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

Abstract

We present new CO(2–1) observations of a representative sample of 24 local (z < 0.02) luminous infrared galaxies (LIRGs) at high spatial resolution (< 100 pc) from the Atacama Large Millimeter/submillimeter Array (ALMA). Our LIRGs lie above the main sequence (MS), with typical stellar masses in the range 1010–1011 M⊙ and SFR ∼ 30 M⊙ yr−1. 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 RCO ∼ 0.7 kpc), which is a factor 2 smaller than the ionized gas (median RHα ∼ 1.4 kpc), and 3.5 times smaller than the stellar size (median Rstar ∼ 2.4 kpc). The molecular size of LIRGs is similar to that of early-type galaxies (ETGs; RCO ∼ 1 kpc) and about a factor of 6 more compact than local spiral galaxies 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 (SFGs) and about a factor of 2–3 lower than submillimeter (submm) galaxies (SMGs). The molecular sizes of high-z MS SFGs and SMGs are larger than those derived for LIRGs by a factor of ∼3 and ∼8, respectively. Contrary to high-z SFGs and SMGs, which have comparable molecular and stellar sizes (median Rstar/RCO = 1.8 and 1.2, respectively), local LIRGs show more centrally concentrated molecular gas distribution (median Rstar/RCO = 3.3). A fraction of the low-z LIRGs and high-z galaxies share a similar range in the size of the ionized gas distribution, from 1 to 4 kpc. However, no LIRGs with a very extended (above 4 kpc) radius are identified, while for high-z galaxies no compact (less than 1 kpc) emission is detected. 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 kiloparsec (kpc) region. High-z galaxies represent a wider population but their star-forming regions are more extended, even covering the entire extent of the 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 sufficiently large molecular gas surface density involving physical mechanisms similar to those identified in the central kpc of LIRGs.

Enhanced Star Formation Efficiency in the Central Regions of Nearby Quasar Hosts

“…The most striking radial variations in gas depletion times are seen in local LIRGs, which can exhibit t dep  0.1 Gyr inside the central kiloparsec, while t dep ≈ 1 Gyr on larger scales (Sánchez-García et al 2022). Molina et al (2021) suggested that the compact molecular gas distributions of the PG quasars are reminiscent of those observed in local LIRGs, whose CO half-light radius is typically 0.8 kpc (Downes & Solomon 1998;Iono et al 2009;Bellocchi et al 2022). Local LIRGs and starbursts also present central sub-kiloparsec-scale star formation that contributes more than 50% of their total IR luminosity (e.g., Soifer et al 2001).…”

High-resolution Hubble Space Telescope Imaging Survey of Local Star-forming Galaxies. I. Spatially Resolved Obscured Star Formation with Hα and Paschen-β Recombination Lines

“…Therefore, by exploiting the high spatial resolution and high signal-to-noise ratio (S/N) of these local systems, we can at the same time aid our closer examination of the distant universe. On the other hand, these local objects appear to be different to higher redshift systems in terms of the distribution of the star formation and the dust temperature (Muzzin et al 2010;Bellocchi et al 2022). Furthermore, high-redshift U/LIRGs are more extended and display a cooler IR SED (Elbaz et al 2011).…”

MIRI/JWST observations reveal an extremely obscured starburst in the z = 6.9 system SPT0311-58

“…Regions of high obscuration, as identified by the compact 160 µm emission clumps, coexist with other regions that have much less dust and/or gas. In some discy low-z luminos infrared galaxies (LIRGs), the cold molecular gas and stellar or ionised gas distribution show different clumpy structures on (sub)kpc scales (Bellocchi et al 2022). Stellar clumps identified in the near-infrared continuum and emission lines are known to exist in low-z LIRG discs (Alonso-Herrero et al 2006;Díaz-Santos et al 2008).…”