Research
Developments in 2009
Older research reports for
2002
- 2003
- 2004
- 2005
- 2006
- 2007
- 2008
New streams indentified in the Galactic halo
The Milky Way galaxy consists of a flat disk of stars and gas,
surrounded by a spherical and very tenuous system of stars called
the 'halo'.
It has long been recognised that these stars are special --
for one thing, they are very old, the oldest in the Galaxy, and
furthermore contain much less of the chemical elements beyond
Helium than the Sun (termed 'metals' collectively) by
astronomers. This is another indication that halo stars were amongst
the first to form in the Galaxy. For many years discusssion has
centered on whether these stars formed 'all at once', pretty much in
their present distribution, or whether they started out by forming
within small galaxies which have subsequently been torn apart, resulting
in their present day distribution.
Evidence in recent years has shown that the latter picture is
probably the correct one. The evidence comes from looking at the motions
of halo stars which happen to be near the Sun.
The stars do not have entirely random velocities: instead, many of them
can be found to share similar motions - in other words they are travelling
on similar orbits around the Galaxy - which is what is expected if the
stars share a common birth origin in a small galaxy which has since been
broken apart. Stars sharing such orbtis are called a stellar 'stream'.
Now, Rainer Klement (Max-Planck-Institut fuer
Astronomie, Heidelberg), working with a team including Chris
Flynn (Tuorla Observatory), Burkhard Fuchs
(Astronomisches-Rechen Institute, Heidelberg) and Hans-Walter
Rix (Max-Planck-Institut fuer Astronomie, Heidelberg) have
searched for such groupings of halo stars - streams - in the
enormous new database of stars created by the Seventh Sloan
Digital Sky Survey. Over 22000 halo stars were found, using the
photometric data on the stars to search for those containing low
amounts of metals, one of the main characteristics of such
stars. Motions through space for the stars were computed and
Galactic orbits for the stars determined.
The authors found 5 streams of stars in the halo. Three of these
have been previously identified, illustrating that the method works well,
and two probable new streams have been found. Streams have a range of
applications, including their use to reconstruct the mass distribution
in the Galaxy (the mass distribution directly affects the orbits the
stars in the stream take as the circle the Galaxy). Many such streams
are expected to be discovered when the ESA satellite GAIA flies in
2012, and this study is a useful precursor to how one might go about
isolating stellar streams in the enormous dataset GAIA will produce.
Preprint is available here
Local stirrings by the Galactic bar
The Milky Way galaxy has a so-called 'bar' in its central regions,
a long, thin system of stars seen in many other galaxies as well.
The amount of matter contained in the bar is not insignificant,
and in the case of the Milky Way could make up as much as 10 percent
of the total matter in the Milky Way disk. As the bar rotates, its
mass will affect the distribution of stars and gas which it encounters
directly, but also further out in the disk, where the stars will feel
regular gravitation tugs from the bar. It is just these tugs which are
thought to have created a 'stream' of similarly affected stars near the
Sun. Called the Hercules Stream, the stellar members have similar orbits
around the Galaxy which are regularly sheparded by long range interaction
with the rotating bar, so that their orbtis remain coherant.
Very recently, observations of stars in the inner Galaxy have shown
that there may actually be two bars -- one short and fat, the other long
and flat. Both bars seem to have about the same mass. If there are two
bars in the inner galaxy, PhD student Esko Gardner, working with
Chris Flynn at Tuorla Observatory, has wondered what the effects
might be on stars near the Sun, feeling the gravitational tug of both.
To answer this, he set up a model of the Galaxy containing a disk,
bulge (i.e. the central roughly spherical region) and the option to include
no bar, one bar, or two. He then investigated how the orbits of stars near
the Sun are affected by the regular tugs they receive from a rotating bar.
Since we don't know how fast the bar rotates, this was a free parameter
in the simualtions, as well as the mass and position angle of the bar.
The simulations reveal that both proposed bars can create features
in the velocities of stars near the Sun quite similar to those actually observed.
This allows the rotation rate of the bar(s) to be estimated, and was found
to be in good agreement with previous studies. The simulations revealed a
special feature that the "long bar" (the newly proposed one) might also
produce near the Sun -- in addition to herding stars into the "Hercules
stream"", the long bar can create a second stream of stars, which for an
appropriate choice of mass, rotation rate and initial angle, matches
a known feature near the Sun, the "Arcturus stream". Until now, most
researchers have speculated that the Arcturus stream is the remnant of a
small galaxy which has been broken up by approaching the Milky Way too
closely: this new research indicates that it could have a different
origin entirely, having been stirred up from certain stars in the disk
of the galaxy through short-range gravitational interactions with the "long bar".
Preprint is available here
Personnel Movements in 2009
Juliet Datson joined us to commence her PhD in
early 2009, and has been working at the NOT for most of the year.