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 The Light Echo around Supernova 2003gd in Messier 74We confirm the discovery of a light echo around the Type II-plateausupernova 2003gd in Messier 74 (NGC 628), seen in images obtained withthe High Resolution Channel of the Advanced Camera for Surveys on boardthe Hubble Space Telescope (HST) as part of a larger Snapshot program onthe late-time emission from supernovae. The analysis of the echo wepresent suggests that it is due to the SN light pulse scattered by asheet of dust grains located ~113 pc in front of the SN, and that thesegrains are not unlike those assumed to be in the diffuse Galacticinterstellar medium, both in composition and in size distribution. Theecho is less consistent with scattering off carbon-rich grains, and ifanything, the grains may be somewhat more silicate rich than theGalactic dust composition. The echo also appears to be more consistentwith a SN distance closer to 7 Mpc than to 9 Mpc. This further supportsthe conclusion we reached elsewhere that the initial mass for the SNprogenitor was relatively low (~8-9 Msolar). The HST shouldbe used to continue to monitor the echo in several bands, particularlyin the blue, to better constrain its origin.Based in part on observations made with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute (STScI),which is operated by the Association of Universities for Research inAstronomy (AURA), Inc., under NASA contract NAS 5-26555. 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. Optical and Near-Infrared Observations of the Peculiar Type ia Supernova 1999acWe present 39 nights of optical photometry, 34 nights of IR photometry,and 4 nights of optical spectroscopy of the Type Ia supernova 1999ac.This supernova was discovered 2 weeks before maximum light, andobservations were begun shortly thereafter. At early times its spectraresembled the unusual SN 1999aa and were characterized by very highvelocities in the Ca II H and K lines but very low velocities in the SiII λ6355 line. The optical photometry showed a slow rise to peakbrightness but, quite peculiarly, was followed by a more rapid declinefrom maximum. Thus, the B- and V-band light curves cannot becharacterized by a single stretch factor. We argue that the best measureof the nature of this object is not the decline rate parameterΔm15(B). The B-V colors were unusual from 30 to 90 daysafter maximum light in that they evolved to bluer values at a muchslower rate than normal Type Ia supernovae. The spectra and bolometriclight curve indicate that this event was similar to thespectroscopically peculiar slow decliner SN 1999aa.Based in part on observations taken at the Cerro Tololo Inter-AmericanObservatory, National Optical Astronomy Observatory, which is operatedby the Association of Universities for Research in Astronomy (AURA),Inc., under cooperative agreement with the National Science Foundation.This paper is also based in part on observations obtained with theApache Point Observatory 3.5 m telescope, which is owned and operated bythe Astrophysical Research Consortium. 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. Classifications of the Host Galaxies of Supernovae, Set IIIA homogeneous sample comprising host galaxies of 604 recent supernovae,including 212 objects discovered primarily in 2003 and 2004, has beenclassified on the David Dunlap Observatory system. Most SN 1991bg-likeSNe Ia occur in E and E/Sa galaxies, whereas the majority of SN1991T-like SNe Ia occur in intermediate-type galaxies. This differenceis significant at the 99.9% level. As expected, all types of SNe II arerare in early-type galaxies, whereas normal SNe Ia occur in all Hubbletypes. This difference is significant at the 99.99% level. A smallnumber of SNe II in E galaxies might be due to galaxy classificationerrors or to a small young-population component in these mainly oldobjects. No significant difference is found between the distributionsover the Hubble type of SNe Ibc and SNe II. This confirms that both ofthese types of objects have similar (massive) progenitors. The presentdata show that in order to understand the dependence of supernova typeon host-galaxy population, it is more important to obtain accuratemorphological classifications than it is to increase the size of thedata sample. Multiband optical photometry and bolometric light curve of the Type Ia supernova 2004SWe present BVRCIC broad-band CCD photometry of theType Ia supernova SN 2004S, which appeared in the galaxy MCG-05-16-021,obtained during 2004 February 12 to March 22. We present multiband andbolometric light curves constructed using our data as well as otheravailable data. The time of the B-band maximum and the peak magnitudesin different bands are obtained using fits of light-curve and colourtemplates. We clearly see a strong shoulder in the