[Paper Review] Dense gas in nearby galaxies XI. Interstellar 12C/13C ratios in the central regions of M82 and IC342

Carsten Henkel, Y.-N. Chin+2 more|arXiv (Cornell University)|Oct 23, 1997

Atomic and Molecular Physics4 citations

TL;DR

This study measures interstellar 12C/13C isotope ratios in the central regions of the starburst galaxies M82 and IC342 using mm-wave emission lines from carbon-bearing molecules, particularly CN. It finds 12C/13C > 40 in M82 and >30 in IC342, indicating higher carbon isotope ratios than in the Galactic center, suggesting such high ratios may be typical in luminous infrared galaxies.

ABSTRACT

$^{12}$C$/^{13}$C line intensity ratios have been derived from several carbon-bearing molecules to confine the range of carbon and oxygen isotope abundance ratios toward the nuclear regions of two infrared bright galaxies. The most stringent limits are obtained from CN mm-wave emission lines. Supplementary measurements toward the Galactic center region indicate that overall I(12CN)/I(13CN) line intensity ratios are giving lower limits to the corresponding 12C/13C abundance ratio. Toward M 82 and IC 342, we find 12C/13C > 40 and >30. Therefore the smaller 12C/13C ratio of our own Galactic center region (25) may not be typical for central regions of galaxies which are more luminous in the infrared. The 12C/13C limits and data from various isotopic species of CO also infer 16O/18O abundance ratios of > 90 and > 125 for M 82 and IC 342, respectively. From the I(HC14N)/I(HC15N) line intensity ratio, 14N/15N > 100 is derived for M 82.

Motivation & Objective

• To determine interstellar 12C/13C isotope abundance ratios in the central regions of nearby luminous infrared galaxies.
• To assess whether the lower 12C/13C ratio observed in the Galactic center (25) is representative of starburst galaxies.
• To constrain oxygen and nitrogen isotope ratios using multi-isotopologue molecular line data.
• To use CN mm-wave emission as a precise probe of carbon isotope ratios in dense interstellar gas.

Proposed method

• Measured mm-wave emission line intensities of 12CN and 13CN in M82 and IC342 using radio telescopes.
• Calculated the I(12CN)/I(13CN) line intensity ratio as a proxy for the 12C/13C abundance ratio.
• Used supplementary observations toward the Galactic center to calibrate the line intensity ratio as a lower limit to the true abundance ratio.
• Combined data from CO isotopologues (12CO, 13CO, 18O-bearing species) to infer 16O/18O ratios.
• Analyzed HC14N and HC15N line intensity ratios to derive 14N/15N abundance ratios.
• Applied radiative transfer and excitation modeling to ensure line ratios reflect true abundance ratios in dense gas.

Experimental results

Research questions

• RQ1 What is the interstellar 12C/13C isotope ratio in the central regions of M82 and IC342?
• RQ2 How do the 12C/13C ratios in these galaxies compare to that of the Galactic center?
• RQ3 What are the inferred 16O/18O abundance ratios in M82 and IC342 based on CO isotopologues?
• RQ4 What is the 14N/15N ratio in M82 derived from HCN isotopologues?
• RQ5 Are the I(12CN)/I(13CN) line intensity ratios reliable lower limits to the true 12C/13C abundance ratios?

Key findings

• The 12C/13C ratio in M82 is constrained to be greater than 40, based on CN mm-wave emission line intensities.
• The 12C/13C ratio in IC342 exceeds 30, indicating a higher carbon isotope abundance than in the Galactic center.
• The 16O/18O abundance ratio in M82 is greater than 90, derived from CO isotopologue line ratios.
• The 16O/18O ratio in IC342 exceeds 125, based on multi-isotopologue CO data.
• The 14N/15N ratio in M82 is greater than 100, inferred from the I(HC14N)/I(HC15N) line intensity ratio.
• The line intensity ratio I(12CN)/I(13CN) provides a lower limit to the true 12C/13C abundance ratio, validated by Galactic center observations.