Morphology and star formation in IllustrisTNG: the build-up of spheroids and discs

Abstract

Using the IllustrisTNG simulations, we investigate the connection between galaxy morphology and star formation in central galaxies with stellar masses in the range 10 9 − 10 11.5 M . We quantify galaxy morphology by a kinematical decomposition of the stellar component into a spheroidal and a disc component (spheroid-to-total ratio, S/T) and by the concentration of the stellar mass density profile (C 82 ). S/T is correlated with stellar mass and star-formation activity, while C 82 correlates only with stellar mass. Overall, we find good agreement with observational estimates for both S/T and C 82 . Low-and high-mass galaxies are dominated by random stellar motion, while only intermediate-mass galaxies (M ≈ 10 10 − 10 10.5 M ) are dominated by ordered rotation. Whereas higher mass galaxies are typical spheroids with high concentrations, lower-mass galaxies have low concentration, pointing to different formation channels. Although we find a correlation between S/T and star-formation activity, in the TNG model galaxies do not necessarily change their morphology when they transition through the green valley or when they cease their star formation, this depending on galaxy stellar mass and morphological estimator. Instead, the morphology (S/T and C 82 ) is generally set during the star-forming phase of galaxies. The apparent correlation between S/T and star formation arises because earlier forming galaxies had, on average, a higher S/T at a given stellar mass. Furthermore, we show that mergers drive in-situ bulge formation in intermediate-mass galaxies and are responsible for the recent spheroidal mass assembly in the massive galaxies with M > 10 11 M . In particular, these massive galaxies assemble about half of the spheroidal mass while star-forming and the other half through mergers while quiescent.

The VIRUS-P Exploration of Nearby Galaxies (VENGA): the stellar populations and assembly of NGC 2903’s bulge, bar, and outer disc

“…(5) Our results in (1) to (3) are consistent with the study by Weinzirl et al (2009), which suggests that most spiral galaxies in the local Universe with low B/T have not had a major merger since z ∼ 2, and have grown their outer disc mostly through gas accretion, minor mergers, and secular processes, based on their comparison of structural properties of spirals (such as the distributions of Sérsic index and B/T of their bulges) to semi-analytic models of galaxy evolution from Hopkins et al (2009), Khochfar & Burkert (2005) and Khochfar & Silk (2006). This is further supported by recent hydrodynamical simulations (e.g., Gargiulo et al 2019;Tacchella et al 2019) that show that galaxies with B/T, Sérsic index, and mass similar to NGC 2903 have not experienced a major merger in the recent past.…”

“…< 0.2) and growth through minor mergers and secular processes. Tacchella et al (2019) used IllustrisTNG hydrodynamical simulations (Pillepich et al 2018) to study the spheroidal and disc components of galaxies. They find that similar-mass galaxies to NGC 2903 have bulges that grow very little between the epoch of disc formation (z∼2) and z=0, which therefore decreases the B/T from ∼0.45 to ∼0.30.…”

From peculiar morphologies to Hubble-type spirals: the relation between galaxy dynamics and morphology in star-forming galaxies at z ∼ 1.5

“…The connection between galaxy morphology and the distribution of angular momentum at z ∼ 0.5 -1.5 was qualitatively established by Swinbank et al (2017), showing that galaxies with 'visually' more disc dominated morphologies had higher angular momentum at fixed stellar mass whilst lower angular momentum galaxies had more peculiar 'complicated' morphologies. This relationship was quantified further by Harrison et al (2017), who parameterised the morphology of the KROSS galaxies with Sérsic profiles, establishing a trend of decreasing specific angular momentum, at fixed stellar mass, with increasing Sérsic index, suggesting there is a causal connection between morphology and angular momentum.…”

“…Marshall et al 2019), whilst the relationship between specific angular momentum and stellar mass increases by 0.5 dex from z = 7 to z = 2, with the dominant morphological fraction of high-redshift galaxies being bulge-dominated systems (e.g. Zoldan et al 2018Zoldan et al , 2019Tacchella et al 2019).…”

The Formation History of Subhalos and the Evolution of Satellite Galaxies

“…One likely reason for the similarity is that, in TNG, galaxy mergers can cause the growth of supermassive black holes and trigger AGN feedback, which can cease star formation in host galaxies. This hints that galaxy mergers play an important (or even dominant) role in triggering AGN feedback and quenching star formation in TNG (see Tacchella et al 2019 for an in depth discussion of this in the TNG100 simulation).…”