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The circumnuclear region of M31, consisting of multiphase interstellarmedium, provides a close-up view of the interaction of the central supermassiveblack hole and surrounding materials. Far-infrared (FIR) line structure linesand their flux ratios can be used as diagnostics of physical properties of theneutral gas in this region. Here we present the first FIR spectroscopic mappingof the circumnuclear region of M31 in [C ii] 158 um, [O i] 63 um and [O iii] 88um lines with the Herschel Space Observatory, covering a ~500 x 500 pc (2' x2') field. Significant emissions of all three lines are detected along theso-called nuclear spiral across the central kpc of M31. The velocity fieldunder a spatial resolution of ~50 pc of the three lines are in broadconsistency and also consistent with previous CO(3-2) line observations in thecentral region. Combined with existing [C ii] and CO(3-2) observations of fiveother fields targeting on the disk, we derived the radial distribution of [Cii]/CO(3-2) flux ratio, and found that this ratio is higher in the center thanthe disk, indicating a low gas density and strong radiation field in thecentral region. We also found that the [C ii]/FIR ratio in the central regionis 5.4 (+-0.8) x 10^-3, which exhibits an increasing trend with thegalactocentric radius, suggesting an increasing contribution from old stellarpopulation to dust heating towards the center.

A Herschel mapping of [C ii], [O i] and [O iii] lines from the circumnuclear region of M31