Neutral gas in the central 2 parsecs of the Galaxy

“…Northern Arm, the former being the inner photoionized boundary of the CND in near circular rotation, and the latter the inner photoionized boundary of a neutral gas feature presumably falling towards the center (the northern intruder; Davidson et al 1992, Jackson et al 1993, Telesco et al 1996).”

“At S0-2’s apoapse distance of 0.01 pc, inferred from the orbital solutions, the Roche density is 1014 cm 3, whereas the maximum density determined for even the nearby circumnuclear disk, located at radii of 1–3 pc, is only about 105–106 cm 3 (e.g., Jackson et al. 1993; Christopher & Scoville 2003).”

“The chosen outer radius is appropriate if the circum-nuclear disk in the Galactic Center — a current reservoir of∼ 104 − 105 M⊙ of gas (e.g., Jackson et al. 1993; Shukla, Yun, & Scoville 2004) — were accreted onto the central black hole.”

“This is several thousand times greater than the density of gas clumps currently found in the atomic and molecular gas clouds in the central 1–2 pc [nðH; H2Þ 106 cm 3; Jackson et al. 1993; Genzel et al. 1994).”

“The Galactic nucleus contains a 4 × 10(6)M⊙ black hole surrounded by a ∼ (10(4) − 10(5))M⊙ molecular gas torus at distances > 1 pc from the SBH [94].”

“The Galactic nucleus contains a 4× 106M⊙ MBH surrounded by an∼ 104M⊙ molecular gas torus (e.g., Jackson et al. 1993).”

“Other planar structures in the Galactic Center such as the minispiral and the CND have too low masses (Jackson et al. 1993; Liszt 2003) to cause significant warps.”

“However, Jackson et al. (1993) calculate densities within the CND cores of 106 - 108 cm−3 from models of the HCN emission, noting that such densities would significantly increase the expected core lifetime.”

“In addition to these virial estimates, similar cloud densities are also derived from multitransition modeling (Jackson et al. 1993) and are also found for clouds in the central 30 pc of Seyferts using analytical modeling (Vollmer, Beckert, & Davies 2008).”

“…one, but rather periodically forms and supplies the GC with star-forming material through instabilities that cause material to fall from its inner radius into the central parsec (Sanders 1999); at present the CND may be accreting about 0:5 10 2 M yr 1 (Güsten et al. 1987; Jackson et al. 1993).”

“The CND, which consists of an inclined disk of orbiting gas, is traced by the distribution of HCN (1–0) and [O I] line emission (Güsten et al. 1987; Jackson et al. 1993) and extends from 1.5 to 17 pc from the Galactic center.”

“…binding energies relative to the black hole that are 100 times larger than that of the nearest observed accumulation of molecular gas (e.g., Jackson et al. 1993) presents a challenge to star formation theories and is a persistent puzzle (e.g., Morris 1993; Ghez et al. 2003; Genzel et al.…”

“Jackson et al. (1993) have suggested that the northern arm is the ionization front on the surface of an atomic cloud seen in the 63 lm [O i] line, which fills the interior of the CND but displays a twin-peak morphology similar to that seen in the integrated 2.12 lm H2 emission line (Yusef-Zadeh et…”

“At present, there is a concentration of molecular gas 2-8 pc from the Galctic Center termed the circumnuclear disk (Guesten et al. 1987; Jackson et al. 1993; Serabyn et al. 1994).”

“While the maximum gas density in the central parsec, a few times 106 cm−3 (Jackson et al. 1993), currently appears to be substantially lower than the minimum density required for a cloud to remain bound, ∼ 6 × 107 cm−3 (Morris 1993), little can now be said about the conditions there when the…”

“This analysis cannot provide stringent constraints on the central mass. Figure 8a is quite compatible with the Northern Arm indeed being the ionized surface of a neutral cloud (as suggested by Jackson et al. 1993; Telesco et al. 1996).”

