We present the detection of hydrogen fluoride , HF , in two luminous nearby galaxies NGCÂ 253 and NGCÂ 4945 using the Heterodyne Instrument for the Far-Infrared ( HIFI ) on board the Herschel Space Observatory .
The HF line toward NGCÂ 253 has a P-Cygni profile , while an asymmetric absorption profile is seen toward NGCÂ 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 Ṁ \sim  6.4 M _ { \odot } yr ^ { -1 } .
In the case of NGCÂ 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 \leq  1.2 M _ { \odot } yr ^ { -1 } , inside a inner radius of \leq 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 .