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 Faint supernovae and supernova impostors: case studies of SN 2002kg/NGC 2403-V37 and SN 2003gmPhotometric and spectroscopic observations of the faint Supernovae (SNe)2002kg and 2003gm, and their precursors, in NGC 2403 and NGC 5334,respectively, are presented. The properties of these SNe are discussedin the context of previously proposed scenarios for faint SNe: low-massprogenitors producing underenergetic SNe; SNe with ejecta constrained bya circumstellar medium; and outbursts of massive Luminous Blue Variables(LBVs). The last scenario has been referred to as Type V SNe', SNimpostors' or fake SNe'.The faint SN 2002kg reached a maximum brightness of MV =-9.6, much fainter than normal Type II SNe. The precursor of SN 2002kgis confirmed to be, as shown in previous work, the LBV NGC 2403-V37.Late-time photometry of SN 2002kg shows it to be only 0.6 mag fainter at500 d than at the epoch of discovery. Two spectra of SN 2002kg, with anapproximately 1-yr interval between observations, show only minordifferences. Strong FeII lines are observed in the spectra of SN 2002kg,similar to both the LBV NGC 2363-V1 and the Type IIn SN 1995G. Thespectrum of SN 2002kg does show strong resolved [NII] atλλ6549,6583 Å. The identified progenitor of SN2003gm is a bright yellow star, consistent with a F5-G2 supergiant,similar to the identified progenitor of SN 2004et. SN 2003gm, at theepoch of discovery, was of similar brightness to the possible fake SN1997bs and the Type IIP SNe 1999br and 2005cs. Photometrically SN 2003gmshows the same decrease in brightness, over the same time period as SN1997bs. The light curve and the spectral properties of SN 2003gm arealso consistent with some intrinsically faint and low-velocity Type IISNe. The early-time spectra of SN 2003gm are dominated by Balmeremission lines, which at the observed resolution, appear similar to SN2000ch. On the basis of the post-discovery photometric and spectroscopicobservations presented here, we suggest that SN 2003gm is a similarevent to SN 1997bs, although the SN/LBV nature of both of these objectsis debated. At 226 d post-discovery the spectrum of SN 2003gm isstrongle contaminated by HII region emission lines, and it cannot beconfirmed that the precursor star has disappeared. The presence ofstrong [NII] lines, near Hα, is suggested as a possible means ofidentifying objects such as SN 2002kg/NGC 2403-V37 as being LBVs -although not as a general classification criterion of all LBVsmasquerading as SNe. 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. 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. 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. Quantitative spectroscopy of BA-type supergiantsLuminous BA-type supergiants have enormous potential for modernastrophysics. They allow topics ranging from non-LTE physics and theevolution of massive stars to the chemical evolution of galaxies andcosmology to be addressed. A hybrid non-LTE technique for thequantitative spectroscopy of these stars is discussed. Thorough testsand first applications of the spectrum synthesis method are presentedfor the bright Galactic objects η Leo (A0 Ib), HD 111613 (A2 Iabe),HD 92207 (A0 Iae) and β Ori (B8 Iae), based on high-resolution andhigh-S/N Echelle spectra. Stellar parameters are derived fromspectroscopic indicators, consistently from multiple non-LTE ionizationequilibria and Stark-broadened hydrogen line profiles, and they areverified by spectrophotometry. The internal accuracy of the methodallows the 1σ-uncertainties to be reduced to 1-2% in T_effand to 0.05-0.10 dex in log g. Elemental abundances are determined forover 20 chemical species, with many of the astrophysically mostinteresting in non-LTE (H, He, C, N, O, Mg, S, Ti, Fe). The non-LTEcomputations reduce random errors and remove systematic trends in theanalysis. Inappropriate LTE analyses tend to systematicallyunderestimate iron group abundances and overestimate the light andα-process element abundances by up to factors of two to three onthe mean. This is because of the different responses of these species toradiative and collisional processes in the microscopic picture, which isexplained by fundamental differences of their detailed atomic structure,and not taken into account in LTE. Contrary to common assumptions,significant non-LTE abundance corrections of ~0.3 dex can be found evenfor the weakest lines (Wλ 10 mÅ). Non-LTEabundance uncertainties amount to typically 0.05-0.10 dex (random) and~0.10 dex (systematic 1σ-errors). Near-solar abundances arederived for the heavier elements in the sample stars, and patternsindicative of mixing with nuclear-processed matter for the lightelements. These imply a blue-loop scenario for η Leo because offirst dredge-up abundance ratios, while the other three objects appearto have evolved directly from the main sequence. In the most ambitiouscomputations several ten-thousand spectral lines are accounted for inthe spectrum synthesis, permitting the accurate reproduction of theentire observed spectra from the visual to near-IR. This prerequisitefor the quantitative interpretation of intermediate-resolution spectraopens up BA-type supergiants as versatile tools for extragalacticstellar astronomy beyond the Local Group. The technique presented hereis also well suited to improve quantitative analyses of less extremestars of similar spectral types. Measuring improved distances to nearby galaxies: Thae Araucaria project.Not Available First Results from THINGS: The HI Nearby Galaxy SurveyWe describe The HI Nearby Galaxy Survey (THINGS), the largestprogramever undertaken at the Very Large Array to perform 21-cm HIobservations of thehighest quality ( 7'', ≤ 5 km s^{-1}resolution) ofnearby galaxies. The goal of THINGS is to investigatekeycharacteristics related to galaxy morphology, star formation andmassdistribution across the Hubble sequence. A sample of 34 objectswithdistances between 3 and 10 Mpc will be observed, covering a widerangeof evolutionary stages and properties. Data from THINGSwillcomplement SINGS, the Spitzer Infrared Nearby Galaxy Survey. Forthe THINGS sample, high-quality observations at comparable resolutionwillthus be available from the X-ray regime through to the radio partofthe spectrum. THINGS data can be used to investigate issues such asthesmall-scale structure of the ISM, its three-dimensional structure,the(dark) matter distribution and processes leading to starformation. Todemonstrate the quality of the THINGS data products, wepresent someprelimary HI maps here of four galaxies from the THINGSsample. 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. Light-to-Mass Variations with EnvironmentLarge and well-defined variations exist between the distribution of massand the light of stars on extragalactic scales. Mass concentrations inthe range 1012-1013 Msolar manifest themost light per unit mass. Group halos in this range are typically thehosts of spiral and irregular galaxies with ongoing star formation. Onaverage M/LB~90 Msolar/Lsolar in thesegroups . More massive halos have less light per unit mass. Within agiven mass range, halos that are dynamically old as measured by crossingtimes and galaxy morphologies have distinctly less light per unit mass.At the other end of the mass spectrum, below 1012Msolar, there is a cutoff in the manifestation of light.Group halos in the range 1011-1012Msolar can host dwarf galaxies but with such low luminositiesthat M/LB values can range from several hundred to severalthousand. It is suspected that there must be completely dark halos atlower masses. Given the form of the halo mass function, the low relativeluminosities of the high-mass halos have the greatest cosmologicalimplications. Of order half the clustered mass may reside in halos withgreater than 1014 Msolar. By contrast, only 5%-10%of clustered mass would lie in entities with less than 1012Msolar. 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. 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. A survey for OB associations in the Sculptor Group spiral galaxy NGC 7793We report on the results from application of an objective algorithm(PLC) to find OB associations, to B and V images of the Sculptor spiralgalaxy NGC 7793, which were obtained with the ESO VLT and FORSinstrument and basically cover the entire spatial extent of the galaxy.We detected 148 associations. Statistical tests show that less than 6 ofthese detections are caused by randomly concentrated blue stars. In thesize distribution, a sharp peak is observed at a value of about 35microradians, which corresponds to a linear diameter of 135 pc, assuminga distance of 3.91 Mpc to the galaxy. We also find 25 much largerobjects. A second application of the PLC technique shows that 20 of themare stellar complexes consisting of multiple sub-associations withtypical sizes on the order of 130 pc. A comparison of the sizedistribution of the detected OB associations in NGC 7793 with observeddistributions in other