Physics
Scientific paper
Mar 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994inpht..35..419p&link_type=abstract
Infrared Physics & Technology, Volume 35, Issue 2-3, p. 419-429.
Physics
2
Scientific paper
We report velocity resolved maps of the 158 μm [CII] fine structure line, of the 63 μm [OI] fine structure line, and of the 88 μm and 52μm [OIII] fine structure lines toward the center region of the Galaxy. Observations were carried out on board the Kuiper Airborne Observatory (KAO) using the MPE/UCB Far-infrared Imaging Fabry-Perot interferometer (FIFI). The angular resolution was 55'' for the [CII] line and 22'' for the oxygen lines. The [CII] emission, tracing the distribution of dense gas dissociated and partially ionized by far-UV photons, is strongest toward the rotating circumnuclear 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 strongly suggesting a direct physical connection between the two. Outside the nuclear region, there is an anticorrelation between the [CII] intensity and the distribution of dense molecular material. The brightest emission occurs near the edges of the massive Galactic Center molecular clouds. We conclude that these clouds are predominantly ionized by external UV photons, partly from the Sgr A West region and partly from OB stars near the center. Our data indicate a physical connection between the center and the massive interstellar clouds in the surrounding 50 pc. Our 88 μm [OIII] maps of the ``sickle'' region within the Radio Arc and of the thermal arches approximately follow the thermal radio continuum. The line width varies substantially over the mapped area; we find a narrow (Δv ~ 60km/s) and a broad (Δv ~ 150km/s) component, with the broad component being more prominent in the sickle. From the 52 μm/88 μm line ratio we derive a density ~ 300 cm-3 and a hydrogen column density ~3 × 1021 cm-1 in the [OIII] emitting medium. This rules out shocks or fast winds from stars as excitation mechaniμs. UV excitation, however, could produce the observed amount of O2+. About 10 to 100 O-stars would be required to ionize the gas in the ``pistol/sickle'' region. The broad linewidth could be due to interaction between the ionized gas and magnetic fields. Our maps of both [OIII] lines toward the circum-nuclear disk show the brightest emission from inside the central, ``ionized cavity'' in the molecular ring. The high 52 μm/88μm ratio requires a density >= 104 cm-3. The derived hydrogen column density, however, is low, which could be understood by a small ionization ratio No2+/No0 or a small beam filling factor of the ionized gas. We infer about 10 msolar of ionized gas inside the cavity, with no indication of an additional low-density component. The [OI] data, on the other hand, reveal the presence of a large amount of neutral atomic gas inside the central cavity, and associated with the ``northern'' and ``eastern'' arms of ionized gas. We estimate about 300 Msolar of dense neutral hydrogen gas within the 1.5 pc radius of the circumnuclear ring, about a factor 10 more than the amount of ionized gas. The northern arm appears to be a bright, ionized rim at the surface of this neutral gas streamer, which is probably in the plane of the circumnuelear disk and falling from a gap in the 1.5pc ring into the central 0.5pc. We infer a mass inflow rate into the central parsec of 0.03Msolar/year.
Geis Norbert
Genzel Reinhard
Harris Andrew I.
Jackson James M.
Krenz Thomas
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