Research
Developments in 2010
Older research reports for
2002
- 2003
- 2004
- 2005
- 2006
- 2007
- 2008
- 2009
Red giants seen in M87
M87 is a giant elliptical galaxy in the Virgo cluster, and is
well known to amateur and professional astronomers alike.
Giant ellipticals are typically very old galaxies, and their
light is dominated by large numbers of so-called "red giant"
stars, those that have exhausted their supply of Hydrogen and
moved off the "main sequence" where most stars, like the Sun,
lie.
PhD student Sarah Bird, with Chris Flynn at
Tuorla Observatory and Bill Harris (McMaster
University) and John Blakeslee (Herzberg Institute of
Astrophysics) have just used archive data at the Hubble
Space Telescope to find red giant stars in this galaxy for
the first time. The galaxy is so distant from us that
detecting such stars, which are about a billion times
fainter than the human eye can see, can only be done with
very long exposures taken in space.
The image that the researchers used was available in the
Hubble Space Telescope archive and represented almost
100,000 seconds of exposure time, more than a day, on this
galaxy. On most of the image, the stars are so close
together that they cannot be separated from each other and
analysed individually, even with the exquisite resolution of
the HST. The stars are simply too tightly packed in this
massive galaxy. However, in the far corners of the image,
they found that the stars were just sufficiently resolved
that they could be individually imaged. The stars, once
their brightnesses and colours had been determined, were
shown to be red giants.
The properties of such stars are quite well understood,
because there are copious quantities of them near the Sun.
Knowing how bright they are near us enabled the researchers
to measure the distance to M87, using a technique called the
"tip of the red giant branch" or TRGB method -- it turns out
to be 55 million light years from us (give or take about 3
million light years). This method is in good accord with
other ways of measuring the distance to this well known
galaxy, and shows that the TRGB method is robust and
reliable. As far as we are aware this is a new distance
record for seeing such stars!
Preprint is available here
Comets and the Galactic Tide
Comets are thought to lie at large distances from the Sun, in
a great reservoir called the "Oort Cloud". The directions
and orbits from which new comets come into the Solar System
indicates that the Oort cloud lies between about 10 thousand
and 40 thousand Astronomical Units from the Sun.
Comets this far out are affected by the tidal forces from the
rest of the Galaxy, in particular as the Sun orbits the
Galactic center, and goes up and down in the Galactic disk.
The long-term dynamics of Oort cloud comets have been studied
by PhD student Esko Gardner and Chris Flynn at
Tuorla Observatory. The influence of both the radial and the
vertical components of the Galactic tidal field. They have
developed an updated model of the Galaxy with a disc, bulge and
dark halo. By computing millions of cometary orbits over a
period of a billion years, they can figure out how many
comets, initially set up in the Oort cloud and at great
distance from the Sun, have their orbits perturbed so that
they come into the Solar system.
The researchers find that the number of comets entering the
Solar system can vary substantially as the Sun goes along its
orbit around the Galaxy. Amplitude of the variations in the
comet flux is of the order of 30 per cent. This sort of
variation in the comet flux is usually ascribed to the
vertical motion of the Sun as it oscillates (up and down) in
the disk, but we find that the radial motion of the Sun (in
and out) is the chief cause of this behaviour.
Preprint is available here
Personnel Movements in 2010
Chris Flynn will spend August 2010 to May 2011 at the
Univerisy of Sydney on sabbatical leave.