EDP Sciences
Free access
Volume 370, Number 3, May II 2001
Page(s) 868 - 874
Section Extragalactic astronomy
DOI http://dx.doi.org/10.1051/0004-6361:20010281

A&A 370, 868-874 (2001)
DOI: 10.1051/0004-6361:20010281

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

K. Mochizuki and T. Onaka

Department of Astronomy, University of Tokyo, Japan

(Received 2 August 2000 / Accepted 15 February 2001 )

We observed the blue compact dwarf galaxy I Zw 36 in the [CII] $158 \mu \mathrm m$ line with the Infrared Space Observatory obtaining an upper limit for the line flux, $F_{\rm [\ion{C}{ii}]} \leq 1.1 10^{-20} \rm W cm^{-2}$ ($3 \sigma$). A comparison with previous CO observations yields an upper limit for the [CII]/CO J = 1-0 line flux ratio, $F_{\rm [\ion{C}{ii}]} / F_{\rm CO} \leq 1.3 10^3$ ($3 \sigma$). This limit indicates that the [CII]/CO J = 1-0 line ratio in I Zw 36 is not higher than those in our Galactic plane ( $I_{\rm [\ion{C}{ii}]} / I_{\rm CO} = 1.1 10^3$) and is at least one order of magnitude lower than those in irregular galaxies ( $I_{\rm [\ion{C}{ii}]} / I_{\rm CO} \simeq 2 10^4$) with low metallicities, in spite of the lower metallicity in I Zw 36 ( $\rm 12 + \log[ O / H ] = 7.9$) 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 ( $10^3 {\rm cm}^{- 3} \la n_{\rm H} \la 10^4 {\rm cm}^{- 3}$) 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.

Key words: ISM: clouds -- dust, extinction -- galaxies: abundances -- galaxies: dwarf -- galaxies: individual: I Zw 36 -- infrared: galaxies

Offprint request: K. Mochizuki, mochi@ir.isas.ac.jp

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