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

Number
10
8
6
4
2
NGC 4365
Optical
Maximum Likelihood
0
0 20 40 60 80 100 120 140 160 180
PA mod 180 (degrees)
Number
6
4
2
NGC 4382
Optical
Maximum Likelihood
0
0 20 40 60 80 100 120 140 160 180
PA mod 180 (degrees)
Fig. 2.7.— Solid histogram: Distribution of the position angles of the X-ray sources
within 3 ′ of the center of NGC 4365 (left) NGC 4382 and (right) in 10 ◦ bins, as
a function of the P.A. (modulo 180 ◦ ). P.A. is measured from north to east. The
curves are the expected number of background sources plus the predicted distribution
based on optical photometry and the best-fit elliptical distribution determined by a
maximum likelihood fit to the observed values.
five background sources in each exposure. We have adopted the Third Reference
Catalogue of Bright Galaxies (RC3) values for the optical photometry’s effective radii,
P.A., and ellipticity (de Vaucouleurs et al. 1992). At an effective radius of 49. ′′ 8,
NGC 4365 has P.A. = 40 ◦ and e = 0.28; while at an effective radius of 54. ′′ 6, NGC 4382
has e = 0.22. There is no P.A. listed for NGC 4382 in RC3. Fisher (1997) found that
NGC 4382 has an isophotal twist, with its P.A. ranging from 60 ◦ at its center to 0 ◦
at large radii. We adopted the latter P.A. for NGC 4382. These elliptical isophotes
contain one-half of the optical light.
In Figure 2.7 we show the observed distributions of position angles (modulo 180 ◦ ,
measuring P.A. from north to east). In addition to the observed distribution and the
expected optical distribution, we have fitted a distribution representing the number
of expected background sources plus the number of sources expected from a set of
sources with constant projected density on elliptical isophotes. For NGC 4365 and
NGC 4382, ∆χ 2 between the optical fits and the maximum likelihood fits indicates
that the optical fits are acceptable at the 83% and 54% confidence level. The best-fit
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