24 research outputs found
Spectroscopic and photometric studies of low-metallicity star-forming dwarf galaxies. I. SBS 1129+576
Spectroscopy and V,I CCD photometry of the dwarf irregular galaxy SBS
1129+576 are presented for the first time. The CCD images reveal a chain of
compact H II regions within the elongated low-surface-brightness (LSB)
component of the galaxy. Star formation takes place mainly in two
high-surface-brightness H II regions. The mean (V-I) colour of the LSB
component in the surface brightness interval between 23 and 26 mag/sq.arcsec is
relatively blue ~0.56+/-0.03 mag, as compared to the (V-I)~0.9-1.0 for the
majority of known dwarf irregular and blue compact dwarf (BCD) galaxies.
Spectroscopy shows that the galaxy is among the most metal-deficient galaxies
with an oxygen abundance 12+log(O/H)= 7.36+/-0.10 in the brightest H II region
and 7.48+/-0.12 in the second brightest H II region, or 1/36 and 1/28 of the
solar value, respectively. Hbeta and Halpha emission lines and Hdelta and
Hgamma absorption lines are detected in a large part of the LSB component. We
use two extinction-insensitive methods based on the equivalent widths of (1)
emission and (2) absorption Balmer lines to put constraints on the age of the
stellar populations in the galaxy. In addition, we use two extinction-dependent
methods based on (3) the spectral energy distribution (SED) and (4) the (V-I)
colour. The observed properties of the LSB component can be reproduced by a
stellar population forming continuously since 10 Gyr ago, provided that the
star formation rate has increased during the last 100 Myr by a factor of 6 to
50 and no extinction is present. However, the observational properties of the
LSB component can be reproduced equally well by continuous star formation which
started not earlier than 100 Myr ago and stopped at 5 Myr, if some extinction
is assumed.(Abridged)Comment: 17 pages, 12 figures, Accepted for publication in A&
The evolutionary status of the low-metallicity blue compact dwarf galaxy SBS 0940+544
We present the results of spectrophotometry and V,R,I, Halpha CCD photometry
of the blue compact dwarf (BCD) galaxy SBS 0940+544. Broad-band images taken
with the 2.1m KPNO and 1.23m Calar Alto telescopes reveal a compact
high-surface-brightness H II region with ongoing star formation, located at the
northwestern tip of the elongated low surface brightness (LSB) main body of the
BCD. High signal-to-noise 4.5m Multiple Mirror Telescope (MMT) and 10m Keck II
telescope long-slit spectroscopy of SBS 0940+544 is used to derive element
abundances of the ionized gas in the brightest H II region and to study the
stellar population in the host galaxy. The oxygen abundance in the brightest
region with strong emission lines is 12+log(O/H) = 7.46-7.50, or 1/29-1/26
solar, in agreement with earlier determinations and among the lowest for BCDs.
Hbeta and Halpha emission lines and Hdelta and Hgamma absorption lines are
detected in a large part of the main body. Three methods are used to put
constraints on the age of the stellar population at different positions along
the major axis. They are based on (a) the equivalent widths of the emission
lines, (b) the equivalent widths of the absorption lines and (c) the spectral
energy distributions (SED). Several scenarios of star formation have been
considered. The observed properties in the main body can be reproduced by a
continuous star formation process which started not earlier than 100 Myr ago,
if a small extinction is assumed. However, the observations can be reproduced
equally well by a stellar population forming continuously since 10 Gyr ago, if
the star formation rate has increased during the last 100 Myr in the main body
of SBS 0940+544 by at least a factor of five. In summary, we find no compelling
evidence which favors either a young or an old age of SBS 0940+544.Comment: 20 pages, 15 Postscript figures, accepted for publication in the
Astronomy and Astrophysic
Applying Iterative and Parallel Methods to Production Planning in the Pharmaceutical Industry ABSTRACT
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