On the left is the CMD in V and I for Sextans A.
The points have been corrected for interstellar reddening;
AV=0.10 and AI=0.05.
The crosses on the left indicate the average error at the given V
The lines near the bottom indicate the completeness at levels 0.25 and 0.5.
The right axis shows the absolute V magnitude assuming a distance
modulus of 25.8.
On the right, the major populations have been outlined and labelled.
These include: the main sequence (MS), blue He-Burning stars (Blue HeB),
red He-Burning stars (red HeB), red giant branch (RGB), red clump (RC), and
assymptotic giant branch (AGB) (from Dohm-Palmer et al. 1997b)
This shows the spatial density of three populations of stars
within Sextans A in both contours and greyscale. The maps have
been convolved with a Gaussian with σ = 50 pc. One star per
convolution beam is 50 kpc-2. The left panel is the MS,
the middle is the blue HeB stars, and the right is the RGB
(from Dohm-Palmer et al. 1997b)
(left) The CMD in V and I for Sextans A compared to
the stellar isochrones presented in Bertelli et al. (1994), with the
logarithm of the age indicated.
The features of the isochrones align well with the observed features.
In particular, the extremes of the blue-loops align with the blue and red
The MS lies along the observed locus of the MS.
The shape of the RGB matches the observed shape and position quite well.
Clearly the observed stars in Sextans A cover a large range of ages.
There are stars as young as 10 Myr and some older than 1 Gyr.
(right) The lines show the tracks of the MS turnoff, the blue HeB stars,
and the red HeB stars for Z=0.001 (Bertelli et al. 1994).
These populations align very well with the observed stars.
The exception is the red HeB which appears slightly too blue compared to
the very few stars with V < 21 (from Dohm-Palmer et al. 1997b)
The SFR for Sextans A over the last 600 Myr, based on the blue HeB stars.
The bins are 25 Myr.
The error bars reflect only random statistical
errors, not systematic errors due to uncertainties in the
models. There are four events that can be pointed out in this
diagram. They are not obvious, but can be spotted in
conjunction with the spatial coherence discussed below.
Region 1 is the young association in the lower right portion of the image.
This appears to peak at about 200 Myr.
Region 2 is a larger region that sits just left of center in the lower
part of the image.
This peaks at roughly 370 Myr.
The small peak at 430 Myr corresponds to a region that runs off the lower
left corner of the image.
Finally, between 500 and 600 Myr corresponds to a large diffuse region
throughout the lower part of the image (see Dohm-Palmer et al. 1997b)
A gallery of nine frames showing the density of blue HeB stars in
Sextans A over time.
Each frame is 1 kpc on a side.
The time shown is the number of years in the past.
The ages were determined from the relations of Bertelli et al. (1994).
Each frame was convolved with a Gaussian with
The series of frames were also convolved in time with
σt=30 Myr (see Dohm-Palmer et al. 1998a)
The star formation history of four nearby dI galaxies based
on the blue HeB luminosity function. The bins are 25 Myr.
The right axis has been labelled assuming
Sextans A consistently has the highest SFR/area, followed by Leo A.
Pegasus and GR 8 have a very similar SFR/area, which is consistently
lower than in the other two galaxies.
From Dohm-Palmer et al. 1998b
A comparison between the CMDs in V and I for Sextans A
(left, from Dohm-Palmer et al. 1997b) and for Pegasus
(right, from Gallagher et al. 1998).
The points have been corrected for interstellar reddening.
Note the paucity of the MS and HeB populations in Pegasus compared
to Sextans A.
Since Pegasus is about half as far away as Sextans A the relative lack
of bright blue stars is even stronger than as appears in this figure
(note the absolute magnitude scales on the right of the figures)
(left) The global SFR for the area of Sextans A within the field of
view determined from the blue HeB stars.
For ages older than ∼600 Myr, there is likely contamination
from RGB stars scattering into that region of the CMD
(from Dohm-Palmer et al. 1997b).
(right) The global SFR for Pegasus over the last 800 Myr, based on
the blue HeB stars.
Note the overall lower level of star formation per area relative to
that found in Sextans A (from Gallagher et al. 1998)
Plot of H II region oxygen abundance versus absolute
blue magnitude following Richer & McCall (1995) (from Skillman et al.
Note the position of Pegasus.
The error bars in oxygen are those implied by the comparison of
the observations with photoionization modeling.
The new position of Leo A is based on our new WFPC2 CMD (Tolstoy et al. 1998)
A comparison of the N/O and O/H in Pegasus with the collection of dwarf
irregular galaxies and H II galaxies assembled by
Kobulnicky & Skillman (1996; see their Table 5 and Fig. 15
for identification of individual points).
Only galaxies without WR emission features and errors in log (N/O) less than
0.2 have been plotted.
Note the positions of Pegasus and Sextans A
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|First version: ||10th||July,||1998
|Last update: ||25th||September,||1998
Jochen M. Braun &