RC bandand a second maximum in the IC band. SN 2004S closelyresembles SN 1992al after maximum. From the peak bolometric luminosity,we estimate the ejected mass of 56 Ni to be 0.41Msolar. Reflections on reflexions - I. Light echoes in Type Ia supernovaeIn the last 10 yr, observational evidence has been steadily growingabout a possible connection between Type Ia supernovae (SNe Ia)properties and the environment where they explode. In this paper, Idiscuss, from a theoretical point of view but with an observer'sperspective, the usage of light echoes (LEs) to probe the circumstellarmatter around SNe Ia because, in principle, they give us a uniqueopportunity for obtaining a three-dimensional description of the SNenvironment. In turn, this can be used to check the often suggestedassociation of some SNe Ia with dusty/star-forming regions, which wouldpoint to a young population for the progenitors. After giving a briefintroduction to the LE phenomenon in the single scatteringapproximation, I derive analytical and numerical solutions for theoptical light and colour curves for a few simple dust geometries. Afully three-dimensional multiple scattering treatment has also beenimplemented in a Monte Carlo (MC) code, which I have used to investigatethe effects of multiple scattering. In particular, I have explored indetail the LE colour dependency from time and dust distribution, becausethis is a promising tool to determine the dust density and derive theeffective presence of multiple scattering from the observed properties.Finally, again by means of Monte Carlo simulations, I have studied theeffects of multiple scattering on the LE linear polarization, analysingthe dependences from the dust parameters and geometry. Both theanalytical formalism and MC codes described in this paper can be usedfor any LE for which the light curve of the central source is known. 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. Molecular Disks in the Elliptical Galaxies NGC 83 and NGC 2320The molecular gas in (some) early-type galaxies holds important clues tothe history and the future of these galaxies. In pursuit of these clues,we have used the BIMA millimeter array to map CO emission in the giantelliptical galaxies NGC 83 and NGC 2320 and to search for CO emissionfrom the S0 galaxy NGC 5838. We also present V and R images of NGC 83and NGC 2320 that trace their dust distributions and enable a search fordisky stellar structures. The molecular gas in NGC 83 is well relaxed,but both CO and dust in NGC 2320 show asymmetric structures that may belinked to a recent acquisition of the gas. However, the specific angularmomentum distribution of molecular gas in NGC 2320 is consistent withthat of the stars. Internal origin of the gas (stellar mass loss)cannot, therefore, be ruled out on angular momentum grounds alone. Wealso consider the evidence for star formation activity and disk growthin these two elliptical galaxies. Radio continuum and FIR fluxes of NGC83 suggest star formation activity. NGC 2320 has bright [O III]emission, but its large radio-FIR flux ratio and the mismatch betweenthe kinematics of CO and [O III] suggest that the ionized gas should notbe attributed to star formation. The origin and future of these twoCO-rich early-type galaxies are thus complex, multifaceted stories. 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. The Link between Star Formation and Accretion in LINERs: A Comparison with Other Active Galactic Nucleus SubclassesWe 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. 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. A Chandra Snapshot Survey of Infrared-bright LINERs: A Possible Link Between Star Formation, Active Galactic Nucleus Fueling, and Mass AccretionWe present results from a high-resolution X-ray imaging study of nearbyLINERs observed by ACIS on board Chandra. This study complements andextends previous X-ray studies of LINERs, focusing on the underexploredpopulation of nearby dust-enshrouded infrared-bright LINERs. The sampleconsists of 15 IR-bright LINERs (LFIR/LB>3),with distances that range from 11 to 26 Mpc. Combining our sample withprevious Chandra studies, we find that ~51% (28/55) of the LINERsdisplay compact hard X-ray cores. The nuclear 2-10 keV luminosities ofthe galaxies in this expanded sample range from ~2×1038to ~2×1044 ergs s-1. We find that the mostextreme IR-faint LINERs are exclusively active galactic nuclei (AGNs).The fraction of LINERs containing AGNs appears to decrease with IRbrightness and increase again at the highest values ofLFIR/LB. We find that of the 24 LINERs showingcompact nuclear hard X-ray cores in the expanded sample that wereobserved at Hα wavelengths, only eight actually show evidence of abroad line. Similarly, of the 14 LINERs showing compact nuclear hardX-ray cores with corresponding radio observations, only