A&A 651, L10 (2021)
https://doi.org/10.1051/0004-6361/202141198

Astronomy&
Astrophysics

c ESO 2021

LETTER TO THE EDITOR

A high pitch angle structure in the Sagittarius Arm
M. A. Kuhn1 , R. A. Benjamin2 , C. Zucker3 , A. Krone-Martins4,5 , R. S. de Souza6 , A. Castro-Ginard7 ,E. E. O. Ishida8 , M. S. Povich9 , and L. A. Hillenbrand1 for the COIN Collaboration
1

California Institute of Technology, Pasadena, CA 91125, USAe-mail: mkuhn@astro.caltech.edu
2
Department of Physics, University of Wisconsin-Whitewater, 800 W Main St, Whitewater, WI 53190, USA
3
Center for Astrophysics | Harvard & Smithsonian, 60 Garden St, Cambridge, MA 02138, USA
4
Donald Bren School of Information and Computer Sciences, University of California, Irvine, CA 92697, USA
5
CENTRA/SIM, Faculdade de Ciéncias, Universidade de Lisboa, Ed. C8, Campo Grande, 1749-016 Lisboa, Portugal
6
Key Laboratory for Research in Galaxies and Cosmology, Shanghai Astronomical Observatory, Chinese Academy of Sciences,
80 Nandan Rd., Shanghai 200030, PR China
7
Institut de Ciéncies del Cosmos, Universitat de Barcelona (IEEC-UB), Martí i Franqués 1, 08028 Barcelona, Spain
8
Université Clermont Auvergne, CNRS/IN2P3, LPC, 63000 Clermont-Ferrand, France
9
Department of Physics and Astronomy, California State Polytechnic University Pomona, 3801 West Temple Avenue, Pomona,
CA 91768, USA
Received 28 April 2021 / Accepted 28 June 2021
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 inferparallaxes 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 suchas M8, M16, M17, and M20. The motions in the structure are remarkably coherent, with velocities in the direction of Galactic rotationof |Vϕ | ≈ 240 ± 3 km s−1 (slightly higher than average) and slight drifts inward (VR ≈ −4.3 km s−1 ) and in the negative Z direction(VZ ≈ −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.
Key words. Galaxy: structure – Galaxy: kinematics and dynamics – galaxies: spiral – ISM: clouds – stars: formation

1. Introduction
Most of our understanding of spiral arms comes from observations of other galaxies, where our outside perspective
allows us to see the full spiral structure (Sandage 1961;
van der Kruit & Freeman 2011). In these other galaxies, spiralarms often have smaller-scale structures, including spurs (luminous stellar features) and feathers (dust features) that extend
from arms to inter-arm regions, as well as branches in the main
arms (Elmegreen 1980; La Vigne et al. 2006). In the Milky Way,it has been more challenging to disentangle such features owing
to our perspective within the highly extincted disk.
In the current picture of the Milky Way, the Sagittarius
Arm is the closest major spiral arm inward from the Sun andhosts several prominent, nearby massive star-forming regions.
Early-type stars in the regions M8, M16, M20, and several others, were used by Morgan et al. (1953) to define the Sagittarius

Arm in the first widely accepted Galactic map to show spiral structure (Appendix A). The currently favored four-armed
model is largely based on H i and CO emission in longitudevelocity (`–v) diagrams (e.g., Dame et al. 2001) supplemented
with very long baseline interferometric (VLBI) parallax measurements of masers (e.g., BeSSeL and VERA; Reid et al. 2019;
VERA Collaboration 2020). Conversion of the `–v diagram
to face-on maps has generally required assumptions regarding circular orbits. Although the maser sample has refined thisapproach, the number of masing targets is small compared to thenumber of star-forming regions and, furthermore, currently limited to Northern Hemisphere targets.
The availability of parallaxes and proper motions for overa billion sources (Gaia Collaboration 2016, 2018, 2021) has
led to a renaissance in investigations of Galactic spiral structure within a few kiloparsecs of the Sun. Zucker et al. (2020)
and Alves et al. (2020) found a coherent, filamentary structure

Article published by EDP Sciences

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Fig. 1. False-color GLIMPSE survey mosaic (Benjamin et al. 2003; Churchwell et al. 2009) of the Sagittarius Arm structure. The three white
contours show the intensity of 12 CO emission (10, 31, and 100 K km s−1 ) from Dame et al. (2001) integrated over the velocity range vlsr = 5–
30 km s−1 . Symbols mark the YSO groups, masers, and the Morgan OB associations (reanalyzed with Gaia) that make up the structure. Smaller
boxes and circles indicate greater distances, and arrows indicate proper motions.

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2.1. Candidate YSOs in Gaia EDR3

Our study is based on stars from the Spitzer/IRAC Candidate
YSO (SPICY) catalog Kuhn et al. (2020, hereafter Paper I).
These objects were identified via mid-infrared indications of circumstellar disks or envelopes, based on photometry from large
Spitzer surveys of the Galactic midplane, including GLIMPSE(Benjamin et al. 2003; Churchwell et al. 2009) and related surveys. The full catalog contains 117 446 YSO candidates, covering nearly all of the first and fourth Galactic quadrants between|b| . 1–2◦ . However, in this Letter we analyze objects between−30◦ ≤ ` < 30◦ .
Paper I divides the YSOs into groups based on clustering in(`, b) coordinates. These groups were defined by the HDBSCAN
algorithm (Campello et al. 2013) requiring ≥30 stars per group.
From these criteria, half the YSO candidates are members ofgroups.
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formed by star-forming clouds, likely associated with the Local
Arm. Xu et al. (2021) and Pantaleoni González et al. (2021)
examined the spatial distribution of previously identified OB
stars, while Zari et al. (2021) and Poggio et al. (2021) havecharacterized a photometrically selected sample of upper-mainsequence stars.
Here, we trace Galactic structure using young stellar objects(YSOs), which provide a link between the stellar content of spiral arms and the molecular clouds in which the YSOs form.
This Letter focuses on a distinct linear feature between Galactic longitudes ` ≈ 4◦ –18◦.5 (Fig. 1). In this region, the suggestion of a linear structure is visible in Galactic maps produced
by several previous studies, including the 3D extinction maps
from Green et al. (2019), the cloud distances from Zucker et al.
(2020), the maser distances from Reid et al. (2019), and the Gaia
Data Release 2 (DR2) cluster distances from Kuhn et al. (2020),
and is even hinted at in the original Morgan et al. (1953) association distances. However, the discrepancy between the high pitch
angle of the structure we trace here and pitch angles used in models of the Sagittarius Arm has never before, to our knowledge,
been remarked upon.

●−20µb [mas yr−1]2●Fig. 2. Scatter plots of stellar astrometry for one of the YSO groups.
Stars identified by the Bayesian model as probable members are black
circles, probable contaminants are indicated with black “×” marks, andstars without Gaia astrometry are gray diamonds. 1σ uncertainties areindicated by the error bars. The red lines show the group’s mean parallax and proper motion, with 95% credible intervals indicated by shading around the lines. (Here, the results are precise enough that theshaded area is difficult to discern.) Negative distances (gray region) areexcluded by the prior.

