Mapping spiral structure on the far side of the Milky Way

MILKY WAY GALAXY
Mapping spiral structure on the far
side of the Milky Way
Alberto Sanna,1
* Mark J. Reid,2
Thomas M. Dame,2
Karl M. Menten,1
Andreas Brunthaler1
Little is known about the portion of the Milky Way lying beyond the Galactic center at distances
of more than 9 kiloparsec from the Sun.These regions are opaque at optical wavelengths
because of absorption by interstellar dust, and distances are very large and hard to measure.
We report a direct trigonometric parallax distance of 20:4þ2:8
À2:2 kiloparsec obtained with the Very
Long Baseline Array to a water maser source in a region of active star formation.These
measurements allow us to shed light on Galactic spiral structure by locating the Scutum-
Centaurus spiral arm as it passes through the far side of the Milky Way and to validate a
kinematic method for determining distances in this region on the basis of transverse motions.
L
arge-scale maps of the Milky Way have had
to exclude a triangular region with the Sun at
its apex and extending ~10° to either side of
the Galactic center [e.g., (1)]. In this region,
the Galaxy is rotating almost perpendicularly
to our line of sight, so the radial velocities that en-
able the estimate of so-called kinematic distances
aredegenerateatvaluesaroundzero.Someprogress
has been made in the nearer parts of this region
with distances determined through stellar spectra
and, more recently, parallax measurements for as-
tronomical sources of maser emission from newly
formed stars (2). But, the region behind the Ga-
lactic center has remained largely out of reach
owing to the large distances (>9 kpc), very high
foreground extinction, and source confusion.
The Bar and Spiral Structure Legacy (BeSSeL)
Survey is a key project of the Very Long Baseline
Array (VLBA), which has been measuring trigo-
nometric parallaxes and motions on the sky plane
(hereafter, proper motions) of methanol and water
masers associated with hundreds of high-mass
star–forming regions in the Galaxy. Recently, the
project focused mainly on distant regions to lo-
cate distant segments of the Galactic spiral arms.
Here, we discuss the most distant of these sources,
G007.47+00.05 (or IRAS 17591À2228), for which
an accurate parallax distance was not previously
available. The source lies on the far side of the
Galaxy and appears to link the prominent Scutum-
Centaurus spiral arm in the fourth Galactic
quadrant with a distant molecular arm recently
discovered in the first quadrant, dubbed the
Outer Scutum-Centaurus (OSC) arm (3).
We used the VLBA to observe strong H2O maser
emission at 22.2 GHz from the star-forming re-
gion G007.47+00.05 between March 2014 and
March 2015 (4). We modeled the relative posi-
tion in time of compact maser spots, with respect
to an extragalactic continuum source (Fig. 1A), as
the sum of the parallax signature (i.e., a sinus-
oid), caused by our changing vantage point as
Earth orbits the Sun, plus a linear motion of the
star-forming region relative to the Sun in the
east-west and north-south directions (Fig. 1B).
After removing the linear motion, the parallax
effect inferred from this analysis (Fig. 1C) yields a
parallax angle of 0.049 ± 0.006 milli-arcsec (mas).
This measurement corresponds to a distance of
RESEARCH
Sanna et al., Science 358, 227–230 (2017) 13 October 2017 1 of 3
1
Max-Planck-Institut für Radioastronomie, Auf dem Hügel
69, 53121 Bonn, Germany. 2
Harvard-Smithsonian Center
for Astrophysics, 60 Garden Street, Cambridge, MA
02138, USA.
*Corresponding author. Email: asanna@mpifr-bonn.mpg.de
Fig. 1. Results of the combined parallax fit for G007.47+00.05. This fit
is obtained from the combination of two point-like maser spots with
respect to the background quasar J175526.28–223210.6 (or [IBR2011]
J1755–2232; table S2). For clarity, we draw only maser positions for the
spot at VLSR of –16.0 km s–1
(red circles). (A) Sky-projected motion of the
maser source with respect to J175526.28–223210.6 with each epoch
labeled (the zero position is arbitrary). The empty circles and the line show
the best-fitting position offsets and the trajectory, respectively. East offset
uncertainties are smaller than the symbol. (B) Decomposed offset
positions for the maser source along the east and north directions versus
time. The best-fitting models in east and north directions are shown as
continuous and dashed lines, respectively. (C) Same as the middle panel
but with fitted proper motions subtracted (µx and µy), yielding the parallax
sinusoid. Positions for the spot at VLSR of –15.6 km s–1
are also shown
for the final parallax (p). The east offset (upper panel) and north offset
(lower panel) data are shown separately on different scales.
onOctober13,2017http://science.sciencemag.org/Downloadedfrom

20:4þ2:8
À2:2 kpc (66,500 light years) with an un-
certainty of less than ±14%. In Fig. 2, we plot
the Galactic position of G007.47þ00.05, super-
posed on a plan view of the Milky Way, which
shows the locations of prominent star-forming
sites determined from trigonometric parallaxes.
