The interface between the galactic disk and halo, known as the disk-halo interface (DHI), plays a critical role in galaxy evolution because the DHI is where halo gas cools and accretes onto the star-forming disk, where metal-poor gas must survive to fuel star formation as observed in stellar populations, and where gas ejected by feedback is recycled back to the disk. Yet studying this very inner region of the CGM has been challenging due to its small scale (~few kpc) compared to the entire CGM (~few hundred kpc). The Milky Way is an ideal laboratory for the DHI, where absorption toward halo stars with known distances isolates gas in this region.
Metallicity serves as an excellent tracer of gas with different origins at the DHI; however, measuring the metallicity of ionized gas remains challenging, primarily due to difficulties in constraining the amount of ionized hydrogen. In our recent work (Choi et al., 2024), we measured and mapped the metallicity of ionized gas clouds at the Milky Way’s DHI using halo star-pulsar pairs, which provide precise ionization corrections. By analyzing HST UV spectra available from MAST, we determined the metal content from absorption spectroscopy of halo stars. The ionized gas content (electron column density) was constrained by the dispersion measures of pulsars that lie in the same globular clusters as these UV-bright halo stars (see the schematic below).
This study reveals a large scatter in metallicity values (0.04–3.2 solar metallicity), suggesting that the DHI comprises a mixture of gaseous structures with multiple origins. Notably, the findings provide clear evidence of gas recycling processes, as indicated by the detection of a supersolar infalling cloud. The estimated timescale for gas recycling processes at the DHI, often referred to as a “Galactic fountain,” is tens of millions of years.
