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 (HIFI) 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(H$_2$)$_{out}$ $\sim$ 1 $\times$ 10$^7$
M$_\odot$ and an outflow rate as large as \.{M} $\sim$ 6.4 M$_\odot$ yr$^{-1}$.
In the case of NGC 4945, the axisymmetric velocity components in the HF line
profile is 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 $\le$ 1.2 M$_\odot$ yr$^{-1}$, inside a
inner radius of $\le$ 200 pc. The gas accretion rate to the central AGN is much
smaller, suggesting that the inflow can 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.