We compare this result with an independent,
statistical (Bayesian), distance estimate (5), de-
rived by taking into account complementary
information about the spatial distribution and
kinematics of giant molecular clouds in the Gal-
axy. In Fig. 3, we plot the probability density
function (PDF) for the source distance, based
on five distinct contributions (listed below), and
the cumulative best estimate for the probability
of any given distance (black solid line). The red
solid line (PDFSA) quantifies the probability that
a set of three values—Galactic longitude (‘), lat-
itude (b), and radial velocity with respect to the
local standard of rest (LSR), VLSR—match those
values expected for a given spiral arm segment,
as traced by CO and H I emission [e.g., (6)]. The
loci of individual spiral arms are then fixed in
position by trigonometric parallaxes, following
previous analysis (2). Also plotted is the kine-
matic distance probability (PDFKD), as inferred
from the radial velocity of the source with respect
to the Sun (VLSR), after assuming a rotation curve
for the Galaxy (2). Because star-forming regions
are expected to lie close to the Galactic plane, we
have also quantified the expectation of finding
a source at high Galactic latitude if it is near the
Sun, and vice versa (PDFGL). Details of how these
contributions are calculated have been previous-
ly discussed (5).
In Fig. 3, we assign two additional distance
PDFs to the source, based on its proper motion in
the Galactic latitude and longitude directions. The
more distant the source, the smaller one expects
the angular motion out of the plane (PDFPM,b). At
the same time, the proper motion along the
plane (i.e., in Galactic longitude) provides an ad-
ditional kinematic distance probability (PDFPM;‘ ),
which makes use of the velocity component (V⊥ )
perpendicular to the line of sight [e.g., (7)]. The
probability density function, PDFPM;‘, is gener-
ated similarly to the PDFKD [(5), section 2.2]. The
observed proper motion components (mx, my) are
rotated about the line of sight into Galactic com-
ponents (m‘, mb), and the component projected
along the Galactic longitude (m‘ ) is compared
with that expected under the assumption of cir-
cular Galactic orbits as a function of distance
[e.g., equation 4 of (7)]. This calculation of kin-
ematic distance using proper motions is most
effective toward the Galactic center and anticen-
ter, the directions in which conventional kine-
matic distances based on radial velocities are
least useful. A similar calculation was previously
followed by (8), who estimated for G007.47+00.05
a far kinematic distance of 20 ± 2 kpc (see sup-
plementary text). These authors made use of the
Japanese VERA (VLBI Exploration of Radio As-
trometry) array to observe the H2O maser emis-
sion between March 2009 and December 2011.
Evaluating the kinematic distance PDFs re-
quires Bayesian priors for the Galactic rotation
curve, QðRÞ, and the fundamental Galactic pa-
rameters of the circular rotation speed Q0 at the
distance R0 of the Sun from the Galactic center.
In Table 1, we list the priors and source input
data used to calculate the PDFs. For the VLSR of
the source, we adopt the average velocity of the
H2O maser emission of –16 ± 4 km s–1
, which is
consistent with the peak of radio recombination
lines at –16.9 ± 0.1 km s–1
(9) and the range of
CS emission at –13.9 ± 4.6 km s–1
(10). For the
proper motion of the star-forming region (mx, my),
we averaged the final values derived from Fig.
1 (–2.42 ± 0.02 mas year–1
, –4:64 ± 0.30 mas
year–1
) with those measured by (8) based on
three masers (–2.46 ± 0.14 mas year–1
, –4.38 ±
0.14 mas year–1
). Because residual atmospheric
delays between the target and calibration source
usually dominate the uncertainty in proper mo-
tion measurements, and the three masers likely
have the same systematic uncertainties, we have
inflated by
ffiffiffi
3
p
the proper motion uncertainties
from (8), to account for fully correlated motions
among the masers. This analysis provides a pro-
per motion in Galactic longitude (m‘) of –5.12 ±
0.18 mas year–1
.
In Fig. 3, we show that the combined Bayesian
distance of 20.4 ± 0.6 kpc agrees well with our
trigonometric parallax measurement. However,
some caveats should be noted. Kinematic dis-
tances hold under the assumption of purely cir-
cular orbits. This condition is not satisfied within
about 4 kpc of the Galactic center, where the
Table 1. Parameters used to calculate the distance PDFs. Column 1 gives the type of Galactic
rotation curve adopted for the calculation [table 5 of (2)]. Columns 2 and 3 report the fundamental
Galactic parameters of circular rotation speed at the solar circle and the distance of the Sun from
the Galactic center, respectively (2). Columns 4 and 5 give the Galactic coordinates of the source in
the longitude and latitude directions, respectively. Columns 6, 7, and 8 give the local standard of rest
velocity of the source, and its motions on the sky plane, respectively.
