Hubble Space Telescope Paα and 1.9 Micron Imaging of Sagittarius a West

We present Hubble Space Telescope/NICMOS images at 0″2 resolution (Nyquist-sampled in the central 19″) of the H I Paα emission line in a 70″ (a) × 90″ (δ) region of the Galactic center centered on the nonthermal radio source Sgr A*. The majority of the emission arises from ionized gas in the minispiral in the central parsec. Paα emission is also seen from 26 stellar sources, presumably early-type stars with mass-loss winds. The new data reveal significant small-scale structure (≤1″ ∼ 1.2 × 10¹⁷ cm) in the ionized gas of the minispiral; low surface brightness emission features are also seen for the first time. The ratio of observed Paa emission to 6 cm radio continuum emission is used to derive an extinction map for the ionized gas over the central parsec. A_V varies from 20 to 50 mag with a median value of A_V = 31.1 mag, in excellent agreement with earlier estimates derived for the stellar sources. Extinctions, independently derived using H92α recombination-line data, were in excellent agreement with those derived from the 6 cm continuum. A broad minimum in the extinction extends ∼30″ northeast-southwest over the area of the 1RS 16 cluster, including the area of peak flux in Paa and radio continuum. Large increases in extinction are seen along the periphery of the ionized gas, particularly in the direction of the western arm, suggesting that the ionized gas is partially extincted by dust in the molecular clouds at the outside of the ionized region; however, the H II cannot be entirely behind the molecular clouds since the extinctions would then be much greater. The minimum Lyman continuum emission rate within 40″ radius of Sgr A*is 3.9 × 10⁵⁰ s^-1 or L _Lyc ≃ 2.7 × 106 L⊙, half of this within 20″ radius. The small-scale, filamentary structures in the ionized gas have a free thermal expansion time of only ∼3000 yr; either magnetic fields or mass-loss winds from the hot emission line stars may contain the ionized filaments. (Eight new emission-line stars were also detected in Paα.) For both the ionized gas and stellar continuum, the centroids of the emission (corrected for extinction) remain within ∼±1″ from a radius of 2″ out to 40″. This provides further evidence that Sgr A*(the putative central black hole) is indeed at or extremely close (≤0.04 pc or 1. 2 × 10¹⁷ cm) to the center of the Galactic nucleus stellar distribution and presumably the dynamical center of the Galaxy. The 1.9 μm surface brightness increases inwards to 0″.9 radius and exhibits a decrease or leveling off closer to Sgr A*, possibly indicating the core radius of the central stellar distribution or depletion of the late-type stars by stellar collisions near the central black hole.