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Constraining Dark Matter Halo Profiles and Galaxy Formation Models Using Spiral Arm Morphology. I. Method Outline
We investigate the use of spiral arm pitch angles as a probe of diskgalaxy mass profiles. We confirm our previous result that spiral armpitch angles (P) are well correlated with the rate of shear (S) in diskgalaxy rotation curves by using a much larger sample (51 galaxies) thanused previously (17 galaxies). We use this correlation to argue thatimaging data alone can provide a powerful probe of galactic massdistributions out to large look-back times. In contrast to previouswork, we show that observed spiral arm pitch angles are similar whenmeasured in the optical (at 0.4 μm) and the near-infrared (at 2.1μm) with a mean difference of 2.3d+/-2.7d. This is then used tostrengthen the known correlation between P and S using B-band images. Wethen use two example galaxies to demonstrate how an inferred shear ratecoupled with a bulge-disk decomposition model and a Tully-Fisher-derivedvelocity normalization can be used to place constraints on a galaxy'sbaryon fraction and dark matter halo profile. We show that ESO 582-G12,a galaxy with a high shear rate (slightly declining rotation curve) at~10 kpc, favors an adiabatically contracted halo, with high initial NFWconcentration (cvir>16) and a high fraction of halobaryons in the form of stars (~15%-40%). In contrast, IC 2522 has a lowshear rate (rising rotation curve) at ~10 kpc and favorsnonadiabatically contracted models with low NFW concentrations(cvir~=2-8) and a low stellar baryon fraction <10%.
| Neon and Oxygen Abundances in M33
We present new spectroscopic observations of 13 H II regions in theLocal Group spiral galaxy M33. The regions observed range from 1 to 7kpc in distance from the nucleus. Of the 13 H II regions observed, the[O III] λ4363 line was detected in six regions. Electrontemperatures were thus able to be determined directly from the spectrausing the [O III] λλ4959, 5007/λ4363 line ratio.Based on these temperature measurements, oxygen and neon abundances andtheir radial gradients were calculated. For neon, a gradient of-0.016+/-0.017 dex kpc-1 was computed, which agrees with theNe/H gradient derived previously from ISO spectra. A gradient of-0.012+/-0.011 dex kpc-1 was computed for O/H, much shallowerthan was derived in previous studies. The newly calculated O/H and Ne/Hgradients are in much better agreement with each other, as expected frompredictions of stellar nucleosynthesis. We examine the correlationbetween the WC/WN ratio and metallicity, and find that the new M33abundances do not impact the observed correlation significantly. We alsoidentify two new He II-emitting H II regions in M33, the first to bediscovered in a spiral galaxy other than the Milky Way. In both casesthe nebular He II emission is not associated with Wolf-Rayet stars.Therefore, caution is warranted in interpreting the relationship betweennebular He II emission and Wolf-Rayet stars when both are observed inthe integrated spectrum of an H II region.
| IC 225: A Dwarf Elliptical Galaxy with a Peculiar Blue Core
We present the discovery of a peculiar blue core in the ellipticalgalaxy IC 225 by using images and spectra from the Sloan Digital SkySurvey (SDSS). The outer parts of the surface brightness profiles of u-,g-, r-, i-, and z-band SDSS images for IC 225 are well fitted with anexponential function. The fitting results show that IC 225 follows thesame relations between the magnitude, scale length, and central surfacebrightness as for dwarf elliptical galaxies and that its absolute bluemagnitude (MB) is -17.14 mag, all of which suggests that IC225 is a typical dwarf elliptical galaxy. The g-r color profileindicates a very blue core with a radius of 2", which is also clearlyseen in the RGB image made of g-, r-, and i-band SDSS images. The SDSSoptical spectrum exhibits strong and very narrow nebular emission lines.The metal abundances derived by the standard methods, which are12+log(O/H)=8.98, log(N/O)=-0.77, and12+log(S+/H+)=6.76, turn out to be significantlyhigher than those predicted by the well-known luminosity-metallicityrelation. After carefully inspecting the central region of IC 225, wefind that there are two distinct nuclei, separated by 1.4" theoff-nuclear one is even bluer than the nucleus of IC 225. The asymmetricline profiles of higher order Balmer lines indicate that the emissionlines are more blueshifted relative to the absorption lines, suggestingthat the line emission arises from the off-center core, a metal-rich HII region. To the best of our knowledge, it is the firsthigh-metallicity H II region detected in a dwarf elliptical galaxy.
