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Metallicity Gradients and Gas Flows in Galaxy Pairs
We present the first systematic investigation into the metallicitygradients in galaxy close pairs. We determine the metallicity gradientsfor eight galaxies in close pairs using H II region metallicitiesobtained with high signal-to-noise multi-slit observations with the KeckLRIS Spectrograph. We show that the metallicity gradients in close pairsare significantly shallower than gradients in isolated spiral galaxiessuch as the Milky Way, M83, and M101. These observations provide thefirst solid evidence that metallicity gradients in interacting galaxiesare systematically different from metallicity gradients in isolatedspiral galaxies. Our results suggest that there is a strong relationshipbetween metallicity gradients and the gas dynamics in galaxyinteractions and mergers.
| Bulges of Nearby Galaxies with Spitzer: Scaling Relations in Pseudobulges and Classical Bulges
We investigate scaling relations of bulges using bulge-diskdecompositions at 3.6 ?m and present bulge classifications for 173E-Sd galaxies within 20 Mpc. Pseudobulges and classical bulges areidentified using Sérsic index, Hubble Space Telescope morphology,and star formation activity (traced by 8 ?m emission). In the near-IRpseudobulges have nb < 2 and classical bulges havenb >2, as found in the optical. Sérsic index andmorphology are essentially equivalent properties for bulgeclassification purposes. We confirm, using a much more robust sample,that the Sérsic index of pseudobulges is uncorrelated with otherbulge structural properties, unlike for classical bulges and ellipticalgalaxies. Also, the half-light radius of pseudobulges is not correlatedwith any other bulge property. We also find a new correlation betweensurface brightness and pseudobulge luminosity; pseudobulges become moreluminous as they become more dense. Classical bulges follow thewell-known scaling relations between surface brightness, luminosity, andhalf-light radius that are established by elliptical galaxies. We showthat those pseudobulges (as indicated by Sérsic index and nuclearmorphology) that have low specific star formation rates are very similarto models of galaxies in which both a pseudobulge and classical bulgeexist. Therefore, pseudobulge identification that relies only onstructural indicators is incomplete. Our results, especially those onscaling relations, imply that pseudobulges are very different types ofobjects than elliptical galaxies.
| The Metal-enriched Outer Disk of NGC 2915
We present optical emission-line spectra for outlying H II regions inthe extended neutral gas disk surrounding the blue compact dwarf galaxyNGC 2915. Using a combination of strong-line R23 and direct oxygenabundance measurements, we report a flat, possibly increasing,metallicity gradient out to 1.2 times the Holmberg radius. We find theouter disk of NGC 2915 to be enriched to a metallicity of 0.4 Zsun. An analysis of the metal yields shows that the outerdisk of NGC 2915 is overabundant for its gas fraction, while the centralstar-forming core is similarly underabundant for its gas fraction. Starformation rates derived from very deep ~14 ks GALEX FUV exposuresindicate that the low level of star formation observed at large radii isnot sufficient to have produced the measured oxygen abundances at thesegalactocentric distances. We consider three plausible mechanisms thatmay explain the metal-enriched outer gaseous disk of NGC 2915: radialredistribution of centrally generated metals, strong galactic winds withsubsequent fallback, and galaxy accretion. Our results have implicationsfor the physical origin of the mass-metallicity relation for gas-richdwarf galaxies.
| The Slope of the Baryonic Tully-Fisher Relation
We present the results of a baryonic Tully-Fisher relation (BTFR) studyfor a local sample of relatively isolated disk galaxies. We derive aBTFR with a slope near 3 measured over about 4 dex in baryon mass forour combined H I and bright spiral disk samples. This BTFR issignificantly flatter and has less scatter than the TFR (stellar massonly) with its slope near 4 reported for other samples and studies. ABTFR slope near 3 is in better agreement with the expected slope fromsimple ?CDM cosmological simulations that include both stellarand gas baryons. The scatter in the TFR/BTFR appears to depend on W20: galaxies that rotate slower have more scatter. The atomicgas-to-stars ratio shows a break near W 20 = 250 kms–1 probably associated with a change in star formationefficiency. In contrast, the absence of such a break in the BTFRsuggests that this relation was probably set at the main epoch of baryondissipation rather than as a product of later galactic evolution.
| Insights into the Cepheid Distance Scale
We present a detailed investigation of the Cepheid distance scale byusing both theory and observations. Through the use of pulsation modelsfor fundamental mode Cepheids, we found that the slope of theperiod-luminosity (P-L) relation covering the entire period range (0.40<= log P <= 2.0) becomes steeper when moving from optical tonear-infrared (NIR) bands, and that the metallicity dependence of theslope decreases from the B- to the K band. The sign of the metallicitydependence for the slopes of the P-LV and P-LIrelation is at odds with some recent empirical estimates. We determinednew homogeneous estimates of V- and I-band slopes for 87 independentCepheid data sets belonging to 48 external galaxies with nebular oxygenabundance 7.5 <= 12 + log (O/H) <= 8.9. By using Cepheid samplesincluding more than 20 Cepheids, the ?2 test indicatesthat the hypothesis of a steepening of the P-L V,I relations with increased metal content can be discarded at the 99%level. On the contrary, the observed slopes agree with the metallicitytrend predicted by pulsation models, i.e., the slope is roughly constantfor galaxies with 12+log (O/H) < 8.17 and becomes shallower in themetal-rich regime, with confidence levels of 62% and 92%, respectively.The ?2 test concerning the hypothesis that the slope doesnot depend on metallicity gives confidence levels either similar(PLV , 62%) or smaller (PLI , 67%). Weinvestigated the dependence of the period-Wesenheit (P-W) relations onthe metal content and we found that the slopes of optical and NIR P-Wrelations in external galaxies are similar to the slopes of LargeMagellanic Cloud (LMC) Cepheids. They also agree with the theoreticalpredictions suggesting that the slopes of the P-W relations areindependent of the metal content. On this ground, the P-W relationsprovide a robust method to determine distance moduli relative to theLMC, but theory and observations indicate that the metallicitydependence of the zero point in the different passbands has to be takeninto account. To constrain this effect, we compared the independent setof galaxy distances provided by Rizzi et al. using the tip of the redgiant branch with our homogeneous set of extragalactic Cepheid distancesbased on the P-W relations. We found that the metallicity correction ondistances based on the P-WBV relation is ? B,V =-0.52 mag dex-1, whereas it is vanishing for thedistances based on the P-WVI and on the P-WJK relations. These findingsfully support Cepheid theoretical predictions.
