Contents
Images
Upload your image
DSS Images Other Images
Related articles
RR Lyrae-based calibration of the Globular Cluster Luminosity Function
We test whether the peak absolute magnitude MV(TO) of theGlobular Cluster Luminosity Function (GCLF) can be used for reliableextragalactic distance determination. Starting with the luminosityfunction of the Galactic Globular Clusters listed in Harris catalogue,we determine MV(TO) either using current calibrations of theabsolute magnitude MV(RR) of RR Lyrae stars as a function ofthe cluster metal content [Fe/H] and adopting selected cluster samples.We show that the peak magnitude is slightly affected by the adoptedMV(RR)-[Fe/H] relation, with the exception of that based onthe revised Baade-Wesselink method, while it depends on the criteria toselect the cluster sample. Moreover, grouping the Galactic GlobularClusters by metallicity, we find that the metal-poor (MP) ([Fe/H]<-1.0, <[Fe/H]>~-1.6) sample shows peak magnitudes systematicallybrighter by about 0.36mag than those of the metal-rich (MR) ([Fe/H]>-1.0, (<[Fe/H]>~-0.6) one, in substantial agreement with thetheoretical metallicity effect suggested by synthetic Globular Clusterpopulations with constant age and mass function. Moving outside theMilky Way, we show that the peak magnitude of the MP clusters in M31appears to be consistent with that of Galactic clusters with similarmetallicity, once the same MV(RR)-[Fe/H] relation is used fordistance determination. As for the GCLFs in other external galaxies,using Surface Brightness Fluctuations (SBF) measurements we giveevidence that the luminosity functions of the blue (MP) GlobularClusters peak at the same luminosity within ~0.2mag, whereas for the red(MR) samples the agreement is within ~0.5mag even accounting for thetheoretical metallicity correction expected for clusters with similarages and mass distributions. Then, using the SBF absolute magnitudesprovided by a Cepheid distance scale calibrated on a fiducial distanceto Large Magellanic Cloud (LMC), we show that the MV(TO)value of the MP clusters in external galaxies is in excellent agreementwith the value of both Galactic and M31 ones, as inferred by an RR Lyraedistance scale referenced to the same LMC fiducial distance. Eventually,adopting μ0(LMC) = 18.50mag, we derive that the luminosityfunction of MP clusters in the Milky Way, M31, and external galaxiespeak at MV(TO) =-7.66 +/- 0.11, - 7.65 +/- 0.19 and -7.67 +/-0.23mag, respectively. This would suggest a value of -7.66 +/- 0.09mag(weighted mean), with any modification of the LMC distance modulusproducing a similar variation of the GCLF peak luminosity.
| Environmental Effects on Late-Type Galaxies in Nearby Clusters
The transformations that take place in late-type galaxies in theenvironment of rich clusters of galaxies at z=0 are reviewed. From thehandful of late-type galaxies that inhabit local clusters, whether theywere formed in situ and survived as such, avoiding transformation oreven destruction, or if they are newcomers that have recently fallen infrom outside, we can learn an important lesson on the latest stages ofgalaxy evolution. We start by reviewing the observational scenario,covering the broadest possible stretch of the electromagnetic spectrum,from the gas tracers (radio and optical) to the star formation tracers(UV and optical), the old star tracers (near-IR), and the dust (far-IR).Strong emphasis is given to the three nearby, well-studied clustersVirgo, A1367, and Coma, which are representative of differentevolutionary stages, from unrelaxed and spiral-rich (Virgo) to relaxedand spiral-poor (Coma). We continue by providing a review of models ofgalaxy interactions that are relevant to clusters of galaxies.Prototypes of various mechanisms and processes are discussed, and theirtypical timescales are given in an appendix. Observations indicate thepresence of healthy late-type galaxies falling into nearby clustersindividually or as part of massive groups. More rare are infallinggalaxies belonging to compact groups, where significant preprocessingmight take place. Once they have entered the cluster, they lose theirgas and quench their star formation activity, becoming anemic.Observations and theory agree in indicating that the interaction withthe intergalactic medium is responsible for the gas depletion. However,this process cannot be the origin of the cluster lenticular galaxypopulation. Physical and statistical properties of S0 galaxies in nearbyclusters and at higher redshift indicate that they originate from spiralgalaxies that have been transformed by gravitational interactions.
