From: foltz@as.arizona.edu (Craig B. Foltz)
Newsgroups: sci.astro
Subject: Quasars in Virgo
Date: 28 Aug 92 19:59:47 GMT
Reply-To: foltz@as.arizona.edu
Organization: MMT Observatory, Univ.of Arizona

The ongoing thread on arguments of the non-cosmological nature of QSOs has
prompted this reply.  Specifically, arguments have been forwarded regarding
the nature of QSOs seen in the direction to the Virgo cluster.  Most of these
can be traced to the work of Arp and collaborators and are nicely summarized
in his book "Quasars, Redshifts, and Controversies" (or some permutation 
thereof).  Basically they claim the existence of a statistical excess of
QSOs near Virgo galaxies.

For the past five years, I and my colleagues have been carrying out the largest
optical survey for QSOs to date, the LBQS.  This survey consists of over 1000
optically-selected QSOs, selected by *objectively-defined* and *consistently-
applied* computer selection algorithms applied to digitized direct and objective-
prism plates taken with the UK Schmidt Telescope and digitized with the Automatic
Plate Measuring Facility at the Institute of Astronomy, Cambridge, UK.  Follow-up
spectroscopy was carried out at the 4.5 m Multiple Mirror Telescope and the 2.5 m
DuPont Telescope at Las Campanas.  The survey consists of positions, accurate
magnitudes, accurate redshifts, and optical spectra for over 1000 QSOs with 
absolute magnitudes brighter than -23 and a smaller number of QSOs and active
galaxies at lower luminosities.  All but about 11 of the original candidate have
been unambiguously identified.  Comparison with earlier surveys in the same
fields show that of all the known QSOs in the LBQS fields, only one was missed
in the LBQS blind selection process.  We are therefore confident that, while the
survey is probably not 'complete,' it certainly has not missed many objects with
spectral energy distributions which would have been classified as QSOs.

Four of our fields covered about 100 square degrees centered on the Virgo Cluster.
In the last few days, I have carried out the following test:  I divided the survey
into the roughly 200 QSOs found in the Virgo fields and the 800 or so in other
fields (distibuted over a range of RA, most on the equator, galactic lattitude
above 40 degrees, none centered on clusters of galaxies) and compared the following 
gross properties of the subsamples:

1. The distribution of redshifts
2. The distribution of apparent magnitudes
3. The surface density of QSOs (number per square degree) as a function of 
magnitude.

If there were an abnormal number of QSOs in Virgo, one might expect to
see a difference between the two samples (e.g. simply a larger number
of QSOs per square degree, or a relatively larger number of bright
QSOs, or a difference in the redshift histogram).

The first two distributions were compared vis a two-sided, two-sample Kolmogorov-
Smirnov test (jargon for a simple, non-parametric statistical test) and the surface
densities and associated errors were simply overplotted.  (I won't include the
details here.  I am considering writing this up and sending it to A.J. of P.A.S.P.)

The results can be stated simply:  WITH RESPECT TO APPARENT BRIGHTNESS,
REDSHIFT, AND SURFACE DENSITY, THERE IS NO STATISTICAL DIFFERENCE
BETWEEN THE TWO SUBSAMPLES.  The coincidences reported by Arp are most
probably due to sample selection and severe selection biases.  Take the
reports of strange alignments of QSOs with galaxies and radio axes in
Virgo with a grain of salt.

If you are really interested in seeing for yourself, get a copy of Arp's book and
compare his plots of the positions of QSOs in Virgo with that in our first LBQS
installment (figure 2 in Foltz et al. 1987 Astronomical Journal, 94, 1423).

---
Craig Foltz                               cfoltz@as.arizona.edu
Multiple Mirror Telescope Observatory     Tel   (602) 621-1269
Univ. of Arizona, Tucson, AZ 85721        Fax   (602) 670-5740
Newsgroups: sci.astro