Understanding the kinematics of Galactic Centre gas

“Peculiar features of the longitude-velocity diagrams of atomic and molecular ISM in our Galaxy are also interpreted as non-circular flows under the influence of the stellar bar (Binney et al. 1991; Combes 1991; Rodriguez-Fernandez & Combes 2008).”

“…indeed expected as for the families of periodic orbits, x1, x2, and x3 (e.g., Contopoulos & Papayannopoulos 1980; van Albada & Sanders 1982; Athanassoula 1992a); these streamlines would appear on − v diagrams as parallelogram-shaped features (e.g., Binney et al. 1991; Bureau & Athanassoula 1999).”

“Their noncircular, elongated motion is thought to be induced by the Galaxy’s barred potential (e.g., Binney et al. 1991; Morris & Serabyn 1996 and references therein), with dynamical friction aiding migration to the central regions of the Galaxy (Stark et al. 1991).”

“Beside the clues provided by the Galaxy (e.g. Blitz & Spergel 1991; Binney et al. 1991; Weiland et al. 1994), little observational data exist to directly test this hypothesis.”

“…Way also has a central bar which can be split into a 3.1 - 3.5 kpc Galactic Bar at an angle of 20◦ with respect to the Galactic Centre-Sun axis (Binney et al. 1991; Blitz & Spergel 1991; Dwek et al. 1995) and a non-symmetric structure, the Long Bar (Hammersley et al. 2000), at an angle of 44◦…”

“The values range from 168, derived from the gas dynamics of the central region (Binney et al. 1991), to 448, derived by Sevenster et al. themselves from radial velocities of OH/IR stars and a Galactic model.”

“The overall position and size of the CMZ is presumably controlled by the gravitational dynamics dictated by the Galactic bar (Binney et al. 1991; Morris & Serabyn 1996; Stark et al. 2004).”

“…are the kinematical data obtained in HI 21-cm emission, CO, and CS (Sanders & Prendergast 1974; Liszt & Burton 1980; Gerhard & Vietri 1986; Binney et al. 1991), the Galactic center stellar distribution with Mira Variables from IRAS (Harmon & Gilmore 1988; Nakada et al. 1991; Weinberg…”

“At this time, a “bar consensus” was developing based on gas kinematics (Binney et al. 1991), infrared light measurements (Dwek et al. 1995), and star counts (Nikolaev & Weinberg 1997).”

“…our Galaxy corresponds approximately to the region within Galactocentric radius R = 3 kpc (e.g. Binney & Merrifield 1998), the ‘circular speed’ can be significantly different from the speed of the gas obtained in simulations, or observed in the Galaxy, at the same radii (e.g. Binney et al. 1991).”

“…ofmodels to observations have shown that gas flow induced by stellar bars can, for example, explain the complex kinematics in our Galactic center (Binney et al. 1991) or the gas flow in the double-barred spiral galaxy M100, where the two nested bars need to be dynamically decoupled…”

“Both have near edge-on viewing angles and owe their gas distributions to a galactic stellar bar (Scoville et al. 1985; Sorai et al. 2000; Binney et al. 1991; Sawada et al. 2004).”

“…bar (a perturbation with azimuthal wavenumber m = 2) eviscerates the interior of the ∼3-kpc molecular ring at the center of our Galaxy (Schwarz 1981; Binney et al. 1991); an eccentric stellar disk (m = 1) may be funneling gas onto the star-forming nucleus of M31 (Chang et al. 2007); and a complex…”

“Besides spiral structure, the Milky Way is known to host a central bar (Blitz & Spergel 1991; Binney et al. 1991) with a mass of about 1010 M⊙ (Zhao & Mao 1996; Weiner & Sellwood 1999) and a semimajor axis of about 3 kpc (Binney, Gerhard, & Spergel 1997; Bissantz & Gerhard 2002).”

