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] ³P₂ --> ³P₁ fine-structure line at 809 GHz. The line was observed toward M82 simultaneously with the ³P₁ --> ³P₀ 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 10⁴ cm^-3. Already at this minimum temperature and density, the beam averaged C I column density is large, 2.1 × 10¹⁸ cm^-2, confirming the high C I/CO abundance ratio of ~0.5 estimated earlier from the 492 GHz line alone. We argue that the [C I] emission from M82 most likely arises in clouds of linear size around a few pc with a density of about 10⁴ cm^-3 or slightly higher and temperatures of from 50 K up to about 100 K.