Far-Infrared Observations of the Center of the Galaxy

Abstract

We report velocity resolved maps of the 158 µm. [CII], the 63 µm [OI], and the 88 µm and 52 µm [OIII] fine structure lines toward the central region of the Galaxy. The angular resolution was 55” for the [CII] line and 22” for the oxygen Unes.

The [CII] emission is strongest toward the the rotating circum-nuclear disk surrounding Sgr A West. A continuous bridge of [CII] line emission connects the Sgr A complex to the thermal radio filaments in the Radio Arc 10’ north of the center, thus suggesting a direct physical connection between the two. Our [OIII] observations toward the thermal arches rule out shocks as ionization mechanism and suggest ionization by UV photons.

Our maps of both [OIII] lines toward the the circum-nuclear disk show the brightest emission from the central “ionized cavity” within the molecular ring. The high observed 52µm/88µm line intensity ratio requires a density ≥ 10⁴ cm^-3. We infer about 10 M_⊙ of ionized gas inside the cavity, with no indication of an additional low-density component.

The [01] data reveal the presence of a large amount (~300 M_⊙) of neutral atomic gas inside the central cavity, and associated with the “northern” and “eastern” arms of ionized gas. The northern arm appears to be an ionized rim at the surface of this neutral gas streamer, which is probably falling from a gap in the 1.5 pc ring into the central 0.5 pc. We infer a mass inflow rate into the central parsec of 0.03 M_⊙/yr.