Research
Developments in 2007
Older research reports for
2002
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The puzzle of the "red halos"
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.
Erik Zackrisson, of Tuorla Observatory, and
colleagues N. Bergvall, G. Ostlin, G. Micheva and
M. Leksell in Stockholm and Uppsala, have taken very
long exposures of a range of galaxies, discovering that
they are frequently embedded in dim halo light which is
very red in colour --- that is to say, light at redder
wavelengths strongly dominates over light at bluer
wavelengths. Dim red halos around galaxies have been
seen in many studies, including some done in space with
the Hubble Space Telescope.
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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). |
What the halo is caused by, and why it should be so
red, is the question. One possibility is that it is
emitted by 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.
Zackrisson has studied some other ideas for what
causes the red halo light, such as its being produced
by stars with a high metal content (all elements other
than Hydrogen and Helium are termed 'metals' by
astronomers) or by nebulosity caused by hot gas clouds
--- but neither of these explanations seem to work for
all the galaxies surveyed.
The study has been published in the Astrophysical Journal.
Preprint is available here
At DARKSTAR, Erik Zackrisson and Chris Flynn are
following up the consequences of red halos, but asking
whether the Milky Way itself could have a red halo
too. Presently available observations of faint stars
seen with HST appear to strongly constrain this
possibility rather tightly, deepening the mystery
further.
Preprint is available here
Star streams in the Galactic halo
Christian Dettbarn, Burkhard Fuchs of
the Astronomisches
Rechen-Institut in Heidelberg, Chris Flynn
(Tuorla Observatory) and Mary Williams
(Potsdam) have analyzed the distribution of velocities
and positions of a sample of about 900 carefully
selected stars of low metallicity which are passing
nearby to the Sun.
Low metallicity stars are amongst the first which were born
in the Milky Way galaxy, and as such contain valuable
information about the early conditions in which our galaxy came
into existance. The stars primarily represent the" thick disk"
and "halo" populations of the Milky Way.
The aim was to identify "star streams" passing close to the
Sun. Such streams are composed of many stars which are
substantially similar orbits as they move around the galaxy,
and may be part of quite extended structures which are left
over from the epoch of the Milky Way's earliest days.
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Simulation of stellar streams forming from a dwarf satellite galaxy as it
is disrupted in its orbit through the Milky Way.
Many such streams are expected to be revealed by the GAIA satellite, when
it flies in the next decade.
Image Source: Johnston et.al. www.sns.ias.edu/~kvj/
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A special technique was developed to search for such stars,
involving their angular momenta and orbital eccentricities, as
a generalisation from a method which works for much more common
stars which are members of the Galaxy's disk.
Besides recovering all well known star streams in the thick
disk, we isolated four statistically significant phase space
overdensities amongst halo stars. One of them is associated
with a previously known halo star stream, but three of them are
novel features, which we propose be also considered as genuine
halo streams.
The study has been published in the Monthly Notices of the
Royal Astronomical Society.
Preprint
is available here
Helium in low mass stars
Efforts to measure the amount of helium in K dwarfs
(stars a little less massive than the Sun) have lead to
very interesting, if partially perplexing, results. The
luminosity of K dwarfs depends mainly on their
temperaure, to a significant extent their metallicity
and to a small extent their helium content.
At Tuorla Luca Casagrande, Laura Portinari
and Chris Flynn have been using K dwarfs with
highly accurate temperature and metallicity
determinations to derive their Helium contents
indirectly. Helium is an important element, being the
second most abundant in the Universe, and providing a
number of interesting diagnostics of the chemical
evolution of the Galaxy and of the models of its
production in the Big Bang. This year we have measured
the ratio of helium to metal production (dY/dZ) from the
luminosities of K dwarfs, with two major improvements
over previous studies: (a) we have complied and analyzed
homogeneously a much larger sample of K dwarfs and (b)
we have recovered from multi-band data their fundamental
parameters of bolometric magnitude and effective
temperature (Casagrande, Portinari & Flynn 2006). We
could thus study for the first time a large homogeneous
sample of the Low Main Sequence stars in the
theoretical, rather than observational, HR diagram,
where the effects of Helium are far more prominent. We
obtain dY/dZ = 2+/-1 from nearby metal rich stars, while
at lower metallicity we find much too low helium
abundances need to be invoked to match the data, which
probably reveal inadequacies in current stellar models
at low metallicity (Casagrande et al. 2007). This
unfortunately prevents a reliable extrapolation of the
primordial helium fraction from local data, but major
improvements are expected from the future comparison of
stellar models with astro-seismology measurements of
helium content in nearby stars.
