1,653 research outputs found
Measuring the Radiative Histories of QSOs with the Transverse Proximity Effect
Since the photons that stream from QSOs alter the ionization state of the gas
they traverse, any changes to a QSO's luminosity will produce
outward-propagating ionization gradients in the surrounding intergalactic gas.
This paper shows that at redshift z~3 the gradients will alter the gas's
Lyman-alpha absorption opacity enough to produce a detectable signature in the
spectra of faint background galaxies. By obtaining noisy (S:N~4) low-resolution
(~7A) spectra of a several dozen background galaxies in an R~20' field
surrounding an isotropically radiating 18th magnitude QSO at z=3, it should be
possible to detect any order-of-magnitude changes to the QSO's luminosity over
the previous 50--100 Myr and to measure the time t_Q since the onset of the
QSO's current luminous outburst with an accuracy of ~5 Myr for t_Q<~50 Myr.
Smaller fields-of-view are acceptable for shorter QSO lifetimes. The major
uncertainty, aside from cosmic variance, will be the shape and orientation of
the QSO's ionization cone. This can be determined from the data if the number
of background sources is increased by a factor of a few. The method will then
provide a direct test of unification models for AGN.Comment: Accepted for publication in the ApJ. 16 page
Constraints on Lorentz Invariance Violation using INTEGRAL/IBIS observations of GRB041219A
One of the experimental tests of Lorentz invariance violation is to measure
the helicity dependence of the propagation velocity of photons originating in
distant cosmological obejcts. Using a recent determination of the distance of
the Gamma-Ray Burst GRB 041219A, for which a high degree of polarization is
observed in the prompt emission, we are able to improve by 4 orders of
magnitude the existing constraint on Lorentz invariance violation, arising from
the phenomenon of vacuum birefringence.Comment: 5 pages, 3 figures, accepted for publication as a Rapid Communication
in Physical Review
The Star Formation Rate Intensity Distribution Function--Implications for the Cosmic Star Formation Rate History of the Universe
We address the effects of cosmological surface brightness dimming on
observations of faint galaxies by examining the distribution of "unobscured"
star formation rate intensities versus redshift. We use the star formation rate
intensity distribution function to assess the ultraviolet luminosity density
versus redshift, based on our photometry and photometric redshift measurements
of faint galaxies in the HDF and the HDF--S WFPC2 and NICMOS fields. We find
that (1) previous measurements have missed a dominant fraction of the
ultraviolet luminosity density of the universe at high redshifts by neglecting
cosmological surface brightness dimming effects, which are important at
redshifts larger than z = 2, (2) the incidence of the highest intensity star
forming regions increases monotonically with redshift, and (3) the ultraviolet
luminosity density plausibly increases monotonically with redshift through the
highest redshifts observed. By measuring the spectrum of the luminosity density
versus redshift, we also find that (4) previous measurements of the ultraviolet
luminosity density at redshifts z < 2 must be reduced by a factor 2 to allow
for the spectrum of the luminosity density between rest-frame wavelengths 1500
and 2800 A. And by comparing with observations of high-redshift damped
Lyman-alpha absorption systems detected toward background QSOs, we further find
that (5) the distribution of star formation rate intensities matches the
distribution of neutral hydrogen column densities at redshifts z = 2 through 5,
which establishes a quantitative connection between high-redshift galaxies and
high column density gas and suggests that high-redshift damped Lyman-alpha
absorption systems trace lower star formation rate intensity regions of the
same galaxies detected in star light in the HDF and HDF--S.Comment: 28 pages, 9 figures; accepted for publication in the Astrophysical
Journa
The Transverse Proximity Effect: A Probe to the Environment, Anisotropy, and Megayear Variability of QSOs
The transverse proximity effect is the expected decrease in the strength of
the Lya forest absorption in a QSO spectrum when another QSO lying close to the
line of sight enhances the photoionization rate above that due to the average
cosmic ionizing background. We select three QSOs from the Early Data Release of
the Sloan Digital Sky Survey that have nearby foreground QSOs, with proper line
of sight tangential separations of 0.50, 0.82, and 1.10 h^{-1} Mpc. We estimate
that the ionizing flux from the foreground QSO should increase the
photoionization rate by a factor (94, 13, 13) in these three cases, which would
be clearly detectable in the first QSO and marginally so in the other two. We
do not detect the transverse proximity effect. Three possible explanations are
provided: an increase of the gas density in the vicinity of QSOs, time
