Astron. Astrophys. 323, 323-336 (1997)

1. Introduction

H II galaxies are actively star forming dwarf galaxies, dominated by one or more giant H II regions. The hot stars which ionize the H II regions have life times of only a few Myrs, thus the presence of H II regions indicates current or very recent star forming activity on a large scale. The age of a star formation event determines the specific amounts of emission from star forming regions across the electromagnetic spectrum.

In Deeg et al. (1993), - hereafter DBDKS -, radio observations of H II galaxies and their unusual radio continuum spectra were reported and several models for the interpretation of the spectra were introduced. Some of these models allowed estimates of the moment, when major changes in the injection rates of relativistic electrons into the ISM occured. The variations in the injection rate are most likely linked to past changes in the rate of Type II supernovae. To support and verify these models, data at additional wavelengths regimes were obtained. This paper (Paper I) presents a homogeneous set of B, R and I broadband, and H narrow band photometry on the eight galaxies, as well as images of them, and discusses their H and optical color morphology. An update on more recent radio continuum measurements is given as well. The interpretation of the data presented here is left to the next paper (Deeg, Brinks & Duric 1997, Paper II), where the results from optical and radio observations are integrated into a consistent picture, based on the star formation histories of these galaxies.

Optical broadband colors allow the derivation of independent estimates for the ages of the stellar populations. Whereas a galaxy's emission in the B -band is dominated by brighter main-sequence stars with a lifetime up to about yrs, I band emission indicates older stellar disk and Halo (Pop. II) populations. Observations with the B, V and I filters were chosen to cover the widest span of stellar colors. Models quoting versus , or colors are frequently used to derive the ages of young stellar populations and the strengths of starbursts (e.g. Krüger et al. 1991; Belfort et al. 1987; Kennicutt 1983; Thuan 1983, 1985; Charlot & Bruzual 1991). U band observations, which were beyond the capabilities of the equipment available, were taken from the literature for all of the galaxies. The advantage of photometric reductions based on CCD imagery, as undertaken for this work, is the availability of surface photometry, which allows derivation of the local colors of a star forming region. Also undertaken were H observations, which yielded morphological information on the size and distribution of the H II regions. The derived H flux is used to set lower limits to the thermal radio emission and serves as an indicator for star formation rates in the last few Myrs, as will be discussed in Paper II II.

The eight sample galaxies - seven of which were introduced by DBDKS - were selected from the study of blue compact dwarf galaxies by Klein et al. (1991) on the basis of strong radio continuum emission. Haro 1 ( NGC 2415 UGC 03930 CGCG 0733.6 3521 MCG 06-17-021 CGCG 177-038 ARK 136) was not included in DBDKS, but was added to this study later, when sufficient radio and optical data became available.

In the following, Sect. 2 covers the optical observations, data reduction, and gives the quantitative results. Additional radio continuum measurements, which improve the frequency coverage in the 8-15 GHz range, are presented in Sect. 3. Section 4 discusses the optical and radio morphology of the galaxies; a summary is given in Sect. 5.

© European Southern Observatory (ESO) 1997

Online publication: June 5, 1998

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