Quokka-based understanding of outflows (QED) -- IV. Limitations of H$\alpha$ as an outflow diagnostic

Abstract

The presence of broad wings in the H$\alpha$ line is commonly used as a diagnostic of the presence and properties of galactic winds from star-forming galaxies. However, the accuracy of this approach has not been subjected to extensive testing. In this paper, we use high-resolution simulations of galactic wind launching to calibrate the extent to which broad H$\alpha$ wings can be used to infer the properties of galactic outflows. For this purpose, we analyse a series of high-resolution wind simulations from the QED suite spanning two orders of magnitude in star formation surface density (${\Sigma }_{\mathrm{SFR}}$). We show that the broad component of H$\alpha$ emission correlates well with the wind mass flux at heights $\sim 1$ kpc above the galactic plane, but that the correlation is poor at larger distances from the plane, and that even at 1 kpc the relationship between mass flux and surface brightness of broad H$\alpha$ is significantly sub-linear. The sub-linear scaling suggests that the electron column density in the wind increases systematically with outflow strength, and that the conventional assumption of constant electron density in the wind leads to a systematic overestimate of how steeply mass loading factors depend on ${\Sigma }_{\mathrm{SFR}}$. We provide empirical scaling relations that observers can apply to correct for this effect when converting H$\alpha$ measurements to mass outflow rates. Finally, we use synthetic observations of the density-diagnostic $[{\mathrm{S}}_{\mathrm{II}}]\lambda \lambda 6716,6731$ doublet to show that using this diagnostic only slightly improves estimates of wind outflow rates compared to the naive assumption of constant electron density, and performs significantly worse than the empirical correlation we provide.

arXiv comment12 pages, 8 figures. Published by MNRAS

References in corpus (42)

• Array Programming with NumPy
• The Astropy Project: Sustaining and Growing a Community-oriented Open-source Project and the Latest Major Release (v5.0) of the Core Package
• Overview of the SDSS-IV MaNGA Survey: Mapping Nearby Galaxies at Apache Point Observatory
• A Multi-Code Analysis Toolkit for Astrophysical Simulation Data
• FIRE-2 Simulations: Physics versus Numerics in Galaxy Formation
• The SINS survey of z~2 galaxy kinematics: properties of the giant star forming clumps
• PyNeb: a new tool for analyzing emission lines. I. Code description and validation of results
• AGN wind scaling relations and the co-evolution of black holes and galaxies
• The Sydney-AAO Multi-object Integral field spectrograph (SAMI)
• Understanding Galaxy Evolution through Emission Lines
• Cool Outflows in Galaxies and their Implications
• Grackle: a Chemistry and Cooling Library for Astrophysics
• The MOSDEF Survey: Electron Density and Ionization Parameter at $z\sim 2.3$
• The Impact of Star Formation on the Interstellar Medium in Dwarf Galaxies: II. The Formation of Galactic Winds
• The Asymmetric Wind in M82
• Figuring Out Gas & Galaxies in Enzo (FOGGIE). I. Resolving Simulated Circumgalactic Absorption at 2 < z < 2.5
• The SINS/zC-SINF survey of z~2 galaxy kinematics: Outflow properties
• Evidence for Wide-Spread AGN Driven Outflows in the Most Massive z~1-2 Star Forming Galaxies
• The KMOS^3D Survey: Demographics and Properties of Galactic Outflows at z = 0.6 - 2.7
• Numerical Simulations of Multiphase Winds and Fountains from Star-Forming Galactic Disks: I. Solar Neighborhood TIGRESS Model
• Metal-enriched galactic outflows shape the mass-metallicity relationship
• Properties of the Multiphase Outflows in Local (Ultra)luminous Infrared Galaxies
• Non-equilibrium chemistry and cooling in the diffuse interstellar medium - II. Shielded gas
• Non-equilibrium chemistry and cooling in the diffuse interstellar medium - I. Optically thin regime
• Quantifying the AGN-driven outflows in ULIRGs (QUADROS) I: VLT/Xshooter observations of 9 nearby objects
• Temperature and Metallicity Gradients in the Hot Gas Outflows of M82
• CLASSY III: The Properties of Starburst-Driven Warm Ionized Outflows
• HI Kinematics Along The Minor Axis of M82
• A Framework for Multiphase Galactic Wind Launching using TIGRESS
• Systematic study of outflows in the Local Universe using CALIFA: I. Sample selection and main properties
• Properties of ionized outflows in MaNGA DR2 galaxies
• Incidence, scaling relations and physical conditions of ionised gas outflows in MaNGA
• Supernova-Driven Outflows in NGC 7552: A Comparison of H-alpha and UV Tracers
• Quantifying the AGN-driven outflows in ULIRGs (QUADROS) III: Measurements of the radii and kinetic powers of 8 near-nuclear outflows
• X-ray Properties of NGC 253's Starburst-Driven Outflow
• Velocities of warm galactic outflows from synthetic Hα observations of star-forming galaxies
• Quokka: A code for two-moment AMR radiation hydrodynamics on GPUs
• The observable properties of cool winds from galaxies, AGN, and star clusters -- II. 3D models for the multiphase wind of M82
• DUVET: Spatially Resolved Observations of Star Formation Regulation via Galactic Outflows in a Starbursting Disk Galaxy
• An Asymptotically-Correct Implicit-Explicit Time Integration Scheme for Finite Volume Radiation-Hydrodynamics
• A Simulated Galaxy Laboratory: Exploring the Observational Effects on UV Spectral Absorption Line Measurements
• Quokka-based understanding of outflows (QED) -- II. X-ray metallicity gradients as a signature of galactic wind metal loading