Roughly one-third of the YSO candidates are matched toGaia Early Data Release 3 (EDR3) counterparts within a radiusof 1001 . We used only stars with renormalized unit weight errors
1This radius is selected to be several times the typical absolute astrometric accuracy of the GLIMPSE catalog. To estimate the spurious
match rate, we shifted right ascensions by 50 and obtained a 3% match
rate. For sources with matches to both the true and shifted positions,the separations were smaller for the true position 90% of the time. Thissuggests that that spurious match rate is small and that true matches willusually override spurious matches.

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Fig. 3. Map of YSO groups (star symbols) and masers (magenta circles) in heliocentric coordinates. Right panel: zoomed-in view; it should be
noted that the Sun is off the left side of both plots. Groups associated with the structure are highlighted in yellow, while others are in black. The
spiral-arm centers defined by Reid et al. (2019) are indicated by the gray bands. The red line indicates the major axis of the feature identified here,with its 56◦ pitch angle illustrated in blue.

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l [deg]Fig. 4. Astrometric quantities (µ`? , µb , and b) versus Galactic longitude` for YSO groups and masers (same symbols as Fig. 3). The missing
error bars are smaller than the symbols.

of ≤1.4 (Lindegren et al. 2021a) and applied the Lindegren et al.
(2021b) parallax zero-point correction, which is estimated as afunction of color, magnitude, and ecliptic latitude. These corrections may be especially useful for YSOs in regions with significant differential absorption that can cause large color differences
between objects in the same cluster (e.g., Kuhn & Hillenbrand2020).
2.2. Estimating mean parallaxes and proper motions forgroupsParallaxes, $, and proper motions in Galactic longitude, µ`? =µ` cos(b), and latitude, µb , can often be more accurately deter-

mined for groups of stars than for individual stars. However, estimates of mean astrometric properties may be affectedby heteroscedastic uncertainties, velocity dispersions withingroups, and contaminants (both YSOs at different distances andnon-YSOs).
To take full advantage of the improved Gaia EDR3 astrometry, we employed a Bayesian model of the astrometric measurements for stars in each YSO group. Outliers can have highleverage on means, so we included a mixture component in
the model to account for contaminants (e.g., Hogg et al. 2010;
Kuhn & Feigelson 2019). Bona fide members are expected tocluster in parallax and proper-motion space, but nonmembers
tend to have broader distributions that depend on ` and b. Thismodel, the details of which are provided in Appendices B and C,
is fit via Markov chain Monte Carlo (MCMC).
Figure 2 shows stars from one of the YSO groups plotted inparallax and proper-motion space and fit with this model. The
probable members form a distinct cluster in this space, while
probable nonmembers have a wider range of values. For the subsequent analysis, we used groups with at least eight probablemembers to ensure that a reliable cluster exists2 . In general, theuncertainties on the mean parallaxes and proper motions of thegroups tend to be smaller than those of the individual stars.
2.3. A kiloparsec-long structure in the Sagittarius Arm

The 3D positions of the YSO groups, based on the group parallaxes, form an elongated linear feature in the first Galactic quadrant. In Fig. 3, the YSO groups (star symbols) are plotted ina heliocentric Cartesian coordinate system, where the Galactic
center is on the positive x axis, the y axis is parallel to the direction of Galactic rotation, and the z axis points out of the planefollowing a right-handed system (Appendix D). The YSO groups2

The results are not particularly sensitive to this threshold; for example, requiring ≥2 members only yields ∼3 more groups that appear associated with the feature analyzed here.
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A&A 651, L10 (2021)

Fig. 5. `–v diagram for objects in the new structure. Groups associated with single velocity peaks are filled symbols, while ambiguous cases areopen symbols with dashed lines connecting plausible vlsr solutions. The shape indicates the survey used to determine the vlsr : SEDIGISM (circle),
FUGIN (square), and COGAL (triangle). Masers are plotted as magenta “x” marks. The underlying image is the Dame et al. (2001) COGAL map
integrated over |b| ≤ 3◦ , with the loci of the Sagittarius Arm (orange line) and Scutum Arm (blue line) from Reid et al. (2016). Each YSO groupin the structure has a possible solution consistent with the Sagittarius Arm.

making up the structure (highlighted in yellow) range from
∼1–2 kpc in heliocentric distance, form a remarkably narrow
band angled relative to our line of sight, and follow coherent
patterns in both proper motion (Fig. 4) and vlsr (Fig. 5).
Several prominent massive star-forming regions are amongthe 25 YSO groups defining the structure. These include
M8 (Lagoon), M16 (Eagle), M17 (Omega), M20 (Trifid),
NGC 6559, and Sharpless 54, but there are also many smaller
YSO groups without ionizing stars. Furthermore, ten masers
associated with massive star formation or red supergiant stars
from the Reid et al. (2019) sample (magenta circles in Figs. 3
and 4) are aligned with this structure. Finally, these objects lie
within a filamentary region of high dust extinction seen in 3D
dust maps (Appendix E). Gaia’s ability to constrain distances
declines beyond a few kiloparsecs, so it is possible that the structure could extend farther inward in galactocentric radius than we
can detect from the EDR3 data.
A notable property of the new structure is its high pitch
angle relative to the commonly assumed low pitch angle of the
Sagittarius Arm. The structure is centered at (x, y, z) = (1470 ±50, 310 ± 30, −8 ± 6) pc. The principal axis of the structure isparallel to (0.85 ± 0.02) x̂ + (0.52 ± 0.04) ŷ + (0.05 ± 0.02) ẑ. The
YSO groups extend ∼950 pc along this axis, and the aspect ratioof the structure is ∼14:2:1, with the structure being narrowest
in the ẑ direction. Based on this orientation, the structure has apitch angle ψ = 56◦ .
Additional evidence for the coherence of this structure comesfrom kinematics. In proper motion versus Galactic longitude(Fig. 4), the objects forming the structure are tightly aligned
in narrow bands; masers confirm these trends. In µ`? versus `,its groups approximately follow the same curve as other YSO
groups and masers. And in µb versus `, the structure is also
coherent, with the near end having more negative µb values.
Furthermore, the near end of the structure has lower averageb. These trends can be partially attributed to perspective effectsof the Sun’s position above the Galactic plane with a positive
vertical velocity (Bland-Hawthorn & Gerhard 2016; Reid et al.
2019).
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Finally, to estimate radial velocities of the YSO groups,
we associated them with molecular gas observed in CO surveys of the Galactic plane (Appendix F), including SEDIGISM(Schuller et al. 2021), FUGIN (Umemoto et al. 2017), and
COGAL data (Dame et al. 2001). On the `–v diagram (Fig. 5),
the groups with known velocities range from vlsr ≈ 10–
30 km s−1 , with a gradual increase with increasing `, and all YSO
groups with uncertain velocities have possible solutions consistent with this trend. The YSO groups that we considered part ofthis new structure roughly follow the band in the COGAL data
thought to be associated with the near Sagittarius Arm3 .

