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# NGC 4536

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 Scalar potential model of redshift and discrete redshiftOn 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. Hαkinematics of the SINGS nearby galaxies survey - I*This is the first part of an Hαkinematics follow-up survey of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample. The data for28galaxies are presented. The observations were done on three differenttelescopes with Fabry-Perot of New Technology for the Observatoire dumont Megantic (FaNTOmM), an integral field photon-counting spectrometer,installed in the respective focal reducer of each telescope. The datareduction was done through a newly built pipeline with the aim ofproducing the most homogenous data set possible. Adaptive spatialbinning was applied to the data cubes in order to get a constantsignal-to-noise ratio across the field of view. Radial velocity andmonochromatic maps were generated using a new algorithm, and thekinematical parameters were derived using tilted-ring models. A Virgo high-resolution Hα kinematical survey - II. The AtlasA 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. Cepheid Distances to SNe Ia Host Galaxies Based on a Revised Photometric Zero Point of the HST WFPC2 and New PL Relations and Metallicity CorrectionsWith 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. Mid-Infrared Spectral Diagnostics of Nuclear and Extranuclear Regions in Nearby GalaxiesMid-infrared diagnostics are presented for a large portion of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample plus archivaldata from ISO and Spitzer. The SINGS data set includes low- andhigh-resolution spectral maps and broadband imaging in the infrared forover 160 nuclear and extranuclear regions within 75 nearby galaxiesspanning a wide range of morphologies, metallicities, luminosities, andstar formation rates. Our main result is that these mid-infrareddiagnostics effectively constrain a target's dominant power source. Thecombination of a high-ionization line index and PAH strength serves asan efficient discriminant between AGNs and star-forming nuclei,confirming progress made with ISO spectroscopy on starbursting andultraluminous infrared galaxies. The sensitivity of Spitzer allows us toprobe fainter nuclear and star-forming regions within galaxy disks. Wefind that both star-forming nuclei and extranuclear regions stand apartfrom nuclei that are powered by Seyfert or LINER activity. In fact, weidentify areas within four diagnostic diagrams containing >90%Seyfert/LINER nuclei or >90% H II regions/H II nuclei. We also findthat, compared to starbursting nuclei, extranuclear regions typicallyseparate even further from AGNs, especially for low-metallicityextranuclear environments. In addition, instead of the traditionalmid-infrared approach to differentiating between AGNs and star-formingsources that utilizes relatively weak high-ionization lines, we showthat strong low-ionization cooling lines of X-ray-dominated regions like[Si II] 34.82 μm can alternatively be used as excellentdiscriminants. Finally, the typical target in this sample showsrelatively modest interstellar electron density (~400 cm-3)and obscuration (AV~1.0 mag for a foreground screen),consistent with a lack of dense clumps of highly obscured gas and dustresiding in the emitting regions. The Hubble Constant from Type Ia Supernovae Calibrated with the Linear and Nonlinear Cepheid Period-Luminosity RelationsIt is well known that the peak brightness of Type Ia supernovaecalibrated with Cepheid distances can be used to determine the Hubbleconstant. The Cepheid distances to the host galaxies of the calibratingsupernovae are usually obtained using the period-luminosity (P-L)relation derived from Large Magellanic Cloud (LMC) Cepheids. However,recent empirical studies provide evidence that the LMC P-L relation isnot linear. Here we determine the Hubble constant using both the linearand nonlinear LMC Cepheid P-L relations as calibrating relations to fourgalaxies that hosted Type Ia supernovae. Our results suggest that theobtained values of the Hubble constant are similar. However, a typicalerror of ~0.03 mag has to be added (in quadrature) to the systematicerror for the Hubble constant when the linear LMC P-L relation is used,assuming that the LMC P-L