A deep , fuly sampled diffraction limited ( FWHM \sim 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 H \alpha + [ 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 ^ { \prime \prime } .1–0 ^ { \prime \prime } .3 = 10–30 pc ) .
The overall surface brightness profile inside a radius \sim 1 \farcs 5 ( 100 pc ) is well represented by a power-law I ( \mu ) \sim \mu ^ { -1.75 } , but when the central \sim 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 } \sim 6 ^ { \circ } for the innermost \sim 1 ^ { \prime \prime } , implying the disk is oriented nearly perpendicular to the synchrotron jet ( PA _ { jet } \sim 291 ^ { \circ } ) .
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 \sim 35 ^ { \circ } .
The sense of rotation combined with the dust obscuration pattern indicate that the spiral arms are trailing .