Several neutral species ( Mg i , Si i , Ca i , Fe i ) have been detected in a weak Mg ii absorption line system ( W _ { r } ( 2796 ) \sim 0.15 ~ { } Å ) at z \sim 0.45 along the sightline toward HE0001-2340 .
These observations require extreme physical conditions , as noted in D ’ Odorico ( 14 ) .
We place further constraints on the properties of this system by running a wide grid of photoionization models , determining that the absorbing cloud that produces the neutral absorption is extremely dense ( \sim 100 - 1000 cm ^ { -3 } ) , cold ( < 100 K ) , and has significant molecular content ( \sim 72 - 94 \% ) .
Structures of this size and temperature have been detected in Milky Way CO surveys , and have been predicted in hydrodynamic simulations of turbulent gas .
In order to explain the observed line profiles in all neutral and singly ionized chemical transitions , the lines must suffer from unresolved saturation and/or the absorber must partially cover the broad emission line region of the background quasar .
In addition to this highly unusual cloud , three other ordinary weak Mg ii clouds ( within densities of \sim 0.005 cm ^ { -3 } and temperatures of \sim 10000 K ) lie within 500 km s ^ { -1 } along the same sightline .
We suggest that the “ bare molecular cloud ” , which appears to reside outside of a galaxy disk , may have had in situ star formation and may evolve into an ordinary weak Mg ii absorbing cloud .