“allow us to infer a unique model, and, unsurprisingly, there is controversy as to the viewing angle, length and pattern speed of the Galactic bar (e.g., Binney et al. 1991, Blitz & Spergel 1991, Sevenster et al. 1998).”

“Binney et al. (1991) interpreted the (l, v) diagram for HI, CO and CS emission using orbits in a barred potential with Ωp = 63 km s −1 kpc−1.”

“But the bar is also quite visible in the pattern speed of the gas (Binney et al, 1991).”

“…one way the emission in the PVD can be used to predict the approximate line-of-sight orientation of the bar in a disk observed near edge-on (Binney et al. 1991), which is especially useful when the line of sight inclination and extinction prohibits direct identification of the bar in the…”

“The size of the nuclear ring is an important parameter to constrain the shape of the potential as the gas in the ring follows x2 orbits (Binney et al. 1991).”

“Non-circular motions caused bya ar and spiral arms are beyond the scope of this review, which are reviewed in the literature (e.g., Binney et al. 1991; Jenkins a d Binney 1994; Athanassoula 1992; Burton and Liszt 1993).”

“…CO component with positive velocities apparently does not contribute to the H2 emission suggests this kind of heating since, at this galactic longitude, shocks are expected at negative velocities due to the intersection of x1 and x2 orbits in the context of a barred potential (Binney et al. 1991).”

“In our own Milky Way, the distribution of molecular gas as measured in CO is offset about 1◦ in longitude and +15 km s−1 in velocity from the center (Binney et al. 1991); other mass concentrations in that region also appear to be off-center (Blitz 1995).”

“The kinematics of the Milky Way along these lines of sight is indeed generally dominated by the Galactic bar (Binney et al. 1991; Ellsworth-Bowers et al. 2015; Li et al. 2016).”

“Models of strong bars, e.g. Binney et al. (1991), can naturally explain the near-infrared rotation curve of gas, sampled at 3 arcsec by Harrison et al. (1998).”

“Thus the outer part of the 180 pc ring appears to mark a transition between H I and H2 (Binney et al. 1991).”

“The Milky Way also has a central bar which can be split into a 3.1 – 3.5 kpc Galactic Bar at an angle of 20◦ with respect to the Galactic Centre-Sun axis (Binney et al. 1991;Blitz & Spergel 1991;Dwek et al. 1995) and a non-axisymmetric structure, the ’Long Bar’ (Hammersley etal.”

“Item (v) is assumed (for galaxies) since stellar dynamics or other methods never give ρ(r) as shallow as r−1.5 (e.g. Binney et al. 1991; Gerhard et al. 2001).”

“…about Galactic structure over the past 50 yr including: a 3.1–3.5 kpc Galactic bar at an angle of 20◦ with respect to the Galactic Centre–Sun axis (Binney et al. 1991; Blitz & Spergel 1991; Dwek et al. 1995), a second non-axisymmetric structure referred to as the ‘Long Bar’ (Hammersley et al.…”

“…Galactic structure over the past fifty years including: a 3.1-3.5 kpc Galactic Bar at an angle of 20◦ with respect to the Galactic Centre-sun axis (Binney et al. 1991; Blitz & Spergel 1991; Dwek et al. 1995), a second non-axisymmetric structure referred to as the “Long Bar” (Hammersley et al.…”

“…provided plenty of evidence that the heart of our Galaxy harbors a triaxial entity that is distinct from the disc (see e.g. Blitz & Spergel 1991; Binney et al. 1991; Dwek et al. 1995; Binney et al. 1997; Stanek et al. 1997; Freudenreich 1998; Alcock et al. 2000; Bissantz & Gerhard 2002; Evans &…”

“The picture changed when non-axisymmetries, found consistently in the HI and CO gas flow (Binney et al. 1991; Englmaier & Gerhard 1999; Fux 1999), in the near-infrared light distribution (Blitz & Spergel 1991; Weiland et al. 1994; Bissantz & Gerhard 2002) and star counts (Nakada et al. 1991; Stanek…”

“Both Binney et al. (1991) and Jenkins & Binney (1994) suggest that the giant and dense Galactic center molecular clouds (for example, the Sgr A, B and C cloud complexes) are likely to reside on the x2 orbits.”

