Compact molecular gas emission in local LIRGs among low- and high-z galaxies

TL;DR

This study reveals that local LIRGs have extremely compact molecular gas regions, similar in size to high-redshift star-forming galaxies, but with more concentrated star formation compared to their high-z counterparts.

Contribution

It provides high-resolution ALMA observations showing the compactness of molecular gas in local LIRGs and compares their sizes and star-forming regions to high-z galaxies, highlighting similarities and differences.

Findings

LIRGs have median CO(2-1) radii of ~0.7 kpc, much smaller than stellar and ionized gas sizes.

Molecular gas regions in LIRGs are comparable to early-type galaxies and more compact than local spirals.

High-z MS SFGs and SMGs have larger molecular sizes than local LIRGs by factors of 3 and 8, respectively.

Abstract

We present new CO(2-1) observations of a representative sample of 24 local (z$<$0.02) luminous infrared galaxies (LIRGs) obtained at high spatial resolution ($<$100 pc) from ALMA. We derive the effective radii of the CO(2-1) and the 1.3 mm continuum emissions using the curve-of-growth method. LIRGs show an extremely compact cold molecular gas distribution (median R$_{CO}$ $\sim$0.7 kpc), which is a factor 2 smaller than the ionized gas, and 3.5 times smaller than the stellar size. The molecular size of LIRGs is similar to that of early-type galaxies (R$_{CO}\sim$1 kpc) and about a factor of 6 more compact than local Spirals of similar stellar mass. Only the CO emission in low-z ULIRGs is more compact than these local LIRGs by a factor of 2. Compared to high-z (1$<$z$<$6) systems, the stellar sizes and masses of local LIRGs are similar to those of high-z MS star-forming galaxies (SFG) and about a factor of 2-3 lower than sub-mm galaxies (SMG). The molecular sizes of high-z MS SFGs and SMGs are larger than those derived for LIRGs by a factor of $\sim$3 and $\sim$8, respectively. These results indicate that while low-z LIRGs and high-z MS-SFGs have similar stellar masses and sizes, the regions of current star formation (traced by the ionized gas) and of potential star-formation (traced by the molecular gas) are substantially smaller in LIRGs, and constrained to the central kpc region. High-z galaxies represent a wider population but their star-forming regions are more extended, even covering the overall size of the host galaxy. High-z galaxies have larger fractions of gas than low-z LIRGs, and therefore the formation of stars could be induced by interactions and mergers in extended disks or filaments with large enough molecular gas surface density involving physical mechanisms similar to those identified in the central kpc of LIRGs.