27 research outputs found
A New Measure of the Clustering of QSO Heavy-Element Absorption-Line Systems
We examine the line-of-sight clustering of QSO heavy-element absorption-line
systems, using a new measure of clustering, called the reduced second moment
measure, that directly measures the mean over-density of absorbers. While
closely related to other second-order measures such as the correlation function
or the power spectrum, this measure has a number of distinct statistical
properties which make possible a continuous exploration of clustering as a
function of scale. From a sample of 352 C IV absorbers with median redshift
2.2, drawn from the spectra of 274 QSOs, we find that the absorbers are
strongly clustered on scales from 1 to 20 Mpc. Furthermore, there appears to be
a sharp break at 20 Mpc, with significant clustering on scales up to 100 Mpc in
excess of that which would be expected from a smooth transition to homogeneity.
There is no evidence of clustering on scales greater than 100 Mpc. These
results suggest that strong C IV absorbers along a line of sight are indicators
of clusters and possibly superclusters, a relationship that is supported by
recent observations of ``Lyman break'' galaxies.Comment: 13 pages (LaTex, uses aaspp4.sty and psfig.sty), with 3 encapsulated
PostScript figures. To appear in The Astrophysical Journal. Extended new
discussion of the statistical properties of the reduced second moment
measure, and a new figure highlighting the excess clustering on comoving
scales greater than 20 Mp
Gravitational Lensing and the Hubble Deep Field
We calculate the expected number of multiply-imaged galaxies in the Hubble
Deep Field (HDF), using photometric redshift information for galaxies with m_I
< 27 that were detected in all four HDF passbands. A comparison of these
expectations with the observed number of strongly lensed galaxies constrains
the current value of Omega_m-Omega_Lambda, where Omega_m is the mean mass
density of the universe and Omega_Lambda is the normalized cosmological
constant. Based on current estimates of the HDF luminosity function and
associated uncertainties in individual parameters, our 95% confidence lower
limit on Omega_m-Omega_Lambda ranges between -0.44, if there are no strongly
lensed galaxies in the HDF, and -0.73, if there are two strongly lensed
galaxies in the HDF. If the only lensed galaxy in the HDF is the one presently
viable candidate, then, in a flat universe (Omega_m+Omega_Lambda=1),
Omega_Lambda < 0.79 (95% C.L.). These limits are compatible with estimates
based on high-redshift supernovae and with previous limits based on
gravitational lensing.Comment: 4 pages (aipproc.sty), 2 figures. To appear in "After the dark ages:
when galaxies were young," proceedings of the 9th Annual October Astrophysics
Conference, eds. S. S. Holt & E. P. Smit
Likelihood Analysis of Repeating in the BATSE Catalogue
I describe a new likelihood technique, based on counts-in-cells statistics,
that I use to analyze repeating in the BATSE 1B and 2B catalogues. Using the 1B
data, I find that repeating is preferred over non-repeating by 4.3:1 odds, with
a well-defined peak at 5-6 repetitions per source. I find that the post-1B data
are consistent with the repeating model inferred from the 1B data, after taking
into account the lower fraction of bursts with well-determined positions.
Combining the two data sets, I find that the odds favoring repeating over
non-repeating are almost unaffected at 4:1, with a narrower peak at 5
repetitions per source. I conclude that the data sets are consistent both with
each other and with repeating, and that for these data sets the odds favor
repeating.Comment: 5 pages including 3 encapsulated figures, as a uuencoded, gzipped,
Postscript file. To appear in Proc. of the 1995 La Jolla workshop ``High
Velocity Neutron Stars and Gamma-Ray Bursts'' eds. Rothschild, R. et al.,
AIP, New Yor
An Excess of C IV Absorbers in Luminous QSOs: Evidence for Gravitational Lensing?
We have compiled a new and extensive catalog of heavy-element QSO absorption
line systems and analyzed the distribution of absorbers in bright and faint
QSOs, to search for gravitational lensing of background QSOs by the matter
associated with the absorbers. There is a highly significant excess of C {\sc
iv} absorbers in bright QSOs in the redshift range , and this excess
increases strongly as a function of QSO absolute magnitude. No significant
excess is found for Mg {\sc ii} absorbers in the redshift range .
We rule out several possible reasons for this effect and argue that the C {\sc
iv} excess could be due to gravitational lensing. If so, then the lensing
masses must be at and within several hundred comoving Mpc of
the QSOs, where the C {\sc iv} absorbers are mainly found. The absence of an
excess in the available Mg {\sc ii} sample would then arise because the Mg {\sc
ii} data does not sample this region of space.Comment: 21 LaTeX pages with 5 encapsulated Postscript figures included, uses
AASTeX (v. 4.0) available at ftp://ftp.aas.org/pubs/ , to appear in The
Astrophysical Journal, Sept. 20, 199
Extreme Ultraviolet Quasar Colours from GALEX Observations of the SDSS DR14Q Catalogue
The rest-frame far to extreme ultraviolet (UV) colour–redshift relationship has been constructed from data on over 480,000 quasars carefully cross-matched between SDSS Data Release 14 and the final GALEX photometric catalogue. UV matching and detection probabilities are given for all the quasars, including dependencies on separation, optical brightness, and redshift. Detection limits are also provided for all objects. The UV colour distributions are skewed redward at virtually all redshifts, especially when detection limits are accounted for. The median GALEX far-UV minus near-UV (FUV − NUV) colour–redshift relation is reliably determined up to z ≈ 2.8, corresponding to rest-frame wavelengths as short as 400 Å. Extreme UV (EUV) colours are substantially redder than found previously, when detection limits are properly accounted for. Quasar template spectra were forward modelled through the GALEX bandpasses, accounting for intergalactic opacity, intrinsic reddening, and continuum slope variations. Intergalactic absorption by itself cannot account for the very red EUV colours. The colour–redshift relation is consistent with no intrinsic reddening, at least for SMC-like extinction. The best model fit has a FUV continuum power-law slope αν, FUV = −0.34 ± 0.03 consistent with previous results, but an EUV slope αν, EUV = −2.90 ± 0.04 that is much redder and inconsistent with any previous composite value (all ≳ −2.0). The EUV slope difference can be attributed in part to the tendency of previous studies to preferentially select UV brighter and bluer objects. The weak EUV flux suggests quasar accretion disc models that include outflows such as disc winds