A search for CO in the Local Group dwarf irregular galaxy WLM

Abstract

Abstract. We present 12 CO J = 1 → 0 and J = 2 → 1 observations of the low metallicity (12 + log(O/H) = 7.74) Local Group dwarf irregular galaxy WLM made with the 15 m SEST and 14 m FCRAO telescopes. Despite the presence a number of HII regions, we find no CO emission. We obtain low upper limits on the integrated intensity (ICO ≤ 0.18 K km s −1 for CO (1 → 0)). The non-detection is consistent with the result of Taylor, Kobulnicky & Skillman (1998), that dwarf galaxies below a metallicity of ∼7.9 are not detected in CO emission. WLM shows that this trend continues for low metallicity galaxies even as their metallicities approach 7.9. These results are consistent with the models of the metal poor ISM by Norman & Spaans (1997). By comparing our CO data with observations of star formation in WLM, we find evidence for a high CO to H2 conversion factor.

A High-Resolution Study of the HiContent of Local Group Dwarf Irregular Galaxy WLM

“…Like many dwarf irregulars, WLM has a low metallicity; its oxygen abundance is 12 þ log (O/H ) ¼ 7:78 8:23 (Skillman et al 1989;Hunter & Hoffman 1999), where 8.7 is solar. Given its metallicity, it is not surprising that Taylor & Klein (2001) did not detect any CO emission in this galaxy, since the CO-to-H 2 ratio is thought to decrease significantly for lowmetallicity galaxies. WLM also has a globular cluster with an age of 14:8 AE 0:6 Gyr on its western edge (Hodge et al 1999), which is very unusual for a faint dwarf irregular ( Billett et al 2002).…”

HiObservations of the Local Group Dwarf WLM

“…A possible explanation of this extinction is the presence of a large molecular gas complex in the southern half of WLM. The only search for any type of molecular gas was that of Taylor & Klein (2001). Their CO observations resulted in a non-detection, however they point out that in low metallicity systems like WLM there is evidence for a high CO to H 2 conversion factor (Taylor, Kobulnicky, & Skillman 1998).…”

“…Since its discovery (Wolf 1910;Melotte 1926), there have been numerous studies of its photometric properties, stellar populations, star formation history, and chemical abundances (van den Bergh 2000). Unfortunately, other than observations of H II regions (Hodge & Miller 1995), an unsuccessful search for CO (Taylor & Klein 2001), and studies of its most basic infrared (Melisse & Israel 1994) and radio (Huchtmeier, Seiradakis & Materne 1981;Huchtmeier & Richter 1986) properties, little has been done to study its interstellar medium.…”

Deep CO Observations and the CO-to-H2 Conversion Factor in DDO 154, a Low Metallicity Dwarf Irregular Galaxy

“…In order to gain decisive information on the metallicity dependence of X CO , observations at low metallicities is necessary. This has been a difficult task, however, because CO has failed to be detected in galaxies with metallicity lower than 12 + log[O/H] ∼ 8.0 (Taylor & Klein 2001;Leroy et al 2005). The lowest metallicity galaxy with detected CO is IZw 36, at 12 + log[O/H] ∼ 7.9 Young et al 1995 which is only tentative.…”