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The Survey for Ionization in Neutral Gas Galaxies. I. Description and Initial Results
We introduce the Survey for Ionization in Neutral Gas Galaxies (SINGG),a census of star formation in H I-selected galaxies. The survey consistsof Hα and R-band imaging of a sample of 468 galaxies selected fromthe H I Parkes All Sky Survey (HIPASS). The sample spans three decadesin H I mass and is free of many of the biases that affect otherstar-forming galaxy samples. We present the criteria for sampleselection, list the entire sample, discuss our observational techniques,and describe the data reduction and calibration methods. This paperfocuses on 93 SINGG targets whose observations have been fully reducedand analyzed to date. The majority of these show a single emission linegalaxy (ELG). We see multiple ELGs in 13 fields, with up to four ELGs ina single field. All of the targets in this sample are detected inHα, indicating that dormant (non-star-forming) galaxies withMHI>~3×107 Msolar are veryrare. A database of the measured global properties of the ELGs ispresented. The ELG sample spans 4 orders of magnitude in luminosity(Hα and R band), and Hα surface brightness, nearly 3 ordersof magnitude in R surface brightness and nearly 2 orders of magnitude inHα equivalent width (EW). The surface brightness distribution ofour sample is broader than that of the Sloan Digital Sky Survey (SDSS)spectroscopic sample, the EW distribution is broader than prism-selectedsamples, and the morphologies found include all common types ofstar-forming galaxies (e.g., irregular, spiral, blue compact dwarf,starbursts, merging and colliding systems, and even residual starformation in S0 and Sa spirals). Thus, SINGG presents a superior censusof star formation in the local universe suitable for further studiesranging from the analysis of H II regions to determination of the localcosmic star formation rate density.
| Cepheid Distances to SNe Ia Host Galaxies Based on a Revised Photometric Zero Point of the HST WFPC2 and New PL Relations and Metallicity Corrections
With this paper we continue the preparation for a forthcoming summaryreport of our experiment with the HST to determine the Hubble constantusing Type Ia supernovae as standard candles. Two problems areaddressed. (1) We examine the need for, and determine the value of, thecorrections to the apparent magnitudes of our program Cepheids in the 11previous calibration papers due to sensitivity drifts and chargetransfer effects of the HST WFPC2 camera over the life time of theexperiment from 1992 to 2001. (2) The corrected apparent magnitudes areapplied to all our previous photometric data from which revised distancemoduli are calculated for the eight program galaxies that are parents tothe calibrator Ia supernovae. Two different Cepheid P-L relations areused; one for the Galaxy and one for the LMC. These differ both in slopeand zero point at a fixed period. The procedures for determining theabsorption and reddening corrections for each Cepheid are discussed.Corrections for the effects of metallicity differences between theprogram galaxies and the two adopted P-L relations are derived andapplied. The distance moduli derived here for the eight supernovaeprogram galaxies, and for 29 others, average 0.20 mag fainter (moredistant) than those derived by Gibson et al. and Freedman et al. intheir 2000 and 2001 summary papers for reasons discussed in this paper.The effect on the Hubble constant is the subject of our forthcomingsummary paper.
| A Comparison of Hα and Stellar Scale Lengths in Virgo and Field Spirals
The scale lengths of the old stars and ionized gas distributions arecompared for similar samples of Virgo Cluster members and field spiralgalaxies via Hα and broad R-band surface photometry. While theR-band and Hα scale lengths are, on average, comparable for thecombined sample, we find significant differences between the field andcluster samples. While the Hα scale lengths of the field galaxiesare a factor of 1.14+/-0.07 longer, on average, than their R-band scalelengths, the Hα scale lengths of Virgo Cluster members are, onaverage, 20% smaller than their R-band scale lengths. Furthermore, inVirgo, the scale length ratios are correlated with the size of thestar-forming disk: galaxies with smaller overall Hα extents alsoshow steeper radial falloff of star formation activity. At the sametime, we find no strong trends in scale length ratio as a function ofother galaxy properties, including galaxy luminosity, inclination,morphological type, central R-band light concentration, or bar type. Ourresults for Hα emission are similar to other results for dustemission, suggesting that Hα and dust have similar distributions.The environmental dependence of the Hα scale length placesadditional constraints on the evolutionary process(es) that cause gasdepletion and a suppression of the star formation rate in clusters ofgalaxies.
