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|Absolute Magnitude Distributions and Light Curves of Stripped-Envelope Supernovae|
The absolute visual magnitudes of three Type IIb, 11 Type Ib, and 13Type Ic supernovae (collectively known as stripped-envelope supernovae)are studied by collecting data on the apparent magnitude, distance, andinterstellar extinction of each event. Weighted and unweighted meanabsolute magnitudes of the combined sample, as well as various subsetsof the sample, are reported. The limited sample size and theconsiderable uncertainties, especially those associated with extinctionin the host galaxies, prevent firm conclusions regarding differencesbetween the absolute magnitudes of supernovae of Types Ib and Ic, andregarding the existence of separate groups of overluminous andnormal-luminosity stripped-envelope supernovae. The spectroscopiccharacteristics of the events of the sample are considered. Three of thefour overluminous events are known to have had unusual spectra. Most butnot all of the normal-luminosity events have had typical spectra. Thelight curves of stripped-envelope supernovae are collected and compared.Because SN 1994I in M51 was very well observed, it often is regarded asthe prototypical Type Ic supernova, but it has the fastest light curvein the sample. Light curves are modeled by means of a simple analyticaltechnique that, combined with a constraint on E/M from spectroscopy,yields internally consistent values of ejected mass, kinetic energy, andnickel mass.
|The Molecular Interstellar Medium of Dwarf Galaxies on Kiloparsec Scales: A New Survey for CO in Northern, IRAS-detected Dwarf Galaxies|
We present a new survey for CO in dwarf galaxies using the ARO Kitt Peak12 m telescope. This survey consists of observations of the centralregions of 121 northern dwarfs with IRAS detections and no known COemission. We detect CO in 28 of these galaxies and marginally detectanother 16, increasing by about 50% the number of such galaxies known tohave significant CO emission. The galaxies we detect are comparable instellar and dynamical mass to the Large Magellanic Cloud, althoughsomewhat brighter in CO and fainter in the far-IR. Within dwarfs, wefind that the CO luminosity LCO is most strongly correlatedwith the K-band and the far-infrared luminosities. There are also strongcorrelations with the radio continuum (RC) and B-band luminosities andlinear diameter. Conversely, we find that far-IR dust temperature is apoor predictor of CO emission within the dwarfs alone, although a goodpredictor of normalized CO content among a larger sample of galaxies. Wesuggest that LCO and LK correlate well because thestellar component of a galaxy dominates the midplane gravitational fieldand thus sets the pressure and density of the atomic gas, which controlthe formation of H2 from H I. We compare our sample with moremassive galaxies and find that dwarfs and large galaxies obey the samerelationship between CO and the 1.4 GHz RC surface brightness. Thisrelationship is well described by a Schmidt law withΣRC~Σ1.3CO. Therefore,dwarf galaxies and large spirals exhibit the same relationship betweenmolecular gas and star formation rate (SFR). We find that this result isrobust to moderate changes in the RC-to-SFR and CO-to-H2conversion factors. Our data appear to be inconsistent with large (orderof magnitude) variations in the CO-to-H2 conversion factor inthe star-forming molecular gas.
|Active and Star-forming Galaxies and Their Supernovae|
To investigate the extent to which nuclear starbursts or other nuclearactivity may be connected with enhanced star formation activity in thehost galaxy, we perform a statistical investigation of supernovae (SNe)discovered in host galaxies from four samples: the Markarian galaxiessample, the Second Byurakan Survey (SBS) sample, the north Galactic pole(NGP) sample of active or star-forming galaxies, and the NGP sample ofnormal galaxies. Forty-seven SNe in 41 Mrk galaxies, 10 SNe in six SBSgalaxies, 29 SNe in 26 NGP active or star-forming galaxies, and 29 SNein 26 NGP normal galaxies have been studied. We find that the rate ofSNe, particularly core-collapse (Types Ib/c and II) SNe, is higher inactive or star-forming galaxies in comparison with normal galaxies.Active or star-forming host galaxies of SNe are generally of latermorphological type and have lower luminosity and smaller linear sizethan normal host galaxies of SNe. The radial distribution of SNe inactive and star-forming galaxies shows a higher concentration toward thecenter of the active host galaxy than is the case for normal hostgalaxies, and this effect is more pronounced for core-collapse SNe.Ib/c-type SNe have been discovered only in active and star-forminggalaxies of our samples. About 78% of these SNe are associated with H IIregions or are located very close to the nuclear regions of these activegalaxies, which are in turn hosting AGNs or starburst nuclei. Besidesthese new results, our study also supports the conclusions of severalother earlier papers. We find that Type Ia SNe occur in all galaxytypes, whereas core-collapse SNe of Types Ib/c and II are found only inspiral and irregular galaxies. The radial distribution of Type Ib SNe intheir host galaxies is more centrally concentrated than that of Type IIand Ia SNe. The radial distances of Types Ib/c and II SNe, from thenuclei of their host galaxies, is larger for barred spiral hosts.Core-collapse SNe are concentrated in spiral arms and are often close toor in the H II regions, whereas Type Ia SNe show only a looseassociation with spiral arms and no clear association with H II regions.
