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Discovery of counter-rotating gas in the galaxies NGC 1596 and 3203 and the incidence of gas counter-rotation in S0 galaxies
We have identified two new galaxies with gas counter-rotation (NGC 1596and 3203) and have confirmed similar behaviour in another one (NGC 128),this using results from separate studies of the ionized-gas and stellarkinematics of a well-defined sample of 30 edge-on disc galaxies. Gascounter-rotators thus represent 10 +/- 5 per cent of our sample, but thefraction climbs to 21 +/- 11 per cent when only lenticular (S0) galaxiesare considered and to 27 +/- 13 per cent for S0 galaxies with detectedionized gas only. Those fractions are consistent with but slightlyhigher than previous studies. A compilation from well-defined studies ofS0 galaxies in the literature yields fractions of 15 +/- 4 and 23 +/- 5per cent, respectively. Although mainly based on circumstantialevidence, we argue that the counter-rotating gas originates primarilyfrom minor mergers and tidally induced transfer of material from nearbyobjects. Assuming isotropic accretion, twice those fractions of objectsmust have undergone similar processes, underlining the importance of(minor) accretion for galaxy evolution. Applications of gascounter-rotators to barred galaxy dynamics are also discussed.
| 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.
| Pseudobulges in the Disk Galaxies NGC 7690 and NGC 4593
We present Ks-band surface photometry of NGC 7690 (Hubbletype Sab) and NGC 4593 (SBb). We find that, in both galaxies, a majorpart of the ``bulge'' is as flat as the disk and has approximately thesame color as the inner disk. In other words, the ``bulges'' of thesegalaxies have disklike properties. We conclude that these are examplesof ``pseudobulges,'' that is, products of secular dynamical evolution.Nonaxisymmetries such as bars and oval disks transport disk gas towardthe center. There star formation builds dense stellar components thatlook like-and often are mistaken for-merger-built bulges, but that wereconstructed slowly out of disk material. These pseudobulges can mosteasily be recognized when, as in the present galaxies, they retaindisklike properties. NGC 7690 and NGC 4593 therefore contribute to thegrowing evidence that secular processes help to shape galaxies. NGC 4593contains a nuclear ring of dust that is morphologically similar tonuclear rings of star formation that are seen in many barred and ovalgalaxies. The nuclear dust ring is connected to nearly radial dust lanesin the galaxy's bar. Such dust lanes are a signature of gas inflow. Wesuggest that gas is currently accumulating in the dust ring andhypothesize that the gas ring will starburst in the future. Theobservations of NGC 4593 therefore suggest that major starburst eventsthat contribute to pseudobulge growth can be episodic.Based on observations made with the Anglo-Australian Telescope.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained from the Data Archive at the Space Telescope ScienceInstitute (STScI). STScI is operated by the Association of Universitiesfor Research in Astronomy, Inc., under NASA contract NAS5-26555. Theobservations of NGC 7690 are associated with program IDs 7331 (NICMOS:M. Stiavelli) and 6359 (WFPC2: M. Stiavelli). The observations of NGC4593 are associated with program IDs 7330 (NICMOS: J. Mulchaey), and5479 (WFPC2: M. Malkan).
| Thermal and non-thermal components of the interstellar medium at sub-kiloparsec scales in galaxies
Aims. We present new radio continuum observations of ten BIMA SONGgalaxies, taken at 1.4 GHz with the Very Large Array. These observationsallow us to extend the study of the relationships between the radiocontinuum (RC) and CO emission to 22 CO luminous galaxies for whichsingle dish CO images have been added to interferometric data. NewSpitzer infrared (IR) images of six of these galaxies have beenreleased. The analysis of these high resolution images allowed us toprobe the RC-IR-CO correlations down to linear scales of a few hundredpc. Methods: .We compare the point-by-point RC, CO and mid-IRintensities across entire galaxy disks, producing radial profiles andspatially resolved images of the RC/CO and RC/mid-IR ratios.Results: .For the 22 galaxies analysed, the RC-CO correlation on scalesfrom ~10 kpc down to ~100 pc is nearly linear and has a scatter of afactor of two, i.e. comparable to that of the global correlations. Thereis no evidence for any severe degradation of the scatter below the kpcscale. This also applies to the six galaxies for which high-resolutionmid-IR data are available. In the case of NGC 5194,we find that the non-thermal radio spectral index is correlated with theRC/FIR ratio. Conclusions: .The scatter of the point-by-pointcorrelations does not increase significantly with spatial resolution. Wethus conclude that we have not yet probed the physical scales at whichthe correlations break down. However, we observe local deviations fromthe correlations in regions with a high star formation rate, such as thespiral arms, where we observe a flat radio spectrum and a low RC/FIRratio. In the intra-arm regions and in the peripheral regions of thedisk, the RC/FIR is generally higher and it is characterized by asteepening of the radio spectrum.
