Chris Flynn : Tuorla Observatory

 disk mass
 black holes
 white dwarfs
 K dwarfs
 local group

Research at Tuorla on Dark Matter is partly funded by the Academy of Finland  
DARKSTAR Team members

Chris Flynn , Team Leader      
Burkhard Fuchs, Corresponding member 

Johan Holmberg (Now at Lund Observatory) 
Laura Portinari, Researcher 
Pasi Nurmi, Researcher 
Erik Zackrisson, Researcher  (now at the University of Stockholm)
Rami Rekola, Researcher 
Luca Casagrande (Now at Garching)
Chris Thom (now at the University of Chicago)
Esko Gardner, Ph.D. student  
Sarah Bird, Ph.D. student  

Research Developments in 2007

Older research reports for 2002 - 2003 - 2004 - 2005 - 2006

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.

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.

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


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.

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.