NGC 4258: A Compact Central Infrared Source Revealed

We present high-resolution (~06) near-infrared Keck observations that probe the optically obscured nucleus of NGC 4258 (M 106), which is believed to contain a 3.6 × 10⁷ M_☉ black hole. The images show a compact central source at K (2.21 μm) and H (1.65 μm). To determine the strength of the compact component against the stellar background, we normalized the J (1.25 μm) surface brightness profile, which is taken to be dominated by the bulge stars, to the K and H profiles at 200 pc and subtracted it out. The normalization constant in each case agrees well with the outer colors of highly inclined spirals. The compact source has a measured K flux of 4.5 mJy and an H flux of 1.1 mJy. After making reasonable assumptions about the intrinsic energy distribution of the central source based on optical polarimetry and X-ray measurements, we derive a reddening to the source of A_V ~ 17 mag. When corrected for this extinction, the IR (1.5-3.5 μm) luminosity is ~2 × 10⁷ L_☉. The corresponding X-ray (2-10 keV) luminosity is ~10⁷ L_☉, while the central source has been estimated at ~10² L_☉ in the radio. We suggest on the basis of the polarimetric evidence that the source of the near-infrared excess is nonthermal, although the possibility of thermal emission by dust grains in the inferred torus cannot be ruled out.