The Galactic Center Environment

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.

Correlation between SiO and X-ray emission in the galactic center

“…From our estimations of N H 2 and N SiO at the five positions of the maxima in the X-ray emission (Table 1), we derive a SiO fractional abundance of ∼10 −9 . Taking into account the value for the elemental abundance of silicon of [19] (X Si =n Si /n H ∼3.55·10 −5 ), considering that the metallicity at the GC is likely twice the solar value [20] and [HI]/[H 2 ] ∼0.05 in the central 500 pc of the Galaxy [21], and assuming that most of the Si is ejected from grains in the form of SiO, we conclude that with only a small fraction of atoms evaporated from small grains (∼0.01%), we could reproduce the observed fractional abundances.…”

The Galactic Center as a point source of neutrons at EeV energies

“…Gas densities in the interstellar medium can reach up to ∼ 10 4.5 cm −3 with high filling factor > 0.1 [8]. Through radio maps from the CO emission [9], one can trace the global distribution of matter inside the region, and create models for the matter distribution therein.…”

The Herschel view of the Galactic center

“…Analyses of the kinematics of CO and Hi emission from the central 3 kpc, in particular the behavior of the radial velocity of the gas as a function of Galactic longitude, led to the conclusion that gas motions in the inner Galaxy are dominated by a stellar bar extending to a galactocentric radius of about 3 kpc (Binney et al 1991;Morris & Serabyn 1996). The bar's major axis is inclined with respect to our line-of-sight by about 20 • to 45 • with the closer part being in the first quadrant (Figure 1).…”

“…The CMZ, which occupies the central 500 pc, contains 5 to 10% of the molecular gas mass of the Galaxy, hosts the largest concentration of massive stars and star clusters, (Morris & Serabyn 1996, Ferriere, Gillard, & Jean 2007 and two of the most massive, densest, and luminous star-forming complexes in the Milky Way, Sgr A and Sgr B2 (YusefZadeh et al 2008;. Giant molecular clouds (GMCs) in the CMZ are one to two orders of magnitude denser and have over an order of magnitude larger line-widths than GMCs in the Galactic plane beyond 3 kpc (Bally et al 1987;1988;Oka et al 1998;Tsuboi et al 1999;Oka et al 2001).…”

“…The CMZ contains ∼3 − 7 × 10 7 M of high-density (<n(H 2 )> > 10 4 cm −3 ) molecular gas (Bally et al 1987(Bally et al , 1988Morris, & Serabyn, 1996;Ferriere, Gillard, & Jean 2007;Longmore et al 2013a,b). Do massive stars and star clusters dominate feedback, regulate the state and energetics of the interstellar medium (ISM), and the rate at which the ISM is converted into stars?…”