Atomic carbon in M82: Physical conditions derived from simultaneous observations of the [C I] fine-structure submillimeter-wave transitions

We report the first extragalactic detection of the neutral carbon [C I] P-3(2) --> P-3(1) fine-structure line at 809 GHz. The line was observed toward M82 simultaneously with the P-3(1) --> P-3(0) line at 492 GHz, providing a precise measurement of the J = 2 --> 1/J = 1 --> 0 integrated line ratio of 0.96 (on a [K km s(-1)] scale). This ratio constrains the [C I] emitting gas to have a temperature of at least 50 K and a density of at least 10(4) cm(-3). Already at this minimum temperature and density, the beam averaged CI column density is large, 2.1 X 10(18) cm(-2), confirming the high CI/CO abundance ratio of approximate to 0.5 estimated earlier from the 492 GHz line alone. We argue that the [CI] emission from M82 most likely arises in clouds of linear size around a few pc with a density of about 10(4) cm(-3) or slightly higher and temperatures of from 50 K up to about 100 K.