“The projected rotation velocity of the inner CND of 100 km s−1 is well determined (Genzel et al. 1985; Serabyn & Lacy 1985; Guesten et al. 1987; Jackson et al. 1993; Christopher et al. 2005), and hence the reason for the discrepancy has to be in the model assumed.”

“More recent HCN observations imply that there is high-density material close to the inner edge (Jackson et al. 1993; Christopher et al. 2005), suggesting that the mass may be closer to 106 M,.”

“2 pc of the Galaxy, however, do not exceed∼106 cm 3 (e.g., Jackson et al. 1993).”

“There also have been attempts to interpret other three minispirals as CNDs which are not necessarily elliptical (Liszt, Burton, van der Hulst 1985; Quinn & Sussman 1985; Lacy et al. 1991; Jackson et al. 1993; Liszt 2003, e. g.).”

“Jackson et al. (1993) report a density of a few times 106cm−3 at a distance of∼ 1 pc, while Christopher et al. (2004) argue that densities approaching the limiting Roche density can be found at a distance of∼ 2 pc; (see the discussion in § 3 below) One possibility is that the gravitational collapse…”

“…inside 1 parsec have substantial non-circular velocities and the molecular ring or CND at 1-3 parsec radius is inclined at 50 - 75◦ to the Galactic plane (Jackson et al. 1993; Christopher et al. 2005) – both of which indicate substantial non-planar, non-circular dynamics for the ISM there.”

“However, it appears that mass leaves the inner edge of the CND in the form of ““ streamers ÏÏ at about the same rate as it is inferred to Ñow through the disk (Jackson et al. 1993).”

“The mass inflow rate of CND gas migrating into the central parsec i ∼ 10−2−10−4 M⊙ yr−1 (Gusten et al. 1987; Jackson et al. 1993; Vollmer et al. 2002;Genzel et al. 2010), while the accretion rate onto the accretion shock (far less gas may actually make it to Sgr A* event horizon) is estimated to be…”

“The dust within a few parsecs of the Galactic Center may originate from infalling molecular clouds/clumps (Jackson et al. 1993) or from in-situ dust formation (Lau et al. 2015).”

“The CND is formed by distinct rotating structures rather than a single structure (Jackson et al. 1993; Wright et al. 2001).”

“The Circumnuclear Disk (CND) is a ring of gas and dust around the central supermassive black hole (SMBH), with an inner radius of ∼ 1.5 pc and an inclination of 70 degrees from the Galactic plane (Güsten et al. 1987; Jackson et al. 1993).”

“…M⊙, a very clumpy distribution with an estimated volume filling factor of ∼ 0.01, a typical clump mass of ∼ 30 M⊙, size ∼ 0.1 pc, and temperature ∼ 100 K (see Gatley et al. 1986; Serabyn et al. 1986; Güsten et al. 1987; DePoy et al. 1989; Sutton et al. 1990; Jackson et al. 1993; Marr et al. 1993).”

“This spatial distribution follows the north-south orientation of the ionized gas and the tongue of neutral cloud along the extension of the northern arm of Sgr A West (e.g., Jackson et al. 1993).”

“…the presence of a dense clumpy molecular ring (also called circum nuclear disk, CND) of warm dust (Zylka et al. 1995) and neutral gas (Güsten et al. 1987; Jackson et al. 1993; Wright et al. 2001; Herrnstein & Ho 2002) surrounding a short-lived central cavity of much lower mean gas density.”

“Furthermore, if the true H i density ≈3 × 105 cm−3 derived by Jackson et al. (1993) is characteristic, the implied mean filling factor of [O i]-emitting atomic gas within the neutral streamers is ∼0.05.”

“The inflow rate deduced from the atomic gas has been estimated as 3 10−2 M⊙ per year (Jackson et al. 1993).”

