The RMS survey: galactic distribution of massive star formation★

Abstract

We have used the well-selected sample of ∼1750 embedded, young, massive stars identified by the Red MSX Source (RMS) survey to investigate the Galactic distribution of recent massive star formation. We present molecular-line observations for ∼800 sources without existing radial velocities. We describe the various methods used to assign distances extracted from the literature, and solve the distance ambiguities towards approximately 200 sources located within the Solar circle using archival H i data. These distances are used to calculate bolometric luminosities and estimate the survey completeness (∼ 2 × 10 4 L ⊙ ). In total, we calculate the distance and luminosity of ∼1650 sources, one third of which are above the survey's completeness threshold. Examination of the sample's longitude, latitude, radial velocities and mid-infrared images has identified ∼120 small groups of sources, many of which are associated with well known star formation complexes, such as G305, G333, W31, W43, W49 and W51.We compare the positional distribution of the sample with the expected locations of the spiral arms, assuming a model of the Galaxy consisting of four gaseous arms. The distribution of young massive stars in the Milky Way is spatially correlated with the spiral arms, with strong peaks in the source position and luminosity distributions at the arms' Galactocentric radii. The overall source and luminosity surface densities are both well correlated with the surface density of the molecular gas, which suggests that the massive star formation rate (SFR) per unit molecular mass is approximately constant across the Galaxy. A comparison of the distribution of molecular gas and the young massive stars to that in other nearby spiral galaxies shows similar radial dependencies.We estimate the total luminosity of the embedded massive star population to be ∼ 0.76 × 10 8 L ⊙ , 30 per cent of which is associated with the ten most active star forming complexes. We measure the scale height as a function of Galactocentric distance and find that it increases only modestly from ∼20-30 pc between 4 and 8 kpc, but much more rapidly at larger distances.

The almost ubiquitous association of 6.7-GHz methanol masers with dust★

“…Although there are a number of massive star-forming regions associated with the Sagittarius arm, the star formation per unit area is relatively modest when compared to the segments of the spiral arms located inside the Solar circle. This is in stark contrast to the high surface density of MYSOs and compact H ii regions reported by Urquhart et al (2014b) found to be associated with the Sagittarius arm and its association with RCW42, NGC3603 and G282.0−1.2 (Carina), which are some of the most active star-forming complexes in the Galaxy.…”

“…itudes (i.e., |b| < 5 • ), also found a low number of MYSOs and compact H ii regions associated with the arms located in the third quadrant (Urquhart et al 2014b). This suggests that little massive star formation is taking place in the outer parts of the Galaxy compared to the inner Galaxy or that star formation in spiral arms is patchy and/or intermittent (Urquhart et al 2014b).…”

“…The remaining 14 sources are located within the Solar circle and are associated with KDAs. We solve 6 of these by assigning an upper limit on their allowed distance from the Galactic mid-plane of 120 pc (i.e., ∼5 times the Galactic scale height of massive stars; Urquhart et al 2014b); if a far distance would place the source above this limit a near distance is considered more likely.…”

“…This suggests that little massive star formation is taking place in the outer parts of the Galaxy compared to the inner Galaxy or that star formation in spiral arms is patchy and/or intermittent (Urquhart et al 2014b). …”

Estimates of black hole natal kick velocities from observations of low-mass X-ray binaries

“…This information makes it possible to integrate the trajectory of the binary ⋆ E-mail: imandel@star.sr.bham.ac.uk backwards in the Galactic potential until it intersects the Galactic plane, where the binary is assumed to have been born. This assumption is consistent with both the observed birth locations of massive stars in a thin disk in the Galactic plane (e.g., Urquhart et al 2014), and more specifically the finding that metal abundances in the secondary in blackhole X-ray binaries is consistent with a thin-disk origin (e.g., González Hernández et al 2008). Population synthesis modeling can then be applied to include the contribution of Blaauw kicks (Blaauw 1961) from mass loss during the supernova and constrain the range of supernova-induced natal kicks required to preserve the binary with current orbital parameters.…”