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

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.

5 pages in emulateapj style, accepted by the Astrophysical Journal

References in corpus (2)

• Molecular line intensities as measures of cloud masses - I. Sensitivity of CO emissions to physical parameter variations
• Molecular Gas in the Low Metallicity, Star Forming Dwarf IC 10

Cited by in corpus (9)

• The Behavior of the Aromatic Features in M101 HII Regions: Evidence for Dust Processing
• Dust and Gas in the Magellanic Clouds from the HERITAGE Herschel Key Project. I. Dust Properties and Insights into the Origin of the Submm Excess Emission
• Revisiting the Integrated Star Formation Law. Paper I: Non-Starbursting Galaxies
• The Weak Carbon Monoxide Emission In An Extremely Metal Poor Galaxy, Sextans A
• Heating of blue compact dwarf galaxies: gas distribution and photoionization by stars in I Zw 18
• Suppressed CO emission and high G/D ratios in z=2 galaxies with sub-solar gas-phase metallicity
• Extremely weak CO emission in IZw 18
• Herschel 158$\mu$m [CII] Observations of "CO-dark" Gas in the Perseus Giant Molecular Cloud
• Weak CS Emission in an Extremely Metal-poor Galaxy DDO 70