relation is indeed nonlinear. This isimportant to minimize the total error on the Hubble constant in the eraof precision cosmology. The Hubble constants calibrated from the linearand nonlinear LMC P-L relations are H0=74.92+/-2.28(random)+/-5.06(systematic) km s-1 Mpc-1 andH0=74.37+/-2.27(random)+/-4.92(systematic) km s-1Mpc-1, respectively. Hubble constants calculated using theGalactic P-L relation are also presented and briefly discussed. Central Star Formation and PAH Profiles in Pseudobulges and Classical BulgesI use Spitzer 3.6-8.0 μm color profiles and surface brightnessprofiles of polycyclic aromatic hydrocarbons (PAHs) to compare theradial structure of star formation in pseudobulges and classical bulges.Pseudobulges are bulges'' that form through secular evolution, ratherthan mergers. In this study, pseudobulges are identified using thepresence of disklike structure in the center of the galaxy (nuclearspirals, nuclear bars, and high ellipticity in bulge); classical bulgesare those galaxy bulges with smooth isophotes that are round compared tothe outer disk and show no disky structure in their bulge. I show thatgalaxies structurally identified as having pseudobulges have highercentral star formation rates than those of classical bulges.Furthermore, I also show that galaxies identified as having classicalbulges have remarkably regular star formation profiles. The colorprofiles of galaxies with classical bulges show a star-forming outerdisk with a sharp change, consistent with a decline in star formationrates, toward the center of the galaxy. Classical bulges have a nearlyconstant inner profile (r<~1.5 kpc) that is similar to ellipticalgalaxies. Pseudobulges in general show no such transition in starformation properties from the outer disk to the central pseudobulge.Thus, I conclude that pseudobulges and classical bulges do in fact formtheir stars via different mechanisms. Furthermore, this adds to theevidence that classical bulges form most of their stars in fast episodicbursts, in a similar fashion to elliptical galaxies, whereaspseudobulges form stars from longer lasting secular processes. Using Line Profiles to Test the Fraternity of Type Ia Supernovae at High and Low RedshiftsUsing archival data of low-redshift (z<0.01 Center for Astrophysicsand SUSPECT databases) Type Ia supernovae (SNe Ia) and recentobservations of high-redshift (0.161.7] SNe Ia, which are also subluminous. Inaddition, we give the first direct evidence in two high-z SN Ia spectraof a double-absorption feature in Ca II λ3945, an event alsoobserved, although infrequently, in low-redshift SN Ia spectra (6 out of22 SNe Ia in our local sample). Moreover, echoing the recent studies ofDessart & Hillier in the context of Type II supernovae (SNe II), wesee similar P Cygni line profiles in our large sample of SN Ia spectra.First, the magnitude of the velocity location at maximum profileabsorption may underestimate that at the continuum photosphere, asobserved, for example, in the optically thinner line S II λ5640.Second, we report for the first time the unambiguous and systematicintrinsic blueshift of peak emission of optical P Cygni line profiles inSN Ia spectra, by as much as 8000 km s-1. All the high-z SNeIa analyzed in this paper were discovered and followed up by the ESSENCEcollaboration and are now publicly available.Based in part on observations obtained at the Cerro TololoInter-American Observatory, which is operated by the Association ofUniversities for Research in Astronomy (AURA), Inc., under cooperativeagreement with the National Science Foundation (NSF); the EuropeanSouthern Observatory, Chile (ESO program 170.A-0519) the GeminiObservatory, which is operated by AURA under a cooperative agreementwith the NSF on behalf of the Gemini partnership (the NSF [UnitedStates], the Particle Physics and Astronomy Research Council [UnitedKingdom], the National Research Council [Canada], CONICYT [Chile], theAustralian Research Council [Australia], CNPq [Brazil], and CONICET[Argentina]) (programs GN-2002B-Q-14, GN-2003B-Q-11, and GS-2003B-Q-11)the Magellan Telescopes at Las Campanas Observatory; the MMTObservatory, a joint facility of the Smithsonian Institution and theUniversity of Arizona; and the F. L. Whipple Observatory, which isoperated by the Smithsonian Astrophysical Observatory. Some of the datapresented herein