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

Abstract

We analyze HCN and HNC emission in the nearby starburst galaxy NGC 253 to investigate its effectiveness in tracing heating processes associated with star formation. This study uses multiple HCN and HNC rotational transitions observed using the Atacama Large Millimeter/submillimeter Array via the ALCHEMI Large Program. To understand the conditions and associated heating mechanisms within NGC 253's dense gas, we employ Bayesian nested sampling techniques applied to chemical and radiative transfer models, which are constrained using our HCN and HNC measurements. We find that the volume density n H 2 and cosmic-ray ionization rate (CRIR) ζ are enhanced by about an order of magnitude in the galaxy’s central regions as compared to those further from the nucleus. In NGC 253's central giant molecular clouds (GMCs), where observed HCN/HNC abundance ratios are the lowest, n ∼ 105.5 cm−3 and ζ ∼ 10−12 s−1 (greater than 104 times the average Galactic rate). We find a positive correlation in the association of both density and CRIR with the number of star formation-related heating sources (supernova remnants, H ii regions, and super hot cores) located in each GMC, as well as a correlation between CRIRs and supernova rates. Additionally, we see an anticorrelation between the HCN/HNC ratio and CRIR, indicating that this ratio will be lower in regions where ζ is higher. Though previous studies suggested HCN and HNC may reveal strong mechanical heating processes in NGC 253's CMZ, we find cosmic-ray heating dominates the heating budget, and mechanical heating does not play a significant role in the HCN and HNC chemistry.

PHANGS–JWST First Results: Rapid Evolution of Star Formation in the Central Molecular Gas Ring of NGC 1365

“…All of these phenomena affect the balance between the self-gravity of the molecular gas and factors (e.g., turbulence, shear, magnetic fields, tidal forces, and cosmic-ray flux) that support the gas against gravitational collapse (e.g., Chevance et al 2020;Girichidis et al 2020). The closest galaxy centers allow for high physical resolution studies to assess the impact of these environmental factors, making them unique targets for testing star formation theories (for recent examples utilizing observations of molecular gas, see, e.g., Callanan et al 2021;Levy et al 2021;Martín et al 2021;Behrens et al 2022;Eibensteiner et al 2022).…”

Tracing the chemical footprint of shocks in AGN-host and starburst galaxies with ALMA multi-line molecular studies

“…40 (submitted); (c) Scourfield et al . 2020 44 – CRIR inferred from multi-J CS emissions in the CND region of NGC 1068; (d) Holdship et al 2021; 45 (e) Harada et al 2021; 46 (f) Holdship et al 2022; 47 (g) Behrens et al 2022; 48 (h) García-Burillo et al . 2019; 3 (i) Westmoquette et al .…”

“…In particular, in the CND of NGC 1068, the inferred CRIR based on multi-transition CS observations ranges between ζ = 1.0–10.0 ζ 0 . 44 And in the CMZ of NGC 253, the measured CRIR based on multi-molecule, multi-transition observations is estimated to be ∼ ζ = 10 3–5 ζ 0 , 45–48 which is a few orders of magnitude higher than NGC 1068. A brief summary of the parameter space explored in our chemical modeling is listed in Table 2.…”

“…We perform modeling cases that cover the physical conditions measured from the CND of NGC 1068 and the CMZ of NGC 253this includes a wide CRIR range (z = 1.0-10 5 z 0 , z 0 = 1.3 × 10 −17 s −1 is the standard galactic CRIR), slow (v s = 10 km s −1 ) versus fast (v s = 50 km s −1 ) shocks, and a wide range of molecular gas densities n H 2 . The chosen CRIR range is based on the existing CRIR measurements [44][45][46][47][48] in these two extragalactic environments. In particular, in the CND of NGC 1068, the inferred CRIR based on multi-transition CS observations ranges between z = 1.0-10.0z 0 .…”

Volume Density Structure of the Central Molecular Zone NGC 253 through ALCHEMI Excitation Analysis

“…molecular gas in their central region, and the giant molecular clouds (GMCs) in their central regions are both in a barred potential and characterized by high temperature and density (Nagai et al 2007;Mangum et al 2013Mangum et al , 2019Rosenberg et al 2014;Gorski et al 2017;Pérez-Beaupuits et al 2018;Tanaka et al 2018), an elevated degree of turbulence (Tsuboi et al 1999;Shetty et al 2012;Rathborne et al 2014;Krieger et al 2020;Tanaka et al 2020;Harada et al 2022;Huang et al 2023), and enhanced cosmic-ray ionization rate (CRIR; Oka et al 2005;Goto et al 2008;Belloche et al 2013;Harada et al 2021;Holdship et al 2021Holdship et al , 2022Behrens et al 2022). However, the dense-gas content with molecular hydrogen volume density (n H 2 ) of 10 4 cm −3 of the GC is dominated by quiescent gas lacking cluster formation, in stark contrast to the NGC 253 CMZ, which harbors 14 embedded young massive clusters (Leroy et al 2018).…”