Compact Star-forming Galaxies as Old Starbursts Becoming Quiescent

Abstract

Optically-compact star-forming galaxies (SFGs) have been proposed as immediate progenitors of quiescent galaxies, although their origin and nature are debated. Were they formed in slow secular processes or in rapid merger-driven starbursts? Addressing this question would provide fundamental insight into how quenching occurs. We explore the location of the general population of galaxies with respect to fundamental star-forming and structural relations, identify compact SFGs based on their stellar core densities, and study three diagnostics of the burstiness of star formation: 1) Star formation efficiency, 2) interstellar medium (ISM), and 3) radio emission. The overall distribution of galaxies in the fundamental relations points towards a smooth transition towards quiescence while galaxies grow their stellar cores, although some galaxies suddenly increase their specific star-formation rate when they become compact. From their star formation efficiencies compact and extended SFGs appear similar. In relation to the ISM diagnostic, by studying the CO excitation, the density of the neutral gas, and the strength of the ultraviolet radiation field, compact SFGs resemble galaxies located in the upper envelope of the SFGs main sequence, although yet based on a small sample size. Regarding the radio emission diagnostic we find that galaxies become increasingly compact as the starburst ages, implying that at least some compact SFGs are old starbursts. We suggest that compact SFGs could be starburts winding down and eventually crossing the main sequence towards quiescence.

CO emission in distant galaxies on and above the main sequence

“…This proved to be a useful distinction and an excellent predictor of several trends (e.g., Sargent et al 2014), but recent results, including our present and previous analysis (Puglisi et al 2019), show that the demarcation between starburst and main-sequence galaxies is more blurred that we previously considered. We do detect starburst-like behaviors in galaxies on the main sequence (Elbaz et al 2018), likely linked to the existence of transitional objects (Popping et al 2017;Barro et al 2017b;Gómez-Guijarro et al 2019;Puglisi et al 2019, and in prep. to limit the references to recent works based on submillimeter observations). Such transition might well imply an imminent increase of the SFR, driving the object in the realm of starbursts (e.g., Barro et al 2017b), or its cessation, bringing the system back onto or even below the main sequence (Gómez-Guijarro et al 2019;Puglisi et al 2019), with the CO properties potentially able to distinguish between these two scenarios.…”

“…We do detect starburst-like behaviors in galaxies on the main sequence (Elbaz et al 2018), likely linked to the existence of transitional objects (Popping et al 2017;Barro et al 2017b;Gómez-Guijarro et al 2019;Puglisi et al 2019, and in prep. to limit the references to recent works based on submillimeter observations). Such transition might well imply an imminent increase of the SFR, driving the object in the realm of starbursts (e.g., Barro et al 2017b), or its cessation, bringing the system back onto or even below the main sequence (Gómez-Guijarro et al 2019;Puglisi et al 2019), with the CO properties potentially able to distinguish between these two scenarios. Regardless of these transitional objects, a definition of starburst based on Σ SFR , rather than ∆MS, would naturally better account for the observed molecular gas excitation properties, dust temperatures and opacities, or SFE (see also Elbaz et al 2011;Rujopakarn et al 2011;Jiménez-Andrade et al 2018;Tacconi et al 2020).…”

“…The same definition of starbursts as galaxies deviating from the main sequence has been recently questioned with the advent of high spatial resolution measurements of their dust and gas emission. Compact galaxies with short depletion timescales typical of SBs are now routinely found on the MS, being possibly on their way to leave the sequence (Barro et al 2017a;Popping et al 2017;Elbaz et al 2018;Gómez-Guijarro et al 2019;Puglisi et al 2019;Jiménez-Andrade et al 2019); or galaxies moving within the MS scatter, due to mergers unable to efficiently boost the star formation (Fensch et al 2017) or owing to gravitational instabilities and gas radial redistribution (Tacchella et al 2016).…”

The Dramatic Size and Kinematic Evolution of Massive Early-type Galaxies

“…Such high SFRs can partly disrupt clumps and molecular clouds (see Murray et al 2010) and may be subject to the Eddington limit for starbursts (e.g., Andrews & Thomson 2011;Simpson et al 2015). All in all, we expect limited, mildly obscured SFRs in the region between R Q and R rot , and a much stronger, obscured SFR in the innermost regions within R rot where most of the stellar mass is accumulated; therefore the SFRs probed by UV and far-IR data are expected to be spatially disconnected (e.g., Gomez-Guijarro et al 2018), with the UV morphology particularly knotty and irregular (e.g., Huertas-Company et al 2015). The above approximate analytical estimates of the SFRs, sizes R rot and velocity ratios (v/σ) rot are consistent with those measured via far-IR/sub-mm and CO line observations of z ∼ 1 − 2 star-forming galaxies (e.g., Barro et al 2016aBarro et al , 2017Hodge et al 2016;Tadaki et al 2017;Talia et al 2018).…”

The e-MERGE Survey (e-MERLIN Galaxy Evolution Survey): overview and survey description

“…As previously suggested by Murphy et al (2017) and Thomson et al (2019), these differences may include frequencydependent cosmic-ray diffusion and/or may be due to the increasing thermal fraction at higher rest-frame radio frequencies, revealing time lags between the production of free-free and synchrotron emission in star-forming galaxies (e.g. Bressan et al 2002;Thomson et al 2014;Gómez-Guijarro et al 2019).…”