HYDROGEN FLUORIDE TOWARD LUMINOUS NEARBY GALAXIES: NGC 253 AND NGC 4945

We present the detection of hydrogen fluoride (HF) in two luminous nearby galaxies, NGC 253 and NGC 4945 using the Heterodyne Instrument for the Far-Infrared on board the Herschel Space Observatory. The HF line toward NGC 253 has a P-Cygni profile, while an asymmetric absorption profile is seen toward NGC 4945. The P-Cygni profile in NGC 253 suggests an outflow of molecular gas with a mass of M(H2)out ∼ 1 × 107 M☉ and an outflow rate as large as Ṁ ∼6.4 M☉ yr−1. In the case of NGC 4945, the axisymmetric velocity components in the HF line profile are compatible with the interpretation of a fast-rotating nuclear ring surrounding the nucleus and the presence of inflowing gas. The gas falls into the nucleus with an inflow rate of ⩽1.2 M☉ yr−1, inside an inner radius of ⩽200 pc. The gas accretion rate to the central active galactic nucleus is much smaller, suggesting that the inflow may be triggering a nuclear starburst. From these results, the HF J = 1–0 line is seen to provide an important probe of the kinematics of absorbing material along the sight-line to nearby galaxies with bright dust continuum and a promising new tracer of molecular gas in high-redshift galaxies.