Faintest of Them All: ZTF 21aaoryiz/SN 2021fcg—Discovery of an Extremely Low Luminosity Type Iax Supernova

We present the discovery of ZTF 21aaoryiz/SN 2021fcg—an extremely low luminosity Type Iax supernova. SN 2021fcg was discovered by the Zwicky Transient Facility in the star-forming galaxy IC0512 at a distance of ≈27 Mpc. It reached a peak absolute magnitude of Mr = −12.66 ± 0.20 mag, making it the least luminous thermonuclear supernova discovered to date. The E(B − V) contribution from the underlying host galaxy is unconstrained. However, even if it were as large as 0.5 mag, the peak absolute magnitude would be Mr = −13.78 ± 0.20 mag—still consistent with being the lowest-luminosity SN. Optical spectra of SN 2021fcg taken at 37 and 65 days post-maximum show strong [Ca ii], Ca ii, and Na i D emission and several weak [Fe ii] emission lines. The [Ca ii] emission in the two spectra has extremely low velocities of ≈1300 and 1000 km s−1, respectively. The spectra very closely resemble those of the very low luminosity Type Iax supernovae SN 2008 ha, SN 2010ae, and SN 2019gsc taken at similar phases. The peak bolometric luminosity of SN 2021fcg is ≈${2.5}_{-0.3}^{+1.5}\times {10}^{40}$ erg s−1, which is a factor of 3 lower than that for SN 2008 ha. The bolometric lightcurve of SN 2021fcg is consistent with a very low ejected nickel mass (M${}_{\mathrm{Ni}}\approx {0.8}_{-0.5}^{+0.4}\times {10}^{-3}$ M⊙). The low luminosity and nickel mass of SN 2021fcg pose a challenge to the picture that low-luminosity SNe Iax originate from deflagrations of near-Mch hybrid carbon–oxygen–neon white dwarfs. Instead, the merger of a carbon–oxygen and oxygen–neon white dwarf is a promising model to explain SN 2021fcg.