| Scalar potential model of redshift and discrete redshift
On the galactic scale the universe is inhomogeneous and redshift z isoccasionally less than zero. A scalar potential model (SPM) that linksthe galaxy scale z to the cosmological scale z of the Hubble Law ispostulated. Several differences among galaxy types suggest that spiralgalaxies are Sources and that early type, lenticular, and irregulargalaxies are Sinks of a scalar potential field. The morphology-radiusand the intragalactic medium cluster observations support the movementof matter from Source galaxies to Sink galaxies. A cell structure ofgalaxy groups and clusters is proposed to resolve a paradox concerningthe scalar potential like the Olber’s paradox concerning light.For the sample galaxies, the ratio of the luminosity of Source galaxiesto the luminosity of Sink galaxies approaches 2.7 ± 0.1. Anequation is derived from sample data, which is anisotropic andinhomogeneous, relating z of and the distance D to galaxies. Thecalculated z has a correlation coefficient of 0.88 with the measured zfor a sample of 32 spiral galaxies with D calculated using Cepheidvariable stars. The equation is consistent with z < 0 observations ofclose galaxies. At low cosmological distances, the equation reduces to z≈ exp(KD)‑1 ≈ KD, where K is a constant, positive value. Theequation predicts z from galaxies over 18 Gpc distant approaches aconstant value on the order of 500. The SPM of z provides a physicalbasis for the z of particle photons. Further, the SPM qualitativelysuggests the discrete variations in z, which was reported by Tifft[Tifft, W.G., 1997. Astrophy. J. 485, 465] and confirmed by others, areconsistent with the SPM.
| Ram pressure stripping of disc galaxies: the role of the inclination angle
We present three-dimensional (3D) hydrodynamical simulations of rampressure stripping of massive disc galaxies in clusters. Studies ofgalaxies that move face-on have predicted that in such a geometry thegalaxy can lose a substantial amount of its interstellar medium. Butonly a small fraction of galaxies is moving face-on. In this work wefocus on a systematic study of the effect of the inclination anglebetween the direction of motion and the galaxy's rotation axis.In agreement with some previous works, we find that the inclinationangle does not play a major role for the mass loss as long as the galaxyis not moving close to edge-on (inclination angle <~60°). Weexplain this behaviour by extending Gunn & Gott's estimate of thestripping radius, which is valid for face-on geometries, to moderateinclinations.The inclination plays a role as long as the ram pressure is comparableto pressures in the galactic plane, which can span two orders ofmagnitude. For very strong ram pressures, the disc will be strippedcompletely, and for very weak ram pressures, mass loss is negligibleindependent of inclination. We show that in non-edge-on geometries thestripping proceeds remarkably similar. A major difference betweendifferent inclinations is the degree of asymmetry introduced in theremaining gas disc.We demonstrate that the tail of gas stripped from the galaxy does notnecessarily point in a direction opposite to the galaxy's direction ofmotion. Therefore, the observation of a galaxy's gas tail may bemisleading about the galaxy's direction of motion.
| A Virgo high-resolution Hα kinematical survey - II. The Atlas
A catalogue of ionized gas velocity fields for a sample of 30 spiral andirregular galaxies of the Virgo cluster has been obtained by using 3Doptical data. The aim of this survey is to study the influence ofhigh-density environments on the gaseous kinematics of local clustergalaxies. Observations of the Hα line by means of Fabry-Perotinterferometry have been performed at the Canada-France-HawaiiTelescope, European Southern Observatory 3.6-m telescope, Observatoirede Haute-Provence 1.93-m telescope and Observatoire du montMégantic telescope at angular and spectral samplings from 0.4 to1.6arcsec and 7 to 16kms-1. A recently developed, automaticand adaptive spatial binning technique is used to reach a nearlyconstant signal-to-noise ratio (S/N) over the whole field of view,allowing us to keep a high spatial resolution in high-S/N regions andextend the detection of signal in low-S/N regions. This paper is part ofa series and presents the integrated emission-line and velocity maps ofthe galaxies. Both Hα morphologies and kinematics exhibit signs ofperturbations in the form of, for example, external filaments, inner andnuclear spiral- and ring-like structures, inner kinematical twists,kinematical decoupling of a nuclear spiral, streaming motions alongspiral arms and misalignment between kinematical and photometricorientation axes.
