Auto-consistent metallicity and star formation history of the nearest blue compact dwarf galaxy NGC 6789

Abstract

We present a detailed auto-consistent study of the nearest blue compact dwarf galaxy NGC 6789 by means of optical and UV archive photometry data and optical long-slit ISIS-WHT spectroscopy observations of the five brightest star-forming knots. The analysis of the spectra in all knots allowed the derivation of ionic chemical abundances of oxygen, nitrogen, sulphur, argon and neon using measures of both the high- and low-excitation electron temperatures, leading to the conclusion that NGC 6789 is chemically homogeneous with low values of the abundance of oxygen in the range 12+log(O/H) = 7.80-7.93, but presenting at the same time higher values of the nitrogen-to-oxygen ratio than expected for its metal regime. We used archival HST/WFPC2 F555W and F814W observations of NGC 6789 to perform a photometric study of the colour-magnitude diagram (CMD) of the resolved stellar populations and derive its star formation history (SFH), which is compatible with the presence of different young and old stellar populations whose metallicities do not necessarily increase with age. We fit the observed optical spectrum in all the five knots using the STARLIGHT code and a combination of single stellar populations following the SFH obtained from the CMD. We compare the resulting stellar masses and the relative fractions of the ionising populations with a non-constrained SFH case. The properties of the younger populations were obtained using CLOUDY photoionisation models, giving similar ages in all the knots in the range 3-6 Myr and the estimation of the dust absorption factor, which correlates with the observed GALEX FUV-NUV colour indices. The total photometric extinction and dust-absorption corrected Hα fluxes were finally used to derive the star formation rates.

18 pages, 6 figures, 10 tables; accepted by MNRAS

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