Understanding the kinematics of Galactic Centre gas

“If the inner bar has a radius of 1 - 2 kpc (Binney et al. 1991; Blitz & Spergel 1991; Binney & Gerhard 1993; Sellwood 1993b; cf. Dwek et al. 1995), then it is more nearly the length of typical nuclear bars than of typical main bars.”

“This corroborates long-standing evidence for a barred potential in the bulge region from noncircular motions seen in HI and CO longitudevelocity (lv) diagrams (Binney et al. 1991, Englmaier & Gerhard 1999).”

“The kinematics of this ring (hereafter referred to as the “180-pc molecular ring”) and its tilt relative to the Galactic plane are continuous with those of the exterior H I nuclear disk, indicating that this structure likely marks the location of an H I/H2 transition (Binney et al 1991).”

“A few percent exhibits non-circular ’forbidden’ velocities, which could be due to non-circular motion in an oval potential (Binney et al. 1991), whereas a question remains why the majority of the gas is regularly rotating.”

“…observed in the inner Galaxy (de Vaucouleurs 1964; Peters 1975; Cohen & Few 1976; Liszt & Burton 1980; Gerhard & Vietri 1986; Mulder & Liem 1986; Binney et al. 1991) and later confirmed by infrared photometry (Blitz & Spergel 1991; Weiland et al. 1994; Dwek et al. 1995; Binney, Gerhard &…”

“The Milky Way would clearly have been detected at the average distance of the survey, though it would have been below the average brightness at that distance.”

“However, the Milky Way is known to have some non-axisymmetric structures, such as the central bar (Blitz & Spergel 1991; Binney et al. 1991), spiral arms (e.g., Drimmel & Spergel 2001), and a potentially triaxial halo (Law et al. 2009), that may influence the measurement performed in this paper.”

“…giant stars have been used as standard candles to probe the Galactic bulge on numerous occasions, beginning with Stanek et al. (1994, 1997), who confirmed evidence from gas kinematics and NIR photometry (Binney et al. 1991; Weiland et al. 1994) that the Galactic bulge hosts a triaxial structure.”

“Although it is well established that the Milky Way hosts a central barred bulge which causes non-axisymmetric gas flow (Peters, III 1975; Binney et al. 1991) and asymmetries in the near-infrared light (Blitz & Spergel 1991; Weiland et al. 1994) and star counts (Nakada et al. 1991; Stanek et al.…”

“Gas kinematics and near-Infrared (NIR) photometry (e.g. Binney et al. 1991; Weiland et al. 1994) have shown that the Galactic bulge at longitudes |l| < 10◦ contains a bar-like structure.”

“The gas kinematics and distribution of near-infrared light indicate that the gravitational potential in the inner Galaxy is dominated by a bar with a major axis inclined by about 20 to 45◦ with respect to our line-of sight (Binney et al. 1991, Benjamin et al. 2003, Minchev et al. 2007).”

“These are better described by models that treat the stellar potential as decoupled from the gas (e.g. Binney et al. 1991; Sormani et al. 2015; Krumholz et al. 2017).”

“There is also some disagreement over the precise size (Weinberg 1992; Dwek et al. 1995) and pattern speed of the bar (Binney et al. 1991; Kalnajs 1996), but it seems unlikely that the value ofDL/aB will turn out to be substantially larger than for other barred galaxies.”

“…often have bars at the center (Barazza et al. 2008) and a thick central portion that is described as a pseudo-bulge (e.g., Kormendy & Kennicutt 2004); the Milky Way fits that pattern, as it has a large-scale stellar bar (Binney et al. 1991; Blitz & Spergel 1991; Weinberg 1992; Binney et al. 1997).”

“Comparing these results to previous studies of the same region in which coherent gas streams were identified (e.g. Bally et al. 1988; Binney et al. 1991; Sofue 1995; Stark et al. 2004), we see that we have obtained a clean sample.”

“At yet higher densities, the CS line traces massive molecular cloud complexes, which are presumably orbiting on x2 orbits inside the inner lindblad resonance (ILR) of the bar (Stark et al. 1991; Binney et al. 1991).”

