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|Scalar potential model of redshift and discrete redshift|
On the galactic scale the universe is inhomogeneous and redshift z isoccasionally less than zero. A scalar potential model (SPM) that linksthe galaxy scale z to the cosmological scale z of the Hubble Law ispostulated. Several differences among galaxy types suggest that spiralgalaxies are Sources and that early type, lenticular, and irregulargalaxies are Sinks of a scalar potential field. The morphology-radiusand the intragalactic medium cluster observations support the movementof matter from Source galaxies to Sink galaxies. A cell structure ofgalaxy groups and clusters is proposed to resolve a paradox concerningthe scalar potential like the Olber’s paradox concerning light.For the sample galaxies, the ratio of the luminosity of Source galaxiesto the luminosity of Sink galaxies approaches 2.7 ± 0.1. Anequation is derived from sample data, which is anisotropic andinhomogeneous, relating z of and the distance D to galaxies. Thecalculated z has a correlation coefficient of 0.88 with the measured zfor a sample of 32 spiral galaxies with D calculated using Cepheidvariable stars. The equation is consistent with z < 0 observations ofclose galaxies. At low cosmological distances, the equation reduces to z≈ exp(KD)‑1 ≈ KD, where K is a constant, positive value. Theequation predicts z from galaxies over 18 Gpc distant approaches aconstant value on the order of 500. The SPM of z provides a physicalbasis for the z of particle photons. Further, the SPM qualitativelysuggests the discrete variations in z, which was reported by Tifft[Tifft, W.G., 1997. Astrophy. J. 485, 465] and confirmed by others, areconsistent with the SPM.
|A non-spherical core in the explosion of supernova SN 2004dj|
An important and perhaps critical clue to the mechanism driving theexplosion of massive stars as supernovae is provided by the accumulatingevidence for asymmetry in the explosion. Indirect evidence comes fromhigh pulsar velocities, associations of supernovae with long-softγ-ray bursts, and asymmetries in late-time emission-line profiles.Spectropolarimetry provides a direct probe of young supernova geometry,with higher polarization generally indicating a greater departure fromspherical symmetry. Large polarizations have been measured for`stripped-envelope' (that is, type Ic; ref. 7) supernovae, whichconfirms their non-spherical morphology; but the explosions of massivestars with intact hydrogen envelopes (type II-P supernovae) have shownonly weak polarizations at the early times observed. Here we reportmulti-epoch spectropolarimetry of a classic type II-P supernova thatreveals the abrupt appearance of significant polarization when the innercore is first exposed in the thinning ejecta (~90days after explosion).We infer a departure from spherical symmetry of at least 30 per cent forthe inner ejecta. Combined with earlier results, this suggests that astrongly non-spherical explosion may be a generic feature ofcore-collapse supernovae of all types, where the asphericity in typeII-P supernovae is cloaked at early times by the massive, opaque,hydrogen envelope.
|The first year of SN 2004dj in NGC 2403|
New BV RI photometry and optical spectroscopy of the Type IIp supernova2004dj in NGC 2403, obtained during the first year since discovery, arepresented. The progenitor cluster, Sandage 96, is also detected onpre-explosion frames. The light curve indicates that the explosionoccurred about 30d before discovery, and the plateau phase lasted about+110 +/- 20 d after that. The plateau-phase spectra have been modelledwith the SYNOW spectral synthesis code using H, NaI, TiII, ScII, FeIIand BaI lines. The SN distance is inferred from the expandingphotosphere method and the standard candle method applicable for SNeIIp.They resulted in distances that are consistent with each other as wellas earlier Cepheid and Tully-Fisher distances. The average distance, D =3.47 +/- 0.29 Mpc is proposed for SN 2004dj and NGC 2403. The nickelmass produced by the explosion is estimated as ~0.02 +/- 0.01Msolar. The spectral energy distribution of the progenitorcluster is reanalysed by fitting population synthesis models to ourobserved BV RI data supplemented by U and JHK magnitudes from theliterature. The χ2 minimization revealed a possible`young' solution with cluster age Tcl = 8 Myr, and an `old'solution with Tcl = 20-30 Myr. The `young' solution wouldimply a progenitor mass M > 20 Msolar, which is higherthan the previously detected progenitor masses for Type II SNe.Based on observations obtained at David Dunlap Observatory (Canada), F.L. Whipple Observatory (USA), Konkoly Observatory and Szeged Observatory(Hungary).E-mail: email@example.com
|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.