were obtained at the W. M. Keck Observatory, which isoperated as a scientific partnership among the California Institute ofTechnology, the University of California, and the National Aeronauticsand Space Administration. The Observatory was made possible by thegenerous financial support of the W. M. Keck Foundation. A Comparison of Hα and Stellar Scale Lengths in Virgo and Field SpiralsThe 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. Examining the Seyfert-Starburst Connection with Arcsecond-Resolution Radio Continuum ObservationsWe compare the arcsecond-scale circumnuclear radio continuum propertiesof five Seyfert and five starburst galaxies, concentrating on the searchfor any structures that could imply a spatial or causal connectionbetween the nuclear activity and a circumnuclear starburst ring. Noevidence is found in the radio emission for a link between thetriggering or feeding of nuclear activity and the properties ofcircumnuclear star formation. Conversely, there is no clear evidence ofnuclear outflows or jets triggering activity in the circumnuclear ringsof star formation. Interestingly, the difference in the angle betweenthe apparent orientation of the most elongated radio emission and theorientation of the major axis of the galaxy is on average larger inSeyfert galaxies than in starburst galaxies, and Seyfert galaxies appearto have a larger physical size scale of the circumnuclear radiocontinuum emission. The concentration, asymmetry, and clumpinessparameters of radio continuum emission in Seyfert galaxies andstarbursts are comparable, as are the radial profiles of radio continuumand near-infrared line emission. The circumnuclear star formation andsupernova rates do not depend on the level of nuclear activity. Theradio emission usually traces the near-infrared Brγ andH2 1-0 S(1) line emission on large spatial scales, butlocally their distributions are different, most likely because of theeffects of varying local magnetic fields and dust absorption andscattering. The extragalactic Cepheid bias: a new test using the period-luminosity-color relationWe 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. The Schmidt Law at High Molecular DensitiesWe 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. Integral Field Spectroscopy of 23 Spiral BulgesWe have obtained integral-field spectroscopy for 23 spiral bulges usingINTEGRAL on the William Herschel Telescope and SPIRAL on theAnglo-Australian Telescope. This is the first two-dimensional surveydirected solely at the bulges of spiral galaxies. Eleven galaxies of thesample do not have previous measurements of the stellar velocitydispersion (σ*). These data are designed to complementour Space Telescope Imaging Spectrograph program for estimating blackhole masses in the range 106-108 Msolarusing gas kinematics from nucleated disks. These observations will serveto derive the stellar dynamical bulge properties using the traditionalMg b and Ca II triplets. We use both cross-correlation and maximumpenalized likelihood to determine projected σ* in thesesystems and present radial velocity fields, major axis rotation curves,curves of growth, and σ* fields. Usingcross-correlation to extract the low-order two-dimensional stellardynamics we generally see coherent radial rotation and irregularvelocity dispersion fields suggesting that σ* is anontrivial parameter to estimate. Ultraluminous X-Ray Sources in Nearby Galaxies from ROSAT High Resolution Imager Observations I. Data AnalysisX-ray observations have revealed in other galaxies a class ofextranuclear X-ray point sources with X-ray luminosities of1039-1041 ergs s-1, exceeding theEddington luminosity for stellar mass X-ray binaries. Theseultraluminous X-ray sources (ULXs) may be powered by intermediate-massblack holes of a few thousand Msolar or stellar mass blackholes with special radiation processes. In this paper, we present asurvey of ULXs in 313 nearby galaxies withD25>1' within 40 Mpc with 467 ROSAT HighResolution Imager (HRI) archival observations. The HRI observations arereduced with uniform procedures, refined by simulations that help definethe point source detection algorithm employed in this survey. A sampleof 562 extragalactic X-ray point sources withLX=1038-1043 ergs s-1 isextracted from 173 survey galaxies, including 106 ULX candidates withinthe D25 isophotes of 63 galaxies