A "new" spiral
arm for the Milky Way?
The structure of the outer disk of the Milky Way has long been a mystery
to Galactic astronomers. Though we know that the neutral hydrogen (HI)
disk extends far beyond the stellar disk, we know very little about its structure
in this Galactic "outback". In
particular, we do not know how far the HI spiral structure of the Galaxy extends.
As part of the Southern Galactic Plane Survey (SGPS; McClure-Griffiths et
al. 2001) we have recently identified a remarkable structure in the far
outer disk of the southern Milky Way. This structure, shown near the top of
Figure 1, is a ridge of emission lying at the far positive-velocity edge of
the SGPS longitude-velocity (l-v) diagram. The
l-v diagram shows HI emission in the Galactic mid-plane
(b = 0°). Emission at negative velocities is interior to
the solar circle and corresponds to gas at two distances, whereas emission
at positive velocities lies exterior to the solar circle and generally corresponds
to only one distance with larger velocities at larger distances. The new ridge
of emission arcs from l = 253°,
v = 102
km s-1 through l = 299°,
v = 110 km s-1 to
l = 321°, v = 88 km
s-1.
Figure 1: HI longitude-velocity
(l-v) diagram of the Southern Galactic Plane showing
the outer arm. The arm is observed as a ridge of emission at the most extreme
positive velocities. The image has an angular resolution of
2 arcmin, a spectral resolution of
0.8 km s-1, and uses a square-root transfer function from 1 to 70 K. The solid lines are
at constant Galactocentric radii of 16 and 24 kpc. (From McClure-Griffiths et al. 2004).
It is relatively cohesive across more than 70 degrees
on the sky, kinematically distinct from its surroundings, and is notably the
last feature before the edge of the Galactic disk. Overlaid on Figure 1 are lines
of constant Galactocentric radius at 16 and 24
kpc, which show that the feature extends from a radius
of Rg ~ 17 kpc at the low-longitude end to
Rg ~ 25 kpc at the high-longitude end, with an estimated
radial width of ~ 2 kpc. The scale height of this feature
is relatively constant along its entire length,
varying between extremes of 1.2 and 1.7 kpc. We
estimate the HI number-density along the mid-plane is
4 × 10-2 cm-3, and by integrating over the
z-height
we find that the average surface-density is
3 × 10-3 Msun
pc-2. Based on the z-height and
velocity width of the feature we estimate that a disk
density of 6 × 10-3
Msun pc-3 would be required to confine
the scale-height of the feature, though there is
no evidence that the stellar disk extends to such
large Galactocentric radii.
A ridge in the longitude-velocity diagram can
be caused by two main physical effects: a gas
density enhancement or a significant velocity
perturbation that causes gas at different distances to appear at
the same velocity. A spiral arm produces a
combination of these effects. Spiral arms typically produce
only small (nominally a factor of two) HI-density enhancements in the
HI disk, but they also induce strong (~10 km
s-1) non-circular velocities, or streaming motions, where gas interior to an arm
is pulled radially by the arm's gravity. Because
the observed feature is not at a constant
Galactocentric radius and appears to be radially confined,
we hypothesised that it might be a very distant
spiral arm. To test this theory we first needed to understand the shape of a spiral arm in
l-v space. We created a simple four-arm spiral model for
the Milky Way with a pitch angle, i, that varies
linearly from 16° at Rg = 4.25
kpc to 10° at Rg = 18
kpc. Using the spiral model and accounting for
streaming motions around the spiral arms, we created
synthetic HI spectra and constructed a
synthetic l-v diagram. The face-on spiral model and synthetic
l-v diagram are shown in Figure 2. The model is by no
means definitive, but it does recreate many of the
dominant features of the observed l-v diagram. In
particular, the synthetic l-v diagram also shows a narrow
ridge of emission at the most positive velocities. From
our model we can show that this ridge maps to the
spiral arm in the face-on diagram that passes through
(x, y) = (-18 kpc, 6 kpc).
Figure 2: Left. Differential HI density (spiral perturbation minus the underlying Toomre disk) for
a simple four-arm Milky Way spiral model. The Sun is at (x,y) = (0 kpc, 8.5 kpc). Overlaid in solid
lines is the spiral model of Cordes & Lazio (2002).
Right. Synthetic l-v diagram created from the
spiral pattern in the left panel. The proposed distant arm in the right panel maps to a ridge of
emission at far positive velocities, as marked. (From McClure-Griffiths et al. 2004)
Though the observed feature can be easily
explained by a spiral arm, we also considered a number
of other explanations. In particular, we
considered whether it might be the result of a pile-up in
velocity space at the projection of the Local Standard of
Rest onto the line-of-sight from a very extended,
smooth HI disk or the signature of elliptical gas-orbits in
the outer Galaxy. However, none of these
explanations fit the data and we therefore concluded on the
basis of agreement with the spiral model that the feature
is best described as a spiral arm with a pitch angle of
i ~ 9°.
Another notable characteristic of this feature is
that it shifts in velocity by about 10 km
s-1 between longitudes l =
275° and 295°, as seen in Figure 1. It is interesting that these longitudes agree well
with the Galactic longitudes of the Large and
Small Magellanic Clouds and the Leading Arm of the Magellanic Stream. In models of the orbits of
the Magellanic Clouds, the Clouds should cross the Galactic plane around
l = 280°. We might expect that the relative proximity of the Clouds would produce
a significant dynamical effect on the outer Milky
Way disk. Some models, such as Weinberg (1995),
even estimate that the orbit of the LMC should produce
a radial shift in the HI at 18 kpc of about 10 km
s-1. The observed shift in the outer arm may be
that effect.
There is still a great deal to understand about
this most distant spiral arm in the Milky Way. For example, it remains to be seen whether there are
any stars associated with the arm. In the future we
also hope to search for evidence of star formation in
the arm. Recent results from the NANTEN telescope
in Chile have revealed some 12CO emission
at coincident positions in the low-longitude end of
the arm (Nakagawa et al., 2004). It is too early to
say whether this gas might have formed stars. We
also hope that future work will be able to
determine whether this feature connects with any
other previously known spiral features, as it appears to
in our simple model.
References
Cordes, J. M., Lazio, T. J. W. 2002, preprint
(astro-ph/0207156)
McClure-Griffiths, N. M., Dickey, J. M.,
Gaensler, B. M., Green, A. J. 2004, ApJ, 607, L127
McClure-Griffiths, N. M., Green, A. J.,
Dickey, J. M., Gaensler, B. M., Haynes, R. F., Wieringa, M. H. 2001, ApJ, 549, 959
Nakagawa, M. et al., 2004, ApJ, submitted.
Weinberg, M. D. 1995, ApJ, 455, L31
N. M. McClure-Griffiths, J. M. Dickey,
B. M. Gaensler and A. J. Green
(Naomi.McClure-Griffiths@csiro.au)