NGC 253 is a local, star-bursting spiral galaxy with strong X-ray emission
from hot gas, as well as many point sources. We have conducted a spectral
survey of the X-ray population of NGC 253 using a deep XMM-Newton
observation.NGC 253 only accounts for ~20% of the XMM-Newton EPIC field of
view, allowing us to identify ~100 X-ray sources that are unlikely to be
associated with NGC\thinspace 253. Hence we were able to make a direct estimate
of contamination from e.g. foreground stars and background galaxies.
X-ray luminosity functions (XLFs) of galaxy populations are often used to
characterise their properties. There are several methods for estimating the
luminosities of X-ray sources with few photons. We have obtained spectral fits
for the brightest 140 sources in the 2003 XMM-Newton observation of NGC 253,
and compare the best fit luminosities of those 69 non-nuclear sources
associated with NGC 253 with luminosities derived using other methods.
We find the luminosities obtained from these various methods to vary
systematically by a factor of up to three for the same data; this is largely
due to differences in absorption.
We therefore conclude that assuming Galactic absorption is probably unwise;
rather, one should measure the absorption for the population.
A remarkable correlation has been reported between the XLFs of galaxies and
their star formation rates. However, the XLFs used in that study were obtained
using several different methods. If the sample galaxies were revisited and a
single method were applied, then this correlation may become stronger still.Comment: Accepted for publication in the Monthly Notices of the Royal
Astronomical Society (MNRAS). 17 pages, 7 figure