Understanding the kinematics of Galactic Centre gas

“…the detailed mechanism for the formation of a massive clump in such strong magnetic field is still unclear, an extreme CCC might take place at stagnation points of molecular gas, which may occur at the foot points of loops and/or at contact points between x1 and x2 orbits (Binney et al. 1991).”

“…pc and Rmax 150 pc, which is roughly consistent with the expected eccentricities of the x2 orbits on which the CMZ gas is believed to be flowing on (e.g. Binney et al. 1991; Englmaier & Gerhard 1999; Sormani et al. 2015a, 2020; Tress et al. 2020), and is also consistent with the eccentricity of…”

“The easiest explanation for the non-circular trajectories is the gas response to a bar potential (Binney et al. 1991; Fux 1999).”

“A few percent of the gas exhibits forbidden velocities, which may be either due to non-circular motion in an oval potential (Binney et al. 1991) or to expanding motion.”

“Bar Model for Maffei 2 The new CO P-V diagrams (Figure 10) have sufficient spatial resolution to reveal that the “oval”-shaped pattern is actually a “parallelogram” like that observed towards the Galactic Center (Bally et al. 1988; Binney et al. 1991).”

“This approach of modelling gas in a bar potential was first used by Binney et al. (1991) in order to explain the distribution of gas in the centre of our own Galaxy.”

“The positions in the 1◦.3 and in Sgr C complexes correspond to the locations where interactions between the X1 and X2 orbits are expected (see, Binney et al. 1991; Stark et al. 2004).”

“We construct a model gas disk based on ideas developed by Paczynski (1977) and applied in many galactic contexts (e.g., Binney et al. 1991; Chang et al. 2007).”

“…the previous two decades, larger values ∼55 km s−1 kpc−1 were favoured, in part on account of a significant underestimate of the length of the bar by Binney et al. (1991) and in part through the Hercules stream being supposed to arise by trapping at the OLR rather than corotation (Dehnen 1998;…”

“Models including a bar (e.g., Binney et al. 1991 ; Zhao 1996) can explain these observational features.”

“…was discovered (Blitz, Spergel 1991; Nakada et al. 1991; Dwek et al. 1995), it has been recognized that non-circular motions must be taken into account when interpreting observational data, such as the CO gas distribution in the central nuclear disk (Binney et al. 1991; Weiner, Sellwood 1999).”

“This value would be an underestimate of the initial radius if the gas velocities were less than ±100 km s−1(e.g., on “x2” orbits; Binney et al. 1991) or angular momentum was lost during expansion.”

“Gas Ñows down the dust lane of the bar to the ILR, then 2120 VEILLEUX, BLAND-HAWTHORN, & CECIL Vol. 118 most sprays back into the bar region at the point of contact of the nuclear ring and the dust lane (Binney et al. 1991 ; Piner et al. 1995 ; Regan et al. 1997).”

“For example, it has been suggested that both the Arches and Quintuplet formed in a collision between gas clouds along the X1 and X2 orbit families in the Galactic bar, which may be a region of highly efficient star formation (Binney et al. 1991; Stolte et al. 2014).”

“Paul Eskridge graciously shared results from the OSU Galaxy Survey prior to publication; Amaya Moro-Martin, Alberto Noriega-Crespo, and Leonardo Testi generously undertook the K ′ imaging at Palomar.”

“…(Blitz & Spergel 1991; Weiland et al. 1994), star counts asymmetries (Nakada et al. 1991; Stanek et al. 1997; Benjamin et al. 2005), gas kinematics (Binney et al. 1991; Englmaier & Gerhard 1999; Fux 1999) and large microlensing optical depth (Udalski et al. 1994; Zhao et al. 1995; Han & Gould…”

“However, the assumption of pure circular orbits does not hold in the innermost 3 kpc, where the bar gravitational potential makes the gas distribution non-axisymmetric and induces non-circular motions (Binney et al. 1991; Sormani & Magorrian 2015; Sormani et al. 2015; Armillotta et al. 2019).”

“As suggested by Binney et al. (1991), the gas tends to be found on families of closed orbits which are not self-intersecting.”

“Jenkins & Binney (1994) examined the hypothesis of Binney et al. (1991) with two-dimensional (2–”

“Binney et al. (1991) made the case for a central bar on dynamical grounds by interpreting the longitude-velocity diagrams of CO and H i emission at longitudes l <∼ 12 ◦ in terms of the two major families of closed orbits in the equatorial plane of a rotating bar.”

