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Faint supernovae and supernova impostors: case studies of SN 2002kg/NGC 2403-V37 and SN 2003gm
Photometric 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.
| On the Role of Continuum-driven Eruptions in the Evolution of Very Massive Stars and Population III Stars
We suggest that the mass lost during the evolution of very massive starsmay be dominated by optically thick, continuum-driven outbursts orexplosions, instead of by steady line-driven winds. In order for amassive star to become a Wolf-Rayet star, it must shed its hydrogenenvelope, but new estimates of the effects of clumping in winds fromO-type stars indicate that line driving is vastly insufficient. Wediscuss massive stars above roughly 40-50 Msolar, which donot become red supergiants and for which the best alternative is massloss during brief eruptions of luminous blue variables (LBVs). Ourclearest example of this phenomenon is the 19th century outburst ofη Carinae, when the star shed 12-20 Msolar or more inless than a decade. Other examples are circumstellar nebulae of LBVs andLBV candidates, extragalactic η Car analogs (the so-called supernovaimpostors), and massive shells around supernovae and gamma-ray bursters.We do not yet fully understand what triggers LBV outbursts or whatsupplies their energy, but they occur nonetheless, and they present afundamental mystery in stellar astrophysics. Since line opacity frommetals becomes too saturated, the extreme mass loss probably arises froma continuum-driven wind or a hydrodynamic explosion, both of which areinsensitive to metallicity. As such, eruptive mass loss could haveplayed a pivotal role in the evolution and ultimate fate of massivemetal-poor stars in the early universe. If they occur in thesePopulation III stars, such eruptions would also profoundly affect thechemical yield and types of remnants from early supernovae andhypernovae thought to be the origin of long gamma-ray bursts.
| Infrared [Fe II] Emission from P Cygni's Nebula: Atomic Data, Mass, Kinematics, and the 1600 AD Outburst
We present moderate- and high-dispersion 1-2.5 μm spectra of the ~10"radius nebula around P Cygni, dominated by bright emission lines of [FeII]. Observed [Fe II] line ratios disagree with theoretical transitionrates in the literature, so we use the spectrum of P Cyg's nebula toconstrain the atomic data for low-lying levels of [Fe II]. Of particularinterest is the ratio [Fe II] λ12567/λ16435, often used asa reddening indicator, for which we empirically derive an intrinsicvalue of 1.49, which is 10%-40% higher than previous estimates.High-dispersion spectra of [Fe II] λ16435 constrain the geometry,detailed structure, and kinematics of P Cyg's nebula, which is the majorproduct of P Cyg's outburst in 1600 AD. We use the [N II]/[N I] lineratio to conclude that the nebula is mostly ionized, with a total massof ~0.1 Msolar, more than the mass lost by the stellar windsince the eruption. For this mass, we would expect a larger infraredexcess than observed. We propose that the dust that obscured the starafter the outburst has since been largely destroyed, releasing Fe intothe gas phase to produce the bright [Fe II] emission. The kinetic energyof this shell is ~1046.3 ergs, far less than the kineticenergy released during the giant eruption of η Car in the 1840s, butclose to the value for η Car's smaller 1890 outburst. In thisrespect, it is interesting that the infrared spectrum of P Cyg's nebularesembles that of the ``Little Homunculus'' around η Car, ejected inthat star's 1890 eruption. The mass and kinetic energy in the nebulae ofη Car and P Cyg give insight into the range of parameters expectedfor extragalactic η Car-like eruptions.