| Hydrogen and helium traces in type Ib-c supernovae
Aims.To investigate the spectroscopic properties of a selected opticalphotospheric spectra of core collapse supernovae (CCSNe). Specialattention is devoted to traces of hydrogen at early phases. The impacton the physics and nature of their progenitors is emphasized.Methods: .The CCSNe-sample spectra are analyzed with the parameterizedsupernova synthetic spectrum code "SYNOW" adopting some simplifyingapproximations. Results: .The generated spectra are found to matchthe observed ones reasonably well, including a list of only 23 candidateions. Guided by SN Ib 1990I, the observed trough near 6300 Å isattributed to Hα in almost all type Ib events, although in someobjects it becomes too weak to be discernible, especially at laterphases. Alternative line identifications are discussed. Differences inthe way hydrogen manifests its presence within CCSNe are highlighted. Intype Ib SNe, the Hα contrast velocity (i.e. line velocity minusthe photospheric velocity) seems to increase with time at early epochs,reaching values as high as 8000 km s-1 around 15-20 daysafter maximum and then remains almost constant. The derived photosphericvelocities, indicate a lower velocity for type II SNe 1987A and 1999emas compared to SN Ic 1994I and SN IIb 1993J, while type Ib eventsdisplay a somewhat larger variation. The scatter, around day 20, ismeasured to be ~5000 km s-1. Following two simple approaches,rough estimates of ejecta and hydrogen masses are given. A mass ofhydrogen of approximately 0.02 M_ȯ is obtained for SN 1990I, whileSNe 1983N and 2000H ejected ~0.008 M_ȯ and ~0.08 M_ȯ ofhydrogen, respectively. SN 1993J has a higher hydrogen mass, ~0.7M_ȯ with a large uncertainty. A low mass and thin hydrogen layerwith very high ejection velocities above the helium shell, is thus themost likely scenario for type Ib SNe. Some interesting and curiousissues relating to oxygen lines suggest future investigations.
| Massive star formation in the central regions of spiral galaxies
Context: . The morphology of massive star formation in the centralregions of galaxies is an important tracer of the dynamical processesthat govern the evolution of disk, bulge, and nuclear activity. Aims. Wepresent optical imaging of the central regions of a sample of 73 spiralgalaxies in the Hα line and in optical broad bands, and deriveinformation on the morphology of massive star formation. Methods. Weobtained images with the William Herschel Telescope, mostly at a spatialresolution of below one second of arc. For most galaxies, no Hαimaging is available in the literature. We outline the observing anddata reduction procedures, list basic properties, and present the I-bandand continuum-subtracted Hα images. We classify the morphology ofthe nuclear and circumnuclear Hα emission and explore trends withhost galaxy parameters. Results. We confirm that late-type galaxies havea patchy circumnuclear appearance in Hα, and that nuclear ringsoccur primarily in spiral types Sa-Sbc. We identify a number ofpreviously unknown nuclear rings, and confirm that nuclear rings arepredominantly hosted by barred galaxies. Conclusions. Other than instimulating nuclear rings, bars do not influence the relative strengthof the nuclear Hα peak, nor the circumnuclear Hα morphology.Even considering that our selection criteria led to an over-abundance ofgalaxies with close massive companions, we do not find any significantinfluence of the presence or absence of a close companion on therelative strength of the nuclear Hα peak, nor on the Hαmorphology around the nucleus.
| Quantitative spectroscopic analysis of and distance to SN1999em
Multi-epoch multi-wavelength spectroscopic observations ofphotospheric-phase type II supernovae (SN) provide information onmassive-star progenitor properties, the core-collapse mechanism, anddistances in the Universe. Following successes of recent endeavors(Dessart & Hillier 2005a, A&A, 437, 667; 2005b, A&A, 439,671) with the non-LTE model atmosphere code CMFGEN (Hillier & Miller1998, ApJ, 496, 407), we present a detailed quantitative spectroscopicanalysis of the type II SN1999em and, using the Expanding PhotosphereMethod (EPM) or synthetic fits to observed spectra, à la Baron etal. (2004, ApJ, 616, 91), we estimate its distance. Selecting eightepochs, which cover the first 38 days after discovery, we obtainsatisfactory fits to optical spectroscopic observations of SN1999em(including the UV and near-IR ranges when available). We use the sameiron-group metal content for the ejecta, the same power-law densitydistribution (with exponent n = 10{-}12), and a Hubble-velocity law atall times. We adopt a H/He/C/N/O abundance pattern compatible withCNO-cycle equilibrium values for a RSG/BSG progenitor, with C/O enhancedand N depleted at later times. The overall evolution of the spectralenergy distribution, whose peak shifts to longer wavelengths as timeprogresses, reflects the steady temperature/ionization-level decrease ofthe ejecta, associated non-linearly with a dramatic shift to ions withstronger line-blocking powers in the UV and optical (Fe ii, Tiii). Inthe parameter space investigated, CMFGEN is very sensitive and providesphotospheric temperatures and velocities, reddenings, and the H/Heabundance ratio with an accuracy of ±500 K, ±10%, 0.05 and50%, respectively. Following Leonard et al. (2002, PASP, 114, 35), andtheir use of correction factors from Hamuy et al. (2001, ApJ, 558, 615),we estimate an EPM distance to SN1999em that also falls 30% short of theCepheid distance of 11.7 Mpc to its host galaxy NGC 1637 (Leonard et al.2003, ApJ, 594, 247). However, using the systematically highercorrection factors of Dessart & Hillier (2005b) removes thediscrepancy. A significant scatter, arising primarily from errors in thecorrection factors and derived temperatures, is seen in distancesderived using different band passes. However, adopting both correctionfactors and corresponding color-temperatures from tailored models toeach observation leads to a good agreement between distance estimatesobtained from different band passes. The need for detailed modelcomputations thus defeats the appeal and simplicity of the original EPMmethod, which uses tabulated correction factors and broadband fluxes,for distance determinations. However, detailed fits to SN opticalspectra, based on tailored models for individual SN observations, offersa promising approach to obtaining accurate distances, either through theEPM or via the technique of Baron et al. (2004). Our bestdistance-estimate to SN1999em is 11.5 ± 1.0 Mpc. We note that toachieve 10-20% accuracy in such distance estimates requires multipleobservations, covering preferentially a range of early epochs precedingthe hydrogen-recombination phase.
