Our RASS II source catalogue only includes sources with a detection likelihood larger than eight. In addition, the number of source photons in the (0.1-2.4 keV) energy band must be larger than six. The latter restriction is obtained from an analysis of the distribution of detection likelihood values versus the ratio of source to background count rates. Sources with five or less source photons show unusually high source to background count ratios for their values of the detection likelihood, compared to sources with a higher detection likelihood. This may be due to uncertainties in the standard analysis software analysis of weak sources. To avoid the uncertainties introduced by the SASS detection algorithm we therefore require the number of source photons to be above some threshold. The RASS II source catalogue thereby constrained has 116 471 sources.
Through a correlation of the positions of 14 315 IRAS galaxies with 116 471 RASS II source positions we obtained a list of 372 possible identifications. Because of its superior positional accuracy, when available we used the IRAS FSC position instead of the IRAS PSC position. Table 2 lists the corresponding FSC and PSC names for such objects.
In Sect. 2.1 we estimate the number of chance coincidences among our correlated IRAS and RASS II sources, as well as the dependence of that number on the positional uncertainty of the RASS II source positions.
In a second step (Sect. 2.2) the candidate identifications were
individually examined through superimposing the X-ray emission
contours on optical images taken from the Palomar Digitized Sky
Survey. In these images we also show the
IRAS 3 
error ellipse.
All objects were classified according to how clearly the
X-rays appear to originate from the optical counterpart of the
respective IRAS source.
This section describes our first identification step, a search for RASS II sources within 5 arcmin around each of the 14 315 IRAS galaxies. We estimate the chance coincidence rate and its dependence on the positional identification uncertainty.
From two random sets of N1 = 14 315 and N2 = 116 471
sources we expect
chance pairs with an angular separation less than 
,
where is measured in arcsec.
Within a separation of 100 arcsec we therefore expect
98 chance coincidences, which account for one quarter of
the source pairs we found.
To empirically verify this number of chance coincidences
we re-correlated the IRAS galaxy positions five times with
RASS II positions that were offset by 
30 arcmin in
right ascension and/or declination.
The number of chance coincidences of separation less than
100 arcsec turned out to be
101 (
),
105 (
),
113 (
.),
105 (
),
102 (, ).
The slightly higher number of pairs in the test correlations may be
due to an excess of correlation below about 150 arcsec separations
that we attribute to a wider spatial correlation between diffuse
cluster X-ray emission and the infrared emission from galaxies within
the clusters (see the discussion in the following section).
The number distribution of pairs drawn from two randomly
distributed sets of N1 and
N2 objects, respectively,
should follow
A least square fit to
the average distribution up to 300 arcsec of
our five test correlations (cf. Fig. 1b) yields
,
which corresponds to 
, in fair
agreement with the
expected value of 
.
We searched for RASS II counterparts within a radius of 5 arcmin
around the position of each of the 14 315 IRAS galaxies
and found 372 possible identifications.
Figure 1 (click here)a shows the distribution of distances between the
infrared position and its nearest X-ray source. In contrast to the
distribution of random associations displayed in Fig. 1 (click here)b, the
actual correlation of the two catalogues shows a strong excess of
source pairs within about 50 arcsec.
Of
the 372 pairs within 100 arcsec the expected number of chance coincidences is
105, with
an uncertainty estimated from the different X-ray catalogue offsets of
about 8.
Thus statistically, we should have found about 267
IRAS sources with X-ray emission in their actual vicinity.
The width of the
distribution of "real'' source pair separations arises from positional
uncertainties in each catalogue, resulting in a Gaussian distribution
with some angular width .
We therefore model the observed
distribution as the sum of 
"real'' pairs, and a background
characterized by the total number, 
, of pairs in two sets of
randomly distributed objects:
A least square fit to the observed distribution
yields 
arcsec and 
(we have fixed the
value of to that obtained from the distribution
of Fig. 1 (click here)b).
Our visual inspection (cf. Sect. 2.3 and our overlays) of all 372 pairs
resulted in 197 secure identification of X-ray emitting
IRAS galaxies, a number in good agreement with the 204
objects attributed to the central Gaussian of the pair distribution
function.
Figure 1: Distribution of separations between the
IRAS PSC and ROSAT All Sky Survey II sources (left pannel) and
an offset X-ray catalogue (right pannel, see text for details).
We chose a maximum angular separation of 100 arcsec to obtain
a first set of 372
candidate identifications.
We estimate between 98 and 113 chance coincidences
among these.