“In equilibrium, a PDR emits ∼ 10−3 erg cm−2 s−1 sr−1 in the 1–0 S(1) line only if the gas density ∼ > 107 cm−3, but then the predicted [OI] 63µm intensity ( ∼ > 0.1 erg cm−2 s−1 sr−1; Burton et al. 1990), is several times higher than observed (Jackson et al. 1993).”

“The strong UV radiation field from stars in the central parsec is thought to dominate the heating of the dust grains and of the neutral atomic gas (Genzel et al. 1985; Jackson et al. 1993).”

“Atomic fine-structure line observations (63 µm [O I]) of the Sgr A West region revealed that the CND indeed circulates around Sgr A* (Genzel et al. 1984; Jackson et al. 1993).”

“…the CND and the mini-spiral are composed of multiple distinct clouds under the gravitational influence of the nucleus easily explains the observed “warping” as well as the complex kinematics and clumpy morphology observed in the region (Dent et al. 1993; Jackson et al. 1993; Wright et al. 2001).”

“This is appealing, as there is convincing observational evidence that the northern arm is an infalling streamer produced in a collision between two clouds somewhere outside the CND (Latvakoski et al. 1999; Herter et al. 1989; Jackson et al. 1993).”

“This event has already been discussed before and was presented first in the paper of Jackson et al. (1993).”

“Within the outer edge of the Sgr A East shell is a well-known north-south ridge of continuum radio emission ∼ 50′′ to the east of Sgr A* (coincident with spectrum F), roughly along the eastern edge of Sgr A West (Pedlar et al. 1989).”

“…there are several distinct components: a warm (>100 K) molecular circumnuclear disk (CND) residing in the inner 1.5–4 pc radii (Güsten et al. 1987; Jackson et al. 1993; Marshall et al. 1995; Chan et al. 1997; Christopher et al. 2005; Montero-Castaño et al. 2009), several ionized mini–spiral…”

“The CND may be undergoing dynamical evolution, or is composed of distinct streams of molecular gas (Jackson et al. 1993; Wright et al. 2001; see Section 4.4).”

“The circumnuclear molecular ring (CMR) extends from 2 to 7 pc from the massive black hole at the center of the Galaxy (Güsten et al. 1987; Jackson et al. 1993; Marshall, Lasenby & Harris 1995; Latvakoski et al. 1999; Yusef-Zadeh et al. 2006; Bradford et al. 2005; Herrnstein & Ho 2005; Schödel et…”

“Star formation via gravitational collapse in regions within 3 pc (75″) of Sgr A* would require gas about five orders of magnitude denser than what is currently observed in the Galactic center (roughly 103–108 cm−3; Jackson et al. 1993; Christopher et al. 2005; Montero-Castaño et al. 2009).”

“We used a background temperature of Tbg = 100 K, close to the values observed in GC molecular and atomic gas (Jackson et al. 1993; Martı́nPintado et al. 1997; Christopher et al. 2005).”

“…hot stars in the IRS 16 complex, this result reflects the complex density and temperature structure within the minispiral that consists of a thermal plasma of ne∼10 4cm−3 at T∼104 K (e.g. Brown, Johnston, & Lo 1981) with denser entities of nH>10 5cm−3 and a few 100 K (e.g. Jackson et al. 1993).”

“Two GMCs that lie near the GC are the best candidates to feed the CND: the 20 km s−1 and the 50 km s−1 GMCs (M−0.13−0.08 and M−0.02−0.07, respectively; Güsten et al. 1981; Güsten & Henkel 1983).”

“…major axis position angle (φ ∼ 25◦) of the CND were determined by fitting the observed radial velocities with a circular rotation model (Güsten et al. 1987; Jackson et al. 1993) while mean values of ī ∼ 61◦ and φ̄ ∼ 19◦ are found by fitting the radial velocities of radio recombination line…”

“There is an almost disklike structure of neutral gas located around the Galactic Center (GC) (Güsten et al. 1987, Jackson et al. 1993) (Serabyn et al. 1986).”