| On the origin of warps and the role of the intergalactic medium
There is still no consensus as to what causes galactic discs to becomewarped. Successful models should account for the frequent occurrence ofwarps in quite isolated galaxies, their amplitude as well as theobserved azimuthal and vertical distributions of the HI layer.Intergalactic accretion flows and intergalactic magnetic fields may bendthe outer parts of spiral galaxies. In this paper we consider theviability of these non-gravitational torques to take the gas off theplane. We show that magnetically generated warps are clearly flawedbecause they would wrap up into a spiral in less than two or threegalactic rotations. The inclusion of any magnetic diffusivity to dilutethe wrapping effect causes the amplitude of the warp to damp. We alsoconsider the observational consequences of the accretion of anintergalactic plane-parallel flow at infinity. We have computed theamplitude and warp asymmetry in the accretion model, for a disc embeddedin a flattened dark matter halo, including self-consistently thecontribution of the modes with azimuthal wavenumbers m= 0 and m= 1.Since the m= 0 component, giving a U-shaped profile, is not negligiblecompared to the m= 1 component, this model predicts quite asymmetricwarps, maximum gas displacements on the two sides in the ratio 3 : 2 forthe preferred Galactic parameters, and the presence of a fraction ~3.5per cent of U-shaped warps, at least. The azimuthal dependence of themoment transfer by the ram pressure would produce a strong asymmetry inthe thickness of the HI layer and asymmetric density distributions in z,in conflict with observational data for the warp in our Galaxy and inexternal galaxies. The amount of accretion that is required to explainthe Galactic warp would give gas scaleheights in the far outer disc thatare too small. We conclude that accretion of a flow with no net angularmomentum cannot be the main and only cause of warps.
| Cepheid Distances to SNe Ia Host Galaxies Based on a Revised Photometric Zero Point of the HST WFPC2 and New PL Relations and Metallicity Corrections
With this paper we continue the preparation for a forthcoming summaryreport of our experiment with the HST to determine the Hubble constantusing Type Ia supernovae as standard candles. Two problems areaddressed. (1) We examine the need for, and determine the value of, thecorrections to the apparent magnitudes of our program Cepheids in the 11previous calibration papers due to sensitivity drifts and chargetransfer effects of the HST WFPC2 camera over the life time of theexperiment from 1992 to 2001. (2) The corrected apparent magnitudes areapplied to all our previous photometric data from which revised distancemoduli are calculated for the eight program galaxies that are parents tothe calibrator Ia supernovae. Two different Cepheid P-L relations areused; one for the Galaxy and one for the LMC. These differ both in slopeand zero point at a fixed period. The procedures for determining theabsorption and reddening corrections for each Cepheid are discussed.Corrections for the effects of metallicity differences between theprogram galaxies and the two adopted P-L relations are derived andapplied. The distance moduli derived here for the eight supernovaeprogram galaxies, and for 29 others, average 0.20 mag fainter (moredistant) than those derived by Gibson et al. and Freedman et al. intheir 2000 and 2001 summary papers for reasons discussed in this paper.The effect on the Hubble constant is the subject of our forthcomingsummary paper.
| Constraining Dark Matter Halo Profiles and Galaxy Formation Models Using Spiral Arm Morphology. I. Method Outline
We investigate the use of spiral arm pitch angles as a probe of diskgalaxy mass profiles. We confirm our previous result that spiral armpitch angles (P) are well correlated with the rate of shear (S) in diskgalaxy rotation curves by using a much larger sample (51 galaxies) thanused previously (17 galaxies). We use this correlation to argue thatimaging data alone can provide a powerful probe of galactic massdistributions out to large look-back times. In contrast to previouswork, we show that observed spiral arm pitch angles are similar whenmeasured in the optical (at 0.4 μm) and the near-infrared (at 2.1μm) with a mean difference of 2.3d+/-2.7d. This is then used tostrengthen the known correlation between P and S using B-band images. Wethen use two example galaxies to demonstrate how an inferred shear ratecoupled with a bulge-disk decomposition model and a Tully-Fisher-derivedvelocity normalization can be used to place constraints on a galaxy'sbaryon fraction and dark matter halo profile. We show that ESO 582-G12,a galaxy with a high shear rate (slightly declining rotation curve) at~10 kpc, favors an adiabatically contracted halo, with high initial NFWconcentration (cvir>16) and a high fraction of halobaryons in the form of stars (~15%-40%). In contrast, IC 2522 has a lowshear rate (rising rotation curve) at ~10 kpc and favorsnonadiabatically contracted models with low NFW concentrations(cvir~=2-8) and a low stellar baryon fraction <10%.
| 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.
