Extremely weak CO emission in IZw 18
  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.
