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|A Dwarf Irregular Galaxy at the Edge of the Local Group: Stellar Populations and Distance of IC 5152|
We have obtained V- and I-band photometry for about 700 stars in thefield of the dwarf irregular galaxy IC 5152, previously considered apossible member of the Local Group. Deep VI color-magnitude diagrams arepresented of the main body of this galaxy and of a nearby field. Weinfer a distance by comparison with the VI color-magnitude diagrams ofsimilar galaxies and with theoretical isochrones. The distance modulusto IC 5152 is found to be m-M_0=26.15+/-0.2, adopting E(V-I)=0. Thisdistance of D=1.7 Mpc implies that IC 5152 is not a certain member ofthe Local Group. We also construct a deep optical luminosity function.By comparison with theoretical isochrones, we find a metallicityZ~0.002, which is lower than previous estimates. Using this metallicityand distance, IC 5152 now fits the metallicity-luminosity relation fordwarf galaxies. The youngest stars in the field studied have an age of~10^7 Gyr, and there is a substantial population of stars withlogt>7.8-8 yr. The central region of IC 5152 is an active site ofstar formation, as found by comparing our optical photometry with HubbleSpace Telescope ultraviolet images. Several candidate globular clustersare found, as well as a candidate for a potential nucleus.
|The WO stars. III. The planetary nebula NGC 5189 and its O VI sequence nucleus.|
Long slit blue and red spectra of the planetary nebula NGC 5189 (PK 307-3°1) with its O VI sequence central object were obtained with theESO 1.52m telescope, in order to establish criteria that allow thediscrimination between the high mass WO stars and lower mass objectshaving similar spectral features. The NGC 5189 stellar nucleus shows avery high ionization spectrum with emission features of He II, C IV, andO V. N V should also be weakly present. The line width suggests amaximum wind velocity of 2800km/s. Long slit spectra on variouspositions across the nebula were used to study the physical structure ofNGC 5189. From the [S II] 671.7/673.2nm line ratio we find that theelectron density varies from 200 to 900cm^-3^. The ionization, asdescribed by the He^++^/He^+^ ratio, largely changes from one region toanother, reaching a maximum of 2.5 in a low density region 20"E of thecentral star, and a minimum in the regions more distant from the star.NGC 5189 appears nitrogen enriched by about a factor 5 with respect tothe cosmic value, while the helium abundance is normal. The chemicalcomposition appears homogeneous throughout the nebula. We thus concludethat, in addition to the wind velocity, the chemical composition andhomogeneity of the nebulae can be used to discriminate between high massWO stars and "O VI sequence" PNN's.
|An image database. II. Catalogue between δ=-30deg and δ=70deg.|
A preliminary list of 68.040 galaxies was built from extraction of35.841 digitized images of the Palomar Sky Survey (Paper I). For eachgalaxy, the basic parameters are obtained: coordinates, diameter, axisratio, total magnitude, position angle. On this preliminary list, weapply severe selection rules to get a catalog of 28.000 galaxies, wellidentified and well documented. For each parameter, a comparison is madewith standard measurements. The accuracy of the raw photometricparameters is quite good despite of the simplicity of the method.Without any local correction, the standard error on the total magnitudeis about 0.5 magnitude up to a total magnitude of B_T_=17. Significantsecondary effects are detected concerning the magnitudes: distance toplate center effect and air-mass effect.
|A generalized Schmidt star formation law: Observational constraints|
This paper aims to see whether a generalized Schmidt star formation lawis consistent with observations in the light of a closed, comoving modelof chemical evolution, where two main phases occurred: contraction(extended component) and equilibrium (flat component). It is shown thatthe empirical relation between present-day gas mass which is beingturned into stars and gas mass, M-dotga proportional toMga, i.e. a linear correlation between the model present-daystar formation rate and gas mass fraction, CD proportional tomua, yields a connection between the ratio of contractiontime to age, Tc/T, and mua. The comparison betweenmodel predictions and observations leads to the following mainconclusions: (1) models where star formation obeys a pure Schmidt lawcannot fit the whole set of observations; (2) if we demand (in dealingwith an averaged CD/mua) Tc/T greaterthan 0 for all the galaxies and Tc/T less than 0.5 for atleast a fraction of large-mass (M greater than 1011 solarmass) galaxies of the sample considered, then a generalized Schmidt starformation law with an exponent n approximately equal to 1 is preferredin respect to n approximately equal to 2; (3) if contraction andequilibrium phase were characterized by different star formation ratesand mass spectra, then a correlation exists between present-day metalcontent and gas mass fraction. Thus a generalized Schmidt star formationlaw with exponent n approximately equal to 1 appears to be consitentwith observations.
|Evolutionary models for disk galaxies|
The evolution of disk galaxies is studied within the framework of a gasturbulence energy balance between the supernovae input energy and theturbulence collisional dissipation. The initial mass, the angularmomentum, and the gas accretion rate determine the evolution of themodels. We predict the present gas content and the star formationhistory of a galaxy. The role of gas accretion is discussed. Thecomparison between models and observations shows that gas accretion maybe very important to explain the high gas content of the dwarf irregulargalaxies. A Schmidt's law with a star formation rate per surface unitproportional to the square of the gas surface density provides a goodagreement with the models at the Hubble time as well as during theirevolution.
|Stochastic star formation and chemical evolution of dwarf irregular galaxies|
The Gerola et al. (1980) stochastic self-propagating star formationmodel is the basis of the present discussion of the observationalproperties of a sample of magellanic and dwarf irregular galaxies.One-zone model galaxies are presented which take into account both thebursting mode of star formation and the inflow of unenriched gas, wherethe rate of star formation is assumed to depend on both a stochasticterm representing the fraction of active cells in the Gerola model andthe current fractionary mass of gas. Models for the SMC and LMC are usedin choosing among different solutions and in fixing an efficiency factorin the star formation rate, the underlying initial mass function, andthe time scale of mass accretion. It is found that the high dispersionrate of observed dwarf irregular galaxy properties cannot be explainedby means of the stochastic star formation mechanism alone.
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