ALMaQUEST -- IV. The ALMA-MaNGA QUEnching and STar formation (ALMaQUEST)
  Survey
  The ALMaQUEST (ALMA-MaNGA QUEnching and STar formation) survey is a program
with spatially-resolved $^{12}$CO(1-0) measurements obtained with the Atacama
Large Millimeter Array (ALMA) for 46 galaxies selected from the Mapping Nearby
Galaxies at Apache Point Observatory (MaNGA) DR15 optical integral-field
spectroscopic survey. The aim of the ALMaQUEST survey is to investigate the
dependence of star formation activity on the cold molecular gas content at kpc
scales in nearby galaxies. The sample consists of galaxies spanning a wide
range in specific star formation rate (sSFR), including starburst (SB),
main-sequence (MS), and green valley (GV) galaxies. In this paper, we present
the sample selection and characteristics of the ALMA observations, and showcase
some of the key results enabled by the combination of spatially-matched stellar
populations and gas measurements. Considering the global (aperture-matched)
stellar mass, molecular gas mass, and star formation rate of the sample, we
find that the sSFR depends on both the star formation efficiency (SFE) and the
molecular gas fraction ($f_{\rm H_{2}}$), although the correlation with the
latter is slightly weaker. Furthermore, the dependence of sSFR on the molecular
gas content (SFE or $f_{\rm H_{2}}$) is stronger than that on either the atomic
gas fraction or the molecular-to-atomic gas fraction, albeit with the small HI
sample size. On kpc scales, the variations in both SFE and $f_{\rm H_{2}}$
within individual galaxies can be as large as 1-2 dex thereby demonstrating
that the availability of spatially-resolved observations is essential to
understand the details of both star formation and quenching processes.