|Self-consistent response of a galactic disc to vertical perturbations|
We study the self-consistent, linear response of a galactic disc tovertical perturbations, as induced, say, by a tidal interaction. Wecalculate the self-gravitational potential corresponding to anon-axisymmetric, self-consistent density response of the disc using theGreen's function method. The response potential is shown to oppose theperturbation potential because the self-gravity of the disc resists theimposed potential, and this resistance is stronger in the inner parts ofa galactic disc. For the m= 1 azimuthal wavenumber, the disc responseopposes the imposed perturbation up to a radius that spans a range of4-6 disc scalelengths, so that the disc shows a net warp only beyondthis region. This physically explains the well known but so farunexplained observation that warps typically set in beyond this range ofradii. We show that the inclusion of a dark matter halo in thecalculation only marginally changes (by ~10 per cent) the radius for theonset of warps. For perturbations with higher azimuthal wavenumbers, thenet signature of the vertical perturbations can only be seen at largerradii - for example, beyond 7 exponential disc scalelengths for m= 10.Also, for the high-m cases, the magnitude of the negative disc responsedue to the disc self-gravity is much smaller. This is shown to result incorrugations of the mid-plane density, which explains the puzzlingscalloping with m= 10 detected in HI in the outermost regions ~30 kpc inthe Galaxy.
|Hαkinematics of the SINGS nearby galaxies survey - I*|
This is the first part of an Hαkinematics follow-up survey of theSpitzer Infrared Nearby Galaxies Survey (SINGS) sample. The data for28galaxies are presented. The observations were done on three differenttelescopes with Fabry-Perot of New Technology for the Observatoire dumont Megantic (FaNTOmM), an integral field photon-counting spectrometer,installed in the respective focal reducer of each telescope. The datareduction was done through a newly built pipeline with the aim ofproducing the most homogenous data set possible. Adaptive spatialbinning was applied to the data cubes in order to get a constantsignal-to-noise ratio across the field of view. Radial velocity andmonochromatic maps were generated using a new algorithm, and thekinematical parameters were derived using tilted-ring models.
|A dynamical model for the extraplanar gas in spiral galaxies|
Recent HI observations reveal that the discs of spiral galaxies aresurrounded by extended gaseous haloes. This extraplanar gas reacheslarge distances (several kiloparsecs) from the disc and shows peculiarkinematics (low rotation and inflow). We have modelled the extraplanargas as a continuous flow of material from the disc of a spiral galaxyinto its halo region. The output of our models is pseudo data cubes thatcan be directly compared to the HI data. We have applied these models totwo spiral galaxies (NGC 891 and NGC 2403) known to have a substantialamount of extraplanar gas. Our models are able to reproduce accuratelythe vertical distribution of extraplanar gas for an energy inputcorresponding to a small fraction (<4 per cent) of the energyreleased by supernovae. However, they fail in two important aspects: (1)they do not reproduce the right gradient in rotation velocity; (2) theypredict a general outflow of the extraplanar gas, contrary to what isobserved. We show that neither of these difficulties can be removed ifclouds are ionized and invisible at 21cm as they leave the disc butbecome visible at some point on their orbits. We speculate that thesefailures indicate the need for accreted material from the intergalacticmedium that could provide the low angular momentum and inflow required.
|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.
|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.