galaxies suggests that the conditions in twoSculptor Group galaxies (NGC 300 and NGC 7793) favour the formation oflarge associations. We provide a catalog giving coordinates and physicalparameters for all the associations and stellar complexes we have foundin our survey. Spectroscopy of Extragalactic Planetary Nebulae as Tracers of Intermediate Age and Old Stellar PopulationsNot Available 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. The Effect of Metallicity on Cepheid-based DistancesWe have used the Wide Field Planetary Camera 2 on board the Hubble SpaceTelescope to obtain V and I images of seven nearby galaxies. For each,we have measured a distance using the tip of the red giant branch (TRGB)method. By comparing the TRGB distances with published Cepheiddistances, we investigate the metallicity dependence of the Cepheidperiod-luminosity relation. Our sample is supplemented by 10 additionalgalaxies for which both TRGB and Cepheid distances are available in theliterature, thus providing a uniform coverage in Cepheid abundancesbetween 1/20 and 2 (O/H)solar. We find that the differencebetween Cepheid and TRGB distances decreases monotonically withincreasing Cepheid abundance, consistent with a mean metallicitydependence of the Cepheid distance moduli ofδ(m-M)/δ[O/H]=-0.24+/-0.05 mag dex-1.Based on observations made with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS5-26555. These observations are associated with programGO-8584. Spectrophotometry of Planetary Nebulae in the Bulge of M31We introduce crowded-field integral field (3D) spectrophotometry as auseful technique for the study of resolved stellar populations in nearbygalaxies. The spectroscopy of individual extragalactic stars, which isnow feasible with efficient instruments and large telescopes, isconfronted with the observational challenge of accurately subtractingthe bright, spatially and wavelength-dependent nonuniform background ofthe underlying galaxy. As a methodological test, we present a pilotstudy with selected extragalactic planetary nebulae (XPNe) in the bulgeof M31, demonstrating how 3D spectroscopy is able to improve the limitedaccuracy of background subtraction that one would normally obtain withclassical slit spectroscopy. It is shown that because of the absence ofslit effects, 3D spectroscopy is a most suitable technique forspectrophometry. We present spectra and line intensities for five XPNein M31, obtained with the MPFS instrument at the Russian 6 m BolshoiTeleskop Azimutal'nij, INTEGRAL at the William Herschel Telescope , andPMAS at the Calar Alto 3.5 m telescope. The results for two of ourtargets, for which data are available in the literature, are comparedwith previously published emission-line intensities. The three remainingPNe have been observed spectroscopically for the first time. One objectis shown to be a previously misidentified supernova remnant. Ourmonochromatic Hα maps are compared with direct Fabry-Pérotand narrowband filter images of the bulge of M31, verifying the presenceof filamentary emission of the interstellar medium in the vicinity ofour objects. We present an example of a flux-calibrated andcontinuum-subtracted filament spectrum and demonstrate how theinterstellar medium component introduces systematic errors in themeasurement of faint diagnostic PN emission lines when conventionalobserving techniques are employed. It is shown how these errors can beeliminated with 3D spectroscopy, using the full two-dimensional spatialinformation and point-spread function (PSF) fitting techniques. Using 3Dspectra of bright standard stars, we demonstrate that the PSF is sampledwith high accuracy, providing a centroiding precision at themilliarcsecond level. Crowded-field 3D spectrophotometry and the use ofPSF fitting techniques is suggested as the method of choice for a numberof similar observational problems, including luminous stars in nearbygalaxies, supernovae, QSO host galaxies, gravitationally lensed QSOs,and others. On the Photometric Variability of Blue Supergiants in NGC 300 and Its Impact on the Flux-weighted Gravity-Luminosity RelationshipWe present a study of the photometric variability of spectroscopicallyconfirmed supergiants in NGC 300, comprising 28 epochs extending over aperiod of 5 months. We find 15 clearly photometrically variable bluesupergiants in a sample of nearly 70 such stars, showing maximum lightamplitudes ranging from 0.08 to 0.23 mag in the V band, and one variablered supergiant. We show their light curves and