DETECTION OF A SUBPARSEC DIAMETER DISK IN THE NUCLEUS OF NGC-4258

We present the first VLBI synthesis map of the luminous H2O maser in t he nucleus of NGC 4258 (M106). The spectral features of the masers nea r the systemic velocity, i.e., between 455 and 528 km s(-1), are distr ibuted along an almost east-west line in the sky (83 degrees +/- 2 deg rees east of north), subtending about 260 microarcseconds (mu as) in a ngle (0.009 pc, at a distance of 7 Mpc). The distribution in the ortho gonal direction is less than 50 mu as in extent. The line-of-sight vel ocity of the maser emission changes with position along the are with a n essentially constant gradient of 3.70 +/- 0.02 mu as km(-1) s or 797 0 +/- 40 km s(-1) pc(-1). The most redshifted emission lies farthest t o the east. The masers clearly delineate a disk structure, viewed edge -on, which is either in solid body rotation or in Keplerian motion whe re 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 ex tragalactic H2O maser that has been mapped with VLBI techniques. The p osition of the maser (referenced to the feature at 465.76 km s(-1)) is alpha(2000) = 12(h)18(m)57.(s)510 +/- 0.(s)004, delta(2000) = 47 degr ees 18'14 ''.27 +/- 0 ''.03, which is coincident with the position of the continuum nuclear source to within 2 pc. The simplest model that e xplains our data is one in which emission from the nucleus NGC 4258 or iginates in a Keplerian molecular ring that is at least 0.014 pc in di ameter and bound by a central mass of about 5000 M circle dot. If we c onsider the recently discovered high-velocity emission at +/-900 km s( -1) offset from the systemic velocity, whose positions we did not meas ure, as well as the apparent centripetal acceleration of the systemic velocity features of about 6-11 km s(-1) yr(-1), then a more comprehen sive model is that of a disk of diameter 0.2 pc, rotating with a veloc ity of 900 km s(-1) (rotational period of 800 yr), which is bound by a central mass of about 2.1 x 10(7) M circle dot. The total gas mass in the disk is limited to 10(6) M circle dot. The maser emission arises in a toroidal region with a fractional thickness of about 0.2. This to roid/disk probably lies near the center of the galactic nucleus. For b oth of these disk models, the mass density of the central source is at least 3.5 x 10(9) M circle dot pc(-3), which could be concentrated in a supermassive object. The maser emission that we observed near the s ystemic velocity has a brightness temperature of at least 10(11) K and is probably unsaturated and amplifies a background, high brightness-t emperature, radio-continuum source known to exist in the nucleus. The beam angle of the emission is probably about 0.015 sr. The VLBI observ ations reported here provide an invaluable dynamical probe of subparse c-scale structure in the nucleus of a nearby active galaxy. The measur ements of centripetal acceleration and the VLBI observations provide a distance estimate for the galaxy of 5.4 +/- 1.3 pc.