3. Kinematics of the structure
To examine the spatial kinematics of the structure, we converted
the observed coordinates into a cylindrical galactocentric coordinate system (R, ϕ, Z) using a reference frame defined by the constants from Reid et al. (2019) (Appendix D). In this system thegalactocentric radii of the groups range from R = 6.2–7.0 kpc,
and their vertical range is Z = −50–70 pc, with a mean of
Z = −2 pc.
Trends in velocity with radius are shown in Fig. 6. The
azimuthal velocities are |Vϕ | ∼ 240 ± 3 km s−1 – slightly fasterthan the expected Galactic rotation – with an increase in velocity
with radius that is moderately statistically significant (p < 0.01
using Kendall’s τ test). Most groups have a negative galactocentric radial velocity (VR = −4.3 ± 3.5 km s−1 ). The mean
VZ is −2.9 km s−1 , with a velocity dispersion of ∼4 km s−1 .
Finally, objects farther from the Galactic center have lower angular velocities than those closer (p < 10−3 ), indicating that thestructure experiences shear of ∆Ω = 4.6 km s−1 kpc−1 . For comparison, Fig. 6 also shows the masers (circles) that were used toestablish the Reid et al. (2019) rotation curve.
Overall, these results suggest that the YSO groups in the feature have a coherent velocity structure, which is only ∼7 km s−1
The maser with a discrepant vlsr of 44 km s−1 is associated with the
runaway red supergiant star IRC -10414 (Gvaramadze et al. 2014).
3

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Fig. 6. Rotational velocity, radial velocity, vertical velocity, and angular velocity versus galactocentric radius. YSO groups (star symbols) andmasers (circles) are color-coded based on the spiral arm categorizations from Reid et al. (2019, see abbreviations therein). The gray lines indicate
their rotation curve.

discrepant from expectations for a purely circular orbit. This
coherence provides further evidence that these groups are associated with one another rather than being a coincidental spatial
arrangement. The presence of shear would explain the feature’s
trailing Galactic orientation. The observed shear implies atimescale of ∼90 Myr for the structure to change pitch angle
by a factor of two (Elmegreen 1980). Ages of young stars within
the structure’s star-forming regions are typically much younger
than this (e.g., ∼1 Myr; Rho et al. 2008; Getman et al. 2014;
Prisinzano et al. 2019), but it remains to be determined whether
any of the ∼12 open clusters in the vicinity of the structure withages 10–100 Myr (Cantat-Gaudin et al. 2020) are connected
to it.

4. Discussion and conclusion
We have demonstrated that many of the first quadrant starforming regions that, historically, have been used to define theSagittarius Arm are part of an elongated structure with a pitch
angle ψ = 56◦ . This is higher than any pitch angle previously
proposed for any portion the Sagittarius Arm (Appendix A). This
structure is traced by star-forming regions, masers, and 3D dust
maps. Its distinct nature has been hiding in plain sight since thestudy of OB associations by Morgan et al. (1953). But thanks to
the dense and clustered sample of infrared-selected YSOs fromthe Spitzer/GLIMPSE program and the direct distance measurements from Gaia, we can be confident that this is a real Galactic
feature.
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A&A 651, L10 (2021)

To envision how this structure would appear if viewed fromoutside the Milky Way, we can compare its star-formation activity to structures observed in other galaxies. Spitzer’s 24 µm bandis a useful tracer of star formation since roughly 25% of the bolometric thermal luminosity of star-forming regions is emitted inthis band (Binder & Povich 2018). The mid-infrared flux of our
feature is dominated by its most significant star-forming regions(e.g., M8, M16, M17, and M20), which have a combined 24 µm
luminosity of 8 × 1039 erg s−1 (Binder & Povich 2018). With alength of ∼950 pc and a width of ∼160 pc, the mean 24 µm surface brightness of the structure would be ∼5×1040 erg s−1 kpc−2 .
This is near the middle of the Calzetti et al. (2007) sample of
H ii knots in nearby galaxies, suggesting that the structure would
appear as a bright stellar feature.
Although this structure is remarkable in the context ofMilky Way models, numerous galaxies contain high pitch
angle structures, some related to an overall spiral pattern andsome whose origin is not so clear. For example, spurs andfeathers in other galaxies have pitch angles ranging from
∼40–80◦ (mean of ∼60◦ ) and lengths ranging from 1–5 kpc(Elmegreen 1980; La Vigne et al. 2006) – similar to the properties of the structure we have examined here. These structures
extend from spiral arms to inter-arm regions, and they typically exhibit quasi-regular spacing with separations from ∼300–
800 pc (La Vigne et al. 2006). Several theoretical models havebeen developed to explain the formation of spur-like structures ingaseous galactic disks, including formation due to gravitational
instabilities and shear (Balbus 1988; Kim & Ostriker 2002) withmagnetohydrodynamical effects explored by Shetty & Ostriker(2006), formation due to hydrodynamics in spiral shocks(Wada & Koda 2004; Dobbs et al. 2006), or expanding superbubbles (Kim et al. 2020). In the gravitational instability models,mass condensations form within the spiral arms, which are thensheared into the inter-arm regions to form spurs. In these models,even within the arm, the mass condensations are elongated withhigh pitch angles and lengths of ∼1 kpc. Thus, it is plausible thatthe feature we examined here corresponds to one of these massconcentrations within the Sagittarius Arm.
Clarifying whether this feature is (i) an isolated structure, (ii)a substructure within the Sagittarius Arm, or (iii) an inter-arm
spur warrants searches for similar structures along the `–v locusof the Sagittarius Arm, using VLBI, Gaia parallaxes, and dust
extinction distances. This new structure in Sagittarius provides
an excellent laboratory for examining star formation on scales
large enough to be compared to extragalactic observations, butwith the ability to resolve the mass function, spatial distribution,and kinematics of the individual sources.
Acknowledgements. This work has made use of data from the European Space
Agency (ESA) mission Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and Analysis Consortium (DPAC, https://www.cosmos.esa.int/web/gaia/dpac/consortium). 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 parton archival data obtained with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology undera contract with NASA. M.A.K. was partially supported by Chandra grantGO9-20002X. 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/FISAST/31546/2017. We thank Butler Burton, Eve Ostriker, Tom Dame, and Debra
Elmegreen for useful discussions, and we thank the referee for a thoroughand thoughtful report on the original manuscript. The Cosmostatistics Initiative(COIN, https://cosmostatistics-initiative.org/) is an international
network of researchers whose goal is to foster interdisciplinarity inspired byAstronomy.

L10, page 6 of 13

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A&A 651, L10 (2021)