| The Supernova Rate-Velocity Dispersion Relation in the Interstellar Medium
We investigate the relationship between the velocity dispersion of thegas and the supernova (SN) rate and feedback efficiency withthree-dimensional numerical simulations of SN-driven turbulence in theinterstellar medium (ISM). Our simulations aim to explore the constancyof the velocity dispersion profiles in the outer parts of galactic disksat ~6-8 km s-1 and the transition to the starburst regime,i.e., high star formation rates (SFRs) associated with high velocitydispersions. With our fiducial value of the SN feedback efficiency(i.e., ε=0.25, corresponding to an injected energy per SN of0.25×1051 ergs), our results show that (1) SN drivingleads to constant velocity dispersions of σ~6 km s-1for the total gas and σHI~3 km s-1 for the HI gas, independent of the SN rate, for values of the rate between 0.01and 0.5 the Galactic value (ηG) (2) the position of thetransition to the starburst regime (i.e., location of sharp increase inthe velocity dispersion) at around SFR/area~=5×10-3 to10-2 Msolar yr-1 kpc-2observed in the simulations is in good agreement with the transition tothe starburst regime in the observations (e.g., NGC 628 and NGC 6949);(3) for the high SN rates, no H I gas is present in the simulations box;however, for the total gas velocity dispersion, there is good agreementbetween the models and the observations; (4) at the intermediate SNrates (η/ηG~0.5-1), taking into account the thermalbroadening of the H I line helps reach a good agreement in that regimebetween the models and the observations; and (5) forη/ηG<0.5, σ and σHI fallbelow the observed values by a factor of ~2. However, a set ofsimulations with different values of ɛ indicates that, forlarger values of the SN feedback efficiencies, velocity dispersions ofthe H I gas of the order of 5-6 km s-1 can be obtained, incloser agreement with the observations. The fact that forη/ηG<0.5, the H I gas velocity dispersions are afactor of ~2 smaller than the observed values could result from the factthat we might have underestimated the SN feedback efficiency. On theother hand, it might also be an indication that other physical processescouple to the stellar feedback in order to produce the observed level ofturbulence in galactic disks.
| A Radio and X-Ray Study of Historical Supernovae in M83
We report the results of 15 years of radio observations of the sixhistorical supernovae (SNe) in M83 using the Very Large Array. We notethe near-linear decline in radio emission from SN 1957D, a Type II SN,which remains a nonthermal radio emitter. The measured flux densitiesfrom SNe 1923A and 1950B have flattened as they begin to fade belowdetectable limits; they are also Type II SNe. The luminosities for thesethree SNe are comparable with the radio luminosities of otherdecades-old SNe at similar epochs. SNe 1945B, 1968L, and 1983N were notdetected in the most recent observations, and these nondetections areconsistent with previous studies. We report the X-ray nondetections ofall six historical SNe using the Chandra X-Ray Observatory, consistentwith previous X-ray searches of other decades-old SNe and low inferredmass-loss rates of the progenitors [M˙~(10-8Msolar yr-1)(vw/10 km s-1)].
| On the X-ray, optical emission line and black hole mass properties of local Seyfert galaxies
We investigate the relation between X-ray nuclear emission, opticalemission line luminosities and black hole masses for a sample of 47Seyfert galaxies. The sample, which has been selected from the Palomaroptical spectroscopic survey of nearby galaxies (Ho et al. 1997a, ApJS,112, 315), covers a wide range of nuclear powers, from L2-10keV ~ 1043 erg/s down to very low luminosities(L2-10 keV ~ 1038 erg/s). Best available data fromChandra, XMM-Newton and, in a few cases, ASCA observations have beenconsidered. Thanks to the good spatial resolution available from theseobservations and a proper modeling of the various spectral components,it has been possible to obtain accurate nuclear X-ray luminosities notcontaminated by off-nuclear sources and/or diffuse emission. X-rayluminosities have then been corrected taking into account the likelycandidate Compton thick sources, which are a high fraction (>30%)among type 2 Seyferts in our sample. The main result of this study isthat we confirm strong linear correlations between 2-10 keV,[OIII]λ5007, Hα luminosities which show the same slope asquasars and luminous Seyfert galaxies, independent of the level ofnuclear activity displayed. Moreover, despite the wide range ofEddington ratios (L/L_Edd) tested here (six orders of magnitude, from0.1 down to ~10-7), no correlation is found between the X-rayor optical emission line luminosities and the black hole mass. Ourresults suggest that Seyfert nuclei in our sample are consistent withbeing a scaled-down version of more luminous AGN.
