Are the smallest galaxies optically invisible?

Summary

Introduction

Recent studies by Tyson (1988) and Tyson & Scalo (1988) suggest the possible existence of a large population of gas-rich dwarf irregular galaxies. Their “bursting dwarf galaxies” model would imply that a large fraction of these dwarfs remains undetected due to observational selection effects (angular diameter, surface brightness). Dekel & Silk (1986), in their cold dark matter biased galaxy formation picture, also predict that the universe is filled more uniformly with dwarf galaxies than with bright ones. Our results on DDO 154 suggest it could be a prototype gas-rich, low surface brightness, small optical diameter galaxy which happens to be relatively nearby (Δ ≤ 4 Mpc based on possible membership to the CVn I cloud and the magnitudes of the brightest blue stars; Carignan & Beaulieu 1989).

Summary of the data

DDO 154 is barely discernible on the Palomar Sky Survey. Its extrapolated central surface brightness is only B(0) = 23.5 mag arcsec⁻². The colours, however, are typical of Im galaxies with (B – V) = 0.32 and (V – R) = 0.30. Its large HI gas content and extent were discovered serendipitously by Krumm & Burstein (1984). From the VLA data, it is found that the HI extends to nearly 5D_HO at a level ˜ 10₁₉ cm⁻² (4D_HO at a level ∼ 10²⁰ cm⁻²). Despite the chaotic optical appearance, the velocity field is very regular and well-defined. The analysis shows that the closing of the isovelocity contours in the outer parts is partly due to the warp of the HI disc.