Appendix A: History of the Sagittarius Arm
The characterization of the high-pitch-angle, kiloparsec-long,star-forming structure described here is unremarkable in the context of spiral galaxies; numerous galaxies show similar structures, some related to an overall spiral pattern and some whose
origin is not so clear. But as we discuss here, this structure isindeed unprecedented in the context of the generally adopted
model of the Milky Way spiral structure. In this appendix weoutline the development of the Sagittarius Arm in the astronomical literature. This is by no means a complete accounting of its
properties, but provides insight into the development of currently
used models.
Our definition of the Sagittarius Arm had its beginnings withthe first generally accepted map of spiral structure, presented by
W. W. Morgan at the 86th meeting of the American Astronomical Society in December 1951. At this meeting, Morgan presented slides of a physical model he had built at Yerkes Observatory: a board into which he had pounded 25 nails, each topped
with white balls to indicate his measured distance to an OB association. The focus of this presentation was a band of star formation near the Sun, later called the Orion Arm, or Local Arm, andregions of star formation in what was later dubbed the Perseus
Arm. This board had a single point inward from the Sun’s position: the start of the Sagittarius Arm (Gingerich 1985).
Although the data from this original presentation were neverpublished, an expanded version of this work was published by
Morgan et al. (1953). By this point, the number of inner Galaxy
OB associations had grown to seven, with measurements foreight additional single stars. Details on the full stellar sample
were published in a companion paper (Morgan et al. 1955). We
recalculated the distances to these stars using Gaia EDR3 andcalculated the average distance to revise the distance of eachassociation (Fig. 1). Modern parallaxes indicate that four stars
are interlopers; these were removed from the averaging. We findthat six of the original associations mapped in Morgan et al.
(1953) belong to the structure that we are characterizing. Surprisingly, the high pitch angle is clear in this original 1953
work but went unremarked. We searched the literature for any
discussion of the depth of this structure and only found two
manuscripts that clearly discuss it, both of which attribute it toa variable thickness for the arm (Avedisova 1989; Gerasimenko
1993). Evidence for a high pitch angle structure in this direction was also provided in the review talk by D. Elmegreen at
IAU Symposium 106: The Milky Way Galaxy and was remarked
upon at that meeting (Elmegreen 1985).
Given the uncertainties in distances, the patchiness of extinction in the inner Galaxy, and the intense focus on finding a spiral structure similar to other spirals – with a lower pitch angle– it is unsurprising that the alignment of these objects did not
draw attention. The advent of 21 cm astronomy, which beganwith the detection of the hyperfine transition of HI in March,May, and July 1951 (Ewen & Purcell 1951; Muller & Oort 1951;
Pawsey 1951), focused the community on the larger-scale structure of spiral arms, beginning with the pioneering paper ofvan de Hulst et al. (1954). Although this paper focused on the
distribution of neutral hydrogen outside the solar circle, theresults were compared with those of Morgan, Whitford, andCode. Van de Hulst, Muller, and Oort – in consultation with
Morgan – proposed the names Perseus Arm, Orion Arm, andSagittarius Arm for the concentrations of star formation mappedby Morgan.
The first explicit linkage between the Sagittarius Arm OB
associations and the HI data came in Kwee et al. (1954). This
L10, page 8 of 13

paper focused on the HI rotation curve, measuring the maximum radial velocity along the line of sight. They found thata plot of this terminal velocity as a function of galactocentric
radius showed three prominent dips, one of which they associated with the same range of galactocentric radius as the Sagittarius OB associations. They speculated that these two regions of
the Galaxy were connected, as illustrated in Fig. 8 of Kwee et al.
(1954). The idea that the l = 49◦ direction marked the point
where the arm went into tangency was reinforced by the discovery of W51 by Westerhout (1958). This optically obscured
but bright radio continuum source has since been frequently
attributed to arising along the tangency of the Sagittarius Arm.
Modern measurements of VLBI parallaxes toward seven objects
in this direction confirm that the star formation is spread out over2 kpc, as one might expect for a spiral arm tangency (Reid et al.
2019). A study of 21 cm emission by Schmidt (1957) claimed
evidence for HI gas in the inner Galaxy that was assumed to
be associated with the Sagittarius Arm, although the association
with the stellar associations was not documented.
Current spiral structure models principally derive from
efforts to identify continuous bands of high intensity emission
as a function of galactic longitude and radial velocity: the `–v diagram. The expectation was to find loops in this diagram,where the turnaround occurs as the arm transitions from nearkinematic distances to far kinematic distances as it reaches tangency. These efforts proceeded using 21 cm emission, and thenemission from CO. The first `–v track tracing the Sagittarius
Arm – to our knowledge – was in Fig. 5 of the 21 cm study of
Burton (1966), which identified a possible loop over the range` = 43◦ to 53◦ . In this paper, Burton noted the presence of emission at higher velocities than those of the Sagittarius Arm loop,
which led to subsequent investigations of the role of streaming
motions due to a spiral density wave – as opposed to overdensities – in creating features in the `–v diagram (Burton 1971, 1972;
Burton & Bania 1974).
The `–v track identified by Burton was extrapolated further into the inner galaxy with the inclusion of models aswell as reference to unpublished data from S. C. Simonson(Burton & Shane 1970). This Sagittarius Arm `–v track was
revived with the CO investigations of Cohen et al. (1980) andextended in the highly influential work of Dame et al. (1986),
which identified 17 large molecular complexes distributed rather
uniformly along a 15 kpc stretch. An oft reproduced image fromthat paper is their Fig. 10, which gives the impression of the
Sagittarius Arm having molecular clouds spaced like “beads ona string” lying along a logarithmic spiral with pitch angle 5.3◦ .
Using VLBI masers parallaxes, this was revised to 6.9 ± 1.6
degrees (Wu et al. 2014; Reid et al. 2014).
A particularly problematic aspect of constructing a spirallocus for the Sagittarius Arm has been determining how it
extends into the fourth quadrant (` = 360 − 270◦ ). This is challenging because the kinematic method of estimating the distanceto sources becomes degenerate toward the Galactic center, disconnecting structures identified in the first and fourth quadrants.
A pivotal moment in this discussion occurred during and after
IAU Symposium No. 38 in Basel in 1969. At this meeting, twomaps constructed from H i observations, one by Kerr (1969) andone by Weaver (1970), appeared to have very different characteristics. At a workshop the following year, it was noted that discrepancies in the inner Galaxy were related to spiral arm tangent
points and the connections between them (Simonson 1970). The
identification of tangency directions has always played an important part in informing models of the spiral structure, starting with

M. A. Kuhn et al.: A high pitch angle structure in the Sagittarius Arm

the work of Mills (1959) and continuing through to the presentday (Hou & Han 2015).
At the 1969 symposium, Kerr chose to connect the Sagittarius Arm tangency at ` = 49◦ to the ` = 305◦ tangency(Centaurus–Crux) in the fourth quadrant, while Weaver linked
the Sagittarius Arm to the Carina tangency at ` = 283◦ . The latter choice ended up propagating through to most current models,
which require a pitch angle of ∼12 degrees.
The tension introduced by fitting local sections of spiral arms
and attempting to fit a larger, suspected log-spiral pattern basedon tangency directions can be seen in the most recent paper ofReid et al. (2019). By introducing a “kink” in the SagittariusCarina spiral arm at a galactocentric angle 24 degrees clockwisefrom the Galactic-center-to-Sun direction, they find a pitch angleof only one degree for the Sagittarius Arm in most of the first
quadrant and a pitch angle of 17 degrees for its extension into
the fourth quadrant (where no VLBI parallax data are available).
The deviation of a Sagittarius-Carina arm from an idealized logspiral had been previously proposed based on measurements ofphotometric distances to the exciting stars of associated H ii
regions. As an example, the well-known Taylor & Cordes (1993)
model of the spatial distribution of free electrons started with
the Georgelin & Georgelin (1976) logarithmic-spiral model but
incorporated a kink based on the H ii regions distance measurements of Downes et al. (1980). However, none of these proposed
revisions seems to have gained general usage. For convenience,
most models have continued to use the log-spiral approximation.

