ALCHEMI Finds a “Shocking” Carbon Footprint in the Starburst Galaxy NGC 253

The centers of starburst galaxies may be characterized by a specific gas and ice chemistry due to their gas dynamics and the presence of various ice desorption mechanisms. This may result in a peculiar observable composition. We analyse the abundances of CO₂, a reliable tracer of ice chemistry, from data collected as part of the Atacama Large Millimeter/submillimeter Array large program ALCHEMI, a wide-frequency spectral scan toward the starburst galaxy NGC 253 with an angular resolution of 1.″6. We constrain the CO₂ abundances in the gas phase using its protonated form HOCO⁺. The distribution of HOCO⁺ is similar to that of methanol, which suggests that HOCO⁺ is indeed produced from the protonation of CO₂ sublimated from ice. The HOCO⁺ fractional abundances are found to be (1-2) × 10⁻⁹ at the outer part of the central molecular zone (CMZ), while they are lower (∼10⁻¹⁰) near the kinematic center. This peak fractional abundance at the outer CMZ is comparable to that in the Milky Way CMZ, and orders of magnitude higher than that in Galactic disk, star-forming regions. From the range of HOCO⁺/CO₂ ratios suggested from chemical models, the gas-phase CO₂ fractional abundance is estimated to be (1-20) × 10⁻⁷ at the outer CMZ, and orders of magnitude lower near the center. We estimate the CO₂ ice fractional abundances at the outer CMZ to be (2-5) × 10⁻⁶ from the literature. A comparison between the ice and gas CO₂ abundances suggests an efficient sublimation mechanism. This sublimation is attributed to large-scale shocks at the orbital intersections of the bar and CMZ.