# The Low CO Content of the Extremely Metal Poor Galaxy I Zw 18

## TL;DR

This study provides sensitive measurements showing that the extremely metal-poor galaxy I Zw 18 has an exceptionally low CO luminosity, indicating a very low molecular gas content relative to its star formation activity.

## Contribution

First sensitive CO observations of I Zw 18 reveal its CO luminosity is much lower than expected, suggesting a very low CO-to-H2 ratio in this metal-poor galaxy.

## Key findings

- CO luminosity is an order of magnitude lower than previous limits.
- I Zw 18's CO content is comparable to or less than that of nearby low-mass irregulars.
- The galaxy likely has a CO-to-H2 ratio about 100 times lower than the Milky Way.

## Abstract

We present sensitive molecular line observations of the metal-poor blue compact dwarf I Zw 18 obtained with the IRAM Plateau de Bure interferometer. These data constrain the CO J=1-0 luminosity within our 300 pc (FWHM) beam to be L_CO < 1 \times 10^5 K km s^-1 pc^2 (I_CO < 1 K km s^-1), an order of magnitude lower than previous limits. Although I Zw 18 is starbursting, it has a CO luminosity similar to or less than nearby low-mass irregulars (e.g. NGC 1569, the SMC, and NGC 6822). There is less CO in I Zw 18 relative to its B-band luminosity, HI mass, or star formation rate than in spiral or dwarf starburst galaxies (including the nearby dwarf starburst IC 10). Comparing the star formation rate to our CO upper limit reveals that unless molecular gas forms stars much more efficiently in I Zw 18 than in our own galaxy, it must have a very low CO-to-H_2 ratio, \sim 10^-2 times the Galactic value. We detect 3mm continuum emission, presumably due to thermal dust and free-free emission, towards the radio peak.