Appendix B: Bayesian model of cluster astrometry
To estimate the basic astrometric properties of a YSO group,
we used a mixture model – a probability distribution made byadding several components – that also takes into account theheteroscedastic measurement uncertainties tabulated by Gaia
EDR3. Each component of this model is astrophysically motivated, with one (or multiple) component(s) corresponding thestar cluster and another component corresponding to nonclustered contaminants found along the same line of sight.
Clustered stars must be at the same distance, and Gaia
parallax measurement errors are approximately Gaussian(Lindegren et al. 2021a). Thus, for each member star, we
assumed identical mean parallaxes, $0 , and standard deviations
obtained from the Gaia EDR3 tables. We also expect member
stars to share similar proper motions, but, in addition to propermotion measurement error, clusters have an internal velocity dispersion that produce small spreads in proper motions. Thus, forproper motions, we used t distributions rather than Gaussian distributions because the heavy tails provide increased robustness
to cluster members with discrepant proper motion. For these t
distributions, we treated the degrees of freedom, ν, as a nuisanceparameter that we marginalized over; ν was fixed to be the samefor both µ`,? and µb .
For star i of a cluster, the probability distribution is
pclust ($i , µ`? ,i , µb,i |$0 , µ`? ,0 , µb,0 ) =φ($i |$0 , σ2$i ) · f (µ`? ,i |µ`? ,0 , σ2µ`? ,0 , νµ ) · f (µb,i |µb,0 , σ2µb,0 , νµ ),(B.1)
where θ = ($0 , µ`? ,0 , µb,0 ) are the mean astrometric values forthe cluster, xi = ($i , µ`? ,i , µb,i ) are the measured values forthe ith star, σi are corresponding uncertainties, φ denotes a
Gaussian distribution, and f denotes a t distribution. In our mixture model, we included either one or two cluster components,

which correspond to the cases where there is either a single cluster or two clusters superimposed along the line of sight.
For contaminants, we expect the distributions of $, µ`,? , andµb to be broader than for cluster members, but these distributions
may shift depending on the direction on the sky. Appendix C
approximates these distributions, pcontam (xi |`, b), as a function of
Galactic coordinates.
For a YSO group that is assumed to be a single cluster, thelikelihood equation isp(xi |θ) = a pclust (xi |θ) + (1 − a) pcontam (xi |`, b),(B.2)

where 0 ≤ a ≤ 1 is the mixing parameter indicating the fraction of stars that are bona fide cluster members rather than nonclustered contaminants. In the case of two clusters, a second
cluster component is added to the above equation.
The prior distributions for our Bayesian model ared ∼ Uniform(0, 8000),µ`? ∼ Uniform(−10, 4),µb ∼ Uniform(−5, 2),a ∼ Beta(2, 0.5),νµ ∼ Gamma(2, 0.2),(B.3)

where d [pc] = 1000/$0 [mas] is the cluster heliocentric distance. For the quantities of interest, we used uniform priors
within reasonable astrophysical ranges. For the mixing parameter, we adopted a prior that mildly favors low contamination rates
because previous studies of clustered YSOs selected in similar
ways have found contamination rates of ∼20% (e.g., Kuhn et al.
2019). For νµ , we adopted a prior that permits t distributions withheavy wings.
For each YSO group, we ran three MCMC chains with
JAGS (Plummer 2019) via the R2jags package (Su 2020). Convergence was assessed by checking that the Gelman & Rubin(1992) statistic is <1.001. The 95% credible intervals for eachastrometric parameter are reported in Table D.1. We also report
the number of stars, Nmem , that have a >50% probability of
belonging to the cluster component of the model.
Among the YSO groups investigated in this Letter, there
are two cases where including a second cluster component significantly improved the model. These are G29.9+2.2 (not partof the structure), where two clusters at different distances arealigned along the line of sight, and G14.1−0.5 (= M17 SWex),
where stars in different parts of the molecular cloud have different proper motions. In both cases the Bayes factor strongly
favored the more complex model (BF > 1010 ).

Appendix C: Astrometric properties ofnonmembers
Young stellar object groups identified by HDBSCAN (`, b) clustering may include nonmembers. Here, we justify our assumption from Appendix B that the astrometric properties of theseobjects can be modeled by t distributions. If these interlopers aremostly non-YSO contaminants, we might expect them to haveproperties similar to the ∼200 000 red infrared sources classified
as non-YSOs in Paper I. Thus, we used this sample to model the
distributions.
Figure C.1 shows the parallax distribution of the full nonYSO sample. This distribution, produced by the combination ofvariations in distance and heteroscedastic measurement errors,
is clearly non-Gaussian, but the heavy tails appear to be welldescribed by the overplotted t distribution. Similar tails are foundL10, page 9 of 13

Appendix D: Properties of objects in the linear
structure

red, non−YSO Spitzer sourcesfrom Paper It distribution model
location = 0.28
scale = 0.31
df = 1.801log counts
2
3

4

A&A 651, L10 (2021)

−6

−4

−2

0
2
parallax [mas]46

Fig. C.1. Histogram of Gaia EDR3 parallaxes for red mid-infrared
sources from Kuhn et al. (2020) classified as non-YSOs. A t distribution (red curve) is able to account for the heavy tails.

for µ`? and µb , but the distribution centers are not as accurately
modeled owing to proper motion variations with Galactic `.
To more accurately approximate the $, µ`? , and µb distributions as functions of (`, b), we divided the survey area intoboxes with ∆` = 10◦ and ∆b = 400 and fit the non-YSOs ineach box with t distributions using the fitdistr function fromthe R library MASS (Venables & Ripley 2002). We then used the
local polynomial regression algorithm loess (Cleveland et al.
1992) to estimate t-distribution parameters as smooth functions of (`, b). Here we adopted the loess implementation
in base R v3.6.0 (R Core Team 2019) with span = 0.2 anddegree = 2. Examination of probability–probability plots (notshown) indicates that this provides excellent approximations for
non-YSOs distributions.

The YSO groups comprising the structure are listed, along with
their properties, in Tables D.1 and D.2. The quantities in Table D.1
include: (i) the number of stars with Gaia data, NGaia , (ii) the
number of those stars classified as probable members, Nmem , (iii)the centers of the groups’ (l0 , b0 ) in Galactic coordinates, (iv) themean parallax, $0 , heliocentric distance, d , and mean propermotions, (µ`? ,0 , µb,0 ), in Galactic coordinates, and (v) the groups’
radial velocities, vlsr , in the “standard” local standard of rest4 .
To facilitate comparison with the positions and kinematicsof the masers of Reid et al. (2019), we adopted the same Galactic parameters here – R0 = 8.15 kpc, Z0 = +5.5 pc and asolar motion of UVW = (10.6, 10.7, 7.6) km s−1 – and compared the space motions of the YSO groups to their derived rotation curve with circular speed, Θ = 236 km s−1 , at the position of the Sun. Other authors prefer different parameters (e.g.,
GRAVITY Collaboration 2019; Eilers et al. 2019), which would
produce a small but systematic change in our derived values.
The quantities in Table D.2 include: (i) the heliocentric
Cartesian coordinates defined as x = d cos(`0 ) cos(b0 ), y =d sin(`0 ) cos(b0 ), and z = d sin(b0 ), (ii) the heliocentric Cartesian velocities (v x , vy , vz ), (iii) the galactocentric Cartesian positions and velocities (X, Y, Z, VX , VY , VZ ), and (iv) the galactocentric positions and velocities in cylindrical coordinates, where ϕ isthe azimuthal angle, VR is the radial velocity with respect to the
Galactic center, and |Vϕ | is the azimuthal velocity. Uncertainties
take into account the astrometric uncertainties and uncertainties
on vlsr described in Appendix F.

