13,959 research outputs found
APM z>4 QSO Survey: Distribution and Evolution of High Column Density HI Absorbers
Eleven candidate damped Lya absorption systems were identified in 27 spectra
of the quasars from the APM z>4 survey covering the redshift range
2.83.5). High resolution echelle spectra (0.8A FWHM)
have been obtained for three quasars, including 2 of the highest redshift
objects in the survey. Two damped systems have confirmed HI column densities of
N(HI) >= 10^20.3 atoms cm^-2, with a third falling just below this threshold.
We have discovered the highest redshift damped Lya absorber known at z=4.383 in
QSO BR1202-0725. The APM QSOs provide a substantial increase in the redshift
path available for damped surveys for z>3. We combine this high redshift sample
with other quasar samples covering the redshift range 0.008 < z < 4.7 to study
the redshift evolution and the column density distribution function for
absorbers with log N(HI)>=17.2. In the HI column density distribution
f(N)=kN^-beta we find evidence for breaks in the power law, flattening for
17.221.2. The column density
distribution function for the data with log N(HI)>=20.3 is better fit with the
form f(N)=(f*/N*)(N/N*)^-beta exp(-N/N*). Significant redshift evolution in the
number density per unit redshift is evident in the higher column density
systems with an apparent decline in N(z) for z>3.5.Comment: To appear in MNRAS. Latex file (10 pages of text) plus 14 separate
postscript figure files. Requires mn.sty. Postscript version with figures
embedded is available at http://www.ociw.edu/~lisa/publications.htm
Evolution of Neutral Gas at High Redshift -- Implications for the Epoch of Galaxy Formation
Though observationally rare, damped Lya absorption systems dominate the mass
density of neutral gas in the Universe. Eleven high redshift damped Lya systems
covering 2.84 QSO Survey,
extending these absorption system surveys to the highest redshifts currently
possible. Combining our new data set with previous surveys we find that the
cosmological mass density in neutral gas, omega_g, does not rise as steeply
prior to z~2 as indicated by previous studies. There is evidence in the
observed omega_g for a flattening at z~2 and a possible turnover at z~3. When
combined with the decline at z>3.5 in number density per unit redshift of
damped systems with column densities log N(HI)>21 atoms cm^-2, these results
point to an epoch at z>3 prior to which the highest column density damped
systems are still forming. We find that over the redshift range 2<z<4 the total
mass in neutral gas is marginally comparable with the total visible mass in
stars in present day galaxies. However, if one considers the total mass visible
in stellar disks alone, ie excluding galactic bulges, the two values are
comparable. We are observing a mass of neutral gas comparable to the mass of
visible disk stars. Lanzetta, Wolfe & Turnshek (1995) found that omega_g(z~3.5)
was twice omega_g(z~2), implying a much larger amount of star formation must
have taken place between z=3.5 and z=2 than is indicated by metallicity
studies. This created a `cosmic G-dwarf problem'. The more gradual evolution of
omega_g we find alleviates this. These results have profound implications for
theories of galaxy formation.Comment: To appear in MNRAS. Latex file (4 pages of text) plus 3 separate
postscript figure files. Requires mn.sty. Postscript version with figures
embedded is available at http://www.ociw.edu/~lisa/publications.htm
Radio Observations of Infrared Luminous High Redshift QSOs
We present Very Large Array (VLA) observations at 1.4 GHz and 5 GHz of a
sample of 12 Quasi-stellar Objects (QSOs) at z = 3.99 to 4.46. The sources were
selected as the brightest sources at 250 GHz from the recent survey of Omont et
al. (2001). We detect seven sources at 1.4 GHz with flux densities, S_{1.4} >
50 microJy. These centimeter (cm) wavelength observations imply that the
millimeter (mm) emission is most likely thermal dust emission. The
radio-through-optical spectral energy distributions for these sources are
within the broad range defined by lower redshift, lower optical luminosity
QSOs. For two sources the radio continuum luminosities and morphologies
indicate steep spectrum, radio loud emission from a jet-driven radio source.
For the remaining 10 sources the 1.4 GHz flux densities, or limits, are
consistent with those expected for active star forming galaxies. If the radio
emission is powered by star formation in these systems, then the implied star
formation rates are of order 1e3 M_solar/year. We discuss the angular sizes and
spatial distributions of the radio emitting regions, and we consider briefly
these results in the context of co-eval black hole and stellar bulge formation
in galaxies.Comment: to appear in the A
- …