The Progenitor Age and Mass of the Black-Hole-Formation Candidate N6946-BH1

TL;DR

This study estimates the age and initial mass of the progenitor of the black hole candidate N6946-BH1 by analyzing surrounding stellar populations and star formation history, providing new constraints on progenitor properties.

Contribution

It combines archival HST imaging, distance measurement via TRGB, and star formation history modeling to infer the progenitor's age and mass, offering a novel approach to progenitor characterization.

Findings

Progenitor age estimated at 10.6 Myr with uncertainties

Initial mass inferred to be approximately 18 solar masses

Results suggest the progenitor was not a runaway star

Abstract

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.