Astron. Astrophys. 363, 493-506 (2000)

6. Conclusions

We can summarize the results of our work as follows:

1. The near-infrared spectrum of SBS 0335-052 is representative of an extreme and young starburst galaxy. The equivalent width indicates a very young age, and is one of the largest ever observed in an extragalactic object. The He I recombination lines support the presence of young massive stars. Recombination and lines are strong, [FeII] and stellar absorption bands are absent, which also implies a strong UV field and a young stellar population; there is no spectroscopic evidence for stars older than 5-6 Myr. Modeling the and K continuum leads to a burst age5 Myr and a star formation rate that can be as high as 10 /yr.

2. The spatial profile of the K continuum is shifted by about 60 pc northwest of the emission line peak and it is much closer to it than the optical peak at a distance of 200 pc. This is explained with the dominant nebular contribution to K. All optical and NIR emission lines coincide and have consistent spatial profiles.

3. The near-infrared photometry of SBS 0335-052 is highly contaminated by nebular emission. The optical and NIR colors are unusual, and after correction for the gas contribution can be only understood as due to a stellar population not older than 4 Myr, together with hot dust at 670 K.

4. On the basis of the NIR colors, the stellar population in SBS 0335-052W appears to be older than that in SBS 0335-052; the colors are redder in and bluer in and correspond to an approximate age of 10-20 Myr according to SB99. However, optical recombination line equivalent widths suggest that the stellar population cannot be older than 10 Myr, and our models suggest that the age is more likely around 5-7 Myr.

5. The gas-corrected spectrum from 2 to 17 µm is well fit by two thermal components (hot dust at 670 K, and cooler dust at 225 K), together with mag of visual extinction in a foreground screen.

6. We do not find evidence at 2 µm for optically hidden star formation.

7. Judging from the uncertainties on the NIR photometry, the possible contribution from an evolved stellar population in SBS 0335-052 cannot exceed 15%.

SBS 0335-052 appears to be an unusual object, as there is no evidence, even in the near-infrared, for an evolved stellar population deriving from an earlier burst. This is in contrast to results for the lowest-metallicity galaxy known, I Zw 18, in which Aloisi et al. (1999) and Östlin (2000) claimed to detect a 1-4 Gyr underlying stellar population. If the star-formation rates derived from our modeling are representative, then stars are forming in SBS 0335-052 extremely rapidly, but how such a rate is connected to the formation of SSCs is an open question. It could be that under certain conditions, star formation, galaxy gas consumption, and evolution proceed very quickly, while in others, evolution is slower. Future work will be aimed at better quantifying what these conditions might be, and how they relate to metallicity.

© European Southern Observatory (ESO) 2000

Online publication: December 11, 2000
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