variability, and anisotropy of the QSO emission. We find that the increase of
gas density near QSOs can be important if they are located in the most massive
halos present at high redshift, but is not enough to fully explain the absence
of the transverse proximity effect. Anisotropy requires an unrealistically
small opening angle of the QSO emission. Variability demands that the
luminosity of the QSO with the largest predicted effect was much lower 10^6
years ago, whereas the transverse proximity effect observed in the HeII Lya
absorption in QSO 0302-003 by Jakobsen et al. (2003) implies a lifetime longer
than 10^7 years. A combination of all three effects may better explain the lack
of Lya absorption reduction. A larger sample of QSO pairs may be used to
diagnose the environment, anisotropy and lifetime distribution of QSOs.Comment: 27 pages, 13 figures, accepted by Ap
Morphological number-count and redshift distributions to I < 26 from the Hubble Deep Field: Implications for the evolution of Ellipticals, Spirals and Irregulars
We combine the photometric redshift data of Fernandez-Soto et al. (1997) with
the morphological data of Odewahn et al. (1996) for all galaxies with I < 26.0
detected in the Hubble Deep Field. From this combined catalog we generate the
morphological galaxy number-counts and corresponding redshift distributions and
compare these to the predictions of high normalization zero- and passive-
evolution models. From this comparison we conclude the following: (1) E/S0s are
seen in numbers and over a redshift range consistent with zero- or minimal
passive- evolution to I = 24. Beyond this limit fewer E/S0s are observed than
predicted implying a net negative evolutionary process --- luminosity dimming,
disassembly or masking by dust --- at I > 24. (2) Spiral galaxies are present
in numbers consistent with zero- evolution predictions to I = 22. Beyond this
magnitude some net- positive evolution is required. Although the number-counts
are consistent with the passive-evolution predictions to I=26.0 the redshift
distributions favor number AND luminosity evolution. (3) There is no obvious
explanation for the late-type/irregular class and this category requires
further subdivision. While a small fraction of the population lies at low
redshift (i.e. true irregulars), the majority lie at redshifts, 1 < z < 3. At z
> 1.5 mergers are frequent and, taken in conjunction with the absence of normal
spirals at z > 2, the logical inference is that they represent the progenitors
of normal spirals forming via hierarchical merging.Comment: Accepted for publication in ApJ Letters, colour plates available from
http://www.phys.unsw.edu.au/~spd/bib.htm
An Aluminum Enhanced Cloud in a CIV Absorber at z = 1.94
In the z=1.94 CIV absorption line system in the spectrum of quasar Q1222+228
(z_em=2.04), we find two clouds which have contrasting physical conditions,
although they are only at a 17 km/s velocity separation. In the first cloud
SiII, SiIV, and CII are detected, and AlII and AlIII column density limits in
conjunction with photoionization models allow us to infer that this cloud has a
large Si abundance and a small Al abundance relative to a solar abundance
pattern. This pattern resembles that of Galactic metal-poor halo stars, which
must have formed from such high redshift gas. The second cloud, in contrast,
has detected AlII and AlIII (also SiIV and CII), but no detected SiII. We
demonstrate, using photoionization models, that Al/Si must be greater than
(Al/Si)_Sun in this unusual cloud. Such a ratio is not found in absorption
profiles looking through Milky Way gas. It cannot be explained by dust
depletion since Al depletes more severely than Si. Comparing to other Al-rich
environments, we speculate about the processes and conditions that could give
rise to this abundance pattern.Comment: 10 pages, 2 figures, AASTEX (aaspp4), to be published in ApJ
Radiation Induced Void in the Spectrum of TOL 1038-2712
Detection of a large void (7 Mpc) is reported between the redshifts
2.16286 and 2.20748 in the Ly~ forest of TOL 1038-2712. This void is
centered near a foreground QSO TOL 1037-2704 which is at a distance 4.4
Mpc away from the void. The estimated probability for the void to occure by
chance in front of the foreground QSO is few times . Various
implications of the void being produced by excess ionization due to foreground
QSO are discussed.Comment: 4 pages, latex 3 figures available on request at [email protected]
Morphological Number Counts and Redshift Distributions to I = 25 from the Hubble Deep Fields: Constraints on Cosmological Models from Early Type Galaxies
We combine magnitude and photometric redshift data on galaxies in the Hubble
Deep Fields with morphological classifications in order to separate out the
distributions for early type galaxies. The updated morphological galaxy number
counts down to I = 25 and the corresponding redshift distributions are used as
joint constraints on cosmological models, in particular on the values of the
density parameter Omega_{0} and normalised cosmological constant Lambda_{0}.
