Dense, Ionized, and Neutral Gas Surrounding Sagittarius A*

Shukla, Hemant; Yun, Min S.; Scoville, N. Z.

doi:10.1086/424868

Cited by 30 publications

(50 citation statements)

References 45 publications

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“…Because radio recombination lines at centimeter wavelengths can be dominated by stimulated emission and affected by pressure broadening, Shukla et al (2004) observed the Sgr A West region in the 92 GHz (3.3 mm) H41α recombination line. At this higher frequency, the continuum and recombination-line emissions are believed to arise mostly in denser ionized gas.…”

Section: The Central Cavitymentioning

confidence: 99%

“…On the other hand, little molecular gas seems to be present, except possibly for a hot and dense molecular component detected in NH 3 (6, 6) emission very near Sgr A * (Herrnstein & Ho 2002). The general lack of molecular gas can be understood if any initially molecular cloud inside the central cavity has been largely photo-dissociated by the intense ambient UV radiation (Jackson et al 1993;Latvakoski et al 1999;Shukla et al 2004).…”

Section: The Central Cavitymentioning

confidence: 99%

“…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.…”

Section: The Circumnuclear Ringmentioning

confidence: 99%

“…), which we take to be smooth and homogeneous. We set the temperatures of the three gases to T wi = 7000 K (Roberts & Goss 1993;Shukla et al 2004), T a = 170 K (Jackson et al 1993) and T h = 1.5 × 10 7 K (Baganoff et al 2003), and we consider that the warm ionized gas has hydrogen completely ionized and helium completely neutral, while the hot ionized gas has both hydrogen and helium fully ionized. ellipsoid l x × l y × l z = 2.9 × 2.9 × 2.1 9.2 centered on Sgr A The extended component of the warm ionized gas is supposed to have a central emission measure of 2.2 × 10 6 pc cm −6 (value derived by Beckert et al (1996) and scaled up to our adopted temperature), which, combined with a line-of-sight dimension l x = 2.9 pc and a filling factor φ ext , yields a true electron density (n e ) ext = (870 cm −3 ) φ −1/2 ext and a space-averaged electron density ( n e ) ext = (870 cm −3 ) φ 1/2 ext .…”

Section: The Central Cavitymentioning

confidence: 99%

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Interstellar gas within ~10 pc of Sagittarius A^∗

Ferrière

2012

A&A

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Aims. We seek to obtain a coherent and realistic three-dimensional picture of the interstellar gas out to about 10 pc of the dynamical center of our Galaxy, which is supposed to be at Sgr A * . Methods. We review the existing observational studies on the different gaseous components that have been identified near Sgr A * , and retain all the information relating to their spatial configuration and/or physical state. Based on the collected information, we propose a three-dimensional representation of the interstellar gas, which describes each component in terms of both its precise location and morphology and its thermodynamic properties. Results. The interstellar gas near Sgr A * can be represented by five basic components, which are, by order of increasing size: (1) a central cavity with roughly equal amounts of warm ionized and atomic gases; (2) a ring of mainly molecular gas; (3) a supernova remnant filled with hot ionized gas; (4) a radio halo of warm ionized gas and relativistic particles; and (5) a belt of massive molecular clouds. While the halo gas fills ≈80% of the studied volume, the molecular components enclose ≈98% of the interstellar mass.

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Section: The Central Cavitymentioning

confidence: 99%

Section: The Central Cavitymentioning

confidence: 99%

Section: The Circumnuclear Ringmentioning

confidence: 99%

Section: The Central Cavitymentioning

confidence: 99%

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Interstellar gas within ~10 pc of Sagittarius A^∗

Ferrière

2012

A&A

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“…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. : Güsten et al 1987;Jackson et al 1993;Marshall et al 1995;Shukla et al 2004;Christopher et al 2005). The CND does not appear to be an equilibrium structure.…”

Section: Introductionmentioning

confidence: 99%

Mapping photodissociation and shocks in the vicinity of Sagittarius A^∗

Amo-Baladrón

Martı́n-Pintado

Martín

2010

A&A

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Aims. We study the chemistry in the harsh environments of galactic nuclei using the nearest one, the Galactic center (GC). Methods. We have obtained maps of the molecular emission within the central five arcminutes (12 pc) of the GC in selected molecular tracers: SiO(2-1), HNCO(5 0,5 -4 0,4 ), and the J = 1 → 0 transition of H 13 CO + , HN 13 C, and C 18 O at an angular resolution of 30 (1.2 pc). The mapped region includes the circumnuclear disk (CND) and the two surrounding giant molecular clouds (GMCs) of the Sgr A complex, known as the 20 and 50 km s −1 molecular clouds. Additionally, we simultaneously observed the J = 2 → 1 and J = 3 → 2 transitions of SiO toward selected positions to estimate the physical conditions of the molecular gas using the large velocity gradient approximation. Results. The SiO(2-1) emission shows all the molecular features identified in previous studies, covering the same velocity range as the H 13 CO + (1-0) emission, which also presents a similar distribution. In contrast, HNCO(5-4) emission appears in a narrow velocity range mostly concentrated in the 20 and 50 km s −1 GMCs. A similar trend follows the HN 13 C(1-0) emission. The HNCO column densities and fractional abundances present the highest contrast, with difference factors of ≥60 and 28, respectively. Their highest values are found toward the cores of the GMCs, whereas the lowest ones are measured at the CND. SiO abundances do not follow this trend, with high values found toward the CND, as well as the GMCs. By comparing our abundances with those of prototypical Galactic sources we conclude that HNCO, similar to SiO, is ejected from grain mantles into gas-phase by nondissociative C-shocks. This results in the high abundances measured toward the CND and the GMCs. However, the strong UV radiation from the Central cluster utterly photodissociates HNCO as we get closer to the center, whereas SiO seems to be more resistant against UV-photons or it is produced more efficiently by the strong shocks in the CND. This UV field could be also responsible for the trend found in the HN 13 C abundance. Conclusions. We discuss the possible connections between the molecular gas at the CND and the GMCs using the HNCO/SiO, SiO/CS, and HNCO/CS intensity ratios as probes of distance to the Central cluster. In particular, the HNCO/SiO intensity ratio is proved to be an excellent tool for evaluating the distance to the center of the different gas components.

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