852 research outputs found
Non-Gaussianity and Excursion Set Theory: Halo Bias
We study the impact of primordial non-Gaussianity generated during inflation
on the bias of halos using excursion set theory. We recapture the familiar
result that the bias scales as on large scales for local type
non-Gaussianity but explicitly identify the approximations that go into this
conclusion and the corrections to it. We solve the more complicated problem of
non-spherical halos, for which the collapse threshold is scale dependent.Comment: 13 pages, 3 figures. v2 references added. Matches published versio
The Luminosity Dependence of Quasar Clustering
We investigate the luminosity dependence of quasar clustering, inspired by
numerical simulations of galaxy mergers that incorporate black hole growth.
These simulations have motivated a new interpretation of the quasar luminosity
function. In this picture, the bright end of the quasar luminosity function
consists of quasars radiating nearly at their peak luminosities, while the
faint end consists mainly of very similar sources, but at dimmer phases in
their evolution. We combine this model with the statistics of dark matter halos
that host quasar activity. We find that, since bright and faint quasars are
mostly similar sources seen in different evolutionary stages, a broad range in
quasar luminosities corresponds to only a narrow range in the masses of quasar
host halos. On average, bright and faint quasars reside in similar host halos.
Consequently, we argue that quasar clustering should depend only weakly on
luminosity. This prediction is in qualitative agreement with recent
measurements of the luminosity dependence of the quasar correlation function
(Croom et al. 2005) and the galaxy-quasar cross-correlation function
(Adelberger & Steidel 2005). Future precision clustering measurements from SDSS
and 2dF, spanning a large range in luminosity, should provide a strong test of
our model.Comment: 9 pages, 4 figures, submitted to Ap
Detecting the Rise and Fall of 21 cm Fluctuations with the Murchison Widefield Array
We forecast the sensitivity with which the Murchison Widefield Array (MWA)
can measure the 21 cm power spectrum of cosmic hydrogen, using radiative
transfer simulations to model reionization and the 21 cm signal. The MWA is
sensitive to roughly a decade in scale (wavenumbers of k ~ 0.1 - 1 h Mpc^{-1}),
with foreground contamination precluding measurements on larger scales, and
thermal detector noise limiting the small scale sensitivity. This amounts
primarily to constraints on two numbers: the amplitude and slope of the 21 cm
power spectrum on the scales probed. We find, however, that the redshift
evolution in these quantities can yield important information about
reionization. Although the power spectrum differs substantially across
plausible models, a generic prediction is that the amplitude of the 21 cm power
spectrum on MWA scales peaks near the epoch when the intergalactic medium (IGM)
is ~ 50% ionized. Moreover, the slope of the 21 cm power spectrum on MWA scales
flattens as the ionization fraction increases and the sizes of the HII regions
grow. Considering detection sensitivity, we show that the optimal MWA antenna
configuration for power spectrum measurements would pack all 500 antenna tiles
as close as possible in a compact core. The MWA is sensitive enough in its
optimal configuration to measure redshift evolution in the slope and amplitude
of the 21 cm power spectrum. Detecting the characteristic redshift evolution of
our models will confirm that observed 21 cm fluctuations originate from the
IGM, and not from foregrounds, and provide an indirect constraint on the
volume-filling factor of HII regions during reionization. After two years of
observations under favorable conditions, the MWA can constrain the filling
factor at an epoch when ~ 0.5 to within roughly +/- 0.1 at 2-sigma.Comment: 14 pages, 9 figures, submitted to Ap
On the Detectability of the Hydrogen 3-cm Fine Structure Line from the EoR
A soft ultraviolet radiation field, 10.2 eV < E <13.6 eV, that permeates
neutral intergalactic gas during the Epoch of Reionization (EoR) excites the 2p
(directly) and 2s (indirectly) states of atomic hydrogen. Because the 2s state
is metastable, the lifetime of atoms in this level is relatively long, which
may cause the 2s state to be overpopulated relative to the 2p state. It has
recently been proposed that for this reason, neutral intergalactic atomic
hydrogen gas may be detected in absorption in its 3-cm fine-structure line
(2s_1/2 -> 2p_3/2) against the Cosmic Microwave Background out to very high
redshifts. In particular, the optical depth in the fine-structure line through
neutral intergalactic gas surrounding bright quasars during the EoR may reach
tau~1e-5. The resulting surface brightness temperature of tens of micro K (in
absorption) may be detectable with existing radio telescopes. Motivated by this
exciting proposal, we perform a detailed analysis of the transfer of Lyman
beta,gamma,delta,... radiation, and re-analyze the detectability of the
fine-structure line in neutral intergalactic gas surrounding high-redshift
quasars. We find that proper radiative transfer modeling causes the
fine-structure absorption signature to be reduced tremendously to tau< 1e-10.
We therefore conclude that neutral intergalactic gas during the EoR cannot
reveal its presence in the 3-cm fine-structure line to existing radio
telescopes.Comment: 7 pages, 4 figures, MNRAS in press; v2. some typos fixe
The Impact of Temperature Fluctuations on the Lyman-alpha Forest Power Spectrum
We explore the impact of spatial fluctuations in the intergalactic medium
temperature on the Lyman-alpha forest flux power spectrum near z ~ 3. We
develop a semianalytic model to examine temperature fluctuations resulting from
inhomogeneous HI and incomplete HeII reionizations. Detection of these
fluctuations might provide insight into the reionization histories of hydrogen
and helium. Furthermore, these fluctuations, neglected in previous analyses,
could bias constraints on cosmological parameters from the Lyman-alpha forest.
We find that the temperature fluctuations resulting from inhomogeneous HI
reionization are likely to be very small, with an rms amplitude of < 5%,
. More important are the temperature fluctuations
that arise from incomplete HeII reionization, which might plausibly be as large
as 50%, . In practice, however, these temperature
fluctuations have only a small effect on flux power spectrum predictions. The
smallness of the effect is possibly due to density fluctuations dominating over
temperature fluctuations on the scales probed by current measurements. On the
largest scales currently probed, k ~ 0.001 s/km (~0.1 h/Mpc), the effect on the
flux power spectrum may be as large as ~10% in extreme models. The effect is
larger on small scales, up to ~20% at k = 0.1 s/km, due to thermal broadening.
Our results suggest that the omission of temperature fluctuations effects from
previous analyses does not significantly bias constraints on cosmological
parameters.Comment: 11 pages, 5 figures, ApJ accepte
Gravitational Lensing as Signal and Noise in Lyman-alpha Forest Measurements
In Lyman-alpha forest measurements it is generally assumed that quasars are
mere background light sources which are uncorrelated with the forest.
Gravitational lensing of the quasars violates this assumption. This effect
leads to a measurement bias, but more interestingly it provides a valuable
signal. The lensing signal can be extracted by correlating quasar magnitudes
with the flux power spectrum and with the flux decrement. These correlations
will be challenging to measure but their detection provides a direct measure of
how features in the Lyman-alpha forest trace the underlying mass density field.
Observing them will test the fundamental hypothesis that fluctuations in the
forest are predominantly driven by fluctuations in mass, rather than in the
ionizing background, helium reionization or winds. We discuss ways to
disentangle the lensing signal from other sources of such correlations,
including dust, continuum and background residuals. The lensing-induced
measurement bias arises from sample selection: one preferentially collects
spectra of magnified quasars which are behind overdense regions. This
measurement bias is ~0.1-1% for the flux power spectrum, optical depth and the
flux probability distribution. Since the effect is systematic, quantities such
as the amplitude of the flux power spectrum averaged across scales should be
interpreted with care.Comment: 22 pages, 8 figures; v2: references added, discussion expanded,
matches PRD accepted versio
The Line-of-Sight Proximity Effect and the Mass of Quasar Host Halos
We show that the Lyman-alpha optical depth statistics in the proximity
regions of quasar spectra depend on the mass of the dark matter halos hosting
the quasars. This is owing to both the overdensity around the quasars and the
associated infall of gas toward them. For a fiducial quasar host halo mass of
(3.0+/-1.6) h^-1 x 10^12 Msun, as inferred by Croom et al. from clustering in
the 2dF QSO Redshift Survey, we show that estimates of the ionizing background
(Gamma^bkg) from proximity effect measurements could be biased high by a factor
of ~2.5 at z=3 owing to neglecting these effects alone. The clustering of
galaxies and other active galactic nuclei around the proximity effect quasars
enhances the local background, but is not expected to skew measurements by more
than a few percent. Assuming the measurements of Gamma^bkg based on the mean
flux decrement in the Ly-alpha forest to be free of bias, we demonstrate how
the proximity effect analysis can be inverted to measure the mass of the dark
matter halos hosting quasars. In ideal conditions, such a measurement could be
made with a precision comparable to the best clustering constraints to date
from a modest sample of only about 100 spectra. We discuss observational
difficulties, including continuum flux estimation, quasar systematic redshift
determination, and quasar variability, which make accurate proximity effect
measurements challenging in practice. These are also likely to contribute to
the discrepancies between existing proximity effect and flux decrement
measurements of Gamma^bkg.Comment: 25 pages, including 14 figures, accepted by Ap
Measuring Galaxy Clustering and the Evolution of [C II] Mean Intensity with Far-IR Line Intensity Mapping during 0.5 < z < 1.5
Infrared fine-structure emission lines from trace metals are powerful diagnostics of the interstellar medium in galaxies. We explore the possibility of studying the redshifted far-IR fine-structure line emission using the three-dimensional (3-D) power spectra obtained with an imaging spectrometer. The intensity mapping approach measures the spatio-spectral fluctuations due to line emission from all galaxies, including those below the individual detection threshold. The technique provides 3-D measurements of galaxy clustering and moments of the galaxy luminosity function. Furthermore, the linear portion of the power spectrum can be used to measure the total line emission intensity including all sources through cosmic time with redshift information naturally encoded. Total line emission, when compared to the total star formation activity and/or other line intensities reveals evolution of the interstellar conditions of galaxies in aggregate. As a case study, we consider measurement of [CII] autocorrelation in the 0.5 < z < 1.5 epoch, where interloper lines are minimized, using far-IR/submm balloon-borne and future space-borne instruments with moderate and high sensitivity, respectively. In this context, we compare the intensity mapping approach to blind galaxy surveys based on individual detections. We find that intensity mapping is nearly always the best way to obtain the total line emission because blind, wide-field galaxy surveys lack sufficient depth and deep pencil beams do not observe enough galaxies in the requisite luminosity and redshift bins. Also, intensity mapping is often the most efficient way to measure the power spectrum shape, depending on the details of the luminosity function and the telescope aperture
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