The Anisotropic Circumgalactic Medium of Sub-L$^*$ Galaxies

TL;DR

This study reveals that lower-mass galaxies exhibit an anisotropic circumgalactic medium with harder ionization along the polar axis at small radii, consistent across a range of stellar masses, indicating potential relic AGN activity or shock ionization.

Contribution

It demonstrates the presence of anisotropic ionization signatures in the CGM of sub-L* galaxies, extending previous findings from massive galaxies, using spectral stacking of emission lines.

Findings

Higher [N II]/Hα ratio in the inner polar region at small radii

Ionization along the polar axis is outside the star formation regime

Signature consistent across different stellar masses

Abstract

Using stacked emission line flux measurements of cool circumgalactic gas (CGM) in lower-mass galaxies ($10^{9.0} \le M_*/M_\odot \le 10^{10.2} $), we measure the dependence of the emission characteristics on orientation relative to the disk plane as a function of radius and compare to that we found previously for massive ($M_* > 10^{10.4} M_\odot$) early-type galaxies. Although the line ratios (the lower [N II]/H$\alpha$ and [O III]/H$\beta$) suggest an overall softer ionizing source than in the more massive galaxies, consistent with previous findings, we find the same ionization hardening signature (a higher [N II]/H$\alpha$ ratio in the inner polar region) along the polar direction at small radii that we found for the more massive galaxies. The line ratio in the inner polar bin is distinct from that measured for the inner planar bin with 99.99% confidence and with $>$ 99.9% confidence we conclude that it lies outside the star formation regime. The effective hardening of the ionization of the CGM along the polar axis, at small radii, could either indicate relic effects of AGN activity or shock ionization. In either case, this signature appears to be ubiquitous across the stellar mass range we are able to explore with our spectral stacking technique and currently available archival data.