| 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.
| Massive star formation in the central regions of spiral galaxies
Context: . The morphology of massive star formation in the centralregions of galaxies is an important tracer of the dynamical processesthat govern the evolution of disk, bulge, and nuclear activity. Aims. Wepresent optical imaging of the central regions of a sample of 73 spiralgalaxies in the Hα line and in optical broad bands, and deriveinformation on the morphology of massive star formation. Methods. Weobtained images with the William Herschel Telescope, mostly at a spatialresolution of below one second of arc. For most galaxies, no Hαimaging is available in the literature. We outline the observing anddata reduction procedures, list basic properties, and present the I-bandand continuum-subtracted Hα images. We classify the morphology ofthe nuclear and circumnuclear Hα emission and explore trends withhost galaxy parameters. Results. We confirm that late-type galaxies havea patchy circumnuclear appearance in Hα, and that nuclear ringsoccur primarily in spiral types Sa-Sbc. We identify a number ofpreviously unknown nuclear rings, and confirm that nuclear rings arepredominantly hosted by barred galaxies. Conclusions. Other than instimulating nuclear rings, bars do not influence the relative strengthof the nuclear Hα peak, nor the circumnuclear Hα morphology.Even considering that our selection criteria led to an over-abundance ofgalaxies with close massive companions, we do not find any significantinfluence of the presence or absence of a close companion on therelative strength of the nuclear Hα peak, nor on the Hαmorphology around the nucleus.
| A comparative analysis of empirical calibrators for nebular metallicity
We present a new analysis of the main empirical calibrators of oxygenabundance for ionized gas nebulae. With that aim we have compiled anextensive sample of objects with emission-line data including thenear-infrared [SIII] lines and the weak auroral lines which allow forthe determination of the gas electron temperature. For all the objectsthe oxygen abundances have been derived in a homogenous way, using themost recent sets of atomic coefficients and taking into the account theeffect of particle density on the temperature of O+. Theresiduals between directly and empirically derived abundances as afunction of abundance have been studied. A grid of photoionizationmodels, covering the range of physical properties of the gas, has beenused to explain the origin of the uncertainties affecting each abundancecalibrator. The range of validity for each abundance parameter has beenidentified and its average uncertainty has been quantified.
| Radio Continuum and Far-infrared Emission from the Galaxies in the Eridanus Group
The Eridanus galaxies follow the well-known radio-FIR correlation. Themajority (70%) of these galaxies have their star formation rates belowthat of the Milky Way. The galaxies that have a significant excess ofradio emission are identified as low luminosity AGNs based on theirradio morphologies obtained from the GMRT observations. There are nopowerful AGNs (L20 cm>1023WHz-1) in the group. The twomost far-infrared and radio luminous galaxies in the group have opticaland HI morphologies suggestive of recent tidal interactions. TheEridanus group also has two far-infrared luminous but radio-deficientgalaxies. It is believed that these galaxies are observed within a fewMyr of the onset of an intense star formation episode after beingquiescent for at least a 100 Myr. The upper end of the radio luminositydistribution of the Eridanus galaxies (L20 cm1022WHz-1) isconsistent with that of the field galaxies, other groups, and late-typegalaxies in nearby clusters.
| The Classification of Galaxies: Early History and Ongoing Developments
"You ask what is the use of classification, arrangement,systematization. I answer you; order and simplification are the firststeps toward the mastery of a subject the actual enemy is the unknown."