|Dwarf and Normal Spiral Galaxies: are they Self-Similar?|
The investigation presented here was focused on clarifying the existenceof dwarf spiral galaxies as a separate group from classical spirals.First, a list of spiral galaxies with small sizes was obtained.Information on colors, luminosities, morphologies and chemical contentwas searched for in the literature for these galaxies. Using thisinformation, it can be concluded that dwarf spirals are not likely to bethe tail of the distribution of classical galaxies. On the contrary,significant differences in some of the most important properties ofspiral galaxies, such as the metallicity gradient and the bar frecuency,were found. In any case, further and more accurate observations areneeded for a definitive answer.
|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.
|[O II] as a Star Formation Rate Indicator|
We investigate the [O II] emission line as a star formation rate (SFR)indicator using integrated spectra of 97 galaxies from the Nearby FieldGalaxies Survey (NFGS). The sample includes all Hubble types andcontains SFRs ranging from 0.01 to 100 Msolaryr-1. We compare the Kennicutt [O II] and Hα SFRcalibrations and show that there are two significant effects thatproduce disagreement between SFR([O II]) and SFR(Hα): reddeningand metallicity. Differences in the ionization state of the interstellarmedium do not contribute significantly to the observed differencebetween SFR([O II]) and SFR(Hα) for the NFGS galaxies withmetallicities log(O/H)+12>~8.5. The Kennicutt [O II]-SFR relationassumes a typical reddening for nearby galaxies; in practice, thereddening differs significantly from sample to sample. We derive a newSFR([O II]) calibration that does not contain a reddening assumption.Our new SFR([O II]) calibration also provides an optional correction formetallicity. Our SFRs derived from [O II] agree with those derived fromHα to within 0.03-0.05 dex. We show that the reddening, E(B-V),increases with intrinsic (i.e., reddening-corrected) [O II] luminosityfor the NFGS sample. We apply our SFR([O II]) calibration withmetallicity correction to two samples: high-redshift 0.8
|H I and CO in Blue Compact Dwarf Galaxies: Haro 2 and Haro 4|
We present Very Large Array H I imaging of two blue compact dwarfgalaxies, Haro 2 and Haro 4, and of the spiral galaxy Haro 26, which isprojected some 22' southwest of Haro 4. We also show a map of theCO(1-0) distribution of Haro 2 obtained with the Owens Valley RadioObservatory Millimeter Array, as well as derive an upper limit forCO(2-1) emission from Haro 4 obtained with the Caltech SubmillimeterObservatory. The H I data of Haro 2 reveal that the kinematical majoraxis lies perpendicular to the photometric major axis, indicating thatthe atomic hydrogen rotates about the major axis of the galaxy. Thisconfirms earlier indications based on CCD photometry that Haro 2 is adust-lane dE rather than a dIrr. We propose that the present neutral andmolecular ISM configuration is due to recent gas accretion or a merger.The H I distribution and dynamics of Haro 4 and the neighboring spiralHaro 26 suggest that they are currently undergoing a tidal interaction,reinforcing the notion that interactions play an important role intriggering the star formation witnessed in blue compact galaxies.
|Globular Clusters as Candidates for Gravitational Lenses to Explain Quasar-Galaxy Associations|
We argue that globular clusters (GCs) are good candidates forgravitational lenses in explaining quasar-galaxy associations. Thecatalog of associations (Bukhmastova 2001) compiled from the LEDAcatalog of galaxies (Paturel 1997) and from the catalog of quasars(Veron-Cetty and Veron 1998) is used. Based on the new catalog, we showthat one might expect an increased number of GCs around irregulargalaxies of types 9 and 10 from the hypothesis that distant compactsources are gravitationally lensed by GCs in the halos of foregroundgalaxies. The King model is used to determine the central surfacedensities of 135 GCs in the Milky Way. The distribution of GCs incentral surface density was found to be lognormal.
