1,741 research outputs found
2dF QSO Redshift Survey
With approximately 6000 QSO redshifts,the 2dF QSO redshift survey is already
the biggest complete QSO survey. The aim for the survey is to have 25000 QSO
redshifts, providing an order of magnitude increase in QSO clustering
statistics. We first describe the observational parameters of the 2dF QSO
survey. We then describe several highlights of the survey so far, including new
estimates of the QSO luminosity function and its evolution. We also review the
current status of QSO clustering analyses from the 2dF data. Finally, we
discuss how the complete QSO survey will be able to constrain the value of
Omega_o by measuring the evolution of QSO clustering, place limits on the
cosmological constant via a direct geometrical test and determine the form of
the fluctuation power-spectrum out to the approximately 1000 Mpc scales only
previously probed by COBE.Comment: 6 pages; to be published in Clustering at High Redshift, Marseille,
June 1999, eds. A. Mazure, O. LeFevre, V. Lebru
The 2dF QSO Redshift Survey
We present preliminary results from the 2-degree Field (2dF) QSO Redshift
Survey currently under way at the Anglo-Australian Telescope. This survey aims
to determine the redshifts of >25000 QSOs over a redshift range of 0.3<z<3.0
with the primary goal of investigating large-scale structure in the Universe to
high redshift and at very large scales (~1000h-1Mpc).
We describe the photometric procedure used to select QSO candidates for
spectroscopic observation. We then describe results from our first 2dF
observations, which have so far measured the redshifts for over 1000 QSOs. We
already find a significant detection of clustering and have also found one
close pair of QSOs (separation 17'') which are gravitational lens candidates.
To keep up to date with the current progress of the survey see:
http://msowww.anu.edu.au/~rsmith/QSO_Survey/qso_surv.htmlComment: 5 pages Latex including 6 figures, To appear in the proceedings of
"Evolution of Large Scale Structure: From Recombination to Garching", held
August 199
The 2dF QSO Redshift Survey - 10K@2K!
With ~10000 QSO redshifts, the 2dF QSO Redshift Survey (2QZ) is already the
biggest individual QSO survey. The aim for the survey is to have ~25000 QSO
redshifts, providing an order of magnitude increase in QSO clustering
statistics. We first describe the observational parameters of the 2dF QSO
survey. We then describe several highlights of the survey so far; we present
new estimates of the QSO luminosity function and the QSO correlation function.
We also present the first estimate of the QSO power spectrum from the 2QZ
catalogue, probing the form of the fluctuation power-spectrum out to the
\~1000h-1Mpc scales only previously probed by COBE. We find a power spectrum
which is steeper than the prediction of standard CDM and more consistent with
the prediction of Lambda-CDM. The best-fit value for the power spectrum shape
parameter for a range of cosmologies is Gamma=0.1+-0.1. Finally, we discuss how
the complete QSO survey will be able to constrain the value of Omega_Lambda by
combining results from the evolution of QSO clustering and from a geometric
test of clustering isotropy.Comment: 11 pages, 6 figures, latex, eso and springer sty files included. To
appear in the proceedings of the MPA/ESO/MPA conference "Mining the Sky",
Garching, July 31 - August 4 2000, eds. A.J. Banday et a
First Results from the 2dF QSO redshift survey
We present some initial results from the 2dF QSO redshift survey. The aim of
the survey is to produce an optically-selected catalogue of 25000 QSOs over the
redshift range 0<z<3 using the 2-degree field at the Anglo-Australian
Telescope.Comment: 7 pages, 6 figures, submitted to proceedings of ESO Deep Fields
conferenc
Understanding the business consequences of ERP use
ERP systems are large integrated packaged software systems used by thousands of major organizations around the world. Yet outcomes from ERP use can be very different, and there is still not an adequate understanding of how and why organizations have such varying outcomes. Using an interpretive case study approach the post implementation periods in four manufacturing companies were examined retrospectively as processes within context over time. Structuration theory was used in the analysis of the cases to identify six themes that explain ’how’ and three contexts that explain ’why’ that form the components of an explanatory framework. The framework provides a foundation for future studies to understand and explain how and why organizations have or have not achieved business benefits from ERP systems
Clustering in the 2dF QSO Redshift Survey
We present clustering results from the 2dF QSO Redshift Survey (2QZ) which
currently contains over 20,000 QSOs at z<3. The two-point correlation function
of QSOs averaged over the entire survey (~1.5) is found to be similar to
that of local galaxies. When sub-dividing the sample as a function of redshift,
we find that for an Einstein-de Sitter universe QSO clustering is constant (in
comoving coordinates) over the entire redshift range probed by the 2QZ, while
in a universe with Omega_0=0.3 and Lambda_0=0.7 there is a marginal increase in
clustering with redshift. Sub-dividing the 2QZ on the basis of apparent
magnitude we find only a slight difference between the clustering of QSOs of
different apparent brightness, with the brightest QSOs having marginally
stronger clustering. We have made a first measurement of the redshift space
distortion of QSO clustering, with the goal of determining the value of