|Magnetic Fields in Starburst Galaxies and the Origin of the FIR-Radio Correlation|
We estimate minimum energy magnetic fields (Bmin) for asample of galaxies with measured gas surface densities, spanning morethan four orders of magnitude in surface density, from normal spirals toluminous starbursts. We show that the ratio of the minimum energymagnetic pressure to the total pressure in the ISM decreasessubstantially with increasing surface density. For the ultraluminousinfrared galaxy Arp 220, this ratio is ~10-4. Therefore, ifthe minimum energy estimate is applicable, magnetic fields in starburstsare dynamically weak compared to gravity, in contrast to normalstar-forming spiral galaxies. We argue, however, that rapid cooling ofrelativistic electrons in starbursts invalidates the minimum energyestimate. We assess a number of independent constraints on the magneticfield strength in starburst galaxies. In particular, we argue that theexistence of the FIR-radio correlation implies that the synchrotroncooling timescale for cosmic-ray electrons is much shorter than theirescape time from the galactic disk; this in turn implies that the truemagnetic field in starbursts is significantly larger thanBmin. The strongest argument against such large fields isthat one might expect starbursts to have steep radio spectra indicativeof strong synchrotron cooling, which is not observed. However, we showthat ionization and bremsstrahlung losses can flatten the nonthermalspectra of starburst galaxies even in the presence of rapid cooling,providing much better agreement with observed spectra. We furtherdemonstrate that ionization and bremsstrahlung losses are likely to beimportant in shaping the radio spectra of most starbursts at GHzfrequencies, thereby preserving the linearity of the FIR-radiocorrelation. We thus conclude that magnetic fields in starbursts aresignificantly larger than Bmin. We highlight severalobservations that can test this conclusion.
|Spitzer Space Telescope IRAC and MIPS Observations of the Interacting Galaxies IC 2163 and NGC 2207: Clumpy Emission|
IC 2163 and NGC 2207 are interacting galaxies that have been wellstudied at optical and radio wavelengths and simulated in numericalmodels to reproduce the observed kinematics and morphological features.Spitzer IRAC and MIPS observations reported here show over 200 brightclumps from young star complexes. The brightest IR clump is amorphologically peculiar region of star formation in the western arm ofNGC 2207. This clump, which dominates the Hα and radio continuumemission from both galaxies, accounts for ~12% of the total 24 μmflux. Nearly half of the clumps are regularly spaced along somefilamentary structure, whether in the starburst oval of IC 2163 or inthe thin spiral arms of NGC 2207. This regularity appears to influencethe clump luminosity function, making it peaked at a value nearly afactor of 10 above the completeness limit, particularly in the starburstoval. This is unlike the optical clusters inside the clumps, which havea luminosity function consistent with the usual power-law form. Thegiant IR clumps presumably formed by gravitational instabilities in thecompressed gas of the oval and the spiral arms, whereas the individualclusters formed by more chaotic processes, such as turbulencecompression, inside these larger scale structures.
|The Disk and Extraplanar Environment of NGC 247|
The stellar content of the spiral galaxy NGC 247 is investigated usingdeep visible and near-infrared images. The main-sequence turnoff (MSTO)in the inner 12 kpc of the disk corresponds to an age of ~6 Myr. A meanstar formation rate (SFR) of 0.1 Msolar yr-1during the past 16 Myr is computed from star counts. The color of thered supergiant plume does not change with radius, suggesting that themean metallicity of young stars does not vary by more than ~0.1 dex. Thenumber of bright main-sequence stars per local stellar mass densityclimbs toward larger radii out to a distance of 12 kpc; the scalelengths that characterize the radial distributions of young and oldstars in the disk thus differ. The density of bright main-sequence starswith respect to projected H I mass gradually drops with increasingradius. The population of very young stars disappears in the outer disk;the MSTO at galactocentric radii between 12 and 15 kpc corresponds to~16 Myr, while between 15 and 18 kpc the age is >=40 Myr. Red giantbranch (RGB) stars are resolved at a projected minor-axis galactocentricdistance of ~12 kpc. There is a broad spread in metallicity among theRGB stars, with a mean [M/H]~-1.2. The RGB tip occurs ati'=24.5+/-0.1, indicating that the distance modulus is27.9+/-0.1. Luminous AGB stars with an age ~3 Gyr are also seen in thisfield.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 (United States), theParticle Physics and Astronomy Research Council (United Kingdom), theNational Research Council of Canada (Canada), CONICYT (Chile), theAustralian Research Council (Australia), CNPq (Brazil), and CONICET(Argentina).This publication makes use of data products from the Two Micron All SkySurvey, which is a joint project of the University of Massachusetts andthe Infrared Processing and Analysis Center/California Institute ofTechnology, funded by the National Aeronautics and Space Administrationand the National Science Foundation.
