Search for the [CII] 158 micron emission toward the blue compact dwarf galaxy I Zw 36^*

Department of Astronomy, University of Tokyo, Japan

Corresponding author: K. Mochizuki, mochi@ir.isas.ac.jp

Received: 2 August 2000
Accepted: 15 February 2001

Abstract

We observed the blue compact dwarf galaxy I Zw 36 in the [Cii] line with the Infrared Space Observatory obtaining an upper limit for the line flux, (). A comparison with previous CO observations yields an upper limit for the [Cii]/CO -0 line flux ratio, (). This limit indicates that the [Cii]/CO -0 line ratio in I Zw 36 is not higher than those in our Galactic plane () and is at least one order of magnitude lower than those in irregular galaxies () with low metallicities, in spite of the lower metallicity in I Zw 36 () 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 () 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.