|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.
|Neutral Hydrogen Mapping of Virgo Cluster Blue Compact Dwarf Galaxies|
A new installment of neutral hydrogen mappings of blue compact dwarf(BCD) galaxies, as defined by optical morphology, in and near the VirgoCluster is presented. The primary motivation was to search for outlyingclouds of H I as potential interactive triggers of enhanced starformation, and therefore the mapped galaxies were selected for large H Imass, large optical diameter, and large velocity profile width.Approximately half the sample proved to have one or more small, lowcolumn density, star-free companion clouds, either detached or appearingas an appendage in our maps, at a resolution on the order of 4 kpc.Comparison is made with a sample of similarly mapped field BCD galaxiesdrawn from the literature; however, the Virgo Cluster sample of mappedBCDs is still too small for conclusive comparisons to be made. We found,on the one hand, little or no evidence for ram-pressure stripping nor,on the other, for extremely extended low column density H I envelopes.The H I rotation curves in most cases rise approximately linearly andslowly, as far out as we can trace the gas.
|A search for Low Surface Brightness galaxies in the near-infrared. I. Selection of the sample|
A sample of about 3800 Low Surface Brightness (LSB) galaxies wasselected using the all-sky near-infrared (J, H and Ks-band)2MASS survey. The selected objects have a mean central surfacebrightness within a 5'' radius around their centre fainter than 18 magarcsec-2 in the Ks band, making them the lowestsurface brightness galaxies detected by 2MASS. A description is given ofthe relevant properties of the 2MASS survey and the LSB galaxy selectionprocedure, as well as of basic photometric properties of the selectedobjects. The latter properties are compared to those of other samples ofgalaxies, of both LSBs and ``classical'' high surface brightness (HSB)objects, which were selected in the optical. The 2MASS LSBs have aBT_c-KT colour which is on average 0.9 mag bluerthan that of HSBs from the NGC. The 2MASS sample does not appear tocontain a significant population of red objects.All tables and Figs. 2a-c are only available in electronic form athttp://www.edpsciences.org
|A Hubble Space Telescope Snapshot Survey of Nearby Supernovae|
We present photometry of 13 recent supernovae (SNe) recovered in aHubble Space Telescope Snapshot program and tie the measurements toearlier ground-based observations in order to study the late-timeevolution of the SNe. Many of the ground-based measurements arepreviously unpublished and were made primarily with a robotic telescope,the Katzman Automatic Imaging Telescope. Evidence for circumstellarinteraction is common among the core-collapse SNe. Late-time declinerates for Type IIn SNe are found to span a wide range, perhaps becauseof differences in circumstellar interaction. An extreme case, SN IIn1995N, declined by only 1.2 mag in V over about 4 yr followingdiscovery. Template images of some SNe must therefore be obtained manyyears after the explosion if contamination from the SN itself is to beminimized. Evidence is found against a previous hypothesis that the TypeIIn SN 1997bs was actually a superoutburst of a luminous blue variablestar. The peculiar SN Ic 1997ef, a ``hypernova,'' declined very slowlyat late times. The decline rate of the SN Ia 2000cx decreased at latetimes, but this is unlikely to have been caused by a light echo.Partially based on observations with the NASA/ESA Hubble SpaceTelescope, obtained at the Space Telescope Science Institute, which isoperated by the Association of Universities for Research in Astronomy(AURA), Inc., under NASA contract NAS 5-26555.
|The Hα and Infrared Star Formation Rates for the Nearby Field Galaxy Survey|
We investigate the Hα and infrared star formation rate (SFR)diagnostics for galaxies in the Nearby Field Galaxy Survey (NFGS). Forthe 81 galaxies in our sample, we derive Hα fluxes (included here)from integrated spectra. There is a strong correlation between the ratioof far-infrared to optical luminosities L(FIR)/L(Hα) and theextinction E(B-V) measured with the Balmer decrement. Before reddeningcorrection, the SFR(IR) and SFR(Hα) are related to each other by apower law: SFR(IR)=(2.7+/-0.3)SFR(Hα)1.30+/-0.06.Correction of the SFR(Hα) for extinction using the Balmerdecrement and a classical reddening curve both reduces the scatter inthe SFR(IR)-SFR(Hα) correlation and results in a much closeragreement between the two SFR indicators;SFR(IR)=(0.91+/-0.04)SFR(Hαcorr)1.07+/-0.03.SFR(IR) and SFR(Hα) agree to ~10%. This SFR relationship spans 4orders of magnitude and holds for all Hubble types with IRAS detectionsin the NFGS. A constant ratio between the SFR(IR) and SFR(Hα) forall Hubble types, including early types (S0-Sab), suggests that the IRemission in all these objects results from a young stellar population.