|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.
|An Initial Look at the Far-Infrared-Radio Correlation within Nearby Star-forming Galaxies Using the Spitzer Space Telescope|
We present an initial look at the far-infrared-radio correlation withinthe star-forming disks of four nearby, nearly face-on galaxies (NGC2403, NGC 3031, NGC 5194, and NGC 6946). Using Spitzer MIPS imaging,observed as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS),and Westerbork Synthesis Radio Telescope (WSRT) radio continuum data,taken for the WSRT SINGS radio continuum survey, we are able to probevariations in the logarithmic 24 μm/22 cm (q24) and 70μm/22 cm (q70) surface brightness ratios across each diskat subkiloparsec scales. We find general trends of decreasingq24 and q70 with declining surface brightness andwith increasing radius. The residual dispersion around the trend ofq24 and q70 versus surface brightness is smallerthan the residual dispersion around the trend of q24 andq70 versus radius, on average by ~0.1 dex, indicating thatthe distribution of star formation sites is more important indetermining the infrared/radio disk appearance than the exponentialprofiles of disks. We have also performed preliminary phenomenologicalmodeling of cosmic-ray electron (CR electron) diffusion using animage-smearing technique and find that smoothing the infrared mapsimproves their correlation with the radio maps. We find that exponentialsmoothing kernels work marginally better than Gaussian kernels,independent of projection for these nearly face-on galaxies. This resultsuggests that additional processes besides simple random walk diffusionin three dimensions must affect the evolution of CR electrons. Thebest-fit smoothing kernels for the two less active star-forming galaxies(NGC 2403 and NGC 3031) have much larger scale lengths than those of themore active star-forming galaxies (NGC 5194 and NGC 6946). Thisdifference may be due to the relative deficit of recent CR electroninjection into the interstellar medium for the galaxies that havelargely quiescent disks.
|The Rest-Frame Far-Ultraviolet Morphologies of Star-forming Galaxies at z ~ 1.5 and 4|
We apply a new approach to quantifying galaxy morphology and identifyinggalaxy mergers to the rest-frame far-ultraviolet images of 82 z~4 Lymanbreak galaxies (LBGs) and 55 1.22.5 and Petrosian radii >0.3". Ten of the 82 LBGs haveM20>=-1.1 and possess bright double or multiple nuclei,implying a major-merger fraction of star-forming galaxies ~10%-25% atMFUV<-20, depending on our incompleteness corrections.Galaxies with bulge-like morphologies (G>=0.55,M20<-1.6) make up ~30% of the z~4 LBG sample, while theremaining ~50% have G- and M20-values higher than expectedfor smooth bulges and disks and may be star-forming disks, minormergers, or postmergers. The star-forming z~1.5 galaxy sample has amorphological distribution that is similar to the UDF z~4 LBGs, with anidentical fraction of major-merger candidates but fewer spheroids. Theobserved morphological distributions are roughly consistent with currenthierarchical model predictions for the major-merger rates andminor-merger-induced starbursts at z~1.5 and ~4. We also examine therest-frame FUV-NUV and FUV-B colors as a function of morphology and findno strong correlations at either epoch.
|Imaging Fabry-Perot Spectroscopy of NGC 5775: Kinematics of the Diffuse Ionized Gas Halo|
We present imaging Fabry-Perot observations of Hα emission in thenearly edge-on spiral galaxy NGC 5775. We have derived a rotation curveand a radial density profile along the major axis by examiningposition-velocity (PV) diagrams from the Fabry-Perot data cube, as wellas a CO 2-1 data cube from the literature. PV diagrams constructedparallel to the major axis are used to examine changes in azimuthalvelocity as a function of height above the midplane. The results of thisanalysis reveal the presence of a vertical gradient in azimuthalvelocity. The magnitude of this gradient is approximately 1 kms-1 arcsec-1, or about 8 km s-1kpc-1, although a higher value of the gradient may beappropriate in localized regions of the halo. The evidence for anazimuthal velocity gradient is much stronger for the approaching half ofthe galaxy, although earlier slit spectra are consistent with a gradienton both sides. There is evidence for an outward radial redistribution ofgas in the halo. The form of the rotation curve may also change withheight, but this is not certain. We compare these results with those ofan entirely ballistic model of a disk-halo flow. The model predicts avertical gradient in azimuthal velocity that is shallower than theobserved gradient, indicating that an additional mechanism is requiredto further slow the rotation speeds in the halo.
