Present-day Galactic Evolution: Low-metallicity, Warm, Ionized Gas Inflow Associated with High-Velocity Cloud Complex A

TL;DR

This study uses WHAM observations to analyze high-velocity cloud Complex A, revealing its ionized and neutral mass, distance, and metallicity, and supporting models of Galactic halo gas accretion and ionizing radiation escape.

Contribution

First full H-alpha map of Complex A and detailed analysis of its ionization, mass, and metallicity, confirming models of Galactic halo gas inflow and ionizing photon escape fraction.

Findings

Mass of neutral and ionized gas exceeds one million solar masses

Ionizing radiation model aligns with the cloud's known distance and geometry

Sub-solar metallicities consistent with previous studies

Abstract

The high-velocity cloud (HVC) Complex A is a probe of the physical conditions in the Galactic halo. The kinematics, morphology, distance, and metallicity of Complex A indicate that it represents new material that is accreting onto the Galaxy. We present Wisconsin H-alpha Mapper (WHAM) kinematically resolved observations of Complex A over the velocity range of -250 to -50 km/s in the local standard of rest reference frame. These observations include the first full H-alpha intensity map of Complex A across (l, b) = (124, 18) to (171, 53) and deep targeted observations in H-alpha, [S II]6716, [N II]6584, and [O I]6300 towards regions with high H I column densities, background quasars, and stars. The H-alpha data imply that the masses of neutral and ionized material in the cloud are similar, both being greater than a million solar masses. We find that the Bland-Hawthorn & Maloney (1999, 2001) model for the intensity of the ionizing radiation near the Milky Way is consistent with the known distance of the high-latitude part of Complex A and an assumed cloud geometry that puts the lower-latitude parts of the cloud at a distance of 7 to 8 kpc. This compatibility implies a 5% ionizing photon escape fraction from the Galactic disk. We also provide the nitrogen and sulfur upper abundance solutions for a series of temperatures, metallicities, and cloud configurations for purely photoionized gas; these solutions are consistent with the sub-solar abundances found by previous studies, especially for temperatures above 10,000 K or for gas with a high fraction of singly-ionized nitrogen and sulfur.