| A Comparison of Hα and Stellar Scale Lengths in Virgo and Field Spirals
The scale lengths of the old stars and ionized gas distributions arecompared for similar samples of Virgo Cluster members and field spiralgalaxies via Hα and broad R-band surface photometry. While theR-band and Hα scale lengths are, on average, comparable for thecombined sample, we find significant differences between the field andcluster samples. While the Hα scale lengths of the field galaxiesare a factor of 1.14+/-0.07 longer, on average, than their R-band scalelengths, the Hα scale lengths of Virgo Cluster members are, onaverage, 20% smaller than their R-band scale lengths. Furthermore, inVirgo, the scale length ratios are correlated with the size of thestar-forming disk: galaxies with smaller overall Hα extents alsoshow steeper radial falloff of star formation activity. At the sametime, we find no strong trends in scale length ratio as a function ofother galaxy properties, including galaxy luminosity, inclination,morphological type, central R-band light concentration, or bar type. Ourresults for Hα emission are similar to other results for dustemission, suggesting that Hα and dust have similar distributions.The environmental dependence of the Hα scale length placesadditional constraints on the evolutionary process(es) that cause gasdepletion and a suppression of the star formation rate in clusters ofgalaxies.
| The extragalactic Cepheid bias: a new test using the period-luminosity-color relation
We use the Period-Luminosity-Color relation (PLC) for Cepheids to testfor the existence of a bias in extragalactic distances derived from theclassical Period-Luminosity (PL) relation. We calculate the parametersof the PLC using several galaxies observed with the Hubble SpaceTelescope and show that this calculation must be conducted with a PLCwritten in a form where the parameters are independent. The coefficientsthus obtained are similar to those derived from theoretical models.Calibrating with a few unbiased galaxies, we apply this PLC to allgalaxies of the Hubble Space Telescope Key Program (HSTKP) and comparethe distance moduli with those published by the HSTKP team. The newdistance moduli are larger (more exactly, the larger the distance thelarger the difference), consistent with a bias. Further, the bias trendthat is observed is the same previously obtained from two independentmethods based either on the local Hubble law or on a theoretical modelof the bias. The results are quite stable but when we force the PLCrelation closer to the classical PL relation by using unrealisticparameters, the agreement with HSTKP distance moduli is retrieved. Thisalso suggests that the PL relation leads to biased distance moduli. Thenew distance moduli reduce the scatter in the calibration of theabsolute magnitude of supernovae SNIa at their maximum. This may alsosuggest that the relation between the amplitude at maximum and the decayof the light curve Δ m15 may not be as strong asbelieved.
| 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 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.
| Chemically Decoupled Nuclei in Five Lenticular Galaxies from SAURON Data
We analyze data from the SAURON integral-field spectrograph of theWilliam Herschel 4-m telescope for five lenticular galaxies in which wepreviously found chemically decoupled nuclei from observations with theMultipupil Fiber Spectrograph of the 6-m Special AstrophysicalObservatory telescope. In a larger field of view, we confirmed thepresence of peaks of the equivalent width of the Mg Ib λ5175absorption line in the nuclei of all five galaxies. However, thestructure of the chemically decoupled regions turned out to be highlyvaried even in such a small sample: from compact unresolved knots todisks with an extent of several hundred parsecs and, in one case, atriaxial compact minibar-type structure. We confirmed the presence of aninner gaseous polar ring in NGC 7280 and found it in NGC 7332. In theirouter parts, the planes of these polar rings are warped toward the planeof stellar rotation in such a way that the gas counterrotates withrespect to the stars. This behavior of the gas in a triaxial potentialwas predicted by several theoretical models.
| Supermassive Black Holes: Relation to Dark Halos
Estimates of the masses of supermassive black holes (M bh ) in thenuclei of disk galaxies with known rotation curves are compared withestimates of the rotational velocities V m and the“indicative” masses of the galaxies M i . Although there isa correlation between M bh and V m or M i , it is appreciably weakerthan the correlation with the central velocity dispersion. The values ofM bh for early-type galaxies (S0-Sab), which have more massive bulges,are, on average, higher than the values for late-type galaxies with thesame rotational velocities. We conclude that the black-hole masses aredetermined primarily by the properties of the bulge and not therotational velocity or the mass of the galaxy.
| The Link between Star Formation and Accretion in LINERs: A Comparison with Other Active Galactic Nucleus Subclasses
We present archival high-resolution X-ray imaging observations of 25nearby LINERs observed by ACIS on board Chandra. This sample builds onour previously published proprietary and archival X-ray observations andincludes the complete set of LINERs with published black hole masses andFIR luminosities that have been observed by Chandra. Of the 82 LINERsobserved by Chandra, 41 (50%) display hard nuclear cores consistent withan AGN. The nuclear 2-10 keV luminosities of these AGN-LINERs range from~2×1038 to ~1×1044 ergss-1. Reinforcing our previous work, we find a significantcorrelation between the Eddington ratio,Lbol/LEdd, and the FIR luminosity,LFIR, as well as the IR brightness ratio,LFIR/LB, in the host galaxy of AGN-LINERs thatextends over 7 orders of magnitude in Lbol/LEdd.Combining our AGN-LINER sample with galaxies from other AGN subclasses,we find that this correlation is reinforced in a sample of 129 AGNs,extending over almost 9 orders of magnitude inLbol/LEdd. Using archival and previously publishedobservations of the 6.2 μm PAH feature from ISO, we find that it isunlikely that dust heating by the AGN dominates the FIR luminosity inour sample of AGNs. Our results may therefore imply a fundamental linkbetween the mass accretion rate (M˙), as measured by the Eddingtonratio, and the star formation rate (SFR), as measured by the FIRluminosity. Apart from the overall correlation, we find that thedifferent AGN subclasses occupy distinct regions in the LFIRand Lbol/LEdd plane. Assuming a constant radiativeefficiency for accretion, our results may imply a variation in theSFR/M˙ ratio as a function of AGN activity level, a result that mayhave significant consequences for our understanding of galaxy formationand black hole growth.
| Secular Evolution via Bar-driven Gas Inflow: Results from BIMA SONG
We present an analysis of the molecular gas distributions in the 29barred and 15 unbarred spirals in the BIMA CO (J=1-0) Survey of NearbyGalaxies (SONG). For galaxies that are bright in CO, we confirm theconclusion by Sakamoto et al. that barred spirals have higher moleculargas concentrations in the central kiloparsec. The SONG sample alsoincludes 27 galaxies below the CO brightness limit used by Sakamoto etal. Even in these less CO-bright galaxies we show that high central gasconcentrations are more common in barred galaxies, consistent withradial inflow driven by the bar. However, there is a significantpopulation of early-type (Sa-Sbc) barred spirals (6 of 19) that have nomolecular gas detected in the nuclear region and have very little out tothe bar corotation radius. This suggests that in barred galaxies withgas-deficient nuclear regions, the bar has already driven most of thegas within the bar corotation radius to the nuclear region, where it hasbeen consumed by star formation. The median mass of nuclear moleculargas is over 4 times higher in early-type bars than in late-type (Sc-Sdm)bars. Since previous work has shown that the gas consumption rate is anorder of magnitude higher in early-type bars, this implies that theearly types have significantly higher bar-driven inflows. The loweraccretion rates in late-type bars can probably be attributed to theknown differences in bar structure between early and late types. Despitethe evidence for bar-driven inflows in both early and late Hubble-typespirals, the data indicate that it is highly unlikely for a late-typegalaxy to evolve into an early type via bar-induced gas inflow.Nonetheless, secular evolutionary processes are undoubtedly present, andpseudobulges are inevitable; evidence for pseudobulges is likely to beclearest in early-type galaxies because of their high gas inflow ratesand higher star formation activity.
| The Pattern Speeds of 38 Barred Galaxies
We estimate the pattern speeds of 38 barred galaxies by simulationmodeling. We construct the gravitational potentials of the galaxies fromnear-IR photometry by assuming that the mass-to-light ratio (M/L) isconstant in the H band and a single pattern speed dominates in thestellar disk. We use the response of gaseous and stellar particle disksto a rigidly rotating potential to determine the pattern speed. If ourassumptions are correct, then the pattern speed depends on themorphological type: the average value of the ratio of the corotationresonance radius to the bar radius, ℛ, increases from about 1.1 intype SB0/a to 1.4 in SBb and 1.7 in SBc. Within the error estimates, allthe bars in galaxies of type SBab or earlier are fast rotators, havingℛ<=1.4, whereas late-type galaxies include both fast and slowrotators.
| The Stellar Populations in the Central Parsecs of Galactic Bulges
We present Hubble Space Telescope blue spectra at intermediate spectralresolution for the nuclei of 23 nearby disk galaxies. These objects wereselected to have nebular emission in their nuclei and span a range ofemission-line classifications, as well as Hubble types. In this paper wefocus on the stellar population as revealed by the continuum spectralenergy distribution measured within the central 0.13" (~8 pc) of thesegalaxies. The data were modeled with linear combinations of single-agestellar population synthesis models. The large majority (~80%) of thesurveyed nuclei have spectra whose features are consistent with apredominantly old (>~5×109 yr) stellar population.Approximately 25% of these nuclei show evidence of a component with ageyounger than 1 Gyr, with the incidence of these stars related to thenebular classification. Successful model fits imply an average reddeningcorresponding to AV~0.4 mag and a stellar metallicity of1-2.5 Zsolar. We discuss the implications of these resultsfor understanding the star formation history in the environment ofquiescent and active supermassive black holes. Our findings reinforcethe picture wherein Seyfert nuclei and the majority of low-ionizationnuclear emission-line regions are predominantly accretion-powered andsuggest that much of the central star formation in H II nuclei isactually circumnuclear.Based on observations obtained with the Hubble Space Telescope, which isoperated by AURA, Inc., under NASA contract NAS5-26555.
| The Hubble Space Telescope View of LINER Nuclei: Evidence for a Dual Population?
