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The GECKOS Survey: Resolved, multiphase observations of mass-loading and gas density in the galactic wind of NGC 4666

Barbara Mazzilli Ciraulo, D. B. Fisher, R. Elliott, A. Fraser-McKelvie, M. R. Hayden, M. Martig, J. van de Sande, A. J. Battisti, J. Bland-Hawthorn, A. D. Bolatto, T. H. Brown, B. Catinella, F. Combes, L. Cortese, T. A. Davis, E. Emsellem, D. A. Gadotti, C. del P. Lagos, X. Lin, A. Marasco, E. Peng, F. Pinna, T. H. Puzia, L. A. Silva-Lima, L. M. Valenzuela, G. van de Ven, J. Wang

TL;DR

This study presents a resolved, multiphase view of the galactic wind in NGC 4666 by combining VLT/MUSE GECKOS data with HI and CO observations. The authors map the ionised and neutral gas in a biconical outflow extending up to ~8 kpc, measure an unusual electron-density profile that stays high ($n_e\sim100-300$ cm$^{-3}$) beyond the disk, and derive mass- and energy-loading factors across phases. They find that HI dominates the mass loading (\dot{M}_{out,HI} ~ 5–13 M$_\odot$ yr$^{-1}$) while the ionised component is an order of magnitude lower, with CO undetected setting strict molecular limits; the wind is unlikely to escape the halo of a Milky Way–mass galaxy. A secondary starburst-driven bubble adds complexity to the feedback, highlighting the need for high-resolution, multiphase observations to test wind theories and improve galaxy-evolution simulations. These results underscore the role of gas cycling and the importance of electron-density diagnostics for accurately quantifying outflow masses in realistic galactic winds.

Abstract

We present a multiphase, resolved study of the galactic wind extending from the nearby starburst galaxy NGC 4666. For this we use VLT/MUSE observations from the GECKOS program and HI data from the WALLABY survey. We identify both ionised and HI gas in a biconical structure extending to at least $z\sim$8 kpc from the galaxy disk, with increasing velocity offsets above the midplane in both phases, consistent with a multiphase wind. The measured electron density, using [SII], differs significantly from standard expectations of galactic winds. We find electron density declines from the galaxy centre to $\sim2$ kpc, then rises again, remaining high ($\sim100-300$ cm$^{-3}$) out to $\sim$5 kpc. We find that HI dominates the mass loading. The total HI mass outflow rate (above $z~>2$ kpc) is between $5-13~M_{\odot}~\rm yr^{-1}$, accounting for uncertainties from disk-blurring and group interactions. The total ionised mass outflow rate (traced by H$α$) is between $0.5~M_{\odot}~\rm yr^{-1}$ and $5~M_{\odot}~\rm yr^{-1}$, depending on $n_e(z)$ assumptions. From ALMA/ACA observations, we place an upper-limit on CO flux in the outflow which correlates to $\lesssim2.9~M_{\odot}~\rm yr^{-1}$. We also show that the entire outflow is not limited to the bicone, but a secondary starburst at the edge generates a more widespread outflow, which should be included in simulations. The cool gas in NGC 4666 wind has insufficient velocity to escape the halo of a galaxy of its mass, especially because most of the mass is present in the slower atomic phase. This strong biconical wind contributes to gas cycling around the galaxy.

The GECKOS Survey: Resolved, multiphase observations of mass-loading and gas density in the galactic wind of NGC 4666

TL;DR

This study presents a resolved, multiphase view of the galactic wind in NGC 4666 by combining VLT/MUSE GECKOS data with HI and CO observations. The authors map the ionised and neutral gas in a biconical outflow extending up to ~8 kpc, measure an unusual electron-density profile that stays high ( cm) beyond the disk, and derive mass- and energy-loading factors across phases. They find that HI dominates the mass loading (\dot{M}_{out,HI} ~ 5–13 M yr) while the ionised component is an order of magnitude lower, with CO undetected setting strict molecular limits; the wind is unlikely to escape the halo of a Milky Way–mass galaxy. A secondary starburst-driven bubble adds complexity to the feedback, highlighting the need for high-resolution, multiphase observations to test wind theories and improve galaxy-evolution simulations. These results underscore the role of gas cycling and the importance of electron-density diagnostics for accurately quantifying outflow masses in realistic galactic winds.