| Determination of Cepheid parameters by light-curve template fitting
We describe techniques to characterize the light curves of regularvariable stars by applying principal component analysis (PCA) to atraining set of high-quality data, and to fit the resulting light-curvetemplates to sparse and noisy photometry to obtain parameters such asperiods, mean magnitudes etc. The PCA approach allows us to efficientlyrepresent the multiband light-curve shapes (LCSs) of each variable, andhence quantitatively describe the average behaviour of the sample as asmoothly varying function of period, and also the range of variationaround this average.In this paper we focus particularly on the utility of such methods foranalysing Hubble Space Telescope (HST) Cepheid photometry, and presentsimulations which illustrate the advantages of our PCA template-fittingapproach. These are: accurate parameter determination, including LCSinformation; simultaneous fitting to multiple passbands; quantitativeerror analysis; objective rejection of variables with non-Cepheid-likelight curves or those with potential period aliases.We also use PCA to confirm that Cepheid LCSs are systematicallydifferent (at the same period) between the Milky Way and the Large andSmall Magellanic Clouds, and consider whether LCS might therefore beused to estimate the mean metallicities of Cepheid samples, thusallowing metallicity corrections to be applied to derived distanceestimates.
| X-ray observations of the edge-on star-forming galaxy NGC 891 and its supernova SN1986J
We present XMM-Newton observations of NGC 891, a nearby edge-on spiralgalaxy. We analyse the extent of the diffuse emission emitted from thedisc of the galaxy, and find that it has a single-temperature profilewith best-fitting temperature of 0.26 keV, though the fit of adual-temperature plasma with temperatures of 0.08 and 0.30 keV is alsoacceptable. There is a considerable amount of diffuse X-ray emissionprotruding from the disc in the north-west direction out toapproximately 6 kpc. We analyse the point-source population using aChandra observation, using a maximum-likelihood method to find that theslope of the cumulative luminosity function of point sources in thegalaxy is -0.77+0.13-0.1. Using a sample of otherlocal galaxies, we compare the X-ray and infrared properties of NGC 891with those of `normal' and starburst spiral galaxies, and conclude thatNGC 891 is most likely a starburst galaxy in a quiescent state. Weestablish that the diffuse X-ray luminosity of spirals scales with thefar-infrared luminosity asLX~L0.87+/-0.07FIR, except for extremestarbursts, and NGC 891 does not fall in the latter category. We studythe supernova SN1986J in both XMM-Newton and Chandra observations, andfind that the X-ray luminosity has been declining with time more steeplythan expected (LX~t-3).
| A study of the Type II-P supernova 2003gd in M74
We present photometric and spectroscopic data of the Type II-P supernova(SN II-P) 2003gd, which was discovered in M74 close to the end of itsplateau phase. SN 2003gd is the first Type II supernova (SN) to have adirectly confirmed red supergiant (RSG) progenitor. We compare SN 2003gdto SN 1999em, a similar SN II-P, and estimate an explosion date of 2003March 18. We determine a reddening towards the SN of E(B-V) = 0.14 +/-0.06, using three different methods. We also calculate three newdistances to M74 of 9.6 +/- 2.8, 7.7 +/- 1.7 and 9.6 +/- 2.2Mpc. Theformer was estimated using the standard candle method (SCM), for Type IIsupernovae (SNe II), and the latter two using the brightest supergiantsmethod (BSM). When combined with existing kinematic and BSM distanceestimates, we derive a mean value of 9.3 +/- 1.8Mpc. SN 2003gd was foundto have a lower tail luminosity compared with other normal Type II-Psupernovae (SNe II-P) bringing into question the nature of this SN. Wepresent a discussion concluding that this is a normal SN II-P, which isconsistent with the observed progenitor mass of8+4-2 Msolar.
