The Systematic Properties of the Warm Phase of Starburst-Driven Galactic Winds (1507.05622v1)

Abstract: Using ultra-violet absorption-lines, we analyze the systematic properties of the warm ionized phase of starburst-driven winds in a sample of 39 low-redshift objects that spans broad ranges in starburst and galaxy properties. Total column densities for the outflows are $\sim$10$^{21}$ cm$^{-2}$. The outflow velocity (v${out}$) correlates only weakly with the galaxy stellar mass (M$$), or circular velocity (v${cir}$), but strongly with both SFR and SFR/area. The normalized outflow velocity (v${out}/v_{cir}$) correlates well with both SFR/area and SFR/M$_$. The estimated outflow rates of warm ionized gas ($\dot{M}$) are $\sim$ 1 to 4 times the SFR, and the ratio $\dot{M}/SFR$ does not correlate with v${out}$. We show that a model of a population of clouds accelerated by the combined forces of gravity and the momentum flux from the starburst matches the data. We find a threshold value for the ratio of the momentum flux supplied by the starburst to the critical momentum flux needed for the wind to overcome gravity acting on the clouds ($R{crit}$). For $R_{crit} >$ 10 (strong-outflows) the outflow momentum flux is similar to the total momentum flux from the starburst and the outflow velocity exceeds the galaxy escape velocity. Neither is the case for the weak-outflows ($R_{crit} <$ 10). For the weak-outflows, the data severely disagree with many prescriptions in numerical simulations or semi-analytic models of galaxy evolution. The agreement is better for the strong-outflows, and we advocate the use of $R_{crit}$ to guide future prescriptions.

Overview of the Systematic Properties of Galactic Winds

The paper "The Systematic Properties of the Warm Phase of Starburst-Driven Galactic Winds" by Timothy M. Heckman et al. rigorously examines the systematic properties of galactic winds driven by starbursts, focusing on the warm ionized phase of these phenomena. Using ultra-violet (UV) absorption-line spectroscopy, this paper investigates a sample of 39 low-redshift starburst galaxies to discern the correlations between wind characteristics and galactic properties. Notably, the paper seeks to inform the treatment of galactic winds in numerical simulations by providing empirical insights, addressing a notable gap in theoretical understanding primarily due to the complex interplay of sub-grid physical processes.

Empirical Findings

A key finding of the paper is the significant correlation between outflow velocity and star formation indicators, namely the star formation rate (SFR) and specific star formation rate (SFR/M*) rather than galaxy mass or circular velocity. More specifically, the velocity of outflows shows a strong positive correlation with both the specific star formation rate (SFR/M*) and SFR per unit area, with the data suggesting a saturation point in outflow velocity at higher SFR/area rates. The outflow velocities often exceed the escape velocities of their host galaxies, which implies an influential role in expelling material beyond galactic boundaries.

Furthermore, the paper identifies three regimes of outflow dynamics based on the ratio of momentum flux provided by the starburst to the critical momentum flux needed to overcome gravitational forces: no-outflow, weak-outflow, and strong-outflow regimes. The empirical evidence indicates that in strong outflows, a significant fraction of the momentum from the starburst is utilized in driving the outflow, contrasting with weak outflows where only a small fraction is effectively used.

Theoretical and Practical Implications

These empirical results have substantive implications for both theoretical models and practical simulations of galaxy evolution. The paper successfully challenges the often-assumed linear scaling of outflow velocity with circular velocity in galaxy evolution models, highlighting the need for more nuanced treatment of stellar feedback. In particular, it questions the efficacy of constant wind models and suggests that the momentum-driven cloud model aligns more closely with observed data. This model better accounts for the variations in outflow properties by considering the momentum flux ratio, providing a framework that could lead to more accurate predictions in simulations.

The reported mass-loading factors, which averagely range from 1 to 4 times the SFR, further suggest that galactic winds are significant conveyors of baryonic material, impacting both the immediate galactic environment and the inter-galactic medium. For models to be consistent with observations, they must integrate variables such as R_crit,c (the ratio of starburst momentum flux to critical momentum density) to predict outflow properties dynamically.

Future Directions

Moving forward, the paper emphasizes the need to reconcile simulation resolutions with empirical data, advocating higher-fidelity simulations to explore the multi-phase nature of galactic winds more effectively. Such studies should focus on conditions that optimize momentum flux utilization and explore the hydrodynamics of cloud acceleration and coupling with supernovae-driven hot wind phases.

This paper establishes a template against which future numerical simulations can be benchmarked, facilitating an enhanced understanding of stellar feedback in galaxy evolution. By adopting nuanced prescriptions informed by these findings, cosmological simulations can better emulate the real dynamic processes within galaxies, thus pushing the boundaries of current astrophysical research.