatial resolution UV monitoring is a valuable means to studyactivity in nearby LLAGNs.
| Infrared Spectral Energy Distributions of Nearby Galaxies
The Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out acomprehensive multiwavelength survey on a sample of 75 nearby galaxies.The 1-850 μm spectral energy distributions (SEDs) are presented usingbroadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. Theinfrared colors derived from the globally integrated Spitzer data aregenerally consistent with the previous generation of models that weredeveloped using global data for normal star-forming galaxies, althoughsignificant deviations are observed. Spitzer's excellent sensitivity andresolution also allow a detailed investigation of the infrared SEDs forvarious locations within the three large, nearby galaxies NGC 3031(M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapesis found within each galaxy, especially for NGC 3031, the closest of thethree targets and thus the galaxy for which the smallest spatial scalescan be explored. Strong correlations exist between the local starformation rate and the infrared colors fν(70μm)/fν(160 μm) and fν(24μm)/fν(160 μm), suggesting that the 24 and 70 μmemission are useful tracers of the local star formation activity level.Preliminary evidence indicates that variations in the 24 μm emission,and not variations in the emission from polycyclic aromatic hydrocarbonsat 8 μm, drive the variations in the fν(8.0μm)/fν(24 μm) colors within NGC 3031, NGC 5194, andNGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sampleare representative of the range present at high redshift, thenextrapolations of total infrared luminosities and star formation ratesfrom the observed 24 μm flux will be uncertain at the factor of 5level (total range). The corresponding uncertainties using theredshifted 8.0 μm flux (e.g., observed 24 μm flux for a z=2source) are factors of 10-20. Considerable caution should be used wheninterpreting such extrapolated infrared luminosities.
| 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.
| Secular Evolution via Bar-driven Gas Inflow: Results from BIMA SONG
We present an analysis of the molecular gas distributions in the 29barred and 15 unbarred spirals in the BIMA CO (J=1-0) Survey of NearbyGalaxies (SONG). For galaxies that are bright in CO, we confirm theconclusion by Sakamoto et al. that barred spirals have higher moleculargas concentrations in the central kiloparsec. The SONG sample alsoincludes 27 galaxies below the CO brightness limit used by Sakamoto etal. Even in these less CO-bright galaxies we show that high central gasconcentrations are more common in barred galaxies, consistent withradial inflow driven by the bar. However, there is a significantpopulation of early-type (Sa-Sbc) barred spirals (6 of 19) that have nomolecular gas detected in the nuclear region and have very little out tothe bar corotation radius. This suggests that in barred galaxies withgas-deficient nuclear regions, the bar has already driven most of thegas within the bar corotation radius to the nuclear region, where it hasbeen consumed by star formation. The median mass of nuclear moleculargas is over 4 times higher in early-type bars than in late-type (Sc-Sdm)bars. Since previous work has shown that the gas consumption rate is anorder of magnitude higher in early-type bars, this implies that theearly types have significantly higher bar-driven inflows. The loweraccretion rates in late-type bars can probably be attributed to theknown differences in bar structure between early and late types. Despitethe evidence for bar-driven inflows in both early and late Hubble-typespirals, the data indicate that it is highly unlikely for a late-typegalaxy to evolve into an early type via bar-induced gas inflow.Nonetheless, secular evolutionary processes are undoubtedly present, andpseudobulges are inevitable; evidence for pseudobulges is likely to beclearest in early-type galaxies because of their high gas inflow ratesand higher star formation activity.
| The Pattern Speeds of 38 Barred Galaxies
We estimate the pattern speeds of 38 barred galaxies by simulationmodeling. We construct the gravitational potentials of the galaxies fromnear-IR photometry by assuming that the mass-to-light ratio (M/L) isconstant in the H band and a single pattern speed dominates in thestellar disk. We use the response of gaseous and stellar particle disksto a rigidly rotating potential to determine the pattern speed. If ourassumptions are correct, then the pattern speed depends on themorphological type: the average value of the ratio of the corotationresonance radius to the bar radius, ℛ, increases from about 1.1 intype SB0/a to 1.4 in SBb and 1.7 in SBc. Within the error estimates, allthe bars in galaxies of type SBab or earlier are fast rotators, havingℛ<=1.4, whereas late-type galaxies include both fast and slowrotators.
| The Vertical Stellar Kinematics in Face-On Barred Galaxies: Estimating the Ages of Bars
In order to perform a detailed study of the stellar kinematics in thevertical axis of bars, we obtained high signal-to-noise spectra alongthe major and minor axes of the bars in a sample of 14 face-on galaxiesand used them to determine the line-of-sight stellar velocitydistribution, parameterized as a Gauss-Hermite series. With these data,we developed a diagnostic tool that allows one to distinguish betweenrecently formed and evolved bars, as well as to estimate their ages,assuming that bars form in vertically thin disks that are recognizableby low values for the vertical velocity dispersion σz.Through N-body realizations of bar unstable disk galaxies we were alsoable to check the timescales involved in the processes that give bars animportant vertical structure. We show that σz inevolved bars is roughly 100 km s-1, which translates to aheight scale of about 1.4 kpc, giving support to scenarios in whichbulges form through disk material. Furthermore, the bars in ournumerical simulations have values for σz generallysmaller than 50 km s-1, even after evolving for 2 Gyr,suggesting that a slow process is responsible for making bars asvertically thick as we observe. We verify theoretically that theSpitzer-Schwarzschild mechanism is quantitatively able to explain theseobservations if we assume that giant molecular clouds are twice asconcentrated along the bar as in the rest of the disk.
| 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 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.
| 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 Distribution of Bar and Spiral Arm Strengths in Disk Galaxies
The distribution of bar strengths in disk galaxies is a fundamentalproperty of the galaxy population that has only begun to be explored. Wehave applied the bar-spiral separation method of Buta and coworkers toderive the distribution of maximum relative gravitational bar torques,Qb, for 147 spiral galaxies in the statistically well-definedOhio State University Bright Galaxy Survey (OSUBGS) sample. Our goal isto examine the properties of bars as independently as possible of theirassociated spirals. We find that the distribution of bar strengthdeclines smoothly with increasing Qb, with more than 40% ofthe sample having Qb<=0.1. In the context of recurrent barformation, this suggests that strongly barred states are relativelyshort-lived compared to weakly barred or nonbarred states. We do notfind compelling evidence for a bimodal distribution of bar strengths.Instead, the distribution is fairly smooth in the range0.0<=Qb<0.8. Our analysis also provides a first look atspiral strengths Qs in the OSUBGS sample, based on the sametorque indicator. We are able to verify a possible weak correlationbetween Qs and Qb, in the sense that galaxies withthe strongest bars tend to also have strong spirals.
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