| Parameters of the classical type-IIP supernova SN 1999em
Based on observations of SN 1999em, we determined the physicalparameters of this supernova using hydrodynamic calculations includingnonequilibrium radiative transfer. Taking the distance to SN 1999emestimated by the expanding photosphere method (EPM) to be D = 7.5 Mpc,we found the parameters of the presupernova: radius R = 450R ȯ,mass M = 15M ȯ, and explosion energy E = 7 × 1050 erg. Forthe distance D = 12 Mpc determined from Cepheids, R, M, and E must beincreased to the following values: R = 1000R ȯ, M = 18M ȯ, andE = 1051 erg. We show that one cannot restrict oneself to using thesimple analytical formulas relating the supernova and presupernovaparameters to obtain reliable parameters for type-IIP presupernovae.
| Cepheid Calibrations from the Hubble Space Telescope of the Luminosity of Two Recent Type Ia Supernovae and a Redetermination of the Hubble Constant
We 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.
| The Distribution of Bar and Spiral Arm Strengths in Disk Galaxies
The distribution of bar strengths in disk galaxies is a fundamentalproperty of the galaxy population that has only begun to be explored. Wehave applied the bar-spiral separation method of Buta and coworkers toderive the distribution of maximum relative gravitational bar torques,Qb, for 147 spiral galaxies in the statistically well-definedOhio State University Bright Galaxy Survey (OSUBGS) sample. Our goal isto examine the properties of bars as independently as possible of theirassociated spirals. We find that the distribution of bar strengthdeclines smoothly with increasing Qb, with more than 40% ofthe sample having Qb<=0.1. In the context of recurrent barformation, this suggests that strongly barred states are relativelyshort-lived compared to weakly barred or nonbarred states. We do notfind compelling evidence for a bimodal distribution of bar strengths.Instead, the distribution is fairly smooth in the range0.0<=Qb<0.8. Our analysis also provides a first look atspiral strengths Qs in the OSUBGS sample, based on the sametorque indicator. We are able to verify a possible weak correlationbetween Qs and Qb, in the sense that galaxies withthe strongest bars tend to also have strong spirals.
| The Opacity of Spiral Galaxy Disks. IV. Radial Extinction Profiles from Counts of Distant Galaxies Seen through Foreground Disks
Dust extinction can be determined from the number of distant fieldgalaxies seen through a spiral disk. To calibrate this number for thecrowding and confusion introduced by the foreground image,González et al. and Holwerda et al. developed the Synthetic FieldMethod (SFM), which analyzes synthetic fields constructed by addingvarious deep exposures of unobstructed background fields to thecandidate foreground galaxy field. The advantage of the SFM is that itgives the average opacity for the area of a galaxy disk without makingassumptions about either the distribution of absorbers or of the diskstarlight. However, it is limited by poor statistics on the survivingfield galaxies, hence the need to combine a larger sample of fields.This paper presents the first results for a sample of 32 deep HubbleSpace Telescope (HST)/WFPC2 archival fields of 29 spiral galaxies. Theradial profiles of average dust extinction in spiral galaxies based oncalibrated counts of distant field galaxies is presented here, both forindividual galaxies and for composites from our sample. The effects ofinclination, spiral arms, and Hubble type on the radial extinctionprofile are discussed. The dust opacity of the disk apparently arisesfrom two distinct components: an optically thicker (AI=0.5-4mag) but radially dependent component associated with the spiral armsand a relatively constant optically thinner disk (AI~0.5mag). These results are in complete agreement with earlier work onocculted galaxies. The early-type spiral disks in our sample show lessextinction than the later types. Low surface brightness galaxies, andpossibly Sd's, appear effectively transparent. The average color of thefield galaxies seen through foreground disks does not appear to changewith radius or opacity. This gray behavior is most likely due to thepatchy nature of opaque clouds. The average extinction of a radialannulus and its average surface brightness seem to correlate for thebrighter regions. This leads to the conclusion that the brighter partsof the spiral disk, such as spiral arms, are also the ones with the mostextinction associated with them.
