Other
Scientific paper
Sep 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997pasp..109.1069v&link_type=abstract
Publications of the Astronomical Society of the Pacific, v.109, p.1069
Other
1
Scientific paper
Blue Dwarf Galaxies (BDG) are objects of low luminosity (M_Vge-18) and very blue visible colors. Both the colors and the presence of narrow emission lines in their spectra are usually interpreted to result from intense episodes of star formation. The metallicity of the BDG is usually low with values in the range 10^{-2}-1~Zodot. Wolf-Rayet (WR) features are detected in many cases in the optical spectra. This is an unexpected characteristic since according to stellar evolution, the number of WR stars in a star forming region should increase with the metallicity while WR features are only rarely detected in solar metallicity galaxies. Different morphologies are found among BDG including compact elliptical, irregular, magellanic, interacting. Due to the low luminosity in most of these objects the starburst (SB) episode dominates the emitted radiation. Therefore starbursts can be studied in more detail in dwarf galaxies than in giant spiral galaxies where the presence of a quiescent stellar population is important in defining the observed characteristics. Moreover BDG are a unique laboratory to study the star formation process in low metallicity environments. We have used new imaging and spectroscopy in the near infrared to study both the star formation history and the excitation mechanism of the near infrared lines in BDG. The observations were taken with the Steward Observatory telescopes. We concentrated on 8 objects selected to be a representative but bright sample of BDG, they are: II Zw 40, He2-10, NGC 3077, NGC 4214, NGC 4449, NGC 1569, NGC 4861 and NGC 5253. The data were used to constrain a starburst model. For II Zw 40 we were able to fit the observations using a gaussian star formation rate with FWHM=5 My and a solar neiborhood IMF. The star formation episode appears to have occured about 4 Myr ago with aproximately one tenth of the total mass of the galaxy involved. Evidence for an evolved population was found in the outer parts of II Zw 40. We also studied the possible periodicity of the starbursting process ruling out episodes comparable to the present one for the last billion years. For all the other galaxies we confirm the tendency of the burst in BDG to be very short, no more than a few million years. The short duration of the burst deduced from our modelling is consistent with the detection of WR features, since short bursts also produce a WR/O ratio higher than a long duration or continous burst. The presence of WR stars is well correlated with the age of the burst. We found a very good correlation between the [FeII] line at 1.64 microns and the age of the burst for a sample of SB galaxies, well explained in terms of a model where all the [FeII] emissin originates from supernova remnants. All the BDG, with the exception of He2-10, show a deficiency in [FeII], in agreement with the low metal content of these objects. Using observations of M 82 we calibrated the [FeII] emission for solar metallicity SB, obtaining good agreement with the supernova remnants observed in the Galaxy. The correlation between molecular hydrogen emission (H_2) at 2.12 microns and age is weaker. In this case there is a minimum emission level associated with supernovae and an additional contribution from different excitation mechanisms. No deviation from the general trend is detected for BDG when compared to normal starburst in agreement with an independency of H_2 on metallicity. Indications for fluorescently excitated H_2 were found in a few cases. We briefly analyzed the FIR-radio relationship finding that BDG follow the trend defined by IR galaxies, despite the low metallicity of the BDG. Finally we use the HeI line at 1.70 microns as an indicator for the presence of high mass stars. Our results show a tendency of commonly used [OIII]/Hβ parameter to overestimate the stellar temperature. The correlation between the 1.70 micron line and the age of the burst is very good. We believe this line is a very reliable indicator of stellar temperature being less affected by the extinction and the electron temperature than the temperature indicators in the visible. (SECTION: Dissertation Summaries)
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