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

TL;DR

This study uses cosmological simulations to explore how red, star-forming galaxies, known as red misfits, form and evolve, revealing their unique properties and the physical processes driving their characteristics.

Contribution

It provides a detailed analysis of the formation mechanisms of red misfits in a {\Lambda}CDM universe, linking their properties to feedback, gas dynamics, and angular momentum.

Findings

Red misfits have less dust and older stars than blue galaxies.

Higher star formation efficiency and early ISM depletion characterize red misfits.

Gas stripping and lower CGM accretion contribute to their evolution.

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.