Radio-continuum radiation from Messier 33

Radio observations of the spiral galaxy Messier 33 are presented at 318 and 606 MHz. These observations were performed with the Arecibo Observatory 1000-foot (305 m) radio telescope. The brightness-temperature distribution at the higher frequency shows substantial structure which could be identified with the northern and southern spiral-arm regions of M33. The radio spectrum of M33 is discussed, and it is shown that its spectral index is in the range -0.17 to -0.20. This somewhat flat spectrum suggests a mixture of thermal and nonthermal radiation. The thermal radiation from M33 is estimated to be at least 10 percent of the total radio emission at frequencies >1 GHz.