“This may be the situation in the inner few kpc of the Milky Way (e.g. Binney et al 1991).”

“◦3 complex and Bania’s Clump 2 are moving along the innermost self-intersecting x1 orbit (Binney et al. 1991; Morris & Serabyn 1996; Rodriguez-Fernandez et al. 2006; Bally et al. 2010).”

“Binney et al. (1991) interpreted the observational longitude-velocity (l, v) diagrams of CO and Hi emissions in terms of the theoretical (l, v) curves generated by non selfintersecting x1 orbits and by x2 orbits.”

“The color contrast of the coherent, large scale dust lane suggests that the molecular gas in the central molecular zone is oriented in the same direction as that of the well-known stellar bar (Binney et al. 1991; Stark et al. 1991).”

“Because of this, inflowing gas has to quickly pass this region: the mechanism described by Binney et al. (1991), where gas moving in from the previous x1-orbits collides with gas on the outermost x2-orbits, producing a spray that then forms an off-axis shock by hitting the far side of the…”

“The Milky Way’s bar was discovered in the distribution of near-infrared emission (Blitz & Spergel 1991) and in the kinematics of gas (Binney et al. 1991) in the central regions.”

“The narrow R(1, 1)l and CO features at this velocity probably do arise in the arm, which merges into the corotating stars in the Galactic Stellar Bar at about 2.4 kpc (Binney et al. 1991; Wada et al. 1994; Mezger et al. 1996).”

“…(CMZ) are consistent with characteristic motions along bar orbits, but the evidence for inflow motions along a bar shock are less clear; the falloff in the signature ‘parallelogram envelope’ with increasing distance from the center in the pv diagram (Binney et al. 1991) is not especially obvious.”

“Binney et al. (1991) provided further evidence for a bar from star counts.”

“The high-velocity arcs are visible as parts of an ellipse corresponding either to parallelogram (Binney et al. 1991) or to an expanding molecular ring (Kaifu et al. 1972; Scoville 1972; Sofue 1995b).”

“…material which circumscribes the CMZ at a galactocentric distance of 150 - 180 pc is hypothesized to be material condensed into molecular form by shocks occurring along the innermost, non-self-intersecting X1 orbit in the Galaxy’s barred potential (Binney et al. 1991; Morris & Serabyn 1996).”

“A kinematic identification is clearly called for, analogous to the use of longitude-velocity diagrams in the Galaxy (e.g., Peters 1975; Mulder & Liem 1986; Binney et al. 1991).”

“This is a typical signature of nuclear bar kinematics (e.g., Binney et al. 1991; Garcia-Burillo & Guelin 1995; Garcia-Burillo et al. 1999).”

“The Galactic bar/bulge causes the non-circular motions in the gas flow seen in HI and CO (Peters, III 1975; Binney et al. 1991; Englmaier & Gerhard 1999; Fux 1999) and is the origin of the asymmetries seen in the near-infrared light distribution (Blitz & Spergel 1991; Weiland et al. 1994; Binney,…”

“…of evidence now point to the fact that the Milky Way hosts a bar (de Vaucouleurs 1964; Peters 1975; Cohen & Few 1976; Liszt & Burton 1980; Gerhard & Vietri 1986; Mulder & Liem 1986; Binney et al. 1991; Nakada et al. 1991; Whitelock & Catchpole 1992; Weiland et al. 1994; Paczynski et al. 1994; ?”

“Our simulations are the very first attempt to model the Milky Way with two nested bars and have been performed assuming a common pattern speed for both bars.”

“…currently known of the structure of our Galaxy, including the 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.…”

“We adopt the corotation value of rmax = 2.4 kpc determined by Binney et al. (1991) in equation (13).”

“Several authors have suggested that such a stellar density decrease may be present in the Galactic disc (Binney et al. 1991, Bertelli et al. 1995, Freudenreich 1998, Lépine & Leroy 2000, López-Corredoira et al. 2004).”

