The Neutral Interstellar Medium of the Dwarf Irregular Galaxy DDO 47 and Its Companion

We present high-resolution H I observations obtained with the Very Large Array, as well as optical broadband imaging, of the dwarf irregular galaxy DDO 47 (UGC 3974, CGCG 087-030). The H I maps show a variety of structures, such as holes and shells, including one supergiant shell, in its neutral inter- stellar medium. We derive an H I mass for DDO 47 of 2.3 ) 108 We cataloged 19 H I holes in M _ . DDO 47, about half of them showing signs of expansion. According to the ìì standard model,ˇˇ their origin is believed to be due to past regions of massive star formation through the eÜects of stellar winds and supernova explosions. A comparison of H II regions with the positions of the H I holes yields that most of the current star-forming regions are situated near the rims of the larger H I holes, indicative of propagating star formation. The mean properties of the H I holes in DDO 47 are very much like those found in other dwarf galaxies. At a projected distance of 20 kpc (adopting a distance to DDO 47 of 4 Mpc), a galaxy that is cataloged as CGCG 087-033 was found to be at almost the same systemic velocity as DDO 47. This object is a likely companion, with an H I mass of 1.4 ) 107 and a M _ dynamical mass of at least 1.2 ) 108 A dynamical study of DDO 47 reveals that this object M_. requires dark matter at a level of about 80% of its dynamical mass to explain the (Mdyn B 5 ) 109 M_) observed rotation (assuming a stellar masstolight ratio of 1 Using a simple mass model, the M_/L B_). scale height of the H I distribution of DDO 47 is derived to be about 500 pc. This high value is similar to values found in other dwarf irregular galaxies and can be explained as being due to a lower gravita- tional potential. In summary, DDO 47 is a gas-rich dwarf irregular galaxy that shows a rather complex structure in its neutral interstellar medium. We speculate that the —yby of the companion may have triggered a starburst in DDO 47 in the past, which may have created the H I holes that are now present.