Hubble Space TelescopePaα and 1.9 Micron Imaging of Sagittarius A West

We present HST/NICMOS images at 0.2" resolution of the HI Paschen Alpha (PaA)emission line in a 70" x 90" region of the Galactic center centered on thenon-thermal radio source Sgr A*. The majority of the emission arises fromionized gas in the mini-spiral in the central parsec. PaA emission is also seenfrom 26 stellar sources, presumably early-type stars with mass-loss winds. Thenew data reveal significant small-scale structure (<1"~0.04pc) in the ionizedgas of the mini-spiral; low surface brightness emission features are also seenfor the first time. Extinction, estimated from the ratio of observed PaAemission to 6-cm continuum emission, varies from 20 to 50 mag with a medianAv=31.1 mag, in excellent agreement with earlier estimates for the stellarsources and indepedent measurements derived using H92alpha recombination linedata. Large increases in extinction are seen along the periphery of the ionizedgas, suggesting that the ionized gas is partially extincted by dust in themolecular clouds at the outside of the ionized regions. The small-scale,filamentary structures in the ionized gas have a free thermal expansion time ofonly ~ 3000 yrs; either magnetic fields or mass-loss winds from the hotemission line stars may contain the ionized filaments. For both the ionized gasand the stellar continuum, the centroids of the emission remain within ~+/- 1"from a radius of 2" out to 40", providing further evidence that Sgr A* isindeed at or extremely close to the dynamical center of the Galactic nucleusstellar distribution. The 1.9 micron surface brightness increases inwards to0.9" and then decreases or levels off closer to Sgr A*, possibly indicating thecore radius of the central stellar distribution or depletion of the late-typestars by stellar collisions near the central black hole.Comment: 43 pages, 15 figures, 2 tables; Accepted to ApJ (9/1/03 issue