Letter to the Editor

Extremely weak CO emission in IZw 18

¹ School of Astronomy and Space Science, Nanjing University, Nanjing 210093, PR China
e-mail: wenjia@nju.edu.cn, yshipku@gmail.com
² Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing 210093, PR China
³ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China

Received: 26 July 2020
Accepted: 9 September 2021

Abstract

Local metal-poor galaxies are ideal analogues of primordial galaxies with the interstellar medium (ISM) barely being enriched with metals. However, it is unclear whether carbon monoxide remains a good tracer and coolant of molecular gas at low metallicity. Based on the observation with the upgraded Northern Extended Millimeter Array, we report a marginal detection of CO J = 2–1 emission in IZw18, pushing the detection limit down to L^′_CO(2-1) = 3.99 × 10³ K km s⁻¹ pc⁻², which is at least 40 times lower than previous studies. As one of the most metal-poor galaxies, IZw18 shows extremely low CO content despite its vigorous star formation activity. Such low CO content relative to its infrared luminosity, star formation rate, and [C II] luminosity, compared with other galaxies, indicates a significant change in the ISM properties at a few percent of the Solar metallicity. In particular, the high [C II] luminosity relative to CO implies a larger molecular reservoir than the CO emitter in IZw18. We also obtain an upper limit of the 1.3 mm continuum, which excludes a sub-millimetre excess in IZw18.