A SEARCH FOR 6.7 GHz METHANOL MASERS IN M33

摘要

We report the negative results from a search for 6.7 GHz methanol masers in the nearby spiral galaxy M33. We observed 14 GMCs in the central 4 kpc of the Galaxy, and found 3 σ upper limits to the flux density of ～4 mJy at a velocity resolution of 0.35 km s~(-1). By velocity shifting and combining the spectra from the positions observed, we obtain an effective 3 σ upper limit on the average emission of ～1 mJy in a 0.25 km s~(-1) channel. These limits lie significantly below what we would expect based on our estimates of the methanol maser luminosity function in the Milky Way. One possible explanation for the absence of detectable methanol masers appears to be the metallicity of M33, which is somewhat less than that of the Milky Way.