eight display acompact flat spectrum radio core. These findings emphasize the need forhigh-resolution X-ray imaging observations in the study of IR-brightLINERs. Finally, we find an intriguing trend in the Eddington ratioversus LFIR and LFIR/LB for theAGN-LINERs in the expanded sample that extends over 7 orders ofmagnitude in L/LEdd. This correlation may imply a linkbetween black hole growth, as measured by the Eddington ratio, and thestar formation rate, as measured by the far-IR luminosity andIR-brightness ratio. If the far-IR luminosity is an indicator of themolecular gas content in our sample of LINERs, our results may furtherindicate that the mass accretion rate scales with the host galaxy's fuelsupply. We discuss the potential implications of our results in theframework of black hole growth and AGN fueling in low-luminosity AGNs. 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. 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. New H2O masers in Seyfert and FIR bright galaxiesUsing the Effelsberg 100-m telescope, detections of four extragalacticwater vapor masers are reported. Isotropic luminosities are ~50, 1000, 1and 230 Lȯ for Mrk 1066 (UGC 2456), Mrk 34, NGC 3556 andArp 299, respectively. Mrk 34 contains by far the most distant and oneof the most luminous water vapor megamasers so far reported in a Seyfertgalaxy. The interacting system Arp 299 appears to show two maserhotspots separated by approximately 20´´. With these newresults and even more recent data from Braatz et al. (2004, ApJ, 617,L29), the detection rate in our sample of Seyferts with known jet-NarrowLine Region interactions becomes 50% (7/14), while in star forminggalaxies with high (S100~μ m>50 Jy) far infrared fluxesthe detection rate is 22% (10/45). The jet-NLR interaction sample maynot only contain “jet-masers” but also a significant numberof accretion “disk-masers” like those seen in NGC 4258. Astatistical analysis of 53 extragalactic H2O sources (excluding theGalaxy and the Magellanic Clouds) indicates (1) that the correlationbetween IRAS Point Source and H2O luminosities, established forindividual star forming regions in the galactic disk, also holds forAGN-dominated megamaser galaxies; (2) that maser luminosities are notcorrelated with 60 μm/100 μm color temperatures; and (3) that onlya small fraction of the luminous megamasers (L_H_2O > 100Lȯ) detectable with 100-m sized telescopes have so farbeen identified. The H2O luminosity function (LF) suggests that thenumber of galaxies with 1 Lȯ < L_H_2O < 10Lȯ, the transition range between“kilomasers” (mostly star formation) and“megamasers” (active galactic nuclei), is small. The overallslope of the LF, ~-1.5, indicates that the number of detectable masersis almost independent of their luminosity. If the LF is not steepeningat very high maser luminosities and if it is possible to find suitablecandidate sources, H2O megamasers at significant redshifts should bedetectable even with present day state-of-the-art facilities. 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). Bar-induced perturbation strengths of the galaxies in the Ohio State University Bright Galaxy Survey - IBar-induced perturbation strengths are calculated for a well-definedmagnitude-limited sample of 180 spiral galaxies, based on the Ohio StateUniversity Bright Galaxy Survey. We use a gravitational torque method,the ratio of the maximal tangential force to the mean axisymmetricradial force, as a quantitative measure of the bar strength. Thegravitational potential is inferred from an H-band light distribution byassuming that the M/L ratio is constant throughout the disc. Galaxiesare deprojected using orientation parameters based on B-band images. Inorder to eliminate artificial stretching of the bulge, two-dimensionalbar-bulge-disc decomposition has been used to derive a reliable bulgemodel. This bulge model is subtracted from an image, the disc isdeprojected assuming it is thin, and then the bulge is added back byassuming that its mass distribution is spherically symmetric. We findthat removing the artificial bulge stretch is important especially forgalaxies having bars inside large bulges. We also find that the massesof the bulges can be significantly overestimated if bars are not takeninto account in the decomposition.Bars are identified using Fourier methods by requiring that the phasesof the main modes (m= 2, m= 4) are maintained nearly constant in the barregion. With such methods, bars are found in 65 per cent of the galaxiesin our sample, most of them being classified as SB-type systems in thenear-infrared by Eskridge and co-workers. We also suggest that as muchas ~70 per cent of the galaxies classified as SAB-types in thenear-infrared might actually be non-barred systems, many of them havingcentral ovals. It is also possible that a small fraction of the SAB-typegalaxies have weak non-classical bars with spiral-like morphologies. 