| Spectral Energy Distributions of Weak Active Galactic Nuclei Associated with Low-Ionization Nuclear Emission Regions
We present a compilation of spectral energy distributions (SEDs) of 35weak active galactic nuclei (AGNs) in low-ionization nuclear emissionregions (LINERs) using recent data from the published literature. Wemake use of previously published compilations of data, aftercomplementing and extending them with more recent data. The mainimprovement in the recent data is afforded by high-spatial-resolutionobservations with the Chandra X-Ray Observatory andhigh-spatial-resolution radio observations utilizing a number offacilities. In addition, a considerable number of objects have beenobserved with the Hubble Space Telescope in the near-IR through near-UVbands since the earlier compilations were published. The data includeupper limits resulting from either non-detections or observations at lowspatial resolution that do not isolate the AGN. For the sake ofcompleteness, we also compute and present a number of quantities fromthe data, such as optical-to-X-ray spectral indices(αox), bolometric corrections, bolometric luminosities,Eddington ratios, and the average SED. We anticipate that these datawill be useful for a number of applications. In a companion paper, weuse a subset of these data ourselves to assess the energy budgets ofLINERs.
| NGC 1300 dynamics - I. The gravitational potential as a tool for detailed stellar dynamics
In a series of papers, we study the stellar dynamics of the grand designbarred-spiral galaxy NGC 1300. In the first paper of this series, weestimate the gravitational potential and we give it in a form suitableto be used in dynamical studies. The estimation is done directly fromnear-infrared observations. Since the 3D distribution of the luminousmatter is unknown, we construct three different general models for thepotential corresponding to three different assumptions for the geometryof the system, representing limiting cases: a pure 2D disc, acylindrical geometry (thick disc) and a third case, where a sphericalgeometry is assumed to apply for the major part of the bar. For thepotential of the disc component on the galactic plane, a Fourierdecomposition method is used, which allows us to express it as a sum oftrigonometric terms. Both even and odd components are considered, sothat the estimated potential accounts also for the observed asymmetriesin the morphology. For the amplitudes of the trigonometric terms, asmoothed cubic interpolation scheme is used. The total potential in eachmodel may include two additional terms (Plummer spheres) representing acentral mass concentration and a dark halo component, respectively. Inall examined models, the relative force perturbation points to astrongly non-linear gravitational field, which ranges from 0.45 to 0.8of the axisymmetric background with the pure 2D being the mostnon-linear one. The force perturbation in each model is found to berobust to small changes of the required parameter values. We present thetopological distributions of the stable and unstable Lagrangian pointsas a function of the pattern speed (?p). Thetopological distribution found deviates in several cases from theclassical paradigm with two stable Lagrangian points at the sides of thebar and two unstable ones close to the ends of the bar. In all threemodels, there is a range of ?p values, where we findmultiple stationary points whose stability affects the overall dynamicsof the system.
| An Assessment of the Energy Budgets of Low-Ionization Nuclear Emission Regions
Using the spectral energy distributions (SEDs) of the weak activegalactic nuclei (AGNs) in 35 low-ionization nuclear emission regions(LINERs) presented in a companion paper, we assess whetherphotoionization by the weak AGN can power the emission-line luminositiesmeasured through the large (few-arcsecond) apertures used inground-based spectroscopic surveys. Spectra taken through such aperturesare used to define LINERs as a class and constrain non-stellarphotoionization models for LINERs. Therefore, our energy budget test isa self-consistency check of the idea that the observed emission linesare powered by an AGN. We determine the ionizing luminosities and photonrates by integrating the observed SEDs and by scaling a template SED. Wefind that even if all ionizing photons are absorbed by the line-emittinggas, more than half of the LINERs in this sample suffer from a deficitof ionizing photons. In 1/3 of LINERs the deficit is severe. If only 10%of the ionizing photons are absorbed by the gas, there is an ionizingphoton deficit in 85% of LINERs. We disfavor the possibility thatadditional electromagnetic power, either obscured or emitted in theunobservable far-UV band, is available from the AGN. Therefore, weconsider other power sources such as mechanical heating by compact jetsfrom the AGN and photoionization by either young or old stars.Photoionization by young stars may be important in a small fraction ofcases. Mechanical heating can provide enough power in most cases but itis not clear how this power would be transferred to the emission-linegas. Photoionization by post asymptotic giant branch stars is animportant power source; it provides more ionizing photons than the AGNin more than half of the LINERs and enough ionizing photons to power theemission lines in 1/3 of the LINERs. It appears likely that theemission-line spectra of LINERs obtained from the ground include the sumof emission from different regions where different power sourcesdominate.
| Grey Milky Way extinction from SDSS stellar photometry
We report results concerning the distribution and properties of galacticextinction at high galactic latitudes derived from stellar statisticsusing the Sloan Digital Sky Survey (SDSS). We use the classical Wolfdiagram method to identify regions with extinction, and derive theextinction and the extinction law of the dust using all five SDSSspectral bands. We estimate the distance to the extinguishing mediumusing simple assumptions about the stellar populations in the line ofsight.We report the identification of three extinguishing clouds, each a fewtens of pc wide, producing 0.2-0.4 mag of g'-band extinction, located1-2 kpc away or 0.5-1 kpc above the Galactic plane. All clouds exhibitgrey extinction, i.e. almost wavelength independent in the limitedspectral range of the SDSS. We discuss the implication of this findingon general astrophysical questions.
| On the Curvature of Dust Lanes in Galactic Bars
We test the theoretical prediction that the straightest dust lanes inbars are found in strongly barred galaxies, or more specifically, thatthe degree of curvature of the dust lanes is inversely proportional tothe strength of the bar. The test uses archival images of barredgalaxies for which a reliable nonaxisymmetric torque parameter (Qb) and the radius at which Q b has been measured(r(Q b)) have been published in the literature. Our resultsconfirm the theoretical prediction but show a large spread that cannotbe accounted for by measurement errors. We simulate 238 galaxies withdifferent bar and bulge parameters in order to investigate the origin ofthe spread in the dust lane curvature versus Q b relation.From these simulations, we conclude that the spread is greatly reducedwhen describing the bar strength as a linear combination of the barparameters Q b and the quotient of the major and minor axesof the bar, a/b. Thus, we conclude that the dust lane curvature ispredominantly determined by the parameters of the bar.
| The Herschel Reference Survey
The Herschel Reference Survey is a Herschel guaranteed time key projectand will be a benchmark study of dust in the nearby universe. The surveywill complement a number of other Herschel key projects including largecosmological surveys that trace dust in the distant universe. We willuse Herschel to produce images of a statistically-complete sample of 323galaxies at 250, 350, and 500 ?m. The sample is volume-limited,containing sources with distances between 15 and 25 Mpc and flux limitsin the band to minimize the selection effects associated with dust andwith young high-mass stars and to introduce a selection in stellar mass.The sample spans the whole range of morphological types (ellipticals tolate-type spirals) and environments (from the field to the center of theVirgo Cluster) and as such will be useful for other purposes than ourown. We plan to use the survey to investigate (i) the dust content ofgalaxies as a function of Hubble type, stellar mass, and environment;(ii) the connection between the dust content and composition and theother phases of the interstellar medium; and (iii) the origin andevolution of dust in galaxies. In this article, we describe the goals ofthe survey, the details of the sample and some of the auxiliaryobserving programs that we have started to collect complementary data.We also use the available multifrequency data to carry out an analysisof the statistical properties of the sample.