The study has been published in the Monthly Notices of the
Royal Astronomical Society.
Preprint
is available here
Clumping in cold dark matter halos
Galaxies are thought to form within so-called "dark halos", which
are quite a bit bigger and heavier than the visible parts of the
galaxy itself. Strong evidence can be found from a number of
independent methods that the halos exist, although direct detection
of the particles of which they are composed remains elusive.
In the standard cosmological model, most (about 85 percent) of
the matter in the Universe is in a "cold" and dark form. Cold
refers to the fact that the particles which make up the matter do
not move at very high speeds, typically at a few 10s to a few
100s of km/s in the halos around galaxies.
The distribution of energies of the particles leads them to
form halos of various sizes, when they are computed as a function
of time in cosmological simulations. Within galactic sized halos,
such as we think out own Milky Way galaxy is embedded in, the
theory predicts large numbers of smaller, sub-halos. Some of
these sub-halos contain ordinary matter, and appear to us as
dwarf galaxy companions to the Milky Way. Theory predicts much
more of these sub-halos than are actually observed, but this is
probably because mnost of them contain little ordinary matter and
are very difficult to detect.
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| Simulation of dark matter
around a Milky Way type galaxy. The dark matter
appears as white, and forms a halo around the visible
galaxy (red). There are a large number of sub-halos,
or clumps of dark matter, which are a fundamental
characteristic of dark halos in the "cold dark matter"
scenario. Some of these contain ordinary matter as
well, and appear as satellite galaxies (shown in red
at upper right). |
One way to detect these sub-halos is to ask what effect they
would have on distant galaxies viewed with many sub-halos
(embedded withing bigger halos) along the line of sight. The
distortions in the background galaxies, or the microlensing of
distant point sources is often computed in theoretical models
without the effects of these sub-halos included.
Now Janne Holopainen, along with Erik
Zackrisson,
Chris Flynn, Pasi Nurmi and Pekka
Heinämäki at Tuorla Observatory and Alexander
Knebe (Astrophysikalisches Institut Potsdam), Stuart
Gill (Columbia University) and Teresa Riehm (Stockholm
University) have investigated the consequences of including
sub-halos in these computations. They find that in cluster-sized
halos triaxiality (non-sphericity) is actually the dominat source
of scatter in the apparent mass density along given lines of
sight, with sub-halos only contributing small extra scatter. A
similar result is found for galaxy-sized halos, with the
non-spherical shapes of the halos dominating over substructure.
An analytical model for the surface mass density scatter as a
function of distance (for redshift ranges of 0 to 1.5) to the
halo centre, halo redshift and halo mass, allowing investigation
of the reliability of results obtained with simplified halo
models. Additionally, it provides the means to add simulated
surface density scatter to analytical density profiles.
As an example application of the technique, is the
impact on the calculation of microlensing optical depths
for massive astrophysical compact halo objects (MACHOS)
in CDM halos. The authors find that sub-halos can cause
30 percent variation in the apparent optical depth for
patches of sky with size of order the microlensing
surveys of the Large Magellanic Cloud. While this is a
quite large scatter along any particukar line of sight,
it is insufficient to revive interest in MACHOS as cold
dark matter candidates, because the number of these
detected in surveys falls far short (by a factor of 500
percent) of the numbers required if the entire dark halo
of the Milky Way were composed of MACHOS.
The study has been published in the Monthly Notices of the
Royal Astronomical Society.
Preprint is available here
Physical parameters for low mass stars
Stars are basic to astronomy, and determining their masses,
temperatures and luminosities a key task for astronomers. Cool
stars are surprisingly difficult to get right in this regard, as
much of their luminosity is releases in the infrared, where data
have been traditionally more difficult to obtain than in the
optical. This situation has altered with the coming on-line of
large area surveys of the sky in the IR, such as 2MASS, meaning that
colours are available for large numbers of cool, low mass stars.
