Astron. Astrophys. Suppl. Ser. 142, 433-441
X-ray and optical observations of three clusters of galaxies:
Abell 901, Abell 1437, and Abell 3570
Astrophysics Research Institute,
Liverpool John Moores University,
Twelve Quays House,
Birkenhead CH41 1LD,
Received September 1, 1999; accepted January 4, 2000
We analyse three clusters of galaxies, Abell 901 (z
=0.17), Abell 1437
=0.13) and Abell 3570 (z
They have low to intermediate X-ray fluxes and
an irregular morphology in the ROSAT All-Sky Survey (RASS).
These clusters are chosen to
test the abilities and limitations of the RASS in terms of cluster
fluxes and cluster morphologies. Therefore some "worst'' cases are
used here. X-ray observations with the
ROSAT/HRI and optical spectroscopic observations are carried out.
The ROSAT/HRI observations, which have a much better spatial
resolution than the RASS, reveal in two of the three cases a
different morphology than seen in the RASS. The reasons are point
sources which could not be resolved in the RASS and were therefore
confused with the cluster emission. For A3570 we could confirm the
relaxed state of the cluster
by the optical determination of a small velocity dispersion.
In the cluster with the lowest flux (Abell 901) the
countrate measurement is strongly affected by point sources, in the two
other cases the countrate measurements of the RASS are reliable,
i.e. they are reproduced by the ROSAT/HRI measurement.
We conclude that for clusters with a flux of a few times 10-12
/s or smaller, which show at the same time a non-relaxed morphology,
the flux measurement of the RASS can be seriously affected by
fore- or background sources. We point out that an all-sky survey of
a second ABRIXAS mission
would provide a much clearer source distinction for low-flux clusters
and thus a much improved countrate determination.
Key words: galaxies: clusters: individual: Abell 901, Abell 1437,
Abell 3570 -- intergalactic medium --
cosmology: observations -- X-rays: galaxies
Copyright The European Southern Observatory (ESO)