determine semiperiods fortwo A2 Ia stars. Assuming that the observed changes correspond tosimilar variations in bolometric luminosity, we test for the influenceof this variability on the flux-weighted gravity-luminosity relationshipand find a negligible effect, showing that the calibration of thisrelationship, which has the potential to measure extragalactic distancesat the Cepheid accuracy level, is not affected by stellar photometricvariability in any significant way.Based on observations obtained with the 2.2 m ESO/MPI telescope at theEuropean Southern Observatory as part of proposal 163.N-0210. Classical Cepheids and the Distances of HST Program GalaxiesThe distances of HST program galaxies are revised using the PL-relationswe have obtained previously along with a different method from thatemployed by Freedman et al. On the average, the resulting distances tothese galaxies have higher internal accuracies than those obtainedbefore by others. In addition, we have used no corrections formetallicity or for the incompleteness of the samples of classicalcepheids in deriving these distances. Despite this, our distance moduli,with a dispersion of ±0m.395, agree with those of Freedman et al.This indicates that these two effects have little or even no effect forthe samples of classical cepheids in the HST program galaxies. A Catalog of Neighboring GalaxiesWe present an all-sky catalog of 451 nearby galaxies, each having anindividual distance estimate D<~10 Mpc or a radial velocityVLG<550 km s-1. The catalog contains data onbasic optical and H I properties of the galaxies, in particular, theirdiameters, absolute magnitudes, morphological types, circumnuclearregion types, optical and H I surface brightnesses, rotationalvelocities, and indicative mass-to-luminosity and H I mass-to-luminosityratios, as well as a so-called tidal index, which quantifies the galaxyenvironment. We expect the catalog completeness to be roughly 70%-80%within 8 Mpc. About 85% of the Local Volume population are dwarf (dIr,dIm, and dSph) galaxies with MB>-17.0, which contributeabout 4% to the local luminosity density, and roughly 10%-15% to thelocal H I mass density. The H I mass-to-luminosity and the H Imass-to-total (indicative) mass ratios increase systematically fromgiant galaxies toward dwarfs, reaching maximum values about 5 in solarunits for the most tiny objects. For the Local Volume disklike galaxies,their H I masses and angular momentum follow Zasov's linear relation,expected for rotating gaseous disks being near the threshold ofgravitational instability, favorable for active star formation. We foundthat the mean local luminosity density exceeds 1.7-2.0 times the globaldensity, in spite of the presence of the Tully void and the absence ofrich clusters in the Local Volume. The mean local H I density is 1.4times its global'' value derived from the H I Parkes Sky Survey.However, the mean local baryon densityΩb(<8Mpc)=2.3% consists of only a half of the globalbaryon density, Ωb=(4.7+/-0.6)% (Spergel et al.,published in 2003). The mean-square pairwise difference of radialvelocities is about 100 km s-1 for spatial separations within1 Mpc, increasing to ~300 km s-1 on a scale of ~3 Mpc. alsoWe calculated the integral area of the sky occupied by the neighboringgalaxies. Assuming the H I size of spiral and irregular galaxies to be2.5 times their standard optical diameter and ignoring any evolutioneffect, we obtain the expected number of the line-of-sight intersectionswith the H I galaxy images to be dn/dz~0.4, which does not contradictthe observed number of absorptions in QSO spectra. Oxygen and nitrogen abundances in nearby galaxies. Correlations between oxygen abundance and macroscopic propertiesWe performed a compilation of more than 1000 published spectra of H IIregions in spiral galaxies. The oxygen and nitrogen abundances in each HII region were recomputed in a homogeneous way, using the P-method. Theradial distributions of oxygen and nitrogen abundances were derived. Thecorrelations between oxygen abundance and macroscopic properties areexamined. We found that the oxygen abundance in spiral galaxiescorrelates with its luminosity, rotation velocity, and morphologicaltype: the correlation with the rotation velocity may be slightlytighter. There is a significant difference between theluminosity-metallicity relationship obtained here and that based on theoxygen abundances determined through the R23-calibrations.The oxygen abundance of NGC 5457 recently determined