4
By historical convention, vlsr are reported in a reference frame thattakes the solar peculiar motion to be 20 km s−1 toward 20h 30◦ (B1900).

L10, page 10 of 13

134
16
463

78

42
31
246
21
37
43
73
9
24
22
49
26
131
14
21
13
119
119
37
15
25

NGaia

121
8
395

56

30
22
225
20
30
37
64
8
21
19
42
19
103
13
17
11
48
40
10
11
11

NMem

16.97
18.09
18.45

16.63

4.07
5.93
6.06
6.67
6.89
6.97
7.32
8.44
8.52
8.52
10.33
10.40
10.87
12.40
12.78
13.34
14.19
14.19
14.56
15.11
15.61

`0
(◦ )

0.533 (0.432, 0.626)−0.19
0.601 (0.577, 0.625)
0.489 (0.261, 0.654)
0.512 (0.499, 0.525)

0.856 (0.803, 0.909)
0.774 (0.734, 0.811)
0.821 (0.810, 0.833)
0.873 (0.811, 0.934)
0.838 (0.787, 0.892)
0.847 (0.811, 0.885)
0.844 (0.816, 0.872)
0.735 (0.157, 1.101)
0.789 (0.747, 0.832)
0.716 (0.652, 0.789)
0.684 (0.633, 0.736)
0.736 (0.616, 0.826)
0.788 (0.769, 0.806)
0.662 (0.513, 0.809)
0.544 (0.437, 0.649)
0.492 (0.316, 0.668)
0.629 (0.572, 0.676)
0.621 (0.584, 0.660)
0.538 (0.471, 0.603)
0.649 (0.587, 0.744)
0.640 (0.458, 0.707)

−1.00
−0.98
−1.35
−1.53
−2.35
−0.25
−2.00
−0.28
−1.00
−0.63
−0.83
2.25
−2.43
0.50
0.42
0.19
−0.53
−0.53
−0.11
−0.59
−0.34

0.81
0.22
1.91

$0
(mas)

b0
(◦ )

1.66 (1.60, 1.73)
2.04 (1.53, 3.84)
1.95 (1.90, 2.01)

1.88 (1.60, 2.31)

1.17 (1.10, 1.24)
1.29 (1.23, 1.36)
1.22 (1.20, 1.23)
1.15 (1.07, 1.23)
1.19 (1.12, 1.27)
1.18 (1.13, 1.23)
1.18 (1.15, 1.23)
1.36 (0.91, 6.35)
1.27 (1.20, 1.34)
1.40 (1.27, 1.53)
1.46 (1.36, 1.58)
1.36 (1.21, 1.62)
1.27 (1.24, 1.30)
1.51 (1.24, 1.95)
1.84 (1.54, 2.29)
2.03 (1.50, 3.17)
1.59 (1.48, 1.75)
1.61 (1.52, 1.71)
1.86 (1.66, 2.12)
1.54 (1.34, 1.70)
1.56 (1.42, 2.19)d(kpc)µ`? ,0(mas yr−1 )−0.583 (−0.730, −0.475)−0.963 (−1.005, −0.921)−0.540 (−0.782, −0.363)−0.642 (−0.675, −0.608)−1.315 (−1.359, −1.265)−0.543 (−1.675, −0.352)−2.127 (−2.157, −2.101)−2.928 (−3.001, −2.801)−2.490 (−2.576, −2.416)−2.193 (−2.244, −2.153)−2.006 (−2.068, −1.937)−2.485 (−2.577, −2.392)−1.568 (−1.735, −1.434)−2.411 (−2.475, −2.342)−0.332 (−0.729, +1.892)−1.927 (−2.164, −1.746)−2.072 (−2.153, −1.969)−1.823 (−1.883, −1.762)−1.557 (−1.683, −1.461)−1.635 (−1.692, −1.574)−1.923 (−2.160, −1.647)−1.329 (−1.597, −1.036)−1.676 (−1.838, −1.520)−0.867 (−0.928, −0.762)−1.600 (−1.649, −1.553)−1.409 (−1.519, −0.958)−1.037 (−1.410, −0.864)−1.052 (−1.167, −0.828)µb,0(mas yr−1 )−1.557 (−1.799, −1.365)−0.725 (−0.837, −0.580)−0.832 (−0.919, −0.758)−1.181 (−1.200, −1.159)−0.411 (−0.481, −0.333)−0.628 (−0.705, −0.546)−1.001 (−1.136, −0.796)−0.388 (−0.433, −0.344)−1.293 (−9.318, −1.135)−0.629 (−0.703, −0.399)−0.799 (−0.879, −0.706)−1.646 (−1.725, −1.582)−1.559 (−1.724, −1.273)−0.684 (−0.761, −0.620)−1.100 (−1.349, −0.758)−0.786 (−1.106, −0.624)−1.435 (−1.616, −1.280)−1.704 (−1.808, −1.567)−1.230 (−1.292, −1.138)−1.024 (−1.107, −0.951)−1.479 (−1.580, −1.413)−1.550 (−1.617, −1.448)

Notes. Ranges for values indicate the 95% credible intervals from Bayesian analysis. All plausible vlsr solutions are listed.

G16.9+0.8 (M16)
G18.0+0.2
G18.4+1.9 (Sh 2-54)

G16.6−0.1

G4.0−1.0
G5.9−0.9
G6.0−1.3 (M8)G6.6−1.5G6.8−2.3 (NGC 6559)
G6.9−0.2 (M20)
G7.3−1.9 (IC 1274)
G8.4−0.2
G8.5−0.9 (Majaess 190)
G8.5−0.6
G10.3−0.8
G10.3+2.2
G10.8−2.4 (LDN 291)
G12.3+0.4
G12.7+0.4 (LBN 49)
G13.3+0.1
G14.1−0.5 A (M17 SWex)
G14.1−0.5 B (M17 SWex)
G14.5−0.1
G15.1−0.5 (M17)
G15.6−0.3

Group

Table D.1. YSO groups in the Sagittarius Arm structure.

