Energizing Star Formation: The Cosmic-Ray Ionization Rate in NGC 253 Derived from ALCHEMI Measurements of H₃O⁺ and SO

Abstract

The cosmic-ray ionization rate (CRIR) is a key parameter in understanding the physical and chemical processes in the interstellar medium. Cosmic rays are a significant source of energy in star formation regions, impacting the physical and chemical processes that drive the formation of stars. Previous studies of the circum-molecular zone of the starburst galaxy NGC 253 have found evidence for a high CRIR value: 103–106 times the average CRIR within the Milky Way. This is a broad constraint, and one goal of this study is to determine this value with much higher precision. We exploit ALMA observations toward the central molecular zone of NGC 253 to measure the CRIR. We first demonstrate that the abundance ratio of H3O+ and SO is strongly sensitive to the CRIR. We then combine chemical and radiative transfer models with nested sampling to infer the gas properties and CRIR of several star-forming regions in NGC 253 from emission from their transitions. We find that each of the four regions modeled has a CRIR in the range (1–80) × 10−14 s−1 and that this result adequately fits the abundances of other species that are believed to be sensitive to cosmic rays, including C2H, HCO+, HOC+, and CO. From shock and photon-dominated/X-ray dominated region models, we further find that neither UV-/X-ray-driven nor shock-dominated chemistry is a viable single alternative as none of these processes can adequately fit the abundances of all of these species.

Tracing Interstellar Heating: An ALCHEMI Measurement of the HCN Isomers in NGC 253

“…It is also important to note that we only consider excitation through collisions with H 2 and therefore ignore electron collisions. A previous ALCHEMI-based study (Holdship et al 2022) found that even at the CRIRs, which will be discussed later in this article (Section 4.4), almost all hydrogen is in its molecular form under these conditions. Holdship et al (2022) found that toward GMCs 3-7 that the fractional abundance of electrons is in the range X (e −1 ) ∼ 10 −4 -10 −5 for volume densities n(H 2 )  10 5.5 cm −3 .…”

“…Only the HNC 4-3 transition is found to have spectral neighbors, which required assessment of the amount of emission contributed by H 2 CO 5 05 -4 04 (362.736048 GHz) and HNC 4-3 v 2 = 1 (362.554351 GHz). Using the procedure described in Holdship et al (2022) we determine that these two interlopers contribute, respectively, at most 4% and 1% to the HNC 4-3 integrated emission. This contamination estimate is consistent with the multispecies LTE analysis of molecular column densities described in Martín et al (2021).…”

“…This paper is one in a series of ALCHEMI projects that analyzes the conditions in the NGC 253 CMZ using molecular signatures (Martín et al 2021;Harada et al 2021;Holdship et al 2021;Haasler et al 2022;Holdship et al 2022;Humire et al 2022). Additionally, in this paper we explore how molecular emission can trace heating processes associated with star formation in this active environment.…”

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

“…Figure 6(b) shows the HOCO + /CO 2 abundance ratios in the gas phase, that vary between 10 −5 and 10 −2 for the most part. Previous ALCHEMI studies have suggested that the cosmicray ionization rates in NGC 253 are at least a few orders of magnitude higher than that in Galactic spiral-arm clouds (Holdship et al 2021;Harada et al 2021;Holdship et al 2022), which increases the HOCO + /CO 2 abundance ratios due to an increased + H 3 abundance. Although the cosmic-ray ionization rates are expected to be high, extremely high rates (ζ  10 −13 s −1 for n = 10 5 cm −3 ) would destroy HOCO + (Figure 6(c)), especially in lower-density regions.…”

“…ALCHEMI is a widefrequency, unbiased spectral scan mosaic toward the CMZ of NGC 253 at a common 1 6 resolution. This survey has discovered high cosmic-ray ionization rates (Holdship et al 2021;Harada et al 2021;Holdship et al 2022), detected a phosphorus-bearing species for the first time in an extragalactic source (Haasler et al 2022), and detected methanol masers (Humire et al 2022) in the CMZ of NGC 253. We utilize the ALCHEMI data to study the multiple transitions of HOCO + .…”