531 research outputs found
The Imprint of Cosmic Reionization on Galaxy Clustering
We consider the effect of reionization on the clustering properties of galaxy
samples at intermediate redshifts (z~0.3-5.5). Current models for the
reionization of intergalactic hydrogen predict that overdense regions will be
reionized early, thus delaying the build up of stellar mass in the progenitors
of massive lower-redshift galaxies. As a result, the stellar populations
observed in intermediate redshift galaxies are somewhat younger and hence
brighter in overdense regions of the Universe. Galaxy surveys would therefore
be sensitive to galaxies with a somewhat lower dark matter mass in overdense
regions. The corresponding increase in the observed number density of galaxies
can be parameterized as a galaxy bias due to reionization. We model this
process using merger trees combined with a stellar synthesis code. Our model
demonstrates that reionization has a significant effect on the clustering
properties of galaxy samples that are selected based on their star-formation
properties. The bias correction in Lyman-break galaxies (including those in
proposed baryonic oscillation surveys at z<1) is at the level of 10-20% for a
halo mass of 10^12 solar masses, leading to corrections factors of 1.5-2 in the
halo mass inferred from measurements of clustering length. The reionization of
helium could also lead to a sharp increase in the amplitude of the galaxy
correlation function at z~3. We find that the reionization bias is
approximately independent of scale and halo mass. However since the traditional
galaxy bias is mass dependent, the reionization bias becomes relatively more
important for lower mass systems. The correction to the bias due to
reionization is very small in surveys of luminous red galaxies at z<1.Comment: 17 pages, 6 figures. Submitted to MNRA
Constraining the Quasar Contribution to the Reionisation of Cosmic Hydrogen
Absorption spectra of high redshift quasars suggest that the reionisation of
cosmic hydrogen was complete near z~6. The dominant sources of ionising photons
responsible for this reionisation are generally thought to be stars and
quasars. In this paper we make a quantitative estimate of the relative
contributions made by these sources. Our approach is to compute the evolution
of the post overlap ionising background radiation by combining semi-analytic
descriptions of reionisation in a clumpy medium with a model for the quasar
luminosity function. Our overall model has two free parameters, the star
formation efficiency and the minimum quasar luminosity. By adjusting these
parameters, we constrain the relative contributions made by stars and quasars
through comparison with reported observations (Fan et al. 2005). We find that
the relative quasar contribution (at z=5.7) to the ionising background was
between 1.4% and 14.5%. The range of uncertainty is dominated by the unknown
minimum quasar luminosity.Comment: 8 pages, 2 figures. Accepted for publication in MNRA
Far-Ultraviolet Observations of NGC 3516 using the Hopkins Ultraviolet Telescope
We observed the Seyfert 1 galaxy NGC 3516 twice during the flight of Astro-2
using the Hopkins Ultraviolet Telescope in March 1995. Simultaneous X-ray
observations were performed with ASCA. Our far-ultraviolet spectra cover the
spectral range 820-1840 A with a resolution of 2-4 A. No significant variations
were found between the two observations. The total spectrum shows a red
continuum, , with an observed flux of at 1450 A, slightly above the historical
mean. Intrinsic absorption in Lyman is visible as well as absorption
from O~vi 1032,1038, N~v 1239,1243, Si~iv 1394,1403, and C~iv 1548,1551. The UV
absorption lines are far weaker than is usual for NGC~3516, and also lie closer
to the emission line redshift rather than showing the blueshift typical of
these lines when they are strong. The neutral hydrogen absorption, however, is
blueshifted by relative to the systemic velocity, and it is
opaque at the Lyman limit. The sharpness of the cutoff indicates a low
effective Doppler parameter, . For
the derived intrinsic column is . As in
NGC~4151, a single warm absorber cannot produce the strong absorption visible
over the wide range of observed ionization states. Matching both the UV and
