1,069 research outputs found
The Implications of Gunn-Peterson Troughs in the HeII Lyman-alpha Forest
Many experts believe that the z ~ 3 HeII Ly-alpha forest will suffer from the
same saturation issues as the z ~ 6 HI Ly-alpha forest and, therefore, will not
be a sensitive probe of HeII reionization. However, there are several factors
that make HeII Ly-alpha absorption more sensitive than HI Ly-alpha. We show
that observations of HeII Ly-alpha and Ly-beta Gunn-Peterson troughs can
provide a relatively model-independent constraint on the volume-averaged HeII
fraction of x_HeII >~ 0.1. This bound derives from first using the most
underdense regions in the HeII forest to constrain the local HeII fraction and,
then, assuming photoionization equilibrium with the maximum allowed
photoionization rate to calculate the ionization state of nearby gas. It is
possible to evade this constraint by a factor of ~2, but only if the HeII were
reionized recently. We argue that HeII Ly-alpha Gunn-Peterson troughs observed
in the spectra of Q0302-003 and HE2347-4342 signify the presence of >~ 10
comoving Mpc patches in which x_HeII > 0.03. This is a factor of 20 improvement
over previous constraints from these spectra and 100 times stronger than the
tightest constraint on the HI volume-filling fraction from the z>6 HI Lyman
forest.Comment: 5 pages, 4 figures, includes some supplementary text not in ApJ
Letter versio
A Flat Photoionization Rate at 2<z<4.2: Evidence for a Stellar-Dominated UV Background and Against a Decline of Cosmic Star Formation Beyond z~3
We investigate the implications of our measurement of the Lyman-alpha forest
opacity at redshifts 2<z<4.2 from a sample of 86 high-resolution quasar spectra
for the evolution of the cosmic ultraviolet luminosity density and its sources.
The derived hydrogen photoionization rate is remarkably flat over this redshift
range, implying an increasing comoving ionizing emissivity with redshift.
Because the quasar luminosity function is strongly peaked near z~2,
star-forming galaxies likely dominate the ionizing emissivity at z>~3. Our
measurement argues against a star formation rate density declining beyond z~3,
in contrast with existing state-of-the-art determinations of the cosmic star
formation history from direct galaxy counts. Stellar emission from galaxies
therefore likely reionized the Universe.Comment: 5 pages, including 1 figure, published by Ap
A Direct Measurement of the IGM Opacity to HI Ionizing Photons
We present a new method to directly measure the opacity from HI Lyman limit
(LL) absorption k_LL along quasar sightlines by the intergalactic medium (IGM).
The approach analyzes the average (``stacked'') spectrum of an ensemble of
quasars at a common redshift to infer the mean free path (MFP) to ionizing
radiation. We apply this technique to 1800 quasars at z=3.50-4.34 drawn from
the Sloan Digital Sky Survey (SDSS), giving the most precise measurements on
k_LL at any redshift. From z=3.6 to 4.3, the opacity increases steadily as
expected and is well parameterized by MFP = (48.4 +/- 2.1) - (38.0 +/-
5.3)*(z-3.6) h^-1 Mpc (proper distance). The relatively high MFP values
indicate that the incidence of systems which dominate k_LL evolves less
strongly at z>3 than that of the Lya forest. We infer a mean free path three
times higher than some previous estimates, a result which has important
implications for the photo-ionization rate derived from the emissivity of star
forming galaxies and quasars. Finally, our analysis reveals a previously
unreported, systematic bias in the SDSS quasar sample related to the survey's
color targeting criteria. This bias potentially affects all z~3 IGM studies
using the SDSS database.Comment: 7 pages, 4 figures; Accepted to ApJ
Systematic Continuum Errors in the Lyman-Alpha Forest and The Measured Temperature-Density Relation
Continuum fitting uncertainties are a major source of error in estimates of
the temperature-density relation (usually parametrized as a power-law, ) of the inter-galactic medium (IGM) through the
flux probability distribution function (PDF) of the Lyman- forest.
Using a simple order-of-magnitude calculation, we show that few percent-level
systematic errors in the placement of the quasar continuum due to e.g. a
uniform low-absorption Gunn-Peterson component, could lead to errors in
of order unity. This is quantified further using a simple
semi-analytic model of the Lya forest flux PDF. We find that
under-(over-)estimates in the continuum level can lead to a lower (higher)
measured value of . Within current observational uncertainties,
continuum biases double the error in from to within our model. We argue that steps
need to be taken to directly estimate the level of continuum bias in order to
make recent claims of an inverted \tdr\ more robust.Comment: 8 pages, 8 figures. Accepted by Ap
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
Effects of Ultraviolet Background and Local Stellar Radiation on the H_I Column Density Distribution
We study the impact of ultraviolet background (UVB) radiation field and the
local stellar radiation on the H_I column density distribution f(N_HI) of
damped Ly-alpha systems (DLAs) and sub-DLAs at z=3 using cosmological smoothed
particle hydrodynamics simulations. We find that, in the previous simulations
with an optically thin approximation, the UVB was sinking into the H_I cloud
too deeply, and therefore we underestimated the f(N_HI) at 19 < log(N_HI) <
21.2 compared to the observations. If the UVB is shut off in the high-density
regions with n_gas > 6 x 10^{-3} cm^{-3}, then we reproduce the observed
f(N_HI) at z=3 very well. We also investigate the effect of local stellar
radiation by post-processing our simulation with a radiative transfer code, and
find that the local stellar radiation does not change the f(N_HI) very much.
Our results show that the shape of f(N_HI) is determined primarily by the UVB
with a much weaker effect by the local stellar radiation and that the optically
thin approximation often used in cosmological simulation is inadequate to
properly treat the ionization structure of neutral gas in and out of DLAs. Our
result also indicates that the DLA gas is closely related to the transition
region from optically-thick neutral gas to optically-thin ionized gas within
dark matter halos.Comment: 5 pages, 3 figures, uses emulateapj. ApJL in press. Corrected a
plotting error in Fig. 3, which reduced the effect of local stellar radiation
on f(N_HI
Stellar feedback sets the universal acceleration scale in galaxies
It has been established for decades that rotation curves deviate from the Newtonian gravity expectation given baryons alone below a characteristic acceleration scale gâ âź10âťâ¸ cm sâťÂ˛â , a scale promoted to a new fundamental constant in MOND. In recent years, theoretical and observational studies have shown that the star formation efficiency (SFE) of dense gas scales with surface density, SFE âź ÎŁ/ÎŁ_(crit) with ÎŁ_(crit)âźâ¨pË/mââŠ/(ĎG)âź1000 M_â pcâťÂ˛ (where â¨pË/mâ⊠is the momentum flux output by stellar feedback per unit stellar mass in a young stellar population). We argue that the SFE, more generally, should scale with the local gravitational acceleration, i.e. that SFE âźg_(tot)/g_(crit) ⥠(GM_(tot)/R²)/â¨pË/mââŠâ , where M_(tot) is the total gravitating mass and g_(crit) = â¨pË/mâ⊠= ĎGÎŁ_(crit) â 10âťâ¸ cm sâťÂ˛ â gâ . Hence, the observed gâ may correspond to the characteristic acceleration scale above which stellar feedback cannot prevent efficient star formation, and baryons will eventually come to dominate. We further show how this may give rise to the observed acceleration scaling g_(obs) âź (g_(baryon)gâ )^(1/2) (where g_(baryon) is the acceleration due to baryons alone) and flat rotation curves. The derived characteristic acceleration gâ can be expressed in terms of fundamental constants (gravitational constant, proton mass, and Thomson cross-section): gâ âź0.1Gmp_/Ď_Tâ
A Measurement of Small Scale Structure in the 2.2 < z < 4.2 Lyman-alpha Forest
The amplitude of fluctuations in the Ly-a forest on small spatial scales is
sensitive to the temperature of the IGM and its spatial fluctuations. The
temperature of the IGM and its spatial variations contain important information
about hydrogen and helium reionization. We present a new measurement of the
small-scale structure in the Ly-a forest from 40 high resolution, high
signal-to-noise, VLT spectra at z=2.2-4.2. We convolve each Ly-a forest
spectrum with a suitably chosen wavelet filter, which allows us to extract the
amount of small-scale structure in the forest as a function of position across
each spectrum. We compare these measurements with high resolution hydrodynamic
simulations of the Ly-a forest which track more than 2 billion particles. This
comparison suggests that the IGM temperature close to the cosmic mean density
(T_0) peaks near z=3.4, at which point it is greater than 20,000 K at 2-sigma
confidence. The temperature at lower redshift is consistent with the fall-off
expected from adiabatic cooling (), after the peak
temperature is reached near z=3.4. At z=4.2 our results favor a temperature of
T_0 = 15-20,000 K. However, owing mostly to uncertainties in the mean
transmitted flux at this redshift, a cooler IGM model with T_0 = 10,000 K is
only disfavored at the 2-sigma level here, although such cool IGM models are
strongly discrepant with the z ~ 3-3.4 measurement. We do not detect large
spatial fluctuations in the IGM temperature at any redshift covered by our data
set. The simplest interpretation of our measurements is that HeII reionization
completes sometime near z ~ 3.4, although statistical uncertainties are still
large [Abridged].Comment: Submitted to ApJ. Best printed in colo
The HeI 584 A Forest as a Diagnostic of Helium Reionization
We discuss the potential of using the HeI 584 A forest to detect and study
HeII reionization. Significant 584 A absorption is expected from intergalactic
HeII regions, whereas there should be no detectable absorption from low density
gas in HeIII regions. Unlike HeII Ly-alpha absorption (the subject of much
recent study), the difficulty with using this transition to study HeII
reionization is not saturation but rather that the absorption is weak. The
Gunn-Peterson optical depth for this transition is tau ~ 0.1 x_{HeII} Delta^2
[(1+z)/5]^{9/2}, where x_{HeII} is the fraction of helium in HeII and Delta is
the density in units of the cosmic mean. In addition, HeI 584 A absorption is
contaminated by lower redshift HI Ly-alpha absorption with a comparable flux
decrement. We estimate the requirements for a definitive detection of
redshifted HeI absorption from low density gas (Delta ~ 1), which would
indicate that HeII reionization was occurring. We find that this objective can
be accomplished (using coeval HI Ly-alpha absorption to mask dense regions and
in cross correlation) with a spectral resolution of 10^4 and a signal-to-noise
ratio per resolution element of ~ 10. Such specifications may be achievable on
a few known z ~ 3.5 quasar sightlines with the Cosmic Origins Spectrograph on
the Hubble Space Telescope. We also discuss how HeI absorption can be used to
measure the hardness of the ionizing background above 13.6 eV.Comment: 12 pages, 5 figures, updated to match published versio
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