| Oxygen Abundance Determination in H II Regions: The Strong Line Intensities-Abundance Calibration Revisited
The problem of oxygen abundance determination in H II regions based onthe ``strong oxygen line intensities-oxygen abundance'' empiricalcalibration is revisited. A compilation of spectroscopic data of H IIregions in spiral and irregular galaxies with a measured [O III]λ4363 line intensity has been carried out, resulting in a samplecontaining more than 700 individual measurements. Methods are devised toselect out only high-precision measurements from that original sample.Te-based oxygen abundances have been recomputed in the sameway for all H II regions with high-precision measurements. That sampleof Te abundances is used to recalibrate the empiricalrelations between the oxygen abundance and the strong oxygen lineintensities for both high (the upper branch of the O/H-R23diagram) and low (the lower branch) metallicities, within the frameworkof the P method, where P is the excitation parameter. Concerninghigh-metallicity H II regions, an alternative way for deriving oxygenabundances using only measurements of the strong nebular oxygen lines isproposed. The method is based on a tight correlation between the flux inthe auroral [O III] λ4363 line and the fluxes in the nebular [OII] λλ3727, 3729 and [O III] λλ4959, 5007lines, called the ff relation. This relation is also used to select outhigh-metallicity H II regions with high-precision(O/H)Te measurements. In contrast to previouswork, the new upper branch P calibration is based only on(O/H)Te abundances. It is found that(O/H)P abundances usually agree well with the(O/H)ff abundances, although faint(logR23<~-0.5) low-excitation (P<~0.25) H II regionsmay show systematic differences that can be as large as ~0.1 dex. As forthe newly derived low-metallicity P calibration, it is shown to berobust. The calibrations derived from the sample containing all(O/H)Te abundance determinations and from thatcontaining only recent (since 1995) measurements are found to be in verygood agreement. For both low- and high-metallicity H II regions, the newcalibration gives (O/H)P abundances that agree with(O/H)Te abundances to within 0.1 dex.
| Emission-Line Spectroscopy of Damped Lyα Systems: The Case of SBS 1543+593/HS 1543+5921
We report HST/STIS spectroscopy and Gemini/GMOS-N imaging of the DampedLyα (DLA) system toward HS 1543+5921 caused by the hoststar-forming galaxy (SFG) SBS 1543+593. The Gemini image shows newmorphological details of this well-resolved DLA galaxy. In combinationwith previous optical spectra, the new UV spectra enable us to compare,for the first time, ionized and neutral gas-phase α-elementabundances derived from emission- and absorption-line spectroscopy, in abona fide DLA galaxy. The abundances we determine using emission-linediagnostics agree with those from absorption-line diagnostics. Wepresent our results on a metallicity versus redshift diagram thatcombines local H II regions and SFGs with high-redshift DLAs, anddiscuss implications for the chemical evolution of galaxies.
| A VLT study of metal-rich extragalactic H II regions. I. Observations and empirical abundances
We have obtained spectroscopic observations from 3600 Åto 9200Åwith FORS at the Very Large Telescope for approximately 70 Hiiregions located in the spiral galaxies NGC 1232, NGC 1365, NGC 2903,NGC 2997 and NGC 5236. These data are part of a project to measure thechemical abundances and characterize the massive stellar content ofmetal-rich extragalactic H iiregions. In this paper we describe ourdataset, and present emission line fluxes for the whole sample. In 32 Hiiregions we measure at least one of the following auroral lines: [S ii]λ4072, [N ii] λ5755, [S iii] λ6312 and [O ii]λ7325. From these we derive electron temperatures, as well asoxygen, nitrogen and sulphur abundances, using classical empiricalmethods (both so-called “Te-based methods” and“strong line methods”). Under the assumption that thetemperature does not introduce severe biases, we find that the mostmetal-rich nebulae with detected auroral lines are found at 12 +log(O/H) ≃ 8.9, i.e. about 60% larger than the adopted solarvalue. However, classical abundance determinations in metal-rich Hiiregions may be severely biased and must be tested with realisticphotoionization models. The spectroscopic observations presented in thispaper will serve as a homogeneous and high-quality database for suchpurposes.