| The Hα Galaxy Survey . III. Constraints on supernova progenitors from spatial correlations with Hα emission
Aims.We attempt to constrain progenitors of the different types ofsupernovae from their spatial distributions relative to star formationregions in their host galaxies, as traced by Hα + [Nii] lineemission. Methods: .We analyse 63 supernovae which have occurredwithin galaxies from our Hα survey of the local Universe. Threestatistical tests are used, based on pixel statistics, Hα radialgrowth curves, and total galaxy emission-line fluxes. Results:.Many type II supernovae come from regions of low or zero emission lineflux, and more than would be expected if the latter accurately traceshigh-mass star formation. We interpret this excess as a 40% "Runaway"fraction in the progenitor stars. Supernovae of types Ib and Ic doappear to trace star formation activity, with a much higher fractioncoming from the centres of bright star formation regions than is thecase for the type II supernovae. Type Ia supernovae overall show a weakcorrelation with locations of current star formation, but there isevidence that a significant minority, up to about 40%, may be linked tothe young stellar population. The radial distribution of allcore-collapse supernovae (types Ib, Ic and II) closely follows that ofthe line emission and hence star formation in their host galaxies, apartfrom a central deficiency which is less marked for supernovae of typesIb and Ic than for those of type II. Core-collapse supernova ratesoverall are consistent with being proportional to galaxy totalluminosities and star formation rates; however, within this total thetype Ib and Ic supernovae show a moderate bias towards more luminoushost galaxies, and type II supernovae a slight bias towardslower-luminosity hosts.
| Outer structure of the Galactic warp and flare: explaining the Canis Major over-density
Aims.In this paper we derive the structure of the Galactic stellar warpand flare. Methods: .We use 2MASS red clump and red giant stars,selected at mean and fixed heliocentric distances ofRȯ≃3, 7 and 17 kpc. Results: .Our resultscan be summarized as follows: (i) a clear stellar warp signature isderived for the 3 selected rings, proving that the warp starts alreadywithin the solar circle; (ii) the derived stellar warp is consistent(both in amplitude and phase-angle) with that for the Galacticinterstellar dust and neutral atomic hydrogen; (iii) the consistency andregularity of the stellar-gaseous warp is traced out to aboutR_GC20 kpc; (iv) the Sun seems not to fall on the line of nodes.The stellar warp phase-angle orientation (φ15°) is closeto the orientation angle of the Galactic bar and this, most importantly,produces an asymmetric warp for the inner Rȯ≃3 and7 kpc rings; (v) a Northern/Southern warp symmetry is observed only forthe ring at Rȯ≃17 kpc, at which the dependency onφ is weakened; (vi) treating a mixture of thin and thick diskstellar populations, we trace the variation with R_GC of the diskthickness (flaring) and derive an almost constant scale-height (~0.65kpc) within R_GC15 kpc. Further out, the disk flaring increasegradually reaching a mean scale-height of ~1.5 kpc at R_GC23 kpc;(vii) the derived outer disk warping and flaring provide further robustevidence that there is no disk radial truncation at R_GC14 kpc. Conclusions: .In the particular case of the Canis Major (CMa)over-density we confirm its coincidence with the Southern stellarmaximum warp occurring near l240° (forRȯ≃7 kpc) which brings down the Milky Waymid-plane by ~3° in this direction. The regularity and consistencyof the stellar, gaseous and dust warp argues strongly against a recentmerger scenario for Canis Major. We present evidence to conclude thatall observed parameters (e.g. number density, radial velocities, propermotion etc) of CMa are consistent with it being a normal Milky Wayouter-disk population, thereby leaving no justification for more complexinterpretations of its origin. The present analysis or outer diskstructure does not provide a conclusive test of the structure or originof the Monoceros Ring. Nevertheless, we show that a warped flared MilkyWay contributes significantly at the locations of the Monoceros Ring.Comparison of outer Milky Way H I and CO properties with those of othergalaxies favors the suggestion that complex structures close to planarin outer disks are common, and are a natural aspect of warped andflaring disks.
| X-ray spectral survey with XMM-Newton of a complete sample of nearby Seyfert galaxies
Results obtained from an X-ray spectral survey of nearby Seyfertgalaxies using XMM-Newton are reported. The sample was opticallyselected, well defined, complete in B magnitude, and distance limited:it consists of the nearest (D 22 Mpc) 27 Seyfert galaxies (9 oftype 1, 18 of type 2) taken from the Ho et al. (1997a, ApJS, 112, 315)sample. This is one of the largest atlases of hard X-ray spectra oflow-luminosity active galaxies ever assembled. All nuclear sourcesexcept two Seyfert 2s are detected between 2 and 10 keV, half for thefirst time ever, and average spectra are obtained for all of them.Nuclear luminosities reach values down to 1038 ergs-1. The shape of the distribution of X-ray parameters isaffected by the presence of Compton-thick objects (30% among type2s). The latter have been identified either directly from their intenseFeK line and flat X-ray spectra, or indirectly with flux diagnosticdiagrams which use isotropic indicators. After taking into account thesehighly absorbed sources, we find that (i) the intrinsic X-ray spectralproperties (i.e., spectral shapes and luminosities above 2 keV) areconsistent between type 1 and type 2 Seyferts, as expected from "unifiedmodels"; (ii) Seyfert galaxies as a whole are distributed fairlycontinuously over the entire range of N_H, between 1020 and1025 cm-2; and (iii) while Seyfert 1s tend to havelower NH and Seyfert 2s tend to have the highest, we find 30%and 10% exceptions, respectively. Overall the sample is of sufficientquality to well represent the average intrinsic X-ray spectralproperties of nearby active galactic nuclei, including a proper estimateof the distribution of their absorbing columns. Finally, we concludethat, with the exception of a few cases, the present study agrees withpredictions of unified models of Seyfert galaxies, and extends theirvalidity down to very low luminosities.