Abstract

We present a multiphase, resolved study of the galactic wind extending from the nearby starburst galaxy NGC 4666. For this we use VLT/MUSE observations from the GECKOS program and HI data from the WALLABY survey. We identify both ionised and HI gas in a biconical structure extending to at least 8 kpc from the galaxy disk, with increasing velocity offsets above the midplane in both phases, consistent with a multiphase wind. The measured electron density, using [SII], differs significantly from standard expectations of galactic winds. We find electron density declines from the galaxy centre to kpc, then rises again, remaining high ( cm) out to 5 kpc. We find that HI dominates the mass loading. The total HI mass outflow rate (above kpc) is between , accounting for uncertainties from disk-blurring and group interactions. The total ionised mass outflow rate (traced by H) is between and , depending on assumptions. From ALMA/ACA observations, we place an upper-limit on CO flux in the outflow which correlates to . We also show that the entire outflow is not limited to the bicone, but a secondary starburst at the edge generates a more widespread outflow, which should be included in simulations. The cool gas in NGC 4666 wind has insufficient velocity to escape the halo of a galaxy of its mass, especially because most of the mass is present in the slower atomic phase. This strong biconical wind contributes to gas cycling around the galaxy.

Paper Structure

This paper contains 25 sections, 3 equations, 16 figures, 2 tables.

Table of Contents

  1. Introduction
  2. Observations, data reduction and methods
  3. MUSE observations and reduction
  4. Continuum subtraction
  5. Emission line fitting
  6. Narrow-band imaging data
  7. ALMA/ACA observations
  8. ASKAP observations
  9. Properties of the ionised gas wind of NGC 4666 and the star formation driving it
  10. Extraplanar H$\alpha$ in NGC 4666
  11. Star formation driving the wind
  12. Kinematics sub-structure of the wind
  13. Electron density profile and Ionisation State of Wind
  14. Outflow rates in NGC 4666
  15. Resolved ionised gas mass outflow rate
  16. ...and 10 more sections

Figures (16)

  • Figure 1: Top left: H$\alpha$ narrow-band image of NGC 4666, rotated such that the disk is horizontal. The image is plotted in log-scale to highlight the low surface brightness components. The biconical superwind is denoted by the red arrows. We identify 4 filaments, called 'limbs', that form an X-shape typical of a biconical wind. The H$\alpha$ gas is brighter inside the limbs than outside. We also identify a smaller bubble to the right on the upper side of the galaxy. Top right: Same rotated H$\alpha$ narrow-band image, with MUSE pointings overlaid. Archival pointing frames are shown in orange, and GECKOS pointing frames are displayed in purple. Bottom: A 4 colour image of MUSE data that combines H$\alpha$ (red), [OIII] 5007 (blue), [NII] 6583 (orange), and R-band continuum (white) is shown. We use a different max and min image cut for the disk and extraplanar regions; both are log scale. There are clear filaments of ionised gas that extend over 6 kpc from the starburst of the galaxy, which connect to a central starburst region (indicated by a dashed box).
  • Figure 2: From left to right, the top panels show the resolved SFR surface density, molecular gas surface density and depletion time. The ACA beam is shown in the bottom-left corner of the molecular gas surface density panel. We assume $\alpha_{\rm CO}=2.5$. The bottom panels show the corresponding major axis radial profiles over the entire minor axis direction. For the $\Sigma_{\rm mol}$ and $t_{\rm depl}$ profiles, the solid lines correspond to a conversion factor $\alpha_{\rm CO}=2.5$, while the dashed lines refer to $\alpha_{\rm CO}=4.36$. The hatching represents the areas that the limbs cover at $\pm$2.5 kpc from the disk midplane (dashed box in Fig. \ref{['fig:ha_bicone']}).
  • Figure 3: The resolved Kennicutt-Schmidt relationship. The measurements of NGC 4666 are shown as blue circles: the filled points are those measured within the wind launching area (dashed box in Fig. \ref{['fig:ha_bicone']}), while the empty points are the other measurements in the disk. The dark golden points correspond to similar spatial-scale measurements of IRAS 08339+6517, presented in ReichardtChu2022b. The grey points represent data from local face-on spirals from the PHANGS sample Sun2023. M 82 Leroy2015, NGC 253 Bolatto2013b and NGC 1482 VeilleuxRupke2002Salak2020 are represented as a purple diamond, a red square and a green hexagon, respectively. The dashed lines indicate where the depletion time $t_{\rm depl}$ would be 1.0 Gyr (bottom line) and 0.1 Gyr (top line).
  • Figure 4: Gas velocity offsets within the upper cone of the NGC 4666 superwind. The dark magenta points denote the H$\alpha$ velocity measurements from VLT/MUSE, by considering the whole outflow pointing (lower points) and the upper right main filament only (upper points). The dashed lines represent the 7th-order polynomial fits to these velocities. The blue curve represents the HI velocity measurements from the WALLABY observations. The velocity values have been de-projected by dividing the observed velocities by cos($i$) (with $i=69.6\degr$ the inclination of NGC 4666).
  • Figure 5: H$\alpha$ velocity dispersion. Top:$\sim$45 pc spatial resolution map (0.6 $\arcsec$ pixel size). Bottom: 500 pc spatial resolution map. The wind filament targeted by our GECKOS 'outflow' pointing shows an enhanced gas velocity dispersion compared to that of the disk.
  • ...and 11 more figures