Early flash-ionization lines in SN 2024ggi revealed by high-resolution spectroscopy

We present an analysis of very early high-resolution spectroscopic observations of the nearby core-collapse (CC) supernova (SN) 2024ggi, a Type II SN that ocurred in the galaxy NGC 3621, at a distance of 7.11 Mpc (z≈0.002435). These observations represent the earliest high-resolution spectroscopy of a CCSN ever made. We analyze the very early-phase spectroscopic evolution of SN 2024ggi obtained in a short interval at 26.6 and 33.8h after the SN first light. Observations were obtained with the high-resolution spectrograph MIKE (R≈22600-28000) at the 6.5m Magellan Clay Telescope, located at the Las Campanas Observatory, during the night of 2024-04-12UT. We constrain emission line features in the early-phase spectroscopic evolution of SN 2024ggi. We analyze the evolution of main spectroscopic features and the occurrence of high-ionization emission lines, by estimating their full width at half maximum (FWHM), equivalent width (EW), and blueshift velocities. We then compare our results to other early-time observations of CCSNe. The spectra show strong and narrow features of Balmer emission lines and of high-ionization species of HeI, HeII, NIII, CIII, together with relatively broader emission features of NIV and CIV. Some of these features become broader or disappear in the interval of 8h, indicating the rapid changes in the early evolution of CCSNe flash-ionization features. The HeII, CIV, NIV and Balmer emission lines have asymmetric Lorentzian profiles, with the HeII λ4686 broad component showing blue wings that extends up to ∼-1000 km s^-1. We also measure a CSM expansion velocity of ∼ 79 kms^-1 from the blueshift in the Hα emission profile, and a total extinction in the line of sight of E(B-V)=0.16 mag. Finally, we note many similarities of SN 2024ggi to the early evolution of SN 2023ixf.