The GECKOS Survey: Extraplanar ionised gas in star-forming galaxies from eDIG to galaxy-scale winds

Abstract

We map the extraplanar gas, with $\sim$50-200 pc resolution, in nine star-forming galaxies using Multi-Unit Spectroscopic Explorer (MUSE) observations from the GECKOS VLT Large Program targeting edge-on galaxies with similar stellar mass as the Milky Way. The narrow range in stellar mass ($\pm0.35$ dex) of the GECKOS sample makes it ideal for studying trends with star formation rate (SFR). We find strong extraplanar emission reaching $\sim$2-8 kpc from the disk midplane in all targets with $\rm{SFR}\geq$1 M$_{\odot}$ yr$^{-1}$. Targets with SFR$\,\geq\,$5 M$_{\odot}$ yr$^{-1}$ have brighter, more extended H$α$ emission compared to lower SFR targets. In high-SFR systems, the gas velocity dispersion ($σ_{\rm Hα}$) shows a biconical morphology, consistent with the expectation of outflows. This agrees with previous works suggesting high velocity dispersion in a biconical shape is a good means to identify outflows. We find mixed results using line diagnostics ([OIII]$_{5007}$/H$β$ - [NII]/H$α$ and $σ_{\rm Hα}$ - [SII]/H$α$) to spatially resolve ionisation mechanisms across the extraplanar gas. The highest [NII]/H$α$ are the extraplanar gas of the highest SFR systems, yet main-sequence galaxies have the highest [OIII]/H$β$. While the morphology of [NII]/H$α$ may be useful to identify outflows, the absolute value of the line ratio alone may not distinguish strong outflows from extraplanar gas of main-sequence galaxies. The ubiquitous extraplanar emission can be interpreted as the result of feedback, in the form of large-scale winds for starbursts or smaller-scale galactic fountains for main-sequence galaxies. Moreover, shock-heating may ionise gas at the interface of the disk and the circumgalactic medium, independent of the source of the gas.

Figures (20)

• Figure 1: A collection of DESI Legacy Imaging Surveys (DR9) and Pan-STARRS-1 (DR2) colour images of the GECKOS galaxies covered in this paper. The images were produced using the $g$, $r$, and $z$ bands with the Astropy package make_lupton_rgbLupton2004 and an asinh stretch. The galaxies were rotated to have the photometric major axis and the dust lane (and photometric major axis) oriented horizontally, and such that the dust lane (if offset) is to the lower side of the image.
• Figure 2: Four colour emission line plus continuum images of GECKOS targets studied in this paper. In all figures, H$\alpha$ emission is shown in red, [NII] 6583 in yellow, [OIII] 5007 in blue, and R band continuum in white. The white line in each panel represents a $\sim$3 kpc scale. Extraplanar emission is visible in all targets, often forming multi-kiloparsec filaments extending out from the disk.
• Figure 3: Maps of log(H$\alpha$) flux (left), H$\alpha$ velocity dispersion that has been corrected for instrumental dispersion (middle-left), and log([NII]/H$\alpha$) flux ratio (middle-right) and velocity (right) made for each galaxy in our sample. Note that the galaxies in this figure are ordered by decreasing SFR. Spatial axes are in units of kpc. The points where the vertical and horizontal axes equal 0 kpc mark the approximate location of the galactic midplane and rotational center respectively. We mark the 1.5 kpc and 3.5 kpc points on the major axis with vertical gray dashed and dotted lines respectively.
• Figure 4: H$\alpha$ velocity dispersion maps made from 500 pc resolution data for each galaxy. The points where the vertical and horizontal axes =0 mark the approximate location of the galactic midplane and rotational axis respectively.
• Figure 5: Illustration of the regions defined for each galaxy in this paper. Brown highlights the disk region, blue highlights the "major axis: $\pm$0-1.5 kpc" region, green highlights the "major axis: $\pm$1.5-3 kpc" region, and the grey area outside of these highlights is the "major axis: $>\pm3$ kpc" region. When we project these regions onto an observation of an edge-on galaxy, the "disk" region aligns with the galactic midplane, and the "0 kpc" mark aligns with the galactic center.