“…al. 2012), carbon stars (Cole & Weinberg 2002), the solar neighborhood velocity field (e.g., Dehnen 2000; Minchev et al. 2007), microlensing (e.g., Gyuk 1999; Rattenbury et al. 2007), and inner Galaxy gas kinematics (e.g., Liszt & Burton 1980; Binney et al. 1991; Fux 1999; Weiner & Sellwood 1999).”

“Pioneering works led by Binney et al. (1991) interpret the CMZ as cold gas switching from x1 orbits to x2 orbits, while for warm gas the x2 orbits would not be occupied (Englmaier & Gerhard 1997).”

“The presence of that gas may be explained by the action of the Galactic bar, driving gas from the galactic disk to the inner regions until the gas settles on stable orbits (e.g. Binney et al. 1991).”

“Such test particle method has been widely used in studying discs in binary systems (Paczynski 1977) and galactic dynamics (Contopoulos 1979; Binney et al. 1991).”

“Furthermore, the projected gas density in the MilkyWay scales as r 1:75 (Binney et al. 1991).”

“…2.4 µm maps, and Nakada et al. (1991), using IRAS stars, detected asymmetries in longitude which they explained by the triaxiality of the outer bulge, also called the bar, with the near end at positive longitudes ; Binney et al. (1991) studied the gas kinematics and came to the same conclusion.”

“…of velocities with Galactic longitudes has established the approximately circular and symmetric rotation of the Galaxy and both tracers have been used to constrain dynamical models (e.g., Peters 1975; Burton & Liszt 1978; Liszt & Burton 1978; Binney et al. 1991; Weiner & Sellwood 1999; Fux 1999).”

“In today’s universe around 70 per cent of all disc galaxies, including the MW (Binney et al. 1991), have stellar bars at their centres (Eskridge et al. 2000).”

“…the center in the same sense as Galactic rotation, but the overall kinematics of this neutral gas layer are complex, asymmetric, and difficult to interpret (Scoville 1972 ; Burton & Liszt 1978 ; Bally et al. 1988 ; Binney et al. 1991 ; von Linden, Duschl, & Biermann 1993 ; Jenkins & Binney 1994).”

“…approximately to the region within Galactocentric radius R = 3kpc (e.g. Binney & Merrifield 1998), the “circular velocity speed” can be significantly different from the speed of the gas obtained in simulations, or observed in the Galaxy, at the same radii (e.g. Binney et al. 1991, SBM15a).”

“…velocity should rise more gently than what can be derived from a simple tangent-point analysis of the l − v diagrams: this was first recognised by Binney et al. (1991), and considered recently by dynamical models of the inner Galactic disc based on the density and kinematics of stars (Portail…”

“Various models have been constructed to explain the observed features, many of which include a bar (e.g., Binney et al. 1991; Weiner & Sellwood 1999).”

“Such a bar had been claimed by Blitz & Spergel (1991) based on asymmetries in surface photometry of the bulge region, and by Binney et al. (1991) from the kinematics of molecular gas.”

“Binney et al.(1991) demonstrated convincingly that many important central gas features on thel − v diagram can be explained with the main orbital families in a barred galaxy.”

“…applied to galaxy NGC 5005 (Sakamoto et al. 2000), and similar models have been successfully used to infer the presence of bars in edge-on galaxies that show characteristic patterns on PV diagrams (Binney et al. 1991; Achtermann & Lacy 1995; Garcı́a-Burillo &Guélin 1995; Kuijken &Merrifield 1995).”

“This is supported by the non-circular features in the Galactic longitude and line-of-sight (LOS) velocity of the gas in the central region (e.g., Binney et al. 1991).”

“Binney et al. (1991) found α = 16 deg for a bar, i.e. a triaxial structure in the centre of the Galaxy, in order to explain the kinematics of the gas in the centre of the Galaxy.”

