689 research outputs found
Comparing Simulations and Observations of the Lyman-Alpha Forest I. Methodology
We describe techniques for comparing spectra extracted from cosmological
simulations and observational data, using the same methodology to link
Lyman-alpha properties derived from the simulations with properties derived
from observational data. The eventual goal is to measure the coherence or
clustering properties of Lyman-alpha absorbers using observations of quasar
pairs and groups. We quantify the systematic underestimate in opacity that is
inherent in the continuum fitting process of observed spectra over a range of
resolution and signal-to-noise ratio. We present an automated process for
detecting and selecting absorption features over the range of resolution and
signal-to-noise of typical observational data on the Lyman-alpha "forest".
Using these techniques, we detect coherence over transverse scales out to 500
h^{-1}_{50} kpc in spectra extracted from a cosmological simulation at z = 2.Comment: 52 pages, includes 14 figures, to appear in ApJ v566 Feb 200
Results from the CASTLES Survey of Gravitational Lenses
We show that most gravitational lenses lie on the passively evolving
fundamental plane for early-type galaxies. For burst star formation models (1
Gyr of star formation, then quiescence) in low Omega_0 cosmologies, the stellar
populations of the lens galaxies must have formed at z_f > 2. Typical lens
galaxies contain modest amounts of patchy extinction, with a median
differential extinction for the optical (radio) selected lenses of E(B-V) =
0.04 (0.07) mag. The dust can be used to determine both extinction laws and
lens redshifts. For example, the z_l=0.96 elliptical lens in MG0414+0534 has an
R_V=1.7 +/- 0.1 mean extinction law. Arc and ring images of the quasar and AGN
source host galaxies are commonly seen in NICMOS H band observations. The hosts
are typically blue, L < L_* galaxies.Comment: 12 pages, 10 figures, from Proceedings of the 9th Annual Astrophysics
Conference in Maryland, After the Dark Ages: When Galaxies Were Youn
PC1643+4631A,B: The Lyman-Alpha Forest at the Edge of Coherence
This is the first measurement and detection of coherence in the intergalactic
medium (IGM) at substantially high redshift (z~3.8) and on large physical
scales (~2.5 h^-1 Mpc). We perform the measurement by presenting new
observations from Keck LRIS of the high redshift quasar pair PC 1643+4631A, B
and their Ly-alpha absorber coincidences. This experiment extends multiple
sightline quasar absorber studies to higher redshift, higher opacity, larger
transverse separation, and into a regime where coherence across the IGM becomes
weak and difficult to detect. We fit 222 discrete Ly-alpha absorbers to
sightline A and 211 to sightline B. Relative to a Monte Carlo pairing test
(using symmetric, nearest neighbor matching) the data exhibit a 4sigma excess
of pairs at low velocity splitting (<150 km/s), thus detecting coherence on
transverse scales of ~2.5 h^-1 Mpc. We use spectra extracted from an SPH
simulation to analyze symmetric pair matching, transmission distributions as a
function of redshift and compute zero-lag cross-correlations to compare with
the quasar pair data. The simulations agree with the data with the same
strength (~4sigma) at similarly low velocity splitting above random chance
pairings. In cross-correlation tests, the simulations agree when the mean flux
(as a function of redshift) is assumed to follow the prescription given by
Kirkman et al. (2005). While the detection of flux correlation (measured
through coincident absorbers and cross-correlation amplitude) is only
marginally significant, the agreement between data and simulations is
encouraging for future work in which even better quality data will provide the
best insight into the overarching structure of the IGM and its understanding as
shown by SPH simulations.Comment: 15 pages, 11 figures; accepted for publication in Astronomical
Journa
Space VLBI Observations of 3C371
We present the first space VLBI observations of 3C~371, carried out at a
frequency of 4.8 GHz. The combination of the high resolution provided by the
orbiting antenna Highly Advanced Laboratory for Communications and Astronomy
(HALCA) and the high sensitivity of the VLBA allows imaging of the jet of
3C~371 with an angular resolution of approximately 0.26 mas, which for this
relatively nearby source corresponds to 0.4 h pc. Comparison
between two epochs separated by 66 days reveals no apparent motions in the
inner 7 mas jet structure above an upper limit of c. This
value, the absence of detectable counterjet emission from the presumably
symmetric jet, plus the presence of extended double-lobe structure, are
consistent with the knots in the jet being stationary features such as standing
shocks. The jet intensity declines with the angular distance from the core as
. This is more gradual than that derived for 3C~120,
, for which there is evidence for strong intereactions between
the jet and ambient medium. This suggests that in 3C~371 there is a greater
level of {\it in situ} acceleration of electrons and amplification of magnetic
field. We interpret sharp bends in the jet at sites of off-center knots as
further evidence for the interaction between the jet and external medium, which
may also be responsible for the generation of standing recollimation shocks.
These recollimation shocks may be responsible for the presumably stationary
components. The radio properties of 3C~371 are intermediate between those of
other radio galaxies with bright cores and those of BL Lacertae objects.Comment: 5 pages, 4 figures. Accepted for publication in Ap
A new method for the estimate of H_0 from quadruply imaged gravitational lens systems
We present a new method to estimate the Hubble constant H_0 from the measured
time delays in quadruply imaged gravitational lens systems. We show how it is
possible to get an estimate of H_0 without the need to completely reconstruct
the lensing potential thus avoiding any a priori hypotheses on the expression
of the galaxy lens model. Our method only needs to assume that the lens
potential may be expressed as r^{\alpha} F(\theta), whatever the shape function
F(\theta) is, and it is thus able to fully explore the degeneracy in the mass
models taking also into account the presence of an external shear. We test the
method on simulated cases and show that it does work well in recovering the
correct value of the slope \alpha of the radial profile and of the Hubble
constant H_0. Then, we apply the same method to the real quadruple lenses
PG1115+080 and B1422+231 obtaining H_0 = 58_{-15}^{+17} km/s/Mpc (68% CL).Comment: 12 pages, 5 figures, accepted for publication on Astronomy &
Astrophysic
Quasars as Absorption Probes of the J0053+1234 Region
We present UBRI photometry and spectra for 60 quasars found within one square
degree centered on the J0053+1234 region, which has been the subject of the
Caltech Faint Galaxy Redshift Survey. Candidate quasars were selected by their
ultraviolet excess with respect to the stellar locus, and confirmed
spectroscopically. The quasars span a wide range in brightness (17.5<B<21.6)
and redshift (0.43<z<2.38). These new quasars comprise a grid of absorption
probes that can be used to study large-scale structure as well as the
correlation between luminous galaxies, non-luminous halos, and Lyman-alpha
absorbers in the direction of the deep pencil-beam galaxy survey. Spectra of 14
emission line galaxies found using the same technique are also presented.Comment: Accepted for publication in the Astronomical Journal; 29 pages,
including 6 tables and 7 figure
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