The observed spiral structure of the Milky Way

Abstract

Context. The spiral structure of the Milky Way is not yet well determined. The keys to understanding this structure are to increase the number of reliable spiral tracers and to determine their distances as accurately as possible. HII regions, giant molecular clouds (GMCs), and 6.7 GHz methanol masers are closely related to high mass star formation, and hence they are excellent spiral tracers. The distances for many of them have been determined in the literature with trigonometric, photometric, and/or kinematic methods. Aims. We update the catalogs of Galactic HII regions, GMCs, and 6.7 GHz methanol masers, and then outline the spiral structure of the Milky Way. Methods. We collected data for more than 2500 known HII regions, 1300 GMCs, and 900 6.7 GHz methanol masers. If the photometric or trigonometric distance was not yet available, we determined the kinematic distance using a Galaxy rotation curve with the current IAU standard, R 0 = 8.5 kpc and Θ 0 = 220 km s −1 , and the most recent updated values of R 0 = 8.3 kpc and Θ 0 = 239 km s −1 , after velocities of tracers are modified with the adopted solar motions. With the weight factors based on the excitation parameters of HII regions or the masses of GMCs, we get the distributions of these spiral tracers.Results. The distribution of tracers shows at least four segments of arms in the first Galactic quadrant, and three segments in the fourth quadrant. The Perseus Arm and the Local Arm are also delineated by many bright HII regions. The arm segments traced by massive star forming regions and GMCs are able to match the HI arms in the outer Galaxy. We found that the models of three-arm and four-arm logarithmic spirals are able to connect most spiral tracers. A model of polynomial-logarithmic spirals is also proposed, which not only delineates the tracer distribution, but also matches the observed tangential directions.

A New Electron-Density Model for Estimation of Pulsar and FRB Distances

“…In a recent study, Hou & Han (2014) updated the catalogs of spiral-arm tracers, including more than 2500 H II regions, 1300 giant molecular clouds, and 900 6.7 GHz methanol masers. They used these data to investigate the spiral structure of the Galaxy, assuming a logarithmic-spiral form:…”

“…For the spiral arms, YMW16 adopts a simple logarithmicspiral pattern based on H II-region distances (Hou & Han 2014), in contrast to the somewhat more complex modified spiral pattern used by TC93 and NE2001. Part of the modified pattern used by these earlier models was to reduce n e in the nominal Sagittarius tangential region.…”

“…For example, these can be used to define the spiral-arm structure in the Galaxy (e.g., Hou & Han 2014) and suggest the presence of a "thick" electron disk with scale height ∼1.5 kpc in addition to the "thin" disk defined by H II regions, which has a scale height ∼70 pc (e.g., Hill et al 2008). However, the parameters describing the n e distribution generally cannot be independently determined with sufficient precision.…”

“…The Galactic part of the model has the same basic structure as TC93 and NE2001, but also some important differences compared to these earlier models. For example, we adopt a four-armed spiral pattern (plus a "local arm") with the location and form of the arms as given by Hou & Han (2014) based on observations of more than 1800 H II regions across the Galaxy. The YMW16 model is fitted to a compilation of 189 independent pulsar distance estimates, using global and local optimization algorithms.…”

Galactic cold cores

“…In the lower panels, the X and Y coordinates define positions in the Galactic plane with the Galactic centre at (X, Y) = (0, 0), the Sun at (X, Y) = (8400 pc, 0), and 0 ≤ l ≤ 180 deg for Y ≤ 0. The shaded arcs are the Carina-Sagittarius arm (I), the local arm (II), and the Perseus arm (III), as parametrised by Hou & Han (2014, their Table 1, three-arm model with all spiral tracers).…”