In a previous paper (Boller et al. 1992a) we presented results
of a first correlation between the ROSAT All-Sky Survey and the
14 708 extragalactic sources selected from the IRAS Point Source Catalogue
(hereafter IRAS PSC).
This initial correlation used the first processing of
the All-Sky Survey data by the Standard Analysis Software System (RASS I)
(Voges et al. 1996a; Downes et al. 1994)
and resulted in a sample of 244 objects. From the distribution of
source separations (see Fig. 4 of Boller et al. 1992a) the number fraction
of spurious sources herein was estimated to about 16
.
Recently, a second processing of the ROSAT All-Sky Survey
(RASS II)
was performed that yielded about 120 000 sources with detection
likelihoods larger 8 (Cruddace et al. (1988) for the definition
of the detection likelihood).
The major differences of the second processing as compared with the first
are:
(i) the photons were not collected in strips but
merged in 1378 sky-fields of 6.4
6.4 degree, which takes full
advantage of the increasing exposure towards the ecliptic poles;
(ii) neighboring fields overlap by at least 0.23 degree in order to ensure
the detection of sources at the field boundaries,
which posed a problem in the first processing;
(iii) a new aspect solution
reduces the number of sources with erroneous positions and morphology.
In this paper we present the results of a correlation of the
14 315 IRAS galaxies with the RASS II source list
.
For consistency we have still taken the IRAS PSC sources, but use
whenever available the IRAS FSC position and fluxes instead of the IRAS PSC
positions and fluxes.
Section 2 describes how the X-ray emitting
IRAS galaxies were identified through a spatial
correlation,
and a subsequent superposition of X-ray emission contours on
optical images. We adopted a classification scheme
to characterize the likelihood that the infrared and X-ray emission
originate from the same object. Thereby we obtained a list of IRAS galaxies
with a very high probability of detected X-ray emission.
Their soft X-ray spectral properties were analyzed and compared with
their infrared emission in Sect. 3.
The aim of this paper is to present the basic data and to quantify the likelihood that the X-rays we measure are associated with the IRAS galaxy. In subsequent papers we shall present results of optical follow-up observations of our galaxy sample and of theoretical models addressing the X-ray and far-infrared emission of galaxies in different states of nuclear activity.
The 197 IRAS galaxies with secure X-ray emission are subject of an optical follow-up observation program. The results will be presented in subsequent papers. The images for the 372 potential identifications and the tables 3 to 5 of the paper are available as postscript files from http://www.rosat.mpe-garching.mpg.de/~bol/iras_rassII or can be retrieved via anonymous ftp from ftp.rosat.mpe-garching.mpg.de in the subdirectories /outgoing/bol/iras_rassII/images and /outgoing/bol/iras_rassII/tables, respectively.