Extremely weak CO emission in IZw 18. (arXiv:2109.07856v1 [astro-ph.GA])
<a href="http://arxiv.org/find/astro-ph/1/au:+Zhou_L/0/1/0/all/0/1">Luwenjia Zhou</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Shi_Y/0/1/0/all/0/1">Yong Shi</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Zhang_Z/0/1/0/all/0/1">Zhi-Yu Zhang</a>, <a href="http://arxiv.org/find/astro-ph/1/au:+Wang_J/0/1/0/all/0/1">Junzhi Wang</a>

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.

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.

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