“Previous determinations of the bar’s shape are based on star counts only (e.g., Blitz & Spergel 1991; Wegg & Gerhard 2013; Wegg et al. 2015) and the bar is also detected clearly in the kinematics of stars and gas in the inner Milky Way (e.g., Binney et al. 1991; Portail et al. 2017).”

“Both of these are still a matter of debate, with the orientation varying between 15◦ and 45◦ (e.g. Peters 1975; Binney et al. 1991; Fux 1999; Englmaier & Gerhard 1999; Rodriguez-Fernandez & Combes 2008), and the halflength between 2 and 5 kpc.”

“Binney et al. (1991) studied the flow of gas in the inner few degrees and also showed that the gas-flow lines followed a bar-like potential with a position angle of 16◦.”

“Resonances will thus shape the orbital structure of the galaxy and are therefore critical ingredients to understand the overall evolution (Binney et al. 1991; Athanassoula 1992; Wada & Habe 1995; Englmaier & Shlosman 2000; Stark et al. 2004). c© 0000 RAS ar X iv :1 41 0.”

“…explained by beam smearing effects, we also expect bar-induced radial inflow and the corresponding conversion of gravitational potential energy to be an additional source of turbulence in the ISM (e.g., through shocks near the center of the stellar bar; see Binney et al. 1991; Sormani et al. 2015).”

“…surface brightness maps of the inner Galaxy (Dwek et al. 1995; Binney, Gerhard & Spergel 1997), the asymmetric distribution of bulge red clump stars (Stanek et al. 1994; 1997) and the noncircular kinematics of 12CO, 13CO and CS gas (Bally et al. 1987; Dame et al. 1987; Binney et al. 1991).”

“…2012; Forbes et al. 2014), and that, once gas reaches the bar, it should be further transported inward along the bar to be deposited in the CMZ (Binney et al. 1991; Kormendy & Kennicutt 2004), where it settles into the disc-like structure that we seek to model (Sormani, Binney & Magorrian…”

“The distribution and presence of molecular gas in the CMZ may in large part be due to the effects of the inner Galactic stellar bar (Liszt & Burton 1980; Mulder & Liem 1986; Binney et al. 1991; Blitz & Spergel 1991; Weiland et al. 1994; Dwek et al. 1995; Stanek et al. 1994).”

“However, this model assumes purely circular motion and takes no account, for example, of the presence of a bar in the Galaxy on the motion of gas near SS 433 (Binney et al 1991).”

“The idea that the Milky Way is a barred spiral galaxy can be traced back at least fifty years to Johnson (1957), de Vaucouleurs (1964), and others, and there is now fairly wide consensus that we are indeed located in a barred spiral (Binney et al. 1991; Blitz & Spergel 1991; Benjamin et al. 2005).”

“The cause of the large velocity dispersion is ascribed either to the gas motions driven by the bar potential (Binney et al. 1991) or to the magnetic instabilities in the strong magnetic field (Suzuki et al. 2015; Kakiuchi et al. 2018; Fukui et al. 2006).”

“Other models incorporating gas kinematics (Binney et al. 1991), metallicity gradients (Zhao et al. 1994), and micro-lensing events (Paczynski 1991; Zhao et al. 1995; Han & Gould 1995), indicate the bulge to have a triaxial symmetry.”

“Binney et al. (1991) (BGSBU) constructed a picture of the flow of gas through the central few kiloparsecs of our Galaxy.”

“Observational evidence for a bar has arisen from several sources, such as the studies of gas kinematics (e.g. Binney et al. 1991), surface brightness (e.g. Blitz & Spergel 1991), star counts (e.g. Nakada et al. 1991; Stanek et al. 1994) and microlensing (e.g. Udalski et al. 1994; see Gerhard 2002…”

“…IRAS point sources (Weinberg 1992 ; Nikolaev & Weinberg 1997), the magnitude o†set of bulge stars at positive and negative longitudes (Stanek 1995 ; Stanek et al. 1997), OH/IR stars (Sevenster 1995), and the gas motions near the Galactic center (e.g., Liszt & Burton 1980 ; Binney et al. 1991).”