After a visual examination of these objects we allow for a maximum
separation of only 30 arcsec, which reduces the number of expected
chance coincidences to 
pairs
The number of 204 sources we attribute to the central Gaussian is smaller than the total number of pairs to 100 arcsec minus the expected random pairs, 372 - 105 = 267. This is probably due to another excess above the expected random distribution and outside the central Gaussian peak, reaching out to about 150 arcsec (cf. Fig. 1 (click here)a). This wider excess could most naturally be explained as coming from IRAS galaxies embedded in clusters with diffuse X-ray emission from the intercluster gas. In such cases the IRAS galaxy is not the source of the X-ray emission, which come from the cluster gas and may peak somewhere within the cluster size away from the IRAS galaxy. The size of nearby clusters from which we would expect RASS-detectable intercluster gas emission is in fact of order 2 arcmin. In our detailed comparison of the ROSAT emission with the optical images we indeed found such clusters that we flagged accordingly (grade 3, see Sect. 2.3). The X-ray emission on the line of sight to IRAS F05537-6653 is a representative example.
For our 372 candidate pairs we searched the NASA/IPAC extragalactic database (NED) for optical identifications within a distance of 2 arcmin around the IRAS PSC positions. Table 3 lists the X-ray and infrared positions, and the positions of the found NED counterparts (we list at most nine of them) with their basic optical properties. Since NED comprises many catalogues, it may identify a given galaxy with several different names and coordinates. When an IRAS galaxy was clearly identified with an optical galaxy, the more accurate optical galaxy position is listed in Table 3 instead of the IRAS position.
We are aware that the spectral classification in NED is collected from the literature without quantifying the likelihood of the classification. For a better and consitent classification we therefore initiated an optical follow-up program the results of which will be presented in a separate paper.
In order to estimate the positional (aspect) errors of the X-ray
sources we selected all IRAS galaxies with optical counterparts, and
correlated the optical and RASS II positions. For accurate optical
positions for the IRAS galaxies we searched the Hubble
Space Telescope (HST) Guide Star Catalogue (GSC) (Lasker et al. 1990;
Russell et al. 1990; Jenkner et al. 1990). Within a search radius of 5
arcmin 16 274 counterparts were found. Figure 2 shows the
distribution of the optical and RASS II position offsets. Under the
reasonable assumption that the accuracy of the HST GSC positions is
much higher than that of the RASS II positions, we estimate the
aspect error of the X-ray positions from the Gaussian width of the
central distribution peak as 
arcsec.
For a Gaussian distribution this results in 68% (90%) of ROSAT
sources found within 10 arcsec (18 arcsec) of the HST GSC position.
These numbers are in good agreement with the results
(12 and 20 arcsec)
found by
Voges & Boller (1997) by correlating the ROSAT Bright Source Catalogue
(Voges et al. 1996b)
with the TYCHO catalogue for bright stars having an error of less than 1 arcsec
(see their Fig. 3).
Figure 2: Distribution of separations between the
HST GSC and RASS II detected IRAS galaxies.
The standard deviation of the Gaussian distribution is 11 arcsec.
The value of 11 arcsec can be used
to estimate the pointing accuracy for RASS II sources
Figure 3: Soft X-ray flux 
derived from a power-law model
with fixed spectral index 
and corrected only
for Galactic absorption, plotted against
the flux 
computed from a best fit power-law spectrum with free
spectral index and absorbing column 
The 14 315 IRAS galaxies were previously correlated with the RASS I
source catalogue and the results were presented in Boller et al. (1992a).
The advantages of the RASS II processing with respect to the RASS I processing
are discussed in the introduction of this paper.
52 objects
from the original RASS I source catalogue are no longer included in
the present catalogue. There are three reasons why objects
were excluded;
(i) the infrared flux
at 100 
is greater than 10 Jansky (see the footnote in our introduction),
26 objects fulfill this criteria;
(ii) the positional offset from the new RASS II processing between
the RASS II position and the infrared position is larger than 100 arcsec and/or
the RASS II detection likelihood is less than eight
(25 objects), and (iii), the number of RASS II source photons is less than
six (one object).
Due to the merging of the original RASS I strips into 1378 sky-fields
of 6.4 
6.4 degree, 78 new objects at fainter X-ray fluxes are
detected in the RASS II processing with respect to the RASS I processing
and these objects appear now also
in our new RASS II catalogue of IRAS galaxies.
The improved RASS II processing and our individual examination of the X-ray emission structure with respect to the optically visible galaxy and its environment together provide a higher degree of reliability of this catalogue compared to the previous RASS I - IRAS correlation. A third release of this catalogue might be called for if major improvements can be made in the SASS processing.
