Structure of the bulge of the galaxy NGC 4258

The superfine structure of the bulge of the galaxy NGC 4258 has been investigated in H₂O maser emission at the epochs on February 4, 2013, and November 29, 2013. The peak intensities of the spectral components reached F ≈ 5 Jy. The emission of the component at v = 476 km s^-1 dominated at the beginning of this period; the second component at v = 487 km s^-1 was observed at the end of the period. The structure is a chain of compact components up to 200 µas or 7mpc in extent. The velocity of the local standard of rest is v _LSR = 482 km s^-1. Two bright compact components with a separation between them Δρ ≈ 35 µas or 1.3 mpc and a pair of components spaced 13 µas apart, whose brightness reaches 30% of the peak value corresponding to a brightness temperature T _b ≈ 1018 K, are located at the center. The sizes of the components are ~2–3 µas. A splitting and a shift of the two pairs of components relative to each other by 8 µas or 0.3 mpc in the 45° direction are observed at the end of the period. The velocity gradient of the structure is dV/dρ = 224 km s^-1 mas^-1, suggesting a solid-body rotation with a period T ≈ 760 years. The compact components correspond to the tangential directions of the arm. Two parallel chains of components corresponding to the tangential directions of the walls of the bipolar outflow carrying away an excess angular momentum are ejected from the central part of the bulge, two sources. The outflow is oriented at an angle X ≈ 15° relative to the disk axis. The brightness of the outflow fragments does not exceed 1.5% of the peak value. The ejection of material from the central part in the northward direction at a level up to 0.2%, T _b ≈ 10¹⁵ K, is observed at the epoch on February 4, 2013, at v = 478 km s^-1. The core structure suggests a double system: parallel disks–vortices spaced 0.25 mpc apart.