3 research outputs found
The Inhomogeneous Ionizing Background Following Reionization
We study the spatial fluctuations in the hydrogen ionizing background in the
epoch following reionization (z ~ 5--6). The rapid decrease with redshift in
the photon mean free path (m.f.p.), combined with the clustering of
increasingly rare ionizing sources, can result in a very inhomogenous ionizing
background during this epoch. We systematically investigate the probability
density functions (PDFs) and power spectra of ionizing flux, by varying several
parameters such as the m.f.p., minimum halo mass capable of hosting stars, and
halo duty cycle. In order to be versatile, we make use of analytic,
semi-numeric and numeric approaches. Our models show that the ionizing
background indeed has sizable fluctuations during this epoch sourced by the
clustering of sources, with the PDFs being a factor of few wide at half of the
maximum likelihood. The distributions also show marked asymmetries, with a
high-value tail set by clustering on small scales, and a shorter low-value tail
which is set by the mean free path. The power spectrum of the ionizing
background is much more sensitive to source properties than the PDF and can be
well-understood analytically with a framework similar to the halo model
(usually used to describe dark matter clustering). Nevertheless, we find that
Lya forest spectra are extremely insensitive to the details of the UVB, despite
marked differences in the PDFs and power spectra of our various ionizing
backgrounds. Assuming a uniform ionizing background only underestimates the
value of the mean ionization rate inferred from the Lya forest by a few
percent. Instead, analysis of the Lya forest is dominated by the uncertainties
in the density field. Thus, our results justify the common assumption of a
uniform ionizing background in Lya forest analysis even during this epoch.Comment: 11 pages, 11 figures, submitted to the MNRA
Probing Reionization with Quasar Spectra: the Impact of the Intrinsic Lyman-alpha Emission Line Shape Uncertainty
Arguably the best hope of understanding the tail end of the reionization of
the intergalactic medium (IGM) at redshift z > 6 is through the detection and
characterization of the Gunn-Peterson (GP) damping wing absorption of the IGM
in bright quasar spectra. However, the use of quasar spectra to measure the IGM
damping wing requires a model of the quasar's intrinsic Lyman-alpha emission
line. Here we quantify the uncertainties in the intrinsic line shapes, and how
those uncertainties affect the determination of the IGM neutral fraction. We
have assembled a catalog of high-resolution HST spectra of the emission lines
of unobscured low-redshift quasars, and have characterized the variance in the
shapes of their lines. We then add simulated absorption from the high-redshift
IGM to these quasar spectra in order to determine the corresponding
uncertainties in reionization constraints using current and future samples of z
> 6 quasar spectra. We find that, if the redshift of the Lyman-alpha emission
line is presumed to coincide with the systemic redshift determined from metal
lines, the inferred IGM neutral fraction is systematically biased to low values
due to a systematic blueshift of the Lyman-alpha line relative to the metal
lines. If a similar blueshift persists in quasars at z > 6, this bias
strengthens previous claims of a significant neutral hydrogen fraction at z ~
6. This technique is capable of making a robust distinction between a highly
ionized (x_IGM ~ 10^-3) and a neutral (x_IGM = 1) IGM with even a few bright
quasars.Comment: accepted for publication in MNRAS, full tables available at
http://www.astro.columbia.edu/~roban
Glimpsing through the high redshift neutral hydrogen fog
We analyze the transmitted flux in a sample of 17 QSOs spectra at
5.74<zem<6.42 to obtain tighter constraints on the volume-averaged neutral
hydrogen fraction, xHI, at z~6. We study separately the narrow transmission
windows (peaks) and the wide dark portions (gaps) in the observed absorption
spectra. By comparing the statistics of these spectral features with Lyalpha
forest simulations, we conclude that xHI evolves smoothly from 10^{-4.4} at
z=5.3 to 10^{-4.2} at z=5.6, with a robust upper limit xHI<0.36 at z=6.3. The
frequency and physical sizes of the peaks imply an origin in cosmic underdense
regions and/or in HII regions around faint quasars or galaxies. In one case
(the intervening HII region of the faint quasar RD J1148+5253 at z=5.70 along
the LOS of SDSS J1148+5251 at z=6.42) the increase of the peak spectral density
is explained by the first-ever detected transverse proximity effect in the HI
Lyalpha forest; this indicates that at least some peaks result from a locally
enhanced radiation field. We then obtain a strong lower limit on the foreground
QSO lifetime of tQ>11 Myr. The observed widths of the peaks are found to be
systematically larger than the simulated ones. Reasons for such discrepancy
might reside either in the photoionization equilibrium assumption or in
radiative transfer effects.Comment: 12 pages, 9 figures, revised to match the accepted version including
a detailed analysis of the foreground QSO redshift and of the relativistic
effects on the HII region shape; MNRAS in pres