14 research outputs found
Triple-ionised carbon associated with the low-density neutral hydrogen gas at 1.7 < z < 3.3: the integrated N(HI)-N(CIV) relation
From the Voigt profile fitting analysis of 183 intervening CIV systems at 1.7
< z < 3.3 in 23 high-quality UVES/VLT and HIRES/Keck QSO spectra, we find that
a majority of CIV systems (~75%) display a well-characterised scaling relation
between integrated column densities of HI and CIV with a negligible redshift
evolution, when column densities of all the HI and CIV components are
integrated within a given (-150, +150) km/sec range centred at the CIV flux
minimum. The integrated CIV column density N(CIV, sys) increases with N(HI,
sys) at log N(HI, sys) = 14.0--15.5 and log N(CIV, sys) = 11.8--14.0, then
becomes almost independent of N(HI, sys) at log N(HI, sys) > 16, with a large
scatter: at log N(HI, sys) = 14--22, log N(CIV, sys) = C1 / (log(NHI, sys) +
C2) + C3, with C1 = -1.90+0.55, C2 = -14.11+0.19 and C3 = 14.76+0.17,
respectively. The steep (flat) part is dominated by SiIV-free (SiIV-enriched)
CIV systems. Extrapolating the N(HI, sys)-N(CIV, sys) relation implies that
most absorbers with log N(HI) < 14 are virtually CIV-free. The N(HI,
sys)-N(CIV, sys) relation does not hold for individual components, clumps or
the integration velocity range less than +-100 km/sec. It is expected if the
line-of-sight extent of CIV is smaller than HI and N(CIV, sys) decreases more
rapidly than N(HI, sys) at the larger impact parameter, regardless of the
location of the HI+CIV gas in the IGM filaments or in the intervening galactic
halos.Comment: Accepted for publication on MNRAS, 26 pages, 20 figures, 4 tables.
On-line materials are found in the submitted civ.tar.gz file: complete Table
2, complete Table 3, complete Table 4, velocity plots civ1.pdf, civ2.pdf,
civ3.pdf, civ4.pdf and civ5.pd
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
Searching for fluctuations in the IGM temperature using the Lyman alpha forest
We propose a statistical method to search for fluctuations in the temperature
of the intergalactic medium (IGM) using the Lyman forest. The power on
small scales (\sim 25 \km/\s) is used as a thermometer and fluctuations of
this power are constrained. The method is illustrated using Q1422+231. We see
no evidence of temperature fluctuations. We show that in a model with two
temperatures that occupy comparable fractions of the spectra, the ratio of
small scale powers is constrained to be smaller than 3.5 (corresponding to a
factor of 2.5 in temperature). We show that approximately ten quasars are
needed constrain factors of two fluctuations in small scale power power.Comment: 9 pages, 10 figure
How Neutral is the Intergalactic Medium at z ~ 6?
Recent observations of high redshift quasar spectra reveal long gaps with
little flux. A small or no detectable flux does not by itself imply the
intergalactic medium (IGM) is neutral. Inferring the average neutral fraction
from the observed absorption requires assumptions about clustering of the IGM,
which the gravitational instability model supplies. Our most stringent
constraint on the neutral fraction at z ~ 6 is derived from the mean Lyman-beta
transmission measured from the z=6.28 SDSS quasar of Becker et al. -- the
neutral hydrogen fraction at mean density has to be larger than 4.7 times
10^{-4}. This is substantially higher than the neutral fraction of ~ 3-5 times
10^{-5} at z = 4.5 - 5.7, suggesting that dramatic changes take place around or
just before z ~ 6, even though current constraints are still consistent with a
fairly ionized IGM at z ~ 6. An interesting alternative method to constrain the
neutral fraction is to consider the probability of having many consecutive
pixels with little flux, which is small unless the neutral fraction is high.
This constraint is slightly weaker than the one obtained from the mean
transmission. We show that while the derived neutral fraction at a given
redshift is sensitive to the power spectrum normalization, the size of the jump
around z ~ 6 is not. We caution that systematic uncertainties include spatial
fluctuations in the ionizing background, and the continuum placement. Tests are
proposed. In particular, the sightline to sightline dispersion in mean
transmission might provide a useful diagnostic. We express the dispersion in
terms of the transmission power spectrum, and develop a method to calculate the
dispersion for spectra that are longer than the typical simulation box.Comment: 20 pages, 5 figures; ApJ accepted version; constraints revised due to
a revised power spectrum normalization in fiducial mode
The Lyman Alpha Forest in the Spectra of QSOs
Observations of redshifted Lyman alpha forest absorption in the spectra of
quasistellar objects (QSOs) provide a highly sensitive probe of the
distribution of gaseous matter in the universe. Over the past two decades
optical spectroscopy with large ground-based telescopes, and more recently
ultraviolet spectroscopy from space have yielded a wealth of information on
what appears to be a gaseous, photoionized intergalactic medium, partly
enriched by the products of stellar nucleosynthesis, residing in coherent
structures over many hundreds of kiloparsecs. Recent progress with cosmological
hydro-simulations based on hierarchical structure formation models has led to
important insights into the physical structures giving rise to the forest. If
these ideas are correct, a truely inter- and proto-galactic medium [at high
redshift (z ~ 3), the main repository of baryons] collapses under the influence
of dark matter gravity into flattened or filamentary structures, which are seen
in absorption against background QSOs. With decreasing redshift, galaxies
forming in the denser regions, may contribute an increasing part of the Lyman
alpha absorption cross-section. Comparisons between large data samples from the
new generation of telescopes and artificial Lyman alpha forest spectra from
cosmological simulations promise to become a useful cosmological tool.Comment: latex plus three postscript figures, uses psfig,sty; Annual Review of
Astronomy and Astrophysics 1998, vol. 36 (in press