| Effects of photon escape on diagnostic diagrams for H II regions
In this article we first outline the mounting evidence that asignificant fraction of the ionizing photons emitted by OB stars withinH ii regions escape from their immediate surroundings, i.e from what isnormally defined as the H ii region, and explain how an H ii regionstructure containing high density contrast inhomogeneities facilitatesthis escape. Next we describe sets of models containing inhomogeneitieswhich are used to predict tracks in the commonly used diagnosticdiagrams (based on ratios of emission lines) whose only independentvariable is the photon escape fraction, ξ. We show that the tracksproduced by the models in two of the most cited of these diagramsconform well to the distribution of observed data points, with themodels containing optically thick inhomogeneities (“CLUMPY”models) yielding somewhat better agreement than those with opticallythin inhomogeneities (“FF” models). We show how variationsin the ionization parameter U, derived from emission line ratios, couldbe due to photon escape, such that for a given region from which 50% ofits ionizing photons leak out we would derive the same value of U as fora region with no photon escape but with an input ionizing flux almost anorder of magnitude higher. This effect will occur whether the individualinhomogeneities are optically thick or thin. Photon escape will alsolead to a change in the derived value of the radiation hardnessparameter, and this change differs significantly between models withoptically thin and optically thick clumps. Using a rather wide range ofassumptions about the filling factor of dense clumps we find, for aselected set of regions observed in M 51 by Díaz et al. (1991) anextreme limiting range of computed photon escape fractions between nearzero and 90%, but with the most plausible values ranging between 30% and50%. We show, using oxygen as the test element, that models withdifferent assumptions about the gas inhomogeneity will tend to givevariations in the abundance values derived from diagnostic diagrams, butdo not claim here to have a fully developed set of diagnostic tools toimprove abundance determinations made in this way. We do present animportant step towards an eventual improvement in abundancedeterminations: the combination of line ratios with the absoluteHα luminosity of a given H ii region, which allows us to determinethe photon escape fraction, and hence resolve the degeneracy between Uand ξ. We use observational data of this type show that a large setof H ii regions in M 101 observed by Cedrés & Cepa (2002) allshow significant photon escape with values of ξ ranging up to 60% inthe “leakiest” cases.
| 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.
| 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}
| The field surrounding NGC 7603: Cosmological or non-cosmological redshifts?
We present new observations of the field surrounding the Seyfert galaxyNGC 7603, where four galaxies with different redshifts - NGC 7603(z=0.029), NGC 7603B (z=0.057) and two fainter emission line galaxies(z=0.245 and z=0.394) - are apparently connected by a narrow filament,leading to a possible case of anomalous redshift. The observationscomprise broad and narrow band imaging and intermediate resolutionspectroscopy of some of the objects in the field. The new data confirmthe redshift of the two emission-line objects found within the filamentconnecting NGC 7603 and NGC 7603B, and settles their type with betteraccuracy. Although both objects are point-like in ground based images,using HST archive images we show that the objects have structure with aFWHM = 0.3-0.4 arcsec. The photometry in the R-band obtained duringthree different campaigns spread over two years does not show any signsof variability in these objects above 0.3-0.4 mag. All the aboveinformation and the relative strength and width of the main spectrallines allow us to classify these as HII galaxies with very vigorous starformation, while the rest of the filament and NGC 7603B lack starformation. We delineate the halo of NGC 7603 out to 26.2mag/arcsec2 in the Sloan r band filter and find evidence forstrong internal distortions. New narrow emission line galaxies atz=0.246, 0.117 and 0.401 are also found at respectively 0.8, 1.5 and 1.7arcmin to the West of the filament within the fainter contour of thishalo. We have studied the spatial distribution of objects in the fieldwithin 1.5 arcmin of NGC 7603. We conclude that the density of QSOs isroughly within the expected value of the limiting magnitude of ourobservations. However, the configuration of the four galaxies apparentlyconnected by the filament appears highly unusual. The probability ofthree background galaxies of any type with apparent B-magnitudes up to16.6, 21.1 and 22.1 (the observed magnitudes, extinction correctionincluded) being randomly projected on the filament of the fourth galaxy(NGC 7603) is ≈ 3× 10-9. Furthermore, the possibledetection of very vigorous star formation observed in the HII galaxiesof the filament would have a low probability if they were backgroundnormal-giant galaxies; instead, the intensity of the lines is typical ofdwarf HII galaxies. Hence, a set of coincidences with a very lowprobability would be necessary to explain this as a fortuitousprojection of background sources. Several explanations in terms ofcosmological or non-cosmological redshifts are discussed.
| New optical spectra and general discussion on the nature of ULXs
We present optical spectroscopic observations of three Ultra LuminousX-ray sources (ULXs). Two of them are very close to the active galaxyNGC 720 and the other is near NGC 1073. The two around NGC 720 turn outto be quasars at z= 2.216 and z= 0.959, the one near NGC 1073 seems tobe associated to an HII region at the redshift of NGC 1073. Weconcentrate our analysis on the two quasars and analyze them inconjunction with a set of 22 additional X-ray sources close to nearbygalaxies which also fit the criteria of ULXs and which also have beenidentified as quasars of medium to high redshift. This sample shows anunusually large fraction of rare BL Lac type objects. The high redshiftsof these ULXs and their close proximity to their low redshift,supposedly parent galaxies is a surprising result in the light ofstandard models. We describe the main properties of each of theseobjects and their parent galaxy, and briefly discuss possibleinterpretations.
