The formation and the stellar populations of dwarf galaxies

Ultra-compact dwarf galaxies (UCDs) are stellar systems with masses similar to those of dwarf elliptical galaxies (dEs), but with characteristic radii that are about a factor of ten smaller than those of dEs. One of the most intriguing properties of UCDs is that the mass-to-light ratios implied by their internal dynamics are rather high, even though it is unlikely that UCDs contain significant amounts of dark matter. This suggests that the reason for the high mass-to-light ratios is a varying stellar initial mass function (IMF) in UCDs, even though the IMF seems remarkably invariant in the Milky Way. This apparently invariant IMF for star forming systems in the Milky Way is known as the canonical IMF. In principle, the mass-to-light ratios of an old stellar population be increased either by additional faint, low-mass stars or by additional remnants of massive stars. In order to decide which one of these two cases applies to the UCDs, the frequency of bright X-ray sources in UCDs was studied. This is because a bright X-ray source in a UCD can be interpreted as a low-mass X-ray binary (LMXB), which is composed of a low-mass star and a stellar remnant. The fraction of the UCDs in the Virgo cluster that contain a bright X-ray source is indeed remarkably high, and the variation of the IMF that explains this frequency of X-ray sources with an overabundance of LMXBs in these UCDs is consistent with their high mass-to-light ratios. This suggests that the UCDs formed with a larger number of massive stars than the canonical IMF would imply. As a consequence, the UCDs would have been extremely dense when star-formation took place in them. These extreme initial conditions may be the reason why the IMF in UCDs would deviate from the canonical IMF in the first place. Regarding the origin of the UCDs, it has been argued before that they are created by the interaction between gas-rich galaxies. However, the formation of dEs may have been triggered by the same process, even though their structural parameters are largely different from those of UCDs. This notion is supported (among other reasons) by the finding that young galaxies that formed through the interaction between gas-rich galaxies would probably evolve into dEs as far as their masses and radii are concerned. This formation scenario for dEs poses however a challenge to the currently prevailing cosmological model.Die Entstehung und die Sternpopulationen von Zwerggalaxien Ultrakompakte Zwerggalaxien (ultra-compact dwarf galaxies, UCDs) sind Sternsysteme, die Massen ähnlich derer von Zwerg-Elliptischen Galaxien (dwarf elliptical galaxies, dEs) haben, aber deren charakteristische Radien um etwa einen Faktor 10 kleiner als die von dEs sind. Eine der erstaunlichsten Eigenschaften von UCDs ist dass ihre interne Dynamik recht hohe Masse-zu-Leuchtkraftverhältnisse implizieren, obwohl es unwahrscheinlich ist dass UCDs nennenswerte Mengen an dunkler Materie enthalten. Das legt nahe dass der Grund für die hohen Masse-zu-Leuchtkraftverhältnisse eine veränderliche stellare Anfangsmassenfunktion (stellar initial mass function, IMF) ist, obwohl die IMF in der Milchstrasse anscheinend bemerkenswert gleichbleibend ist. Diese anscheinend gleichbleibende IMF wird als die kanonische IMF bezeichnet. Prinzipiell kann das Masse-zu-Leuchtkraftverhältnis einer alten Sternpopulation entweder durch zusätzliche leichte, leuchtschwache Sterne, oder durch zusätzliche Überreste massiver Sterne erhöht werden. Um zu entscheiden welcher der beiden Fälle auf die UCDs zutrifft, wurde die Häufigkeit heller Röntgenquellen in UCDs untersucht. Der Grund dafür ist dass eine solche Röntgenquelle als ein leichter Röntgendoppelstern (low-mass X-ray binary, LMXB) interpretiert werden kann, der aus einem Neutronenstern und einem leichten Stern besteht. Der Bruchteil von UCDs im Virgohaufen, der eine helle Röntgenquelle enthält ist in der Tat bemerkenswert hoch, und die veränderliche IMF, welche diese Häufigkeit von Röntgenquellen mit einer großen Anzahl von LMXBs erklären kann, ist auch konsistent mit den hohen Masse-zu-Leuchtkraftverhältnissen dieser UCDs. Das legt nahe dass UCDs mit mehr massiven Sternen entstehen als die kanonische IMF impliziert. Als Konsequenz wären die UCDs zu dem Zeitpunkt, an dem Sternentstehung in ihnen stattgefunden hat, extrem dicht gewesen. Falls die IMF in UCDs von der kanonischen IMF abweicht, können diese extremen Anfangsbedingungen der eigentliche Grund dafür sein. Hinsichtlich des Ursprunges UCDs ist argumentiert worden dass sie durch Wechselwirkungen zwischen gasreichen Galaxien entstanden sein könnten. Die Bildung von dEs könnte allerdings durch denselben Prozess ausgelöst werden, obwohl sich ihre strukturellen Parameter erheblich von denen von UCDs unterscheiden. Diese Sichtweise wird (unter anderem) dadurch gestützt dass junge Galaxien, die durch die Wechselwirkung zwischen gasreichen Galaxien entstanden sind, sich hinsichtlich ihrer Massen und Radien wahrscheinlich zu dEs entwickeln. