Deriving the shape of the Galactic stellar disc (Forwarded)

Journal Astronomy & Astrophysics
Paris, France

Contact persons:

Science:

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

Press office:

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 (0l180 degrees) and downwards in the third and fourth
quadrants (180l360 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 cent 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 disc¹s 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 colleagues¹ 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.

[1] The team includes Y. Momany, S. Zaggia, G.P. Piotto (Italy), G.
Gilmore, F. De Angeli (United Kingdom), G. Carraro (Chile) and L. Bedin
(Germany).

Outer structure of the Galactic warp and fla 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
number:10.1051/0004-6361:20054081)

Full article available in PDF format,

http://www.edpsciences.org/articles/...PRAA200605.pdf

IMAGE CAPTIONS:

[Fig. 1:
http://hubblesite.org/newscenter/new...001/23/image/]
An edge-on view of the ESO 510-G13 warped galaxy. Courtesy of NASA, the
Hubble Heritage Team, and C. Conselice.

[Fig. 2:
http://www.edpsciences.org/papers/aa...00605/fig2.jpg
(76KB)]
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).

[Fig. 3:
http://www.edpsciences.org/papers/aa...00605/fig3.jpg
(40KB)]
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).