Various models of the gas and dust distribution in the GC exist: a simple bar model (Morris & Serabyn 1996); variations of a spiral arm model in which the apparent ring is formed by the inner part of two spiral arms (Sofue 1995; Sawada et al. 2004; Rodríguez-Fernández & Combes 2008; Rodríguez-Fernández 2011; Ridley et al. 2017); a closed, twisted elliptical ring detected in dust emission (Molinari et al. 2011), and a sequence of open-ended gas streams (Kruijssen et al. 2015, hereafter K15). K15 highlight the impossibility of closed orbits in extended gravitational potentials and provide a better fit to single-dish ammonia emission in position–position–velocity (PPV) space, as was confirmed by Henshaw et al. (2016b) for three molecular species in single-dish observations. Thus, we focus on this model and call it the “stream model” in contrast to the “ring model” and the “spiral arm” model. According to the stream model, the stream of GC molecular clouds oscillates radially and vertically, and can be traced for ∼1.5 revolutions around the GC. The radial oscillation periodically brings dense molecular gas closer (r ∼ 60 pc at pericenter) to the gravitational center (traced by Sgr A*) and deeper into the potential, whereas the apocenter lies at r ∼ 120 pc. As the CMZ clouds slowly evolve toward low virial ratios as the turbulent energy dissipates (Krumholz & Kruijssen 2015; Walker et al. 2015; Henshaw et al. 2016a), it is statistically more likely that cloud collapse occurs at the pericenter where the compressive tidal forces are strongest along the orbit and thus a cloud receives the final nudge for the transition to self-gravitation. Subsequent SF stages will then occur downstream from the pericenter passages and could potentially be observed as an SF sequence if the orbit is sufficiently sampled with molecular clouds that start to collapse at a similar point of their orbits. This model of triggered star formation was first proposed by Longmore et al. (2013b) based on the observation of different evolutionary SF stages along the dust ridge. The two young stellar clusters in the GC, Arches and Quintuplet, may also support this model as their orbits and ages are consistent with formation at a common point after collapse of their respective parent clouds at a pericenter passage (Figure 1 of Stolte et al. 2014; Kruijssen et al. 2015). In addition to stars and star formation tracers, cloud properties are also expected to show evolutionary behavior along the gas streams; this has not been thoroughly tested yet. Weak hints toward rising gas temperatures in the dust ridge are suggested by Ginsburg et al. (2016), while a recent paper by Kauffmann et al. (2017b) based on N2H+ data from the Galactic Center Molecular Cloud Survey can neither confirm nor exclude the possibility of triggered evolution when examining mass-size relation and SF suppression. Both analyses were based on a low number of measurements (∼35 measurements in dust ridge clouds in Ginsburg et al. 2016 and six clouds in Kauffmann et al. 2017b) and lack the statistical power to detect potential evolution in the presence of scatter.
Citation: Henshaw et al. 2016a
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Citation: Longmore et al. (2013b)
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Citation: Kauffmann et al. (2017b)
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Citation: Kauffmann et al. 2017b
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