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

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 $\sim 2$ kpc, then rises again, remaining high ($\sim 100-300$ 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 $5-13M_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$, accounting for uncertainties from disk-blurring and group interactions. The total ionised mass outflow rate (traced by H$\alpha$) is between $0.5M_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ and $5M_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$, depending on $n_e(z)$ assumptions. From ALMA/ACA observations, we place an upper-limit on CO flux in the outflow which correlates to $\lesssim 2.9M_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$. 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.