The Photon Event (PET) Files from the RASS II processing were used to
produce X-ray images and contour plots.
To obtain an optimal spatial resolution of the X-ray images, the
PET files were binned in 5 arcsec width bins.
The resulting images were then smoothed with a 
arcsec
Gaussian filter, corresponding to a full-width at half maximum of 45 arcsec,
which is the
expected width of the point spread function in the ROSAT All-Sky Survey.
X-ray contours were computed in units of source photons per FWHM detection cell.
The background photon density 
is known at each source position
from the RASS II processing.
To detect at least one source
photon within a circular area of 45 arcsec diameter requires a mean
photon density of 
.
The source plus background photon density is known from the PET
files and the exposure map.
The lowest contour line
was chosen to represent two source photons per FWHM detection cell.
The higher contour levels represent 3, 5, 9, 17
(doubling the contour value difference) and then 2n
(for 
) photons per FWHM detection cell. This choice of
contours appears to best trace the source structure over
their dynamic range.
The X-ray photon flux contours were overlaid on optical images,
and a cross marks the centroid X-ray position obtained by the
RASS II processing. Rectangles were drawn
to mark the positions of NED sources, and for the IRAS FSC or PSC
sources the 3 error ellipse.
The likelihood grade of the association of the X-ray emission with the
IRAS galaxy (see next section)
is printed in the lower right corner of each image.
For a visual inspection we consider all 372 pairs with a separation of 100 arcsec as potentially "real'' correlations subject to further analysis.
This does however not necessarily imply that the IRAS galaxy itself is
responsible for the X-ray emission. Since galaxies tend to be
clustered (see our discussion in Sect. 2.1.1), the IR and X-ray emission may
arise from neighboring galaxies, or
from
extended intercluster gas.
An visual inspection of all 372 sources is therefore necessary to assess the
emission structure in relation to the optically visible galaxy and its
environment.
In the visual examination we finally allow for a maximum
separation of only 30 arcsec
between the X-ray and the IR position,
which reduces the number of expected
chance coincidences to pairs.
We adopted a classification scheme with the following six grades to characterize the quality and likelihood of an identification of the RASS II emission with an IRAS galaxy.
1) The X-ray emission is spatially clearly coincident with an optical counterpart to the IRAS galaxy. There is no significant surrounding X-ray emission peak within the 5 arcmin detection cell used in the RASS II processing. Objects with a classification grade 1 have the highest likelihood that the X-ray emission is actually originating from the IRAS galaxy.
2) The X-ray emission is spatially coincident with an optical counterpart of the IRAS galaxy. However, there is significant surrounding X-ray emission within the RASS II detection cell. In this case the RASS II count rate was corrected by subtraction of the surrounding emission peaks, using the PET files. The number of source photons after subtraction of the secondary peaks is still required to be equal or greater then six.
2db) The IRAS galaxy may be a blend of two or more galaxies not resolved in the IRAS FSC or IRAS PSC. The X-ray emission is spatially coincident with the infrared position. The spatial resolution of the ROSAT All-Sky Survey does not allow a unique identification of one of the optical counterparts with the X-ray emission.
3) The X-ray emission shows complex and diffuse structure and cannot be uniquely associated with the IRAS galaxy. The complex emission may originate from intercluster gas, background or foreground objects. An identification with the IRAS galaxy is uncertain.
4)
There is significant X-ray emission near the IRAS position, but
it peaks at a distance larger than about 30 arcsec
from the IRAS galaxy, which is about 
from the
X-ray peak. This appears too far, but in some cases may be
due to pointing errors in RASS II.
5) There is no spatial association of the X-ray emission with the IRAS galaxy. The RASS II centroid position only fortuitously coincides with the IRAS galaxy position.
9)
Other:
(i) a bright foreground star close to the position of the IRAS galaxy
is the most likely source of the X-ray emission;
(ii) less than 2 source photons are detected at the position of the RASS II
centroid position, i.e. no contour lines are plotted (cf. IRAS 05576-7655).
Only objects with a classification grade 1, 2 or 2db are considered secure identifications of X-ray emission with an IRAS galaxy.