| GHASP: an Hα kinematic survey of spiral and irregular galaxies - IV. 44 new velocity fields. Extension, shape and asymmetry of Hα rotation curves
We present Fabry-Perot observations obtained in the frame of the GHASPsurvey (Gassendi HAlpha survey of SPirals). We have derived the Hαmap, the velocity field and the rotation curve for a new set of 44galaxies. The data presented in this paper are combined with the datapublished in the three previous papers providing a total number of 85 ofthe 96 galaxies observed up to now. This sample of kinematical data hasbeen divided into two groups: isolated (ISO) and softly interacting(SOFT) galaxies. In this paper, the extension of the Hα discs, theshape of the rotation curves, the kinematical asymmetry and theTully-Fisher relation have been investigated for both ISO and SOFTgalaxies. The Hα extension is roughly proportional toR25 for ISO as well as for SOFT galaxies. The smallestextensions of the ionized disc are found for ISO galaxies. The innerslope of the rotation curves is found to be correlated with the centralconcentration of light more clearly than with the type or thekinematical asymmetry, for ISO as well as for SOFT galaxies. The outerslope of the rotation curves increases with the type and with thekinematical asymmetry for ISO galaxies but shows no special trend forSOFT galaxies. No decreasing rotation curve is found for SOFT galaxies.The asymmetry of the rotation curves is correlated with themorphological type, the luminosity, the (B-V) colour and the maximalrotational velocity of galaxies. Our results show that the brightest,the most massive and the reddest galaxies, which are fast rotators, arethe least asymmetric, meaning that they are the most efficient withwhich to average the mass distribution on the whole disc. Asymmetry inthe rotation curves seems to be linked with local star formation,betraying disturbances of the gravitational potential. The Tully-Fisherrelation has a smaller slope for ISO than for SOFT galaxies.
| Recent Star Formation in the Extreme Outer Disk of M83
Ultraviolet imaging with the Galaxy Evolution Explorer (GALEX) hasrevealed an extensive sample of UV-bright stellar complexes in theextreme outer disk of M83, extending to about 4 times the radius atwhich the majority of H II regions are detected(RHII=5.1m, or 6.6 kpc). These sources are typicallyassociated with large-scale filamentary H I structures in the warpedouter disk of M83 and are distributed beyond the galactocentric radii atwhich molecular interstellar medium has yet been detected. We presentmeasured properties of these stellar complexes, including far-UV andnear-UV magnitudes and local gas surface density. Only a subset of theouter-disk UV sources have corresponding H II regions detected inHα imaging, consistent with a sample of mixed age in which somesources are a few megayears old and others are much more evolved(~108 yr).
| 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.
| A survey for OB associations in the Sculptor Group spiral galaxy NGC 7793
We report on the results from application of an objective algorithm(PLC) to find OB associations, to B and V images of the Sculptor spiralgalaxy NGC 7793, which were obtained with the ESO VLT and FORSinstrument and basically cover the entire spatial extent of the galaxy.We detected 148 associations. Statistical tests show that less than 6 ofthese detections are caused by randomly concentrated blue stars. In thesize distribution, a sharp peak is observed at a value of about 35microradians, which corresponds to a linear diameter of 135 pc, assuminga distance of 3.91 Mpc to the galaxy. We also find 25 much largerobjects. A second application of the PLC technique shows that 20 of themare stellar complexes consisting of multiple sub-associations withtypical sizes on the order of 130 pc. A comparison of the sizedistribution of the detected OB associations in NGC 7793 with observeddistributions in other galaxies suggests that the conditions in twoSculptor Group galaxies (NGC 300 and NGC 7793) favour the formation oflarge associations. We provide a catalog giving coordinates and physicalparameters for all the associations and stellar complexes we have foundin our survey.
