1,241 research outputs found
Large-Scale Correlations in the Lyman-alpha Forest at z = 3-4
We present a study of the spatial coherence of the intergalactic medium
toward two pairs of high-redshift quasars with moderate angular separations
observed with Keck/ESI, Q1422+2309A/Q1424+2255 (z_em = 3.63, theta = 39") and
Q1439-0034A/B (z_em = 4.25, theta = 33"). The crosscorrelation of transmitted
flux in the Lyman-alpha forest shows a 5-7 sigma peak at zero velocity lag for
both pairs. This strongly suggests that at least some of the absorbing
structures span the 230-300/h_70 proper kpc transverse separation between
sightlines. We also statistically examine the similarity between paired spectra
as a function of transmitted flux, a measure which may be useful for comparison
with numerical simulations. In investigating the dependence of the correlation
functions on spectral characteristics, we find that photon noise has little
impact for S/N >~ 10 per resolution element. However, the agreement between the
autocorrelation along the line sight and the crosscorrelation between
sightlines, a potential test of cosmological geometry, depends significantly on
instrumental resolution. Finally, we present an inventory of metal lines. These
include a a pair of strong C IV systems at z ~ 3.4 appearing only toward
Q1439B, and a Mg II + Fe II system present toward Q1439 A and B at z = 1.68.Comment: 33 pages, 13 figures, submitted to Ap
Ionizing radiation fluctuations and large-scale structure in the Lyman-alpha forest
We investigate the large-scale inhomogeneities of the hydrogen ionizing
radiation field in the Universe at redshift z=3. Using a raytracing algorithm,
we simulate a model in which quasars are the dominant sources of radiation. We
make use of large scale N-body simulations of a LambdaCDM universe, and include
such effects as finite quasar lifetimes and output on the lightcone, which
affects the shape of quasar light echoes. We create Lya forest spectra that
would be generated in the presence of such a fluctuating radiation field,
finding that the power spectrum of the Lya forest can be suppressed by as much
as 15 % for modes with k=0.05-1 Mpc/h. This relatively small effect may have
consequences for high precision measurements of the Lya power spectrum on
larger scales than have yet been published. We also investigate another
radiation field probe, the cross-correlation of quasar positions and the Lya
forest. For both quasar lifetimes which we simulate (10^7 yr and 10^8 yr), we
expect to see a strong decrease in the Lya absorption close to other quasars
(the ``foreground'' proximity effect). We then use data from the Sloan Digital
Sky Survey First Data Release to make an observational determination of this
statistic. We find no sign of our predicted lack of absorption, but instead
increased absorption close to quasars. If the bursts of radiation from quasars
last on average < 10^6 yr, then we would not expect to be able to see the
foreground effect. However, the strength of the absorption itself seems to be
indicative of rare objects, and hence much longer total times of emission per
quasar. Variability of quasars in bursts with timescales > 10^4yr and < 10^6 yr
could reconcile these two facts.Comment: Submitted to ApJ, 21 pages, 17 postscript figures, emulateapj.st
On the Search for Quasar Light Echoes
The UV radiation from a quasar leaves a characteristic pattern in the
distribution of ionized hydrogen throughout the surrounding space. This pattern
or light echo propagates through the intergalactic medium at the speed of
light, and can be observed by its imprint on the Ly-alpha forest spectra of
background sources. As the echo persists after the quasar has switched off, it
offers the possibility of searching for dead quasars, and constraining their
luminosities and lifetimes. We outline a technique to search for and
characterize these light echoes. To test the method, we create artificial
Ly-alpha forest spectra from cosmological simulations at z=3, apply light
echoes and search for them. We show how the simulations can also be used to
quantify the significance level of any detection. We find that light echoes
from the brightest quasars could be found in observational data. With
absorption line spectra of 100 redshift z~3-3.5 quasars or galaxies in a 1
square degree area, we expect that ~10 echoes from quasars with B band
luminosities L_B=3x10^45 ergs/s exist that could be found at 95% confidence,
assuming a quasar lifetime of ~10^7 yr. Even a null result from such a search
would have interesting implications for our understanding of quasar
luminosities and lifetimes.Comment: 9 pages, 7 figures, ApJ in pres
A Multi-Species Model for Hydrogen and Helium Absorbers in Lyman-Alpha Forest Clouds
We have performed a multi-species hydrodynamical simulation of the formation
and evolution of Lyman alpha clouds in a flat CDM dominated universe with an
external flux of ionizing radiation. We solve the fully coupled non-equilibrium
rate equations for the following species: H, H^+, H^-, H_2, H_2^+, He, He^+,
He^{++}, and e^-. The statistical properties of the distribution and evolution
of both hydrogen and helium absorption lines are extracted and compared to
observed data. We find excellent agreement for the following neutral hydrogen
data: the distribution of column densities is fit well by a power law with
exponent beta=1.55 with a possible deficiency of lines above column density
10^{15} cm^{-2}; the integrated distribution matches observed data over a broad
range of column densities 10^{13} to 10^{17} cm^{-2}; a Gaussian statistical
fit to the Doppler parameter distribution yields a median of 35.6 km s^{-1};
the evolution of the number of clouds with column densities larger than 10^{14}
cm^{-2} follows a power law with exponent gamma=2.22. Analogous calculations
are presented for HeII absorption lines and we find the ratio of Doppler
parameters b_{HeII}/b_{HI} = 0.87. Our data also suggests that Ly
clouds may belong to two morphologically different groups: small column density
clouds which tend to reside in sheets or filamental structures and are very
elongated and/or flattened, and the large column density clouds which are
typically found at the intersections of these caustic structures and are
quasi-spherical.Comment: 14 pages, 4 postscript figure
High-Resolution Spectroscopy from 3050 to 10000 A of the HDF-S QSO J2233-606 with UVES at the ESO VLT
We report on high-resolution observations () of the Hubble
Deep Field South QSO J2233-606 obtained with the VLT UV-Visual Echelle
Spectrograph (UVES). We present spectral data for the wavelength region \AA. The ratio of the final spectrum is about 50 per
resolution element at 4000 \AA, 90 at 5000 \AA, 80 at 6000 \AA, 40 at 8000 \AA.
