PhD Thesis (PDF) - Department of Astronomy - University of Virginia

tected sources may be foreground or (more likely) background objects unrelated to
NGC 1600. If we consider the fluxes of all our detected sources, we expect ≈48 unre-
lated sources based on the source counts in Brandt et al. (2000) and Mushotzky et al.
(2000). Using the minimum detected ≥3 σ flux, we expect ≈15 unrelated sources.
Incompleteness will reduce both of these estimates, especially the first number. Un-
related sources should be spread out fairly uniformly over the S3 image (Fig. 3.1),
except for the reduced sensitivity at the center due to bright diffuse emission and at
the outer edges of the field due to reduced exposure and increased PSF. Sources close
to NGC 1600 are more likely to be associated with the galaxy, while sources far from
NGC 1600 are more likely unrelated to it.
3.4.2 Identifications
Sources in Table 3.1 were cross-correlated against optical/IR catalogs to identify
possible counterparts and to improve the absolute astrometry of the observations. We
used the Tycho-2 Catalog (Høg et al. 2000), the 2MASS 10 Point Source and Extended
Source Catalogs, 11 and the USNO-B Catalog (Monet et al. 2003) to identify seven
optical/IR counterparts to the X-ray sources. Four of these sources were used to de-
termine the astrometry: source 18 corresponds to USNO-B1 0849−0044132, an R =
19.4 mag object with a non-stellar PSF; source 23 corresponds to NGC 1601, a nearby
lenticular galaxy north of NGC 1600; source 33 corresponds to
2MASS 04313613−0503081, a J = 16.8 mag star; and source 50 corresponds to
Tycho-2 4742-254-1, a B = 11.7 mag star. After correcting for an astrometric shift
of about 0. ′′ 5, the residual astrometric errors are ≈ 0. ′′ 5.
We did not use the remaining three sources for astrometry since they were as-
10 See http://www.ipac.caltech.edu/2mass/releases/second/doc/explsup.html.
11 When a source appeared in both 2MASS catalogs, the Point Source Catalog positions were used.
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