X-ray absorption simultaneously requires absorbers spanning a range of
in both ionization parameter and column density.Comment: 18 pages, 4 PostScript figures, uses aaspp4.sty To appear in the
August 20, 1996, issue of The Astrophysical Journa
Far-UV Observations of NGC 4151 during the ORFEUS-SPAS II Mission
We observed the Seyfert 1 galaxy NGC 4151 on eleven occasions at 1-2 day
intervals using the Berkeley spectrometer during the ORFEUS-SPAS II mission in
1996 November. The mean spectrum covers 912-1220 A at ~0.3 A resolution with a
total exposure of 15,658 seconds. The mean flux at 1000 A was 4.7e-13
erg/cm^2/s/A. We identify the neutral hydrogen absorption with a number of
components that correspond to the velocity distribution of \ion{H}{1} seen in
our own Galaxy as well as features identified in the CIV 1549 absorption
profile by Weymann et al. The main component of neutral hydrogen in NGC 4151
has a total column density of log N_HI = 18.7 +/- 1.5 cm^{-2} for a Doppler
parameter b=250 +/- 50 km/s, and it covers 84 +/- 6% of the source. This is
consistent with previous results obtained with the Hopkins Ultraviolet
Telescope. Other intrinsic far-UV absorption features are not resolved, but the
CIII* 1176 absorption line has a significantly higher blueshift relative to NGC
4151 than the CIII 977 resonance line. This implies that the highest velocity
region of the outflowing gas has the highest density. Variations in the
equivalent width of the CIII* 1176 absorption line anticorrelate with continuum
variations on timescales of days. For an ionization timescale <1 day, we set an
upper limit of 25 pc on the distance of the absorbing gas from the central
source. The OVI 1034 and HeII 1085 emission lines also vary on timescales of
1-2 days, but their response to the continuum variations is complex. For some
continuum variations they show no response, while for others the response is
instantaneous to the limit of our sampling interval.Comment: 4 pages, 2 PostScript figures, uses emulateapj.sty, apjfonts.sty. To
appear in the Astrophysical Journal (Letters) special issue for ORFEU
Simultaneous Ultraviolet and X-ray Observations of the Seyfert Galaxy NGC 4151. I. Physical Conditions in the X-ray Absorbers
We present a detailed analysis of the intrinsic X-ray absorption in the
Seyfert 1 galaxy NGC 4151 using Chandra/HETGS data obtained 2002 May, as part
of a program which included simultaneous UV spectra using HST/STIS and FUSE.
NGC 4151 was in a relatively low flux state during the observations reported
here, although roughly 2.5 times as bright in the 2 --10 keV band as during a
Chandra observation in 2000. The soft X-ray band was dominated by emission
lines, which show no discernible variation in flux between the two
observations. The 2002 data show the presence of a very highly ionized
absorber, in the form of H-like and He-like Mg, Si, and S lines, as well as
lower ionization gas via the presence of inner-shell absorption lines from
lower-ionization species of these elements. The former is too highly ionized to
be radiatively accelerated in a sub-Eddington source such as NGC 4151. We find
that the lower ionization gas had a column density a factor of ~ 3 higher
during the 2000 observation. If due to bulk motion, we estimate that this
component must have a velocity of more than 1250 km/sec transverse to our
line-of-sight. We suggest that these results are consistent with a
magneto-hydrodynamic flow.Comment: 42 pages, 14 figures. Accepted for publication in The Astrophysical
Journa
A Composite HST Spectrum of Quasars
We construct a composite quasar spectrum from 284 HST FOS spectra of 101
quasars with redshifts z > 0.33. The spectrum covers the wavelengths between
350 and 3000 A in the rest frame. There is a significant steepening of the
continuum slope around 1050 A. The continuum between 1050 and 2200 A can be
modeled as a power law with alpha = -0.99. For the full sample the power-law
index in the extreme ultraviolet (EUV) between 350 and 1050 A is alpha = -1.96.
The continuum flux in the wavelengths near the Lyman limit shows a depression
of about 10 percent. The break in the power-law index and the slight depression
of the continuum near the Lyman limit are features expected in Comptonized
accretion-disk spectra.Comment: 10 figures To appear in the February 1, 1997, issue of the Ap.