using directmeasurements of Te (Kennicutt et al. \cite{Kennicutt2003})agrees with the luminosity-metallicity relationship derived in thispaper, but is in conflict with the luminosity-metallicity relationshipderived with the R23-based oxygen abundances. The obtainedluminosity-metallicity relation for spiral galaxies is compared to thatfor irregular galaxies. Our sample of galaxies shows evidence that theslope of the O/H - MB relationship for spirals (-0.079± 0.018) is slightly more shallow than that for irregulargalaxies (-0.139 ± 0.011). The effective oxygen yields wereestimated for spiral and irregular galaxies. The effective oxygen yieldincreases with increasing luminosity from MB  -11 toMB  -18 (or with increasing rotation velocity fromVrot  10 km s-1 to Vrot  100km s-1) and then remains approximately constant. Irregulargalaxies from our sample have effective oxygen yields lowered by afactor of 3 at maximum, i.e. irregular galaxies usually keep at least1/3 of the oxygen they manufactured during their evolution.Appendix, Tables \ref{table:refero}, \ref{table:referV}, and Figs.\ref{figure:sample2}-\ref{figure:sample5} are only available inelectronic form at http://www.edpsciences.org} The structure and environment of young stellar clusters in spiral galaxiesA search for stellar clusters has been carried out in 18 nearby spiralgalaxies, using archive images from the Wide Field Planetary Camera 2 onboard the Hubble Space Telescope. All of the galaxies have previouslybeen imaged from the ground in UBVI. A catalogue of structuralparameters, photometry and comments based on visual inspection of theclusters is compiled and used to investigate correlations betweencluster structure, environment, age and mass. Least-squares fits to thedata suggest correlations between both the full-width at half-maximum(FWHM) and half-light radius (Reff) of the clusters and theirmasses (M) at about the 3σ level. Although both relations show alarge scatter, the fits have substantially shallower slopes than for aconstant-density relation (size ∝ M1/3). However, manyof the youngest clusters have extended halos which make theReff determinations uncertain. There is no evidence forgalaxy-to-galaxy variations in the mean cluster sizes. In particular,the mean sizes do not appear to depend on the host galaxy star formationrate surface density. Many of the youngest objects (age <107 years) are located in strongly crowded regions, and about1/3-1/2 of them are double or multiple sources. The HST images are alsoused to check the nature of cluster candidates identified in a previousground-based survey. The contamination rate in the ground-based sampleis generally less than about 20%, but some cluster identificationsremain ambiguous because of crowding even with HST imaging, especiallyfor the youngest objects.Full Tables \ref{tab:all}-\ref{tab:hstphot}, and \ref{tab:gb} are onlyavailable in electronic form at the CDS via anonymous ftp tocdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/416/537Based on observations obtained with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute, which is operated bythe Association of Universities for Research in Astronomy, Inc., underNASA contract NAS 5-26555. Stellar astrophysics in the Local Group and beyond with the GTCIn this review I discuss the capabilities that the GTC, and inparticular the OSIRIS spectrometer, will bring to studying massivestellar populations within Local Group galaxies and even beyond. Byobserving massive stars in other dwarf irregular and spiral galaxies onecan probe star formation and stellar evolution in extreme environments,the wind properties of massive luminous stars, and determine distancesto an accuracy of  10%. SINGS: The SIRTF Nearby Galaxies SurveyThe SIRTF Nearby Galaxy Survey is a comprehensive infrared imaging andspectroscopic survey of 75 nearby galaxies. Its primary goal is tocharacterize the infrared emission of galaxies and their principalinfrared-emitting components, across a broad range of galaxy propertiesand star formation environments. SINGS will provide new insights intothe physical processes connecting star formation to the interstellarmedium properties of galaxies and provide a vital foundation forunderstanding infrared observations of the distant universe andultraluminous and active galaxies. The galaxy sample and observingstrategy have been designed to maximize the scientific and archivalvalue of the data set for the SIRTF user community at large. The SIRTFimages and spectra will