“The mass of the ionized gas in the mini-spiral and neutral gas in the central parsec are estimated to be ∼ 100 and 300 M , respectively (Jackson et al. 1993; Christopher et al. 2005).”

“However, the central velocities of ionized in the N arm are ∼100 km s−1 and decrease to 0 km s−1 as the ionized gas approaches closer to Sgr A* (e.g., Roberts and Goss 1993; Jackson et al. 1993; Zhao et al. 2011).”

“There is evidence of disturbed neutral gas and shocked molecular gas based on OH 1720 MHz maser emission at the interface of the extension of the Northern and Eastern Arms of Sgr A West and a “gap” in the CND (Yusef-Zadeh et al. 1996; Jackson et al. 1993).”

“The region of good compatibilityalso includes an approximate orientation of the CND as determined from observations, (Ω′CND, i ′ CND) = (25 ◦, 70◦), e.g. by Jackson et al. (1993).”

“It has been well recognized that the tidal shear from the central potential is quickly rising within the central 10 pc (Güsten & Downes 1980; Genzel et al. 1985; Jackson et al. 1993; Shukla et al. 2004; Christopher et al. 2005).”

“The CND is formed by distinct rotating structures rather than a single structure (Jackson et al. 1993; Wright et al. 2001).”

“The CND (inclination ≈ 70 ◦ ) and ionized streams (inclination ≈ 50 ◦ ) are significantly misaligned with the large scale Galactic plane (inclination = 90 ◦ ) (Jackson et al. 1993; Paumard et al. 2004; Zhao et al. 2009; Genzel et al. 2010), an important issue I will come back to below.”

“The masses of atomic hydrogen inside the cavity (dominated by the minispiral NA) range between 50 - 300 M (from FIR and O i observations; Telesco et al. 1996; Latvakoski et al. 1999; Jackson et al. 1993), while the molecular gas mass is most likely only a fraction of this.”

“The southern extension seems to be better described by a pure Keplerian rotation at a radius of 2.5 pc, which is consistent with the results of Jackson et al. (1993) and Martin et al. (2012).”

“…disk (CND) in the Galactic Center (GC) has remained unclear in spite of intensive studies over the past decades (e.g., Güsten et al. 1987; Jackson et al. 1993; Amo-Baladrón et al. 2011; Harris et al. 1985; Mezger et al. 1989; Etxaluze et al. 2011; Lau et al. 2013; Dent et al. 1993;…”

“(CO, CS), Güsten et al. (1987) (HCN), DePoy et al. (1989) (H2), Sutton et al. (1990) (CO), Jackson et al. (1993) (Oi, HCN), Marr et al. (1993) (HCN), Coil & Ho (1999, 2000) (NH3), and Wright et al. (2001) (HCN).”

“Using the velocity information, we can also isolate and examine other kinematic features proposed to be associated with the CND, such as the ‘‘+50 km s 1 streamer’’ and the ‘‘+70 km s 1 feature’’ (see Jackson et al. 1993).”

“Neutral atomic gas is also present in the inner few pc (Jackson et al. 1993).”

“Previous studies proposed that the CNR formed through the collision of two molecular clouds (e.g. Guesten et al. 1987) or through the assembly of multiple dynamically different streamers (e.g. Jackson et al. 1993).”

“The accuracy of the HCN(3-2) peak brightness temperatures is stated by Jackson et al. (1993) to be ±30% due to the relatively large beam size of about 12”, compared to the average clump size of 6.5”, this also contributed to the large FWHM line widths for this transition compared to the other…”

“3 The CND is a ring of dense molecular gas located ≈ 1.5 pc from Sgr A*, with a scale height of ∼ 0.5 pc (Jackson et al. 1993; Christopher et al. 2005).”

“Based on 63µm [OI] 3P1→ 3P2 emission line measurements, Jackson et al. (1993) find a total of 300 M⊙ of neutral atomic hydrogen within the 1.5 pc radius circum nuclear ring.”