11.7
13.0
11.7
13.0
11.7
8.0, 18.5
9.1
16.5, 36.8
15.6
16.9
18.2, 28.6
6.5, 23.4
11.7
18.0, 31.0
19.0
18.0
19.9
19.9
25.5, 39.8
19.3
16.0, 20.5,
40.0, 58.0
24.3, 47.5,
56.3
20.6
22.5
27.3

vlsr
(km s−1 )M. A. Kuhn et al.: A high pitch angle structure in the Sagittarius Arm

L10, page 11 of 13

L10, page 12 of 13y(pc)1165 ± 35
83 ± 21284 ± 31
133 ± 31210 ± 8
128 ± 11137 ± 40
133 ± 51183 ± 38
143 ± 51171 ± 25
143 ± 31174 ± 19
151 ± 21346 ± 382 200 ± 57
1254 ± 34
188 ± 51381 ± 63 207 ± 101438 ± 57 262 ± 101335 ± 90 245 ± 171245 ± 15239 ± 31475 ± 170 324 ± 37
1791 ± 181 406 ± 41
1976 ± 359 468 ± 85
1541 ± 63 389 ± 16
1560 ± 51 394 ± 13
1799 ± 115 467 ± 301488 ± 85 402 ± 231506 ± 144 421 ± 40
1799 ± 176 537 ± 53
1592 ± 32 486 ± 101944 ± 398 635 ± 130
1852 ± 25
618 ± 8x(pc)vx
(km s−1 )vy(km s−1 )−20 ± 10.5 ± 5.2 −4.0 ± 0.5−22 ± 1
1.6 ± 5.0 −5.0 ± 0.6−29 ± 10.5 ± 4.8 −6.8 ± 0.5−30 ± 1
1.2 ± 4.7 −2.1 ± 0.6−49 ± 2 −0.2 ± 5.1 −3.6 ± 0.6−5 ± 1 −3.4 ± 5.8 −6.1 ± 0.8−41 ± 1 −3.0 ± 4.9 −2.6 ± 0.7−7 ± 25.3 ± 8.9 −7.6 ± 13.0
−22 ± 1
3.6 ± 5.0 −3.3 ± 0.9−15 ± 15.1 ± 4.9 −4.6 ± 0.8−21 ± 17.2 ± 5.7 −10.3 ± 1.1
53 ± 4 −4.4 ± 7.4 −11.0 ± 1.7−54 ± 1 −0.6 ± 4.8 −4.3 ± 0.9
13 ± 26.3 ± 5.9 −6.7 ± 2.0
13 ± 17.0 ± 5.0 −5.4 ± 1.77±17.5 ± 5.1 −12.4 ± 2.7−15 ± 19.1 ± 5.3 −10.9 ± 1.5−15 ± 18.2 ± 4.5 −7.6 ± 1.3−4 ± 1 13.6 ± 6.1 −5.8 ± 1.6−16 ± 18.0 ± 4.7 −9.0 ± 1.4−9 ± 1 5.0 ± 15.4 −10.5 ± 4.3−6 ± 1 13.6 ± 11.9 −10.4 ± 3.7
23 ± 19.0 ± 4.9 −8.1 ± 1.58±2
9.2 ± 4.5 −2.5 ± 3.5
65 ± 1 18.2 ± 4.5 −14.7 ± 1.6z(pc)

Heliocentric−16.2 ± 0.6−15.3 ± 0.5−12.7 ± 0.2−10.9 ± 0.4−14.0 ± 0.6−8.8 ± 0.5−13.4 ± 0.3−2.2 ± 4.4−11.6 ± 0.7−13.8 ± 0.7−12.7 ± 0.5−10.3 ± 0.8−9.8 ± 0.3−13.7 ± 1.8−11.5 ± 1.7−16.1 ± 3.0−6.6 ± 0.5−12.3 ± 0.4−12.4 ± 1.5−7.6 ± 1.1−7.8 ± 1.0−5.2 ± 0.7−7.5 ± 0.2−5.2 ± 1.6−5.5 ± 0.2vz(km s−1 )Y(pc)−6985 ± 35
83 ± 2−6866 ± 31
133 ± 3−6940 ± 8
128 ± 1−7013 ± 40
133 ± 5−6967 ± 38
143 ± 5−6979 ± 25
143 ± 3−6976 ± 19
151 ± 2−6804 ± 382 200 ± 57
−6896 ± 34
188 ± 5−6769 ± 63 207 ± 10−6712 ± 57 262 ± 10−6815 ± 90 245 ± 17−6905 ± 15239 ± 3−6675 ± 170 324 ± 37
−6359 ± 181 406 ± 41
−6174 ± 359 468 ± 85−6609 ± 63 389 ± 16−6590 ± 51 394 ± 13
−6351 ± 115 467 ± 30
−6662 ± 85 402 ± 23−6644 ± 144 421 ± 40
−6351 ± 176 537 ± 53
−6558 ± 32 486 ± 10−6206 ± 398 635 ± 130−6298 ± 25
618 ± 8X
(pc)

VY
(km s−1 )VZ(km s−1 )Galactocentric−15 ± 1 11.1 ± 5.2 242.7 ± 0.5 −8.6 ± 0.6−17 ± 1 12.2 ± 5.0 241.7 ± 0.6 −7.7 ± 0.5−23 ± 1 11.1 ± 4.8 239.9 ± 0.5 −5.1 ± 0.2−25 ± 1 11.8 ± 4.7 244.6 ± 0.6 −3.3 ± 0.4−43 ± 2 10.4 ± 5.1 243.1 ± 0.6 −6.4 ± 0.6
0±1
7.2 ± 5.8 240.6 ± 0.8 −1.2 ± 0.5−36 ± 17.6 ± 4.9 244.1 ± 0.7 −5.8 ± 0.3−1 ± 2 15.9 ± 8.9 239.1 ± 13.0 5.4 ± 4.4−17 ± 1 14.2 ± 5.0 243.4 ± 0.9 −4.0 ± 0.7−10 ± 1 15.7 ± 4.9 242.1 ± 0.8 −6.2 ± 0.7−16 ± 1 17.8 ± 5.7 236.4 ± 1.1 −5.1 ± 0.5
59 ± 46.2 ± 7.4 235.7 ± 1.7 −2.7 ± 0.8−48 ± 1 10.0 ± 4.8 242.4 ± 0.9 −2.2 ± 0.3
19 ± 2 16.9 ± 5.9 240.0 ± 2.0 −6.1 ± 1.8
19 ± 1 17.6 ± 5.0 241.3 ± 1.7 −3.9 ± 1.7
12 ± 1 18.1 ± 5.1 234.3 ± 2.7 −8.5 ± 3.0−9 ± 1 19.7 ± 5.3 235.8 ± 1.5 1.0 ± 0.5−9 ± 1 18.8 ± 4.5 239.1 ± 1.3 −4.7 ± 0.4
2 ± 1 24.2 ± 6.1 240.9 ± 1.6 −4.8 ± 1.5−10 ± 1 18.6 ± 4.7 237.7 ± 1.4 −0.0 ± 1.1−4 ± 1 15.6 ± 15.4 236.2 ± 4.3 −0.2 ± 1.0−1 ± 1 24.2 ± 11.9 236.3 ± 3.7 2.4 ± 0.7
29 ± 1 19.6 ± 4.9 238.6 ± 1.5 0.1 ± 0.2
13 ± 2 19.8 ± 4.5 244.2 ± 3.5 2.4 ± 1.6
71 ± 1 28.8 ± 4.5 232.0 ± 1.6 2.1 ± 0.2

CartesianZ
VX(pc)(km s−1 )6986 ± 35
6867 ± 31
6941 ± 87014 ± 406968 ± 38
6980 ± 256978 ± 196807 ± 379
6899 ± 34
6773 ± 63
6717 ± 57
6820 ± 906909 ± 146683 ± 168
6372 ± 178
6192 ± 352
6620 ± 62
6601 ± 50
6368 ± 112
6675 ± 846658 ± 141
6374 ± 171
6576 ± 31
6239 ± 381
6328 ± 24

