The Earliest Stage of Galactic Star Formation

TL;DR

This paper introduces a new diagram based on gas temperature and specific star formation rate to study early galaxy evolution, revealing a core formation stage that precedes disk formation and quiescence.

Contribution

It proposes a novel $T_ extrm{SF}$-sSFR diagram to trace galaxy evolution and identifies an early core-forming stage in galactic star formation history.

Findings

Core-forming galaxies have younger stellar populations.

These galaxies are more common at higher redshifts.

The diagram reveals a morphological sequence in galaxy evolution.

Abstract

Using a recently-developed technique to estimate gas temperatures ($T_\textrm{SF}$) in star-forming regions from large photometric surveys, we propose a diagram, analogous to the Hertzsprung-Russell diagram for individual stars, to probe the evolution of individual galaxies. On this $T_\textrm{SF}$-sSFR (specific star formation rate) diagram, a small fraction of star-forming galaxies appear to be dominated by different feedback mechanisms than typical star-forming galaxies. These galaxies generically have younger stellar populations, lower stellar masses and increase in relative abundance towards higher redshifts, so we argue that these objects are in an earlier stage of galactic star formation. Further, Hubble observations find that these "core-forming" galaxies also exhibit distinct morphology, and that tracks on the $T_\textrm{SF}$-sSFR diagram are also a morphological sequence. Thus, unlike starburst phases which can be triggered environmentally, these earliest, core-forming galaxies, appear to be a stage that typical galaxies go through early in their star formation history. We therefore argue that most galaxies first go through a core formation stage, then subsequently disk formation, and finally become quiescent.