2018ApJ...860..117M

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 and infer a distance modulus of 29.47 ± 0.079 to NGC 6946, or a metric distance of 7.83 ± 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_–5.9^+14.5 Myr. Assuming that the progenitor evolved effectively as a single star, this corresponds to an initial mass of 17.9_–7.6^+29.9M_☉. Previous spectral energy distribution (SED) modeling of the progenitor suggests a mass of ∼27 M_☉. Formally, the SED-derived mass falls within our narrowest 68% confidence interval; however, 91% of the probability distribution function we measure lies below that mass, putting some tension between the age and the direct-imaging results.