Redshifts, column densities and Doppler widths of the absorption features have
been determined with Voigt-profile fitting. A total of 621 lines have been
measured. In particular 270 Ly-alpha lines, 41 Ly-beta and 24 systems
containing metal lines have been identified. Together with other data in the
literature, the present spectrum confirms that the evolution of the number
density of Ly-alpha lines with \huno has an upturn at .Comment: 34 pages Latex, with 3 PostScript figures. Astronomical Journal, in
press. A few revised upper limit
Temperature and Kinematics of CIV Absorption Systems
We use Keck HIRES spectra of three intermediate redshift QSOs to study the
physical state and kinematics of the individual components of CIV selected
heavy element absorption systems. Fewer than 8 % of all CIV lines with column
densities greater than 10^{12.5} cm^{-2} have Doppler parameters b < 6 km/s. A
formal decomposition into thermal and non-thermal motion using the simultaneous
presence of SiIV gives a mean thermal Doppler parameter b_{therm}(CIV) = 7.2
km/s, corresponding to a temperature of 38,000 K although temperatures possibly
in excess of 300,000 K occur occasionally. We also find tentative evidence for
a mild increase of temperature with HI column density. Non-thermal motions
within components are typically small (< 10 km/s) for most systems, indicative
of a quiescent environment. The two-point correlation function (TPCF) of CIV
systems on scales up to 500 km/s suggests that there is more than one source of
velocity dispersion. The shape of the TPCF can be understood if the CIV systems
are caused by ensembles of objects with the kinematics of dwarf galaxies on a
small scale, while following the Hubble flow on a larger scale. Individual high
redshift CIV components may be the building blocks of future normal galaxies in
a hierarchical structure formation scenario.Comment: submitted to the ApJ Letters, March 16, 1996 (in press); (13 Latex
pages, 4 Postscript figures, and psfig.sty included
Small Scale Structure at High Redshift: II. Physical Properties of the CIV Absorbing Clouds
Keck HIRES spectra were obtained of the separate images of three
gravitationally lensed QSOs (UM 673, Q1104-1804, and Q1422+2309). We studied
the velocity and column density differences in CIV doublets in each QSO. Unlike
the low ionization gas clouds typical of the interstellar gas in the Galaxy or
damped Ly alpha galaxies, the spatial density distribution of CIV absorbing gas
clouds turns out to be mostly featureless on scales up to a few hundred
parsecs, with column density differences rising to 50 percent or more over
separations beyond a few kpc. Similarly, velocity shear becomes detectable only
over distances larger than a few hundred pc, rising to 70 km/s at a few kpc.
The energy transmitted to the gas is substantially less than in present day
star-forming regions, and the gas is less turbulent on a given spatial scale
than, e.g., local HII regions. The quiescence of CIV clouds, taken with their
probable low density, imply that these objects are not internal to galaxies.
The CIV absorbers could be gas expelled recently to large radii and raining
back onto its parent galaxy, or pre-enriched gas from an earlier (population
III) episode of star formation, falling into the nearest mass concentration.
However, while the metals in the gas may have been formed at higher redshifts,
the residual turbulence in the clouds and the minimum coherence length measured
here imply that the gas was stirred more recently, possibly by star formation
events recurring on a timescale on the order of 10-100 Million years (abstract
abbreviated).Comment: latex file plus 15 postscript figures (45 pages in total); to be
published in the ApJ, June 20, 2001 issu
The Statistics of Density Peaks and the Column Density Distribution of the Lyman-Alpha Forest
We develop a method to calculate the column density distribution of the
Lyman-alpha forest for column densities in the range . The Zel'dovich approximation, with appropriate smoothing, is used to
compute the density and peculiar velocity fields. The effect of the latter on
absorption profiles is discussed and it is shown to have little effect on the
column density distribution. An approximation is introduced in which the column
density distribution is related to a statistic of density peaks (involving its
height and first and second derivatives along the line of sight) in real space.
We show that the slope of the column density distribution is determined by the
temperature-density relation as well as the power spectrum on scales . An expression relating the three is given. We
find very good agreement between the column density distribution obtained by
applying the Voigt-profile-fitting technique to the output of a full
hydrodynamic simulation and that obtained using our approximate method for a
test model. This formalism then is applied to study a group of CDM as well as
CHDM models. We show that the amplitude of the column density distribution
depends on the combination of parameters , which is not well-constrained by independent observations. The
slope of the distribution, on the other hand, can be used to distinguish
between different models: those with a smaller amplitude and a steeper slope of
the power spectrum on small scales give rise to steeper distributions, for the
range of column densities we study. Comparison with high resolution Keck data
is made.Comment: match accepted version; discussion added: the effect of the shape of
the power spectrum on the slope of the column density distributio
Radiative Transfer Effects during Photoheating of the Intergalactic Medium
The thermal history of the intergalactic medium (IGM) after reionization is
to a large extent determined by photoheating. Here we demonstrate that
calculations of the photoheating rate which neglect radiative transfer effects
substantially underestimate the energy input during and after reionization. The
neglect of radiative transfer effects results in temperatures of the IGM which
are too low by a factor of two after HeII reionization. We briefly discuss
implications for the absorption properties of the IGM and the distribution of
baryons in shallow potential wells.Comment: 4 pages, 2 figures, submitted to ApJ
- âŠ