We find that an Einstein - de Sitter universe with simple passive evolution
gives an excellent fit to the counts and redshift data at all magnitudes. An
open, low Omega_{0}, model with no net evolution (and conservation of the
number of ellipticals), which fits the counts equally well, is somewhat less
successful, predicting slightly lower mean redshifts and, more significantly,
the lack of a high--z tail. A number conserving model with a dominant
contribution from Lambda_{0}, on the other hand, is far less successful,
predicting a much narrower distribution than seen. More complex models are
obviously possible, but we conclude that if large scale transmutation between
types does {\it not} occur, then the lambda-dominated models provide a very
poor fit to the current data.Comment: Accepted for publication in MNRA
A Uniform Analysis of the Ly-alpha Forest at z=0 - 5: V. The extragalactic ionizing background at low redshift
In Paper III of our series "A Uniform Analysis of the Ly-alpha forest at z=0
- 5", we presented a set of 270 quasar spectra from the archives of the Faint
Object Spectrograph on the Hubble Space Telescope. A total of 151 of these
spectra, yielding 906 lines, are suitable for using the proximity effect
signature to measure J(\nu_0), the mean intensity of the hydrogen-ionizing
background radiation field, at low redshift. Using a maximum likelihood
technique and the best estimates possible for each QSO's Lyman limit flux and
systemic redshift, we find J(\nu_0)= 7.6^+9.4_-3.0 x 10^-23 ergs s^-1 cm^-2
Hz^-1 sr^-1 at at 0.03 < z < 1.67. This is in good agreement with the mean
intensity expected from models of the background which incorporate only the
known quasar population. When the sample is divided into two subsamples,
consisting of lines with z 1, the values of J(\nu_0) found are
6.5^+38._-1.6 x 10^-23 ergs s^-1 cm^-2 Hz^-1 sr^-1, and 1.0^+3.8_-0.2 x 10^-22
ergs s^-1 cm^-2 Hz^-1 sr^-1, respectively, indicating that the mean intensity
of the background is evolving over the redshift range of this data set.
Relaxing the assumption that the spectral shapes of the sample spectra and the
background are identical, the best fit HI photoionization rates are found to be
6.7 x 10^-13 s^-1 for all redshifts, and 1.9 x 10^-13 s^-1 and 1.3 x 10^-12
s^-1 for z 1, respectively. This work confirms that the evolution
of the number density of Ly-alpha lines is driven by a decrease in the ionizing
background from z ~ 2 to z ~ 0 as well as by the formation of structure in the
intergalactic medium. (Abridged)Comment: 71 LaTeX pages, 20 encapsulated Postscript figures, Accepted for
publication in ApJ, Figure 4 available at
http://lithops.as.arizona.edu/~jill/QuasarSpectra/ or
http://hea-www.harvard.edu/QEDT/QuasarSpectra
- …