Unveiling the main sequence to starburst transition region with a sample of intermediate redshift luminous infrared galaxies

Abstract

We present a CO(3-2) study of four systems composed of six (ultra) luminous infrared galaxies (U/LIRGs), located at 0.28 < z < 0.44, that straddle the transition region between regular star forming galaxies and starbursts. These galaxies benefit from previous multi-wavelength analysis allowing in depth exploration of an understudied population of U/LIRGs at a time when the universe is experiencing a rapid decline in star formation rate density. We detect CO(3-2) emission in four targets and these galaxies fall between the loci of regular star forming galaxies and starbursts on the Kennicutt-Schmidtt relation. Compared to low luminosity LIRGs and high luminosity ULIRGs at similar redshifts, we find they all have similar molecular gas budgets with the difference in their star formation rates (SFR) driven by the star formation efficiency (SFE). This suggests that at these redshifts large molecular gas reservoirs must coincide with an increased SFE to transition a galaxy into the starburst regime. We studied the structure and kinematics and found our four detections are either interacting or have disturbed morphology which may be driving the SFE. One of the CO(3-2) non-detections has a strong continuum detection, and has been previously observed in Hα, suggesting an unusual interstellar medium for a ULIRG. We conclude that our sample of transitioning U/LIRGs fill the gap between regular star forming galaxies and starbursts, suggest a continuous change in SFE between these two populations and the increased SFE may be driven by morphology and differing stages of interaction.

Main Sequence to Starburst Transitioning Galaxies: Gamma-Ray Burst Hosts at z ∼ 2

“…the intermediate-z transitioning galaxies (Cheng et al 2018;Hogan et al 2022). Furthermore, their low [C I]/CO ratios point to a high-density environment observed in the collisional fronts of mergers (Valentino et al 2020b;Michiyama et al 2020;Michiyama et al 2021).…”

“…In what follows we put forward two nonexclusive scenarios to explain this feature. The first one is that the GRB hosts are caught at their transition from the main sequence toward the starburst phase (Cheng et al 2018;Michiyama et al 2021;Hogan et al 2022). The second possibility is that they are [C I]dark galaxies, which may explain their low [C I]/CO brightness temperature ratio (Jiao et al 2017;Michiyama et al 2020).…”

“…In order to place our two GRB hosts into a general perspective we compiled a sample of galaxies spanning several orders of magnitude in stellar mass, SFR, and gas mass. In particular, this compilation contains normal SFGs (Bourne et al 2019;Valentino et al 2020b;Dunne et al 2021), (U) LIRGs (Lu et al 2017;Valentino et al 2020b;Michiyama et al 2021), intermediate-z isolated LIRGs (Lee et al 2017) and merging starburst ULIRGs (Combes et al 2011), high-z starbursts (Shi et al 2018), and transitioning isolated and merging galaxies (Cheng et al 2018;Hogan et al 2022). For all sources with [C I] data, M mol has been estimated here in a consistent manner (see Section 2.4).…”

“…In Figure 3 we show the estimated molecular gas masses of our GRB hosts based on ¢ L C I using our two different methods (see Section 2.4 for details). We include the same data as in Figure 2, 12 (Shi et al 2018), and lowand intermediate-z transitioning isolated and merging galaxies (Cheng et al 2018;Hogan et al 2022) with the molecular mass estimated from CO emission (i.e., these have the same M mol in both panels, and are shown for comparison only). Using the SFG sample from the literature (gray empty markers) we find the best-fit relation between SFR and [C I]-based M mol for each of the methods.…”

“…The so-called starburst galaxies are found above the MS with extremely high SFRs for a given stellar mass (Combes et al 2011;Rodighiero et al 2011;Larson et al 2016). It is not clear whether an increase in star formation efficiency (SFE; Cheng et al 2018;Hogan et al 2022) or gas mass fraction (Lee et al 2017;Valentino et al 2020a) drives the departure from the MS. As pointed out by Gao & Solomon (2004b), the global SFR depends mainly on the amount of dense molecular gas, which can be traced for example by a hydrogen cyanide (HCN) line. This dependence remains nearly the same (with a slope of about 1) for normal and starburst galaxies, including ultraluminous galaxies (ULIRGs).…”