|Bar Galaxies and Their Environments|
The prints of the Palomar Sky Survey, luminosity classifications, andradial velocities were used to assign all northern Shapley-Ames galaxiesto either (1) field, (2) group, or (3) cluster environments. Thisinformation for 930 galaxies shows no evidence for a dependence of barfrequency on galaxy environment. This suggests that the formation of abar in a disk galaxy is mainly determined by the properties of theparent galaxy, rather than by the characteristics of its environment.
|An H I Survey of Actively Star-forming Dwarf Galaxies|
We present the results of H I 21 cm observations of 139 activelystar-forming dwarf galaxies obtained with the 305 m radio telescope atArecibo Observatory. Our sample consists of all objects cataloged inobjective-prism surveys for UV-excess or emission-line galaxiespublished prior to the start of the survey that have luminosities belowMB=-17.0 and that are located within the declination limitsof the Arecibo telescope. Galaxies from the Markarian, Michigan, Case,Wasilewski, Haro, and Zwicky lists are included. The sample spans a widerange of both H I gas content and star formation levels. A total of 122objects (88%) were detected; 82 galaxies have been observed for thefirst time in H I. The median velocity width for our sample is 88 kms-1, and the median H I gas mass is 3.0×108Msolar. In general, the sample galaxies are gas-rich, with anaverage MHI/LB=1.3 after correcting for theluminosity enhancement due to the starburst. The progenitors of many ofthe star-forming dwarfs have higher MHI/LB thantypically seen in samples of nearby ``normal'' galaxies, emphasizingtheir distinct nature.
|Neutral hydrogen and optical observations of edge-on galaxies: Hunting for warps|
We present 21-cm HI line and optical R-band observations for a sample of26 edge-on galaxies. The HI observations were obtained with theWesterbork Synthesis Radio Telescope, and are part of the WHISP database(Westerbork HI Survey of Spiral and Irregular Galaxies). We present HImaps, optical images, and radial HI density profiles. We have alsoderived the rotation curves and studied the warping and lopsidedness ofthe HI disks. 20 out of the 26 galaxies of our sample are warped,confirming that warping of the HI disks is a very common phenomenon indisk galaxies. Indeed, we find that all galaxies that have an extendedHI disk with respect to the optical are warped. The warping usuallystarts around the edge of the optical disk. The degree of warping variesconsiderably from galaxy to galaxy. Furthermore, many warps areasymmetric, as they show up in only one side of the disk or exhibitlarge differences in amplitude in the approaching and receding sides ofthe galaxy. These asymmetries are more pronounced in rich environments,which may indicate that tidal interactions are a source of warpasymmetry. A rich environment tends to produce larger warps as well. Thepresence of lopsidedness seems to be related to the presence of nearbycompanions. Full Fig. 13 is only available in electronic form athttp://www.edpsciences.org
|Warps and correlations with intrinsic parameters of galaxies in the visible and radio|
From a comparison of the different parameters of warped galaxies in theradio, and especially in the visible, we find that: a) No large galaxy(large mass or radius) has been found to have high amplitude in thewarp, and there is no correlation of size/mass with the degree ofasymmetry of the warp. b) The disc density and the ratio of dark toluminous mass show an opposing trend: smaller values give moreasymmetric warps in the inner radii (optical warps) but show nocorrelation with the amplitude of the warp; however, in the externalradii is there no correlation with asymmetry. c) A third anticorrelationappears in a comparison of the amplitude and degree of asymmetry in thewarped galaxies. Hence, it seems that very massive dark matter haloeshave nothing to do with the formation of warps but only with the degreeof symmetry in the inner radii, and are unrelated to the warp shape forthe outermost radii. Denser discs show the same dependence.
