Abstract
Abstract We present results from the EDGE survey, a spatially resolved CO(1−0) follow-up to CALIFA, an optical Integral Field Unit survey of local galaxies. By combining the data products of EDGE and CALIFA, we study the variation in molecular gas depletion time (τ dep) on kiloparsec scales in 52 galaxies. We divide each galaxy into two parts: the center, defined as the region within , and the disk, defined as the region between 0.1 and . We find that 14 galaxies show a shorter τ dep (∼1 Gyr) in the center relative to that in the disk (τ dep ∼ 2.4 Gyr), which means the central region in those galaxies is more efficient at forming stars per unit molecular gas mass. This finding implies that the centers with shorter τ dep resemble the intermediate regime between galactic disks and starburst galaxies. Furthermore, the central drop in τ dep is correlated with a central increase in the stellar surface density, suggesting that a shorter τ dep is associated with molecular gas compression by the stellar gravitational potential. We argue that varying the CO-to-H2 conversion factor only exaggerates the central drop of τ dep.
Highlights
Galactic stellar masses grow through a combination of mergers and the formation of stars from their gas reservoir over cosmic time
In galactic regions with low mean gas volume and low surface density, local gas compression by spiral arms or self-gravity may be needed for molecules to form, whereas in galactic regions of high mean gas volume and surface density, most of the gas is already molecular
We focus on the second part of the star-formation processes; we study how the relation between molecular gas and star-formation rate (SFR) changes between the galactic centers and the disks
Summary
Galactic stellar masses grow through a combination of mergers and the formation of stars from their gas reservoir over cosmic time. Star formation involves two processes: (1) the conversion of diffuse, atomic gas into molecular gas in well-shielded regions of high density and (2) the dynamical collapse of self-gravitating regions within the molecular component to form stars. One would expect that the relevant timescale of this process is the free-fall time (tff) of the total gas (atomic and molecular), which is inversely proportional to the square-root of gas volume density (r-ga0s.5). The implication of this simple scenario is that SFR relates to the amount of gas as rSFR μ rgas /tff μ r1g.a5s.13. The implication of this simple scenario is that SFR relates to the amount of gas as rSFR μ rgas /tff μ r1g.a5s.13 In general, the relation between SFR and total gas density is called the Kennicutt–Schmidt (KS) relation, after the seminal papers by Schmidt (1959) and Kennicutt (1998b).
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