| Chandra X-Ray Imaging of the Interacting Starburst Galaxy System NGC 7714/7715: Tidal Ultraluminous X-Ray Sources, Emergent Wind, and Resolved H II Regions
We present high spatial resolution X-ray imaging data for theinteracting galaxy pair NGC 7714/7715 (Arp 284) from the Chandra X-raytelescope. In addition to the unresolved starburst nucleus, a variablepoint source with LX~1040 ergs s-1 wasdetected 1.5" (270 pc) to the northwest of the nucleus, coincident witha blue, extremely optically luminous (MV~-14.1) point sourceon Hubble Space Telescope images. Eleven other candidate pointlikeultraluminous X-ray sources (ULXs) were also detected in the vicinity ofNGC 7714/7715, two of which exceed 1040 ergs s-1.Ten of these appear to be associated with interaction-induced features,but only two are associated with star formation regions. We also founddiffuse emission with LX~3×1040 ergss-1 extending 11" (1.9 kpc) to the north of the nucleus. Itsspectrum can be fitted with either a two-temperature MEKAL function(kT=0.59+0.05-0.06 and8+10-3 keV) or a 0.6 keV MEKAL function plus apower law (Γ=1.8+/-0.2). The hard component may be due tohigh-mass X-ray binaries (HMXBs) with possible contributions frominverse Compton radiation, while the soft component is likely from asuperwind. Superbubble models imply an expansion age of ~15 Myr,supporting previous assertions of an intermediate-age nuclear stellarpopulation in addition to a 5 Myr starburst. We also detected extendedX-ray emission associated with four extranuclear H II region complexes.The emission from these H II regions and the nuclear starburst could bedue to either an enhanced population of HMXBs relative to Local Groupgalactic averages or to diffuse gas heated by winds from supernovae, ifthe X-ray production efficiency LX/Lmech is high(~5%). To estimate LX/Lmech, we collectedpublished data for well-studied H II regions and superbubbles in nearbygalaxies. For H II regions with ages less than 3.5 Myr, the medianLX/Lmech~0.02%, while for older star formationregions, LX/Lmech~0.2%-7%. Thus, it is possiblethat gas heating by supernovae may be sufficient to account for theobserved X-rays from these H II regions. In galaxies much more distantthan NGC 7714, for example, the Cartwheel galaxy, H II region complexessimilar to those in NGC 7714 will be unresolved by Chandra and willmimic ULXs. No X-ray emission was detected from the Type Ib supernova SN1999dn, with an upper limit of ~2×1038 ergss-1.
| The opacity of spiral galaxy disks. V. Dust opacity, HI distributions and sub-mm emission
The opacity of spiral galaxy disks, from counts of distant galaxies, iscompared to HI column densities. The opacity measurements are calibratedusing the "Synthetic Field Method" from González et al. (1998,ApJ, 506, 152), Holwerda et al. (2005a, AJ, 129, 1381). When comparedfor individual disks, the HI column density and dust opacity do not seemto be correlated as HI and opacity follow different radial profiles. Toimprove statistics, an average radial opacity profile is compared to anaverage HI profile. Compared to dust-to-HI estimates from theliterature, more extinction is found in this profile. This differencemay be accounted for by an underestimate of the dust in earliermeasurements due to their dependence on dust temperature. Since the SFMis insensitive to the dust temperature, the ratio between the SFMopacity and HI could very well be indicative of the true ratio. Earlierclaims for a radially extended cold dust disk were based on sub-mmobservations. A comparison between sub-mm observations and counts ofdistant galaxies is therefore desirable. We present the best currentexample of such a comparison, M 51, for which the measurements seem toagree. However, this remains an area where improved counts of distantgalaxies, sub-mm observations and our understanding of dust emissivityare needed.
| Distance determinations using type II supernovae and the expanding photosphere method
Due to their high intrinsic brightness, caused by the disruption of theprogenitor envelope by the shock-wave initiated at the bounce of thecollapsing core, hydrogen-rich (type II) supernovae (SN) can be used aslighthouses to constrain distances in the Universe using variants of theBaade-Wesselink method. Based on a large set of CMFGEN models (Hillier& Miller 1998) covering the photospheric phase of type II SN, westudy the various concepts entering one such technique, the ExpandingPhotosphere Method (EPM). We compute correction factors ξ needed toapproximate the synthetic Spectral Energy Distribution (SED) with thatof a blackbody at temperature T. Our ξ, although similar, aresystematically greater, by ~0.1, than the values obtained by Eastman etal. (1996) and translate into a systematic enhancement of 10-20% inEPM-distances. We find that line emission and absorption, not directlylinked to color temperature variations, can considerably alter thesynthetic magnitude: in particular, line-blanketing attributable to Feii and Ti ii is the principal cause for above-unity correction factorsin the B and V bands in hydrogen-recombining models. Following thedominance of electron-scattering opacity in type II SN outflows, theblackbody SED arising at the thermalization depth is diluted, by afactor of approximately 0.2 to 0.4 for fully- or partially-ionizedmodels, but rising to unity as hydrogen recombines for effectivetemperatures below 9000 K. For a given effective temperature, modelswith a larger spatial scale, or lower density exponent, have a largerelectron-scattering optical depth at the photosphere and consequentlysuffer enhanced dilution. We also find that when lines are present inthe emergent spectrum, the photospheric radius in the correspondingwavelength range can be enhanced by a factor of 2-3 compared to the casewhen only continuum opacity is considered. Lines can thus nullify theuniqueness of the photosphere radius and invalidate the Baade method atthe heart of the EPM. Both the impact of line-blanketing on the SED andthe photospheric radius at low T suggest that the EPM is best used atearly times, when the outflow is fully ionized and line-opacity mostlyconfined to the UV range. We also investigate how reliably one canmeasure the photospheric velocity from P-Cygni line profiles. Contraryto the usually held belief, the velocity at maximum absorption in theP-Cygni trough of optically-thick lines can both overestimate orunderestimate the photospheric velocity, with a magnitude that dependson the SN outflow density gradient and the optical thickness of theline. This stems from the wavelength-shift, toward line-center, of thelocation of maximum line-absorption for rays with larger impactparameters. This has implications for the measurement of expansion ratesin SN outflows, especially at earlier times when only fewer, broader,and blue-shifted lines are present. This investigation should facilitatemore reliable use of the EPM and the determination of distances in theUniverse using type II SN.