|The Westerbork HI survey of spiral and irregular galaxies. II. R-band surface photometry of late-type dwarf galaxies|
R-band surface photometry is presented for 171 late-type dwarf andirregular galaxies. For a subsample of 46 galaxies B-band photometry ispresented as well. We present surface brightness profiles as well asisophotal and photometric parameters including magnitudes, diameters andcentral surface brightnesses. Absolute photometry is accurate to 0.1 magor better for 77% of the sample. For over 85% of the galaxies the radialsurface brightness profiles are consistent with published data withinthe measured photometric uncertainty. For most of the galaxies in thesample H I data have been obtained with the Westerbork Synthesis RadioTelescope. The galaxies in our sample are part of the WHISP project(Westerbork H I Survey of Spiral and Irregular Galaxies), which aims atmapping about 500 nearby spiral and irregular galaxies in H I. Theavailability of H I data makes this data set useful for a wide range ofstudies of the structure, dark matter content and kinematics oflate-type dwarf galaxies. Based on observations made with INT operatedon the island of La Palma by the Isaac Newton Group in the SpanishObservatorio del Roque de los Muchachos of the Instituto de Astrofisicade Canarias. The tables in Appendix A are only available in electronicform at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (126.96.36.199)or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/390/863. Thefigures in Appendix B are only available in electronic formhttp://www.edpsciences.org
|Bipolar Supernova Explosions|
We discuss the optical spectropolarimetry of several core-collapsesupernovae, SN 1996cb (Type IIB), SN 1997X (Type Ic), and SN 1998S (TypeIIn). The data show polarization evolution of several spectral featuresat levels from 0.5% to above 4%. The observed line polarization isintrinsic to the supernovae and not of interstellar origin. These datasuggest that the distribution of ejected matter is highly aspherical. Inthe case of the Type IIn SN 1998S, the major-to-minor axis ratio must belarger than 2.5 if the polarization is 3% from an oblate spheroidalejecta seen edge-on. A well-defined symmetry axis can be deduced fromspectropolarimetry for SN 1998S, but the Type IIB events SN 1993J and SN1996cb seem to possess much more complicated geometries withpolarization position angles showing larger irregular variations acrossspectral features; the latter may be associated with large-scaleclumpiness of the ejecta. The observed degree of polarization of theType Ic SN 1997X is above 4%. The data reveal a trend that the degree ofpolarization increases with decreasing envelope mass and with the depthwithin the ejecta. The high axial ratio of the ejecta is difficult toexplain in terms of the conventional neutrino-driven core-collapsemodels for Type II explosions. Highly asymmetric explosion mechanismssuch as the formation of bipolar jets during core collapse may be anecessary ingredient for models of all core-collapse supernovae.
|Redshifts for 2410 Galaxies in the Century Survey Region|
The Century Survey strip covers 102 deg2 within the limits8h5<=α<=16h5, 29.0d<=δ<=30.0d, equinoxB1950.0. The strip passes through the Corona Borealis supercluster andthe outer region of the Coma cluster. Within the Century Survey region,we have measured 2410 redshifts that constitute four overlappingcomplete redshift surveys: (1) 1728 galaxies with Kron-CousinsRph<=16.13 covering the entire strip, (2) 507 galaxieswith Rph<=16.4 in right ascension range8h32m<=α<=10h45m, equinox B1950.0, (3) 1251 galaxies withabsorption- and K-corrected RCCDc<=16.2 (where ``c''indicates ``corrected'') covering the right ascension range8h5<=α<=13h5, equinox B1950.0, and (4) 1255 galaxieswith absorption- and K-corrected VCCDc<=16.7 also coveringthe right ascension range 8h5<=α<=13h5, equinoxB1950.0. All these redshift samples are more than 98% complete to thespecified magnitude limit. We derived samples 1 and 2 from scans of thePOSS1 red (E) plates calibrated with CCD photometry. We derived samples3 and 4 from deep V and R CCD images covering the entire region. Weinclude coarse morphological types for all the galaxies in sample 1. Thedistribution of (V-R)CCD for each type correspondsappropriately with the classification. Work reported here is basedpartly on observations obtained at the Michigan-Dartmouth-MITObservatory.