| SN 2002kg - the brightening of LBV V37 in NGC 2403
SN 2002kg is a type IIn supernova, detected in October 2002 in thenearby spiral galaxy NGC 2403. We show that the position of SN 2002kgagrees within the errors with the position of the LBV V37. Ground basedand HST ACS images however show that V37 is still present after the SN2002kg event. We compiled a lightcurve of V37 which underlines thevariablity of the object, and shows that SN 2002kg was the brighteningof V37 and not a supernova. The recent brightening is not a gianteruption, but more likely part of an S Dor phase. V37 shows strongHα +[N II] emission in recent images and in the SN2002kg spectrum, which we interprete as the signature of the presence ofan LBV nebula. A historic spectrum lacks emission, which may hint thatwe are witnessing the formation of an LBV nebula.
| A catalogue of ultraluminous X-ray sources in external galaxies
We present a catalogue of ultraluminous X-ray sources (ULXs) in externalgalaxies. The aim of this catalogue is to provide easy access to theproperties of ULXs, their possible counterparts at other wavelengths(optical, IR, and radio), and their host galaxies. The cataloguecontains 229 ULXs reported in the literature until April 2004. Most ULXsare stellar-mass-black hole X-ray binaries, but it is not excluded thatsome ULXs could be intermediate-mass black holes. A small fraction ofthe candidate ULXs may be background Active Galactic Nuclei (AGN) andSupernova Remnants (SNRs). ULXs with luminosity above 1040ergs s-1 are found in both starburst galaxies and in thehalos of early-type galaxies.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/429/1125
| Control of star formation by supersonic turbulence
Understanding the formation of stars in galaxies is central to much ofmodern astrophysics. However, a quantitative prediction of the starformation rate and the initial distribution of stellar masses remainselusive. For several decades it has been thought that the star formationprocess is primarily controlled by the interplay between gravity andmagnetostatic support, modulated by neutral-ion drift (known asambipolar diffusion in astrophysics). Recently, however, bothobservational and numerical work has begun to suggest that supersonicturbulent flows rather than static magnetic fields control starformation. To some extent, this represents a return to ideas popularbefore the importance of magnetic fields to the interstellar gas wasfully appreciated. This review gives a historical overview of thesuccesses and problems of both the classical dynamical theory and thestandard theory of magnetostatic support, from both observational andtheoretical perspectives. The outline of a new theory relying on controlby driven supersonic turbulence is then presented. Numerical modelsdemonstrate that, although supersonic turbulence can provide globalsupport, it nevertheless produces density enhancements that allow localcollapse. Inefficient, isolated star formation is a hallmark ofturbulent support, while efficient, clustered star formation occurs inits absence. The consequences of this theory are then explored for bothlocal star formation and galactic-scale star formation. It suggests thatindividual star-forming cores are likely not quasistatic objects, butdynamically collapsing. Accretion onto these objects varies depending onthe properties of the surrounding turbulent flow; numerical models agreewith observations showing decreasing rates. The initial massdistribution of stars may also be determined by the turbulent flow.Molecular clouds appear to be transient objects forming and dissolvingin the larger-scale turbulent flow, or else quickly collapsing intoregions of violent star formation. Global star formation in galaxiesappears to be controlled by the same balance between gravity andturbulence as small-scale star formation, although modulated by coolingand differential rotation. The dominant driving mechanism instar-forming regions of galaxies appears to be supernovae, whileelsewhere coupling of rotation to the gas through magnetic fields orgravity may be important.
| Discovery and Evolution of an Unusual Luminous Variable Star in NGC 3432 (Supernova 2000ch)
We present photometric and spectroscopic observations of SN 2000ch, anunusual and extremely luminous variable star located in the galaxy NGC3432. The object was discovered on 2000 May 3.2 during the course of theLick Observatory Supernova Search, at an unfiltered magnitude of about17.4. Prediscovery images obtained in 1997, 1998, and 2000 April showthe object with R=19.2-19.5 mag. Optical spectra obtained beginning on2000 May 6 show a smooth, flat continuum and strong, broad hydrogenBalmer emission lines at wavelengths consistent with the catalogedredshift of NGC 3432, strengthening the association of the variable withthe galaxy. Photometric monitoring reveals a complex and erratic lightcurve over a time span of ~10 days. Subsequent optical spectra over thenext ~3 months continued to show strong Balmer emission lines with amean full width at half-maximum intensity ~1550 km s-1 and adistinct red asymmetry. A spectrum obtained 9 months after the outburstis similar to the previous spectra, but the integrated flux in Hαis nearly half that observed during the outburst. The object'sphotometric behavior, spectrum, and luminosity suggest that it is a verymassive and luminous variable star and might be related to some luminousblue variable stars such as η Carinae and SN 1997bs in NGC 3627. Thebrightest apparent magnitude implies an absolute magnitude ofMV~-12.7 at the distance of NGC 3432, a value that iscomparable to η Car during its outburst in the mid-19th century.