In previous research, Luca Casagrande,
Laura Portinari and Chris Flynn of Tuorla Observatory
have obtained a sample of about 100 bright G and K
dwarfs with accurate parallax and photometric data in
BVRIJH and K. They derived an empirical effective
temperature and bolometric luminosity calibration for G
and K dwarfs, by applying their own implementation of
the Infrared Flux Method to multiband photometry.
Now this work has been extended to M type dwarfs, in
collaboration with Mike Bessell (Mount Stromlo) and Chris Koen
(Cape) and we have found that, perhaps unexpectedly, the method
works as well for these stars as it did for the G and K
types. The internal consistency of the temperature scale for the
M types remains as good as for the hotter stars (observational
errors leading to uncertainties of well under 100 K). There are
very few good temperatures for M dwarfs with which to compare our
temperature scale externally, so the situation with systematic
offsets of up to 100 K, seen in the G and K dwarfs, between other
temperature scales, is less clear. Most interestingly, a rough
metallicity can be extracted from the optical to near-IR
photometry, on the basis of a theoretical calibration using model
spectra. Again, metallicities for M dwarfs are notoriously
difficult to measure, but a comparison to M dwarfs which are part
of systems in which a more massive primary star has a good
metallicity indicates the system works quite well.
The research has been submitted to the Monthly Notices of the
Royal Astronomical Society.
The Milky Way and the Tully-Fisher relation
Our earlier
results on the local M*/L ratio and the global
luminosity of the Milky Way allow us to compare it to
the Tully-Fisher (TF) relation of external disc
galaxies, namely the relation between their rotation
velocity and total luminosity. Our Galaxy turns out to
be less luminous (and less massive, in terms of stellar
mass) than expected for a typical disc galaxy of the
same rotation velocity, and it lies below the TF
relation by about one standard deviation, compared to
the mean for many hundreds of galaxies. While not
statistically worrisome, this offset may hint to a
problem with the luminosity zero-point of the TF
relation, or to the possibility that the stellar Initial
Mass Function (IMF) in disc galaxies on average is
somewhat different (lighter) than that of the Milky Way.
These conclusions are reinforced by comparison to
cosmological simulations of disc galaxies, simulated
assuming a Solar Neighbourhood-like IMF; the
corresponding TF relation agrees well with the location
of the Milky Way and shows the same 1 sigma offset from
the observed TF relation (Portinari & Sommer-Larsen
2007). This raises the insteresting question that the
offset may not be a problem of the current cosmological
scenario, but rather of the stellar IMF or of the
empirical zero-point of the TF relation.
Our cosmological simulations of the TF relation also
predict its evolution with redshift, to be compared to
the wealth of (yet still controversial) observational
results that are being obtained in this field. Our
results suggest a strong luminosity evolution of the
relation (by almost 1 B-mag between redshift 1 and 0),
but a negligible mass evolution, namely the TF relation
between stellar mass and rotation velocity is constant
with redshift. This does not mean that an individual
disc galaxy does not evolve in stellar mass; rather, as
its stellar mass grows in time, its rotation velocity
also increases so that the galaxy keeps lying on the
same relation at different epochs.
The study has been published in the Monthly Notices of the
Royal Astronomical Society.
Preprint is available here
Personnel Movements in 2007
Janne Holopainen completed
his PhD in mid-2007 and has left us to pursue
other professional interests, amongst them film
production. Esko Gardner spent two months at
York University in Toronto visiting Professor Kimmo
Innanen. Laura Dunn spent six months at
DARKSTAR doing distance measurements to dwarf
galaxies, and returned to Australia in late 2007.
Three longer term visitors to DARKSTAR in 2007 were
Rainer Klement (Max-Planck Institute,
Heidelberg) who is working on disc galaxy dynamics,
Professor Ali Talib Mohammed (Baghdad
University, Department of Astronomy) who spoke with us
about exo-planetary imaging, and Regner
Trampedach (Aarhus University) with whom we
discussed the latest results in 3-D models of stellar
convection.