| Lopsided spiral galaxies: evidence for gas accretion
We quantify the degree of lopsidedness for a sample of 149 galaxiesobserved in the near-infrared from the OSUBGS sample, and try to explainthe physical origin of the observed disk lopsidedness. We confirmprevious studies, but for a larger sample, that a large fraction ofgalaxies have significant lopsidedness in their stellar disks, measuredas the Fourier amplitude of the m=1 component normalised to the averageor m=0 component in the surface density. Late-type galaxies are found tobe more lopsided, while the presence of m=2 spiral arms and bars iscorrelated with disk lopsidedness. We also show that the m=1 amplitudeis uncorrelated with the presence of companions. Numerical simulationswere carried out to study the generation of m=1 via different processes:galaxy tidal encounters, galaxy mergers, and external gas accretion withsubsequent star formation. These simulations show that galaxyinteractions and mergers can trigger strong lopsidedness, but do notexplain several independent statistical properties of observed galaxies.To explain all the observational results, it is required that a largefraction of lopsidedness results from cosmological accretion of gas ongalactic disks, which can create strongly lopsided disks when thisaccretion is asymmetrical enough.
| Quantitative spectroscopy of photospheric-phase type II supernovae
We present first results on the quantitative spectroscopic analysis ofthe photospheric-phase of type II supernovae (SN). The analyses arebased on the model atmosphere code, CMFGEN, of Hillier & Miller(1998) which solves the radiative transfer and statistical equilibriumequations in expanding outflows under the constraint of radiativeequilibrium. A key asset of CMFGEN is its thorough treatment ofline-blanketing due to metal species. From its applicability to hot starenvironments, the main modifications to the source code were to allow alinear velocity law, a power-law density distribution, an adaptive gridto handle the steep H recombination/ionization front occurring in someSN models, and a routine to compute the gray temperature structure inthe presence of large velocities. In this first paper we demonstrate theability of CMFGEN to reproduce, with a high level of accuracy, the UVand optical observations of a sample of well observed type II SN, i.e.SN1987A and SN1999em, at representative stages of their photosphericevolution. Two principal stages of SN are modeled that where hydrogenis fully ionized, and that in which H is only partially ionized. Formodels with an effective temperature below ~8000 K, hydrogen recombinesand gives rise to a steep ionization front. The effect of varying thelocation of the outer grid radius on the spectral energy distribution(SED) is investigated. We find that going to 5-6 times theoptically-thick base radius is optimal, since above that, the modelbecomes prohibitively large, while below this, significant differencesappear because of the reduced line-blanketing (which persists even farabove the photosphere) and the truncation of line-formation regions. Toconstrain the metallicity and the reddening of SN, the UV spectralregion of early-time spectra is essential. We find that the density ofthe photosphere and effect of line blanketing decline as the spatialscale of the SN increases. The density distribution is found to have astrong impact on the overall flux distribution as well as line profiles.For a given base density, the faster the density drops, the higher theeffective temperature of the model. We also find in cool models that theset of Ca ii lines, near 8500 Å is strongly sensitive to thedensity gradient. They show a weaker and narrower profile for steeperdensity distributions. Hydrogen Balmer lines are very well reproduced infully or partially ionized models, but underestimated when hydrogenrecombines. A reduced turbulent velocity or a flatter density layout arefound to partially, but not fully, cure this persistent problem instudies of type II SN. He i lines observed in early-time spectra arevery well reproduced, even for very modest helium enrichments, likelyresulting from treatment of important non-LTE effects. At similar earlyepochs CMFGEN predicts, unambiguously, the presence of N ii lines in theblue-wing of both Hβ and He I 5875 Å. These lines have beenobserved but so far have generally been associated with peculiaremission, from locations far above the photosphere, in the strongadjacent lines. Finally, we present a pedagogical investigation onP-Cygni profile formation in type II SN. Hα is found to form veryclose to the photosphere and thus presents a significant flux-deficit inthe red, made greater by the rapidly declining density distribution.This provides a clear explanation for the noticeable blue-shift ofP-Cygni profiles observed in early-time spectra of type II SN. Futurestudies based on CMFGEN modeling will focus on using type II SN for thecalibration of distances in the Universe, as well as on detailedspectroscopic analyses for the determination of progenitor properties.
| Detection of a Red Supergiant Progenitor Star of a Type II-Plateau Supernova
We present the discovery of a red supergiant star that exploded assupernova 2003gd in the nearby spiral galaxy M74. The Hubble SpaceTelescope (HST) and the Gemini Telescope imaged this galaxy 6 to 9months before the supernova explosion, and subsequent HST images confirmthe positional coincidence of the supernova with a single resolved starthat is a red supergiant of 8+4-2 solar masses. Thisconfirms both stellar evolution models and supernova theories predictingthat cool red supergiants are the immediate progenitor stars of typeII-plateau supernovae.
