What Suppresses Star Formation in Bulge-Dominated Early-Type Galaxies?

TL;DR

The paper tackles why star formation is suppressed in bulge-dominated, gas-rich ETGs by leveraging five high-resolution GalactISM simulations with AREPO and applying a modified virial theorem to cloud-scale overdensities. This approach separates internal gravity, external gravity, and rotational (Coriolis) effects, revealing that a dynamically suppressed ETG experiences strong rotational support due to a compact bulge and enhanced shear, which inhibits collapse even when molecular gas is abundant and Toomre $Q$ is not markedly elevated. The study shows that overdensities in the suppressed galaxy have weak self-gravity and a disproportionately large Coriolis contribution, whereas other galaxies are governed mainly by kinetic energy and self-gravity. These results imply a dynamical suppression mechanism that does not require gas removal and align with observations of rotating GMCs in ETGs, suggesting the need for a revised, cloud-scale criterion for star formation suppression. The work highlights the importance of galaxy morphology and rotation in regulating star formation and provides a framework (MVT) to interpret GMC dynamics under realistic, multiphase ISM conditions.

Abstract

We investigate the physical origin of star formation suppression in gas-rich early-type galaxies using five high-resolution hydrodynamical idealized galaxy simulations, performed with the moving-mesh code AREPO. These simulations include one Milky Way-like galaxy and four early-type galaxies, of which one early-type galaxy is found to have significantly less star formation despite a substantial molecular gas reservoir. We apply a modified virial theorem to the overdensities in each galaxy to quantify the forces regulating their stability and thus star formation. We find evidence that, in the suppressed galaxy, strong Coriolis forces driven by elevated galactic shear may inhibit gravitational collapse. This is caused by the galaxy's high central compactness, providing a physical mechanism for the suppression of star formation that does not require the removal of molecular gas. In contrast, less compact early-type galaxies host more gravity-dominated clouds and therefore exhibit higher star formation rates. However, we find that this gravitational stability occurs without significantly increasing the classical Toomre-Q parameter, and therefore a new criterion for suppressed star formation may be needed. We also discuss the impact of our choice of overdensity scale and connections to observations of molecular clouds.

Figures (13)

• Figure 1: Stellar and gas surface density distributions across all galaxies in this paper (columns), with the ETGs increasing in mass from left to right, and the right-most column being the MW-like galaxy. First row depicts a face-on view of the galaxy while the second row is edge-on. The stellar distribution is represented by orange (first colorbar) and gas is grey/white (second colorbar). The MW-like galaxy has the thickest gas disk that extends beyond the stellar component. The suppressed ETG ($\rm 10^{11} \, M_{\odot}$) contains the smoothest and least disrupted gas disk.
• Figure 2: 2-D histograms of overdensity properties across all galaxies in this paper (columns), with the ETGs increasing in mass from left to right, and the right-most column being the MW-like galaxy. First row depicts the Sobolev length $\ell$ (cloud radius) versus total mass enclosed $M$, second row is the cloud freefall time $t_{\rm ff}$ versus crossing time $t_{\rm cross}$, third row is the classical virial parameter $\alpha_{\rm vir}$ (from Equation \ref{['eqn::virial_param']}) versus SFR $\dot M_\star$, and last row is the classical Toomre-$Q$ parameter $Q_{\rm gas}$ and radius of the overdensity from the galactic center $R$. The dashed grey line in the second row represents where $t_{\rm cross}=t_{\rm ff}$, while in the third row it represents where $\alpha_{\rm vir}=2$. Cyan line in the bottom row represents the median value in the gas across the galaxy, similar to Figure 7 of Jeffreson2024b. The second colorbar denotes data where the SFE is at least $0.01 \;\rm Myr^{-1}$, ideally being more representative of realistic gravitationally-bound clouds. The suppressed ETG shows larger Sobolev lengths $\ell$. Both the suppressed ETG ($\rm 10^{11} \, M_{\odot}$) and highest-mass ETG ($\rm 10^{11.5} \, M_{\odot}$) show no overdensities with crossing times larger than the freefall time. The suppressed ETG is the only galaxy to have no overdensities with $\alpha_{\rm vir}\leq 2$.
• Figure 3: Distributions of total kinetic energy (first row), self-gravitational energy (second row), axisymmetric external midplane energy (third row), and energy from the Coriolis force (fourth row), from Equations \ref{['eqn::MVT4_kineticenergy']}, \ref{['eqn::MVT6_selfgravity']}, \ref{['eqn::MVT8_Rextax']}, and \ref{['eqn::MVT9_Rextnax']}, respectively. Lighter filled histograms represent all overdensities, and the dashed vertical line is the median. Darker unfilled histograms (solid lines) show the distribution for all overdensities with a star formation efficiency (SFE) greater than 0.01 $\rm Myr^{-1}$, which are more likely to represent gravitationally-bound systems, while the solid vertical line is the median for such systems.
• Figure 4: 2-D histograms of the classical virial parameter from Equation \ref{['eqn::virial_param']}, $\alpha_{\rm vir}$, versus the energy term from the Coriolis force, colored by the mean SFR in each bin. Unfilled contours show the 50%, 75%, and 95% data inclusion regions, while the solid line is the least squares regression. Dashed grey line is where the Coriolis energy term is 0. The top row is all overdensities, while the bottom row is limited to overdensities with SFE $> 0.01 \; {\rm Myr^{-1}}$. The suppressed ETG ($\rm 10^{11}M_{\odot}$) is the only galaxy with an energy contribution from the Coriolis force that is almost entirely positive, and maintains a much steeper slope than the other ETGs. The MW-like galaxy shows little variation in the Coriolis force with $\alpha_{\rm vir}$, remaining extremely flat.
• Figure 5: 2-D histograms of the self-gravity in the midplane direction versus the Coriolis energy term. Pixels are colored based on the sum of the force terms (x- and y-axis), where reds represent a positive sum (support against collapse) and greens are a negative sum (collapsing overdensity). The dashed black line represents where the two axes are equal. As in Figure \ref{['fig:axi_anticorrelation']}, the top row is all overdensities, while the bottom row is limited to overdensities with SFE $> 0.01 \; {\rm Myr^{-1}}$. The suppressed ETG ($\rm 10^{11} M_{\odot}$) shows the least amount of collapsing overdensities (green).