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. Supernovae 2004gn and 2004goIAUC 8448 available at Central Bureau for Astronomical Telegrams. Circumnuclear Structure and Black Hole Fueling: Hubble Space Telescope NICMOS Imaging of 250 Active and Normal GalaxiesWhy are the nuclei of some galaxies more active than others? If mostgalaxies harbor a central massive black hole, the main difference isprobably in how well it is fueled by its surroundings. We investigatethe hypothesis that such a difference can be seen in the detailedcircumnuclear morphologies of galaxies using several quantitativelydefined features, including bars, isophotal twists, boxy and diskyisophotes, and strong nonaxisymmetric features in unsharp-masked images.These diagnostics are applied to 250 high-resolution images of galaxycenters obtained in the near-infrared with NICMOS on the Hubble SpaceTelescope. To guard against the influence of possible biases andselection effects, we have carefully matched samples of Seyfert 1,Seyfert 2, LINER, starburst, and normal galaxies in their basicproperties, taking particular care to ensure that each was observed witha similar average scale (10-15 pc pixel-1). Severalmorphological differences among our five different spectroscopicclassifications emerge from the analysis. The H II/starburst galaxiesshow the strongest deviations from smooth elliptical isophotes, whilethe normal galaxies and LINERs have the least disturbed morphology. TheSeyfert 2s have significantly more twisted isophotes than any othercategory, and the early-type Seyfert 2s are significantly more disturbedthan the early-type Seyfert 1s. The morphological differences betweenSeyfert 1s and Seyfert 2s suggest that more is at work than simply theviewing angle of the central engine. They may correspond to differentevolutionary stages. Unified One-Dimensional Simulations of Gamma-Ray Line Emission from Type Ia SupernovaeThe light curves of Type Ia supernovae (SNe Ia) are powered by gammarays emitted by the decay of radioactive elements such as56Ni and its decay products. These gamma rays aredownscattered, absorbed, and eventually reprocessed into the opticalemission that makes up the bulk of all SN observations. Detection of thegamma rays that escape the expanding star provide the only direct meansto study this power source for SN Ia light curves. Unfortunately,disagreements between calculations for the gamma-ray lines have made itdifficult to interpret any gamma-ray observations. Here we present adetailed comparison of the major gamma-ray line transport codes for aseries of one-dimensional SN Ia models. Discrepancies in past resultswere due to errors in the codes, and the corrected versions of the sevendifferent codes yield very similar results. This convergence of thesimulation results allows us to infer more reliable information from thecurrent set of gamma-ray observations of SNe Ia. The observations of SN1986G, SN 1991T, and SN 1998bu are consistent with explosion modelsbased on their classification: subluminous, superluminous, and normallyluminous, respectively. Hα Morphologies and Environmental Effects in Virgo Cluster Spiral GalaxiesWe describe the various Hα morphologies of Virgo Cluster andisolated spiral galaxies and associate the Hα morphologies withthe types of environmental interactions that have altered the clustergalaxies. The spatial distributions of Hα and R-band emission areused to divide the star formation morphologies of the 52 Virgo Clusterspiral galaxies into several categories: normal (37%), anemic (6%),enhanced (6%), and (spatially) truncated (52%). Truncated galaxies arefurther subdivided on the basis of their inner star formation rates intotruncated/normal (37%), truncated/compact (6%), truncated/anemic (8%),and truncated/enhanced (2%). The fraction of anemic galaxies isrelatively small (6%-13%) in both environments, suggesting thatstarvation is not a major factor in the reduced star formation rates ofVirgo spiral galaxies. The majority of Virgo spiral galaxies have theirHα disks truncated (52%), whereas truncated Hα disks arerarer in isolated galaxies (12%). Most of the Hα-truncatedgalaxies have relatively undisturbed stellar disks and normal toslightly enhanced inner disk star formation rates, suggesting thatintracluster medium-interstellar medium (ICM-ISM) stripping is the mainmechanism causing the reduced star formation rates of Virgo spiralgalaxies. Several of the truncated galaxies are peculiar, with enhancedcentral star formation rates, disturbed stellar disks, and barlikedistributions of luminous H II complexes inside the central 1 kpc but nostar formation beyond, suggesting that recent tidal interactions orminor mergers have also influenced their morphology. Two highly