4,993 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
Discovery of excess O I absorption towards the z = 6.42 QSO SDSS J1148+5251
We present a search for O I in the spectra of nine 4.9 < z_qso < 6.4 QSOs
taken with Keck/HIRES. We detect six systems with N(O I) > 10^13.7 cm^{-2} in
the redshift intervals where O I 1302 falls redward of the Ly-alpha forest.
Four of these lie towards SDSS J1148+5251 (z_qso = 6.42). This imbalance is
unlikely to arise from variations in sensitivity among our data or from a
statistical fluctuation. The excess O I occurs over a redshift interval that
also contains transmission in Ly-alpha and Ly-beta. Therefore, if these O I
systems represent pockets of neutral gas, then they must occur within or near
regions of the IGM that are highly ionized. In contrast, no O I is detected
towards SDSS J1030+0524 (z_qso = 6.30), whose spectrum shows complete
absorption in Ly-alpha and Ly-beta over \Delta z ~ 0.2. Assuming no ionization
corrections, we measure mean abundance ratios = -0.04 +/- 0.06,
= -0.31 +/- 0.09, and = -0.34 +/- 0.07 (2 sigma), which are
consistent with enrichment dominated by Type II supernovae. The O/Si ratio
limits the fraction of silicon in these systems contributed by metal-free very
massive stars to < 30%, a result which is insensitive to ionization
corrections. The ionic comoving mass densities along the z_qso > 6.2
sightlines, including only the detected systems, are \Omega(O I) = (7.0 +/-
0.6) * 10^{-8}, \Omega(Si II) = (9.6 +/- 0.9) * 10^{-9}, and \Omega(C II) =
(1.5 +/- 0.2) * 10^{-8}.Comment: Submitted to ApJ, with changes to reflect referee's comment
Observations of Chemically Enriched QSO Absorbers near z ~ 2.3 Galaxies: Galaxy-Formation Feedback Signatures in the IGM
We present a study of galaxies and intergalactic gas toward the z=2.73 quasar
HS1700+6416, to explore the effects of galaxy formation feedback on the IGM.
Our observations and ionization simulations indicate that the volume within
100-200 h_71^{-1} physical kpc of high-redshift galaxies contains very small,
dense, and metal-rich absorption-line regions. These systems often contain
shock-heated gas seen in OVI, and may exhibit [Si/C] abundance enhancements
suggestive of Type II supernova enrichment. We argue that the absorbers
resemble thin sheets or bubbles, whose physical properties can be explained
with a simple model of radiatively efficient shocks propegating through the
IGM. Their high metallicities suggest that these shocks are being expelled
from--rather than falling into--star forming galaxies. There is a dropoff in
the IGM gas density at galaxy impact parameters beyond ~300 physical kpc that
may trace boundaries of gas structures where the galaxies reside. The local
heavy-element enhancement covers 100-200 kpc; beyond this the observed
abundances blend into the general IGM. Supernova-driven winds or dynamical
stripping of interstellar gas appears to affect the IGM near massive galaxies,
even at R>~100 kpc. However, these feedback systems represent only a few
percent of the Lya forest mass at z~2.5. Their mass could be larger if the more
numerous metal-poor CIV systems at >~200 kpc are tepid remnants of very
powerful winds. Based on present observations it is not clear that this
scenario is to be favored over one involving pre-enrichment by smaller galaxies
at z>~6.Comment: Accepted for publication in the Astrophysical Journal. 26 pages
emulateapj, incl. 5 pages tables, 15 figure
A first direct measurement of the intergalactic medium temperature around a quasar at z=6
The thermal state of the intergalactic medium (IGM) provides an indirect
probe of both the HI and HeII reionisation epochs. Current constraints on the
IGM temperature from the Lya forest are restricted to the redshift range
2<z<4.5, limiting the ability to probe the thermal memory of HI reionisation
toward higher redshift. In this work, we present the first direct measurement
of the IGM temperature around a z=6 quasar by analysing the Doppler widths of
Lya absorption lines in the proximity zone of SDSS J0818+1722. We use a high
resolution (R= 40000) Keck/HIRES spectrum in combination with detailed
numerical modelling to obtain the temperature at mean density,
T_0=23600\pm^5000_6900K (\pm^9200_9300K) at 68 (95) per cent confidence
assuming a prior probability 13500K<T_0<38500 K following HI and HeII
reionisation. This enables us to place an upper limit on the redshift of HI
reionisation, z_H, within 33 comoving Mpc of SDSS J0818+1722. If the quasar
reionises the HeII in its vicinity, then in the limit of instantaneous
reionisation we infer z_H<9.0 (11.0) at 68 (95) per cent confidence assuming
photoheating is the dominant heat source and that HI reionisation is driven by
ionising sources with soft spectra, typical of population II stars. If the HI
and HeII in the IGM around SDSS J0818+1722 are instead reionised simultaneously
by a population of massive metal-free stars, characterised by very hard
ionising spectra, we obtain a tighter upper limit of z_H<8.4 (9.4). Initiating
reionisation at higher redshifts produces temperatures which are too low with
respect to our constraint unless the HI ionising sources or the quasar itself
have spectra significantly harder than typically assumed.Comment: 15 pages, 9 figures, accepted to MNRA
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