| Disk-bulge decompositions of spiral galaxies in UBVRI
A sample of 26 bright spiral galaxies (Btot < 12.7) withlow to medium inclination and without a bar was observed with UBVRIfilters. The CAFOS focal reducer camera at the Calar Alto 2.2 mtelescope was used. The surface-brightness distributions were fittedusing a 2-dimensional algorithm with corresponding functions for thedisk- and bulge-structure. For the disks an exponential function wasused, for the bulges a Sérsic Rβ law, was appliedwith the concentration parameter β = 1/n as another fit variable.Correlations of the resulting structural parameters of disks and bulgesin UBVRI are investigated, giving clues to the formation history of thebulges.We confirm that the large and bright bulges of early-type spirals aresimilar to elliptical galaxies. They were probably formed prior to thedisks in a monolithic collapse or via early mergers. Late-type spiralshave tiny and faint bulges with disk-like profiles. These bulges wereprobably formed after the disk in secular evolution processes, e.g. froma disk instability. The comparison of the color indices of correspondingspirals and bulges with population synthesis computations support aboveformation scenarios.Tables 2-4 are only available in electronic form at the CDS viaanonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or viahttp://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/415/63
| Optical and near-IR spectroscopy of low excitation H II regions with the GTC
The determination of chemical abundances in ionized-gas nebulae is basedon the detection of auroral emission lines, from which it is possible todeduce the line temperatures of several ions. These lines are weak inlow excitation objects, so their measurement is only possible in largeaperture telescopes. The GTC would allow us to plan a comprehensivestudy of chemical abundances in such objects from mid to high resolutionspectra, which would give place accurate observational constraints onmodels of chemical evolution.
| Spectrophotometry of massive star forming regions with the GTC
In the last few years, the detection of Wolf-Rayet (WR) stars in Giant HII Regions (GHRs) has yielded several questions about our currentunderstanding of massive stars evolution and hot expanding atmospheres,the age of the ionizing populations, and their impact on the physicalproperties of GHRs. Here, we present spectrophotometric observations offour extragalactic GHRs which show WR features in their spectra. Ourgoal is to reproduce simultaneously the observed WR properties and theemission line spectra with the help of current evolutionary synthesismodels. Finally, we address the main advantages that the GTC willprovide to our better understanding of massive star forming regions.
| Stellar population gradients in Seyfert 2 galaxies: northern sample
We use high signal-to-noise ratio long-slit spectra in theλλ3600-4700 range of the 20 brightest northern Seyfert 2galaxies to study the variation of the stellar population properties asa function of distance from the nucleus. In order to characterize thestellar population and other continuum sources (e.g. featurelesscontinuum, FC) we have measured the equivalent width, W, of sixabsorption features, four continuum colours and their radial variations,and performed spectral population synthesis as a function of distancefrom the nucleus. About half of the sample has CaIIK and G band W valuessmaller at the nucleus than at 1 kpc from it, owing to a youngerpopulation and/or FC. The stellar population synthesis shows that, whileat the nucleus, 75 per cent of the galaxies present contribution >20per cent of ages <=100 Myr and/or of an FC, this proportion decreasesto 45 per cent at 3 kpc. In particular, 55 per cent of the galaxies havea contribution >10 per cent of the 3-Myr/FC component (a degeneratecomponent in which one cannot separate what is caused by an FC or by a3-Myr stellar population) at the nucleus, but only 25 per cent of themhave this contribution at 3 kpc. As a reference, the stellar populationof 10 non-Seyfert galaxies, spanning the Hubble types of the Seyfert(from S0 to Sc) was also studied. A comparison between the stellarpopulation of the Seyferts and that of the non-Seyferts shows systematicdifferences: the contribution of ages younger than 1 Gyr is in mostcases larger in the Seyfert galaxies than in non-Seyferts, not only atthe nucleus but up to 1 kpc from it.