inclinedHα-truncated spiral galaxies have numerous extraplanar H IIregions and are likely in an active phase of ICM-ISM stripping. Severalspiral galaxies have one-sided Hα enhancements at the outer edgeof their truncated Hα disks, suggesting modest local enhancementsin their star formation rates due to ICM-ISM interactions. Low-velocitytidal interactions and perhaps outer cluster H I accretion seem to bethe triggers for enhanced global star formation in four Virgo galaxies.These results indicate that most Virgo spiral galaxies experienceICM-ISM stripping, many experience significant tidal effects, and manyexperience both. Optical and Infrared Photometry of the Type Ia Supernovae 1991T, 1991bg, 1999ek, 2001bt, 2001cn, 2001cz, and 2002boWe present optical and/or infrared photometry of the Type Ia supernovaeSN 1991T, SN 1991bg, SN 1999ek, SN 2001bt, SN 2001cn, SN 2001cz, and SN2002bo. All but one of these supernovae have decline rate parameters,Δm15(B), close to the median value of 1.1 for the wholeclass of Type Ia supernovae. The addition of these supernovae to therelationship between the near-infrared absolute magnitudes andΔm15(B) strengthens the previous relationships we havefound in that the maximum light absolute magnitudes are essentiallyindependent of the decline rate parameter. (SN 1991bg, the prototype ofthe subclass of fast-declining Type Ia supernovae, is a special case.)The dispersion in the Hubble diagram in JHK is only ~0.15 mag. Thenear-infrared properties of Type Ia supernovae continue to be excellentmeasures of the luminosity distances to the supernova host galaxiesbecause of the need for only small corrections from the epoch ofobservation to maximum light, low dispersion in absolute magnitudes atmaximum light, and the minimal reddening effects in the near-infrared. Nuclear Properties of a Sample of Nearby Spiral Galaxies from Hubble Space Telescope STIS ImagingWe present surface photometry for the central regions of a sample of 48spiral galaxies (mostly unbarred and barred of type Sbc or Sc) observedwith the Space Telescope Imaging Spectrograph on board the Hubble SpaceTelescope. Surface brightness profiles (SBPs) were derived and modeledwith a Nuker law. We also analyzed archival Wide Field Planetary Camera2 images with a larger field of view, which are available for 18galaxies in our sample. We modeled the extracted bulge SBPs with anexponential, an r1/4, or an rn profile. Inagreement with previous studies, we find that bulges of Sbc galaxiesfall into two categories: bulges well described by an exponentialprofile and those well described by an r1/4 profile. Only onegalaxy requires the use of a more general Sérsic profile toproperly describe the bulge. Nuclear photometrically distinct componentsare found in ~55% of the galaxies. For those that we classify as starclusters on the basis of their resolved extent, we find absolutemagnitudes that are brighter on average than those previously identifiedin spiral galaxies. This might be due to a bias in our sample towardstar-forming galaxies, combined with a trend for star-forming galaxiesto host brighter central clusters.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, Inc., under NASAcontract NAS 5-26555. A New Nonparametric Approach to Galaxy Morphological ClassificationWe present two new nonparametric methods for quantifying galaxymorphology: the relative distribution of the galaxy pixel flux values(the Gini coefficient or G) and the second-order moment of the brightest20% of the galaxy's flux (M20). We test the robustness of Gand M20 to decreasing signal-to-noise ratio (S/N) and spatialresolution and find that both measures are reliable to within 10% forimages with average S/N per pixel greater than 2 and resolutions betterthan 1000 and 500 pc, respectively. We have measured G andM20, as well as concentration (C), asymmetry (A), andclumpiness (S) in the rest-frame near-ultraviolet/optical wavelengthsfor 148 bright local normal'' Hubble-type galaxies (E-Sd) galaxies, 22dwarf irregulars, and 73 0.05 Deprojecting spiral galaxies using Fourier analysis. Application to the Ohio sampleWe use two new methods developed recently (Barberàet al.\cite{bar03}, A&A, 415, 849), as well as information obtained fromthe literature, to calculate the orientation parameters of the spiralgalaxies in the Ohio State University Bright Galaxy Survey. We comparethe results of these methods with data from the literature, and find ingeneral good agreement. We provide a homogeneous set of mean orientationparameters which can be used to approximately deproject the disks of thegalaxies and facilitate a number of statistical studies of galaxyproperties.Table 1 is only available in electronic form at the CDS via anonymousftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/421/595
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