We study a complete, distance-limited sample of 25 LINERs, 21 of whichhave been imaged with the Hubble Space Telescope. In nine objects wedetect an unresolved nucleus. To study their physical properties, wecompare the radio and optical properties of the nuclei of LINERs withthose of other samples of local active galactic nuclei (AGNs), namely,Seyfert galaxies and low-luminosity radio galaxies (LLRGs). Our resultsshow that the LINER population is not homogeneous, as there are twosubclasses: (1) the first class is similar to the LLRG class, as itextends the population of radio-loud nuclei to lower luminosities; (2)the second is similar to Seyfert galaxies and extends the properties ofradio-quiet nuclei toward the lowest luminosities. The objects areoptimally discriminated in the plane formed by the black hole massversus nuclear radio loudness: all radio-loud LINERs haveMBH>~108Msolar, while Seyfertgalaxies and radio-quiet LINERs haveMBH<~108Msolar. The different natureof the various classes of local AGNs are best understood when thefraction of the Eddington luminosity they irradiate,Lo/LEdd, is plotted against the nuclearradio-loudness parameter: Seyfert galaxies are associated withrelatively high radiative efficienciesLo/LEdd>~10-4 (and high accretionrates onto low-mass black holes); LLRGs are associated with lowradiative efficiencies (and low accretion rates onto high-mass blackholes); all LINERs have low radiative efficiency (and accretion rates)and can be radio-loud or radio-quiet, depending on their black holemass.Based on observations obtained at the Space Telescope Science Institute,which is operated by the Association of Universities for Research inAstronomy, Inc., under NASA contract NAS5-26555.
| 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.
| The Distribution of Bar and Spiral Arm Strengths in Disk Galaxies
The distribution of bar strengths in disk galaxies is a fundamentalproperty of the galaxy population that has only begun to be explored. Wehave applied the bar-spiral separation method of Buta and coworkers toderive the distribution of maximum relative gravitational bar torques,Qb, for 147 spiral galaxies in the statistically well-definedOhio State University Bright Galaxy Survey (OSUBGS) sample. Our goal isto examine the properties of bars as independently as possible of theirassociated spirals. We find that the distribution of bar strengthdeclines smoothly with increasing Qb, with more than 40% ofthe sample having Qb<=0.1. In the context of recurrent barformation, this suggests that strongly barred states are relativelyshort-lived compared to weakly barred or nonbarred states. We do notfind compelling evidence for a bimodal distribution of bar strengths.Instead, the distribution is fairly smooth in the range0.0<=Qb<0.8. Our analysis also provides a first look atspiral strengths Qs in the OSUBGS sample, based on the sametorque indicator. We are able to verify a possible weak correlationbetween Qs and Qb, in the sense that galaxies withthe strongest bars tend to also have strong spirals.
| The Opacity of Spiral Galaxy Disks. IV. Radial Extinction Profiles from Counts of Distant Galaxies Seen through Foreground Disks
Dust extinction can be determined from the number of distant fieldgalaxies seen through a spiral disk. To calibrate this number for thecrowding and confusion introduced by the foreground image,González et al. and Holwerda et al. developed the Synthetic FieldMethod (SFM), which analyzes synthetic fields constructed by addingvarious deep exposures of unobstructed background fields to thecandidate foreground galaxy field. The advantage of the SFM is that itgives the average opacity for the area of a galaxy disk without makingassumptions about either the distribution of absorbers or of the diskstarlight. However, it is limited by poor statistics on the survivingfield galaxies, hence the need to combine a larger sample of fields.This paper presents the first results for a sample of 32 deep HubbleSpace Telescope (HST)/WFPC2 archival fields of 29 spiral galaxies. Theradial profiles of average dust extinction in spiral galaxies based oncalibrated counts of distant field galaxies is presented here, both forindividual galaxies and for composites from our sample. The effects ofinclination, spiral arms, and Hubble type on the radial extinctionprofile are discussed. The dust opacity of the disk apparently arisesfrom two distinct components: an optically thicker (AI=0.5-4mag) but radially dependent component associated with the spiral armsand a relatively constant optically thinner disk (AI~0.5mag). These results are in complete agreement with earlier work onocculted galaxies. The early-type spiral disks in our sample show lessextinction than the later types. Low surface brightness galaxies, andpossibly Sd's, appear effectively transparent. The average color of thefield galaxies seen through foreground disks does not appear to changewith radius or opacity. This gray behavior is most likely due to thepatchy nature of opaque clouds. The average extinction of a radialannulus and its average surface brightness seem to correlate for thebrighter regions. This leads to the conclusion that the brighter partsof the spiral disk, such as spiral arms, are also the ones with the mostextinction associated with them.