| VLA Imaging of Virgo Spirals in Atomic Gas (VIVA). I. The Atlas and the H I Properties
We present the results of a new VLA H I Imaging survey of Virgogalaxies, the VLA Imaging survey of Virgo galaxies in Atomic gas (VIVA).The survey includes high-resolution H I data of 53 carefully selectedlate type galaxies (48 spirals and five irregular systems). The goal isto study environmental effects on H I gas properties of cluster galaxiesto understand which physical mechanisms affect galaxy evolution indifferent density regions, and to establish how far out the impact ofthe cluster reaches. As a dynamically young cluster, Virgo containsexamples of galaxies experiencing a variety of environmental effects.Its nearness allows us to study each galaxy in great detail. We haveselected Virgo galaxies with a range of star formation properties in lowto high density regions (at projected distances from M87, d87 = 0.3-3.3 Mpc). Contrary to previous studies, more thanhalf of the galaxies in the sample (~60%) are fainter than 12 mag inBT . Overall, the selected galaxies represent the late typeVirgo galaxies (S0/a to Sd/Irr) down to mp lsim 14.6 fairlywell in morphological type, systemic velocity, subcluster membership, HI mass, and deficiency. The H I observations were done in C short (CS)configuration of the VLA radio telescope, with a typical spatialresolution of 15'' and a column density sensitivity of ≈3-5 ×1019 cm–2 in 3σ per 10 kms–1 channel. The survey was supplemented with data ofcomparable quality from the NRAO archive, taken in CS or Cconfiguration. In this paper, we present H I channel maps, totalintensity maps, velocity fields, velocity dispersions, global/radialprofiles, position-velocity diagrams and overlays of H I/1.4 GHzcontinuum maps on the optical images. We also present H I propertiessuch as total flux (S H I ), H I mass (M H I ),linewidths (W 20 and W 50), velocity (V H I), deficiency (def H I ), and size (D effH I and D iso H I ), and describe theH I morphology and kinematics of individual galaxies in detail. Thesurvey has revealed details of H I features that were never seen before.In this paper, we briefly discuss differences in typical H I morphologyfor galaxies in regions of different galaxy densities. We confirm thatgalaxies near the cluster core (d 87 lsim 0.5 Mpc) have H Idisks that are smaller compared to their stellar disks (D H I/D 25 < 0.5). Most of these galaxies in the corealso show gas displaced from the disk, which is either currently beingstripped or falling back after a stripping event. At intermediatedistances (d 87 ~ 1 Mpc) from the center, we find aremarkable number of galaxies with long one-sided H I tails pointingaway from M87. In a previous letter, we argue that these galaxies arerecent arrivals, falling into the Virgo core for the first time. In theoutskirts, we find many gas-rich galaxies, with gas disks extending farbeyond their optical disks. Interestingly, we also find some galaxieswith H I disks that are smaller compared to their stellar disks at largeclustercentric distances.
| Massive star formation in Wolf-Rayet galaxies. II. Optical spectroscopy results
Aims. We have performed a comprehensive multiwavelength analysis of asample of 20 starburst galaxies that show the presence of a substantialpopulation of very young massive stars, most of them classified asWolf-Rayet (WR) galaxies. In this paper, the second of the series, wepresent the results of the analysis of long-slit intermediate-resolutionspectroscopy of star-formation bursts for 16 galaxies of our sample. Methods: We study the spatial localization of the WR stars in eachgalaxy. We analyze the excitation mechanism and derive the reddeningcoefficient, physical conditions, and chemical abundances of the ionizedgas. We study the kinematics of the ionized gas to check therotation/turbulence pattern of each system. When possible, tentativeestimates of the Keplerian mass of the galaxies are calculated.Results: Aperture effects and the exact positioning of the slit onto theWR-rich bursts seem to play a fundamental role in their detection. Wecheck that the ages of the last star-forming burst estimated usingoptical spectra agree with those derived from Hα imagery. Ouranalysis shows that a substantial fraction of the galaxies exhibitevidence of perturbed kinematics. With respect to the results found inindividual galaxies, we remark the detection of objects with differentmetallicity and decoupled kinematics in Haro 15 and Mkn 1199, thedetection of tidal streams in IRAS 08208+2816, Tol 9, and perhaps in SBS1319+579, and the development of a merging process in both SBS 0926+606A and Tol 1457-262. Conclusions: All of these results - incombination with those obtained in Paper I - reinforce the hypothesisthat interactions with or between dwarf objects is a very importantmechanism in triggering massive-star formation in starburst galaxies,especially in dwarf ones. It must be highlighted that only deep and verydetailed observations - as presented in this paper - can provide clearevidences that these subtle interaction processes are taking place.Based on observations made with NOT (Nordic Optical Telescope), INT(Isaac Newton Telescope) and WHT (William Herschel Telescope) operatedon the island of La Palma jointly by Denmark, Finland, Iceland, Norwayand Sweden (NOT) or the Isaac Newton Group (INT, WHT) in the SpanishObservatorio del Roque de Los Muchachos of the Instituto deAstrofísica de Canarias.Table 1 and Figs. 1-4, 6-10, 12, 13, 15-30, 32-35 and Appendix are onlyavailable in electronic form at http://www.aanda.org
| The anticorrelation between the hard X-ray photon index and the Eddington ratio in low-luminosity active galactic nuclei
We find a significant anticorrelation between the hard X-ray photonindex Γ and the Eddington ratio Lbol/LEddfor a sample of low-ionization nuclear emission-line regions and localSeyfert galaxies, compiled from literature with Chandra or XMM-Newtonobservations. This result is in contrast with the positive correlationfound in luminous active galactic nuclei (AGN), while it is similar tothat of X-ray binaries (XRBs) in the low/hard state. Our result isqualitatively consistent with the spectra produced fromadvection-dominated accretion flows (ADAFs). It implies that the X-rayemission of low-luminosity active galactic nuclei (LLAGN) may originatefrom the Comptonization process in ADAF, and the accretion process inLLAGN may be similar to that of XRBs in the low/hard state, which isdifferent from that in luminous AGN.
