The timing and duration of the reionization epoch is crucial to the emergence and evolution of structure in the universe. The relative roles that star-forming galaxies, active galactic nuclei and ...quasars play in contributing to the metagalactic ionizing background across cosmic time remains uncertain. Deep quasar counts provide insights into their role, but the potentially crucial contribution from star-formation is highly uncertain due to our poor understanding of the processes that allow ionizing radiation to escape into the intergalactic medium (IGM). The fraction of ionizing photons that escape from star-forming galaxies is a fundamental free parameter used in models to "fine-tune" the timing and duration of the reionization epoch that occurred somewhere between 13.4 and 12.7 Gyrs ago (redshifts between 12 > z > 6). However, direct observation of Lyman continuum (LyC) photons emitted below the rest frame \ion{H}{1} ionization edge at 912 \AA\ is increasingly improbable at redshifts z > 3, due to the steady increase of intervening Lyman limit systems towards high z. Thus UV and U-band optical bandpasses provide the only hope for direct, up close and in depth, observations of the types of environment that favor LyC escape. By quantifying the evolution over the past 11 billion years (z < 3) of the relationships between LyC escape and local and global parameters ..., we can provide definitive information on the LyC escape fraction that is so crucial to answering the question of, how did the universe come to be ionized? Here we provide estimates of the ionizing continuum flux emitted by "characteristic" (L_{uv}^*) star-forming galaxies as a function of look back time and escape fraction, finding that at z = 1 (7.6 Gyrs ago) L_{uv}^* galaxies with an escape fraction of 1% have a flux of 10^{-19} ergs cm^{-2} s^{-1} \AA^{-1}.
Astrophys.J.598:458-473,2003 The challenge in searching for non-radio-pulsing isolated neutron stars
(INSs) is in excluding association with objects in the very large error boxes
(~13", 1 sigma ...radius) typical of sources from the largest X-ray all-sky
survey, the ROSAT All-Sky-Survey/Bright Source Catalog (RASS/BSC). We search
for candidate INSs using statistical analysis of optical (USNO-A2), infrared
(IRAS), and radio (NVSS) sources near the ROSAT X-ray localization, and show
that this selection would find 20% of the INSs in the RASS/BSC. This selection
finds 32 candidates at declinations greater than -39 deg, among which are two
previously known INSs, seventeen sources which we show are not INSs, and
thirteen the classification of which are as yet undetermined. These results
require a limit of <67 INSs (90% confidence, full sky, assuming isotropy) in
the RASS/BSC. This limit modestly constrains a naive and optimistic model for
cooling NSs in the galaxy.
We present new measurements of the spatial distribution and kinematics of
neutral hydrogen in the circumgalactic and intergalactic medium surrounding
star-forming galaxies at z ~ 2. Using the spectra ...of ~ 3000 galaxies with
redshifts +/- 0.4 from the Keck Baryonic Structure Survey (KBSS), we
assemble a sample of more than 200,000 distinct foreground-background pairs
with projected angular separations of 3 - 500 arcsec and spectroscopic
redshifts, with <$z_{fg}$> = 2.23 and <$z_{bg}$> = 2.57. The ensemble of
sightlines and foreground galaxies is used to construct a 2D map of the mean
excess Ly$\alpha$ optical depth relative to the intergalactic mean as a
function of projected galactocentric distance (20 < $D_{tran}$/pkpc < 4000) and
line-of-sight velocity. We provide information on the line-of-sight kinematics
of H I gas as a function of projected distance $D_{tran}$. We compare the map
with cosmological zoom-in simulation, finding qualitative agreement between
them. A simple two-component (accretion, outflow) analytical model generally
reproduces the observed line-of-sight kinematics and projected spatial
distribution of H I. The best-fitting model suggests that galaxy-scale outflows
with initial velocity $v_{out}$ ~ 600 km/s dominate the kinematics of
circumgalactic H I out to $D_{tran}$ ~ 50 kpc, while H I at $D_{tran}$ > 100
kpc is dominated by infall with characteristic $v_{in}$ < $v_c$, where $v_c$ is
the circular velocity of the host halo ($M_h$ ~ $10^{12} M_\odot$). Over the
impact parameter range 80 < $D_{tran}$/pkpc < 200, the H I line-of-sight
velocity range reaches a minimum, with a corresponding flattening in the
rest-frame Ly$\alpha$ equivalent width. These observations can be naturally
explained as the transition between outflow-dominated and accretion-dominated
flows. Beyond $D_{tran}$ ~ 300 kpc, the line of sight kinematics are dominated
by Hubble expansion.
The challenge in searching for non-radio-pulsing isolated neutron stars (INSs) is in excluding association with objects in the very large error boxes (~13", 1 sigma radius) typical of sources from ...the largest X-ray all-sky survey, the ROSAT All-Sky-Survey/Bright Source Catalog (RASS/BSC). We search for candidate INSs using statistical analysis of optical (USNO-A2), infrared (IRAS), and radio (NVSS) sources near the ROSAT X-ray localization, and show that this selection would find 20% of the INSs in the RASS/BSC. This selection finds 32 candidates at declinations greater than -39 deg, among which are two previously known INSs, seventeen sources which we show are not INSs, and thirteen the classification of which are as yet undetermined. These results require a limit of <67 INSs (90% confidence, full sky, assuming isotropy) in the RASS/BSC. This limit modestly constrains a naive and optimistic model for cooling NSs in the galaxy.
We present observations of Q1549-C25, an ~L* star-forming galaxy at z=3.15
for which Lyman-continuum (LyC) radiation is significantly detected in deep
Keck/LRIS spectroscopy. We find no evidence for ...contamination from a
lower-redshift interloper close to the line of sight in the high
signal-to-noise spectrum of Q1549-C25. Furthermore, the morphology of Q1549-C25
in V_606, J_125, and H_160 Hubble Space Telescope (HST) imaging reveals that
the object consists of a single, isolated component within 1". In combination,
these data indicate Q1549-C25 as a clean spectroscopic detection of LyC
radiation, only the second such object discovered to date at z~3. We model the
spectral energy distribution (SED) of Q1549-C25, finding evidence for
negligible dust extinction, an age (assuming continuous star formation) of ~1
Gyr, and a stellar mass of M_*=7.9x10^9 M_sun. Although it is not possible to
derive strong constraints on the absolute escape fraction of LyC emission,
f_esc(LyC), from a single object, we use simulations of intergalactic and
circumgalactic absorption to infer f_esc(LyC)>=0.51 at 95% confidence. The
combination of deep Keck/LRIS spectroscopy and HST imaging is required to
assemble a larger sample of objects like Q1549-C25, and obtain robust
constraints on the average f_esc(LyC) at z~3 and beyond.
We present new measurements of the spatial distribution and kinematics of neutral hydrogen in the circumgalactic and intergalactic medium surrounding star-forming galaxies at z ~ 2. Using the spectra ...of ~ 3000 galaxies with redshifts +/- 0.4 from the Keck Baryonic Structure Survey (KBSS), we assemble a sample of more than 200,000 distinct foreground-background pairs with projected angular separations of 3 - 500 arcsec and spectroscopic redshifts, with <\(z_{fg}\)> = 2.23 and <\(z_{bg}\)> = 2.57. The ensemble of sightlines and foreground galaxies is used to construct a 2D map of the mean excess Ly\(\alpha\) optical depth relative to the intergalactic mean as a function of projected galactocentric distance (20 < \(D_{tran}\)/pkpc < 4000) and line-of-sight velocity. We provide information on the line-of-sight kinematics of H I gas as a function of projected distance \(D_{tran}\). We compare the map with cosmological zoom-in simulation, finding qualitative agreement between them. A simple two-component (accretion, outflow) analytical model generally reproduces the observed line-of-sight kinematics and projected spatial distribution of H I. The best-fitting model suggests that galaxy-scale outflows with initial velocity \(v_{out}\) ~ 600 km/s dominate the kinematics of circumgalactic H I out to \(D_{tran}\) ~ 50 kpc, while H I at \(D_{tran}\) > 100 kpc is dominated by infall with characteristic \(v_{in}\) < \(v_c\), where \(v_c\) is the circular velocity of the host halo (\(M_h\) ~ \(10^{12} M_\odot\)). Over the impact parameter range 80 < \(D_{tran}\)/pkpc < 200, the H I line-of-sight velocity range reaches a minimum, with a corresponding flattening in the rest-frame Ly\(\alpha\) equivalent width. These observations can be naturally explained as the transition between outflow-dominated and accretion-dominated flows. Beyond \(D_{tran}\) ~ 300 kpc, the line of sight kinematics are dominated by Hubble expansion.
Thesis (Ph. D.) -- California Institute of Technology, 2010.
Title from home page (viewed 05/10/10). Advisor and committee chair names found in the thesis' metadata record in the digital repository. ...Includes bibliographical references.
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