10
G.
et
al.
1991. The optical component has a peculiar square shape with star
formation activity in two corners while the neutral gas disk is organized in a
horseshoe-like shape around a central minimum. The gas disk rotates as a solid
body Skillman
et
al.
1988. From H observations of the galaxy, Hodge
et
al.
1994 identify 25 HII regions whose sizes and size distribution are similar to other
galaxies, thus suggesting that the physical process of star formation is normal.
Peaks in the HI density correspond to the two main regions of star formation.
Hunter & Plummer 1996 remark that these peaks are regions of higher radially
averaged gas density relative to the critical density for stability of gas. However,
since the ratio of observed to critical gas density in regions of active star formation
is lower in comparison to that of star forming spiral galaxies, Sextans A must be
marginally stable against collapse. Hunter & Plummer 1996 estimate a current
star formation rate of only 0.006 M yr
,
1
. It is plausible to assume that the current
rate can be maintained over a large portion of the galaxy's lifetime given that HI
synthesis observations show roughly half of the HI associated with Sextans A is
contained in a low density reservoir Skillman
et
al.
1988. Recent Wide Field
Planetary Camera WFPC2 observations of the galaxy allow Dohm-Palmer
et
al.
1997 to identify two distinct stellar populations. They conclude that Sextans
A is currently undergoing an intense episode of star formation that is a factor of
20 times the past average rate. This period of activity began roughly 80 Myr
ago and was preceded by an episode 100
,
600 Myr ago of relatively constant
star formation, a factor of 6 times the average rate. The major axis of the HI
distribution is misaligned with the kinematical major axis by 43 which could
be indicative of a bar Skillman
et
al.
1988. The star formation in Sextans A is
likely governed by the gas dynamics to a large extent. Table 1.3 summarizes the
adopted parameters for the galaxy.