| 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.
| The SAI Catalog of Supernovae and Radial Distributions of Supernovae of Various Types in Galaxies
We describe the Sternberg Astronomical Institute (SAI) catalog ofsupernovae. We show that the radial distributions of type-Ia, type-Ibc,and type-II supernovae differ in the central parts of spiral galaxiesand are similar in their outer regions, while the radial distribution oftype-Ia supernovae in elliptical galaxies differs from that in spiraland lenticular galaxies. We give a list of the supernovae that arefarthest from the galactic centers, estimate their relative explosionrate, and discuss their possible origins.
| A Nursery of Young Objects: Intergalactic H II Regions in Stephan's Quintet
We have discovered four intergalactic H II regions in Stephan's quintet,which is more than a 25 kpc projected distance from the center of thenearest group galaxy, with no apparent optical connection to it. Theyhave MB ranging from -11.9 to -12.5 mag, colors B-R=0.7-1.1mag, radial velocities from 6565 to 6651 km s-1, and they aresuperposed onto the H I tail east of NGC 7319, with a mean radialvelocity of 6610 km s-1. In addition, they have metallicitiesof the order of 12+log(O/H)=8.58+/-0.25, which suggests that they wereformed from preenriched material. We derive a mean age of 4.6+/-0.6 Myrand a mean stellar mass of (2.9+/-1.4)×104Msolar for the four objects. The masses, ages, colors,velocities, metallicities, and location of the objects suggest that theyare H II regions that were formed far away from the galaxies throughcompression of the intergalactic H I gas by galaxy collisions.Based on observations obtained at the Gemini Observatory, which isoperated by the Association of Universities for Research in Astronomy,Inc., under a cooperative agreement with the NSF on behalf of the Geminipartnership: the National Science Foundation (US), the Particle Physicsand Astronomy Research Council (UK), the National Research Council(Canada), CONICYT (Chile), the Australian Research Council (Australia),CNPq (Brazil), and CONICET (Argentina).
| Gravitational Bar and Spiral Arm Torques from Ks-band Observations and Implications for the Pattern Speeds
We have obtained deep near-infrared Ks-band William HerschelTelescope observations of a sample of 15 nearby spiral galaxies having arange of Hubble types and apparent bar strengths. The near-infraredlight distributions are converted into gravitational potentials, and themaximum relative gravitational torques due to the bars and the spiralsare estimated. We find that spiral strength, Qs, and barstrength, Qb, correlate well with other measures of spiralarm and bar amplitudes and that spiral and bar strengths also correlatewell with each other. We also find a correlation between the positionangle of the end of the bar and the position angle of the inner spiral.These correlations suggest that the bars and spirals grow together withthe same rates and pattern speeds. We also show that the strongest barstend to have the most open spiral patterns. Because open spirals implyhigh disk-to-halo mass ratios, bars and spirals most likely growtogether as a combined disk instability. They stop growing for differentreasons, however, giving the observed variation in bar-spiralmorphologies. Bar growth stops because of saturation when most of theinner disk is in the bar, and spiral growth stops because of increasedstability as the gas leaves and the outer disk heats up.
