8,191 research outputs found
New Measurements of the Ionizing Ultraviolet Background over 2 < z < 5 and Implications for Hydrogen Reionization
We present new measurements of the intensity of the ionizing ultraviolet
background and the global emissivity of ionizing photons over 2 < z < 5. Our
results are based on a suite of updated measurements of physical properties of
the high-redshift intergalactic medium (IGM), including gas temperatures and
the opacity of the IGM to Ly-alpha and ionizing photons. Consistent with
previous works, we find a relatively flat hydrogen photoionization rate over 2
< z < 5, although our measurements are roughly a factor of two higher than the
2008 values of Faucher-Giguere et al., due primarily to our lower gas
temperatures. The ionizing emissivity we derive is also generally higher than
other recent estimates due to a combination of lower gas temperatures, higher
ionizing opacity, and an accounting of cosmological radiative transfer effects.
We find evidence that the emissivity increases from z~3 to 5, reaching ~5
ionizing photons per atom per gigayear at z=4.75 for realistic galaxy spectra.
We further find that galaxies must dominate the emissivity near 1 Ryd at z > 4,
and possibly at all redshifts z > 2.4. Our results suggest that the
globally-averaged ionizing "efficiency" of star-forming galaxies increases
substantially with redshift over 3.2 < z < 4.75. This trend is consistent with
the conclusion often drawn from reionization models that the ionizing
efficiency of galaxies must be higher during reionization in order for galaxies
to reionize the IGM by z=6. Our emissivity values at z~5 suggest that ionizing
photons may have been a factor of two more abundant during the final stages of
reionization than previously indicated. The evolution of the ionizing
emissivity over 2 < z < 5 suggests, moreover, that the steep decline in the
photoionization rate from z~5 to 6 may indicate a rapid evolution in the mean
free path at z > 5.Comment: 19 pages, 14 figures, MNRAS, in pres
Cosmic Reionization On Computers. Properties of the Post-reionization IGM
We present a comparison between several observational tests of the
post-reionization IGM and the numerical simulations of reionization completed
under the Cosmic Reionization On Computers (CROC) project. The CROC simulations
match the gap distribution reasonably well, and also provide a good match for
the distribution of peak heights, but there is a notable lack of wide peaks in
the simulated spectra and the flux PDFs are poorly matched in the narrow
redshift interval 5.5<z<5.7, with the match at other redshifts being
significantly better, albeit not exact. Both discrepancies are related:
simulations show more opacity than the data.Comment: Accepted by Ap
Determining the Nature of Late Gunn-Peterson Troughs with Galaxy Surveys
Recent observations have discovered long (up to ~110 Mpc/h), opaque
Gunn-Peterson troughs in the z ~ 5.5 Lyman-alpha forest, which are challenging
to explain with conventional models of the post-reionization intergalactic
medium. Here we demonstrate that observations of the galaxy populations in the
vicinity of the deepest troughs can distinguish two competing models for these
features: deep voids where the ionizing background is weak due to fluctuations
in the mean free path of ionizing photons would show a deficit of galaxies,
while residual temperature variations from extended, inhomogeneous reionization
would show an overdensity of galaxies. We use large (~550 Mpc/h) semi-numerical
simulations of these competing explanations to predict the galaxy populations
in the largest of the known troughs at z ~ 5.7. We quantify the strong
correlation of Lyman-alpha effective optical depth and galaxy surface density
in both models and estimate the degree to which realistic surveys can measure
such a correlation. While a spectroscopic galaxy survey is ideal, we also show
that a relatively inexpensive narrowband survey of Lyman-alpha-emitting
galaxies is ~90% likely to distinguish between the competing models.Comment: 12 pages, 16 figures. Submitted to Ap
Star-forming Galactic Contrails at z=3.2 as a Source of Metal Enrichment and Ionizing Radiation
A spectroscopically detected Lyman alpha emitting halo at redshift 3.216 in
the GOODS-N field is found to reside at the convergence of several Lyman alpha
filaments. HST images show that some of the filaments are inhabited by
galaxies. Several of the galaxies in the field have pronounced head-tail
structures, which are partly aligned with each other. The blue colors of most
tails suggest the presence of young stars, with the emission from at least one
of the galaxies apparently dominated by high equivalent width Lyman alpha.
Faint, more diffuse, and similarly elongated, apparently stellar features, can
be seen over an area with a linear extent of at least 90 kpc. The region within
several arcseconds of the brightest galaxy exhibits spatially extended emission
by HeII, NV and various lower ionization metal lines. The gas-dynamical
features present are strongly reminiscent of ram-pressure stripped galaxies,
including evidence for recent star formation in the stripped contrails. Spatial
gradients in the appearance of several galaxies may represent a stream of
galaxies passing from a colder to a hotter intergalactic medium. The stripping
of gas from the in-falling galaxies, in conjunction with the occurrence of star
formation and stellar feedback in the galactic contrails suggests a mechanism
for the metal enrichment of the high redshift intergalactic medium that does
not depend on long-range galactic winds, at the same time opening a path for
the escape of ionizing radiation from galaxies.Comment: 12 pages, 9 figures, submitted to MNRA
Right to Counsel in Criminal Cases an Inquiry into the History and Practice in England and America
- …