| 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*.
| The AMIGA sample of isolated galaxies. II. Morphological refinement
We present a refinement of the optical morphologies for galaxies in theCatalog of Isolated Galaxies that forms the basis of the AMIGA (Analysisof the interstellar Medium of Isolated GAlaxies) project. Uniformreclassification using the digitized POSS II data benefited from thehigh resolution and dynamic range of that sky survey. Comparison withindependent classifications made for an SDSS overlap sample of more than200 galaxies confirms the reliability of the early vs. late-typediscrimination and the accuracy of spiral subtypes within Δ T =1-2. CCD images taken at the Observatorio de Sierra Nevada were alsoused to solve ambiguities in early versus late-type classifications. Aconsiderable number of galaxies in the catalog (n = 193) are flagged forthe presence of nearby companions or signs of distortion likely due tointeraction. This most isolated sample of galaxies in the local Universeis dominated by two populations: 1) 82% are spirals (Sa-Sd) with thebulk being luminous systems with small bulges (63% between types Sb-Sc)and 2) a significant population of early-type E-S0 galaxies (14%). Mostof the types later than Sd are low luminosity galaxies concentrated inthe local supercluster where isolation is difficult to evaluate. Thelate-type spiral majority of the sample spans a luminosity rangeMB-corr = -18 to -22 mag. Few of the E/S0 population are moreluminous than -21.0 marking the absence of the often-sought superL* merger (e.g. fossil elliptical) population. The rarity ofhigh luminosity systems results in a fainter derived M* forthis population compared to the spiral optical luminosity function(OLF). The E-S0 population is from 0.2 to 0.6 mag fainter depending onhow the sample is defined. This marks the AMIGA sample as unique amongsamples that compare early and late-type OLFs separately. In othersamples, which always involve galaxies in higher density environments,M^*_E/S0 is almost always 0.3-0.5 mag brighter than M^*_S, presumablyreflecting a stronger correlation between M* andenvironmental density for early-type galaxies.
| 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.
| Molecular hydrogen and [FeII] in active galactic nuclei - II. Results for Seyfert 2 galaxies
Near-infrared spectroscopy is used to study the kinematics andexcitation mechanisms of H2 and [FeII] lines in a sampledominated by Seyfert 2 galaxies. The spectra simultaneously cover theJHK bands, allowing us to compare line fluxes emitted in the interval0.8-2.4 μm and avoiding aperture and seeing effects. TheH2 lines are systematically narrower than the narrow-lineregion lines, suggesting that, very likely, the H2 does notoriginate from the same parcel of gas that forms the narrow-line region.Emission-line ratios between H2 lines favour thermalexcitation mechanisms for the molecular gas in active galactic nuclei.It was found that non-thermal excitation contributes, at most, 30 percent of the observed H2. Thermal excitation is also confirmedby the rather similar vibrational and rotational temperatures in theobjects (~2000 K). The mass of hot H2 ranges from102 to 103Msolar, with nearly half ofobjects showing values of <500 Msolar. It shows that thefraction of molecular mass present in the nuclear region and emitting inthe near-infrared is a very small fraction of the warm molecular masspresent in the centre. A diagnostic diagram composed of the line ratiosH2/Brγ and [FeII]/Paβ proves to be a useful toolin the near-infrared for separating emission-line objects by theirdegree of nuclear activity. We found that active galactic nuclei arecharacterized by H2 2.121 μm/Brγ and [FeII] 1.257μm/Paβ flux ratios between 0.6 and 2. Starburst/HII galaxiesdisplay line ratios <0.6 while low-ionization nuclear emission-lineregions are characterized by values larger than 2 in either ratio.