“A mild noncircularity is expected, as this disc is born from gas on orbits of the x2 family and affected by the rotating bar (Binney et al. 1991), some might derive from possible spiral arms within this disc.”

“In the Milky Way’s CMZ, the bulk of the gas at high column density is found in a ring of giant molecular clouds, orbiting the central supermassive black hole at radii of 50 to 100 pc (Molinari et al. 2011; Kruijssen et al. 2015), on so-called ‘x2’ orbits (Binney et al. 1991).”

“It is well known that the gas dynamics differs considerably from the stellar dynamics in the Galaxy (e.g., Binney et al. 1991).”

“…mass distribution, in which the density varies radially as R−1.8 – this behaviour is consistent with what is inferred for the centre of the Milky Way (Binney et al., 1991; Genzel et al., 1997) and is characteristic of the luminosity profiles of a large class of elliptical galaxies (Gebhardt et…”

“The radio lines have often been interpreted as arising near the outer edge of the CMZ (Kaifu et al. 1972; Scoville 1972; Sofue 1995), in a shell or ring-like structure (see Binney et al. 1991, for an alternative explanation).”

“The much-studied 3-kpc arm is probably an inner ring, connected to the IUHR±CR region rather than OLR (Yuan 1984; Binney et al. 1991) or ILR (Amaral & Lepine 1997).”

“A similar di†erentiation seems to be in place in the Milky Way (e.g., Bally et al. 1988 ; Stark et al. 1989, 1991 ; Binney et al. 1991 ; Spergel & Blitz 1992).”

“…of the SiO emission was related to the large-scale gas dynamics in the Galactic center region where the movement of the gas can be understood as the response of a rapidly rotating bar potential (Binney et al. 1991), and the higher abundances of SiO can be identified with the collision region.”

“Binney et al. (1991) (see also Sormani et al. 2015a) predicts that the major axis of the ring should be equal to the minor axis of the cusped x1 orbit.”

“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).”

“The inner Lindblad resonance (ILR) locates at ∼ 1 kpc in this model, which is quite different from some previous models (e.g., Binney et al. 1991; Bissantz et al. 2003) that place the ILR at ∼ 100 pc.”

“The orbits trapped in the inner ring resemble the cuspy x1 orbits of the bar, responsible for the maximum in the terminal velocity curves (Binney et al. 1991).”

“These orbits have proven essential in explaining the (l, v) diagram of gas towards the Galactic Centre (Binney et al. 1991) and are thus expected to be an important ingredient for explaining any observation of the kinematics of bar stars.”

“We note here that the center of our Galaxy also exhibits a tilted rotating ring of ionized and neutral gas (Burton & Liszt 1978; Lacy et al. 1979), which may be understood in terms of a barred potential (Binney et al. 1991).”

“…forbidden velocities in the (ℓ, v) diagram for 12CO and the similar results for H i 21 cm (see e.g., Dame et al. 2001) are also a direct signature of non-circular orbits in the inner Galaxy, which is interpreted as inner Galaxy gas flows caused by a barred Galactic potential (Binney et al. 1991).”

“In addition, the multitude of “forbidden” (e.g. sign opposite to galactic rotation) velocity components in the GC region are thought to represent the response of the molecular gas in the GC to the Galaxy’s strong stellar bar (Binney et al. 1991; Bally et al. 1988).”

“This conclusion can be readily established from the gas kinematics in the inner Galaxy (e.g., de Vaucouleurs 1964; Binney et al. 1991), as well as from near-infrared photometry (Binney et al. 1997).”

“However, this difference is expected in barred galaxies where the gas has a non-circular motion with in-falling streams of gas, as shown by Kim et al. (2012, see MNRAS 000, 1–30 (2018) their Figure 5, see also Gerhard & Vietri 1986; Binney et al. 1991; Li et al. 2015; Chemin et al. 2015).”