ar X iv : a st ro - p h / 98 03 17 8 v 1 1 6 M ar 1 99 8 Morphology of the Nuclear Disk in M 87

A deep, fuly sampled diffraction limited (FWHM ∼ 70 mas) narrow-band image of the central region in M87 was obtained with the Wide Filed and Planetary Camera 2 of the Hubble Space Telescope using the dithering technique. The Hα+[N ii] continuum subtracted image reveals a wealth of details in the gaseous disk structure described earlier by Ford et al. (1994). The disk morphology is dominated by a well defined three-arm spiral pattern. In addition, the major spiral arms contain a large number of small " arclets " covering a range of sizes (0 ′′ .1–0 ′′ .3 = 10–30 pc). The overall surface brightness profile inside a radius ∼ 1. ′′ 5 (100 pc) is well represented by a power-law I(µ) ∼ µ −1.75 , but when the central ∼ 40 pc are excluded it can be equally well fit by an exponential disk. The major axis position angle remains constant at about PA disk ∼ 6 • for the innermost ∼ 1 ′′ , implying the disk is oriented nearly perpendicular to the synchrotron jet (PAjet ∼ 291 •). At larger radial distances the isophotes twist, reflecting the gas distribution in the filaments connecting to the disk outskirts. The ellipticity within the same radial range is e = 0.2 − 0.4, which implies an inclination angle of i ∼ 35 •. The sense of rotation combined with the dust obscuration pattern indicate that the spiral arms are trailing.