Galactic structure & dynamics from BeSSeL

There is currently much ambiguity in our knowledge of the Milky Way’s structure and or- bital properties. No consensus exists as to the number of spiral arms, their positions, or rotational attributes (e.g. Urquhart et al. 2014a; Reid & Honma 2014; Purcell et al. 2011). Determining the Milky Way’s structure has been listed as a primary objective for the Gaia mission, launched by the European Space Agency (ESA) in 2014. Even though Gaia aims to measure parallaxes of approximately 1 billion stars by 2022, the thick dust clouds which permeate the Galactic Plane and where much of the spiral structure exists, will be obscured from this advanced optical facility due to extinction of visible light. The VLBI observations of methanol (and water) maser targets from BeSSeL will complement the results from Gaia, as radio waves are able to penetrate the disk and central barred regions of the Galaxy allowing these sources to be detected through the Galactic Plane.

Spiral arm locations

The discovery of the 21 cm Hydrogen (Hi) emission line brought with it the opportunity

to characterise the shape of the Milky Way, as coherent arcs and loops in the longitude– velocity (`−V) plots hinted at its spiral structure (Westerhout 1957; Kerr et al. 1959). While Galactic Hi was deemed to be an excellent atomic tracer, the H2 molecule is symmetric and

does not radiate in the radio range. Therefore astronomers rely on the Carbon monoxide (CO) molecule as a molecular tracer instead (Dame et al. 1987; Burton et al. 1975; Schwartz et al. 1973). Molecular gas clouds are an important tool in mapping out individual spiral arms (e.g. Dame & Thaddeus 2008), as they contain around 50% of the ISM mass within a small fraction of volume (e.g. Ade et al. 2014). While CO gas is regarded as a good spiral arm tracer, the`−V diagrams give little information about the distances to individual arms or the extent of the separation between them.

In Reid et al. (2014), the authors present the latest results from over 100 trigonometric parallaxes and proper motions of masers associated with HMSFRs from the BeSSeL survey. The HMSFRs are assigned to spiral arms by association with CO and Hi emission features.

In doing so, any subjective judgments of spiral arm assignment based on parallax distances alone are avoided. The spiral arms are then modeled using a log–periodic spiral pattern in equation (4.1) for radius R from the Galactic Centre, with longitude β and pitch angle (which measures of the chirality of the spiral arm)ψ. The results from Reid et al. (2014) are summarized in Table 4.3 and described in Figure 4.3.

ln R Rref =−(β−βref) tanψ (4.1) Galactic dynamics

Using the complete three–dimensional location and velocity parameters (position (`,b), par- allax distance _π1, proper motion (µx, µy) and Doppler shift) of&100 HMSFRs in the Milky

Arm N βref β range Rref Width ψ (◦) (◦) (kpc) (kpc) (◦) Scutum 17 27.6 +3 to 101 5.0±0.1 0.17±0.02 19.8±2.6 Sagittarius 18 25.6 −2 to 68 6.6±0.1 0.26±0.02 6.9±1.6 Local 25 8.9 −8 to 27 8.4±0.1 0.33±0.01 12.8±2.7 Perseus 24 14.2 −21 to 88 9.9±0.1 0.38±0.01 9.4±1.4 Outer 6 18.6 −6 to 56 13.0±0.3 0.63±0.18 13.8±3.0

Table 4.3: Spiral arm characteristics from parallax measurements of HMSFRs in Reid et al. (2014) and described in Figure 4.3.

Figure 4.3: The Milky Way with spiral arms, adopted from Reid et al. (2014). The data are from trigonometric parallax measurements to HMSFRs using the VLBA, VERA and the EVN. The Galactic Centre is at (0 , 0) and the Sun is at (0 , 8.34). The inner Galaxy sources are described in yellow; the Scutum arm in cyan octagons; the Sagittarius arm in magenta hexagons the Local arm in blue pentagons; the Perseus arm in black squares and the Outer arm in red triangles. The open black circles are sources for which arm assignment was unclear. The data are concentrated in the 1st _{and 2}nd _{quadrants, showing the need for}

Way, Reid et al. (2014, 2009b) construct a model of the Milky Way as a flat disk as described in equation (4.2). The circular disk velocity at a given radius Θ(R) is described as a function of the circular orbit of the Sun Θ0 around the Galactic Centre, rate of change of orbital ve-

locity with radius d_{d R}Θ and Solar distance from the Galactic CentreR0. In this model, sources

close to the Galactic Centre were excluded due to complications in their orbits resulting from the gravitational potential of the central bar(s).

Θ(R) = Θ0+

dΘ

d R(R−R0) (4.2)

A Bayesian fitting approach was used in conjunction witha priori values for the peculiar (deviations from circular) components of Solar motion, U = 11.1 ±1.2 km s−1 (radial

inwards);V= 15±10 km s−1 (in the direction of Galactic rotation);W= 7.2±1.1 km s−1

(vertically upwards; Bovy et al. 2012; Sch¨onrich et al. 2010; Reid et al. 2009b), and Us = 3±10 km s−1 and Vs−3±10 km s−1 (Reid et al. 2009b) for the average peculiar motions of HMSFRs.

Due to large uncertainties in V, Us and Vs, Reid et al. (2014) adopted a loose prior forV which resulted in a chi–squared (χ2) value of 224.9 for 232 degrees of freedom. They

report a distance to the Galactic Centre ofR0= 8.3±0.16 kpc, circular rotation speed of the

spiral arms of Θ0 = 240±8 km s−1 and a flat rotation curve between radii of ∼5−16 kpc

from the Galactic Centre of d_{d R}Θ = 0.2±0.4 km s−1 kpc−1. The International Astronomical Union (IAU) recommended value for Θ0 is 220 km s−1 (Kerr & Lynden-Bell 1986) and has

not been updated for almost 30 years. The implications for an updated value for Θ0 in Reid

et al. (2014) means that the Milky Way is larger than has been assumed and has a more substantial dark matter halo.