2023MNRAS.519.6098L

The extraplanar diffuse ionized gas (eDIG) represents the cool/warm ionized gas reservoir around galaxies. We present spatial analysis of the Hα images of 22 nearby edge-on spiral galaxies taken with the Apache Point Observatory 3.5-m telescope (eDIG-CHANGES). We conduct an exponential fit to the vertical Hα intensity profiles of the galaxies, of which 16 can be decomposed into thin + thick disk components. The median value of the Hα scale height of the thick disk is $\langle h_{\rm H\alpha }\rangle =1.13\pm 0.14\rm ~kpc$. We further examine the dependence of h_Hα on the stellar mass, SFR, and SFR surface density (SFR_SD) of the galaxies. We find a tight sublinear correlation between h_Hα and SFR, expressed in h_Hα ∝ SFR^α, where α ≈ 0.29. Moreover, the offset of individual galaxies from the best-fit SFR-h_Hα relation, expressed in h_Hα/SFR^α, shows significant anti-correlation with SFR_SD. We further compare the vertical extension of the eDIG to multi-wavelength measurements of other CGM phases. We find the eDIG slightly more extended than the neutral gas. This indicates the existence of some extended ionizing sources, in addition to the leaking photons from the disk star formation regions. Most galaxies have an X-ray scale height smaller than Hα, suggesting the majority of the X-ray photons are actually from the thick disk instead of the extended CGM. h_Hα is comparable to the L-band radio continuum scale height. This indicates that the thermal and non-thermal electrons have similar spatial distributions, a natural result if both are transported outwards by a galactic wind. This further indicates the thermal gas, cosmic rays, and magnetic field may be close to energy equipartition.