| The IRAS Revised Bright Galaxy Sample
IRAS flux densities, redshifts, and infrared luminosities are reportedfor all sources identified in the IRAS Revised Bright Galaxy Sample(RBGS), a complete flux-limited survey of all extragalactic objects withtotal 60 μm flux density greater than 5.24 Jy, covering the entiresky surveyed by IRAS at Galactic latitudes |b|>5°. The RBGS includes629 objects, with median and mean sample redshifts of 0.0082 and 0.0126,respectively, and a maximum redshift of 0.0876. The RBGS supersedes theprevious two-part IRAS Bright Galaxy Samples(BGS1+BGS2), which were compiled before the final(Pass 3) calibration of the IRAS Level 1 Archive in 1990 May. The RBGSalso makes use of more accurate and consistent automated methods tomeasure the flux of objects with extended emission. The RBGS contains 39objects that were not present in the BGS1+BGS2,and 28 objects from the BGS1+BGS2 have beendropped from RBGS because their revised 60 μm flux densities are notgreater than 5.24 Jy. Comparison of revised flux measurements forsources in both surveys shows that most flux differences are in therange ~5%-25%, although some faint sources at 12 and 25 μm differ byas much as a factor of 2. Basic properties of the RBGS sources aresummarized, including estimated total infrared luminosities, as well asupdates to cross identifications with sources from optical galaxycatalogs established using the NASA/IPAC Extragalactic Database. Inaddition, an atlas of images from the Digitized Sky Survey with overlaysof the IRAS position uncertainty ellipse and annotated scale bars isprovided for ease in visualizing the optical morphology in context withthe angular and metric size of each object. The revised bolometricinfrared luminosity function, φ(Lir), forinfrared-bright galaxies in the local universe remains best fit by adouble power law, φ(L)~Lα, withα=-0.6(+/-0.1) and α=-2.2(+/-0.1) below and above the``characteristic'' infrared luminosityL*ir~1010.5Lsolar,respectively. A companion paper provides IRAS High Resolution (HIRES)processing of over 100 RBGS sources where improved spatial resolutionoften provides better IRAS source positions or allows for deconvolutionof close galaxy pairs.
| On the origin of nitrogen
The problem of the origin of nitrogen is considered within the frameworkof an empirical approach. The oxygen abundances and nitrogen to oxygenabundances ratios are derived in H II regions of a number of spiralgalaxies through the recently suggested P-method using more than sixhundred published spectra. The N/O-O/H diagram for H II regions inirregular and spiral galaxies is constructed. It is found that the N/Ovalues in H II regions of spiral galaxies of early morphological typesare higher than those in H II regions with the same metallicity inspiral galaxies of late morphological types. This suggests along-time-delayed contribution to the nitrogen production. The N/O ratioof a galaxy can then be used as an indicator of the time that haselapsed since the bulk of star formation occurred, or in other words ofthe nominal ``age'' of the galaxy as suggested by Edmunds & Pagelmore than twenty years ago. The scatter in N/O values at a given O/H canbe naturally explained by differences in star formation histories ingalaxies. While low-metallicity dwarf galaxies with low N/O do notcontain an appreciable amount of old stars, low-metallicity dwarfgalaxies with an appreciable fraction of old stars have high N/O.Consideration of planetary nebulae in the Small Magellanic Cloud and inthe Milky Way Galaxy suggests that the contribution of low-mass stars tothe nitrogen production is significant, confirming the conclusion thatthere is a long-time-delayed contribution to the nitrogen production.
| A Comprehensive Study of High-Metallicity Giant Extragalactic H II Regions: Ionizing Populations
Theoretically, the evolution of a young stellar population depends onmetallicity at least through two very important effects: the increasingopacity of the stellar material and the dependence of mass loss on metalcontent in high mass stars. As a consequence of the first, the effectivetemperature of ionizing stars should be lower in regions of highermetallicity (see for example McGaugh 1991) but, on the other hand, as aconsequence of the second, that is if the strength of stellar windsincreases with metallicity, the loss of the outer envelopes of the mostmassive stars can increase their surface temperature to very highvalues. These highly evolved massive O stars are identified with theWolf-Rayet population.
| A Comprehensive Study of High Metallicity Giant Extragalactic H II Regions: Chemical Abundances
We have made long-slit spectrophotometric observations in the opticaland near infrared of 15 H II regions in different spiral galaxies (NGC628, NGC 925, NGC 1232 and NGC 1637). These spectrophotometricobservations were performed with a wide spectral coverage and at aresolution high enough to detect and measure both weak auroral forbiddenlines and Wolf-Rayet features. Electron temperatures have been derivedin order to investigate the ionization structure and to derive thechemical composition of the gas in these regions. Therefore, we haveselected from the literature (Van Zee et al. 1998) those H II regionswith solar or oversolar abundance, as deduced from empiricalcalibrations based on the optical oxygen forbidden lines.