be supplemented by a comprehensivemultiwavelength library of ancillary and complementary observations,including radio continuum, H I, CO, submillimeter, BVRIJHK, Hα,Paα, ultraviolet, and X-ray data. This paper describes the mainastrophysical issues to be addressed by SINGS, the galaxy sample and theobserving strategy, and the SIRTF and other ancillary data products. Quantitative Spectroscopy of SupergiantsBlue supergiants of spectral types B and A are the visually brighteststars in spiral and irregular galaxies, with their most luminous members(at M_V=-10) outshining entire dwarf galaxies. This characteristicallows us to use them as probes to study the Local Universe in greatdetail. In principle, already the existing large telescopes andinstrumentation facilitate quantitative spectroscopy of these objects asfar as the Virgo and Fornax clusters of galaxies. Beyond theirchallenging stellar atmospheres and opportunities for testingsophisticated non-LTE physics they offer numerous applications to modernastrophysics. Quantitative spectroscopy of supergiants can contribute toimprove our understanding of massive star evolution. Galactic abundancegradients and abundance patterns, as can be obtained from studies oflarge ensembles of supergiants in our own and other galaxies, willfoster the understanding of galactochemical evolution. Finally, they arepromising independent indicators for calibrating the extragalacticdistance scale, by application of the wind momentum-luminosity and theflux-weighted gravity-luminosity relationships. In view of this largepotential, the objective of this thesis is to improve the status ofquantitative spectroscopy of BA-type supergiants and to provide firstapplications on a sample of Galactic and extragalactic targets, withinthe Local Group and beyond. Characteristics and classification of A-type supergiants in the Small Magellanic CloudWe address the relationship between spectral type and physicalproperties for A-type supergiants in the Small Magellanic Cloud (SMC).First, we construct a self-consistent classification scheme for Asupergiants, employing the calcium K to Hɛ line ratio as atemperature-sequence discriminant. Following the precepts of the MKprocess', the same morphological criteria are applied to Galactic andSMC spectra, with the understanding that there may not be acorrespondence in physical properties between spectral counterparts indifferent environments. Then we discuss the temperature scale,concluding that A supergiants in the SMC are systematically cooler thantheir Galactic counterparts at the same spectral type, by up to ~10 percent. Considering the relative line strengths of Hγ and the CH Gband, we extend our study to F- and early G-type supergiants, for whichsimilar effects are found. We note the implications for analyses ofluminous extragalactic supergiants, for the flux-weightedgravity-luminosity relationship and for population synthesis studies inunresolved stellar systems. The column density distribution function at z= 0 from HI selected galaxiesWe have measured the column density distribution function, f(NHI), at z= 0 using 21-cm HI emission from galaxies selected from ablind HI survey. f(NH I) is found to be smaller and flatterat z= 0 than indicated by high-redshift measurements of damped Lymanα (DLA) systems, consistent with the predictions of hierarchicalgalaxy formation. The derived DLA number density per unit redshift,dNDLA/dz= 0.058, is in moderate agreement with valuescalculated from low-redshift QSO absorption line studies. We use twodifferent methods to determine the types of galaxies which contributemost to the DLA cross-section: comparing the power-law slope off(NH I) to theoretical predictions and analysingcontributions to dNDLA/dz. We find that comparison of thepower-law slope cannot rule out spiral discs as the dominant galaxy typeresponsible for DLA systems. Analysis of dNDLA/dz however, ismuch more discriminating. We find that galaxies with log MHI< 9.0 make up 34 per cent of dNDLA/dz Irregular andMagellanic types contribute 25 per cent; galaxies with surfacebrightness account for 22 per cent and sub-L* galaxiescontribute 45 per cent to dNDLA/dz. We conclude that a largerange of galaxy types give rise to DLA systems, not just large spiralgalaxies as previously speculated.
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 קבוצת-כוכבים: נחש מים התרוממות ימנית: 11h18m16.20s סירוב: -32Â°48'43.0" גודל גלוי: 10.965′ × 4.786′

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