ALMA $^{12}$CO (J=1--0) imaging of the nearby galaxy M83: Variations in the efficiency of star formation in giant molecular clouds

Abstract

We present results of the $^{12}$CO (1--0) mosaic observations of the nearby barred-spiral galaxy M83 obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). The total flux is recovered by combining the ALMA data with single-dish data obtained using the Nobeyama 45-m telescope. The combined map covers a $\sim$13 kpc$^{2}$ field that includes the galactic center, eastern bar, and spiral arm with a resolution of \timeform{2''.03} $\times$ \timeform{1''.1} ($\sim$45 pc $\times$ $\sim$25 pc). With a resolution comparable to typical sizes of giant molecular clouds (GMCs), the CO distribution in the bar and arm is resolved into many clumpy peaks that form ridge-like structures. Remarkably, in the eastern arm, the CO peaks form two arc-shaped ridges that run along the arm and exhibit a distinct difference in the activity of star formation: the one on the leading side has numerous HII regions associated with it, whereas the other one on the trailing side has only a few. To see whether GMCs form stars with uniform star formation efficiency (SFE) per free-fall time (SFEff), GMCs are identified from the data cube and then cross-matched with the catalog of HII regions to estimate the star formation rate for each of them. 179 GMCs with a median mass of 1.6 $\times$ 10$^{6}$ $M_{\odot}$ are identified. The mass-weighted average SFEff of the GMCs is $\sim$9.4 $\times$ 10$^{-3}$, which is in agreement with models of turbulence regulated star formation. Meanwhile, we find that SFEff is not universal within the mapped region. In particular, one of the arm ridges shows a high SFEff with a mass-weighted value of $\sim$2.7 $\times$ 10$^{-2}$, which is higher by more than a factor of 5 compared to the inter-arm regions. This large regional variation in SFEff favors the recent interpretation that GMCs do not form stars at a constant rate within their lifetime.