Bound Gas, Dense Gas, and Star Formation: A deceptively simple braid

 Most molecular clouds are observed to be bound alpha_virial < 1-2 but the overall star formation efficiency per free-fall time (e_ff) is observed to be 0.01. Some observations also indicate that star formation is less efficient in higher surface density regions. I present intuitive reasons explaining both of these unintuitive points, based on identifying bound and/or dense gas in galactic simulations. Gas can appear bound without being bound, for example due to tidal effects. Bound gas indeed forms stars at high efficiency. Star formation efficiency of gas at a fixed density decreases within higher density ambient gas, suggesting that star formation depends on relative density.