  We present {\it Hubble Space Telescope} Advanced Camera for Surveys
multicolor coronagraphic images of the recently discovered edge-on debris disk
around the nearby ($\sim10$ pc) M dwarf AU Microscopii. The disk is seen
between $r = $0\farcs 75 -- 15'' (7.5 -- 150 AU) from the star. It has a thin
midplane with a projected full-width-at-half-maximum (FWHM) thickness of 2.5 --
3.5 AU within $r < 50$ AU of the star that increases to 6.5 -- 9 AU at $r \sim
75$ AU. The disk's radial brightness profile is generally flat for $r < 15$ AU,
then decreases gradually ($I \propto r^{-1.8}$) out to $r \approx 43$ AU,
beyond which it falls rapidly ($I \propto r^{-4.7}$). Within 50 AU the midplane
is straight and aligned with the star, and beyond that it deviates by
$\sim3^{\circ}$, resulting in a bowed appearance that was also seen in
ground-based images. Three-dimensional modelling of the disk shows that the
inner region ($r < 50$ AU) is inclined to the line-of-sight by $<1^{\circ}$ and
the outer disk by $\sim3^{\circ}$. The inclination of the outer disk and
moderate forward scattering ($g \approx 0.4$) can explain the apparent bow. The
intrinsic, deprojected FWHM thickness is 1.5 -- 10 AU, increasing with radius.
The models indicate that the disk is clear of dust within $\sim12$ AU of the
star, in general agreement with the previous prediction of 17 AU based on the
infrared spectral energy distribution. The disk is blue, being 60% brighter at
$B$ than $I$ relative to the star. One possible explanation for this is that
there is a surplus of very small grains compared to other imaged debris disks
that have more neutral or red colors. This may be due to the low radiation
pressure exerted by the late-type star. Observations at two epochs show that an
extended source seen along the midplane is a background galaxy.

Astrophysics