Journal Astronomy & Astrophysics
Dr. Momany Yazan
Email: momany (at) pd.astro.it
Phone: +39 340 6156797
Dr. Zaggia Simone
Email: zaggia (at) oats.inaf.it
Phone: +39 040 3199212
Dr. Jennifer Martin
Journal Astronomy & Astrophysics
61, avenue de l'Observatoire
75014 Paris, France
Phone: +33 1 43 29 05 41
Press Release: March 16th, 2006
This press release is issued as a collaboration with the Italian
Institute for Astrophysics (INAF) and Astronomy & Astrophysics.
Deriving the shape of the Galactic stellar disc
While analysing the complex structure of the Milky Way, an international
team of astronomers from Italy and the United Kingdom has recently
derived the shape of the Galactic outer stellar disc, and provided the
strongest evidence that, besides being warped, it is at least 70% more
extended than previously thought. Their findings will be reported in an
upcoming issue of Astronomy & Astrophysics, and is a new step in
understanding the large-scale structure of our Galaxy.
Using the 2MASS all-sky near infrared catalogue, Yazan Momany and his
collaborators reconstructed the outer structure of the Galactic stellar
disc, in particular, its warp. Their work will soon be published in
Astronomy & Astrophysics. Observationally, the warp is a bending of the
Galactic plane upwards in the first and second Galactic longitude
quadrants (0<l<180 degrees) and downwards in the third and fourth
quadrants (180<l<360 degrees). Although the origin of the warp remains
unknown, this feature is seen to be a ubiquitous property of all spiral
galaxies. As we are located inside the Galactic disc, it is difficult to
unveil specific details of its shape. To appreciate a warped stellar
disc one should, therefore, look at other galaxies. Figure 1 shows a
good example of what a warped galaxy looks like.
The disc of our Galaxy is made up of three major components: the
stellar, the gaseous, and the interstellar dust components. The warping
of the gas and dust component has been well established and documented.
In particular, the gaseous component is known to be warped and to extend
out to 25,000 parsecs (pc). In contrast, the true extent of this stellar
warping is still being debated. Over the past years, there has been
changing evidence of a difference in the warp amplitude between stars
and gas. These studies have led to the idea that the Milky Way stellar
disc is truncated beyond 14,000 pc from the Galactic centre.
The new analysis by Momany and his team provides the first clear and
complete view of the outer stellar disc warp. They analyzed the
distribution of over 115 million stars from the all-sky 2MASS catalogue
that comprise the totality of the Galactic disc. Among the many
different stellar types, M-giant stars were found to be the ideal
stellar tracer for reconstructing the outer disc structure. They are, in
fact, highly luminous but relatively cool and evolved stars, and these
unique properties allow better determination of their distance. The
analysis also shows that M-giants stars located at distances between
3,000 and of 17,000 pc from the Sun draw the same stellar warp
signature. This means that a global and large-scale Milky Way feature
has been identified to about 25,000 pc from the Galactic centre: the
team thus clearly demonstrates that there is no truncation of the
stellar disc beyond 14,000 pc. The figures below illustrate the shape of
the Galactic outer stellar disc. Figure 2 shows the density maps as
derived from the 2MASS M-giant sample at 14,000 pc from the Galactic
centre. The presence of the warp is quite clear at both ends of the
stellar disc. Figure 3 quantitatively shows the amplitude and
orientation of the disc’s stellar warp as a function of the Galactic
longitude. It also shows the consistency of the warp signature in the
three disc components (gas, dust, and stars). It is a natural
consequence of the close physical correlation between these three
Galactic disc components, and proves once more the existence of a global
and regular warp signature for the Galactic disc.
Last but not least, this new evidence of an extended and warped Milky
Way stellar disc allows the team to solve a heated debate among
astronomers. In the past years, astronomers have identified
over-densities in the opposite direction to the Galactic centre. Located
in the Galactic plane, they stretch over 100 degrees in Monoceros
constellation. Known as the Monoceros Ring, this over-density was
believed to be the remnant of a dwarf satellite galaxy cannibalised by
the Milky Way. Another well-known example exists in the Sagittarius
constellation of how the Milky Way halo is continuously building up by
means of cannibalised smaller galaxies. Recently, an over-density
located in Canis Major was associated to the Monoceros Ring and
identified as the core of a satellite galaxy currently being accreted
into the Galactic plane. Momany and colleagues’ work, however, casts
serious doubt on this scenario. They show that the Canis Major
over-density is easily explained by the imprint of the Galactic warp.
They may also be able to explain the Monoceros Ring by the complex
structure of the outer disc, but they cannot offer a definite conclusion
about this issue yet, as very little is known about the Monoceros Ring.
It seems, however, that the Sagittarius dwarf remains the only example
we have for the moment of how our Milky Way is still growing by
cannibalising smaller galaxies.
 The team includes Y. Momany, S. Zaggia, G.P. Piotto (Italy), G.
Gilmore, F. De Angeli (United Kingdom), G. Carraro (Chile) and L. Bedin
Outer structure of the Galactic warp and flare: explaining the Canis
Major over-density, by Y. Momany, S. Zaggia, G. Gilmore, G.P. Piotto,
G. Carraio, L. Bedin & F. De Angeli.
To be published in Astronomy & Astrophysics (DOI
Full article available in PDF format,
An edge-on view of the ESO 510-G13 warped galaxy. Courtesy of NASA, the
Hubble Heritage Team, and C. Conselice.
The density maps and contours of the M-giant sample at 14,000 pc from
the Galactic centre. The stellar disc in the Southern hemisphere is
bending downwards (right of the figure) while in the Northern hemisphere
it bends upwards (left of the figure).
The stellar warp as derived from 2MASS M-giant stars at 14,000 pc from
the Galactic centre (black filled squares) compared to the gaseous and
interstellar dust warp (green triangles and asterisks).