| Core collapse supernovae
The current study stays for supernovae, especially for core collapsesupernovae are reviewed in this paper. The definition and classificationof supernovae are introduced, in particular, focused to core collapsesupernovae, and analysed their nature in detail. Finally the theoreticalstudy of supernovae is briefly introduced.
| Extragalactic binaries as core-collapse supernova progenitors
Binary star systems are likely the progenitors of many core-collapse(Type II, Type Ib/c) supernovae (SNe). We present observationalinvestigations using ground-based and Hubble Space Telescope opticalimaging and radio monitoring of SNe and their environments, which eitherindicate or attempt to constrain the possible binary nature of the SNprogenitors. For example, from radio observations with the Very LargeArray of the Type II-linear SN 1979C in M100 we conclude that theprogenitor was possibly in a massive, highly eccentric binary, similarto the VV Cephei systems. The Type IIb SN 1993J in M81 is presumed tohave a massive progenitor in an interacting binary system, and fromHubble imaging we cannot yet constrain the nature of the presumedmassive, blue companion. We will present additional results for otherType Ib/c and II SNe.
| Bar-induced perturbation strengths of the galaxies in the Ohio State University Bright Galaxy Survey - I
Bar-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.
| [OIII]/[NII] as an abundance indicator at high redshift
Among `empirical' methods of estimating oxygen abundances inextragalactic HII regions, the use of the ratio of nebular lines of[OIII] and [NII], first introduced by Alloin et al., is reappraised withmodern calibration data and shown to have certain advantages overR23≡ ([OII]+[OIII])/Hβ and N2≡[NII]λ6583/Hα, particularly when applied tostar-forming galaxies at high redshifts.
| BUDDA: A New Two-dimensional Bulge/Disk Decomposition Code for Detailed Structural Analysis of Galaxies
We present BUDDA (Bulge/Disk Decomposition Analysis), a new code devotedto perform a two-dimensional bulge/disk decomposition directly from theimages of galaxies. The bulge component is fitted with a generalizedSérsic profile, whereas disks have an exponential profile. Noother components are included. Bars and other substructures, likelenses, rings, inner bars, and inner disks, are studied with theresidual images obtained through the subtraction of bulges and disksfrom the original images. This means that a detailed structural analysisof galaxies may be performed with a small number of parameters, andsubstructures may be directly studied with no a priori assumptions. Ashas been already shown by several studies, two-dimensional fitting ismuch more reliable than one-dimensional profile fitting. Moreover, ourcode has been thoroughly tested with artificial data, and we demonstrateit to be an accurate tool for determining structural parameters ofgalaxies. We also show that our code is useful in various kinds ofstudies, including galaxies of, e.g., different morphological types, andinclinations, which also may be observed at different spatialresolutions. Thus, the code has a broader range of potentialapplications than most of the previous codes, which are developed totackle specific problems. To illustrate its usefulness, we present theresults obtained with a sample of 51 mostly early-type galaxies (butcovering the whole Hubble sequence). These results show some of theapplications in which the code may be used: the determination ofparameters for fundamental plane and structural studies, quantitativemorphological classification of galaxies, and the identification andstudy of hidden substructures. We have determined the structuralparameters of the galaxies in our sample and found many examples ofhidden inner disks in ellipticals, secondary bars, nuclear rings anddust lanes in lenticulars and spirals, and also wrong morphologicalclassification cases. We now make BUDDA generally available to theastronomical community.Based on observations made at the Pico dos Dias Observatory(PDO/LNA-CNPq), Brazil.
| Type IIP Supernovae as Cosmological Probes: A Spectral-fitting Expanding Atmosphere Model Distance to SN 1999em
Because of their intrinsic brightness, supernovae make excellentcosmological probes. We describe the spectral-fitting expandingatmosphere method (SEAM) for obtaining distances to Type IIP supernovae(SNe IIP) and present a distance to SN 1999em for which a Cepheiddistance exists. Our models give results consistent with the Cepheiddistance, even though we have not attempted to tune the underlyinghydrodynamical model but have simply chosen the best fits. This is incontradistinction to the expanding photosphere method (EPM), whichyields a distance to SN 1999em that is 50% smaller than the Cepheiddistance. We emphasize the differences between the SEAM and the EPM. Weshow that the dilution factors used in the EPM analysis weresystematically too small at later epochs. We also show that the EPMblackbody assumption is suspect. Since SNe IIP are visible to redshiftsas high as z<~6, with the James Webb Space Telescope, the SEAM may bea valuable probe of the early universe.