and 110 ULX candidatesbetween 1D25 and 2D25 of 64 galaxies, from which aclean sample of 109 ULXs is constructed to minimize the contaminationfrom foreground or background objects. The strong connection betweenULXs and star formation is confirmed based on the striking preference ofULXs to occur in late-type galaxies, especially in star-forming regionssuch as spiral arms. ULXs are variable on timescales over days to yearsand exhibit a variety of long term variability patterns. Theidentifications of ULXs in the clean sample show some ULXs identified assupernovae (remnants), H II regions/nebulae, or young massive stars instar-forming regions, and a few other ULXs identified as old globularclusters. In a subsequent paper, the statistic properties of the surveywill be studied to calculate the occurrence frequencies and luminosityfunctions for ULXs in different types of galaxies to shed light on thenature of these enigmatic sources. Chemistry and Star Formation in the Host Galaxies of Type Ia SupernovaeWe study the effect of environment on the properties of Type Iasupernovae by analyzing the integrated spectra of 57 local Type Iasupernova host galaxies. We deduce from the spectra the metallicity,current star formation rate, and star formation history of the host andcompare these to the supernova decline rates. Additionally, we comparethe host properties to the difference between the derived supernovadistance and the distance determined from the best-fit Hubble law. Fromthis we investigate possible uncorrected systematic effects inherent inthe calibration of Type Ia supernova luminosities using light-curvefitting techniques. Our results indicate a statistically insignificantcorrelation in the direction of higher metallicity spiral galaxieshosting fainter Type Ia supernovae. However, we present qualitativeevidence suggesting that progenitor age is more likely to be the sourceof variability in supernova peak luminosities than is metallicity. We donot find a correlation between the supernova decline rate and hostgalaxy absolute B magnitude, nor do we find evidence of a significantrelationship between decline rate and current host galaxy star formationrate. A tenuous correlation is observed between the supernova Hubbleresiduals and host galaxy metallicities. Further host galaxyobservations will be needed to refine the significance of this result.Finally, we characterize the environmental property distributions forType Ia supernova host galaxies through a comparison with two larger,more general galaxy distributions using Kolmogorov-Smirnov tests. Theresults show the host galaxy metallicity distribution to be similar tothe metallicity distributions of the galaxies of the NFGS and SDSS.Significant differences are observed between the SN Ia distributions ofabsolute B magnitude and star formation histories and the correspondingdistributions of galaxies in the NFGS and SDSS. Among these is an abruptupper limit observed in the distribution of star formation histories ofthe host galaxy sample, suggesting a Type Ia supernovae characteristicdelay time lower limit of approximately 2.0 Gyr. Other distributiondiscrepancies are investigated and the effects on the supernovaproperties are discussed. Infrared Spectral Energy Distributions of Nearby GalaxiesThe Spitzer Infrared Nearby Galaxies Survey (SINGS) is carrying out acomprehensive multiwavelength survey on a sample of 75 nearby galaxies.The 1-850 μm spectral energy distributions (SEDs) are presented usingbroadband imaging data from Spitzer, 2MASS, ISO, IRAS, and SCUBA. Theinfrared colors derived from the globally integrated Spitzer data aregenerally consistent with the previous generation of models that weredeveloped using global data for normal star-forming galaxies, althoughsignificant deviations are observed. Spitzer's excellent sensitivity andresolution also allow a detailed investigation of the infrared SEDs forvarious locations within the three large, nearby galaxies NGC 3031(M81), NGC 5194 (M51), and NGC 7331. A wide variety of spectral shapesis found within each galaxy, especially for NGC 3031, the closest of thethree targets and thus the galaxy for which the smallest spatial scalescan be explored. Strong correlations exist between the local starformation rate and the infrared colors fν(70μm)/fν(160 μm) and fν(24μm)/fν(160 μm), suggesting that the 24 and 70 μmemission are useful tracers of the local star formation activity level.Preliminary evidence indicates that variations in the 24 μm emission,and not variations in the emission from polycyclic aromatic hydrocarbonsat 8 μm, drive the variations in the fν(8.0μm)/fν(24 μm) colors within NGC 3031, NGC 5194, andNGC 7331. If the galaxy-to-galaxy variations in SEDs seen in our sampleare representative of the range present at high redshift, thenextrapolations of total infrared luminosities and star formation ratesfrom the observed 24 μm flux will be uncertain at the factor of 5level (total range). The corresponding uncertainties using theredshifted 8.0 μm flux (e.g., observed 24 μm flux for a z=2source) are factors of 10-20. Considerable caution should be used wheninterpreting such extrapolated infrared luminosities. The Central Region of Barred Galaxies: Molecular Environment, Starbursts, and Secular EvolutionStellar bars drive gas into the circumnuclear (CN) region of galaxies.To investigate the fate of the CN gas and star formation (SF), we studya sample of barred nonstarbursts and starbursts with high-resolution CO,optical, Hα, radio continuum, Brγ, and HST data, and findthe following. (1) The inner kiloparsec of bars differs markedly fromthe outer disk. It hosts molecular gas surface densitiesΣgas-m of 500-3500 Msolar pc-2,gas mass fractions of 10%-30%, and epicyclic frequencies of several100-1000 km s-1 kpc-1. Consequently, in the CNregion gravitational instabilities can only grow at high gas densitiesand on short timescales, explaining in part why powerful starburstsreside there. (2) Across the sample, we find bar pattern speeds withupper limits of 43-115 km s-1 pc-1 and outer innerLindblad resonance radii of >500 pc. (3) Barred starbursts andnonstarbursts have CN SF rates of 3-11 and 0.1-2 Msolaryr-1, despite similar CN gas masses. TheΣgas-m value in the starbursts is larger (1000-3500Msolar pc-2) and close to the Toomre criticaldensity over a large region. (4) Molecular gas makes up 10%-30% of theCN dynamical mass and fuels large CN SF rates in the starbursts,building young, massive, high-V/σ components. Implications forsecular evolution along the Hubble sequence are discussed. Cepheid Calibrations from the Hubble Space Telescope of the Luminosity of Two Recent Type Ia Supernovae and a Redetermination of the Hubble ConstantWe report observations of two nearby Type Ia supernovae (SNe Ia) forwhich observations of Cepheid variables in the host galaxies have beenobtained with the Hubble Space Telescope: SN 1994ae in NGC 3370 and SN1998aq in NGC 3982. For NCG 3370, we used the Advanced Camera forSurveys to observe 64 Cepheids that yield a distance of 29 Mpc, thefarthest direct measurement of Cepheids. We have measured emission linesfrom H II regions in both host galaxies that providemetallicity-dependent corrections to their period-luminosity relations.These two SNe Ia double the sample of ideal'' luminosity calibrators:objects with well-observed and well-calibrated light curves of typicalshape and with low reddening. By comparing them to all similarlywell-measured SNe Ia in the Hubble flow, we find thatH0=73+/-4(statistical)+/-5(systematic) km s-1Mpc-1. A detailed analysis demonstrates that most of the pastdisagreement over the value of H0 as determined from SNe Iais abated by the replacement of past, problematic data by more accurateand precise, modern data.Based on observations with the NASA/ESA Hubble Space Telescope, obtainedat the Space Telescope Science Institute, which is operated by AURA,Inc., under NASA contract NAS 5-26555. Reddening, Absorption, and Decline Rate Corrections for a Complete Sample of Type Ia Supernovae Leading to a Fully Corrected Hubble Diagram to v < 30,000 km s-1Photometric (BVI) and redshift data corrected for streaming motions arecompiled for 111 Branch-normal,'' four 1991T-like, seven 1991bg-like,and two unusual supernovae of Type Ia (SNe Ia). Color excessesE(B-V)host of normal SNe Ia, due to the absorption of thehost galaxy, are derived by three independent methods, giving excellentagreement leading to the intrinsic colors at maximum of(B-V)00=-0.024+/-0.010 and (V-I)00=-0.265+/-0.016if normalized to a common decline rate of Δm15=1.1. Thestrong correlation between redshift absolute magnitudes (based on anarbitrary Hubble constant of H0=60 km s-1Mpc-1), corrected only for the extrinsic Galactic absorption,and the derived E(B-V)host color excesses leads to thewell-determined yet abnormal absorption-to-reddening ratios ofRBVI=3.65+/-0.16, 2.65+/-0.15, and 1.35+/-0.21.Comparison with the canonical Galactic values of 4.1, 3.1, and 1.8forces the conclusion that the law of interstellar absorption in thepath length to the SN in the host galaxy is different from the localGalactic law, a result consistent with earlier conclusions by others.Improved correlations of the fully corrected absolute magnitudes (on thesame arbitrary Hubble constant zero point) with host galaxymorphological type, decline rate, and intrinsic color are derived. Werecover the result that SNe Ia in E/S0 galaxies are ~0.3 mag fainterthan in spiral galaxies for possible reasons discussed in the text. Thenew decline rate corrections to absolute magnitudes are smaller thanthose by some authors for reasons explained in the text. The fourspectroscopically peculiar 1991T-type SNe are significantly overluminousas compared to Branch-normal SNe Ia. The overluminosity of the seven1999aa-like SNe is less pronounced. The seven 1991bg types in the sampleconstitute a separate class of SNe Ia, averaging in B 2 mag fainter thanthe normal Ia. New Hubble diagrams in B, V, and I are derived out to~30,000 km s-1 using the fully corrected magnitudes andvelocities, corrected for streaming motions. Nine solutions for theintercept magnitudes in these diagrams show extreme stability at the0.02 mag level using various subsamples of the data for both low andhigh extinctions in the sample, proving the validity of the correctionsfor host galaxy absorption. We shall use the same precepts for fullycorrecting SN magnitudes for the luminosity recalibration of SNe Ia inthe forthcoming final review of our Hubble Space Telescope Cepheid-SNexperiment for the Hubble constant. Nuclear Properties of Nearby Spiral Galaxies from Hubble Space Telescope NICMOS Imaging and STIS SpectroscopyWe investigate the central regions of 23 spiral galaxies using SpaceTelescope Imaging Spectrograph (STIS) spectroscopy and archivalNear-Infrared Camera and Multi-Object Spectrometer (NICMOS) imaging. Thesample is taken from our program to determine the masses of centralmassive black holes (MBHs) in 54 nearby spiral galaxies. Stars arelikely to contribute significantly to any dynamical central massconcentration that we find in our MBH program, and this paper is part ofa series to investigate the nuclear properties of these galaxies. We usethe Nuker law to fit surface brightness profiles, derived from theNICMOS images, to look for nuclear star clusters and find possibleextended sources in three of the 23 galaxies studied (13%). The factthat this fraction is lower than that inferred from optical Hubble SpaceTelescope studies is probably due to the greater spatial resolution ofthose studies. Using R-H and J-H colors and equivalent widths ofHα emission (from the STIS spectra), we investigate the nature ofthe stellar population with evolutionary models. Under the assumption ofhot stars ionizing the gas, as opposed to a weak active galactic nucleus(AGN), we find that there are young stellar populations (~10-20 Myr);however, these data do not allow us to determine what percentage of thetotal nuclear stellar population they form. In addition, in an attemptto find any unknown AGN, we use [N II] and [S II] line flux ratios(relative to Hα) and find tentative evidence for weak AGNs in NGC1300 and NGC 4536.Based on observations with the NASA/ESA Hubble Space Telescope obtainedat the Space Telescope Science Institute, which is operated by theAssociation of Universities for Research in Astronomy (AURA), Inc.,under NASA contract NAS 5-26555. The Opacity of Spiral Galaxy Disks. IV. Radial Extinction Profiles from Counts of Distant Galaxies Seen through Foreground DisksDust 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. III. Automating the Synthetic Field MethodDust extinction in spiral disks can be estimated from the counts ofbackground field galaxies, provided the deleterious effects of confusionintroduced by structure in the image of the foreground spiral disk canbe calibrated. González et al. developed a method for thiscalibration, the Synthetic Field Method (SFM), and applied this conceptto a Hubble Space Telescope (HST)/Wide Field Planetary Camera 2 image ofNGC 4536. The SFM estimates the total extinction through the diskwithout requiring assumptions about the distribution of absorbers ordisk light. The poor statistics, however, result in large errors inindividual measurements. We report on improvements to and automation ofthe SFM that render it suitable for application to large archival datasets. To illustrate the strengths and weaknesses of this new method, theresults on NGC 1365, an SBb galaxy, and NGC 4536, an SABbc, arepresented. The extinction estimate for NGC 1365 isAI=0.6+0.6-0.7 at 0.45R25,and for NGC 4536 it is AI=1.6+1.0-1.3at 0.75R25. The results for NGC 4536 are compared with thoseof González et al. The automation is found to limit the maximumdepth to which field galaxies can be found. Taking this into account,our results agree with those of González et al. We conclude thatthis method can only give an inaccurate measure of extinction for afield covering a small solid angle. An improved measurement of diskextinction can be done by averaging the results over a series of HSTfields, thereby improving the statistics. This can be achieved with theautomated method, trading some completeness limit for speed. The resultsfrom this set of fields are reported in a companion paper by Holwerda etal. The opacity of spiral galaxy disks. VII. The accuracy of galaxy counts as an extinction probeThe "Synthetic Field Method" (SFM) was introduced by González etal. (1998, ApJ, 506, 152) to calibrate numbers of distant galaxies as aprobe of extinction in a foreground spiral disk. González et al.(2003, AJ, 125, 1182) studied the effect of the foreground disk on thesenumbers using simulations of current and future instruments for fieldsin the LMC, M 31 and NGC 4536, a galaxy in Virgo. They concludedthat: (1) the brighter centers of disks were unsuitable; (2) thegranularity of the disk at a fixed surface brightness is the limitingfactor in the detection of distant galaxies; and (3) the optimumdistance for measurements would be that of the Virgo cluster for thecurrent instruments on board HST. At this distance the foreground diskis smoothed with distance, improving detection of distant backgroundgalaxies. Holwerda et al. (2005a, AJ, 129, 1381) automated the SFM andHolwerda et al. (2005b, AJ, 129, 1396) applied it to a large set ofWFPC2 fields. In this paper, the quality of the extinction measurementin these fields is compared to their distance, granularity, surfacebrightness and structure.  The average surface brightness of theof a field is shown to directly influence the accuracy of the SFM. Thisrestricts meaningful measurements to the disks of spiral galaxies. Largestructures such as spiral arms have a similar effect. The granularity orsmall scale structure in a field influences the detection of distantgalaxies, limiting the SFM measurements in nearby disks. From the trendsin the accuracy and maximum practical field-of-view considerations, theminimum and maximum distance for SFM application, approximately 5 and 35Mpc respectively. Using the same instrument and detection method, therelations with SFM parameters and field characteristics can be used toforgo the synthetic fields altogether. For the wealth of ACS fieldsbecoming available in the archive, these relations can be used to selectthose fields based on expected SFM accuracy. The opacity of spiral galaxy disks. VI. Extinction, stellar light and colorIn 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 emissionThe 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 scaleThe 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. Completing H I observations of galaxies in the Virgo clusterHigh 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 Spectral homogeneity of type Ia supernovaeI present a newly defined set of spectral indicators for type Ia SNe. Iuse these indicators to characterize the evolution of these objects withphase and to study the issue of spectral homogeneity from a quantitativepoint of view. I present an example of a new spectral parameter and itscorrelation to the secondary lightcurve-shape parameter of the standardcandle, Δm15(B). Cepheid calibration of Type Ia supernovae and the Hubble constant We investigate how a different calibration of the Cepheidperiod-luminosity (PL) relation, taking into account metallicitycorrections, affects the absolute magnitude