Friday

Fluorine is one of the most interesting elements for nuclear and stellar astrophysics. Fluorine abundance was first measured for stars other than the Sun in 1992, then for a handful of metal-poor stars, which are likely to have formed in the early Universe. The main production sites of fluorine have been debated for more than two decades, and include asymptotic giant branch (AGB) stars, \nu-process in core-collapse supernovae, and Wolf-Rayet (WR) stars. Due to the difference in mass, and therefore lifetime, between the progenitor stars (2-4 M_sun for AGB stars, > 10 M_sun for the \nu-processs, and >20 M_sun for WR stars), high redshift observations of fluorine abundance can help constrain the mechanism of fluorine production in massive galaxies, with the molecule HF providing a convenient observable tracer. I report the most distant detection of HF in absorption in a gravitationally lensed dusty star-forming galaxy at z=4.4 with ALMA. With an estimated lensing magnification of µ~5, we show that N(HF)/N(H2) is as high as 4 x 10^-9, indicating a very fast ramp-up of the chemical enrichment in this high-z galaxy. At z=4.4 only 1.4Gyrs have elapsed since the Big Bang, such that AGB stars with masses of a few M_sun are very unlikely to dominate the enrichment. Instead, we show that WR stars are required to produce the observed fluorine abundance at this time, with other production mechanisms becoming important at later times. These observations therefore provide an insight into the underlying processes driving the `ramp up' phase of chemical enrichment alongside rapid stellar mass assembly in a young massive galaxy.