Molecular gas in the outer disks of galaxies

Abstract

Molecular gas has still only been detected beyond the R₂₅ radius in a few galaxies. Is this due to the low H₂ content or to the difficulty of using Carbon Monoxide (CO) to trace H₂? Similarly, star formation (SF) decreases sharply in the outer disks of spirals although HI is often plentiful; is the decrease in SF because there is little H₂ or because the SF is very inefficient in the outer disk environment?

Existing observations suggest that while outer disk clouds tend to be smaller (steeper mass function), their CO brightness temperature is only slightly lower than in the inner disk, at least when observed with sufficiently high angular resolution. In near-solar metallicity galaxies (Z ⩾ 0.5Z_sol), the CO does not become intrinsically difficult to detect when H₂ is present, even in the outer disk. While more observations of CO or other means of tracing H₂ in the outer disks are necessary, current data tend to show that the SF rate per unit H² remains approximately constant with galactocentric distance, indicating that the star formation proceeds normally but the transformation of HI into H₂ is very slow in the outer disk.