363 research outputs found
The Reionization of Carbon
Observations suggest that CII was more abundant than CIV in the intergalactic
medium towards the end of the hydrogen reionization epoch. This transition
provides a unique opportunity to study the enrichment history of intergalactic
gas and the growth of the ionizing background (UVB) at early times. We study
how carbon absorption evolves from z=10-5 using a cosmological hydrodynamic
simulation that includes a self-consistent multifrequency UVB as well as a
well-constrained model for galactic outflows to disperse metals. Our predicted
UVB is within 2-4 times that of Haardt & Madau (2012), which is fair agreement
given the uncertainties. Nonetheless, we use a calibration in post-processing
to account for Lyman-alpha forest measurements while preserving the predicted
spectral slope and inhomogeneity. The UVB fluctuates spatially in such a way
that it always exceeds the volume average in regions where metals are found.
This implies both that a spatially-uniform UVB is a poor approximation and that
metal absorption is not sensitive to the epoch when HII regions overlap
globally even at column densites of 10^{12} cm^{-2}. We find, consistent with
observations, that the CII mass fraction drops to low redshift while CIV rises
owing the combined effects of a growing UVB and continued addition of carbon in
low-density regions. This is mimicked in absorption statistics, which broadly
agree with observations at z=6-3 while predicting that the absorber column
density distributions rise steeply to the lowest observable columns. Our model
reproduces the large observed scatter in the number of low-ionization absorbers
per sightline, implying that the scatter does not indicate a partially-neutral
Universe at z=6.Comment: 16 pages, 14 figures, accepted to MNRA
The redshift evolution of the baryonic Tully-Fisher relation in Simba
The baryonic Tully-Fisher relation (BTFR) is an important tool for
constraining galaxy evolution models. As 21-cm HI emission studies have been
largely restricted to low redshifts, the redshift evolution of the BTFR is yet
to be fully studied. The upcoming LADUMA survey (Looking At the Distant
Universe with the MeerKAT Array) will address this. As preparation for LADUMA,
we use the Simba hydrodynamical galaxy formation simulation from the
Simba-hires (25 h Mpc) run to generate rotational velocity
measures from galaxy rotation curves () and HI spectral line
profile widths ( and ) at three different redshifts ( = 0, 0.5,
and 1). Using these measures, together with the dark matter velocity
dispersion, we consider the redshift evolution of the BTFR of Simba galaxies.
We find that LADUMA will be successful in detecting redshift evolution of the
BTFR, provided that auxiliary data is used to distinguish galaxies with disky
morphologies. spectral line widths give lower scatter and more pronounced
redshift evolution compared to . We also compare these rotational velocity
measures to the dark matter velocity dispersion across redshift and galaxy
morphology. We find weak redshift evolution between rotational velocity and the
dark matter halo mass, and provide fits for estimating a galaxy's dark matter
halo mass from HI spectral line widths. This study with Simba showcases the
importance of upcoming, deep SKA pathfinder surveys such as LADUMA, and
provides predictions to compare with redshift evolution of the BTFR and galaxy
dark matter content from HI rotational velocity measures.Comment: 15 pages, 7 figures. Submitted to MNRA
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
Simulations of AGN feedback in galaxy clusters and groups: impact on gas fractions and the Lx-T scaling relation
Recently, rapid observational and theoretical progress has established that
black holes (BHs) play a decisive role in the formation and evolution of
individual galaxies as well as galaxy groups and clusters. In particular, there
is compelling evidence that BHs vigorously interact with their surroundings in
the central regions of galaxy clusters, indicating that any realistic model of
cluster formation needs to account for these processes. This is also suggested
by the failure of previous generations of hydrodynamical simulations without BH
physics to simultaneously account for the paucity of strong cooling flows in
clusters, the slope and amplitude of the observed cluster scaling relations,
and the high-luminosity cut-off of central cluster galaxies. Here we use
high-resolution cosmological simulations of a large cluster and group sample to
study how BHs affect their host systems. We focus on two specific properties,
the halo gas fraction and the X-ray luminosity-temperature scaling relation,
both of which are notoriously difficult to reproduce in self-consistent
hydrodynamical simulations. We show that BH feedback can solve both of these
issues, bringing them in excellent agreement with observations, without
alluding to the `cooling only' solution that produces unphysically bright
central galaxies. By comparing a large sample of simulated AGN-heated clusters
with observations, our new simulation technique should make it possible to
reliably calibrate observational biases in cluster surveys, thereby enabling
various high-precision cosmological studies of the dark matter and dark energy
content of the universe.Comment: 4 pages, 2 figures, minor revisions, ApJL in pres
Impedance Analysis for Oxygen Reduction in a Lithium Carbonate Melt
Oxygen reduction on a smooth gold electrode in a pure lithium carbonate melt was investigated by electrochemical impedance spectroscopy and cyclic voltammetry. The impedance data were analyzed using the Randles-Ershler equivalent circuit to determine parameters such as the charge-transfer resistance, Warburg coefficient, double-layer capacity, and uncompensated electrolyte resistance. The parameters estimated by complex plane plots and a complex nonlinear least squares method are in good agreement. Cyclic voltammetric measurements showed that oxygen reduction in a lithium carbonate melt is very rapid. A mass transfer parameter, DC0, estimated by the cyclic voltammetry concurred withthat calculated by the electrochemical impedance spectroscopy technique. The temperature dependences of the exchange current density and the product DC0 were examined, and the apparent activation energies were determined to be 121.7 ± 24.4 and 181.0 ± 7.0 kJ/mol, respectively
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