A high pitch angle structure in the Sagittarius Arm

Abstract

Context. In spiral galaxies, star formation tends to trace features of the spiral pattern, including arms, spurs, feathers, and branches. However, in our own Milky Way, it has been challenging to connect individual star-forming regions to their larger Galactic environment owing to our perspective from within the disk. One feature in nearly all modern models of the Milky Way is the Sagittarius Arm, located inward of the Sun with a pitch angle of ∼12 • . Aims. We map the 3D locations and velocities of star-forming regions in a segment of the Sagittarius Arm using young stellar objects (YSOs) from the Spitzer/IRAC Candidate YSO (SPICY) catalog to compare their distribution to models of the arm. Methods. Distances and velocities for these objects are derived from Gaia EDR3 astrometry and molecular line surveys. We infer parallaxes and proper motions for spatially clustered groups of YSOs and estimate their radial velocities from the velocities of spatially associated molecular clouds. Results. We identify 25 star-forming regions in the Galactic longitude range ∼ 4. • 0-18. • 5 arranged in a narrow, ∼1 kpc long linear structure with a high pitch angle of ψ = 56 • and a high aspect ratio of ∼7:1. This structure includes massive star-forming regions such as M8, M16, M17, and M20. The motions in the structure are remarkably coherent, with velocities in the direction of Galactic rotation of |V ϕ | ≈ 240 ± 3 km s −1 (slightly higher than average) and slight drifts inward (V R ≈ −4.3 km s −1 ) and in the negative Z direction (V Z ≈ −2.9 km s −1 ). The rotational shear experienced by the structure is ∆Ω = 4.6 km s −1 kpc −1 . Conclusions. The observed 56 • pitch angle is remarkably high for a segment of the Sagittarius Arm. We discuss possible interpretations of this feature as a substructure within the lower pitch angle Sagittarius Arm, as a spur, or as an isolated structure.

The Effect of Molecular Cloud Properties on the Kinematics of Stars Formed in the Trifid Region

“…(See cautionary note by Bailer-Jones et al 2021, their Section 4.3. ) fid derived by Kuhn et al (2021b) using Gaia EDR3 -this distance is corroborated by our Chandra+Gaia analysis here. Nevertheless, the complicated molecular cloud velocity structure along the line of sight suggests the possibility of the coincidental alignment of multiple star-forming regions with differences distances.…”

“…Intriguingly, most of the rejected sources (7 out of 8) lie very close to the 3σ boundary. Given that the Trifid Nebula is part of a larger kpc-scale star-forming structure in the Sagittarius Arm (Kuhn et al 2021b), it is possible that some of these stars may be part of this larger scale structure, albeit not part of Trifid.…”

“…In the vicinity of the Sun, many of the major starforming regions are associated with long, filamentary, molecular-cloud structures (e.g., Jackson et al 2010;Goodman et al 2014;Zucker et al 2015;Green et al 2019;Lallement et al 2019Lallement et al , 2022Zucker et al 2020;Alves et al 2020). One of these filaments is a kpc-long collection of molecular clouds and star-forming regions, traditionally considered to be part of the Milky Way's Sagittarius Arm, but shown by Gaia to form a discrete substructure with a high-pitch angle relative to the arm (Kuhn et al 2021b). This structure contains several of the nearest massive star-forming regions, including Messier 8 (the Lagoon Nebula), 16 (the Eagle Nebula), 17 (the Omega Nebula), and 20 (the Trifid Nebula).…”

“…Gaia's measurements have dramatically improved the situation, indicating that Trifid is closer than previously thought. We adopt a distance 1 of 1180±25 pc for Tri-1 The Trifid distance estimate of 1180±25 pc from Kuhn et al (2021b) is based on a joint probabilistic Bayesian model of Gaia EDR3 astrometry for Trifid member stars, adopting the parallax zero-point corrections from Lindegren et al (2021). Subsequently, Kalari (2021) published a slighly farther, but less precise, distance estimate of 1257 +190 −98 pc, derived by first estimating Bayesian distances to individual stars using a prior similar to that of Bailer-Jones et al (2018), then estimating the mean of the distribution of stellar distances.…”

A Kpc Scale Molecular Wave in the Inner Galaxy: Feather of the Milky Way?

“…If we exclude the emission matching with Norma and 3-kpc arms, the filament is extending over 500 pc in the inter-arm region (assuming a minimum heliocentric distance of 4.5 kpc). Kuhn et al (2021) recently identified a 1 kpc, high pitch angle and high aspect ratio (7:1) structure associated with the Sagittarius arm. This feature is believed to be either an isolated entity, a sub-structure within the spiral arm or an inter-arm spur.…”