“If we take V (R) = V (R⊙)(R/R⊙)0.1 km s−1 (Binney et al. 1991), this leads to R = R⊙ ( 1 + vrV (R⊙) sin l )1.11 .”

“In fact, the l-v diagram reveals elliptical orbits of the molecular gas in this region (Binney et al. 1991; Stark et al. 2004), which are understood as x2 orbits expected near or inside the inner Lindblad resonance (Contopoulos & Papayannopoulos 1980; Athanassoula 1992).”

“…bulge in the COBE NIR light distribution (Blitz & Spergel 1991; Freudenreich 1998), a non-axisymmetric signature in the gas kinematics (Binney et al. 1991; Weiner & Sellwood 1999), OGLE photometry showing a magnitude offset of the bulge red clump giants at positive and negative…”

“Binney et al. (1991) proposed that the molecular ring could be due to the outer Lindblad resonance of the bar.”

“…regions within a few degrees of the Galactic plane where the nuclear disk, 3-kpc arm, and other features with large non-circular velocities (e.g., Oort 1977; Burton & Liszt 1978; Binney et al. 1991) make it impossible to identify clouds associated with expansion of the Fermi Bubbles unambiguously.”

“One predestined location where cloud collisions might trigger cluster formation are the regions where instreaming clouds on x1 orbits from the outer Galactic bar collide with CMZ clouds on orbits around the bar’s minor axis (x2 orbits, Binney et al. 1991).”

“The MW bar was initially identified in near-infrared data (Blitz & Spergel 1991) and in the study of gas kinematics (Binney et al. 1991).”

“It is certainly formed by the large-scale bar (Binney et al. 1991; Morris & Serabyn 1996; Sawada et al. 2004) and has a large amount of molecular gas 5- 10 × 107 M⊙ (Serabyn & Morris 1995).”

“The rotation curve of the exponen- tial model clearly exceeds the approximate fit to the measured circular speed that is given by the short-dashed curve: vc(R) ∼ 220(R/R0)0.1 kms−1 (Binney et al., 1991).”

“The gas then flows towards the center and finally set-tles into a ring-like structure closely following the x 2 orbits and thus forming the CMZ-like rings in galaxies (Binney et al. 1991; Athanassoula 1992; Sormani et al. 2015).”

“The motion of this molecular gas is best explained by the gas dynamics in a barred gravitational potential (Binney et al. 1991).”

“The bar like gravitational potential is considered to be a viable mechanims to explain the radial motions of the gas in the central few 100 pc of the Galaxy (Binney et al. 1991).”

“When a galaxy contains a bar in the central region, gas far from resonances tends to settle on periodic orbits including x1 orbits that are elongated along the major axis of bars (Contopoulos & Papayannopoulos 1980; Binney et al. 1991; Morris & Serabyn 1996).”

“…is confirmed by numerous infra-red observations (Blitz & Spergel 1991; Benjamin et al. 2005; Cabrera-Lavers et al. 2007; Churchwell et al. 2009, González-Fernández et al. 2012) and by gas kinematics in the central region (Binney et al. 1991; Englmaier & Gerhard 1999; Weiner & Sellwood 1999).”

“The choice of a constant ∆v1 = 50 km s −1 for ` ≤ 5◦ limits the accuracy of this simulation at R ≤ 0.75, but in any case our results are incomplete so close to the nucleus where strong non-circular motions make kinematic analysis difficult (e.g., Burton & Liszt 1978; Binney et al. 1991; Rodriguez-Fernandez & Combes 20…”

“As in other barred galaxies, the Milky Way (MW)’s bar funnels gas inwards (Binney et al. 1991; Weiner & Sellwood 1999; Fux 1999).”

“Infrared observations clearly show that the Galactic bulge hosts a prominent bar and that the bulge isophotes are “boxy” (e.g., Binney et al. 1991; Blitz & Spergel 1991; Dwek et al. 1995; see Figure 8).”

“The Milky Way is, however, not axisymmetric: It harbours both a bar (de Vaucouleurs 1964; Binney et al. 1991; Binney, Gerhard & Spergel 1997; Wegg, Gerhard & Portail 2015; Monari et al. 2017a,b) and spiral arms, the exact number, dynamics and nature of which are still under debate (Sellwood &…”

“This method of explaining gas kinematics in a bar potential was first introduced by Binney et al. (1991) for the distribution of gas in the Galactic Center.”

