Emergence of red, star-forming galaxies (red misfits) in a {\Lambda}CDM universe

Abstract

We investigate the formation of red misfits (RM) using a cosmological, hydrodynamical simulation from the $\rm \small EAGLE$ project. Similar to observations, the RM possess less dust, higher stellar metallicities, and older stellar populations compared to blue, star-forming galaxies (BA) at the same $M_\star$. Lagrangian particle-tracking reveals that the older ages of RM have resulted from a combined effect of higher star formation efficiency (SFE), and the earlier onset and faster net depletion of their interstellar medium (ISM). For the centrals, the latter was partially due to higher efficiency of escape from ISM, driven by stronger stellar and/or AGN feedback (depending on the mass). There was an additional contribution to this escape from gas stripping for satellite RM, as suggested by the higher group masses ($\gtrsim 0.5$ dex) and ${\rm H_2}/{\rm H\,{\small I}}$ ratios ($\gtrsim 0.3$ dex). Moreover, accretion of circumgalactic gas (CGM) onto the galaxy has been less efficient for the satellites. On the metallicity front, the offsets are largely due to the disparity in SFE, causing varying degrees of enrichment through the mass-transfers associated with stellar winds and supernovae. We ascribe this SFE disparity to the lower specific angular momentum ($j$) of freshly accreted CGM for RM, which ultimately manifested in the ISM kinematics due to interactions with cooling flows. The impact on $j_{\rm ism}$ was further intensified by poorer alignment with the flow's $\vec{j}$, particularly for the satellites. Our results illuminate potential origins of RM, and motivate further exploration of this peculiar class through a synergy between observations and simulations.