Priors Input data
Q(R)
Q0
(km s–1
)
R0
(kpc)
‘
(°)
b
(°)
VLSR
(km s–1
)
µx
(mas year–1
)
µy
(mas year–1
)
Universal 241 ± 8 8.34 ± 0.16 7.471 0.058 –16 ± 4 –2.44 ± 0.07 –4.51 ± 0.17.....................................................................................................................................................................................................................
Fig. 2. Plan view of
the Milky Way
showing the location
of G007.47+00.05 and
other maser sources
determined via
trigonometric parallaxes
[table 1 of (2)]. The
reference system gives
offset positions from the
Galactic center (red aster-
isk). Colored symbols desig-
nate distinct regions in the
Galaxy: inner Galaxy sour-
ces (empty yellow circles);
Scutum-Centaurus arm
(solid cyan circles);
Sagittarius arm (magenta
hexagons); Local arm (blue
pentagons); Perseus arm
(black squares); and Outer
arm (red triangles). Error
bars correspond to distance
uncertainties of 1s.The
background gray disks,
centered on the Galactic center, show regions within Galactocentric radii of 4, 8, 12, and 16 kpc.The Sun is
located at (0, 8.34) kpc. Galactic quadrants, centered on the Sun, are indicated with dashed lines. Solid
lines trace log-periodic spiral fits to sources grouped by arms; dotted lines correspond to 1s widths.Two
logarithmic spirals are drawn for the Scutum-Centaurus arm, one with a pitch angle (y) of 14° (solid),
constrained by the Scutum and Centaurus tangent directions (3), and the other with a pitch of 20°
(dashed), based on maser sources in the inner first quadrant (14).The Scutum and Centaurus tangent
directions are also shown. [Adapted from figure 1 of (2)]
RESEARCH | REPORT

gravitational effects of the Milky Way’s central
bar cause highly noncircular motions [e.g., (11, 12)].
A line of sight from the Sun at ‘ ¼ 7:47° passes
through this region between 4.7 and 12.3 kpc;
hence, this range of distances cannot be excluded
on the basis of kinematic PDFs. Our direct par-
allax measurement does not suffer this limitation.
The spiral arm PDF (PDFSA) has two main peaks
between 4 and 5 kpc, because the target source
has a VLSR between that of the Norma (~10 km s–1
)
and Near 3-kpc (À20 km s–1
) arms at that longi-
tude [e.g., figure 7 of (12)]; the current parallax
measurement unambiguously rules out such small
distances. Distances to spiral arms beyond about
10 kpc are a substantial extrapolation because of
the current lack of direct parallax measurements.
The spiral arm PDF shows a small peak at ~16 kpc
based on a long extrapolation of the Outer arm
(red curve in Fig. 2) through the first Galactic quad-
rant. However, locating G007.47+00.05 at this dis-
tance can be excluded by the parallax distance,
with a probability of >95%, as well as by kinemat-
ic distance values, because this portion of the
Outer arm is well outside the Galactic center
region.
Close to the Galactic plane at a longitude of
7.5° [e.g., figure 3 of (3)], the OSC arm has a VLSR
(–12 km s–1
) close to that of the target source. If
the Scutum-Centaurus arm is a logarithmic spi-
ral with constant pitch angle (y) of 14°, which is
constrained by the tangent directions of the
Scutum ‘ ¼ 31° and Centaurus ‘ ¼ 309° arms
in addition to nearby parallax measurements (3),
the distance to the OSC arm at the longitude of
the maser source would be in the range of 20 to
21 kpc. In Fig. 2, we show the logarithmic spiral
of the OSC arm under these conditions. This
arm also coincides, within the uncertainties,
with the farthest arm traced by (13), taking into
account the value of R0 adopted by those authors.
This distance estimate determines a third peak
of the PDFSA which is consistent with the trigo-
nometric parallax distance. In Fig. 3, the com-
bination of spiral and (both) kinematic PDFs
strongly constrains a narrow range of distances
(absolute peak of the black solid line), which
matches the trigonometric distance of 20.4 kpc.
This result has two further consequences. First,
the agreement of the parallax and the kinematic
distances can be interpreted as evidence that the
approximation of circular motions holds at Ga-
lactocentric radii near 12 kpc in the far outer
Galaxy. Second, it calls into question the assump-
tion of constant pitch angle of the Outer Scutum-
Centaurus arm along its winding around the
Galaxy. In Fig. 2, we extend the track of the Scu-
tum segment from the first Galactic quadrant
through the fourth quadrant, by assuming a loga-
rithmic spiral with y of 20°. This pitch angle pro-
vides the best spiral fit to trigonometric distances
of star-forming regions in the Scutum arm be-
tween ‘ of 5.9° and 32.0° (14, 15). Because the
near segment of the Scutum-Centaurus arm (i.e.,
the Scutum arm in the first quadrant) has a much
larger pitch angle than that determined from the
Scutum and Centaurus tangents and the location
of G007.47+00.05 (14°), it is clear that a single
logarithmic spiral cannot describe the full com-
plexity of the Scutum-Centaurus-OSC arm. This
result agrees with a recent analysis of the spiral
arm morphology in four external face-on galaxies,
where pitch angles can vary along individual arms
by more than 10° (16).