| A bar signature and central disc in the gaseous and stellar velocity fields of NGC 5448
We analyse Spectrographic Areal Unit for Research on Optical Nebulae(SAURON) kinematic maps of the inner kiloparsec of the early-type (Sa)barred spiral galaxy NGC 5448. The observed morphology and kinematics ofthe emission-line gas is patchy and perturbed, indicating cleardepartures from circular motion. The kinematics of the stars is moreregular, and display a small inner disc-like system embedded in alarge-scale rotating structure. We focus on the [OIII] gas, and use aharmonic decomposition formalism to analyse the gas velocity field. Thehigher order harmonic terms and the main kinematic features of theobserved data are consistent with an analytically constructed simple barmodel. The bar model is derived using linear theory, considering an m= 2perturbation mode, and with bar parameters that are consistent with thelarge-scale bar detected via imaging. We also study optical and nearinfrared images to reveal the asymmetric extinction in NGC 5448, and werecognize that some of the deviations between the data and theanalytical bar model may be due to these complex dust features. Ourstudy illustrates how the harmonic decomposition formalism can be usedas a powerful tool to quantify non-circular motions in observed gasvelocity fields.
| The stellar populations of low-luminosity active galactic nuclei - III. Spatially resolved spectral properties
In a recently completed survey of the stellar population properties oflow-ionization nuclear emission-line regions (LINERs) and LINER/HIItransition objects (TOs), we have identified a numerous class ofgalactic nuclei which stand out because of their conspicuous108-9 yr populations, traced by high-order Balmer absorptionlines and other stellar indices. These objects are called `young-TOs',because they all have TO-like emission-line ratios. In this paper weextend this previous work, which concentrated on the nuclear properties,by investigating the radial variations of spectral properties inlow-luminosity active galactic nuclei (LLAGNs). Our analysis is based onhigh signal-to-noise ratio (S/N) long-slit spectra in the 3500-5500Å interval for a sample of 47 galaxies. The data probe distancesof typically up to 850 pc from the nucleus with a resolution of ~100 pc(~1 arcsec) and S/N ~ 30. Stellar population gradients are mapped by theradial profiles of absorption-line equivalent widths and continuumcolours along the slit. These variations are further analysed by meansof a decomposition of each spectrum in terms of template galaxiesrepresentative of very young (<=107 yr), intermediate age(108-9 yr) and old (1010 yr) stellar populations.This study reveals that young-TOs also differ from old-TOs andold-LINERs in terms of the spatial distributions of their stellarpopulations and dust. Specifically, our main findings are as follows.(i) Significant stellar population gradients are found almostexclusively in young-TOs. (ii) The intermediate age population ofyoung-TOs, although heavily concentrated in the nucleus, reachesdistances of up to a few hundred pc from the nucleus. Nevertheless, thehalf width at half-maximum of its brightness profile is more typically100 pc or less. (iii) Objects with predominantly old stellar populationspresent spatially homogeneous spectra, be they LINERs or TOs. (iv)Young-TOs have much more dust in their central regions than otherLLAGNs. (v) The B-band luminosities of the central <~1 Gyr populationin young-TOs are within an order of magnitude of MB=-15,implying masses of the order of ~107-108Msolar. This population was 10-100 times more luminous in itsformation epoch, at which time young massive stars would have completelyoutshone any active nucleus, unless the AGN too was brighter in thepast.