|Masses of the local group and of the M81 group estimated from distortions in the local velocity field|
Based on high precision measurements of the distances to nearby galaxieswith the Hubble telescope, we have determined the radii of the zerovelocity spheres for the local group, R0 =0.96±0.03Mpc, and for the group of galaxies around M 81/M 82,0.89±0.05Mpc. These yield estimates of MT =(1.29±0.14)· 1012 Mȯ and(1.03±0.17)· 1012 Mȯ,respectively, for the total masses of these groups. The R0method allows us to determine the mass ratios for the two brightestmembers in both groups, as well. By varying the position of the centerof mass between the two principal members of a group to obtain minimalscatter in the galaxies on a Hubble diagram, we find mass ratios of0.8:1.0 for our galaxy and Andromeda and 0.54:1.00 for the M82 and M81galaxies, in good agreement with the observed ratios of the luminositiesof these galaxies.
|Associations of Dwarf Galaxies|
The Hubble Space Telescope Advanced Camera for Surveys has been used todetermine accurate distances for 20 galaxies from measurements of theluminosity of the brightest red giant branch stars. Five associations ofdwarf galaxies that had originally been identified based on strongcorrelations on the plane of the sky and in velocity are shown to beequally well correlated in distance. Two more associations with similarproperties have been discovered. Another association is identified thatis suggested to be unbound through tidal disruption. The associationshave the spatial and kinematic properties expected of bound structureswith (1-10)×1011 Msolar. However, theseentities have little light, with the consequence that the mass-to-lightratios are in the range 100-1000 MsolarL-1solar. Within a well-surveyed volume extendingto a 3 Mpc radius, all but one known galaxy lie within one of the groupsor associations that have been identified.
|Objective Classification of Spiral Galaxies Having Extended Rotation Curves Beyond the Optical Radius|
We carry out an objective classification of four samples of spiralgalaxies having extended rotation curves beyond the optical radius. Amultivariate statistical analysis (viz., principal component analysis[PCA]) shows that about 96% of the total variation is due to twocomponents, one being the combination of absolute blue magnitude andmaximum rotational velocity beyond the optical region and the otherbeing the central density of the halo. On the basis of PCA a fundamentalplane has been constructed that reduces the scatter in the Tully-Fisherrelation up to a maximum of 16%. A multiple stepwise regression analysisof the variation of the overall shape of the rotation curves shows thatit is mainly determined by the central surface brightness, while theshape purely in the outer part of the galaxy (beyond the optical radius)is mainly determined by the size of the galactic disk.
|Optical Photometry of the Type II-P Supernova 2004dj in NGC 2403|
We present photometric data of the Type II-P supernova (SN) 2004dj inNGC 2403. The multicolor light curves cover the SN from ~60 to 200 daysafter explosion and are measured with a set of intermediate-band filtersthat have the advantage of tracing the strength variations of somespectral features. The light curves show a flat evolution in the middleof the plateau phase, then decline exponentially at the late times, witha rate of 0.10+/-0.03 mag (10 days)-1 in most of the filters.In the nebular phase, the spectral energy distribution of SN 2004djshows a steady increase in the flux near 6600 and 8500 Å, whichmay correspond to the emission lines of Hα and the Ca II near-IRtriplet, respectively. The photometric behavior suggests that SN 2004djis a normal SN II-P. Compared with the light curves of another typicalSN II-P, 1999em, we estimate the explosion date to be 2004 June 10+/-21UT (JD 2,453,167+/-21) for SN 2004dj. We also estimate the ejectednickel mass during the explosion to be M(56Ni)=0.023+/-0.005Msolar from two different methods, which is typical for a SNII-P. We derive the explosion energy to beE~0.75+0.56-0.38×1051 ergs, theejecta mass to be M~10.0+7.4-5.2Msolar, and the initial radius to beR~282+253-122 Rsolar for thepresupernova star of SN 2004dj, which are consistent with other typicalSNe II-P.