| Star Formation Properties of a Large Sample of Irregular Galaxies
We present the results of Hα imaging of a large sample ofirregular galaxies. Our sample includes 94 galaxies with morphologicalclassifications of Im, 26 blue compact dwarfs (BCDs), and 20 Sm systems.The sample spans a large range in galactic parameters, includingintegrated absolute magnitude (MV of -9 to -19), averagesurface brightness (20-27 mag arcsec-2), current starformation activity (0-1.3 Msolar yr-1kpc-2), and relative gas content(0.02-5Msolar/LB). The Hα images were usedto measure the integrated star formation rates, determine the extents ofstar formation in the disks, and compare azimuthally averaged radialprofiles of current star formation to older starlight. The integratedstar formation rates of Im galaxies normalized to the physical size ofthe galaxy span a range of a factor of 104 with 10% Imgalaxies and one Sm system having no measurable star formation at thepresent time. The BCDs fall, on average, at the high star formation rateend of the range. We find no correlation between star formation activityand proximity to other cataloged galaxies. Two galaxies located in voidsare similar in properties to the Sm group in our sample. The H IIregions in these galaxies are most often found within the Holmbergradius RH, although in a few systems H II regions are tracedas far as 1.7RH. Similarly, most of the star formation isfound within three disk scale lengths RD, but in somegalaxies H II regions are traced as far as 6RD. A comparisonof Hα surface photometry with V-band surface photometry shows thatthe two approximately follow each other with radius in Sm galaxies, butin most BCDs there is an excess of Hα emission in the centers thatdrops with radius. In approximately half of the Im galaxies Hα andV correspond well, and in the rest there are small to large differencesin the relative rate of falloff with radius. The cases with stronggradients in the LHα/LV ratios and with highcentral star formation rate densities, which include most of the BCDs,require a significant fraction of their gas to migrate to the center inthe last gigayear. We discuss possible torques that could have causedthis without leaving an obvious signature, including dark matter barsand past interactions or mergers with small galaxies or H I clouds.There is now a substantial amount of evidence for these processes amongmany surveys of BCDs. We note that such gas migration will also increasethe local pressure and possibly enhance the formation of massive denseclusters but conclude that the star formation process itself does notappear to differ much among BCD, Im, and Sm types. In particular, thereis evidence in the distribution function for Hα surface brightnessthat the turbulent Mach numbers are all about the same in these systems.This follows from the Hα distribution functions corrected forexponential disk gradients, which are log-normal with a nearly constantdispersion. Thus, the influence of shock-triggered star formation isapparently no greater in BCDs than in Im and Sm types.
| Inner-truncated Disks in Galaxies
We present an analysis of the disk brightness profiles of 218 spiral andlenticular galaxies. At least 28% of disk galaxies exhibit innertruncations in these profiles. There are no significant trends oftruncation incidence with Hubble type, but the incidence among barredsystems is 49%, more than 4 times that for nonbarred galaxies. However,not all barred systems have inner truncations, and not allinner-truncated systems are currently barred. Truncations represent areal dearth of disk stars in the inner regions and are not an artifactof our selection or fitting procedures nor the result of obscuration bydust. Disk surface brightness profiles in the outer regions are wellrepresented by simple exponentials for both truncated and nontruncateddisks. However, truncated and nontruncated systems have systematicallydifferent slopes and central surface brightness parameters for theirdisk brightness distributions. Truncation radii do not appear tocorrelate well with the sizes or brightnesses of the bulges. Thissuggests that the low angular momentum material apparently missing fromthe inner disk was not simply consumed in forming the bulge population.Disk parameters and the statistics of bar orientations in our sampleindicate that the missing stars of the inner disk have not simply beenredistributed azimuthally into bar structures. The sharpness of thebrightness truncations and their locations with respect to othergalactic structures suggest that resonances associated with diskkinematics, or tidal interactions with the mass of bulge stars, might beresponsible for this phenomenon.
| Distance of the hypernova SN 2002ap via the expanding photosphere method
New optical photometric and spectroscopic data of the bright hypernovaSN 2002ap are presented. The obtained BVRI light curves as well as theoptical spectra agree well with other published data series. Thedistance has been inferred by applying the Expanding Photosphere Methodfor the data around maximum. The derived 6.7 Mpc is in good agreementwith recent photometric distances of M 74. However, the total (randomplus systematic) uncertainty of the EPM-distance is at least ±4.5 Mpc (about 70%). The physical parameters of the SN have beendetermined via simplified analytic models of the light and velocitycurves. It is confirmed that SN 2002ap was a less energetic hypernova,the kinetic energy was 4-8 × 1051 erg, and thereddening-free absolute bolometric magnitude reached -16.63 mag(corresponding to Lbol = 3.47 × 108Lȯ), about 2 mag less than the prototype hypernova SN1998bw.Based on observations obtained at the David Dunlap Observatory,University of Toronto.
| Oxygen and nitrogen abundances in nearby galaxies. Correlations between oxygen abundance and macroscopic properties
We 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}
| Deprojecting spiral galaxies using Fourier analysis. Application to the Ohio sample
We 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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