| First Results from THINGS: The HI Nearby Galaxy Survey
We describe The HI Nearby Galaxy Survey (THINGS), the largestprogramever undertaken at the Very Large Array to perform 21-cm HIobservations of thehighest quality ( 7'', ≤ 5 km s^{-1}resolution) ofnearby galaxies. The goal of THINGS is to investigatekeycharacteristics related to galaxy morphology, star formation andmassdistribution across the Hubble sequence. A sample of 34 objectswithdistances between 3 and 10 Mpc will be observed, covering a widerangeof evolutionary stages and properties. Data from THINGSwillcomplement SINGS, the Spitzer Infrared Nearby Galaxy Survey. Forthe THINGS sample, high-quality observations at comparable resolutionwillthus be available from the X-ray regime through to the radio partofthe spectrum. THINGS data can be used to investigate issues such asthesmall-scale structure of the ISM, its three-dimensional structure,the(dark) matter distribution and processes leading to starformation. Todemonstrate the quality of the THINGS data products, wepresent someprelimary HI maps here of four galaxies from the THINGSsample.
| 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.
| 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.
| On the Relevance of the Tremaine-Weinberg Method Applied to an Hα Velocity Field: Pattern Speed Determination in M100 (NGC 4321)
The relevance of the Tremaine-Weinberg (TW) method is tested formeasuring bar, spiral, and inner structure pattern speeds using agaseous velocity field. The TW method is applied to various simulatedbarred galaxies in order to demonstrate its validity in seven differentconfigurations, including star formation and/or dark matter halo. Thereliability of the different physical processes involved and of thevarious observational parameters is also tested. The simulations showthat the TW method could be applied to gaseous velocity fields to get agood estimate of the bar pattern speed, under the condition that regionsof shocks are avoided and measurements are confined to regions where thegaseous bar is well formed. We successfully apply the TW method to theHα velocity field of the Virgo Cluster galaxy M100 (NGC 4321) andderive pattern speeds of 55+/-5 km s-1 kpc-1 forthe nuclear structure, 30+/-2 km s-1 kpc-1 for thebar, and 20+/-1 km s-1 kpc-1 for the spiralpattern, in full agreement with published determinations using the samemethod or alternative ones.
| 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.
| A General Theory of Turbulence-regulated Star Formation, from Spirals to Ultraluminous Infrared Galaxies
We derive an analytic prediction for the star formation rate inenvironments ranging from normal galactic disks to starbursts and ULIRGsin terms of the observables of those systems. Our calculation is basedon three premises: (1) star formation occurs in virialized molecularclouds that are supersonically turbulent; (2) the density distributionwithin these clouds is lognormal, as expected for supersonic isothermalturbulence; and (3) stars form in any subregion of a cloud that is sooverdense that its gravitational potential energy exceeds the energy inturbulent motions. We show that a theory based on this model isconsistent with simulations and with the observed star formation rate inthe Milky Way. We use our theory to derive the Kennicutt-Schmidt lawfrom first principles and make other predictions that can be tested byfuture observations. We also provide an algorithm for estimating thestar formation rate that is suitable for inclusion in numericalsimulations.
| Nuclear Accretion in Galaxies of the Local Universe: Clues from Chandra Observations
In order to find an explanation for the radiative quiescence ofsupermassive black holes in the local universe, the most accurateestimates for a sample of nearby galaxies are collected for the mass ofa central black hole (MBH), the nuclear X-ray luminosityLX,nuc, and the circumnuclear hot gas density andtemperature, by using Chandra data. The nuclear X-ray luminosityLX,nuc varies by ~3 orders of magnitude and does not show arelationship with MBH or with the Bondi mass accretion rateM˙B LX,nuc is always much lower than expectedif M˙B ends in a standard accretion disk with highradiative efficiency (this instead can be the case of the active nucleusof Cen A). Radiatively inefficient accretion as in the standardadvection-dominated accretion flow (ADAF) modeling may explain the lowluminosities of a few cases; for others, the predicted luminosity isstill too high, and, in terms of Eddington-scaled quantities, it isincreasingly higher than that observed for increasingM˙B. Variants of the simple radiatively inefficientscenario including outflow and convection may reproduce the low emissionlevels observed, since the amount of matter actually accreted is reducedconsiderably. However, the most promising scenario includes feedbackfrom accretion on the surrounding gas; this has the important advantagesof naturally explaining the observed lack of relationship amongLX,nuc, MBH, and M˙B, and evadingthe problem of the fate of the material accumulating in the centralgalactic regions over cosmological times.