|Optical Spectroscopy of Type IB/C Supernovae|
We present 84 spectra of Type Ib/c and Type IIb supernovae (SNe),describing the individual SNe in detail. The relative depths of thehelium absorption lines in the spectra of the SNe Ib appear to provide ameasurement of the temporal evolution of the SN, with He I λ5876and He I λ7065 growing in strength relative to He I λ6678over time. Light curves for three of the SNe Ib provide a sequence forcorrelating the helium line strengths. We find that some SNe Ic showevidence for weak helium absorption, but most do not. Aside from thepresence or absence of the helium lines, there are other spectroscopicdifferences between SNe Ib and SNe Ic. On average, the O I λ7774line is stronger in SNe Ic than in SNe Ib. In addition, the SNe Ic havedistinctly broader emission lines at late times, indicating aconsistently larger explosion energy and/or lower envelope mass for SNeIc than for SNe Ib. While SNe Ib appear to be basically homogeneous, theSNe Ic are quite heterogeneous in their spectroscopic characteristics.Three SNe Ic that may have been associated with gamma-ray bursts arealso discussed; two of these have clearly peculiar spectra, while thethird seems fairly typical.
|Spectrophotometry of Nearby Field Galaxies: The Data|
We have obtained integrated and nuclear spectra as well as U, B, Rsurface photometry for a representative sample of 196 nearby galaxies.These galaxies span the entire Hubble sequence in morphological type, aswell as a wide range of luminosities (MB=-14 to -22). Here wepresent the spectrophotometry for these galaxies. The selection of thesample and the U, B, R surface photometry is described in a companionpaper. Our goals for the project include measuring the current starformation rates and metallicities of these galaxies, and elucidatingtheir star formation histories, as a function of luminosity andmorphology. We thereby extend the work of Kennicutt to lower luminositysystems. We anticipate that our study will be useful as a benchmark forstudies of galaxies at high redshift. We describe the observing, datareduction, and calibration techniques and demonstrate that ourspectrophotometry agrees well with that of Kennicutt. The spectra spanthe range 3550-7250 Å at a resolution (FWHM) of ~6 Å andhave an overall relative spectrophotometric accuracy of ~+/-6%. Wepresent a spectrophotometric atlas of integrated and nuclear rest-framespectra as well as tables of equivalent widths and synthetic colors. Theatlas and tables of measurements will be made available electronically.We study the correlations of galaxy properties determined from thespectra and images. Our findings include: (1) galaxies of a givenmorphological class display a wide range of continuum shapes andemission-line strengths if a broad range of luminosities are considered,(2) emission-line strengths tend to increase and continua tend to getbluer as the luminosity decreases, and (3) the scatter on the generalcorrelation between nuclear and integrated Hα emission-linestrengths is large.
|Surface Photometry of Nearby Field Galaxies: The Data|
We have obtained integrated spectra and multifilter photometry for arepresentative sample of ~200 nearby galaxies. These galaxies span theentire Hubble sequence in morphological type, as well as a wide range ofluminosities (MB=-14 to -22) and colors (B-R=0.4-1.8). Herewe describe the sample selection criteria and the U, B, R surfacephotometry for these galaxies. The spectrophotometric results will bepresented in a companion paper. Our goals for the project includemeasuring the current star formation rates and metallicity of thesegalaxies, and elucidating their star formation histories, as a functionof luminosity and morphology. We thereby extend the work of Kennicutt tolower luminosity systems. We anticipate that our study will be useful asa benchmark for studies of galaxies at high redshift. We discuss theobserving, data reduction, and calibration techniques and show that ourphotometry agrees well with previous work in those cases in whichearlier data are available. We present an atlas of images, radialsurface brightness profiles, and color profiles as well as tables ofderived parameters. The atlas and tables of measurements will be madeavailable electronically. We study the correlations of galaxy propertiesdetermined from the galaxy images. Our findings include the following:(1) colors determined within the effective radius correlate better withmorphological type than with MB and (2) 50% of thelow-luminosity galaxies are bluest in their centers.
|Nearby Optical Galaxies: Selection of the Sample and Identification of Groups|
In this paper we describe the Nearby Optical Galaxy (NOG) sample, whichis a complete, distance-limited (cz<=6000 km s-1) andmagnitude-limited (B<=14) sample of ~7000 optical galaxies. Thesample covers 2/3 (8.27 sr) of the sky (|b|>20deg) andappears to have a good completeness in redshift (97%). We select thesample on the basis of homogenized corrected total blue magnitudes inorder to minimize systematic effects in galaxy sampling. We identify thegroups in this sample by means of both the hierarchical and thepercolation ``friends-of-friends'' methods. The resulting catalogs ofloose groups appear to be similar and are among the largest catalogs ofgroups currently available. Most of the NOG galaxies (~60%) are found tobe members of galaxy pairs (~580 pairs for a total of ~15% of objects)or groups with at least three members (~500 groups for a total of ~45%of objects). About 40% of galaxies are left ungrouped (field galaxies).We illustrate the main features of the NOG galaxy distribution. Comparedto previous optical and IRAS galaxy samples, the NOG provides a densersampling of the galaxy distribution in the nearby universe. Given itslarge sky coverage, the identification of groups, and its high-densitysampling, the NOG is suited to the analysis of the galaxy density fieldof the nearby universe, especially on small scales.
