Fading into Darkness: A Weak Mass Ejection and Low-efficiency Fallback Accompanying Black Hole Formation in M31-2014-DS1

Abstract Stellar-mass black holes (BHs) can form from the near-complete collapse of massive stars, causing them to abruptly disappear. The star M31-2014-DS1 in the Andromeda galaxy was reported to exhibit such a disappearance between 2014 and 2022, with properties consistent with the failed explosion of an ≈12–13 M ⊙ yellow supergiant leading to the formation of an ≈5 M ⊙ BH. We present mid-infrared (MIR) observations of the remnant obtained with the James Webb Space Telescope (JWST) and X-ray observations from the Chandra X-ray Observatory in 2024. The JWST Mid-infrared Instrument/NIRSpec data reveal an extremely red source, showing strong blueshifted absorption from molecular gas (CO, CO 2 , H 2 O, SO 2 ) and deep silicate dust features. Modeling the dust continuum confirms continued bolometric fading of the central source to <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>log</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>L</mml:mi> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>L</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>⊙</mml:mo> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≈</mml:mo> <mml:mn>3.88</mml:mn> </mml:math> (≈7%–8% of the progenitor luminosity), surrounded by a dust shell spanning ≈40–200 au. Modeling of the molecular gas indicates ∼0.1 M ⊙ of gas expanding at ≈100 km s −1 near the inner edge of the dust shell. No X-ray source is detected down to a luminosity limit of L X ≲ 1.5 × 10 35 erg s −1 . We show that the panchromatic observations are explained by (i) a low-energy (≈10 46 erg) ejection of the outer H-rich progenitor envelope and (ii) a fading central BH powered by inefficient (∼0.1% in mass) accretion of loosely bound fallback material. The analysis robustly establishes the bolometric fading of M31-2014-DS1 following its &gt;30 yr archival record of constant optical/infrared brightness, and provides the first cohesive insights into BH formation via low-energy explosions and long-term fallback.