science highlights ngc 5253

One of the areas of extragalactic research which makes great use of ALMA's resolution and sensitivity is the study of the molecular gas properties of dwarf galaxies. In a recent study by Dr. Jean Turner and her collaborators, they make use of Band 7 ALMA observations to detect warm ¹²CO(3-2) and ¹³CO(3-2) emission (Cloud D1) from the core of a giant star-forming region, in the dwarf galaxy NGC 5253. This "supernebula" is the source of one-third of the galaxy’s infrared luminosity and is in proximity to optical clusters with measured stellar ages of ~ 1 Myr. From radio recombination line analysis, the region is estimated to have 1400-1800 O stars. The authors use the CO data to measure a cloud radius of 2.8 pc and a dynamical mass of 2.5x10⁹ solar masses. The overall gas mass content of this stellar mass-dominated region is estimated to be < 35%, and the derived mass-to-light ratio is consistent with a top heavy IMF. Comparisons with the infrared luminosity-to-molecular gas mass ratio (= the luminosity of massive, imbedded young stars per unit star-forming molecular gas mass) of its closest Galactic analog, W49N, shows the NGC 5253 supernebula region to be ~ 50 times more efficient at forming stars.

Fig 1. ALMA CO J=3-2 emission in NGC 5253, in color and contours, superimposed on an archival HST Hα image. The ALMA beam is 0.33" × 0.27", p.a. -90.0 (6 pc × 5 pc). The larger CO cloud to the southwest of D1 is ∼15-20 pc away and redshifted with respect to D1. Cloud D1, which is the focus of this paper, is coincident with the radio continuum nebula emission.