Molecular Tracers of the Central 12 Parsecs of the Galactic Center

Abstract

We have used the BIMA array to image the Galactic Center with a 19-pointing mosaic in HCN(1-0), HCO + (1-0), and H 42α emission with 5 km s −1 velocity resolution and 13 ′′ × 4 ′′ angular resolution. The 5 ′ field includes the circumnuclear ring (CND) and parts of the 20 and 50 km s −1 clouds. HCN(1-0) and HCO + trace the CND and nearby giant molecular clouds while the H 42α emission traces the ionized gas in Sgr A West. We find that the CND has a definite outer edge in HCN and HCO + emission at ∼ 45 ′′ radius and appears to be composed of two or three distinct streams of molecular gas rotating around the nucleus.Outside the CND, HCN and HCO + trace dense clumps of high-velocity gas in addition to optically thick emission from the 20 and 50 km s −1 clouds. A molecular ridge of compressed gas and dust, traced in NH 3 emission and self-absorbed HCN and HCO + , wraps around the eastern edge of Sgr A East. Just inside this ridge are several arcs of gas which have been accelerated by the impact of Sgr A East with the 50 km s −1 cloud.HCN and HCO + emission trace the extension of the northern arm of Sgr A West which appears to be an independent stream of neutral and ionized gas and dust originating outside the CND. Broad line widths and OH maser emission mark the intersection of the northern arm and the CND.Comparison to previous NH 3 and 1.2mm dust observations shows that HCN and HCO + preferentially trace the CND and are weaker tracers of the GMCs than NH 3 and dust. We discuss possible scenarios for the emission mechanisms and environment at the Galactic center which could explain the differences in these images.

Mapping photodissociation and shocks in the vicinity of Sagittarius A∗

“…The accelerated molecular gas appears on the front and the back sides of the expanding radio shell. Wright et al (2001) support this scenario from their interferometric HCO + and HCN maps, together with other near blue and redshifted clumps of gas inside of the compressed ridge. A different interpretation was proposed by Liszt & Burton (1995), who detected this feature with very strong emission in CO(3-2) at v ≤ −40 km s −1 , but they considered this feature as an extension of the high-longitude forbidden-velocity part of the CND, as they see a connection between it and the emission arising from the southwestern region of the CND, concluding that Cloud A was not an isolated feature.…”

“…These studies have characterized the CND as dense (∼10 5−6 cm −3 ; Marr et al 1993;Marshall et al 1995), clumpy, and very turbulent (with large linewidths). It extends from its inner edge, 1.6 pc from Sgr A * , to > ∼ 2 pc, according to interferometric studies (Wright et al 2001;Christopher et al 2005), and it has an inclination (rotation axis from the line of sight; LOS) of about 60 • −70 • , with the major axis aligned approximately along the Galactic plane (position angle of ∼30 • east of north; Güsten et al 1987;Jackson et al 1993;A&A 526, A54 (2011) Gatley et al (1986) Yusef Telesco et al (1996) Atomic species Liszt et al (1985) (H i, [O Genzel et al (1985) Lugten et al ) Jackson et al (1993) Serabyn et al (1994 Marshall et al 1995). The gas is moving in circular orbits, rotating around the nucleus with a velocity of 110 km s −1 (Liszt et al 1985;Marr et al 1993), but it also presents noncircular motions (e.g.…”

Surviving the hole

“…However, the CND appears not to be a single symmetric structure forming a coherent ring (Marshall et al 1995). It has been suggested that the CND is a warped structure (Guesten et al 1987) or, because of the incompleteness of the ring, formed by A&A 539, A29 (2012) distinct rotating structures (Jackson et al 1993;Wright et al 2001). Most of the observed molecular clumps forming the CND, with diameters of ∼7 , are tidally stable and in virial equilibrium (Christopher et al 2005).…”

“…The mini-spiral of ionized gas consisting of three streamers known as Sgr A West (Lo & Claussen 1983;Ekers et al 1983) is likely feeding the central source. Surrounding Sgr A West, the so-called circumnuclear disk (CND) is a clumpy ringlike structure rotating counterclockwise around the dynamical center (Guesten et al 1987;Wright et al 2001;Christopher et al 2005). The molecular ring extends from its inner sharp edge at a projected distance of 1.6 pc from Sgr A * , out to a smoother edge at ∼2 pc (Christopher et al 2005).…”

“…So far, the only species with highresolution aperture synthesis observations are the HCN 1−0 (Guesten et al 1987;Wright et al 2001;Christopher et al 2005), 3−2 (Marshall et al 1995), and 4−3 (Marshall et al 1995;Montero-Castaño et al 2009), the 1−0 transitions of H 13 CN and HCO + (Marr et al 1993;Wright et al 2001;Christopher et al 2005), the CS 7−6 (Montero-Castaño et al 2009), and NH 3 (1, 1), (2, 2), (3, 3), (6, 6) Herrnstein & Ho 2005). The emission from the high-density tracers like HCN, HCO + , and CN nicely trace all molecular features of the CND, though significant differences are observed in the line ratios.…”