NH₃in the Central 10 pc of the Galaxy. I. General Morphology and Kinematic Connections between the Circumnuclear Disk and Giant Molecular Clouds

Abstract

New VLA images of NH 3 (1,1), (2,2), and (3,3) emission in the central 10 parsecs of the Galaxy trace filamentary streams of gas, several of which appear to feed the circumnuclear disk (CND). The NH 3 images have a spatial resolution of 16.5 ′′ × 14.5 ′′ and have better spatial sampling than previous NH 3 observations. The images show the "southern streamer," "50 km s −1 cloud," and new features including a "western streamer", 6 parsecs in length, and a "northern ridge" which connects to the CND. NH 3 (3,3) emission is very similar to 1.2 mm dust emission indicating that NH 3 traces column density well. Ratios of the NH 3 (2,2) to (1,1) line intensities give an estimate of the temperature of the gas and indicate high temperatures close to the nucleus and CND. The new data cover a velocity range of 270 km s −1 , including all velocities observed in the CND, with a resolution of 9.8 km s −1 . Previous NH 3 observations with higher resolution did not cover the entire range of velocities seen in the CND. The large-scale kinematics of the CND do not resemble a coherent ring or disk. We see evidence for a high velocity cloud within a projected distance of 50 ′′ (2 pc) which is only seen in NH 3 (3,3) and is likely to be hot. Comparison to 6 cm continuum emission reveals that much of the NH 3 emission traces the outer edges of Sgr A East and was probably pushed outward by this expanding shell. The connection between the northern ridge (which appears to be swept up by Sgr A East) and the CND indicates that Sgr A East and the CND are in close proximity to each other. Kinematic evidence for these connections is presented in this paper, while the full kinematic analysis of the central 10 pc will be presented in Paper II.

Mapping photodissociation and shocks in the vicinity of Sagittarius A∗

“…It has been observed in several transitions of NH 3 (Okumura et al 1991;Ho et al 1991;Dent et al 1993;Coil & Ho 1999McGary et al 2001;Herrnstein & Ho 2002, and in methanol . The HNCO/SiO ratio along the Southern Streamer shows the largest gradient with a variation of a factor of ∼18 between the ratio in the GMC (∼2.6 in the intermediate-velocity range) and in the CND (∼0.15 in the high-velocity range).…”

“…10) suggest there are shocks, as well as UV-radiation, which also explain the lack of NH 3 and 1.2 mm dust emission, since the ammonia molecules and dust grains are destroyed by photodissociation and shocks, respectively (see Fig. 9 of McGary et al 2001). Shocks could be caused by the expansion of Sgr A East SNR into this cloud, whereas UV-radiation could originat in the Central cluster, locating this cloud close to the nuclear region.…”

“…McGary et al (2001) reports a velocity gradient of 1 km s −1 arcsec −1 along its 150 length. The southern half has a negative velocity whereas the northern half has a positive velocity, consistent with a ridge of gas highly inclined to the LOS that is pushed outward by the expansion of the SNR.…”

Interstellar gas within ~10 pc of Sagittarius A∗

“…Becklin et al 1982;Mezger et al 1989;Davidson et al 1992;Dent et al 1993;Telesco et al 1996;Latvakoski et al 1999) as well as in a wide variety of atomic and molecular tracers, including the 21-cm line of H i (Liszt et al 1983), finestructure lines of [O i] and [C ii] Poglitsch et al 1991), and various transitions of H 2 (Gatley et al 1984(Gatley et al , 1986Depoy et al 1989;Burton & Allen 1992;Yusef-Zadeh et al 2001), CO (Liszt et al 1985;Genzel et al 1985;Harris et al 1985;Serabyn et al 1986;Güsten et al 1987;Sutton et al 1990;Bradford et al 2005), OH , CS (Serabyn et al 1986(Serabyn et al , 1989Montero-Castaño et al 2009), HCN (Güsten et al 1987;Marr et al 1993;Jackson et al 1993;Marshall et al 1995;Wright et al 2001;Christopher et al 2005;Montero-Castaño et al 2009), HCO + (Marr et al 1993;Wright et al 2001;Shukla et al 2004;Christopher et al 2005), NH 3 (Coil & Ho 1999;McGary et al 2001;, etc. Collectively, these tracers lead to the picture of an asymmetric, extremely clumpy, torus-like CNR, with a sharp inner boundary at radius ≈(1−1.5) pc and a more blurry, irregular outer boundary at radius ≈(2.5−3) pc to the northeast and ≈(4−7) pc to the southwest (see Fig.…”

“…A follow-up study of the central 10 pc of the Galaxy was performed by McGary et al (2001), based on spectral observations of the NH 3 (1, 1), (2, 2) and (3, 3) transitions (all between 23.7 GHz and 23.9 GHz). In addition to the southern streamer and the molecular ridge clearly visible in the (1, 1) and (2, 2) maps of Coil & Ho (1999, the (3, 3) map brings out two new features:…”