Starburst Energy Feedback Seen through HCO⁺/HOC⁺ Emission in NGC 253 from ALCHEMI

Abstract

Molecular abundances are sensitive to the UV photon flux and cosmic-ray ionization rate. In starburst environments, the effects of high-energy photons and particles are expected to be stronger. We examine these astrochemical signatures through multiple transitions of HCO+ and its metastable isomer HOC+ in the center of the starburst galaxy NGC 253 using data from the Atacama Large Millimeter/submillimeter Array large program ALMA Comprehensive High-resolution Extragalactic Molecular inventory. The distribution of the HOC+(1−0) integrated intensity shows its association with “superbubbles,” cavities created either by supernovae or expanding H ii regions. The observed HCO+/HOC+ abundance ratios are ∼10–150, and the fractional abundance of HOC+ relative to H2 is ∼1.5 × 10−11–6 × 10−10, which implies that the HOC+ abundance in the center of NGC 253 is significantly higher than in quiescent spiral arm dark clouds in the Galaxy and the Galactic center clouds. Comparison with chemical models implies either an interstellar radiation field of G 0 ≳ 103 if the maximum visual extinction is ≳5, or a cosmic-ray ionization rate of ζ ≳ 10−14 s−1 (3–4 orders of magnitude higher than that within clouds in the Galactic spiral arms) to reproduce the observed results. From the difference in formation routes of HOC+, we propose that a low-excitation line of HOC+ traces cosmic-ray dominated regions, while high-excitation lines trace photodissociation regions. Our results suggest that the interstellar medium in the center of NGC 253 is significantly affected by energy input from UV photons and cosmic rays, sources of energy feedback.

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

“…The column densities we obtain are typically an order of magnitude lower than those obtained from measurements of the dust (Mangum et al 2019). However, they are consistent with the H 2 column densities derived from observations of C 18 O (Harada et al 2021). This agreement is important as RADEX relies on species column densities.…”

“…The range of likely density values for each region overlaps very well with those derived from the ALCHEMI observations of C 2 H (Holdship et al 2021). They also fall well within the range of 1 × 10 5 -1 × 10 6 cm −3 found in other ALCHEMI work (Harada et al 2021) and elsewhere (Leroy et al 2018). Given how strongly correlated ζ is with this parameter, it is key that these values are accurate.…”

“…However, an attempt to use chemical modeling to measure the CRIR found that it could only be constrained in the range 1 × 10 −14 -1 × 10 −11 s −1 due to the low sensitivity of the C 2 H abundance to the CRIR. Meanwhile, Harada et al (2021) found that HCO + and HOC + measurements indicated a CRIR >1 × 10 −14 s −1 confirming-but not further constraining-the previous measurement. Despite these large uncertainties, it is clear the CRIR in these regions is much higher than the Galactic value, which is typically measured to be in the range of 1 × 10 −17 -10 × 10 −17 s −1 (Padovani et al 2009;Indriolo et al 2015).…”

“…We obtain a large CRIR that is nevertheless consistent with previous measurements. In previous ALCHEMI studies, Harada et al (2021) found that the CRIR must be larger than 1 × 10 3 ζ 0 across the GMCs in the CMZ of NGC 253 for chemical models to adequately explain the measured HCO + and HOC + emission. Holdship et al (2021) also find that a CRIR of ζ = 1 × 10 3 -1 × 10 6 ζ 0 is required to explain high column densities of C 2 H. With H 3 O + and SO, we are now able to constrain the CRIR to within an order of magnitude for the first time, with all GMCs having a CRIR ∼1 × 10 4 ζ 0 or 1 × 10 −13 s −1 .…”

“…Finally, a prior distribution p(θ) must be chosen. The large amount of previous work on NGC 253 provides a fantastic opportunity to set informed priors on the density (Leroy et al 2018), column density (Mangum et al 2019), and CRIR (Harada et al 2021;Holdship et al 2021). Despite this, we chose in the first instance to use uninformative priors that are uniform within limits based on those works.…”