“…2010 RAS, MNRAS 000, 1–?? sumably under the action of the Galactic stellar bar, possibly assisted by dynamical friction and magnetic viscosity effects (Binney et al. 1991; Blitz et al. 1993; Serabyn & Morris 1996; Morris & Serabyn 1996; Beck et al. 1999; Stark et al. 2004) – over timescales…”

“The major axis of the bar is thought to have an angle with respect to our line-of-sight between 20◦ to 45◦(Binney et al. 1991; Bissantz et al. 2003; Ferrière et al. 2007; Pohl et al. 2008).”

“Binney et al. (1991) interpreted the longitude-velocity feature that corresponds to the central condensation as a projection of gaseous materials in x2 orbits.”

“The potential used in Jenkins & Binney (1994) and SBM2015a differs from the potential used in Binney et al. (1991) only by the addition of an axisymmetric disc component.”

“The Galaxy has a significant bar, though (Binney et al. 1991; Weiland et al. 1994).”

“…and dynamics of the bar and the spiral arms of the Milky Way relies both on the gas, and notably on its observed longitudevelocity diagram (Binney et al. 1991; Bissantz et al. 2003; Englmaier et al. 2011) or masers in high mass star forming regions (Reid et al. 2009, and references…”

“Galactic Center gas tha t Binney et al. (1991) identify as being on x% orbits has a density near 1 0 3 5 c m ~ 3 , which renders it only marginally stable against gravitational coagulation into one or two giant clouds (Elmegreen 1994).”

“This mass flux is provided by material that is removed from its circular orbit by the Galactic bar and transported inwards to settle into the CMZ (Binney et al. 1991; Kormendy & Kennicutt 2004; Crocker 2012; Sormani, Binney & Magorrian 2015).”

“Although their statistics were not very good, they claim that the asymmetry is best fitted with the interpolation of the models developed by Blitz & Spergel (1991) and Binney et al. (1991, 1997) for the large-scale bar.”

“First, projection e†ects (in both longitude and latitude) mean that two lines of sight on each side of the Galactic center reach correspondingly different parts of the bar (e.g., Binney et al. 1991).”

“…et al. 1997), asymmetries in the distribution or magnitude of stars (Whitelock & Catchpole 1992; Weinberg 1992; Nikolaev & Weinberg 1997; Stanek 1995; Sevenster 1996; Stanek et al. 1997), and gas velocities (Liszt & Burton 1980; Binney et al. 1991; Englmaier & Gerhard 1999; Weiner & Sellwood 1999).”

“Not all of the gas entering the nuclear region remains there, since some of the gas sprays back into the bar region at the contact point of the dust lane and the nuclear ring (Binney et al. 1991).”

“Physically, first shocks could be caused by close encounters or mergers between galaxies, or collisions of giant molecular cloud complexes (for example, following the bar potential toward the centre of the galaxy, Binney et al. 1991).”

“…bulges, such as the one hosted by our own Milky Way (e.g., Combes & Sanders 1981; de Souza & Dos Anjos 1987; Shaw 1987; Combes et al. 1990; Binney et al. 1991; Kuijken & Merrifield 1995; Merrifield & Kuijken 1999; Bureau & Freeman 1999; Lütticke et al. 2000; Chung & Bureau 2004; Bureau &…”

“Most present-day spirals are barred (e.g., Eskridge et al. 2000; Laurikainen et al. 2004; Marinova & Jogee 2007; Menéndez-Delmestre et al. 2007), including our own Milky Way (Petters 1975; Blitz & Spergel 1991; Binney et al. 1991; Weiland et al. 1994).”

“…that the total density distribution in the central regions of elliptical galaxies is well approximated by isothermal, r 2 (Rix et al. 1997; Gerhard et al. 2001), while the study of the dynamics of the gas in the center of our Galaxy shows that total density scales as r 1.75 (Binney et al. 1991).”

“See Binney et al. 1991 for effects of changes in the inclination angle on the (l, vlos) diagram. line-of-sight distances above 200 pc in front of or behind the GC appear less likely because of the expected high initial velocities, vlos > 170 km/s, significantly larger than the observed cloud…”