The Star Formation–Gas Density Relation in Four Galactic GMCs: Effects of Stellar Feedback

Abstract

We present maps of four Galactic giant molecular clouds (GMCs) in the J = 2-1 emission of both CO and 13 CO. We use an LTE analysis to derive maps of the CO excitation temperature and column density and the distribution of total molecular gas column density, Σ gas . The depletion of CO by freeze-out onto cold dust grains is accounted for by an approximation to the results of Lewis et al., which were derived from far-IR observations with Herschel. The surface density of young stellar objects (YSOs) is obtained from published catalogs. The mean YSO surface density exhibits a power-law dependence on Σ gas , with exponents in the range 0.9–1.9. Gas column density probability distribution functions show power-law tails extending to high column densities. The distributions of sonic Mach number, M S , are sharply peaked at M S ∼ 5–8 for 3 GMCs; a fourth has a broad distribution up to M S = 30, possibly a result of feedback effects from multiple OB stars. An analysis following the methodology of Pokhrel et al. finds that our sample of GMCs shows power-law relations that are somewhat shallower than those found by Pokhrel et al. for the star formation rate versus 〈Σ gas 〉 and versus 〈Σ gas 〉/ t ff in a different sample of clouds. We discuss possible differences in the two samples of star-forming clouds and the effects of stellar feedback on the relation between gas density and star formation rate.