“The presence of a bar in the inner regions of the Milky Way is well-known (e.g. Binney et al. 1991; Weiner & Sellwood 1999; Hammersley et al. 2000; Benjamin et al. 2005)—see a more detailed discussion in Barbuy, Chiappini, & Gerhard (2018, and references therein).”

“In the case of a barred system, the bar efficiently funnels gas toward to center, where it set- Corresponding author: Rebecca C. Levy rebeccalevy@email.arizona.edu ∗ NSF Astronomy and Astrophysics Postdoctoral Fellow † Jansky Fellow of the National Radio Astronomy Observatory tles onto a nuclear ring/disk with typical…”

“1972) or – more recently – to the gravitational effects of a Galactic bar potential (Binney et al. 1991).”

“Moreover, position–velocity diagrams of the CO, 13CO, and H I gas distributions clearly indicate the presence of such inner-bar structure (Binney et al. 1991; Weiner, Sellwood 1999).”

“Using Hipparcos data and dynamical modeling, Dehnen (2000) has shown how the Galactic Bar (Blitz & Spergel 1991; Binney et al. 1991; Dwek et al. 1995) could have influenced the velocity distribution in the solar neighborhood.”

“Binney et al. (1991) showed that the 180-pc molecular ring is formed by shock compression of the atomic gas at the inner edge of the innermost x1 orbit and that the molecular gas rapidly flows into the inner x2 orbits.”

“Binney et al.(1991) showed that the gas motions in the inner Galaxy implied that theGalaxy is barred.”

“These data have constrained extensive models of the dynamics of the GC, including those incorporating the effects of the bar(s) (e.g., Peters 1975; Binney et al. 1991; Fux 1999; Rodriguez-Fernandez et al. 2006; Romero-Gómez et al. 2011).”

“In order to investigate a realistic example of a strong bar we consider a simulation we used in Sormani et al. (2015), performed in the Binney et al. (1991) barred potential.”

“While the non-bifurcated x1 family becomes more eccentric as one moves inward, as noted by Binney et al. (1991), the x1b family remains highly elongated even to the end of the bar, a point which may have observational implications for the MW. Additionally, although we have not discussed the x2…”

“The angle between the line Sun–Galactic Centre and the major axis of the bar is usually assumed to be in the range 15◦–30◦ (see e.g. Binney et al. 1991; Wegg & Gerhard 2013) on the trailing side of the bar.”

“Binney et al. (1991) were the first to link the motions of gas towards the Bulge to orbits in (planar) elliptical potentials.”

“Much of the gas in the Galactic center region ( o l o\ 6¡) occurs at velocities forbidden to circular orbits and is likely to be in eccentric orbits about the center (Bally et al. 1987 ; Binney et al. 1991).”

“…improve our understanding of the properties of molecular clouds and large-scale processes that govern star formation, molecular cloud evolution, feedback, etc.(e.g., Binney et al. 1991; Dame et al. 2001; Li et al. 2013; Zhang et al. 2014; Zhou et al. 2014; Ragan et al. 2014; Su et al. 2014, 2015).”

“This configuration would be however inconsistent with the dynamical studies predicting that the barred gravitational potential produces a family of closed x2 orbits perpendicular to the bar (e.g. Binney et al. 1991; Athanassoula 1992).”

“Binney et al. (1991) have shown that the parallelogram structure of CO clouds can be generated by the cusped orbit (the smallest nonÈself-intersecting orbit of ContopoulosÏ sequence x1) in a simple, nonaxisymmetric rotating poten- tial.”

“The gas flow in barred galaxies is also directly related to the periodic orbits (Binney et al. 1991; Athanassoula 1992b; Sormani et al. 2015).”

“Through the COBE/DIRBE survey the presence of a Galactic bulge/bar was established (Binney et al. 1991; Weiland et al. 1994).”

“This ring could be associated with a system of orbits called x2, which would be enclosed within another system of orbits called x1, aligned along the Galactic bar (Binney et al. 1991).”

“A number of papers have argued that the bulge is triaxial with the near end in the first quadrant at an angle of 128 to 308 to the GC–Sun line of sight (Binney et al. 1991; Dwek et al. 1995; Freudenreich 1997; López-Corredoira et al. 1997).”

“For the lens stars residing in the Galactic bulge, the density distribution is nl ∝ R−sg , (45) where the power law index is either s = 1.75 (Binney et al. 1991) or s = 1.85 (Kent 1992).”