Star-forming complexes and the spiral structure of our Galaxy

Abstract

Abstract.We have carried out a multiwavelength study of the plane of our Galaxy in order to establish a star-forming-complex catalogue which is as complete as possible. Features observed include Hα, H109α, CO, the radio continuum and absorption lines. For each complex we have determined the position, the systemic velocity, the kinematic distance and, when possible, the stellar distance and the corresponding uncertainties. All of these parameters were determined as homogeneously as possible, in particular all the stellar distances have been (re)calculated with the same calibration and the kinematic distances with the same mean Galactic rotation curve. Through the complexes with stellar distance determination, a rotation curve has been fitted. It is in good agreement with the one of Brand & Blitz (1993). We also investigated the residual velocities relative to the circular rotation model. We find that departures exist over large areas of the arms, with different values from one arm to another. From our data and in good agreement with previous studies, the Galactic warp is observed. It does not seem correlated with the departures from circular rotation. Finally, as segment-like features are noted from the complexes' distribution, we tried to find if they are indicative of a larger underlying structure. Then, we attempted to interpret the complexes' distribution in terms of spiral structure by fitting models with two, three and four logarithmic spiral arms. The four-arm model seems more appropriate to represent the grand design of our Galaxy. In this model the Norma arm and the external arm appear as being the two extremities of a single arm called the Norma-Cygnus arm. The new data and fitted model confirm the four-segment model of Georgelin & Georgelin (1976), clarifying the arms' design and extension and doubling their known length.

A general catalogue of 6.7 GHz methanol masers

“…2 is at a galactocentric distance where the spiral pattern of the Galaxy is not yet clearly visible (molecular ring, see e.g. Russeil 2003). The latter argument is also supported in Fux (1999), where it is shown that the existence of a bar in our Galaxy does not significantly affect the dynamics outside the molecular ring.…”

“…In the case of galactic sources, kinematic distances from spectral line observations are becoming more accurate thanks to improved modelling of the Galactic rotation curve (e.g. Brand & Blitz 1993 as well as Russeil 2003). The difficulty resides in the discrimination between near-and farheliocentric distances for all sources on orbits internal to the Solar orbit around the Galactic Centre.…”

Spectral classification and distance determination of stars in nine southern Galactic $\ion{H}{ii}$ regions

“…The heliocentric photometric distance d was calculated adopting the calibration of the absolute magnitude M V as a function of the spectral type and luminosity class given by Russeil (2003). The error in distance was estimated to be…”

Evidence of the Galactic outer ring R 1 R 2 ′ $R_{1}R_{2}'$ from young open clusters and OB-associations

“…the review by Vallée 2013). The main achievement of this purely spiral model is that it can explain the distribution of HII regions in the Galactic disk (Russeil 2003). This model became more physical after incorporation of the bar into it (Englmaier & Gerhard 1999).…”