|Advanced Camera for Surveys Imaging of 25 Galaxies in Nearby Groups and in the Field|
We present Hubble Space Telescope Advanced Camera for Surveys images andcolor-magnitude diagrams for 25 nearby galaxies with radial velocitiesVLG<500 km s-1. Distances are determined basedon the luminosities of stars at the tip of the red giant branch thatrange from 2 to 12 Mpc. Two of the galaxies, NGC 4163 and IC 4662, arefound to be the nearest known representatives of blue compact dwarfobjects. Using high-quality data on distances and radial velocities of110 nearby field galaxies, we derive their mean Hubble ratio to be 68 kms-1 Mpc-1 with a standard deviation of 15 kms-1 Mpc-1. Peculiar velocities of most of thegalaxies, Vpec=VLG-68D, follow a Gaussiandistribution with σv=63 km s-1 but with atail toward high negative values. Our data display the known correlationbetween peculiar velocity and galaxy elevation above the LocalSupercluster plane. The small observed fraction of galaxies with highpeculiar velocities, Vpec<-500 km s-1, may beunderstood as objects associated with nearby groups (Coma I, Eridanus)outside the local volume.
|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.
|The Star Formation Threshold in NGC 6822|
We investigate the star formation threshold in NGC 6822, a nearby LocalGroup dwarf galaxy, on subkiloparsec scales using high-resolution,wide-field, deep H I, Hα, and optical data. In a study of the H Ivelocity profiles we identify cool and warm neutral components in theinterstellar medium of NGC 6822. We show that the velocity dispersion ofthe cool component (~4 km s-1) when used with a ToomreQ-criterion gives an optimal description of ongoing star formation inNGC 6822 superior to that using the more conventional dispersion valueof 6 km s-1. However, a simple constant surface densitycriterion for star formation gives an equally superior description. Wealso investigate the two-dimensional distribution of Q and the starformation threshold and find that these results also hold locally. Therange in gas density in NGC 6822 is much larger than the range incritical density, and we argue that the conditions for star formation inNGC 6822 are fully driven by this density criterion. Star formation islocal, and in NGC 6822 global rotational or shear parameters areapparently not important.
|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.
|The AMIGA sample of isolated galaxies. II. Morphological refinement|
We present a refinement of the optical morphologies for galaxies in theCatalog of Isolated Galaxies that forms the basis of the AMIGA (Analysisof the interstellar Medium of Isolated GAlaxies) project. Uniformreclassification using the digitized POSS II data benefited from thehigh resolution and dynamic range of that sky survey. Comparison withindependent classifications made for an SDSS overlap sample of more than200 galaxies confirms the reliability of the early vs. late-typediscrimination and the accuracy of spiral subtypes within Δ T =1-2. CCD images taken at the Observatorio de Sierra Nevada were alsoused to solve ambiguities in early versus late-type classifications. Aconsiderable number of galaxies in the catalog (n = 193) are flagged forthe presence of nearby companions or signs of distortion likely due tointeraction. This most isolated sample of galaxies in the local Universeis dominated by two populations: 1) 82% are spirals (Sa-Sd) with thebulk being luminous systems with small bulges (63% between types Sb-Sc)and 2) a significant population of early-type E-S0 galaxies (14%). Mostof the types later than Sd are low luminosity galaxies concentrated inthe local supercluster where isolation is difficult to evaluate. Thelate-type spiral majority of the sample spans a luminosity rangeMB-corr = -18 to -22 mag. Few of the E/S0 population are moreluminous than -21.0 marking the absence of the often-sought superL* merger (e.g. fossil elliptical) population. The rarity ofhigh luminosity systems results in a fainter derived M* forthis population compared to the spiral optical luminosity function(OLF). The E-S0 population is from 0.2 to 0.6 mag fainter depending onhow the sample is defined. This marks the AMIGA sample as unique amongsamples that compare early and late-type OLFs separately. In othersamples, which always involve galaxies in higher density environments,M^*_E/S0 is almost always 0.3-0.5 mag brighter than M^*_S, presumablyreflecting a stronger correlation between M* andenvironmental density for early-type galaxies.