| 12CO(J = 1 – 0) On-the-Fly Mapping Survey of the Virgo Cluster Spirals. I. Data and Atlas
We have performed an On-The-Fly (OTF) mapping survey of12CO(J = 1-0) emission in 28 Virgo cluster spiral galaxiesusing the Five College Radio Astronomy Observatory (FCRAO) 14 mtelescope. This survey aims to characterize the CO distribution,kinematics, and luminosity of a large sample of galaxies covering thefull extents of stellar disks, rather than sampling only the inner disksor the major axis as was done by many previous single dish andinterferometric CO surveys. CO emission is detected in 20 galaxies amongthe 28 Virgo spirals observed. An atlas consisting of global measures,radial measures, and maps is presented for each detected galaxy. A notesummarizing the CO data is also presented along with relevantinformation from the literature. The CO properties derived from our OTFobservations are presented and compared with the results from the FCRAOExtragalactic CO Survey by Young et al. which utilizedposition-switching observations along the major axis and a model fittingmethod. We find that our OTF-derived CO properties agree well with theYoung et al. results in many cases, but the Young et al.measurements are larger by a factor of 1.4-2.4 for seven (out of 18)cases. We will explore further the possible causes for the discrepancyin the analysis paper currently under preparation.
| Fractal Dimension of Galaxy Isophotes
In this paper we investigate the use of the fractal dimension of galaxyisophotes in galaxy classification. We have applied two differentmethods for determining fractal dimensions to the isophotes ofelliptical and spiral galaxies derived from CCD images. We conclude thatfractal dimension alone is not a reliable tool but that combined withother parameters in a neural net algorithm the fractal dimension couldbe of use. In particular, we have used three parameters to segregate theellipticals and lenticulars from the spiral galaxies in our sample.These three parameters are the correlation fractal dimension Dcorr, the difference between the correlation fractaldimension and the capacity fractal dimension D corr – Dcap, and, thirdly, the B – V color of the galaxy.
| A Search for "Dwarf" Seyfert Nuclei. VII. A Catalog of Central Stellar Velocity Dispersions of Nearby Galaxies
We present new central stellar velocity dispersion measurements for 428galaxies in the Palomar spectroscopic survey of bright, northerngalaxies. Of these, 142 have no previously published measurements, mostbeing relatively late-type systems with low velocity dispersions(lsim100 km s–1). We provide updates to a number ofliterature dispersions with large uncertainties. Our measurements arebased on a direct pixel-fitting technique that can accommodate compositestellar populations by calculating an optimal linear combination ofinput stellar templates. The original Palomar survey data were takenunder conditions that are not ideally suited for deriving stellarvelocity dispersions for galaxies with a wide range of Hubble types. Wedescribe an effective strategy to circumvent this complication anddemonstrate that we can still obtain reliable velocity dispersions forthis sample of well-studied nearby galaxies.
| The Ks-band Tully-Fisher Relation — A determination of the Hubble parameter from 218 ScI galaxies and 16 galaxy clusters
The value of the Hubble Parameter (H0) is determined using themorphologically type dependent Ks-band Tully-Fisher Relation (K-TFR).The slope and zero point are determined using 36 calibrator galaxieswith ScI morphology. Calibration distances are adopted from directCepheid distances, and group or companion distances derived with theSurface Brightness Fluctuation Method or Type Ia Supernova. Distancesare determined to 16 galaxy clusters and 218 ScI galaxies with minimumdistances of 40.0 Mpc. From the 16 galaxy clusters a weighted meanHubble Parameter of H0=84.2 +/-6 km s-1 Mpc-1 is found. From the 218 ScIgalaxies a Hubble Parameter of H0=83.4 +/-8 km s-1 Mpc-1 is found. Whenthe zero point of the K-TFR is corrected to account for recent resultsthat find a Large Magellanic Cloud distance modulus of 18.39 +/-0.05 aHubble Parameter of 88.0 +/-6 km s-1 Mpc-1 is found. A comparison withthe results of the Hubble Key Project (Freedman et al 2001) is made anddiscrepancies between the K-TFR distances and the HKP I-TFR distancesare discussed. Implications for Lamda-CDM cosmology are considered withH0=84 km s-1 Mpc-1. (Abridged)
| Radiatively Inefficient Accretion in Nearby Galaxies
We use new central stellar velocity dispersions and nuclear X-ray andHα luminosities for the Palomar survey of nearby galaxies toinvestigate the distribution of nuclear bolometric luminosities andEddington ratios for their central black holes (BHs). This informationhelps to constrain the nature of their accretion flows and the physicaldrivers that control the spectral diversity of nearby active galacticnuclei. The characteristic values of the bolometric luminosities andEddington ratios, which span over 7-8 orders of magnitude, from Lbol lsim 1037 to 3 × 1044 ergs–1 and L bol/L Edd≈10–9 to 10–1, vary systematicallywith nuclear spectral classification, increasing along the sequenceabsorption-line nuclei → transition objects → low-ionizationnuclear emission-line regions → Seyferts. The Eddington ratio alsoincreases from early-type to late-type galaxies. We show that the verymodest accretion rates inferred from the nuclear luminosities can bereadily supplied through local mass loss from evolved stars and Bondiaccretion of hot gas, without appealing to additional fueling mechanismssuch as angular momentum transport on larger scales. Indeed, we arguethat the fuel reservoir generated by local processes should produce farmore active nuclei than is actually observed. This genericluminosity-deficit problem suggests that the radiative efficiency inthese systems is much less than the canonical value of 0.1 fortraditional optically thick, geometrically thin accretion disks. Theobserved values of L bol/L Edd, all substantiallybelow unity, further support the hypothesis that massive BHs in mostnearby galaxies reside in a low or quiescent state, sustained byaccretion through a radiatively inefficient mode.
| A Census of X-ray Nuclear Activity in Nearby Galaxies
We have studied the X-ray nuclear activity of 187 nearby (distance lessthan 15 Mpc) galaxies observed with Chandra/ACIS. We found that 86of them have a pointlike X-ray core, consistent with an accreting blackhole (BH). We argue that the majority of them are nuclear BHs, ratherthan X-ray binaries. The fraction of galaxies with an X-ray-detectednuclear BH is higher (≈60%) for ellipticals and early type spirals (Eto Sb), and lower (≈30%) for late-type spirals (Sc to Sm). There isno preferential association of X-ray cores with the presence of alarge-scale bar; in fact, strongly barred galaxies appear to haveslightly lower detection fraction and luminosity for their nuclear X-raysources, compared with nonbarred or weakly barred galaxies of similarHubble types. The cumulative luminosity distribution of the nuclearsources in the 0.3-8 keV band is a power law with slope ≈ –0.5,from ≈2 × 1038 erg s–1 to≈1042 erg s–1. The Eddington ratio islower for ellipticals (L X/L Edd ~10–8) and higher for late-type spirals (up to LX/L Edd ~ 10–4), but in allcases, the accretion rate is low enough to be in the radiativelyinefficient regime. The intrinsic absorbing column density is generallylow, especially for the less luminous sources: there appear to be noType 2 nuclear BHs at luminosities lsim1039 ergs–1. The lack of a dusty torus or of other sources ofintrinsic absorption (e.g., an optically thick disk wind) may bedirectly related to the lack of a standard accretion disk around thosefaint nuclear BHs. The fraction of obscured sources increases with thenuclear BH luminosity: two-thirds of the sources with L X> 1040 erg s–1 have a fitted columndensity greater than 1022 cm–2. This isin contrast to the declining trend of the obscured fraction withincreasing luminosities, observed in more luminous active galacticnuclei and quasars.