In summary, we have measured the distance
to a water maser source in the OSC arm using
both a direct trigonometric parallax and a sta-
tistical Bayesian analysis. Our parallax distance
agrees with an indirect distance estimate for the
source inferred from its measured (proper) mo-
tion on the sky (8), thus providing support for the
determination of kinematic distances based on
proper motions rather than radial motions alone.
These measurements allow us to trace the Scutum-
Centaurus arm nearly one full turn of Galactic
azimuth and out to large Galactocentric radii on
the far side of the Milky Way, and show that we
can map spiral structure throughout the Galaxy.
REFERENCES AND NOTES
1. J. H. Oort, F. J. Kerr, G. Westerhout, Mon. Not. R. Astron. Soc.
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2. M. J. Reid et al., Astrophys. J. 783, 130 (2014).
3. T. M. Dame, P. Thaddeus, Astrophys. J. 734, L24 (2011).
4. Materials and methods are available as supplementary materials.
5. M. J. Reid, T. M. Dame, K. M. Menten, A. Brunthaler,
Astrophys. J. 823, 77 (2016).
6. T. M. Dame, D. Hartmann, P. Thaddeus, Astrophys. J. 547,
792–813 (2001).
7. Y. Sofue, Publ. Astron. Soc. Jpn. 63, 813–821 (2011).
8. A. Yamauchi et al., Publ. Astron. Soc. Jpn. 68, 60 (2016).
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Mon. Not. R. Astron. Soc. 252, 210–218 (1991).
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ACKNOWLEDGMENTS
A.S. gratefully acknowledges financial support by the Deutsche
Forschungsgemeinschaft (DFG) Priority Program 1573. The
National Radio Astronomy Observatory is a facility of the National
Science Foundation operated under cooperative agreement by
Associated Universities, Inc. This work made use of the Swinburne
University of Technology software correlator, developed as part
of the Australian Major National Research Facilities Programme
and operated under license. We thank M. Honma for fruitful
discussions in preparation. All data used in the paper are available
through the National Radio Astronomy Observatory (NRAO)
archive at https://archive.nrao.edu/archive/advquery.jsp under
program BR198. The spectrum of the maser emission on March 18
2014 is available at http://bessel.vlbi-astrometry.org/first_epoch.
SUPPLEMENTARY MATERIALS
www.sciencemag.org/content/358/6360/227/suppl/DC1
Materials and Methods
Supplementary Text
Figs. S1 and S2
Tables S1 to S3
References (17–31)
28 April 2017; accepted 5 September 2017
10.1126/science.aan5452
Fig. 3. Distance prob-
ability density function
(PDF) for the star-
forming region
G007.47+00.05. Individ-
ual PDFs are indicated by
subscripts and refer to
the spiral arm assign-
ment of the source (SA),
its kinematic distance
(KD), Galactic latitude
(GL), and its proper
motion in latitude (PM, b)
and longitude (PM, ‘).
Input parameters for the
PDFs are given in Table 1.
Gray areas mark the
expected regions for each
spiral arm at a Galactic
longitude of 7.5°. The
combined PDF (black
solid line) strongly favors
an association with the
Outer Scutum-Centaurus
(OSC) arm at a distance
of 20.4 ± 0.6 kpc. This
estimate agrees with the
trigonometric parallax
distance of 20:4þ2:8
À2:2 kpc
(hatched black area).
RESEARCH | REPORT

Mapping spiral structure on the far side of the Milky Way
Alberto Sanna, Mark J. Reid, Thomas M. Dame, Karl M. Menten and Andreas Brunthaler
DOI: 10.1126/science.aan5452
(6360), 227-230.358Science
, this issue p. 227Science
of the Milky Way's spiral arms through almost an entire rotation.
method of inferring distances from transverse motions, which produced the same answer. This allowed them to trace one
to directly determine the parallax distance to a star-forming region on the far side of the Galaxy. They also used a
used radio interferometryet al.parallax is very small and interstellar dust blocks optical light from those regions. Sanna
for relatively nearby objects. The far side of the Milky Way has been impossible to measure accurately, because the
Direct measurements of distances to astronomical sources rely on parallax, which can usually only be measured
Measuring the far side of the Galaxy
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Mapping spiral structure on the far side of the Milky Way

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