Compact Star-forming Galaxies as Old Starbursts Becoming Quiescent

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? Answering 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 toward a smooth transition toward 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, study of the CO excitation, the density of the neutral gas, and the strength of the ultraviolet radiation field shows that compact SFGs resemble galaxies located in the upper envelope of the main sequence of SFGs, although this is 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 starbursts winding down and eventually crossing the main sequence toward quiescence.