| The Hα galaxy survey. VII. The spatial distribution of star formation within disks and bulges
Aims. We analyse the current build-up of stellar mass within the disksand bulges of nearby galaxies through a comparison of the spatialdistributions of forming and old stellar populations. Methods: Hαand R-band imaging are used to determine the distributions of young andold stellar populations in 313 S0a-Im field galaxies out to ~40 Mpc.Concentration indices and mean normalised light profiles are calculatedas a function of galaxy type and bar classification. Results: The meanprofiles and concentration indices show a strong and smooth dependenceon galaxy type. Apart from a central deficit due to bulge/bar light insome galaxy types, mean Hα and R-band profiles are very similar.Mean profiles within a given type are remarkably constant even givenwide ranges in galaxy luminosity and size. SBc, SBbc, and particularlySBb galaxies have profiles that are markedly different from those ofunbarred galaxies. The Hα emission from individual SBb galaxies isstudied in detail; virtually all show resolved central components andconcentrations of star formation at or just outside the bar-end radius. Conclusions: Galaxy type is an excellent predictor of R-band lightprofile. In field galaxies, star formation has the same radialdistribution as R-band light, i.e. stellar mass is building atapproximately constant morphology, with no strong evidence for outertruncation or inside-out disk formation. Bars have a strong impact onthe radial distribution of star formation, particularly in SBb galaxies.Based on observations made with the Jacobus Kapteyn Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias.
| The Imperial IRAS-FSC Redshift Catalogue
We present a new catalogue, the Imperial IRAS-FSC Redshift Catalogue(IIFSCz), of 60303 galaxies selected at 60?m from the IRAS FaintSource Catalogue (FSC). The IIFSCz consists of accurate position,optical, near-infrared and/or radio identifications, spectroscopicredshift (if available) or photometric redshift (if possible), predictedfar-infrared (FIR) and submillimetre (submm) fluxes ranging from 12 to1380?m based upon the best-fitting infrared template. About 55 percent of the galaxies in the IIFSCz have spectroscopic redshifts, and afurther 20 per cent have photometric redshifts obtained through eitherthe training set or the template-fitting method. For S(60) > 0.36 Jy,the 90 per cent completeness limit of the FSC, 90 per cent of thesources have either spectroscopic or photometric redshifts. Scientificapplications of the IIFSCz include validation of current and forthcominginfrared and submm/mm surveys such as AKARI, Planck and Herschel,follow-up studies of rare source populations, large-scale structure andgalaxy bias, local multiwavelength luminosity functions and sourcecounts. The catalogue is publicly available athttp://astro.imperial.ac.uk/~mrr/fss/.
| Pattern Corotation Radii from Potential-Density Phase-Shifts for 153 OSUBGS Sample Galaxies
The potential-density phase-shift method is an effective new tool forinvestigating the structure and evolution of galaxies. In this paper, weapply the method to 153 galaxies in the Ohio State University BrightGalaxy Survey (OSUBGS) to study the general relationship between patterncorotation radii and the morphology of spiral galaxies. The analysis isbased on near-infrared H-band images that have been deprojected anddecomposed assuming a spherical bulge. We find that multiple patternspeeds are common in disk galaxies. By selecting those corotation radiiclose to or slightly larger than the bar radius as being the barcorotation (CR) radius, we find that the average and standard deviationof the ratio \cal R = r(CR)/r(bar), is 1.20 ± 0.52 for 101galaxies having well-defined bars. There is an indication that thisratio depends weakly on galaxy type in the sense that the average rangesfrom 1.03 ± 0.37 for 65 galaxies of type Sbc and earlier, to 1.50± 0.63 for 36 galaxies of type Sc and later. Our bar corotationradii are on average smaller than those estimated fromsingle-pattern-speed numerical simulations, most likely because thesesimulations tend to find the pattern speed which generates a densityresponse in the gas that best matches the morphology of the outer spiralstructure. Although we find CR radii in most of the sample galaxies thatsatisfy conventional ideas about the extent of bars, we also considerthe alternative interpretation that in many cases the bar CR is actuallyinside the bar and that the bar ends close to its outer Lindbladresonance instead of its CR. These "superfast" bars are the mostcontroversial finding from our study. We see evidence in the phase-shiftdistributions for ongoing decoupling of patterns, which hints at theformation pathways of nested patterns, and which in turn further hintsat the longevity of the density wave patterns in galaxies. We alsoexamine how uncertainties in the orientation parameters of galaxies andin the shapes of bulges affect our results.
| The M-σ and M-L Relations in Galactic Bulges, and Determinations of Their Intrinsic Scatter
We derive improved versions of the relations between supermassive blackhole mass (M BH) and host-galaxy bulge velocity dispersion(σ) and luminosity (L; the M-σ and M-L relations), based on49 M BH measurements and 19 upper limits. Particularattention is paid to recovery of the intrinsic scatter(epsilon0) in both relations. We find log(M BH/Msun) = α + βlog(σ/200 kms–1) with (α, β, epsilon0) =(8.12 ± 0.08, 4.24 ± 0.41, 0.44 ± 0.06) for allgalaxies and (α, β, epsilon0) = (8.23 ±0.08, 3.96 ± 0.42, 0.31 ± 0.06) for ellipticals. Theresults for ellipticals are consistent with previous studies, but theintrinsic scatter recovered for spirals is significantly larger. Thescatter inferred reinforces the need for its consideration whencalculating local black hole mass function based on the M-σrelation, and further implies that there may be substantial selectionbias in studies of the evolution of the M-σ relation. We estimatethe M-L relationship as log(M BH/M sun) = α+ βlog(LV /1011 L sun,V ) of(α, β, epsilon0) = (8.95 ± 0.11, 1.11± 0.18, 0.38 ± 0.09); using only early-type galaxies.These results appear to be insensitive to a wide range of assumptionsabout the measurement errors and the distribution of intrinsic scatter.We show that culling the sample according to the resolution of the blackhole's sphere of influence biases the relations to larger mean masses,larger slopes, and incorrect intrinsic residuals.
