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A high pitch angle structure in the Sagittarius Arm

Kuhn, M. A. and Benjamin, R. A. and Zucker, C. and Krone-Martins, A. and de Souza, R. S. and Castro-Ginard, A. and Ishida, E. E. O. and Povich, M. S. and Hillenbrand, L. A. (2021) A high pitch angle structure in the Sagittarius Arm. Astronomy and Astrophysics, 651 . Art. No. L10. ISSN 0004-6361. doi:10.1051/0004-6361/202141198.

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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 ℓ ∼ 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 |Vφ| ≈ 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 ΔΩ = 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.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper ItemCosmostatistics Initiative (COIN)
Kuhn, M. A.0000-0002-0631-7514
Benjamin, R. A.0000-0002-8109-2642
Zucker, C.0000-0002-2250-730X
Krone-Martins, A.0000-0002-2308-6623
de Souza, R. S.0000-0001-7207-4584
Castro-Ginard, A.0000-0002-9419-3725
Ishida, E. E. O.0000-0002-0406-076X
Povich, M. S.0000-0001-9062-3583
Additional Information:© ESO 2021. Article published by EDP Sciences. Received 28 April 2021; Accepted 28 June 2021; Published online 21 July 2021. This work has made use of data from the European Space Agency (ESA) mission Gaia (, processed by the Gaia Data Processing and Analysis Consortium (DPAC, Funding for the DPAC has been provided by national institutions, in particular the institutions participating in the Gaia Multilateral Agreement. This work is based in part on archival data obtained with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. M.A.K. was partially supported by Chandra grant GO9-20002X. R.A.B. would like to acknowledge support from NASA grant NNX17AJ27G. AKM acknowledges the support from the Portuguese Fundação para a Ciência e a Tecnologia (FCT) grants UID/FIS/00099/2019, PTDC/FIS-AST/31546/2017. We thank Butler Burton, Eve Ostriker, Tom Dame, and Debra Elmegreen for useful discussions, and we thank the referee for a thorough and thoughtful report on the original manuscript. The Cosmostatistics Initiative (COIN, is an international network of researchers whose goal is to foster interdisciplinarity inspired by Astronomy.
Group:Astronomy Department
Funding AgencyGrant Number
Gaia Multilateral AgreementUNSPECIFIED
NASA ChandraGO9-20002X
Fundação para a Ciência e a Tecnologia (FCT)UID/FIS/00099/2019
Fundação para a Ciência e a Tecnologia (FCT)PTDC/FIS-AST/31546/2017
Subject Keywords:Galaxy: structure – Galaxy: kinematics and dynamics – galaxies: spiral – ISM: clouds – stars: formation
Record Number:CaltechAUTHORS:20210803-162712639
Persistent URL:
Official Citation:A high pitch angle structure in the Sagittarius Arm. M. A. Kuhn, R. A. Benjamin, C. Zucker, A. Krone-Martins, R. S. de Souza, A. Castro-Ginard, E. E. O. Ishida, M. S. Povich and L. A. Hillenbrand. A&A, 651 (2021) L10; DOI:
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:110131
Deposited By: Tony Diaz
Deposited On:04 Aug 2021 18:30
Last Modified:04 Aug 2021 18:33

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