Search for the [C{{II}}] 158 micron emission toward the blue compact dwarf galaxy I Zw 36

Abstract

We observed the blue compact dwarf galaxy I Zw 36 in the [C Ii] 158 mu m line with the Infrared Space Observatory obtaining an upper limit for the line flux, F_[ion {C{ii}]} <= 1.1 10^-20 W cm^-2 (3 sigma ). A comparison with previous CO observations yields an upper limit for the [C Ii]/CO J = 1-0 line flux ratio, F_[ion {C{ii}]} / F_CO <= 1.3 10³ (3 sigma ). This limit indicates that the [C Ii]/CO J = 1-0 line ratio in I Zw 36 is not higher than those in our Galactic plane (I_[ion {C{ii}]} / I_CO = 1.1 10³) and is at least one order of magnitude lower than those in irregular galaxies (I_[ion {C{ii}]} / I_CO =~ 2 10⁴) with low metallicities, in spite of the lower metallicity in I Zw 36 (12 + log [ O / H ] = 7.9) than in the irregulars. The observed difference in the line ratio between I Zw 36 and the irregulars can be accounted for by a higher gas density (10³ cm^-3 <~ n_H <~ 10⁴ cm^-3) in I Zw 36, because the higher CO-formation rate at the higher density enables CO molecules to survive even against the photodissociation enhanced by the low dust abundance. The expected higher gas density in I Zw 36 may be related to change in large-scale gravitational potential with galactic evolution, if blue compact dwarfs and irregulars have evolutionary links. Based on observations with ISO, an ESA project with instruments funded by ESA member States (especially the PI countries: France, Germany, the Netherlands and the UK) and with the participation of ISAS and NASA.