R
(pc)

|Vϕ |
(km s−1 )179.3 −8.1 ± 5.2 242.8 ± 0.5
178.9 −7.3 ± 4.9 241.9 ± 0.7
179.0 −6.5 ± 4.8 240.1 ± 0.6
178.9 −7.0 ± 4.7 244.8 ± 0.7
178.9 −5.3 ± 5.1 243.3 ± 0.7
178.8 −2.1 ± 5.8 240.7 ± 0.9
178.8 −2.2 ± 4.8 244.2 ± 0.8
178.4 −8.7 ± 8.8 239.4 ± 13.0
178.5 −7.4 ± 5.0 243.7 ± 1.0
178.3 −8.1 ± 4.8 242.5 ± 0.9
177.8 −8.4 ± 5.6 236.9 ± 1.3
178.0
2.5 ± 7.4 235.8 ± 1.8
178.1 −1.4 ± 4.8 242.6 ± 1.1
177.3 −5.0 ± 6.0 240.6 ± 2.2
176.4 −1.9 ± 5.3 241.9 ± 2.0
175.8 −0.1 ± 5.8 235.0 ± 2.9
176.7 −5.6 ± 5.1 236.5 ± 1.8
176.7 −4.3 ± 4.5 239.8 ± 1.5
175.9 −6.1 ± 6.1 242.0 ± 2.0
176.6 −4.0 ± 4.8 238.4 ± 1.7
176.5 −0.3 ± 15.1 236.7 ± 5.3
175.3 −3.8 ± 11.2 237.5 ± 4.7
175.9 −1.6 ± 4.7 239.4 ± 1.9
174.3
5.5 ± 8.1 244.9 ± 3.7
174.5 −5.7 ± 4.4 233.7 ± 2.0

Cylindrical
ϕ
VR
(◦ )
(km s−1 )Notes. The entries in this table are derived from the values in Table D.1, with conversion to galactocentric coordinates using the Galactic model parameters from Reid et al. (2019). Uncertainties
correspond to one standard deviation and do not include systematic errors in the coordinate system.

G4.0−1.0
G5.9−0.9
G6.0−1.3
G6.6−1.5G6.8−2.3
G6.9−0.2
G7.3−1.9G8.4−0.2
G8.5−0.9
G8.5−0.6
G10.3−0.8
G10.3+2.2
G10.8−2.4
G12.3+0.4
G12.7+0.4
G13.3+0.1
G14.1−0.5 A
G14.1−0.5 B
G14.5−0.1
G15.1−0.5
G15.6−0.3
G16.6−0.1
G16.9+0.8
G18.0+0.2
G18.4+1.9

Group

Table D.2. Kinematics of YSO groups.

A&A 651, L10 (2021)

M. A. Kuhn et al.: A high pitch angle structure in the Sagittarius Arm

Appendix E: Comparison with the dust extinction

Fig. F.1. Plots of 13 CO J=1–0 emission from the SEDIGISM survey(Schuller et al. 2021) for G14.1−0.5 (= M17 SWex). The left column is
integrated emission over two different 10 km s−1 velocity ranges, and theright column is the CO spectrum, with the velocity range centered at 19.9
and 38.8 km s−1 (yellow shading). The grouped SPICY objects (yellowdots) are plotted on the integrated emission map for comparison with thespatial distribution of CO gas. Here the YSOs show the strongest spatial
correlation with the lower velocity component at 20 km s−1 .
Fig. E.1. YSO groups and masers (same symbols as Fig. 3) overplottedon the 3D dust reddening map from Green et al. (2019), integrated fromz = −300 pc to 300 pc. The YSO groups follow a dust feature in thereddening map with a similar pitch angle. The dust map is truncated by
the limits of the Pan-STARRS survey (bottom edge) and by our |`| < 30◦limits in this Letter (top edge).

Three-dimensional dust extinction maps produced by
Lallement et al. (2019), Green et al. (2019), and Zucker et al.
(2020) reveal a filament-shaped region of high extinction thatcoincides with the structure formed by the YSO groups that wehave examined in this paper (Fig. E.1). The results from theextinction maps are effectively independent from our analysis of
YSO group distances because they are based on photometry andGaia parallaxes of different sets of stars, with the 3D dust map
based on main-sequence stars and our distances based on YSOs.
The 3D dust maps support the view that the structure is a narrow
linear feature with a width of ≈200 pc and a length of ≈1 kpc.

Appendix F: CO analysis
For each YSO group, we characterized the velocity structure of
associated molecular gas using available CO spectral-line surveys. We favored the 13 CO line whenever possible since it is
optically thinner than 12 CO. We prioritized the use of the 13 CO
survey based on angular and spectral resolution, first checking whether a group exists in the SEDIGISM (Schuller et al.
2021) footprint, followed by FUGIN (Umemoto et al. 2017) and
THrUMMS (Barnes et al. 2015). For YSO groups that lie outside the boundaries of existing 13 CO surveys, we supplemented
the 13 CO data with 12 CO data from the 1.2 m CfA CO survey(Dame et al. 2001), offering full coverage of the Galactic plane.
To extract a spectrum, we started by defining a concave hullon the plane of the sky using the stellar members of each group,
as delineated by the yellow polygon on the left hand side ofFig. F.1 for YSO group G14.5−1.0. To do so, we employed
the Alpha Shape Python package (Bellock et al. 2021), adopting a uniform alpha parameter of 5.0. We then identified thepeak velocity component of the spectrum averaged over the area

inside the concave hull. Since the YSO group is not necessarily associated with the highest intensity gas component, we
employed a peak finding algorithm to identify all potentially relevant velocity maxima. To do so, we used the find_peaks algorithm in the SciPy signal processing module, with the conservative criteria that any sub-peak must have a height of at least 25%of the main peak, have a width of at least 2 km s−1 , and be atleast 2 km s−1 away from neighboring peaks. Then for each peak
we created a custom zeroth moment map, integrated ±5 km s−1
from the peak velocity, which we contextualized in light of thespatial distribution of YSOs on the plane of the sky. The spectrum for G14.5−1.0 is shown on the right hand side of Fig. F.1.
Group G14.5−1.0 has two velocity peaks, at 19.9 km s−1 and36.3 km s−1 , but only the lower velocity component shows
strong spatial correlation with the YSO structure and group concave hull boundary on the plane of the sky.
We repeated the procedure shown in Fig. F.1 for the remaining YSO groups, with three authors evaluating the spatial correspondence between the YSO groups and the zeroth moment
maps integrated around each velocity peak. All groups associated with the structure show a plausible component at the velocity of the near Sagittarius arm, but many also show additional
strong peaks at higher velocities, consistent with the near Scutum arm at larger distances (see Fig. 5). Given the significant
confusion along the line of sight, we are making all the potential velocities for each group publicly available, and the mostplausible velocities are plotted in Fig. 5. Future targeted observations of YSOs in the structure obtained with high-resolution
stellar spectroscopy should allow for more refined estimates of
its velocity structure.
To transform to galactocentric coordinates (e.g., Fig. 6), we
assumed velocity uncertainties of ±5 km s−1 for unambiguous
vlsr solutions. This is similar to the peak widths observed formany clouds and is also the velocity range used for the integrated emission maps in Fig. F.1. For the six ambiguous cases,
we picked the solution most consistent with the expected Sagittarius Arm velocity with an uncertainty equal to the difference
between the maximum and minimum possible vlsr solutions.
L10, page 13 of 13