A Technique Fof Determining the Extragalactic Distance Scale

We propose a method of distance determination based on the internal structure and dynamics of disk galaxies. The method relies on the universal luminosity profile of a stellar disk represented by an exponential law. Calibrating nearby galaxies with known distances, it is found that the scale length of the disk is tightly correlated with the specific combination of central surface brightness {\it and} rotational velocity at a characteristic radius of 2.15 scale lengths from the center. This suggests that the scale length of the disk may be used as an indicator for extragalactic distance scale. The application of this relation to M51 and M100 allows us to arrive at the distances of about 6 Mpc and 14 Mpc, respectively, implying a Hubble constant of $H_0 = 92 \sim 94$ km s$^{-1}$ Mpc$^{-1}$.Comment: 12pp + 2 figures, included as uuencoded postscript file, to appear in ApJ (Part1), TH94092

Chemical Evolution of Spheroidal Systems(Proceedings of Japan-France Seminar on Chemical Evolution of Galaxies with Active Star Formation)

application/pdfA model of chemical evolution for spheroidal systems is constructed assuming a relation between binding energy and mass of a system Ω_G∝M_G^ and a universal IMF of stars dn/dlogm∝m. Taking into account the effect of stopping the star formation due to gas removal by a galactic wind, we successfully reproduced the chemical and structural properties of gE galaxies, dE galaxies, and globular clusters. The average metallicity of stars in a system declines from gEs through dEs monotonically, while the galactic mass loss expands dEs most significantly among others, causing the distinct sequences of gEs, dEs, and globulars in the diameter-surface brightness diagram. It is suggested that all the spheroidal systems were formed as a one-parameter family of mass.紀要類(bulletin)524728 bytesdepartmental bulletin pape

Three-dimensional orbits of metal-poor halo stars and the formation of the Galaxy

We present the three-dimensional orbital motions of metal-poor stars in conjunction with their metal abundances, for the purpose of getting insight into the formation process of the Galaxy. Our sample stars, which include metal-deficient red giants and RR Lyrae variables observed by the Hipparcos satellite, are least affected by known systematics, stemmed from kinematic bias, metallicity calibration, and secondary metal contamination of stellar surface. We find, for the stars in the metallicity range of [Fe/H]<-1, that there is no evidence for the correlation between [Fe/H] and their orbital eccentricities e. Even for [Fe/H]<-1.6, about 16% of the stars have e less than 0.4. We show that the e distribution of orbits for [Fe/H]<-1.6 is independent of the height |z| away from the Galactic plane, whereas for [Fe/H]>-1.6 the stars at |z|>1 kpc are systematically devoid of low-e orbits with e<0.6. This indicates that low-e stars with [Fe/H]<-1.6 belong to the halo component, whereas the rapidly-rotating thick disk with a scale height about 1 kpc has a metal-weak tail in the range of -1.6<[Fe/H]<-1. The fraction of this metal-weak thick disk appears to be only less than 20%. The significance of these results for the early evolution of the Galaxy is briefly discussed.Comment: 11 pages, 3 figures, AASTeX, to appear in ApJ Letter

A new measurement of the baryonic fraction using the sparse NGC 3258 group of galaxies

New X-ray observations of the sparse NGC 3258 group of galaxies made by the ASCA satellite with good spectral and spatial resolution has revealed that this group has a gravitational potential deep enough to prohibit significant mass removal from the system. The baryonic fraction within 240 kpc is found to be 0.065 +0.051/-0.020 for h_{50}=1, where h_{50}=H_0/50 km/s/Mpc, in good agreement with the universal value of 0.05 +/-0.01 predicted by standard Big Bang nucleosynthesis for a Universe with Omega_0=1 and h_{50}=1. Since the deep potential of the NGC 3258 group ensures that all pristine intragroup gas has been retained, the baryonic fraction of the NGC 3258 group is indicative of the universal value. Consequently it seems premature to rule out a critical Universe.Comment: 19 pages Latex, using aasms4.sty, paper also available at http://www.dsri.dk/~kristian To appear in Astrophysical Journal Letter

The Interpretation of Near-Infrared Star Counts at the South Galactic Pole

We present new deep $K'$ counts of stars at the South Galactic Pole (SGP) taken with the NAOJ PICNIC camera to $K'=17.25$. Star-galaxy separation to $K'=17.5$ was accomplished effectively using image profiles because the pixel size we used is 0.509 arcsec. We interpret these counts using the SKY (Cohen 1994) model of the Galactic point source sky and determine the relative normalization of halo-to-disk populations, and the location of the Sun relative to the Galactic plane, within the context of this model. The observed star counts constrain these parameters to be: halo/disk $\sim$ 1/900 and z$_\odot$=16.5$\pm$2.5 pc. These values have been used to correct our SGP galaxy counts for contamination by the point source Galactic foreground.Comment: accepted for publication in AJ, 15 pages with 2 figure