| Molecular Hydrogen Deficiency in H I-poor Galaxies and its Implications for Star Formation
We use a sample of 47 homogeneous and high-sensitivity CO images takenfrom the Nobeyama and BIMA surveys to demonstrate that, contrary tocommon belief, a significant number (~40%) of H I-deficient nearbyspiral galaxies are also depleted in molecular hydrogen. While H Ideficiency by itself is not a sufficient condition for molecular gasdepletion, we find that H2 reduction is associated with theremoval of H I inside the galaxy optical disk. Those H I-deficientgalaxies with normal H2 content have lost H I mainly fromoutside their optical disks, where the H2 content is low inall galaxies. This finding is consistent with theoretical models inwhich the molecular fraction in a galaxy is determined primarily by itsgas column density. Our result is supported by indirect evidence thatmolecular deficient galaxies form stars at a lower rate or have dimmerfar infrared fluxes than gas rich galaxies, as expected if the starformation rate is determined by the molecular hydrogen content. Ourresult is consistent with a scenario in which, when the atomic gascolumn density is lowered inside the optical disk below the criticalvalue required to form molecular hydrogen and stars, spirals becomequiescent and passive evolving systems. We speculate that this processwould act on the timescale set by the gas depletion rate and might be afirst step for the transition between the blue and red sequence observedin the color-magnitude diagram.
| Galaxies Correlating with Ultra-High Energy Cosmic Rays
The Pierre Auger Observatory reported that 20 of the 27 highest energycosmic rays have arrival directions within 3fdg2 of a nearby galaxy inthe Veron-Cetty and Veron (VCV) Catalog of Quasars and Active GalacticNuclei (12th ed.), with a 1% probability that this would be due tochance if the cosmic ray directions were isotropic. In this paper, weexamine the correlated galaxies to gain insight into the possibleultra-high energy cosmic ray (UHECR) sources. We find that 14 of the 21correlated VCV galaxies are active galactic nuclei (AGNs) and wedetermine their bolometric luminosities. The remaining seven areprimarily star-forming galaxies. The bolometric luminosities of thecorrelated AGNs are all greater than 5 × 1042 ergs-1. This may explain the absence of UHECRs from the Virgoregion in spite of the large number of VCV galaxies in Virgo, since mostof the VCV galaxies in the Virgo region are low-luminosity AGNs.Interestingly, the bolometric luminosities of most of the AGNs aresignificantly lower than that required to satisfy the minimum conditionfor UHECR acceleration in a continuous jet. If a UHECR-AGN correlationis substantiated with further statistics, our results lend support tothe recently proposed "giant AGN flare" mechanism for UHECRacceleration.
| Bulge n and B/T in High-Mass Galaxies: Constraints on the Origin of Bulges in Hierarchical Models
We use the bulge Sérsic index n and bulge-to-total mass ratio(B/T) to explore the fundamental question of how bulges form. We performtwo-dimensional bulge-disk-bar decomposition on H-band images of 143bright, high-mass (M sstarf >= 1.0 × 1010M sun) low-to-moderately inclined (i < 70°) spirals.Our results are as follows. (1) Our H-band bar fraction (~58%) isconsistent with that from ellipse fits. (2) 70% of the stellar mass isin disks, 10% in bars, and 20% in bulges. (3) A large fraction (~69%) ofbright spirals have B/T<= 0.2, and ~76% have low n <= 2 bulges.These bulges exist in barred and unbarred galaxies across a wide rangeof Hubble types. (4) About 65% (68%) of bright spirals with n <= 2(B/T <= 0.2) bulges host bars, suggesting a possible link betweenbars and bulges. (5) We compare the results with predictions from a setof ΛCDM models. In the models, a high-mass spiral can have abulge with a present-day low B/T<= 0.2 only if it did not undergo amajor merger since z <= 2. The predicted fraction (~ 1.6%) ofhigh-mass spirals, which have undergone a major merger since z <= 4and host a bulge with a present-day low B/T <= 0.2, is a factor ofover 30 smaller than the observed fraction (~66%) of high-mass spiralswith B/T <= 0.2. Thus, contrary to common perception, bulges builtvia major mergers since z <= 4 seriously fail to account for thebulges present in ~66% of high mass spirals. Most of these present-daylow B/T <= 0.2 bulges are likely to have been built by a combinationof minor mergers and/or secular processes since z <= 4.
| Understanding the H2/HI ratio in galaxies
We revisit the mass ratio ηgalaxy between molecularhydrogen (H2) and atomic hydrogen (HI) in different galaxiesfrom a phenomenological and theoretical viewpoint. First, the localH2 mass function (MF) is estimated from the local COluminosity function (LF) of the FCRAO Extragalactic CO Survey, adoptinga variable CO-to-H2 conversion fitted to nearby observations.This implies an average H2 density and in the local Universe.Secondly, we investigate the correlations between ηgalaxyand global galaxy properties in a sample of 245 local galaxies. Based onthese correlations we introduce four phenomenological models forηgalaxy, which we apply to estimate H2 massesfor each HI galaxy in the HIPASS catalogue. The resulting H2MFs (one for each model for ηgalaxy) are compared to thereference H2 MF derived from the CO LF, thus allowing us todetermine the Bayesian evidence of each model and to identify a clearbest model, in which, for spiral galaxies, ηgalaxynegatively correlates with both galaxy Hubble type and total gas mass.Thirdly, we derive a theoretical model for ηgalaxy forregular galaxies based on an expression for their axially symmetricpressure profile dictating the degree of molecularization. This model isquantitatively similar to the best phenomenological one at redshift z =0, and hence represents a consistent generalization while providing aphysical explanation for the dependence of ηgalaxy onglobal galaxy properties. Applying the best phenomenological model forηgalaxy to the HIPASS sample, we derive the firstintegral cold gas MF (HI + H2 + helium) of the localuniverse.
