Abstract.
UVI photometry of the stars brighter than
MI ∼ -2
was obtained from
HST PC images of
the probably nearest BCDG VII Zw 403.
We found a well populated red giant branch.
This BCDG contains many old stars.
The tip of the red giant branch allowed a distance estimate.
The spatial distribution of young and old stars is discussed and the stellar
color-magnitude diagrams are compared to isochrones.
1. Introduction
It is an unsolved problem how to link the highly actively star forming
blue compact dwarf galaxies (BCDGs) into a common evolutionary scheme
with the other types of dwarf galaxies.
It is also a long-standing (and related) question whether or not BCDGs are
young galaxies in the sense that they are forming their first (major)
population of stars or if they started long ago to form stars
(Davies & Phillipps 1988; Bergvall & Jörsäter 1988;
Lacey & Silk 1991; Blanchard et al. 1992; Thuan et al. 1997 and
Thuan,
this conference).
Earlier approaches to find old stars include
JHK integral photometry
(Thuan 1983), searches for color gradients of the stellar population
underlying the burst region (Telles et al. 1996), and SED fitting
techniques (Krüger et al. 1995).
All these approaches yield indirect evidence for old stars in some of the
BCDGs, but they are hampered by observational problems related to the severe
surface brightness difference between the star forming complex in the BCDGs
and the underlying galaxies.
A far more direct approach is possible for the very nearby (
D <
6...7 Mpc) BCDGs with the high resolution capability of
HST through the detection of red giant
branch stars (RGB) in the color-magnitude diagrams (CMD) of individual stars.
Our study of VII Zw 403 = UGC 6456 (Schulte-Ladbeck et al. 1998)
showed the presence of a RGB in this BCDG, a clear evidence for an old
population.
[Click here to see Fig. 1!]
2. VII Zw 403 from the ground
VII Zw 403 belongs to the 10 Mpc sample of Kraan-Korteweg & Tammann
(1979) and is an isolated galaxy.
According to Tully et al. (1981) and Izotov et al. (1996), it shows the typical
spectrum of a BCDG (see also
Fig. 1).
A chemical abundance of about 1/15 solar was determined.
VII Zw 403 still has a fair amount of neutral gas
(4·10
7 Msun, Thuan & Martin 1981)
and Papaderos et al. (1994) announced the detection of hot gaseous outflows
from ROSAT images.
Ground-based CCD images show a regular, dE-like galaxy, but with several
central knots, many of them prominent in H
alpha.
The H
alpha flux indicates a recent star formation rate of
∼10
-3 Msun yr
-1
(see Schulte-Ladbeck et al. 1998 and Hopp & Schulte-Ladbeck 1995 for
details).
3. Spatial distribution
We performed DAOPHOT photometry of the resolved individual stars in the
HST-PC images in
U,V,I and
detected several 10
3 stars.
The limits of this photometry are described by Schulte-Ladbeck et al. (1998).
Foreground contamination is negligible due to the tiny field of view.
The CMD in
I,
V-I is shown in
Fig. 2.
It shows all the features which are typical for irregular galaxies (e.g.
Gallart et al. 1996;
Aparicio,
this conference).
VII Zw 403 shows - as expected for a BCDG - more
blue plume
stars than an irregular galaxy like NGC 6822 (Gallart et al. 1996) and
even more than the post-starburst galaxy NGC 1596 (Vallenari & Bomans
1996; Greggio et al. 1998).
But we also easily see the
red tail (AGB stars) and the
red tangle (mostly RGB).
A comparison of the
observed RGB in galactic globular clusters with our
CMD (
Fig. 2) points to a large number
of RGs in VII Zw 403.
Schulte-Ladbeck et al. (1998) were able to detect the tip of the RG branch and
derived a distance of 4.8 Mpc for VII Zw 403, clearly behind
the M81 group of galaxies and far more distant than assumed before.
[Click here to see Fig. 2!]
We devided the detected stars into a young star sample (blue plume
and red super giants (RSG)) and in old stars
(V-I > 0.7 -2 < MI < -4) and performed star
counts in the two subsamples.
The derived radial profiles are shown in
Fig. 3 which clearly point to a high
concentration of the young stars and a relatively homogeneous distribution of
the old ones.
A CMD constructed only from the inner part (r ∼ 7"
(185 pc)) is dominated by the blue plume and the RSGs while few AGB
and very few RGs are visible.
Outside (r > 11" (295 pc)), the CMD is dominated by
the red tangle, no RSG and only a few fainter blue plume stars
are visible.
The slope of the luminosity function of blue plume objects is - within
the range of complete data - consistent with the one derived by Freedman (1985)
for nearby irregular galaxies.
[Click here to see Fig. 3!]
4. Observed CMDs and isochrones
As a first step for a further interpretation of the CMDs, we compared
the observed distribution of stars with isochrones of Bertelli et
al. (1994)
*.
We broadened the Padua isochrones with the 1σ observational errors
function (magnitudes and colors).
To fill up the observed distribution, some combinations of age and
metallicities are allowed, some are necessary while others are excluded.
Internal reddening was neglected as we have little evidence from
UVI
color-color diagrams.
Crowding can still affect the analysis especially in the blue part
(associations, clusters).
Even with
HST PC data the spatial
resolution of the data already corresponds to ∼2.5 pc in the distance
of VII Zw 403.
A more detailed analysis, taking into account the star formation history,
the IMF, but also the observational selection effects and errors, therefore
needs a discussion following the work presented during this conference e.g.
by
Aparicio and
Gallart,
Tolstoy,
and
Tosi
(see
their contributions).
Within the framework of the I,V-I CMD, old stars
(log(t) > 9) of Z = 0.004 are needed to explain the
red tail and red tangle.
Older stars are possible if more metal-poor.
The RSGs need intermediate age isochrones (log(t) ∼ 7-8) while stars
with log(t) < 7.1 are needed for the blue plume (Z
always 0.004; Fig. 4).
There is no hint for metal-enrichment through the stellar
generations, an indication for blow-out of the metals produced by
earlier massive stars (see Mac Low,
this conference)?
The V,U-V CMD shows intermediate age to very young stars as
expected (6.5 < log(t) < 7.9).
The strong Halpha emission indicates ongoing formation of
massive stars.
[Click here to see Fig. 4!]
*We selected the Padua tracks as Greggio et al.
(1998; see also Tosi, this
conference) were slightly more successful with them than with the Geneva
tracks for the post-starburst dwarf NGC 1569.
Acknowledgments.
This work was funded through HST archival grant AR-06404.01-95A.
U.H. acknowledges support from the SFB 375 and by the DfG (Ho 1812/2-1).
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| First version: | 01st | August, | 1998
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| Last update: | 08th | October, | 1998
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Jochen M. Braun &
Tom Richtler
(E-Mail: jbraun|richtler@astro.uni-bonn.de)