2013ApJ...773...20N

We report Suzaku results for soft X-ray emission to the south of the Galactic center (GC). The emission (hereafter "GC South") has an angular size of ∼42'x16' centered at (l, b) ∼ (0.°0, - 1.°4) and is located in the largely extended Galactic ridge X-ray emission (GRXE). The X-ray spectrum of GC South exhibits emission lines from highly ionized atoms. Although the X-ray spectrum of the GRXE can be well fitted with a plasma in collisional ionization equilibrium (CIE), that of GC South cannot be fitted with a plasma in CIE, leaving hump-like residuals at ∼2.5 and 3.5 keV, which are attributable to the radiative recombination continua of the K-shells of Si and S, respectively. In fact, GC South spectrum is well fitted with a recombination-dominant plasma model; the electron temperature is 0.46 keV while atoms are highly ionized (kT = 1.6 keV) in the initial epoch, and the plasma is now in a recombining phase at a relaxation scale (plasma densityxelapsed time) of 5.3x10¹¹ s/cm3. The absorption column density of GC South is consistent with that toward the GC region. Thus, GC South is likely to be located in the GC region (∼8 kpc distance). The size of the plasma, the mean density, and the thermal energy are estimated to be ∼97 pcx37 pc, 0.16/cm3, and 1.6x10⁵¹ erg, respectively. We discuss possible origins of the recombination-dominant plasma as a relic of past activity in the GC region.