EDP Sciences
Free access
Issue
A&A
Volume 407, Number 1, August III 2003
Page(s) 75 - 90
Section Galactic structure and dynamics
DOI http://dx.doi.org/10.1051/0004-6361:20030806


A&A 407, 75-90 (2003)
DOI: 10.1051/0004-6361:20030806

Spectroscopic and photometric studies of low-metallicity star-forming dwarf galaxies

I. SBS 1129+576
N. G. Guseva1, P. Papaderos2, Y. I. Izotov1, R. F. Green3, K. J. Fricke2, T. X. Thuan4 and K. G. Noeske2

1  Main Astronomical Observatory, Ukrainian National Academy of Sciences, Zabolotnoho 27, Kyiv 03680, Ukraine
2  Universitäts-Sternwarte, Geismarlandstraße 11, 37083 Göttingen, Germany
3  National Optical Astronomy Observatory, Tucson, AZ 85726, USA
4  Astronomy Department, University of Virginia, Charlottesville, VA 22903, USA

(Received 21 November 2002 / Accepted 16 May 2003 )

Abstract
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 $\mu_V$ between 23 and 26  mag arcsec -2 is relatively blue ~ $0.56 \pm 0.03$ mag, as compared to the $(V-I) \sim 0.9$-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 $12 + \log\, \rm (O/H) = 7.36 \pm 0.10$ in the brightest H II region and $7.48 \pm 0.12$ in the second brightest H II region, or 1/36 and 1/28 of the solar value, respectively. H $\beta$ and H $\alpha$ emission lines and H $\delta$ and H $\gamma$ 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. Several scenarios of star formation were explored using all 4 methods. 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 in SBS 1129+576 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. Hence, the ground-based spectroscopic and photometric observations are not sufficient for distinguishing between a young and an old age for SBS 1129+576.


Key words: galaxies: fundamental parameters -- galaxies: starburst -- galaxies: abundances -- galaxies: photometry -- galaxies: individual: SBS 1129+576

Offprint request: N. G. Guseva, guseva@mao.kiev.ua

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