| Bar strengths in spiral galaxies estimated from 2MASS images
Non-axisymmetric forces are presented for a sample of 107 spiralgalaxies, of which 31 are barred (SB) and 53 show nuclear activity. As adata base we use JHK images from the 2 Micron All-sky Survey, and thenon-axisymmetries are characterized by the ratio of the tangential forceto the mean axisymmetric radial force field, following Buta & Block.Bar strengths have an important role in many extragalactic problems andtherefore it is important to verify that the different numerical methodsapplied for calculating the forces give mutually consistent results. Weapply both direct Cartesian integration and a polar grid integrationutilizing a limited number of azimuthal Fourier components of density.We find that the bar strength is independent of the method used toevaluate the gravitational potential. However, because of thedistance-dependent smoothing by Fourier decomposition, the polar methodis more suitable for weak and noisy images. The largest source ofuncertainty in the derived bar strength appears to be the uncertainty inthe vertical scaleheight, which is difficult to measure directly formost galaxies. On the other hand, the derived bar strength is ratherinsensitive to the possible gradient in the vertical scaleheight of thedisc or to the exact model of the vertical density distribution,provided that the same effective vertical dispersion is assumed in allmodels. In comparison with the pioneering study by Buta & Block, thebar strength estimate is improved here by taking into account thedependence of the vertical scaleheight on the Hubble type: we find thatfor thin discs bar strengths are stronger than for thick discs by anamount that may correspond to as much as one bar strength class. Weconfirm the previous result by Buta and co-workers showing that thedispersion in bar strength is large among all the de Vaucouleurs opticalbar classes. In the near-infrared 40 per cent of the galaxies in oursample have bars (showing constant phases in the m= 2 Fourier amplitudesin the bar region), while in the optical band one-third of these barsare obscured by dust. Significant non-axisymmetric forces can also beinduced by the spiral arms, generally in the outer parts of the galacticdiscs, which may have important implications on galaxy evolution.Possible biases of the selected sample are also studied: we find thatthe number of bars identified drops rapidly when the inclination of thegalactic disc is larger than 50°. A similar bias is found in theThird Reference Catalogue of Bright Galaxies, which might be of interestwhen comparing bar frequencies at high and low redshifts.
| A comprehensive study of reported high-metallicity giant HII regions - II. Ionizing stellar populations
The ionizing stellar populations of 11 HII regions in the spiralgalaxies: NGC 628, 925, 1232 and 1637, all of them reported to havesolar or oversolar abundances according to empirical calibrations, havebeen analysed using stellar population synthesis models. Four of theobserved regions in the sample show features which indicate the presenceof a population of Wolf-Rayet (WR) stars with ages between 2.3 and 4.1Myr. This population is sufficient to explain the emission line spectrumof the low-metallicity region H13 in NGC 628, taking into account theuncertainties involved in both observations and model computations. Thisis not the case for the rest of the regions for which a second ionizingpopulation is required to simultaneously reproduce both the WR featuresand the emission line spectrum. Composite populations are also found forhalf of the regions without WR features, although in this case, theresult is based only on the emission line spectrum analysis. For two ofthe regions showing WR features, no consistent solution is found, as thepopulation containing WR stars produces a spectral energy distributionwhich is too hard to explain the emission of the gas. Several solutionsare proposed to solve this problem.
| A comprehensive study of reported high-metallicity giant H II regions - I. Detailed abundance analysis
We present long-slit observations in the optical and near-infrared of 14HII regions in the spiral galaxies NGC 628, 925, 1232 and 1637, all ofthem reported to have solar or oversolar abundances according toempirical calibrations. For seven of the observed regions, ion-weightedtemperatures from optical forbidden auroral to nebular line ratios areobtained and, for six of them, the oxygen abundances derived by standardmethods turn out to be significantly lower than solar. The other one,named CDT1 in NGC 1232, shows an oxygen abundance of12+log(O/H)=8.95+/-0.20, and constitutes, to the best of our knowledge,the first high-metallicity HII region for which accurate linetemperatures, and hence elemental abundances, have been derived. For therest of the regions no line temperature measurements could be made, andthe metallicity has been determined by means of both detailedphotoionization modelling and the sulphur abundance parameterS23. Only one of these regions shows values of O23and S23 implying a solar or oversolar metallicity. Accordingto our analysis, only two of the observed regions can therefore beconsidered as of high metallicity. These two fit the trends previouslyfound in other high-metallicity HII regions, i.e., N/O and S/O abundanceratios seem to be higher and lower than solar respectively.