| The Nature of SN 1961V
The nature of SN 1961V has been uncertain. Its peculiar optical lightcurve and slow expansion velocity are similar to those of superoutburstsof luminous blue variables (LBVs), but its nonthermal radio spectralindex and declining radio luminosity are consistent with decades-oldsupernovae (SNe). We have obtained Hubble Space Telescope STIS imagesand spectra of the stars in the vicinity of SN 1961V and find object 7identified by Filippenko and coworkers to be closest to the optical andradio positions of SN 1961V. Object 7 is the only point source detectedin our STIS spectra, and only its Hα emission is detected; itcannot be the SN or its remnant because of the absence of forbiddenlines. While the Hα line profile of object 7 is remarkably similarto that of η Carinae, the blue color (similar to an A2 Ibsupergiant) and lack of appreciable variability are unlike knownpostoutburst LBVs. We have also obtained Very Long Baseline Arrayobservations of SN 1961V at 18 cm. The nondetection of SN 1961V places alower limit on the size of the radio-emitting region, 7.6 mas or 0.34pc, which implies an average expansion velocity in excess of 4400 kms-1, much higher than the optical expansion velocity measuredin 1961. We conclude the following: (1) An SN occurred in the vicinityof SN 1961V a few decades ago. (2) If the SN 1961V light maximumoriginates from a giant eruption of a massive star, object 7 is the mostprobable candidate for the survivor, but its blue color and lack ofsignificant variability are different from a postoutburst η Car. (3)The radio SN and object 7 could be physically associated with each otherthrough a binary system. (4) Object 7 needs to be monitored to determineits nature and relationship to SN 1961V.
| 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}
| Interstellar turbulence driven by the magnetorotational instability
The occurrence of the magnetorotational instability (MRI) in verticallystratified galactic disks is considered. Global 3D nonlinear MHDsimulations with the ZEUSMP code are performed in a cylindriccomputational domain. Due to the evolution of the MRI toroidal andpoloidal components of the mean magnetic fields are generated. Theresults are also applied to very young galaxies which are assumed topossess strong magnetic fields already after a few 108 years.The dependence of MRI growth rate on the shear strength is shown. Thevelocity dispersion grows with height and reaches values of about 5 kms-1 in good agreement with observations and close to thepredictions of Sellwood & Balbus (\cite{Sellwood99}). For strongmagnetic fields the MRI is suppressed but it is not suppressed byturbulence initially present in the disk.
| 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
| The Hα galaxy survey. I. The galaxy sample, Hα narrow-band observations and star formation parameters for 334 galaxies
We discuss the selection and observations of a large sample of nearbygalaxies, which we are using to quantify the star formation activity inthe local Universe. The sample consists of 334 galaxies across allHubble types from S0/a to Im and with recession velocities of between 0and 3000 km s-1. The basic data for each galaxy are narrowband H\alpha +[NII] and R-band imaging, from which we derive starformation rates, H\alpha +[NII] equivalent widths and surfacebrightnesses, and R-band total magnitudes. A strong correlation is foundbetween total star formation rate and Hubble type, with the strongeststar formation in isolated galaxies occurring in Sc and Sbc types. Moresurprisingly, no significant trend is found between H\alpha +[NII]equivalent width and galaxy R-band luminosity. More detailed analyses ofthe data set presented here will be described in subsequent papers.Based on observations made with the Jacobus Kapteyn Telescope operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto deAstrofísica de Canarias.The full version of Table \ref{tab3} is available in electronic form atthe CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/414/23 Reduced image datafor this survey can be downloaded fromhttp://www.astro.livjm.ac.uk/HaGS/
| Explosion energies, nickel masses and distances of Type II plateau supernovae
The hydrodynamical modelling of Type II plateau supernova (SNIIP) lightcurves predicts a correlation between three observable parameters(plateau duration, absolute magnitude and photospheric velocity at themiddle of the plateau) on the one hand, and three physical parameters(explosion energy E, mass of the envelope expelled and pre-supernovaradius R) on the other. The correlation is used, together with adopteddistances from the expanding photosphere method, to estimate and R for adozen well-observed SNIIP. For this set of supernovae, the resultingvalue of E varies within a factor of 6 (0.5 <~E/1051 erg<~ 3), whereas the envelope mass remains within the limits . Thepre-supernova radius is typically 200-600 Rsolar, but canreach >~1000 Rsolar for the brightest supernovae (e.g. SN1992am).A new method of determining the distance of SNIIP is proposed. It isbased on the assumption of a correlation between the explosion energy Eand the 56Ni mass required to power the post-plateau lightcurve tail through 56Co decay. The method is useful for SNIIPwith well-observed bolometric light curves during both the plateau andradioactive tail phases. The resulting distances and future improvementsare discussed.