In Table 1 (click here) we list sources considered as secure identifications which have an source extent that exceeds the point spread function. The likelihood of the source extent is required to be at least 10, a value obtained from a verification process of that SASS parameter.
| (1) | (2) | (3) |
| IRAS name | extent | likelihood |
| F07387+4955 | 27 | 186 |
| F13518+6933 | 13 | 23 |
| F14157+2522 | 10 | 26 |
| F14400+3539 | 12 | 38 |
| F16136+6550 | 11 | 19 |
| F17020+4544 | 12 | 11 |
| F18011+4246 | 75 | 12 |
| F18216+6419 | 23 | 485 |
| F20240-5233 | 67 | 16 |
| F22402+2927 | 9 | 26 |
| 23566-0424 | 19 | 10 |
| PSC name | FSC name | PSC name | FSC name | PSC name | FSC name | PSC name | FSC name |
| 00127+2817 | 00128+2817 | 00360-2432 | 00361-2432 | 00488+2907 | 00489+2908 | 00540-0133 | 00541-0133 |
| 01134+3046 | 01134+3045 | 01464+1249 | 01464+1248 | 02025+0941 | 02025+0940 | 02093+3714 | 02092+3714 |
| 02223-1922 | 02223-1921 | 02537-1641 | 02536-1641 | 03208-3723 | 03207-3723 | 03229-0618 | 03229-0619 |
| 03398-2124 | 03398-2123 | 03543-7216 | 03544-7216 | 04105-6811 | 04105-6810 | 04339-1028 | 04340-1028 |
| 04384-4848 | 04384-4849 | 04470-6227 | 04469-6227 | 04503+0114 | 04504+0114 | 04576+0912 | 04575+0912 |
| 05264-3936 | 05263-3937 | 05291-2608 | 05290-2608 | 05335-7359 | 05334-7359 | 05399-8345 | 05400-8345 |
| 05339-5804 | 05340-5804 | 05576-7655 | 05577-7655 | 05581-5907 | 05580-5907 | 06280+6342 | 06279+6342 |
| 06295+5743 | 06296+5743 | 07388+4955 | 07387+4955 | 07451+5543 | 07452+5543 | 08080-6109 | 08081-6109 |
| 08066-1905 | 08068-1906 | 08082+7900 | 08080+7900 | 08162+2717 | 08162+2716 | 09162+2628 | 09161+2628 |
| 09241+5735 | 09242+5735 | 09571+8435 | 09572+8435 | 10213+0644 | 10214+0644 | 10291+6517 | 10290+6517 |
| 11029+3130 | 11028+3130 | 11033+7250 | 11034+7250 | 11058+7159 | 11060+7158 | 11161+6020 | 11162+6020 |
| 11395+1033 | 11396+1033 | 11419+2022 | 11419+2023 | 12055+6527 | 12056+6527 | 12175+2933 | 12176+2933 |
| 12409+7823 | 12409+7824 | 12446-4058 | 12446-4057 | 12551+2040 | 12552+2039 | 12561+2752 | 12561+2751 |
| 12566+3507 | 12566+3506 | 12584+2803 | 12585+2803 | 13003+8017 | 13004+8017 | 13150+2051 | 13150+2052 |
| 13177-2021 | 13176-2020 | 13428+6652 | 13429+6652 | 13446+1121 | 13445+1121 | 13467+4014 | 13468+4013 |
| 13503+6104 | 13504+6104 | 13519+6933 | 13518+6933 | 13510+0442 | 13509+0442 | 13578+0516 | 13577+0517 |
| 14156+2522 | 14157+2522 | 14457+2308 | 14457+2309 | 15480+6822 | 15480+6821 | 15519+1444 | 15519+1445 |
| 16040+1818 | 16042+1824 | 17166-7536 | 17167-7536 | 17520+3250 | 17520+3249 | 17550+6520 | 17549+6520 |
| 17552+6209 | 17551+6209 | 17511-6542 | 17512-6542 | 18011+4247 | 18011+4246 | 18130+5703 | 18129+5703 |
| 18216+6418 | 18216+6419 | 18396-6225 | 18402-6224 | 19073-5257 | 19074-5257 | 19290+5830 | 19289+5830 |
| 19463-5843 | 19462-5843 | 19519-8141 | 19518-8142 | 20051-1117 | 20050-1117 | 20069+5929 | 20068+5929 |
| 20044-6114 | 20045-6114 | 20448+2515 | 20448+2514 | 20546-4849 | 20547-4849 | 21236-6013 | 21235-6013 |
| 21467-5530 | 21468-5530 | 22403+2927 | 22402+2927 | 22453-1744 | 22454-1744 | 22482-7027 | 22481-7028 |
| 22537-6511 | 22537-6512 | 23016-5144 | 23017-5144 | 23229+2835 | 23229+2834 | 23252+2318 | 23251+2318 |
| 23410+0228 | 23411+0228 | - | - | - | - | - | - |