NGC 4449 is a prototypical dwarf irregular galaxy of the Magellanic type, located at a distance of 3.7 Mpc (Bajaja et al., 1994). It is the best studied low-mass galaxy beyond the Magellanic Clouds, with a host of data available over essentially the whole electromagnetic spectrum (citing even the most relevant papers would be beyond the scope of this contribution). The only gap of missing information was left in the mm/submm window (exept for measurements at selected positions), which was recently filled by completely mapping the CO line (IRAM 30-m) and the dust continuum (JCMT).

Low-metallicity galaxies such as NGC 4449 are generally deemed dust-poor, but quantitative answers to this conjecture have not been given so far. We have therefore embarked on a project to fully map the molecular gas and dust in this template galaxy. SCUBA observations were carried out in January 1999. A total integration time of 2 hours was obtained to yield final maps at 850  and 450 um, respectively. The dust emission is clearly seen in both maps. In Fig.1 we show contours of the 850 um continuum, superimposed onto a grey-scale representation of the Halpha emission. It is evident that the strongest dust emission follows the distribution of intense star formation (as expected). There is also significant dust emission away from the star-forming regions, but this is also accompanied by CO radiation. Fig.2 shows that there is an overall correspondence of dust and molecular gas, though naturally not obeying a one-to-one relation. The big caveat here is that the CO emissivity depends on local excitation conditions (stellar radiation field, X-ray and CR heating). However, since dust and molecular hydrogen are closely coupled, the data presented here provide an important basis for a study of the molecular gas. This is facilitated by the comprehensive data base that we have at our disposal: HI, 850 and 450 um, CO(1-0) and (2-1), radio continuum at several frequencies, X-rays, H alpha, optical and UV.

A couple of relevant numbers resulting from our ongoing study are worth being mentioned here. The total flux densities at 850 and 450 um are S₈₅₀ =1.5 +/- 0.2 Jy (after corrections for CO) and S₄₅₀ =16.5 +/- 3.3 Jy, respectively. We fitted a three-component dust model to the FIR distribution (Fig.3) and get temperatures of 16, 38 and 170K, resulting in a total dust mass of M_d ~ 1.7 ^. 10⁶ M_o , where an  absorption coefficient of k_d850 = 1.5 cm² g^-1 has been adopted. The CO luminosity of L_CO = 7.9 ^. 10⁶ K km s^-1 pc² leads to a total H₂ mass of M_H2 = 4.4 ^. 10⁸ M_o , with a conversion ratio of CO intensity to H₂ columm density of X_CO ~ 14 X_gal , with X_gal =2.8 ^. 10²⁰ cm^-2 (K km s^-1)^-1 (Kohle 1999). Combining this with the HI mass in the same integration area, this yields a total gas mass of M_HI+H2 =6 ^. 10⁸ M_o . The resulting gas-to-dust ratio is ~ 400. This is  a factor of ~ 3 higher than the canonical value of  ~ 150 (e.g. Schmitd & Boller, 1993) for the Milky Way. It is in accord with the lower metallicity of NGC 4449, viz 12+log(O/H) = 8.3 (Lequeux et al., 1979), a factor of  ~ 3 below the Milky Way value.

Finally, we point out that studies of nearby low-metallicity galaxies (i.e. dwarfs) may provide important clues to a proper interpretation of galaxy observations at cosmological redshifts, hence to a better understanding of galaxy evolution.
 

Acknowledgements

We are grateful to Gerald Moriarty-Schieven for performing the observations.
 

References

Bajaja, E., Huchtmeier, W. K., Klein, U., 1994 A&A 285, 385

Hunter, D. A., Gillett, F. C., Gallagher, J. S., Rice, W. L., Low, F. J., 1986, ApJ 303, 171

Kohle S., 1999 Ph.D. thesis, University of Bonn

Kohle S., Klein U., Henkel C.: 2000, A&A subm.

Lequeux, J., Peimbert, M., Rayo, J. F., Serrano, A., Torres-Peimbert, S., 1979, A&A 80, 155

Schmidt, K.-H., Boller, T., 1993, Astron. Nachr. 314, 361

Thronson, H. A., Hunter, D. A., Telesco, C. M., Harper, D. A., Decher, R., 1987, ApJ 317, 180
 

Figures