Ultraviolet Broad Absorption Features and the Spectral Energy Distribution of the QSO PG 1351+64
We present a moderate-resolution (~20 km/s) spectrum of the mini
broad-absorption-line QSO PG1351+64 between 915-1180 A, obtained with the Far
Ultraviolet Spectroscopic Explorer (FUSE). Additional spectra at longer
wavelengths were also obtained with the HST and ground-based telescopes. Broad
absorption is present on the blue wings of CIII 977, Ly-beta, OVI 1032,1038,
Ly-alpha, NV 1238,1242, SiIV 1393,1402, and CIV 1548,1450. The absorption
profile can be fitted with five components at velocities of ~ -780, -1049,
-1629, -1833, and -3054 km/s with respect to the emission-line redshift of z =
0.088. All the absorption components cover a large fraction of the continuum
source as well as the broad-line region. The OVI emission feature is very weak,
and the OVI/Lyalpha flux ratio is 0.08, one of the lowest among low-redshift
active galaxies and QSOs. The UV continuum shows a significant change in slope
near 1050 A in the restframe. The steeper continuum shortward of the Lyman
limit extrapolates well to the observed weak X-ray flux level. The absorbers'
properties are similar to those of high-redshift broad absorption-line QSOs.
The derived total column density of the UV absorbers is on the order of 10^21
cm^-2, unlikely to produce significant opacity above 1 keV in the X-ray. Unless
there is a separate, high-ionization X-ray absorber, the QSO's weak X-ray flux
may be intrinsic. The ionization level of the absorbing components is
comparable to that anticipated in the broad-line region, therefore the
absorbers may be related to broad-line clouds along the line of sight.Comment: 23 pages, Latex, 5 figure
ASCA Observations of the Composite Warm Absorber in NGC 3516
We obtained X-ray spectra of the Seyfert 1 galaxy NGC~3516 in March 1995
using ASCA. Simultaneous far-UV observations were obtained with HUT on the
Astro-2 shuttle mission. The ASCA spectrum shows a lightly absorbed power law
of energy index 0.78. The low energy absorbing column is significantly less
than previously seen. Prominent O~vii and O~viii absorption edges are visible,
but, consistent with the much lower total absorbing column, no Fe K absorption
edge is detectable. A weak, narrow Fe~K emission line from cold
material is present as well as a broad Fe~K line. These features are
similar to those reported in other Seyfert 1 galaxies. A single warm absorber
model provides only an imperfect description of the low energy absorption. In
addition to a highly ionized absorber with ionization parameter and
a total column density of , adding a lower
ionization absorber with and a total column of significantly improves the fit. The contribution of
resonant line scattering to our warm absorber models limits the Doppler
parameter to at 90\% confidence. Turbulence at the sound
speed of the photoionized gas provides the best fit. None of the warm absorber
models fit to the X-ray spectrum can match the observed equivalent widths of
all the UV absorption lines. Accounting for the X-ray and UV absorption
simultaneously requires an absorbing region with a broad range of ionization
parameters and column densities.Comment: 14 pages, 4 Postscript figures, uses aaspp4.sty To appear in the
August 20, 1996, issue of The Astrophysical Journa
Simultaneous Ultraviolet and X-ray Observations of the Seyfert Galaxy NGC 4151. II. Physical Conditions in the UV Absorbers
We present a detailed analysis of the intrinsic absorption in the Seyfert 1
galaxy NGC 4151 using UV spectra from the HST/STIS and FUSE, obtained 2002 May
as part of a set of contemporaneous observations that included Chandra/HETGS
spectra. In our analysis of the Chandra spectra, we determined that the soft
X-ray absorber was the source of the saturated UV lines of O VI, C IV, and N V
associated with the absorption feature at a radial velocity of ~ -500 km/sec,
which we referred to as component D+E. In the present work, we have derived
tighter constrains on the the line-of-sight covering factors, densities, and
radial distances of the absorbers. We find that the Equivalent Widths (EWs) of
the low-ionization lines associated with D+E varied over the period from 1999
July to 2002 May. The drop in the EWs of these lines between 2001 April and
2002 May are suggestive of bulk motion of gas out of our line-of-sight. If
these lines from these two epochs arose in the same sub-component, the
transverse velocity of the gas is ~ 2100 km/sec. Transverse velocities of this
order are consistent with an origin in a rotating disk, at the roughly radial
distance we derived for D+E.Comment: 51 pages, including 12 figures. Accepted for publication in ApJ
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