CHEMICAL ABUNDANCE OF CO AND HCO+ IN NGC 253

We present a study of the molecular gas distribution in the nearby starburst galaxy NGC 253 using observations from the Atacama Large Millimeter/submillimeter Array (ALMA) as part of the ALMA Comprehensive High-Resolution Extragalactic Molecular Inventory (ALCHEMI) Large Program. The dataset includes observations of the CO (1--0), (2--1), (3--2) and HCO+ (1--0), (2--1), (3--2), (4--3) transitions. We generate integrated intensity (moment-0) maps for each transition and we selected three representative regions of low, medium, and high brightness for spectral extraction. We studied CO and HCO+ to distinguish between neutral and ionic gas components.
By comparing the observed line intensities and CO/HCO+ ratios with a grid of non-LTE radiative transfer models, we constrained the physical conditions of the gas across the different regions. We find that CO tends to trace lower-density gas (with nH2 ~ 2.78 x 10^3 cm^-3 in all regions and Tkin reaching 206 K), while HCO+ traces slightly denser and warmer gas, especially in the medium- and high-brightness regions (with nH2 up to 7.74 x 10^3 cm^-3 and Tkin reaching 283 K). The highest inferred abundance of HCO+ (3.59 x 10^-7) is found in the low-emission region, though this may reflect modeling limitations or visual region selection biases. Our findings highlight how chemical and physical conditions vary across NGC 253’s central molecular zone and underscore the need to account for optical depth effects and photoionization-driven chemistry in interpreting molecular line emission.