| Upper Limits on the Masses of 105 Supermassive Black Holes from Hubble Space Telescope/Space Telescope Imaging Spectrograph Archival Data
Based on the modeling of the central emission-line width measured oversubarcsecond apertures with the Hubble Space Telescope, we presentstringent upper bounds on the mass of the central supermassive blackhole, M •, for a sample of 105 nearby galaxies (D <100 Mpc) spanning a wide range of Hubble types (E Sc) and values of thecentral stellar velocity dispersion, σc (58-419 kms 1). For the vast majority of the objects, the derived M• upper limits run parallel and above the well-known M•-σc relation independently of thegalaxy distance, suggesting that our nebular line-width measurementstrace rather well the nuclear gravitational potential. For values ofσc between 90 and 220 km s 1, 68% of ourupper limits falls immediately above the M•-σc relation without exceeding theexpected M • values by more than a factor 4.1. Nosystematic trends or offsets are observed in this σcrange as a function of the galaxy Hubble type or with respect to thepresence of a bar. For 6 of our 12 M • upper limits withσc <90 km s 1, our line-widthmeasurements are more sensitive to the stellar contribution to thegravitational potential, either due to the presence of a nuclear stellarcluster or because of a greater distance compared to the other galaxiesat the low-σc end of the M•-σc relation. Conversely, our M• upper bounds appear to lie closer to the expected M• in the most massive elliptical galaxies with values ofσc above 220 km s 1. Such a flattening ofthe M •-σc relation at itshigh-σc end would appear consistent with a coevolutionof supermassive black holes and galaxies driven by dry mergers, althoughbetter and more consistent measurements for σc andK-band luminosity are needed for these kinds of objects beforesystematic effects can be ruled out.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat STScI, which is operated by the Association of Universities forResearch in Astronomy, Incorporated, under NASA contract NAS5-26555.
| A Spitzer Spectroscopic Survey of Low-Ionization Nuclear Emission-Line Regions: Characterization of the Central Source
We have conducted a comprehensive mid-infrared (IR) spectroscopicinvestigation of 67 low-ionization nuclear emission line regions(LINERs) using archival observations from the high-resolution modules ofthe Infrared Spectrograph on board the Spitzer Space Telescope. Usingthe [Ne V] 14 and 24 μm lines as active galactic nuclei (AGNs)diagnostics, we detect active black holes in 39% of the galaxies in oursample, many of which show no signs of activity in either the optical orX-ray bands. In particular, a detailed comparison of multiwavelengthdiagnostics shows that optical studies fail to detect AGNs in galaxieswith large far-IR luminosities. These observations emphasize that thenuclear power source in a large percentage of LINERs is obscured in theoptical. Indeed, the majority of LINERs show mid-IR [Ne V] 14/[NeV] 24 μm flux ratios well below the theoretical low-densitylimit, suggesting that there is substantial extinction toward even the[Ne V]-emitting region. Combining optical, X-ray, and mid-IRdiagnostics, we find an AGN detection rate in LINERs of 74%, higher thanpreviously reported statistics of the fraction of LINERs hosting AGNs.The [Ne V] 24 μm/[O IV] 26 μm mid-IR line fluxratio in AGN-LINERs is similar to that of standard AGNs, suggesting thatthe spectral energy distribution of the intrinsic optical/UV continuumis similar in the two. This result is in contrast to previoussuggestions of a UV deficit in the intrinsic broadband continuumemission in AGN-LINERs. Consistent with our finding of extinction to the[Ne V]-emitting region, we propose that extinction may also beresponsible for the observed optical/UV deficit seen in at least someAGN-LINERs.
| The relationship between gas content and star formation rate in spiral galaxies. Comparing the local field with the Virgo cluster
Context: Despite many studies of star formation in spiral galaxies, acomplete and coherent understanding of the physical processes thatregulate the birth of stars has not yet been achieved, nor has unanymousconsensus been reached, despite the many attempts, on the effects of theenvironment on the star formation in galaxy members of rich clusters. Aims: We focus on the local and global Schmidt law and we investigatehow cluster galaxies have their star formation activity perturbed. Methods: We collect multifrequency imaging for a sample of spiralgalaxies, members of the Virgo cluster and of the local field; wecompute the surface density profiles for the young and for the bulk ofthe stellar components, for the molecular and for the atomic gas. Results: Our analysis shows that the bulk of the star formationcorrelates with the molecular gas, but the atomic gas is important oreven crucial in supporting the star formation activity in the outer partof the disks. Moreover, we show that cluster members that suffer from amoderate HI removal have their molecular component and their SFRquenched, while highly perturbed galaxies show an additional truncationin their star forming disks. Conclusions: Our results are consistentwith a model in which the atomic hydrogen is the fundamental fuel forthe star formation, either directly or indirectly through the molecularphase; therefore galaxies whose HI reservoirs have been depleted sufferfrom starvation or even from truncation of their star formationactivity.
| The morphology of galactic rings exterior to evolving bars: test-particle simulations
The morphology of the outer rings of early-type spiral galaxies iscompared to integrations of massless collisionless particles initiallyin nearly circular orbits. Particles are perturbed by a quadrupolargravitational potential corresponding to a growing and secularlyevolving bar. We find that outer rings with R1R2 morphology andpseudo-rings are exhibited by the simulations even though they lackgaseous dissipation. Simulations with stronger bars form pseudo-ringsearlier and more quickly than those with weaker bars. We find that theR1 ring, perpendicular to the bar, is fragile and dissolves after a fewbar rotation periods if the bar pattern speed increases by more than ~8per cent, bar strength increases (by >~140 per cent) after bar growthor the bar is too strong (QT > 0.3). If the bar slows downafter formation, pseudo-ring morphology persists and the R2 ringperpendicular to the bar is populated due to resonance capture. The R2ring remains misaligned with the bar and increases in ellipticity as thebar slows down. The R2 ring becomes scalloped and does not resemble anyringed galaxies if the bar slows down more than 3.5 per cent suggestingthat bars decrease in strength before they slow down this much. Wecompare the morphology of our simulations to B-band images of nineringed galaxies from the Ohio State University Bright Spiral GalaxySurvey, and we find a reasonable match in morphologies to R1R2'pseudo-rings seen within a few bar rotation periods of bar formation.Some of the features previously interpreted in terms of dissipativemodels may be due to transient structure associated with recent bargrowth and evolution.
| Molecular Gas Distribution in Barred and Nonbarred Galaxies along the Hubble Sequence
We present results from a survey of 12 CO (J=1-0) spectraobtained for the central regions of 68 nearby galaxies at an angularresolution of 16" using the Nobeyama Radio Observatory 45 m telescope,aimed at characterizing the properties of star-forming molecular gas.Combined with observations of similar resolution in the literature, thecompiled sample set of 166 galaxies span a wide range of galacticproperties. NGC 4380, which was previously undetected in CO, wasdetected. This initial paper of a series will focus on the data and thegaseous properties of the samples, and particularly on the degree ofcentral concentration of molecular gas in a range of morphologicaltypes, from early (S0/Sa) to late (Sd/Sm) galaxies with and withoutbars. The degree of molecular central concentration in the centralkiloparsec, compared with the central several kiloparsecs of galaxies,is found to vary smoothly with Hubble type, so that early-type galaxiesshow larger central concentration. The comparison of barred andnonbarred galaxies within early- and late-type galaxies suggest thatdifference in Hubble type, representing the effect of bulges, is themore important factor in concentrating gas into the central regions thanbars.