| The Luminosity-Metallicity Relation, Effective Yields, and Metal Loss in Spiral and Irregular Galaxies
I present results on the correlation between galaxy mass, luminosity,and metallicity for a sample of spiral and irregular galaxies havingwell-measured abundance profiles, distances, and rotation speeds.Additional data for low surface brightness galaxies from the literatureare also included for comparison. These data are combined to study themetallicity-luminosity and metallicity-rotation speed correlations forspiral and irregular galaxies. The metallicity-luminosity correlationshows its familiar form for these galaxies, a roughly uniform change inthe average present-day O/H abundance of about a factor of 100 over 11mag in B luminosity. However, the O/H-Vrot relation shows achange in slope at a rotation speed of about 125 km s-1. Atfaster Vrot, there appears to be no relation between averagemetallicity and rotation speed. At lower Vrot, themetallicity correlates with rotation speed. This change in behaviorcould be the result of increasing loss of metals from the smallergalaxies in supernova-driven winds. This idea is tested by looking atthe variation in effective yield, derived from observed abundances andgas fractions assuming closed box chemical evolution. The effectiveyields derived for spiral and irregular galaxies increase by a factor of10-20 from Vrot~5 to 300 km s-1, asymptoticallyincreasing to approximately constant yeff forVrot>~150 km s-1. The trend suggests thatgalaxies with Vrot<~100-150 km s-1 may lose alarge fraction of their supernova ejecta, while galaxies above thisvalue tend to retain metals.
| On the Large Escape of Ionizing Radiation from Giant Extragalactic H II Regions
A thorough analysis of well-studied disk giant H II regions, for whichwe know the ionizing stellar population, gas metallicity, and Wolf-Rayetpopulation, leads to photoionization models that can only match allobserved line intensity ratios ([O III], [O II], [N II], [S II], and [SIII] with respect to the intensity of Hβ), as well as the Hβluminosity and equivalent width if one allows for an important escape ofenergetic ionizing radiation. For the three regions presented here, thefractions of escaping Lyman continuum photons amount to 10%-73%, and inall cases, the larger fraction of escaping photons has energies ofbetween 13.6 and 24.4 eV. These escaping photons clearly must have animportant impact as sources of ionization of the diffuse ionized gasfound surrounding many galaxies, as well as of the intergalactic medium.
| The H I Line Width/Linear Diameter Relationship as an Independent Test of the Hubble Constant
The relationship between corrected H I line widths and linear diameters(LW/LD) for spiral galaxies is used as an independent check on the valueof the Hubble constant. After calibrating the Tully-Fisher (TF) relationin both the B and I bands, the B-band relation is used for galaxies ofmorphological/luminosity types Sc I, Sc I.2, Sc I.3, Sab, Sb, Sb I-II,and Sb II to derive the LW/LD relation. We find that for this sample thescatter in the LW/LD is smallest with a Hubble constant of 90-95 kms-1 Mpc-1. Lower values of the Hubble constantproduce a separation in the LW/LD relation that is a function ofmorphological type. Since a Hubble constant of 90-95 is significantlylarger than the final Key Project value of 72 km s-1Mpc-1, a comparison of TF, surface brightness fluctuation(SBF), and fundamental plane (FP) is made. This comparison indicatesthat the Key Project TF distances to 21 clusters may be too large. For asample of 11 clusters, the Key Project TF distances provide anunweighted mean Hubble constant of 77 km s-1Mpc-1, while a combination of the FP, SBF, and our TFdistances for the same 11 clusters gives H0=91 kms-1 Mpc-1. A more subtle result in our data is amorphological dichotomy in the Hubble constant. The data suggest that ScI galaxies follow a Hubble constant of 90-95 while Sb galaxies follow aHubble constant closer to 75 km s-1 Mpc-1.Possible explanations for this result are considered, but it is shownthat this Sb/Sc I Hubble flow discrepancy is also present in the VirgoCluster and is consistent with previous investigations that indicatethat some galaxies carry a component of age-related intrinsic redshift.
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