| Giant Molecular Clouds in M64
We investigate the properties of giant molecular clouds (GMCs) in themolecule-rich galaxy M64 (NGC 4826). In M64, the mean surface density ofmolecular gas is 2N(H2)~1022 cm-2 overa 2 kpc region, equal to the surface densities of individual GMCs in theMilky Way. We observed the J=1-->0 transitions of CO,13CO, and HCN. The line ratio WCO/W13COfor 200pc
| Light and Motion in the Local Volume
Using high-quality data on 149 galaxies within 10 Mpc, I find nocorrelation between luminosity and peculiar velocity at all. There is nounequivocal sign on scales of 1-2 Mpc of the expected gravitationaleffect of the brightest galaxies, in particular infall toward groups, orof infall toward the supergalactic plane on any scale. Either darkmatter is not distributed in the same way as luminous matter in thisregion, or peculiar velocities are not due to fluctuations in mass. Thesensitivity of peculiar velocity studies to the background model ishighlighted.
| EGRET Upper Limits and Stacking Searches of Gamma-Ray Observations of Luminous and Ultraluminous Infrared Galaxies
We present a stacking analysis of EGRET γ-ray observations at thepositions of luminous and ultraluminous infrared galaxies. The latterwere selected from the recently presented HCN survey, which is thoughtto contain the most active star-forming regions of the universe.Different sorting criteria are used, and since there is no positivecollective detection of γ-ray emission from these objects, wedetermined both collective and individual upper limits. The uppermostexcess we find appears in the case of ULIRGs ordered by redshift, at avalue of 1.8 σ.
| 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.
| Probing Halos of Galaxies at Very Large Radii Using Background QSOs
Gaseous halos of nine nearby galaxies (with redshifts cz<6000 kms-1) were probed at large galactocentric radii usingbackground quasars observed with the Hubble Space Telescope Goddard HighResolution Spectrograph and the Space Telescope Imaging Spectrograph.The projected quasar-galaxy separations range from 55 to 387h-175 kpc. Lyα absorption lines weresuccessfully detected in the spectra of five quasars, at impactparameters of up to ~170 h-175 kpc from the centerof the nearby galaxy, and in each case at wavelengths consistent withthe galaxy's redshift. Our observations include the lowest redshiftLyα lines detected to date. H I velocity fields were obtained atthe Very Large Array for three of the galaxies in our sample (in onecase the velocity field was available from the literature) to derivetheir rotation curves. When comparing the inner rotation curves of thegalaxies with the velocity at large radius provided by the Lyαline, it is apparent that it is very difficult to explain the observedLyα velocity as due to gas in an extended rotating disk. In mostcases, one would need to invoke large warps in the outer gas disks andalso thick gas disks to reconcile the observed velocities with thepredicted ones. Indeed, in one case, the Lyα line velocityindicates, in fact, counterrotation with respect to the inner diskrotation. In light of these results, we conclude that in a typicalgalaxy there is no longer detectable atomic gas corotating in anextended disk at radii greater than 35α-1, whereα-1 is the stellar disk exponential scale length. Thecosmic web is the most likely origin for the detected Lyα lines.Our observations confirm the recent Bowen et al. correlation ofequivalent widths with the local volume density of galaxies around thesight line, and the observed equivalent widths of the lines areconsistent with expectations of the cosmic web.Based on observations with the NASA ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS5-26555.
| Gas and Stars in an H I-Selected Galaxy Sample
We present the results of a J-band study of the H I-selected AreciboDual-Beam Survey and Arecibo Slice Survey galaxy samples using TwoMicron All Sky Survey data. We find that these galaxies span a widerange of stellar and gas properties. However, despite the diversitywithin the samples, we find a very tight correlation between luminosityand size in the J band, similar to that found in a previous paper byRosenberg & Schneider between the H I mass and size. We also findthat the correlation between the baryonic mass and the J-band diameteris even tighter than that between the baryonic mass and the rotationalvelocity.
| 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.
