Beyond Galaxy Bimodality: The Complex Interplay between Kinematic Morphology and Star Formation in the Local Universe

Abstract

It is generally assumed that galaxies are a bimodal population in both star formation and structure; star-forming galaxies are disks, while passive galaxies host large bulges or are entirely spheroidal. Here we test this scenario by presenting a full census of the kinematic morphologies of a volume-limited sample of galaxies in the local universe extracted from the MaNGA galaxy survey. We measure the integrated stellar line-of-sight velocity to velocity dispersion ratio (V/σ) for 4574 galaxies in the stellar mass range 9.75 < log M ⋆ [ M ⊙ ] < 11.75 . We show that at fixed stellar mass, the distribution of V/σ is not bimodal, and that a simple separation between fast and slow rotators is oversimplistic. Fast rotators are a mixture of at least two populations, referred to here as dynamically cold disks and intermediate systems, with disks dominating in both total stellar mass and number. When considering star-forming and passive galaxies separately, the star-forming population is almost entirely made up of disks, while the passive population is mixed, implying an array of quenching mechanisms. Passive disks represent ∼30% (both in number and mass) of passive galaxies, nearly a factor of two higher than that of slow rotators, reiterating that these are an important population for understanding galaxy quenching. These results paint a picture of a local universe dominated by disky galaxies, most of which become somewhat less rotation-supported upon or after quenching. While spheroids are present to a degree, they are certainly not the evolutionary end point for the majority of galaxies.

ALMA 1.1 mm Observations of a Conservative Sample of High-redshift Massive Quiescent Galaxies in SHELA

“…Galaxies that are disk dominated and star forming show strong blue colors, whereas spheroid dominated galaxies that have stopped forming stars and have predominantly old stellar populations are generally red (Baldry et al 2004;Balogh et al 2004;Bell et al 2004). While recent results have pointed to a more nuanced understanding of galaxy colors and populations, and exceptions exist within this bimodality (Newman et al 2018;Fraser-McKelvie & Cortese 2022), the majority of galaxy populations still fall within this dichotomy, providing us with a powerful tool for characterizing galaxies. The most-massive galaxies in the local universe are also quiescent, forming stars well below the stellar mass and star formation relation.…”