THE GALACTIC CENTER ENVIRONMENT

TLDR

The central half kiloparsec of the Milky Way hosts unique phenomena, with gas states and star formation processes that differ from the disk, and gas not consumed either escapes as a hot X‑ray halo wind or moves inward through circumnuclear disks toward the central parsec. The review investigates the structure and activity of the interstellar medium in the Galactic center, focusing on its inevitable inflow toward the Galactic gravitational potential well. It examines this inflow by analyzing the observed ISM structure and activity within the central region. Dissipative processes concentrate gas into a ~200‑pc Central Molecular Zone with high densities, velocity dispersions, temperatures, and strong magnetic fields, while in the central parsec SgrA* accretes only a tiny fraction of inflowing material, likely due to a limit cycle where inflow triggers star formation that temporarily halts inflow via outflows.

Abstract

▪ Abstract The central half kiloparsec region of our Galaxy harbors a variety of phenomena unique to the central environment. This review discusses the observed structure and activity of the interstellar medium in this region in terms of its inevitable inflow toward the center of the Galactic gravitational potential well. A number of dissipative processes lead to a strong concentration of gas into a “Central Molecular Zone” of about 200-pc radius, in which the molecular medium is characterized by large densities, large velocity dispersions, high temperatures, and apparently strong magnetic fields. The physical state of the gas and the resultant star formation processes occurring in this environment are therefore quite unlike those occurring in the large-scale disk. Gas not consumed by star formation either enters a hot X ray–emitting halo and is lost as a thermally driven galactic wind or continues moving inward, probably discontinuously, through the domain of the few parsec-sized circumnuclear disks and eventually into the central parsec. There, the central radio source SgrA * currently accepts only a tiny fraction of the inflowing material, likely as a result of a limit cycle wherein the continual inflow of matter provokes star formation, which in turn can temporarily halt the inflow via mass-outflow winds.