| The opacity of spiral galaxy disks. VI. Extinction, stellar light and color
In this paper we explore the relation between dust extinction andstellar light distribution in disks of spiral galaxies. Extinctioninfluences our dynamical and photometric perception of disks, since itcan distort our measurement of the contribution of the stellarcomponent. To characterize the total extinction by a foreground disk,González et al. (1998, ApJ, 506, 152) proposed the "SyntheticField Method" (SFM), which uses the calibrated number of distantgalaxies seen through the foreground disk as a direct indication ofextinction. The method is described in González et al. (1998,ApJ, 506, 152) and Holwerda et al. (2005a, AJ, 129, 1381). To obtaingood statistics, the method was applied to a set of HST/WFPC2 fields(Holwerda et al. 2005b, AJ, 129, 1396) and radial extinction profileswere derived, based on these counts. In the present paper, we explorethe relation of opacity with surface brightness or color from 2MASSimages, as well as the relation between the scalelengths for extinctionand light in the I band. We find that there is indeed a relation betweenthe opacity (AI) and the surface brightness, particularly atthe higher surface brightnesses. No strong relation between nearinfrared (H-J, H-K) color and opacity is found. The scalelengths of theextinction are uncertain for individual galaxies but seem to indicatethat the dust distribution is much more extended than the stellar light.The results from the distant galaxy counts are also compared to thereddening derived from the Cepheids light-curves (Freedman et al. 2001,ApJ, 553, 47). The extinction values are consistent, provided theselection effect against Cepheids with higher values of AI istaken into account. The implications from these relations for diskphotometry, M/L conversion and galaxy dynamical modeling are brieflydiscussed.
| The opacity of spiral galaxy disks. V. Dust opacity, HI distributions and sub-mm emission
The opacity of spiral galaxy disks, from counts of distant galaxies, iscompared to HI column densities. The opacity measurements are calibratedusing the "Synthetic Field Method" from González et al. (1998,ApJ, 506, 152), Holwerda et al. (2005a, AJ, 129, 1381). When comparedfor individual disks, the HI column density and dust opacity do not seemto be correlated as HI and opacity follow different radial profiles. Toimprove statistics, an average radial opacity profile is compared to anaverage HI profile. Compared to dust-to-HI estimates from theliterature, more extinction is found in this profile. This differencemay be accounted for by an underestimate of the dust in earliermeasurements due to their dependence on dust temperature. Since the SFMis insensitive to the dust temperature, the ratio between the SFMopacity and HI could very well be indicative of the true ratio. Earlierclaims for a radially extended cold dust disk were based on sub-mmobservations. A comparison between sub-mm observations and counts ofdistant galaxies is therefore desirable. We present the best currentexample of such a comparison, M 51, for which the measurements seem toagree. However, this remains an area where improved counts of distantgalaxies, sub-mm observations and our understanding of dust emissivityare needed.
| The extragalactic Cepheid bias: significant influence on the cosmic distance scale
The unique measurements with the Hubble Space Telescope of Cepheidvariable stars in nearby galaxies led to extragalactic distances thatmade the HST Key Project conclude that the Hubble constant isH0 = 72 km s-1 Mpc-1. The idea thatH0 is now known is widely spread among the astronomicalcommunity. Some time ago, we suggested that a strong selection effectmay still exist in the Cepheid method, resulting in too short distances.Using a model similar to traditional bias corrections, we deduce herenew estimates of distances from HST and previous ground-basedobservations which are both affected by this effect, showing the sametrend which starts at different distances. The recent measurement of M83 with the VLT is unbiased. Revisiting the calibration of HSTKP's withour new scale, makes long-range distance criteria more concordant andreduces the value of H0 to ≈60 km s-1Mpc-1. Locally, the corrected Cepheid distances giveHlocal=56 km s-1 Mpc-1 and reduce thevelocity dispersion in the Hubble flow. These numbers are indicative ofthe influence of the suggested Cepheid bias in the context of the HSTKPstudies and are not final values.
