The Star Formation Efficiency in Nearby Galaxies: Measuring Where Gas Forms Stars Effectively

Abstract

We measure the star formation efficiency (SFE), the star formation rate per unit gas, in 23 nearby galaxies and compare it to expectations from proposed star formation laws and thresholds. We use H I maps from THINGS and derive H 2 maps from CO measured by HERACLES and BIMA SONG. We estimate the star formation rate by combining GALEX FUV maps and SINGS 24µm maps, infer stellar surface density profiles from SINGS 3.6µm data, and use kinematics from THINGS. We measure the SFE as a function of: the free-fall and orbital timescales; midplane gas pressure; stability of the gas disk to collapse (including the effects of stars); the ability of perturbations to grow despite shear; and the ability of a cold phase to form. In spirals, the SFE of H 2 alone is nearly constant at 5.25 ± 2.5 × 10 −10 yr −1 (equivalent to an H 2 depletion time of 1.9 × 10 9 yr) as a function of all of these variables at our 800 pc resolution. Where the ISM is mostly H I, on the other hand, the SFE decreases with increasing radius in both spiral and dwarf galaxies, a decline reasonably described by an exponential with scale length 0.2-0.25 r 25 . We interpret this decline as a strong dependence of GMC formation on environment. The ratio of molecular to atomic gas appears to be a smooth function of radius, stellar surface density, and pressure spanning from the H 2 -dominated to H I-dominated ISM. The radial decline in SFE is too steep to be reproduced only by increases in the free-fall time or orbital time. Thresholds for large-scale instability suggest that our disks are stable or marginally stable and do not show a clear link to the declining SFE. We suggest that ISM physics below the scales that we observe -phase balance in the H I, H 2 formation and destruction, and stellar feedback -governs the formation of GMCs from H I.

SDSS-IV MaNGA-resolved Star Formation and Molecular Gas Properties of Green Valley Galaxies: A First Look with ALMA and MaNGA

“…Top panels: the relation between the SFR surface density and the gas surface density for Galaxy 1, Galaxy 2, and Galaxy 3 (from left to right). The dashed line corresponds to a constant SFE of -10 9.1 yr −1 , the averaged result from the HERACLES sample (Leroy et al 2008), while the upper and lower dotted-dashed lines correspond to constant gas fractions of -10 8.6 and -10 9.6 yr −1 , respectively. Bottom panel: the star formation efficiency as a function of stellar mass surface density.…”

Redshift evolution of stellar mass versus gas fraction relation in 0 <z< 2 regime: observational constraint for galaxy formation models

“…We adopted the Galactic conversion factor of αCO = 4.35 M⊙(K km s −1 pc 2 ) −1 (including the contribution of helium; Bolatto et al 2013) for all the sample galaxies 2 considered in this study to estimate the molecular gas mass as where L ′ CO represents the luminosity of the 12 CO(J = 1−0) line in units of K km s −1 pc 2 . The CO data of Leroy et al (2008), Bothwell et al (2014), Tacconi et al (2013), and Daddi et al (2010) are not based on 12 CO(J = 1 − 0) observations but based on 12 CO(J = 3 − 2) or 12 CO(J = 2 − 1). The conversions to 12 CO(J = 1 − 0) intensity assumed in these studies are described in the following lists.…”

“…Bigiel et al 2008;Leroy et al 2008). These theoretical models need to consider both star formation and molecule formation.…”

“…pressure-based model is an empirical method based on the observed relation between the molecule fraction of cold gas and the surface densities of stellar and gas components in nearby disc galaxies (Blitz & Rosolowsky 2004, 2006Leroy et al 2008). In contrast, the metallicity-based model uses an analytical formulae of the molecule fraction as a function of the gas density, metallicity, and radiation field (Krumholz et al 2008(Krumholz et al , 2009a.…”

GMC Collisions as Triggers of Star Formation. II. 3D Turbulent, Magnetized Simulations

“…It is an attractive mechanism because it is a process that is expected to create ∼parsec-scale dense gas clumps that are prone to gravitational instability and are the precursors to star clusters, while at the same time being sensitive to global galactic dynamics, such as the shear rate (Tan 2000(Tan , 2010Tasker & Tan 2009;Suwannajak et al 2014) and the presence of spiral arms (Dobbs 2008). Such a connection to orbital shear naturally explains the dynamical Kennicutt-Schmidt relation (Kennicutt 1998;Leroy et al 2008), S µ S W…”