| The Contribution of H I-rich Galaxies to the Damped Lyα Absorber Population at z = 0
We present a study of the expected properties of the low-redshift dampedLyα absorber population determined from a sample of H I-selectedgalaxies in the local universe. Because of a tight correlation betweenthe H I mass and H I cross section, which we demonstrate spans allgalaxy types, we can use our H I-selected sample to predict theproperties of the absorption-line systems. We use measurements of thenumber density and H I cross section of galaxies to show that the totalH I cross section at column densities sufficient to produce dampedLyα absorption is consistent with no evolution of the absorberpopulation. We also find that the dN/dz distribution is dominated bygalaxies with H I masses near 109 Msolar. However,because of the large dispersion in the correlation between H I mass andstellar luminosity, we find that the distribution of dN/dz as a functionof LJ is fairly flat. In addition, we examine the line widthsof the H I-selected galaxies and show that there may be evolution in thekinematics of H I-rich galaxies, but it is not necessary for the higherredshift population to contain a greater proportion of high-massgalaxies than we find locally.
| The WSRT wide-field H I survey. I. The background galaxy sample
We have used the Westerbork array to carry out an unbiased wide-fieldsurvey for H I emission features, achieving an RMS sensitivity of about18 mJy/Beam at a velocity resolution of 17 km s-1 over 1800deg2 and between -1000 < VHel <+6500 kms-1. The primary data consists of auto-correlation spectrawith an effective angular resolution of 49' FWHM, althoughcross-correlation data were also acquired. The survey region is centeredapproximately on the position of Messier 31 and is Nyquist-sampled over60x 30o in RA x Dec. More than 100 distinct features aredetected at high significance in each of the two velocity regimes(negative and positive LGSR velocities). In this paper we present theresults for our H I detections of external galaxies at positive LGSRvelocity. We detect 155 external galaxies in excess of 8sigma inintegrated H I flux density. Plausible optical associations are foundwithin a 30' search radius for all but one of our H I detections in DSSimages, although several are not previously cataloged or do not havepublished red-shift determinations. Our detection without a DSSassociation is at low galactic latitude. Twenty-three of our objects aredetected in H I for the first time. We classify almost half of ourdetections as ``confused'', since one or more companions is catalogedwithin a radius of 30' and a velocity interval of 400 km s-1.We identify a handful of instances of significant positional offsetsexceeding 10 kpc of unconfused optical galaxies with the associated H Icentroid, possibly indicative of severe tidal distortions or uncatalogedgas-rich companions. A possible trend is found for an excess of detectedH I flux in unconfused galaxies within our large survey beam relative tothat detected previously in smaller telescope beams, both as function ofincreasing distance and increasing gas mass. This may be an indicationfor a diffuse gaseous component on 100 kpc scales in the environment ofmassive galaxies or a population of uncataloged low mass companions. Weuse our galaxy sample to estimate the H I mass function from our surveyvolume. Good agreement is found with the HIPASS BGC results, but onlyafter explicit correction for galaxy density variations with distance.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/406/829 and Fig. 3 is onlyavailable in electronic form at http://www.edpsciences.org
| A new catalogue of ISM content of normal galaxies
We have compiled a catalogue of the gas content for a sample of 1916galaxies, considered to be a fair representation of ``normality''. Thedefinition of a ``normal'' galaxy adopted in this work implies that wehave purposely excluded from the catalogue galaxies having distortedmorphology (such as interaction bridges, tails or lopsidedness) and/orany signature of peculiar kinematics (such as polar rings,counterrotating disks or other decoupled components). In contrast, wehave included systems hosting active galactic nuclei (AGN) in thecatalogue. This catalogue revises previous compendia on the ISM contentof galaxies published by \citet{bregman} and \citet{casoli}, andcompiles data available in the literature from several small samples ofgalaxies. Masses for warm dust, atomic and molecular gas, as well asX-ray luminosities have been converted to a uniform distance scale takenfrom the Catalogue of Principal Galaxies (PGC). We have used twodifferent normalization factors to explore the variation of the gascontent along the Hubble sequence: the blue luminosity (LB)and the square of linear diameter (D225). Ourcatalogue significantly improves the statistics of previous referencecatalogues and can be used in future studies to define a template ISMcontent for ``normal'' galaxies along the Hubble sequence. The cataloguecan be accessed on-line and is also available at the Centre desDonnées Stellaires (CDS).The catalogue is available in electronic form athttp://dipastro.pd.astro.it/galletta/ismcat and at the CDS via anonymousftp to\ cdsarc.u-strasbg.fr (130.79.128.5) or via\http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/405/5
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