Astronomical Applications Department, U.S. Naval Observatory thesis Page 23
9
shell is estimated to be 1000
500 pc in size and expanding at 25 km s
,
1
. Soft
X-ray emission is detected at its center indicating some presence of massive star
formation. They estimate the energy needed for its creation to be the equivalent
of roughly 100 Type II supernovae. The number of HII regions surrounding this
HI hole is predicted to the right order of magnitude by Martimbeau
et
al.
1994
using a model for triggered star formation via an expanding giant shell. A distance
of 3.05 Mpc for IC2574 is adopted. This is the average of ve independent distance
determinations assumed by Martimbeau
et
al.
1994 and derived by Tyson &
Scalo 1988. To facilitate comparisons with HoII, this value is adopted despite
a more recent estimate of 3.6 Mpc Tikhonov
et
al.
1991. Table 1.2 lists the
assumed parameters for the galaxy.
Parameter
Value
Reference
Optical Center
1950
10
h
24
m
41
:
s
3 RC3, de Vaucouleurs
et
al.
1991
1950
68 40
0
18
00
Dynamical Center
1950
10
h
24
m
57
s
Westpfahl 1998
1950
68 40
0
40
:
00
3
Distance
3.05 Mpc
Hoessel
et
al.
1998
M
B
,
16.6
RC3, de Vaucouleurs
et
al.
1991
Inclination
75
Martimbeau
et
al.
1994
Position Angle
52
Axial Ratio, q=b a
0.25
A
B
0.07
RC3, de Vaucouleurs
et
al.
1991
A
FUV
0.14
This study
Table 1.2. Adopted Parameters for IC2574
1.2.3 Sextans A
Dwarf irregular galaxy Sextans A DDO 75 = UGC 205 is one of the most
well studied dwarf galaxies in the Local Group. It is a low metallicity system with
Z Z
'
0
:
05 Skillman
et
al.
1989 and classi ed as a IBm galaxy de Vaucouleurs,
