Observations of Cold and Warm CO in the Irregular Galaxy NGC 4449

Abstract

We present observations of cold 12CO (1–0) in H I concentrations located in and around the peculiar irregular galaxy NGC 4449. The observed positions range from the center out to 18 kpc, or nearly 4 times the Holmberg radius, but most are in the large H I complexes that encircle the optical galaxy. We have detected CO (1–0) in two positions outside the center of the galaxy, including one that has no detectable sign of star formation. No CO was detected in the H I concentrations located far from the galaxy center. In general, CO emission was detected at positions where approximately σHI,max ≥ 2 × 1021 atoms cm-2, a factor of 3 higher than expected for self-shielding for the metallicity of the galaxy, and R ≤ 5 kpc from the center of the galaxy. In the H I complexes surrounding the galaxy, M/MHI,max is approximately constant. In the center of the galaxy, there is more Hα emission relative to M than in the outer H I complexes. Generally, there is no relationship between the presence or amount of Hα emission and M. We have also presented observations of warm molecular 12CO (2–1) gas in the center of the galaxy where CO (1–0) had already been detected. In the center of the galaxy, the ratio ICO (2–1)/ICO (1–0) is unusually high, indicating the presence of a significant amount of hot, optically thin gas. This is generally consistent with the unusual activity that has taken place in the central regions of the galaxy, but the ratio is higher than is observed in other galaxies with high star formation rates.