Extremely weak CO emission in IZw 18

TL;DR

This study reports an extremely weak CO emission in the metal-poor galaxy IZw 18, suggesting that CO is a poor tracer of molecular gas in such low-metallicity environments, with implications for understanding primordial galaxy ISM.

Contribution

First detection of CO $J$=2-1 emission in IZw 18 at levels 40 times lower than previous limits, revealing the scarcity of CO in extremely metal-poor galaxies.

Findings

CO emission is at least 40 times weaker than previous limits.

High [CII] luminosity indicates a larger molecular reservoir than CO suggests.

No sub-millimetre excess detected in IZw 18.

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 (NOEMA), we report a marginal detection of CO $J$=2-1 emission in IZw18, pushing the detection limit down to $L^\prime_{\rm CO(2-1)}$=3.99$\times$10$^3$ K km$^{-1}$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 [CII] 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 [CII] luminosity relative to CO implies a larger molecular reservoir than the CO emitter in IZw18. We also obtain an upper limit of the 1.3mm continuum, which excludes a sub-millimetre excess in IZw18.