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

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 similar to 42' x 16' centered at (l, b) similar to (0 degrees.0, -1 degrees.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 similar to 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 density x elapsed time) of 5.3 x 10(11) s cm(-3). 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 (similar to 8 kpc distance). The size of the plasma, the mean density, and the thermal energy are estimated to be similar to 97 pc x 37 pc, 0.16 cm(-3), and 1.6 x 10(51) erg, respectively. We discuss possible origins of the recombination-dominant plasma as a relic of past activity in the GC region.