cosmological parameters (in partcular Lambda_0) from geometric distortions. The
current data do not allow us to discriminate between models, however, in
combination with constraints from the evolution of mass clustering we find
Omega_0=1-Lambda_0=0.23 +0.44-0.13 and beta(z~1.4)=0.39 +0.18-0.17. The full
2QZ data set will provide further cosmological constraints.Comment: 5 pages, 7 figures. Contributed to the 'Where's the Matter'
conference in Marseille 25-29 June 200
Spatial Correlation Function of X-ray Selected AGN
We present a detailed description of the first direct measurement of the
spatial correlation function of X-ray selected AGN. This result is based on an
X-ray flux-limited sample of 219 AGN discovered in the contiguous 80.7 deg^2
region of the ROSAT North Ecliptic Pole (NEP) Survey. Clustering is detected at
the 4 sigma level at comoving scales in the interval r = 5-60 h^-1 Mpc. Fitting
the data with a power law of slope gamma=1.8, we find a correlation length of
r_0 = 7.4 (+1.8, -1.9) h^-1 Mpc (Omega_M=0.3, Omega_Lambda=0.7). The median
redshift of the AGN contributing to the signal is z_xi=0.22. This clustering
amplitude implies that X-ray selected AGN are spatially distributed in a manner
similar to that of optically selected AGN. Furthermore, the ROSAT NEP
determination establishes the local behavior of AGN clustering, a regime which
is poorly sampled in general. Combined with high-redshift measures from optical
studies, the ROSAT NEP results argue that the AGN correlation strength
essentially does not evolve with redshift, at least out to z~2.2. In the local
Universe, X-ray selected AGN appear to be unbiased relative to galaxies and the
inferred X-ray bias parameter is near unity, b_X~1. Hence X-ray selected AGN
closely trace the underlying mass distribution. The ROSAT NEP AGN catalog,
presented here, features complete optical identifications and spectroscopic
redshifts. The median redshift, X-ray flux, and X-ray luminosity are z=0.41,
f_X=1.1*10^-13 cgs, and L_X=9.2*10^43 h_70^-2 cgs (0.5-2.0 keV), respectively.
Unobscured, type 1 AGN are the dominant constituents (90%) of this soft X-ray
selected sample of AGN.Comment: 17 pages, 8 figures, accepted for publication in ApJ, a version with
high-resolution figures is available at
http://www.eso.org/~cmullis/papers/Mullis_et_al_2004b.ps.gz, a
machine-readable version of the ROSAT NEP AGN catalog is available at
http://www.eso.org/~cmullis/research/nep-catalog.htm
Quasar Clustering and the Lifetime of Quasars
Although the population of luminous quasars rises and falls over a period of
10^9 years, the typical lifetime of individual quasars is uncertain by several
orders of magnitude. We show that quasar clustering measurements can
substantially narrow the range of possible lifetimes with the assumption that
luminous quasars reside in the most massive host halos. If quasars are
long-lived, then they are rare phenomena that are highly biased with respect to
the underlying dark matter, while if they are short-lived they reside in more
typical halos that are less strongly clustered. For a given quasar lifetime, we
calculate the minimum host halo mass by matching the observed space density of
quasars, using the Press-Schechter approximation. We use the results of Mo &
White to calculate the clustering of these halos, and hence of the quasars they
contain, as a function of quasar lifetime. A lifetime of t_Q = 4 x 10^7 years,
the e-folding timescale of an Eddington luminosity black hole with accretion
efficiency eps=0.1, corresponds to a quasar correlation length r_0 ~ 10 Mpc/h
in low-density cosmological models at z=2-3; this value is consistent with
current clustering measurements, but these have large uncertainties.
High-precision clustering measurements from the 2dF and Sloan quasar surveys
will test our key assumption of a tight correlation between quasar luminosity
and host halo mass, and if this assumption holds then they should determine t_Q
to a factor of three or better. An accurate determination of the quasar
lifetime will show whether supermassive black holes acquire most of their mass
during high-luminosity accretion, and it will show whether the black holes in
the nuclei of typical nearby galaxies were once the central engines of
high-luminosity quasars.Comment: ApJ Accepted (Feb 2001). 30 pages, 8 embedded ps figures, AASTEX5.
Added discussion of quasar luminosity evolution. Also available at
http://www.ociw.edu/~martini/pubs
QSO clustering and the AAT 2dF redshift survey
We review previous results on the clustering and environments of QSOs. We
show that the correlation length for QSOs derived from existing surveys is
r~5/h Mpc, similar to the observed correlation length for field galaxies at the
present epoch. The galaxy environment for z<1 radio-quiet QSOs is also
consistent with field galaxies. The evolution of the QSO correlation length
with redshift is currently uncertain, largely due to the small numbers of QSOs
(~2000) in surveys suitable for clustering analysis. We report on intial
progress with the AAT 2dF QSO redshift survey, which, once completed will
comprise almost 30000 QSOs. With over 1000 QSOs already observed, it is already
the largest single homogeneous QSO survey. We discuss prospects for deriving
limits on cosmological parameters from this survey, and on the evolution of
large-scale structure in the Universe.Comment: Invited talk at RS meeting on 'Large Scale Structure in the Universe'
held at the Royal Society on 25-26 March 1998 14 pages, 11 figre
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