| A survey for OB associations in the Sculptor Group spiral galaxy NGC 7793
We report on the results from application of an objective algorithm(PLC) to find OB associations, to B and V images of the Sculptor spiralgalaxy NGC 7793, which were obtained with the ESO VLT and FORSinstrument and basically cover the entire spatial extent of the galaxy.We detected 148 associations. Statistical tests show that less than 6 ofthese detections are caused by randomly concentrated blue stars. In thesize distribution, a sharp peak is observed at a value of about 35microradians, which corresponds to a linear diameter of 135 pc, assuminga distance of 3.91 Mpc to the galaxy. We also find 25 much largerobjects. A second application of the PLC technique shows that 20 of themare stellar complexes consisting of multiple sub-associations withtypical sizes on the order of 130 pc. A comparison of the sizedistribution of the detected OB associations in NGC 7793 with observeddistributions in other galaxies suggests that the conditions in twoSculptor Group galaxies (NGC 300 and NGC 7793) favour the formation oflarge associations. We provide a catalog giving coordinates and physicalparameters for all the associations and stellar complexes we have foundin our survey.
| Ram pressure stripping of disk galaxies. From high to low density environments
Galaxies in clusters and groups moving through the intracluster orintragroup medium (abbreviated ICM for both) are expected to lose atleast a part of their interstellar medium (ISM) by the ram pressure theyexperience. We perform high resolution 2D hydrodynamical simulations offace-on ram pressure stripping (RPS) of disk galaxies to compile acomprehensive parameter study varying galaxy properties (mass, verticalstructure of the gas disk) and covering a large range of ICM conditions,reaching from high density environments like in cluster centres to lowdensity environments typical for cluster outskirts or groups. We findthat the ICM-ISM interaction proceeds in three phases: firstly theinstantaneous stripping phase, secondly the dynamic intermediate phase,thirdly the quasi-stable continuous viscous stripping phase. In thefirst phase (time scale 20 to 200~Myr) the outer part of the gas disk isdisplaced but only partially unbound. In the second phase (10 times aslong as the first phase) a part of the displaced gas falls back (about10% of the initial gas mass) despite the constant ICM wind, but mostdisplaced gas is now unbound. In the third phase the galaxy continues tolose gas at a rate of about 1~Mȯ~~yr-1 byturbulent viscous stripping. We find that the stripping efficiencydepends slightly on the Mach number of the flow, however, the mainparameter is the ram pressure. The stripping efficiency does not dependon the vertical structure and thickness of the gas disk. We discussuncertainties in the classic estimate of the stripping radius of Grunn& Gott (1972, ApJ, 176, 1), which compares the ram pressure to thegravitational restoring force. In addition, we adapt the estimate usedby Mori & Burkert (2000, ApJ, 538, 559) for spherical galaxies,namely the comparison of the central pressure with ram pressure. We findthat the latter estimate predicts the radius and mass of the gas diskremaining at the end of the second phase very well, and better than theGrunn & Gott (1972, ApJ, 176, 1) criterion. From our simulations weconclude that gas disks of galaxies in high density environments areheavily truncated or even completely stripped, but also the gas disks ofgalaxies in low density environments are disturbed by the flow andback-falling material, so that they should also be pre-processed.
| Completing H I observations of galaxies in the Virgo cluster
High sensitivity (rms noise 0.5 mJy) 21-cm H I line observationswere made of 33 galaxies in the Virgo cluster, using the refurbishedArecibo telescope, which resulted in the detection of 12 objects. Thesedata, combined with the measurements available from the literature,provide the first set of H I data that is complete for all 355 late-type(Sa-Im-BCD) galaxies in the Virgo cluster with mp ≤ 18.0mag. The Virgo cluster H I mass function (HIMF) that was derived forthis optically selected galaxy sample is in agreement with the HIMFderived for the Virgo cluster from the blind HIJASS H I survey and isinconsistent with the Field HIMF. This indicates that both in this richcluster and in the general field, neutral hydrogen is primarilyassociated with late-type galaxies, with marginal contributions fromearly-type galaxies and isolated H I clouds. The inconsistency betweenthe cluster and the field HIMF derives primarily from the difference inthe optical luminosity function of late-type galaxies in the twoenvironments, combined with the HI deficiency that is known to occur ingalaxies in rich clusters.Tables \ref{t1, \ref{sample_dat} and Appendix A are only available inelectronic form at http://www.edpsciences.org
|
Submit a new article
Related links
Submit a new link
Member of following groups:
|
Observation and Astrometry data
Catalogs and designations:
|
