84 research outputs found
The Lyman-alpha Forest at z~4: Keck HIRES Observations of Q 0000-26
This paper describes a study of the Lyman-alpha forest absorption clouds
along the quasar sightline Q0000-26 (zem=4.1). The spectrum was obtained with
the High Resolution Spectrometer on the 10m Keck telescope. We derive accurate
H I column density and Doppler width distributions for the clouds from Voigt
profile fitting. We also analyze simulated Lyman-alpha forest spectra of
matching characteristics in order to gauge the effects of line
blending/blanketing and noise in the data. The results are compared with
similar studies at lower redshifts in order to study any possible evolution in
the clouds' properties. We also estimate the mean intensity of the UV
background at z=4 from an analysis of the proximity effect.Comment: plain TeX containing 23 PS pages, 3 PS tables, and 9 PS figures, ApJ,
Dec 1, 1996 issue replacing an earlier version which contains an corrupted
table
The HST Key Project on the Extragalactic Distance Scale XXVI. The Calibration of Population II Secondary Distance Indicators and the Value of the Hubble Constant
A Cepheid-based calibration is derived for four distance indicators that
utilize stars in the old stellar populations: the tip of the red giant branch
(TRGB), the planetary nebula luminosity function (PNLF), the globular cluster
luminosity function (GCLF) and the surface brightness fluctuation method (SBF).
The calibration is largely based on the Cepheid distances to 18 spiral galaxies
within cz =1500 km/s obtained as part of the HST Key Project on the
Extragalactic Distance Scale, but relies also on Cepheid distances from
separate HST and ground-based efforts. The newly derived calibration of the SBF
method is applied to obtain distances to four Abell clusters in the velocity
range between 3800 and 5000 km/s, observed by Lauer et al. (1998) using the
HST/WFPC2. Combined with cluster velocities corrected for a cosmological flow
model, these distances imply a value of the Hubble constant of H0 = 69 +/- 4
(random) +/- 6 (systematic) km/s/Mpc. This result assumes that the Cepheid PL
relation is independent of the metallicity of the variable stars; adopting a
metallicity correction as in Kennicutt et al. (1998), would produce a (5 +/-
3)% decrease in H0. Finally, the newly derived calibration allows us to
investigate systematics in the Cepheid, PNLF, SBF, GCLF and TRGB distance
scales.Comment: Accepted for publication in the Astrophysical Journal. 48 pages
(including 13 figures and 4 tables), plus two additional tables in landscape
format. Also available at http://astro.caltech.edu/~lff/pub.htm K' SBF
magnitudes have been update
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