http://arxiv.org/abs/1803.00024
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 {\it 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 parsecs. This suggests that the progenitor is not a run-away star. From these measurements, we infer an age for the black hole progenitor of $10.6^{+14.4}{-6.1}$ Myr. Assuming that the progenitor evolved effectively as a single star, this corresponds to an initial mass of $17.9^{+33.8}{-7.6}$ 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, 89\% of the PDF we measure lies below that mass, putting some tension between the age and the direct imaging results.
J. Murphy, R. Khan, B. Williams, et. al.
Fri, 2 Mar 18
39/61
Comments: 12 pages, 7 Figures, submitted to ApJ. Figure 3 presents the main result
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