|The C star population of DDO 190|
We have carried out deep R, I, CN, TiO observations of the dwarfirregular galaxy DDO 190. We confirm the existence of anintermediate-age population around this galaxy. The identification of 47carbon stars seen up to 5 arcmin from the centre of the galaxy impliesthat the population distribution of DDO 190 is similar to those found insome other Local Group dIrr galaxies. An estimate of the metallicity,[Fe/H] = -1.55 ± 0.12, is obtained based on the observed C/Mratio. From the analysis of star counts, corrected for the radialvariation of the incompleteness level, we determine a scale-lengthα = 40 ± 5'', in agreement with the recent literature.
|Balance of Dark and Luminous Mass in Rotating Galaxies|
A fine balance between dark and baryonic mass is observed in spiralgalaxies. As the contribution of the baryons to the total rotationvelocity increases, the contribution of the dark matter decreases by acompensating amount. This poses a fine-tuning problem for ΛCDMgalaxy formation models, and may point to new physics for dark matterparticles or even a modification of gravity.
|Detection of Neutrinos from Supernovae in Nearby Galaxies|
While existing detectors would see a burst of many neutrinos from aMilky Way supernova, the supernova rate is only a few per century. As analternative, we propose the detection of 1 neutrino per supernovafrom galaxies within 10 Mpc, in which there were at least 9core-collapse supernovae since 2002. With a future 1 Mton scaledetector, this could be a faster method for measuring the supernovaneutrino spectrum, which is essential for calibrating numerical modelsand predicting the redshifted diffuse spectrum from distant supernovae.It would also allow a ≳104 times more precise triggertime than optical data alone for high-energy neutrinos and gravitationalwaves.
|Classifications of the Host Galaxies of Supernovae, Set III|
A homogeneous sample comprising host galaxies of 604 recent supernovae,including 212 objects discovered primarily in 2003 and 2004, has beenclassified on the David Dunlap Observatory system. Most SN 1991bg-likeSNe Ia occur in E and E/Sa galaxies, whereas the majority of SN1991T-like SNe Ia occur in intermediate-type galaxies. This differenceis significant at the 99.9% level. As expected, all types of SNe II arerare in early-type galaxies, whereas normal SNe Ia occur in all Hubbletypes. This difference is significant at the 99.99% level. A smallnumber of SNe II in E galaxies might be due to galaxy classificationerrors or to a small young-population component in these mainly oldobjects. No significant difference is found between the distributionsover the Hubble type of SNe Ibc and SNe II. This confirms that both ofthese types of objects have similar (massive) progenitors. The presentdata show that in order to understand the dependence of supernova typeon host-galaxy population, it is more important to obtain accuratemorphological classifications than it is to increase the size of thedata sample.
|Supernova 1954J (Variable 12) in NGC 2403 Unmasked|
We have confirmed that the precursor star of the unusual supernova 1954J(also known as Variable 12) in NGC 2403 survived what appears to havebeen a superoutburst, similar to the 1843 Great Eruption of ηCarinae in the Galaxy. The apparent survivor has changed little inbrightness and color over the last 8 years, and a Keck spectrum revealscharacteristics broadly similar to those of η Car. This is furthersuggested by our identification of the actual outburst-surviving star inhigh-resolution images obtained with the Advanced Camera for Surveys onthe Hubble Space Telescope. We reveal this ``supernova impostor'' as ahighly luminous (M0V~-8.0 mag), very massive(Minitial>~25 Msolar) eruptive star, nowsurrounded by a dusty (AV~4 mag) nebula, similar to ηCar's famous Homunculus.Based in part on observations with the NASA/ESA Hubble Space Telescope,obtained at the Space Telescope Science Institute (STScI), which isoperated by AURA, Inc., under NASA contract NAS5-26555.
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