A Supernova Search with Public Astronomical Observatories in Japan

Details A Supernova Search with Public Astronomical Observatories in Japan A Supernova Search with Public Astronomical Observatories in Japan

Dec 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996jkass..29..213y&link_type=abstract

Journal of the Korean Astronomical Society, v. 29 Supplement, p.S213

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Scientific paper

The evolution of the neutral hydrogen content of galaxies as a function of time is an important constraint on processes in galactic evolution. A comprehensive, statistical description of the HI content and distribution within galaxies at the present epoch is presented and the statistics are compared with the properties of HI associated with high -redshift "damped Lyman alpha" absorption systems observed in the spectra of QSOs. It is found that spiral galaxies contain 92% of the HI mass density at the current epoch. The cosmological mass density of H I is estimated to be Omega _{rm HI}(z=0)=(2.4+/- 0.6)times 10^{-4}h_sp{75}{ -1}, where h75 is the Hubble constant in units of 75 km s ^{-1} Mpc^{ -1}. Spirals are also found to offer the largest cross-section to line-of-sight absorption of light from QSOs. A complete sample of 27 large-diameter spirals is used to estimate the number density of absorbers per unit redshift at the present epoch, n(z = 0), and the column density distribution of absorbers, f(N). n(z = 0) is found to be 0.015 +/- 0.004, a factor of about 17 smaller than n(z~ 2.5). f(N) is also shown to have significantly evolved over this redshift interval. The incidence of damped Lyalpha absorbers in the redshift interval 0 < z < 1.65 is investigated with a new method that uses the statistical properties of the well-studied Mg II absorption systems. It is found that n_ {rm DLyalpha}(z = 0.81) <= 0.06, which is consistent with the trend at higher z as well as with the number density at the current epoch. The data at z > 1.65 combined with the constraint at z = 0 result in n_{ rm DLyalpha}(z) (0.015 +/- 0.004)(1 + z)^{2.27 +/- 0.25} for 0 < z < 4. The evolution of the cosmological mass density of neutral gas with redshift is also studied. The redshift dependence of the neutral gas density is fitted by a power law given by Omega_{rm g}(z) = (3.2+/- 0.8)times 10^{ -4} [ {2over 3}(1 + z)^{3/2} + {1over 3} ]^{1.21+/- 0.15}. It is valid from a redshift of 4 to the present epoch. However, the relation is shown to lead to inconsistencies with the star formation rates observed in nearby galaxies. Processes that can cause changes in the HI content of galaxies are dicussed and simple models are proposed to explain the decline in Omegag with time.

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