[Paper Review] Stochastic Star Formation & Feedback: Mapping Low-Mass Galaxies to Dark Matter Haloes

G. A. Wynn, Geraint F. Lewis+3 more|arXiv (Cornell University)|Jun 27, 2014

Galaxies: Formation, Evolution, Phenomena4 references6 citations

TL;DR

The paper proposes that stochastic star formation and supernova feedback in low-mass haloes—driven by inefficient high-mass star formation and gas disruption—explain the observed stochastic occupation of Milky Way satellite galaxies in dark matter subhaloes. Using a Monte Carlo model of halo assembly, it shows that low-mass haloes ($\lesssim 10^9\,M_\odot$) lack atomic cooling and are prone to gas unbinding, stalling galaxy formation in a way dependent on assembly history and feedback timing.

ABSTRACT

Comparison of observed satellite galaxies of the Milky Way (hereafter MW) with dark matter subhaloes in cosmological $N$-body simulations of MW-mass haloes suggest that such subhaloes, if they exist, are occupied by satellites in a stochastic fashion. We examine how inefficient massive star formation and associated supernova feedback in high-redshift progenitors of present-day low-mass subhaloes might contribute to this stochasticity. Using a Monte Carlo approach to follow the assembly histories of present-day low-mass haloes with $10^7 \lesssim M \leq 10^{10}$ ${ m M}_{\odot}$, we identify when cooling and star formation is likely to proceed, and observe that haloes with present-day masses $\lesssim 10^9 { m M}_{\odot}$ never grow sufficiently massive to support atomic hydrogen line cooling. Noting that the star formation timescale decreases sharply with stellar mass as $t_{ m PMS} \propto m_{\ast}^{-2.5}$, we argue that, should the conditions for high mass star formation arise in low-mass haloes, the ensuing supernovae are likely to disrupt ongoing lower-mass star formation and unbind gas within the halo. This potentially star-forming gas is unlikely to be replenished in lower mass haloes because of, e.g. cosmological reionization, and so we expect galaxy formation to be stymied in a manner that depends on host halo assembly history and the efficiency and timing of star formation in proto-galaxies, which we illustrate using a Monte Carlo model. Based on these simple physical arguments, we assert that stochasticity of star formation and feedback is an essential but overlooked ingredient in modelling galaxy formation on the smallest scales.

Motivation & Objective

• To explain the stochastic occupation of Milky Way satellite galaxies in dark matter subhaloes by examining feedback processes in low-mass haloes.
• To investigate why low-mass haloes ($10^7 \lesssim M \leq 10^{10}\,M_\odot$) fail to form sustained galaxies despite hosting gas.
• To assess how inefficient high-mass star formation and supernova feedback disrupt gas and halt star formation in low-mass haloes.
• To model the dependence of galaxy formation on halo assembly history and feedback timing using a Monte Carlo approach.
• To argue that stochastic feedback is a critical, yet overlooked, factor in modeling galaxy formation at the smallest scales.

Proposed method

• Employing a Monte Carlo simulation to trace the assembly histories of present-day low-mass haloes with $10^7 \lesssim M \leq 10^{10}\,M_\odot$.
• Modeling cooling and star formation conditions based on halo mass and redshift, identifying when atomic hydrogen cooling becomes viable.
• Applying the PMS timescale relation $t_{\text{PMS}} \propto m_*^{-2.5}$ to estimate how quickly low-mass stars form relative to high-mass stars.
• Assessing the impact of supernova feedback on gas unbinding, particularly in haloes where cooling is inefficient.
• Evaluating gas replenishment constraints due to cosmological reionization, especially in low-mass systems.
• Simulating the stochastic outcome of galaxy formation by coupling feedback timing, halo history, and gas retention.

Experimental results

Research questions

• RQ1 Why do low-mass haloes ($\lesssim 10^9\,M_\odot$) fail to form sustained galaxies despite hosting gas?
• RQ2 How does the timing and efficiency of high-mass star formation affect gas retention and subsequent star formation in low-mass haloes?
• RQ3 To what extent does supernova feedback disrupt star formation and unbind gas in haloes that lack atomic hydrogen cooling?
• RQ4 How does the stochasticity of star formation and feedback depend on the halo's assembly history?
• RQ5 Why is galaxy formation in low-mass haloes so sensitive to initial conditions and feedback timing?

Key findings

• Haloes with present-day masses $\lesssim 10^9\,M_\odot$ never grow massive enough to support atomic hydrogen line cooling, limiting their ability to form stars.
• The rapid PMS timescale ($t_{\text{PMS}} \propto m_*^{-2.5}$) implies that high-mass stars form quickly, triggering supernovae before lower-mass stars can stabilize the system.
• Supernova feedback in low-mass haloes is likely to unbind gas and disrupt ongoing star formation due to inefficient cooling and low gravitational potential.
• Gas replenishment is unlikely in low-mass haloes due to cosmological reionization, leading to irreversible quenching of galaxy formation.
• The stochasticity in galaxy formation arises from the interplay between halo assembly history, feedback timing, and star formation efficiency, not just subhalo abundance.
• Stochastic feedback is identified as a key, previously overlooked mechanism governing galaxy formation in the smallest dark matter haloes.