218 research outputs found
The Carbon Content of Intergalactic Gas at z=4.25 and its Evolution Toward z=2.4
This paper presents ionization-corrected measurements of the carbon abundance
in intergalactic gas at 4.0 < z < 4.5, using spectra of three bright quasars
obtained with the MIKE spectrograph on Magellan. By measuring the CIV strength
in a sample of 131 discrete HI-selected quasar absorbers with
\rho/\bar{\rho}>1.6, we derive a median carbon abundance of [C/H]=-3.55, with
lognormal scatter of approximately ~0.8 dex. This median value is a factor of
two to three lower than similar measurements made at z~2.4 using CIV and OVI.
The strength of evolution is modestly dependent on the choice of UV background
spectrum used to make ionization corrections, although our detection of an
abundance evolution is generally robust with respect to this model uncertainty.
We present a framework for analyzing the effects of spatial fluctuations in the
UV ionizing background at frequencies relevant for CIV production. We also
explore the effects of reduced flux between 3-4 Rydbergs (as from HeII Lyman
series absorption) on our abundance estimates. At HeII line absorption levels
similar to published estimates the effects are very small, although a larger
optical depth could reduce the strength of the abundance evolution. Our results
imply that ~50% of the heavy elements seen in the IGM at z~2.4 were deposited
in the 1.3 Gyr between z~4.3 and z~2.4. The total implied mass flux of carbon
into the Lyman alpha forest would constitute ~30% of the IMF-weighted carbon
yield from known star forming populations over this period.Comment: Accepted for publication in the Astrophysical Journal. 23 pages, 24
figures, 2 table
Detection of Massive Forming Galaxies at Redshifts Greater than One
The complex problem of when and how galaxies formed has not until recently
been susceptible of direct attack. It has been known for some time that the
excessive number of blue galaxies counted at faint magnitudes implies that a
considerable fraction of the massive star formation in the universe occurred at
z < 3, but, surprisingly, spectroscopic studies of galaxies down to a B
magnitude of 24 found little sign of the expected high-z progenitors of current
massive galaxies, but rather, in large part, small blue galaxies at modest
redshifts z \sim 0.3. This unexpected population has diverted attention from
the possibility that early massive star-forming galaxies might also be found in
the faint blue excess. From KECK spectroscopic observations deep enough to
encompass a large population of z > 1 field galaxies, we can now show directly
that in fact these forming galaxies are present in substantial numbers at B
\sim 24, and that the era from redshifts 1 to 2 was clearly a major period of
galaxy formation. These z > 1 galaxies have very unusual morphologies as seen
in deep HST WFPC2 images.Comment: 10 pages LaTeX + 5 PostScript figures in uuencoded gzipped tar file;
aasms4.sty, flushrt.sty, overcite.sty (the two aastex4.0 and overcite.sty
macros are available from xxx.lanl.gov) Also available (along with style
files) via anonymous ftp to ftp://hubble.ifa.hawaii.edu/pub/preprints .
E-print version of paper adds citation cross-references to other archived
e-prints, where available. To appear in Nature October 19, 199
Faint M-dwarfs and the structure of the Galactic disk
We use broadband photometry and low-resolution spectra of a complete sample
of late-K and M dwarfs brighter than I=22 in three fields at high galactic
latitude to study issues relating to galactic structure and large scale
abundance gradients in the Galaxy. The observed starcounts in each field are a
good match to the predictions of models based on deep starcount data in other
intermediate-latitude fields, and these models identify the late-type stars as
members of the Galactic disk. Abundances for these late type stars are
estimated via narrowband indices that measure the strength of the TiO and CaH
bands in their spectra. Our results show that the average abundance in the
Galactic disk remains close to solar even at heights of more than 2 kpc above
the Plane.Comment: to appear in PASP; 17 pages, including 7 embedded, postscript figures
and 1 embedded table; uses AAS LaTeX style files (not included); also
available at http://astro.caltech.edu/~map/map.bibliography.htm
Photon Conserving Radiative Transfer around Point Sources in multi-dimensional Numerical Cosmology
Many questions in physical cosmology regarding the thermal and ionization
history of the intergalactic medium are now successfully studied with the help
of cosmological hydrodynamical simulations. Here we present a numerical method
that solves the radiative transfer around point sources within a three
dimensional cartesian grid. The method is energy conserving independently of
resolution: this ensures the correct propagation speeds of ionization fronts.
We describe the details of the algorithm, and compute as first numerical
application the ionized region surrounding a mini-quasar in a cosmological
density field at z=7.Comment: 5 pages, 4 figures, submitted to ApJ
Constraints on the Universal CIV Mass Density at z~6 from Early IR Spectra Obtained with the Magellan FIRE Spectrograph
We present a new determination of the intergalactic CIV mass density at 4.3 <
z < 6.3. Our constraints are derived from high signal-to-noise spectra of seven
quasars at z > 5.8 obtained with the newly commissioned FIRE spectrograph on
the Magellan Baade telescope, coupled with six observations of northern objects
taken from the literature. We confirm the presence of a downturn in the CIV
abundance at =5.66 by a factor of 4.1 relative to its value at =4.96, as
measured in the same sightlines. In the FIRE sample, a strong system previously
reported in the literature as CIV at z=5.82 is re-identified as MgII at z=2.78,
leading to a substantial downward revision in for these prior
studies. Additionally we confirm the presence of at least two systems with
low-ionization CII, SiII, and OI absorption but relatively weak signal from
CIV. The latter systems systems may be of interest if the downward trend in
at high redshift is driven in part by ionization effects.Comment: 14 pages, 6 figures, 2 tables. Submitted to Ap
Detection of Lyman-alpha Emitting Galaxies at Redshift z=4.55
Studies of the formation and early history of galaxies have been hampered by
the difficulties inherent in detecting faint galaxy populations at high
redshift. As a consequence, observations at the highest redshifts (3.5 < z < 5)
have been restricted to objects that are intrinsically bright. These include
quasars, radio galaxies, and some Ly alpha-emitting objects that are very close
to (within ~10 kpc) -- and appear to be physically associated with -- quasars.
But the extremely energetic processes which make these objects easy to detect
also make them unrepresentative of normal (field) galaxies. Here we report the
discovery using Keck spectroscopic observations of two Ly alpha-emitting
galaxies at redshift z = 4.55, which are sufficiently far from the nearest
quasar (~700 kpc) that radiation from the quasar is unlikely to provide the
excitation source of the Ly alpha emission. Instead, these galaxies appear to
be undergoing their first burst of star formation, at a time when the Universe
was less than one billion years old.Comment: 8 pages, 1 landscape table, and 3 PostScript figures. Uses
aaspp4.sty, flushrt.sty, aj_pt4.sty, overcite.sty (style macros available
from xxx.lanl.gov) Figure 1 is bitmapped to 100 dpi. The original PostScript
version of Fig. 1 is available via anonymous ftp to
ftp://hubble.ifa.hawaii.edu/pub/preprints To appear in Natur
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