The electron density in M82 from the S III mid-infrared line ratio

Strong detections have been made of both the 18.7 microns and 33.4 microns lines of S III in the irregular galaxy M82. This represents the first detection of the 33.4 microns line in an extragalactic object. These measurements sample the ionized gas component which dominates the observed infrared forbidden line and infrared hydrogen recombination line emission. From the ratio of the line fluxes an electron density of 120 (+ 280, - 120) per cu cm is deduced. This result is in agreement with previous estimates of the density derived from high-frequency radio continuum measurements and an assumed emitting volume. They impose further constraints on possible starburst models.