Research
Developments in 2008
Older research reports for
2002
- 2003
- 2004
- 2005
- 2006
- 2007
Schmidt-Kennicutt star formation law seen in Milky Way
Measurents of mow many stars have formed in galaxies as a
function of time is an important factor in understanding how
galaxies form and evolve.
Determining the star formation history of our own galaxy, the
Milky Way, has turned out to be surprisingly difficult, and
although many attempts have been made to do so over the years
using a variety of techniques, the studies do not agree with
each other very satisfactorily. Deciding whether stars have
formed in major bursts has been difficult to tease out of the
data, although the general trend is that of a slowly declining
star formation rate over the last few Gyr.
Burkhard Fuchs, Hartmut Jahreiss
(Astronomisches-Rechen Institute, Heidelberg) and Chris
Flynn (Tuorla Observatory) have used a new method to trace
backward the star formation history of the Milky Way disk, using
a sample of M dwarfs. Such stars are used because they show
Hydrogen emission until a particular age, which is a function of
their absolute magnitudes, permiting us to reconstruct the rate
at which disk stars have been born over about half the disk's
lifetime, or about 5 Gyr. Our star formation rate (SFR) agrees
well with those obtained using other methods and seems to rule
out a constant SFR.
The principal result of this study is to show that a relation
of the Schmidt-Kennicut type (which relates the SFR to the gas
content of galaxies) has been found in the Milky Way disk during
the last 5 Gyr, and of the same type as is seen in other
galaxies. This result adds considerable value to the standard
assumption used in galaxy evolution studies that the
Schmidt-Kennicut applies not just in present day galaxies, but
also stretching back over the last 5 Gyr -- at least in the
Milky Way!
Preprint is available here
No "red halo" around the Milky Way galaxy
Galaxies are typically composed of stars, gas and dust
arranged either into a flattened system (disk galaxies) or
something which is roughly spherical (elliptical galaxies). Most
of the light is to be found in the prominant, central regions of
galaxies, but they can also be surrounded by a "halo" of dim,
red light which can extend to very large distances from the
center. What causes this "red halo" is at present something of a
mystery.
What the halo is caused by, and why it should be so red, is
the question. It might be made up of very large numbers of low
mass stars, which are known to be both very dim and very red
from studies of such stars near the Sun. If the red halos are
caused by dim stars, then there must be very many of them indeed
to explain the feeble light they contribute to the red
halos. Such stars would be almost as plentiful as stars of all
other types, perhaps making them a significant contributor to
the total mass of galaxies. The complication here is that the
total mass of galaxies is quite a mystery anyway, due to the
apparent presence of large amounts of dark matter, and perhaps
red halos are providing us with an important clue (rather than a
red herring!) on this aspect of galaxies as well.
Erik Zackrisson and Chris Flynn, of Tuorla
Observatory, have examined whether our own Milky Way might have
such a red halo. Presently available observations of faint stars
seen with the Hubble Space Telescope appear to strongly
constrain this possibility rather tightly, so that if the Milky
Way is embedded in such a structure, it is very dim indeed.
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Optical image of the blue compact galaxy He 2-10,
taken with the ESO New Technology Telescope. To
demonstrate the size and projected shape of its red
halo, the latter has been subtracted away, leaving the
dark "shadow" surrounding the central galaxy. Image
copyright N. Bergvall (Uppsala Observatory). |
Preprint is available here
Twinkle, twinkle any star -- The Sun is not so special
Astronomers working in Australia and Finland have found there
is nothing special about the Sun after conducting the most
comprehensive comparison of it with other stars - adding weight
to the idea that life could be common in the universe.
Scientists have long argued about whether or not the Earth
has some special characteristics that led to the evolution of
life. Jose Robles, Charley Lineweaver, Chas Egan and Michael
Pracey (Australian National University), along with Chris Flynn,
Esko Gardner and Johan Holmberg (Tuorla
Observatory), and Daniel Grether (University of New South
Wales), contend that this is a difficult question to answer
because we don't have information about other Earth-like
planets.
"Yet the question 'How special is the Sun?' is easier to
address because we do have observations of thousands of other
Sun-like stars," explains Dr Lineweaver.
Rather than guess what properties a star should have to
enable life, the researchers decided to compare the Sun - which
already hosts a life bearing planet - to other stars.
"Our research goes further than previous work which only
looked at single properties such as mass or iron content," says
Robles, who is the lead author on the research paper. "We looked
at 11 properties that could plausibly be connected with life and
did an analysis of these properties: The upshot is that there
doesn't seem to be anything special about the Sun. It seems to
be a random star that was blindly pulled out of the bag of all
stars."
The researchers found that the Sun's mass is the most
anomalous of its properties; the Sun is more massive than 95 per
cent of stars. The Sun's orbit around the centre of the galaxy
is also more circular than the orbits of 93 per cent of its
peers. "But when analysing the 11 properties together, the Sun
shows up as a star selected at random, rather than one selected
for some life-enhancing property," Robles says.
The research is part of the ongoing scientific understanding
of our place in the universe. "Those who are searching for
justification for their beliefs that terrestrial life and
humanity in particular are special, will probably interpret this
result as a humiliating dethronement," says Dr
Lineweaver. "Those who believe we are the scum of the universe,
may find our non-special status uplifting."
The research paper, A comprehensive comparison of the Sun
to other stars: searching for self-selection effects has
been accepted for publication in The Astrophysical Journal and
is available online at http://arxiv.org/abs/0805.2962.
Personnel Movements in 2008
Sarah Bird joined us to commence her PhD in
mid-2008. Luca Casagrande finished his PhD in mid-2008
and has left us to his first postdoc position at the MPIA in
Garching. Erik Zackrisson finished his postdoctoral
position and has taken up a position at Stockholm Observatory.
Two longer term visitors to DARKSTAR in 2008 were
Rainer Klement (Max-Planck Institute, Heidelberg) who is
working on disc galaxy dynamics, Professor Burkhard
Fuchs (Astronomisches Rechen-Institut, Heidelberg ) was
again a very welcome visitor in 2008.