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

LMXBs has an effective radius of 129 ′′ . Both fits indicate that there are more X-ray
sources at larger radii than expected from the optical light distribution. Radial fits
from circular isophotes yield similar results. Since NGC 4382 is an S0 galaxy, this
disagreement may reflect the contribution of X-ray sources associated with the disk
of the galaxy. However, this would not explain the difference between the LMXB
distribution and the optical light distribution in NGC 4365.
We note that a significant fraction of the LMXBs in NGC 4365 are associated
with globular clusters (§ 2.4.2), as is generally true of elliptical galaxies (Angelini,
Loewenstein, & Mushotzky 2001; Sarazin et al. 2000). In elliptical galaxies (including
NGC 4365), the globular cluster population is more broadly distributed than that
of the optical light and field stars (e.g., Harris 1991). This might help to explain
the broader distribution of the LMXBs. Figure 2.8 suggests that there may be two
populations of LMXBs, one with a small effective radius perhaps consistent with the
optical distribution, and one with a larger effective radius. It is possible that these
two distributions represent the LMXBs formed in globular clusters and those formed
from field binary star systems. However, we note that in NGC 4697, the LMXBs
followed the optical light distribution (Sarazin et al. 2001). More elliptical galaxies
need to be observed to resolve this question.
To test whether the LMXBs in these galaxies could discriminate between such
models, we added either a second de Vaucouleurs model or an exponential model to the
optical light distribution. For either additional component, there were only minimal
improvements to the fits (∆χ 2 −0.6 for NGC 4365 and −0.8 for NGC 4382,
with one less dof compared to the single profile), indicating that the current data
cannot statistically discriminate between single de Vaucouleurs distributions with
large effective radii or multiple component distributions.
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