|Arcsecond Positions of UGC Galaxies|
We present accurate B1950 and J2000 positions for all confirmed galaxiesin the Uppsala General Catalog (UGC). The positions were measuredvisually from Digitized Sky Survey images with rms uncertaintiesσ<=[(1.2")2+(θ/100)2]1/2,where θ is the major-axis diameter. We compared each galaxymeasured with the original UGC description to ensure high reliability.The full position list is available in the electronic version only.
|The I-Band Tully-Fisher Relation for SC Galaxies: 21 Centimeter H I Line Data|
A compilation of 21 cm line spectral parameters specifically designedfor application of the Tully-Fisher (TF) distance method is presentedfor 1201 spiral galaxies, primarily field Sc galaxies, for which opticalI-band photometric imaging is also available. New H I line spectra havebeen obtained for 881 galaxies. For an additional 320 galaxies, spectraavailable in a digital archive have been reexamined to allow applicationof a single algorithm for the derivation of the TF velocity widthparameter. A velocity width algorithm is used that provides a robustmeasurement of rotational velocity and permits an estimate of the erroron that width taking into account the effects of instrumental broadeningand signal-to-noise. The digital data are used to establish regressionrelations between measurements of velocity widths using other commonprescriptions so that comparable widths can be derived throughconversion of values published in the literature. The uniform H I linewidths presented here provide the rotational velocity measurement to beused in deriving peculiar velocities via the TF method.
|The I-Band Tully-Fisher Relation for SC Galaxies: Optical Imaging Data|
Properties derived from the analysis of photometric I-band imagingobservations are presented for 1727 inclined spiral galaxies, mostly oftypes Sbc and Sc. The reduction, parameter extraction, and errorestimation procedures are discussed in detail. The asymptotic behaviorof the magnitude curve of growth and the radial variation in ellipticityand position angle are used in combination with the linearity of thesurface brightness falloff to fit the disk portion of the profile. TotalI-band magnitudes are calculated by extrapolating the detected surfacebrightness profile to a radius of eight disk scale lengths. Errors inthe magnitudes, typically ~0.04 mag, are dominated by uncertainties inthe sky subtraction and disk-fitting procedures. Comparison is made withthe similar imaging database of Mathewson, Ford, & Buchhorn, both aspresented originally by those authors and after reanalyzing theirdigital reduction files using identical disk-fitting procedures. Directcomparison is made of profile details for 292 galaxies observed incommon. Although some differences occur, good agreement is found,proving that the two data sets can be used in combination with onlyminor accommodation of those differences. The compilation of opticalproperties presented here is optimized for use in applications of theTully-Fisher relation as a secondary distance indicator in studies ofthe local peculiar velocity field.
