SIMBAD references

The metal-deficient starburst galaxy I Zw 18 has been observed with the Far Ultraviolet Spectroscopic Explorer (FUSE) in a search for H₂ molecules. The spectrum obtained with an aperture covering the full galaxy shows no absorption lines of diffuse H₂ at the radial velocity of the galaxy. The upper limit for the diffuse H₂ column density is found to be very low, N(H₂)≲10¹⁵ cm^–2 (10 σ), unlike our Galaxy where H₂ is generally present for even low H I column densities. Although the H I column density here is as high as N(H I) ~ 2x10²¹ cm^–2, we observe 2N(H₂)/N(H I) ≲ 10^–6. We cannot exclude the possibility that some H₂ could be in very dense, small, and discrete clumps that cannot be detected with the present observation. However, the remarkable absence of diffuse H₂ in this metal-poor galaxy can be explained by the low abundance of dust grains (needed to form this molecule from H atoms), the high ultraviolet flux, and the low density of the H I cloud surrounding the star-forming regions. Thus, having eliminated diffuse H₂ as a significant contributor to the total mass, it appears that the gas of the galaxy is dominated by H I and that the high dynamical mass is not composed of cold and diffuse baryonic dark matter.