A Search for 6.7 GHz Methanol Masers in M33

TL;DR

This study conducted a search for 6.7 GHz methanol masers in M33 but found no detections, suggesting that lower metallicity in M33 may inhibit maser formation compared to the Milky Way.

Contribution

First search for 6.7 GHz methanol masers in M33, providing upper limits and insights into environmental effects on maser occurrence.

Findings

No methanol masers detected in M33 within sensitivity limits.

Upper limits are below expected luminosity based on Milky Way estimates.

Metallicity may influence methanol maser presence in external galaxies.

Abstract

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 sigma upper limits to the flux density of ~9 mJy in spectral channels having a velocity width of 0.069 km/s. By velocity shifting and combining the spectra from the positions observed, we obtain an effective 3sigma upper limit on the average emission of ~1mJy in a 0.25 km/s channel. These limits lie significantly below what we would expect based on our estimates of the methanol maser luminosity function in the Milky Way. The most likely explanation for the absence of detectable methanol masers appears to be the metallicity of M33, which is modestly less than that of the Milky Way.