Number Density Distribution of Near-Infrared Sources on a Sub-Degree Scale in the Galactic Center: Comparison with the Fe XXV Ka Line at 6.7 keV

Abstract

The stellar distribution derived from an $H$ and $K_{\mathrm{S}}$-band survey of the central region of our Galaxy is compared with the Fe XXV K$\alpha$ (6.7 keV) line intensity observed with the Suzaku satellite. The survey is for the Galactic coordinates $|l|\lesssim 3^{\circ }.0$ and $|b|\lesssim 1^{\circ }.0$ (equivalent to 0.8 kpc $\times$ 0.3 kpc for $R_0=8$ kpc), and the number-density distribution $N(K_{\mathrm{S},0};l,b)$ of stars is derived using the extinction-corrected magnitude $K_{\mathrm{S},0}=10.5$. This is deep enough to probe the old red giant population and in turn to estimate the ($l$, $b$) distribution of faint X-ray point sources such as coronally active binaries and cataclysmic variables. In the Galactic plane ($b=0^{\circ }$), $N(10.5;l,b)$ increases to the Galactic center as $|l{|}^{-0.30\pm 0.03}$ in the range of $-0^{\circ }.1\ge l\ge -0^{\circ }.7$, but this increase is significantly slower than the increase ($|l{|}^{-0.44\pm 0.02}$ ) of the Fe XXV K$\alpha$ line intensity. If normalized with the ratios in the outer region $1^{\circ }.5\le |l|\le 2^{\circ }.8$, where faint X-ray point sources are argued to dominate the diffuse Galactic X-ray ridge emission, the excess of the Fe XXV K$\alpha$ line intensity over the stellar number density is at least a factor of two at $|l|=0^{\circ }.1$. This indicates that a significant part of the Galactic center diffuse emission arises from a truly diffuse optically-thin thermal plasma, and not from an unresolved collection of faint X-ray point sources related to the old stellar population.

12 pages, 4 figures; accepted for publication in PASJ. Some bold-font characters were replaced

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• A Chandra Catalog of X-ray Sources in the Central 150 pc of the Galaxy
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• 3D CMZ V: A new orbital model of our Galaxy's Center, informed by data across the electromagnetic spectrum
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