| Molecular gas in NUclei of GAlaxies (NUGA). IV. Gravitational torques and AGN feeding
We discuss the efficiency of stellar gravity torques as a mechanism toaccount for the feeding of the central engines of four low luminosityActive Galactic Nuclei (AGN): NGC 4321 (HII nucleus/LINER), NGC 4826(HII nucleus/LINER), NGC 4579 (LINER 1.9/Seyfert 1.9) and NGC 6951(Seyfert 2). These galaxies have been observed as part of the NUclei ofGAlaxies-(NUGA) CO project, aimed at the study of AGN fuelingmechanisms. Our calculations allow us to derive the characteristictime-scales for gas flows and discuss whether torques from the stellarpotentials are efficient enough to drain the gas angular momentum in theinner 1 kpc of these galaxies. The stellar potentials are derived usinghigh-resolution near infrared (NIR) images and the averaged effectivetorques on the gas are estimated using the high-resolution(~0.5´´-2´´) CO maps of the galaxies. Resultsindicate paradoxically that feeding should be thwarted close to theAGNs: in the four cases analyzed, gravity torques are mostly positiveinside r 200 pc, resulting in no inflow on these scales. As apossible solution for the paradox, we speculate that the agentresponsible for driving inflow to still smaller radii is transient andthus presently absent in the stellar potential. Alternatively, thegravity torque barrier associated with the Inner Lindblad Resonance ofthe bars in these galaxies could be overcome by other mechanisms thatbecome competitive in due time against gravity torques. In particular,we estimate on a case-by-case basis the efficiency of viscosity versusgravity torques to drive AGN fueling. We find that viscosity cancounteract moderate-to-low gravity torques on the gas if it acts on anuclear ring of high gas surface density contrast and ~a few 100 pcsize. We propose an evolutionary scenario in which gravity torques andviscosity act in concert to produce recurrent episodes of activityduring the typical lifetime of any galaxy. In this scenario therecurrence of activity in galaxies is indirectly related to that of thebar instabilities although the active phases are not necessarilycoincident with the maximum strength of a single bar episode. Thegeneral implications of these results for the current understanding offueling of low-luminosity AGN are discussed.
| The molecular connection to the FIR-radio continuum correlation in galaxies
We have studied the relationships between the radio continuum (RC) andCO emission for a set of galaxies selected from the BIMA Survey ofNearby Galaxies. We find that the global CO-RC correlation is as tightas the global FIR-RC correlation for the 24 galaxies studied. Within 9galaxies with ~6´´ CO and RC data available, the CO and RCemission is as tightly correlated as its global value; the radiallyaveraged correlation is nearly linear, extends over four order ofmagnitude and holds down to the smallest linear resolution of theobservations, which is ~100 pc. We define q_CO/RC as the log of theratio of the CO to RC flux as a way to characterize the CO-RCcorrelation. Combining 6´´ pixel-by-pixel comparisons acrossall sources yields an average small-scale correlation of q_CO/RC = 1.1± 0.28; that is, the spatially resolved correlation has adispersion that is less than a factor of 2. There are however systematicvariations in the CO/RC ratio; the strongest organized structures inq_CO/RC tend to be found along spiral arms and on size scales muchlarger than the resolution of the observations. We do not measure anysystematic trend in CO/RC ratio as a function of radius in galaxies. Theconstancy of the CO/RC ratio stands in contrast to the previouslymeasured decrease in the FIR/RC ratio as a function of radius ingalaxies. We suggest that the excellent correlation between the CO, RCand FIR emission in galaxies is a consequence of regulation byhydrostatic pressure; this model links all three emissions withoutinvoking an explicit dependence on a star formation scenario.
