The Progenitor Age and Mass of the Black-Hole-Formation Candidate
  N6946-BH1
  The failed supernova N6946-BH1 likely formed a black hole (BH); we age-date
the surrounding population and infer an age and initial mass for the progenitor
of this BH formation candidate. First, we use archival Hubble Space Telescope
imaging to extract broadband photometry of the resolved stellar populations
surrounding this event. Using this photometry, we fit stellar evolution models
to the color-magnitude diagrams to measure the recent star formation history
(SFH). Modeling the photometry requires an accurate distance; therefore, we
measure the tip of the red giant branch (TRGB) and infer a distance modulus of
$29.47 \pm 0.079$ to NGC~6946, or a metric distance of $7.83 \pm 0.29$ Mpc. To
estimate the stellar population's age, we convert the SFH and uncertainties
into a probabilistic distribution for the progenitor's age. The region in the
immediate vicinity of N6946-BH1 exhibits the youngest and most vigorous star
formation for several hundred pc. This suggests that the progenitor is not a
runaway star. From these measurements, we infer an age for the BH progenitor of
$10.6^{+14.5}_{-5.9}$ Myr. Assuming that the progenitor evolved effectively as
a single star, this corresponds to an initial mass of $17.9^{+29.9}_{-7.6}$
$M_{\odot}$. Previous spectral energy distribution (SED) modeling of the
progenitor suggests a mass of $\sim$27 $M_{\odot}$. Formally, the SED-derived
mass falls within our narrowest 68\% confidence interval; however, $91\%$ of
the probability distribtuion function we measure lies below that mass, putting
some tension between the age and the direct-imaging results.