|The Study of a Type IIB Supernova: SN 1996CB|
We present spectroscopic and photometric results of SN 1996cb. Thesupernova was independently discovered in NGC 3510 by M. Aoki, T. Cho,& K. Toyama of Japan and Qiao et al. of Beijing AstronomicalObservatory on 1996 December 15 and 18, respectively. The results coverabout 6 months following the discovery. The first few spectra showedstrong Balmer lines with obvious P Cygni profiles, offering evidence ofa Type II supernova. The emergence of He I lines could be inferred inthese spectra. That the He I lines became quite prominent in the spectranear optical maximum confirmed that SN 1996cb was definitely a Type IIbsupernova, like SN 1987K and SN 1993J. The photometric results showedthat the B-I color evolution was very similar to that of SN 1993J.Comparing two color curves, we were able to estimate that the explosionof SN 1996cb occurred on UT 1996 December 12. Although the overall lightcurves resembled that of SN 1993J, they showed some differences,especially for the B band. SN 1996cb had a broad peak in the lightcurves, and it declined somewhat slowly and, compared with SN 1993J,exhibited a plateau-like shape between 20 and 50 days after the maximumfor the B and V bands. This indicates that there was relatively morehydrogen in the outer envelope of the progenitor of SN 1996cb. Thespectral evolution of SN 1996cb displayed further differences from SN1993J. In the case of SN 1996cb, the Balmer lines showed strong P Cygniprofiles at a very early time, resembling the early spectra of SN 1987A,a supernova resulting from a compact blue supergiant star. The dramaticchanges of expansion velocities at early times indicated that thephotosphere of SN 1996cb receded more quickly than it did in SN 1993J.The He I lines emerged much earlier and evolved more dramatically,causing SN 1996cb to display the features of a Type Ib supernova beforemaximum. This might be the result of a dramatic recession of thephotosphere at an early time; the He I lines and [O I] lines showedconspicuous blueshifts when they emerged, resembling the blueshifts ofthe [O I] lines that appeared in the late spectra of SN 1993J. This isprobably observational evidence of Rayleigh-Taylor instabilitiesoccurring at the interfaces between the H and He and the He and O+Clayers, respectively; Hα emission and absorption components,especially the latter, were conspicuous 100 days after the explosion. Wealso conclude that the outer envelope of SN 1996cb had relatively morehydrogen than was the case for SN 1993J, even though the amount remainedmuch less than is typical of other Type II supernovae. This finding isconsistent with the results of the photometry. The [O I] and O linesemerged very late and exhibited weak emission, indicating that theHe-rich layer was relatively thick. Combining the analyses ofphotometric and spectroscopic evolution, we conclude that the progenitorof SN 1996cb, like that of SN 1993J, was a stripped massive starexhibiting some special features: it was probably a more compact starwith a thick helium layer and a relatively more massive hydrogenenvelope.
|Radio Supernovae as Distance Indicators|
Long-term monitoring of the radio emission from supernovae with the VeryLarge Array (VLA) shows that the radio "light curves" evolve in asystematic fashion with a distinct peak flux density (and thus, incombination with a distance, a peak spectral luminosity) at eachfrequency and with a well-defined time from explosion to that peak.Studying these two quantities at 6 cm wavelength, peak spectralluminosity (L6 cm peak), and time after explosion date(t0) to reach that peak (t6 cm peak - t0), we find thatthey appear related. In particular, based on two objects, Type Ibsupernovae may be approximate radio "standard candles" with a 6 cm peakluminosity of L6 cm peak ~ 19.9 x 1026 ergs s-1 Hz-1;also, based on two objects, Type Ic supernovae may be approximate radiostandard candles with a 6 cm peak luminosity of L6 cm peak~ 6.5 x 1026 ergs s-1 Hz-1; and, based on 12 objects, Type II supernovaeappear to obey a relation L6 cm peak ~= 5.5 x1023(t6 cm peak - t0)1.4 ergs s-1 Hz-1, with time measuredin days. If these relations are supported by further observations, theyprovide a means for determining distances to supernovae, and thus totheir parent galaxies, from purely radio continuum observations. Withcurrently available sensitivity of the VLA, it is possible to employthese relations for objects further than the Virgo Cluster out to ~100Mpc. With planned improvements to the VLA and the possible constructionof more sensitive radio telescopes, these techniques could be extendedto z ~ 1 for some classes of bright radio supernovae.
|Dust-to-Gas Ratio and Metal Abundance in Dwarf Galaxies|
We have compared the metallicity (represented by oxygen abundance), Xo,and the dust-to-gas ratio, D , in a sample of dwarf galaxies. For dwarfirregulars (dIrrs) we find a good correlation between the twoquantities, with a power-law index of 0.52 +/- 0.25. Blue compact dwarf(BCD) galaxies do not show such a good correlation; in addition, boththe dust-to-gas ratio and the metallicity tend to be higher than fordIrrs. We have then developed a simple but physical analytical model forthe above relation. Comparing the model results with the data, weconclude that (i) for low values of D , the D-Xo relation isquasi-linear, whereas for higher values, the curve strongly deviatesfrom the linear behavior, implying that the commonly used power-lawapproximation is very poor; (ii) the deviation from the linear behaviordepends critically on the parameter chi , the "differential" massoutflow rate from the galaxy in units of the star formation rate, psi ;(iii) the shape of the D-Xo curve does not depend on psi ,but only on chi ; however, the specific location of a given galaxy onthe curve does depend on psi ; and (iv) the BCD metallicity segregationis the result of a higher psi , together with a significant differentialmass outflow. Thus, the lack of correlation can be produced by largelydifferent star formation rates and values of chi in these objects.
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