calibration of Type Iasupernovae (SNe Ia) and, in turn, the determination of the Hubbleconstant H0. We use SN Ia light curves from the literatureand previously unpublished data to establish theMB-Δm15(B) relation, and calibrate the zeropoint by means of nine SNe Ia with Cepheid-measured distances. Thisrelation is then used to establish the Hubble diagram, and in turn toderive H0. In the attempt to correct for the host-galaxyextinction, we find that the data suggest a value for the total toselective absorption ratio of RB= 3.5, which is smaller thanthe standard value for our own Galaxy of RB= 4.315.Depending on the metallicity correction for the Cepheid PL relation, thevalue of RB, and SN sample selection criteria, the value ofthe Hubble constant H0 takes a value in the range 68-74 kms-1 Mpc-1, with associated uncertainties of theorder of 10 per cent.Unpublished photometry is also presented for 18 SNe of our sample(1991S, 1991T, 1992A, 1992K, 1993H, 1993L, 1994D, 1994M, 1994ae, 1995D,1995ac, 1995bd, 1996bo, 1997bp, 1997br, 1999aa, 1999dk, 2000cx). Thesedata are the results of a long-standing effort in supernova monitoringat ESO - La Silla and Asiago observatories. The dispersion in the Cepheid period-luminosity relation and the consequences for the extragalactic distance scaleUsing published Hubble Space Telescope (HST) Cepheid data from 25galaxies, we have found a correlation between the dispersion in theCepheid period-luminosity (P-L) relation and host galaxy metallicity,which is significant at the ~3σ level in the V band. In the I bandthe correlation is less significant, although the tighter intrinsicdispersion of the P-L relation in I may make it harder to detect such acorrelation in the HST sample. One possibility is that low metallicitygalaxies have smaller metallicity gradients than high metallicitygalaxies; if the Cepheid P-L relation has a significant dependence onmetallicity then this might explain the higher P-L dispersion in thehigher metallicity galaxies. A second possibility is that the increasedP-L dispersion is driven by metallicity dispersion but now due to arelation between metallicity and Cepheid colour rather than luminosity.A third possibility is that the increased P-L dispersion is caused by anincrease in the width of the instability strip with metallicity.Whatever the explanation, the high observed dispersions in the HSTCepheid P-L relations have the important consequence that the bias dueto incompleteness in the P-L relation at faint magnitudes is moresignificant than previously thought. Using a maximum likelihoodtechnique which takes into account the effect on the P-L relations oftruncation by consistently defined magnitude completeness limits, werederive the Cepheid distances to the 25 galaxies. In the case of thegalaxies with the highest P-L dispersion at the largest distances, wefind that the published distance modulus underestimates the truedistance modulus by up to ~0.5 mag.When both metallicity and magnitude incompleteness corrections are made,a scale error in the published Cepheid distances is seen in the sensethat the published distance moduli are increasingly underestimated atlarger distances. This results in the average distance modulus to thefour galaxies in the Virgo cluster core increasing from(m-M)0= 31.2 +/- 0.19 to (m-M)0= 31.4 +/- 0.19 ifthe γVI=-0.24 mag dex-1 metallicitycorrection of Kennicutt et al. is assumed. For the 18 HST galaxies withgood Tully-Fisher (TF) distances and (m-M)0 > 29.5 theCepheid-TF distance modulus average residual increases from 0.44 +/-0.09 to 0.63 +/- 0.1 mag with γVI=-0.24. This indicatesa significant scale error in TF distances, which reduces the previousPierce & Tully TF estimate of H0= 85 +/- 10 kms-1 Mpc-1 to H0= 63 +/- 7 kms-1 Mpc-1, assuming γVI=-0.24 anda still uncertain Virgo infall model. Finally, for the eight HSTgalaxies with Type Ia supernovae (SNIa), the metallicity andincompleteness corrected Cepheid distances marginally suggest there maybe a metallicity dependence of SNIa peak luminosity in the sense thatmetal-poor hosts have lower luminosity SNIa. Thus, SNIa Hubble diagramestimates of both H0 and q0 may therefore alsorequire significant corrections for metallicity, once the exact sizes ofthe Cepheid metallicity corrections become better established.
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