ALMA Observations toward the starburst dwarf galaxy NGC 5253: I. Molecular cloud properties and scaling relations

TL;DR

This study uses high-resolution ALMA CO(2--1) observations to analyze molecular cloud properties in NGC 5253, revealing unique scaling relations, high star formation efficiencies, and the impact of stellar feedback on cloud dispersal in a starburst dwarf galaxy.

Contribution

First detailed measurement of molecular cloud properties and the CO-to-H2 conversion factor in NGC 5253, highlighting differences from quiescent galaxies and insights into starburst-driven feedback.

Findings

Identified 118 molecular clouds with typical radii of 4.3 pc.

Derived the CO-to-H2 conversion factor as 4.1×10^{20} cm^{-2}(K km/s)^{-1}.

Found higher line-widths and mass surface densities near the galaxy's center.

Abstract

We present high-spatial-resolution ($\sim 0\farcs2$, or $\sim$3\,pc) CO(2--1) observations of the nearest young starburst dwarf galaxy, NGC\,5253, taken with the Atacama Large Millimeter/submillimeter Array. We have identified 118 molecular clouds with average values of 4.3\,pc in radius and 2.2\,\kms\, in velocity dispersion, which comprise the molecular cloud complexes observed previously with $\sim$100\,pc resolution. We derive for the first time in this galaxy the $I{\rm (CO)}$--$N$(H$_2$) conversion factor, $X$ = $4.1^{+5.9}_{-2.4}\times10^{20}$\,cm$^{-2}$(K\,\kms)$^{-1}$, based on the virial method. The line-width and mass-to-size relations of the resolved molecular clouds present an offset on average toward higher line-widths and masses with respect to quiescent regions in other nearby spiral galaxies and our Galaxy. The offset in the scaling relation reaches its maximum in regions close to the central starburst, where velocity dispersions are $\sim$ 0.5 dex higher and gas mass surface densities are as high as $\Sigma_{\rm H_2}$ = 10$^3$\,\Msol\,pc $^{-2}$. These central clouds are gravitationally bound despite the high internal pressure. A spatial comparison with star clusters found in the literature enables us to identify six clouds that are associated with young star clusters. Furthermore, the star formation efficiencies (SFEs) of some of these clouds exceed those found in star-cluster-forming clouds within our Galaxy. We conclude that once a super star cluster is formed, the parent molecular clouds are rapidly dispersed by the destructive stellar feedback, which results in such a high SFE in the central starburst of NGC\,5253.