| New H2O masers in Seyfert and FIR bright galaxies
Using the Effelsberg 100-m telescope, detections of four extragalacticwater vapor masers are reported. Isotropic luminosities are ~50, 1000, 1and 230 Lȯ for Mrk 1066 (UGC 2456), Mrk 34, NGC 3556 andArp 299, respectively. Mrk 34 contains by far the most distant and oneof the most luminous water vapor megamasers so far reported in a Seyfertgalaxy. The interacting system Arp 299 appears to show two maserhotspots separated by approximately 20´´. With these newresults and even more recent data from Braatz et al. (2004, ApJ, 617,L29), the detection rate in our sample of Seyferts with known jet-NarrowLine Region interactions becomes 50% (7/14), while in star forminggalaxies with high (S100~μ m>50 Jy) far infrared fluxesthe detection rate is 22% (10/45). The jet-NLR interaction sample maynot only contain “jet-masers” but also a significant numberof accretion “disk-masers” like those seen in NGC 4258. Astatistical analysis of 53 extragalactic H2O sources (excluding theGalaxy and the Magellanic Clouds) indicates (1) that the correlationbetween IRAS Point Source and H2O luminosities, established forindividual star forming regions in the galactic disk, also holds forAGN-dominated megamaser galaxies; (2) that maser luminosities are notcorrelated with 60 μm/100 μm color temperatures; and (3) that onlya small fraction of the luminous megamasers (L_H_2O > 100Lȯ) detectable with 100-m sized telescopes have so farbeen identified. The H2O luminosity function (LF) suggests that thenumber of galaxies with 1 Lȯ < L_H_2O < 10Lȯ, the transition range between“kilomasers” (mostly star formation) and“megamasers” (active galactic nuclei), is small. The overallslope of the LF, ~-1.5, indicates that the number of detectable masersis almost independent of their luminosity. If the LF is not steepeningat very high maser luminosities and if it is possible to find suitablecandidate sources, H2O megamasers at significant redshifts should bedetectable even with present day state-of-the-art facilities.
| Studies of Extragalactic Formaldehyde and Radio Recombination Lines
We present the most sensitive and extensive survey yet performed ofextragalactic H2CO 6 cm (4.829 GHz) emission/absorption.Sixty-two sources were observed with the C-band system of the AreciboTelescope to a 1 σ rms noise level of ~0.3 mJy. We report a newdetection of H2CO 6 cm absorption toward NGC 520 and theconfirmation of H2CO 6 cm absorption toward several sources.We report confirmation of H2CO 6 cm emission toward the OHmegamasers Arp 220, IC 860, and IRAS 15107+0724. At present these arethe only extragalactic H2CO 6 cm emitters independentlyconfirmed. A characterization of the properties of formaldehydeabsorbers and emitters based on infrared properties of the galaxies isdiscussed. We also conducted a simultaneous survey of the H110αhydrogen recombination line toward a sample of 53 objects. We report thedetection of H110α toward the giant extragalactic H II region NGC604 in M33.
| HCN Survey of Normal Spiral, Infrared-luminous, and Ultraluminous Galaxies
We report systematic HCN J=1-0 (and CO) observations of a sample of 53infrared (IR) and/or CO-bright and/or luminous galaxies, including sevenultraluminous infrared galaxies, nearly 20 luminous infrared galaxies,and more than a dozen of the nearest normal spiral galaxies. This is thelargest and most sensitive HCN survey of galaxies to date. All galaxiesobserved so far follow the tight correlation between the IR luminosityLIR and the HCN luminosity LHCN initially proposedby Solomon, Downes, & Radford, which is detailed in a companionpaper. We also address here the issue of HCN excitation. There is noparticularly strong correlation between LHCN and the 12 μmluminosity; in fact, of all the four IRAS bands, the 12 μm luminosityhas the weakest correlation with the HCN luminosity. There is also noevidence of stronger HCN emission or a higher ratio of HCN and COluminosities LHCN/LCO for galaxies with excess 12μm emission. This result implies that mid-IR radiative pumping, orpopulating, of the J=1 level of HCN by a mid-IR vibrational transitionis not important compared with the collisional excitation by densemolecular hydrogen. Furthermore, large velocity gradient calculationsjustify the use of HCN J=1-0 emission as a tracer of high-densitymolecular gas (>~3×104/τcm-3) andgive an estimate of the mass of dense molecular gas from HCNobservations. Therefore, LHCN may be used as a measure of thetotal mass of dense molecular gas, and the luminosity ratioLHCN/LCO may indicate the fraction of moleculargas that is dense.
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