HST Palpha Survey of the Galactic Center -- Searching the missing young
  stellar populations within the Galactic Center
  We present preliminary results of our \hst Pa$\alpha$ survey of the Galactic
Center (\gc), which maps the central 0.65$\times$0.25 degrees around Sgr A*.
This survey provides us with a more complete inventory of massive stars within
the \gc, compared to previous observations. We find 157 Pa$\alpha$ emitting
sources, which are evolved massive stars. Half of them are located outside of
three young massive star clusters near Sgr A*. The loosely spatial distribution
of these field sources suggests that they are within less massive star
clusters/groups, compared to the three massive ones. Our Pa$\alpha$ mosaic not
only resolves previously well-known large-scale filaments into fine structures,
but also reveals many new extended objects, such as bow shocks and H II
regions. In particular, we find two regions with large-scale Pa$\alpha$ diffuse
emission and tens of Pa$\alpha$ emitting sources in the negative Galactic
longitude suggesting recent star formation activities, which were not known
previously. Furthermore, in our survey, we detect $\sim$0.6 million stars, most
of which are red giants or AGB stars. Comparisons of the magnitude distribution
in 1.90 $\mu$m and those from the stellar evolutionary tracks with different
star formation histories suggest an episode of star formation process about 350
Myr ago in the \gc .
