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

Rongjun Huang, Aditi Vijayan, Mark R. Krumholz

TL;DR

This study interrogates the reliability of using broad Hα wings as tracers of galactic winds by leveraging high-resolution QED simulations to synthesize Hα spectra across environments with varying $Σ_{\rm SFR}$. The authors show that broad Hα emission tracks near-disc mass flux at $|z|\sim1$ kpc with a sub-linear scaling caused by electron column density increasing with outflow strength, quantified as $N_e \propto (Σ_{\rm H\alpha,broad})^{0.54}$. They derive an empirical calibration for $N_e$ as a function of broad Hα surface brightness, $N_e = N_{e,40}(Σ_{\rm H\alpha,broad}/10^{40})^{0.54}$, to correct mass-outflow rate estimates, and demonstrate that [S II]-based densities provide only marginal improvement and often overestimate low true densities. Collectively, the results urge using the proposed $N_e(Σ_{\rm H\alpha,broad})$ calibration when converting Hα measurements to outflow rates, and they caution against relying on constant $N_e$ or [S II]-derived densities for accurate estimations in local and low-$z$ galaxies.

Abstract

The presence of broad wings in the H$α$} 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$α$} 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 ($Σ_\mathrm{SFR}$). We show that the broad component of H$α$} emission correlates well with the wind mass flux at heights $\sim1$ 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$α$} 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 $Σ_\mathrm{SFR}$. We provide empirical scaling relations that observers can apply to correct for this effect when converting H$α$} measurements to mass outflow rates. Finally, we use synthetic observations of the density-diagnostic $[\mathrm{S_{II}}]\,λ\lambda6716,6731$ 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.

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

TL;DR

This study interrogates the reliability of using broad Hα wings as tracers of galactic winds by leveraging high-resolution QED simulations to synthesize Hα spectra across environments with varying . The authors show that broad Hα emission tracks near-disc mass flux at kpc with a sub-linear scaling caused by electron column density increasing with outflow strength, quantified as . They derive an empirical calibration for as a function of broad Hα surface brightness, , to correct mass-outflow rate estimates, and demonstrate that [S II]-based densities provide only marginal improvement and often overestimate low true densities. Collectively, the results urge using the proposed calibration when converting Hα measurements to outflow rates, and they caution against relying on constant or [S II]-derived densities for accurate estimations in local and low- galaxies.

Abstract

The presence of broad wings in the H} 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} 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 (). We show that the broad component of H} emission correlates well with the wind mass flux at heights 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} 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 . We provide empirical scaling relations that observers can apply to correct for this effect when converting H} measurements to mass outflow rates. Finally, we use synthetic observations of the density-diagnostic 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.

Paper Structure

This paper contains 16 sections, 17 equations, 8 figures, 1 table.

Table of Contents

  1. Introduction
  2. Data & Methods
  3. QED III Data
  4. Generating H$\alpha$ spectra
  5. H$\alpha$-inferred and true outflow rates
  6. Electron densities: [Sii] doublet estimates and true values
  7. Results
  8. Origin of broad H$\alpha$ emission
  9. Correlation between broad H$\alpha$ emission and outflow rate
  10. Calibration for the electron column density
  11. Electron densities from the [Sii] diagnostic
  12. Discussion
  13. Implications for the interpretation of observations
  14. Comparison with previous simulations
  15. Limitations and Future Work
  16. ...and 1 more sections

Figures (8)

  • Figure 1: Slices through the three simulation runs used in this study: (a) $\rm \Sigma13-Z1-H150$ at $t=142$ Myr, (b) $\rm \Sigma13-Z0.2-H150$ at $t=230$ Myr and (c) $\rm \Sigma2.5-Z1-H1500$ at $t=153$ Myr. Each panel shows, from top to bottom, the log gas density (in g cm$^{-3}$), log temperature (in K), log total H$\alpha$ emissivity (in erg s$^{-1}$ cm$^{-3}$), and log of the broad-wing ($50\le|v_z|/\mathrm{km\,s}^{-1}\le200$) H$\alpha$ emissivity (also in erg s$^{-1}$ cm$^{-3}$). Note that to enhance readability, we show only half of the simulation domain in these plots.
  • Figure 2: Example synthetic H$\alpha$ spectrum from the fiducial run $\Sigma13$--Z1--H150 at $t=142$ Myr (panel (a) from \ref{['fig:vis']}). The dashed vertical lines at $v_z = \pm 50$ km s$^{-1}$ indicate the boundary between the wing region (red/blue solid lines) that we use to fit the broad line profile and the central narrow bump (green solid line) that we mask when fitting. The black dashed curve is the best-fitting Gaussian model to the broad component. We report the dispersion $\sigma_{\rm broad}$ and total integrated luminosity $\Sigma_{\mathrm{H}\alpha,\mathrm{broad}}$ of this fit in Columns 10 and 11 of \ref{['tab:params']}.
  • Figure 3: Left and right columns show the histograms of density and temperature, respectively, weighted by H$\alpha$ wing luminosity (i.e., luminosity emitted at velocities $50 \leq |v_z|/\mathrm{km\,s}^{-1}\leq 200$). Colours denote same snapshots as \ref{['fig:vis']}: magenta is $\Sigma13$–$Z1$–$H150$ at $t=142$ Myr, green is $\Sigma13$–$Z0.2$–$H150$ at $t=230$ Myr and blue is $\Sigma2.5$–$Z1$–$H1500$ at $t=153$ Myr. The vertical dashed line marks $T=10^4$ K.
  • Figure 4: Cumulative fraction of the H$\alpha$ wing luminosity originating below a given $|z|$. Green, magenta and blue curves correspond to the $\rm \Sigma13-Z1-H150$, $\rm \Sigma13-Z0.2-H150$ and $\rm \Sigma13-Z1-H150$ runs, respectively. Note here that the horizontal axis is expressed in units of the simulation domain half-height, which is 4 kpc for the two $\Sigma13$ runs but 8 kpc for $\rm \Sigma13-Z1-H150$.
  • Figure 5: Relation between H$\alpha$ broad component surface brightness and mass-outflow rate surface density for all simulations (see legend). Rows show different distances $|z|$ from the galactic plane, as indicated by the labels to the right, while the left and right columns show outflow rates for all gas (left) and only for "warm" gas at temperatures $1 \le T/10^4\,\mathrm{K} \le 3$ (right). Best-fit slopes and Pearson correlation coefficients $r_\mathrm{P}$ are indicated in each panel.
  • ...and 3 more figures