DISCOVERY OF THE RECOMBINING PLASMA IN THE SOUTH OF THE GALACTIC CENTER: A RELIC OF THE PAST GALACTIC CENTER ACTIVITY?

Abstract

We report Suzaku results for soft X-ray emission to the south of the Galactic\ncenter (GC). The emission (hereafter "GC South") has an angular size of ~42' x\n16' centered at (l, b) ~ (0.0, -1.4), and is located in the largely extended\nGalactic ridge X-ray emission (GRXE). The X-ray spectrum of GC South exhibits\nemission lines from highly ionized atoms. Although the X-ray spectrum of the\nGRXE can be well fitted with a plasma in collisional ionization equilibrium\n(CIE), that of GC South cannot be fitted with a plasma in CIE, leaving\nhump-like residuals at ~2.5 and 3.5 keV, which are attributable to the\nradiative recombination continua of the K-shells of Si and S, respectively. In\nfact, GC South spectrum is well fitted with a recombination-dominant plasma\nmodel; the electron temperature is 0.46 keV while atoms are highly ionized (kT\n= 1.6 keV) in the initial epoch, and the plasma is now in a recombining phase\nat a relaxation scale (plasma density x elapsed time) of 5.3 x 10^11 s cm^-3.\nThe absorption column density of GC South is consistent with that toward the GC\nregion. Thus GC South is likely to be located in the GC region (~8 kpc\ndistance). The size of the plasma, the mean density, and the thermal energy are\nestimated to be 97 pc x 37 pc, 0.16 cm^-3, and 1.6 x 10^51 erg, respectively.\nWe discuss possible origins of the recombination-dominant plasma as a relic of\npast activity in the GC region.\n