| Abstract | We present the first VLBI synthesis map of the luminous H2O maser in the nucleus of NGC 4258 (M106). The spectral features of the masers near the systemic velocity, i.e., between 455 and 528 km s-1, are distributed along an almost east-west line in the sky (83±2 east of north), subtending about 260 microarcseconds (μas) in angle (0.009 pc, at a distance of 7 Mpc). The distribution in the orthogonal direction is less than 50 μas in extent. The line-of-sight velocity of the maser emission changes with position along the arc with an essentially constant gradient of 3.70±0.02 μas km-1 s or 7970±40 km s-1 pc-1. The most redshifted emission lies farthest to the east. The masers clearly delineate a disk structure, viewed edge-on, which is either in solid body rotation or in Keplerian motion where the masers are confined to an annulus having a fractional width of less than 0.3. This masing region is about an order of magnitude more compact than the one in NGC 3079, the only other extremely luminous extragalactic H2O maser that has been mapped with VLBI techniques. The position of the maser (referenced to the feature at 465.76 km s-1) is α2000 = 12h 18m 57s.510± 0s.004, δ2000 = 47°18'14".27±0".03 which is coincident with the position of the continuum nuclear source to within 2 pc. The simplest model that explains our data is one in which emission from the nucleus NGC 4258 originates in a Keplerian molecular ring that is at least 0.014 pc in diameter and bound by a central mass of about 5000 Msun. If we consider the recently discovered high-velocity emission at ±900 km s-1 offset from the systemic velocity, whose positions we did not measure, as well as the apparent centripetal acceleration of the systemic velocity features of about 6-11 km s-1 yr-1, then a more comprehensive model is that of a disk of diameter 0.2 pc, rotating with a velocity of 900 km s-1 (rotational period of 800 yr), which is bound by a central mass of about 2.1 × 107 Msun. The total gas mass in the disk is limited to 106 Msun. The maser emission arises in a toroidal region with a fractional thickness of about 0.2. This toroid/disk probably lies near the center of the galactic nucleus. For both of these disk models, the mass density of the central source is at least 3.5 × 109 Msun pc-3, which could be concentrated in a supermassive object. The maser emission that we observed near the systemic velocity has a brightness temperature of at least 1011 K and is probably unsaturated and amplifies a background, high brightness-temperature, radio-continuum source known to exist in the nucleus. The beam angle of the emission is probably about 0.015 sr. The VLBI observations reported here provide an invaluable dynamical probe of subparsec-scale structure in the nucleus of a nearby active galaxy. The measurements of centripetal acceleration and the VLBI observations provide a distance estimate for the galaxy of 5.4±1.3 pc. |
