The JWST Discovery of the Triply Imaged Type Ia "Supernova H0pe" and Observations of the Galaxy Cluster PLCK G165.7+67.0

A Type Ia supernova (SN) at z = 1.78 was discovered in James Webb Space Telescope Near Infrared Camera imaging of the galaxy cluster PLCK G165.7+67.0 (G165; z = 0.35). The SN is situated 1.5–2 kpc from the host-galaxy nucleus and appears in three different locations as a result of gravitational lensing by G165. These data can yield a value for Hubble's constant using time delays from this multiply imaged SN Ia that we call "SN H0pe." Over the cluster, we identified 21 image multiplicities, confirmed five of them using the Near-Infrared Spectrograph, and constructed a new lens model that gives a total mass within 600 kpc of (2.6 ± 0.3) × 10¹⁴ M _⊙. The photometry uncovered a galaxy overdensity coincident with the SN host galaxy. NIRSpec confirmed six member galaxies, four of which surround the SN host galaxy with relative velocity ≲900 km s^‑1 and projected physical extent ≲33 kpc. This compact galaxy group is dominated by the SN host galaxy, which has a stellar mass of (5.0 ± 0.1) × 10¹¹ M _⊙. The group members have specific star formation rates of 2–260 Gyr^‑1 derived from the Hα-line fluxes corrected for stellar absorption, dust extinction, and slit losses. Another group centered on a strongly lensed dusty star-forming galaxy is at z = 2.24. The total (unobscured and obscured) SFR of this second galaxy group is estimated to be (≳ 100 M _⊙ yr^‑1), which translates to a supernova rate of ∼1 SNe yr^‑1, suggesting that regular monitoring of this cluster may yield additional SNe.