| The Fractal Distribution of H II Regions in Disk Galaxies
It is known that interstellar gas has a fractal structure in a widerange of spatial scales with a fractal dimension that seems to be aconstant around Df~=2.7. It is expected that stars formingfrom this fractal medium exhibit similar fractal patterns. Here weaddress this issue by quantifying the degree to which star-formingevents are clumped. We develop, test, and apply a precise and accuratetechnique to calculate the correlation dimension Dc of thedistribution of H II regions in a sample of disk galaxies. We find thatthe determination of Dc is limited by the number of H IIregions, since if there are <~100 regions available, then a biastending to underestimate the dimension is produced. The reliable resultsare distributed in the range 1.5<~Dc<~2.0 with anaverage value =1.81. This corresponds to athree-dimensional dimension of =2.73, very similarto the value measured in the interstellar clouds. However, we getsignificant variations in the fractal dimension among galaxies, contraryto the universal picture sometimes claimed in literature. The fractaldimension exhibits a weak but significant correlation with the absolutemagnitude and, to a lesser extent, with the galactic radius. Thefaintest galaxies tend to distribute their H II regions in moreclustered (less uniform) patterns. The fractal dimension for thebrightest H II regions within the same galaxy seems to be smaller thanfor the faintest ones, suggesting some kind of evolutionary effect, butthe obtained correlation remains unchanged if only the brightest regionsare taken into account.
| Model-based pattern speed estimates for 38 barred galaxies
We have modelled 38 barred galaxies by using near-infrared and opticaldata from the Ohio State University Bright Spiral Galaxy Survey. Weconstructed the gravitational potentials of the galaxies from H-bandphotometry, assuming a constant mass-to-light ratio. The halo componentwe choose corresponds to the so-called universal rotation curve. In eachcase, we used the response of gaseous and stellar particle disc torigidly rotating potential to determine the pattern speed.We find that the pattern speed of the bar depends roughly on themorphological type. The average value of corotation resonance radius tobar radius, , increases from 1.15 +/- 0.25 in types SB0/a-SBab to 1.44+/- 0.29 in SBb and 1.82 +/- 0.63 in SBbc-SBc. Within the errorestimates for the pattern speed and bar radius, all galaxies of typeSBab or earlier have a fast bar , whereas the bars in later typegalaxies include both fast and slow rotators. Of 16 later type galaxieswith a nominal value of , there are five cases, where the fast-rotatingbar is ruled out by the adopted error estimates.We also study the correlation between the parameter and other galacticproperties. The clearest correlation is with the bar size: the slowestbars are also the shortest bars when compared to the galaxy size. Aweaker correlation is seen with bar strength in a sense that slow barstend to be weaker. These correlations leave room for a possibility thatthe determined pattern speed in many galaxies corresponds to actuallythat of the spiral, which rotates more slowly than the bar. No clearcorrelation is seen with either the galaxy luminosity or the colour.
| Ram-pressure stripping of disc galaxies orbiting in clusters - II. Galactic wakes
We present 3D hydrodynamical simulations of ram-pressure stripping of adisc galaxy orbiting in a galaxy cluster. In this paper, we focus on theproperties of the galaxies' tails of stripped gas. The galactic wakesshow a flaring width, where the flaring angle depends on the gas disc'scross-section with respect to the galaxy's direction of motion. Thevelocity in the wakes shows a significant turbulent component of a few .The stripped gas is deposited in the cluster rather locally, i.e. withinfrom where it was stripped. We demonstrate that the most importantquantity governing the tail density, length and gas mass distributionalong the orbit is the galaxy's mass-loss per orbital length. This inturn depends on the ram pressure as well as the galaxy's orbitalvelocity.For a sensitivity limit of in projected gas density, we find typicaltail lengths of . Such long tails are seen even at large distances (0.5to ) from the cluster centre. At this sensitivity limit, the tails showlittle flaring, but a width similar to the gas disc's size.Morphologically, we find good agreement with the HI tails observed inthe Virgo cluster by Chung et al. 2007. However, the observed tails showa much smaller velocity width than predicted from the simulation. Thefew known X-ray and Hα tails are generally much narrower and muchstraighter than the tails in our simulations. Thus, additional physicslike a viscous intracluster medium (ICM), the influence of cooling andtidal effects may be needed to explain the details of the observations.We discuss the hydrodynamical drag as a heat source for the ICM butconclude that it is not likely to play an important role, especially notin stopping cooling flows.
| Colors and Mass-to-Light Ratios of Bulges and Disks of Nearby Spiral Galaxies
We investigate colors and mass-to-light ratios ($M/L$s) of the bulgesand disks for 28 nearby spiral galaxies with various morphological typesof Sab to Scd, using images in optical and near-infrared ($V$, $I$, and$J$) bands and published rotation curves. It is shown that the observedcolors and $M/L$s generally agree with the galaxy formation model withan exponentially declining star formation rate and shallow slope (ex.Scalo) initial mass function (IMF) for both the bulges and the disks. Wefind that the bulge $M/L$ is generally higher than the disk $M/L$ andthat the galaxies with larger bulge-to-total luminosity ratio tend tohave a smaller bulge $M/L$. The fact indicates that theluminosity-weighted average age of bulges for early-type spirals isyounger than that of later-type spirals. These results support aformation scenario that produces young stars for the bulges ofmiddle-type and early-type spirals.
| Pre-peak ram pressure stripping in the Virgo cluster spiral galaxy NGC 4501
VIVA Hi observations of the Virgo spiral galaxy NGC 4501 are presented.The Hi disk is sharply truncated to the southwest, well within thestellar disk. A region of low surface-density gas, which is moreextended than the main Hi disk, is discovered northeast of the galaxycenter. These data are compared to existing 6 cm polarized radiocontinuum emission, Hα, and optical broad band images. We observea coincidence between the western Hi and polarized emission edges, onthe one hand, and a faint Hα emission ridge, on the other. Thepolarized emission maxima are located within the gaps between the spiralarms and the faint Hα ridge. Based on the comparison of theseobservations with a sample of dynamical simulations with differentvalues for maximum ram pressure and different inclination angles betweenthe disk and the orbital plane, we conclude that ram pressure strippingcan account for the main observed characteristics. NGC 4501 is strippednearly edge-on, is heading southwest, and is 200{-}300 Myr before peakram pressure, i.e. its closest approach to M 87. The southwestern ridgeof enhanced gas surface density and enhanced polarized radio-continuumemission is due to ram pressure compression. It is argued that the faintwestern Hα emission ridge is induced by nearly edge-on rampressure stripping. NGC 4501 represents an especially clear example ofearly stage ram pressure stripping of a large cluster-spiral galaxy.
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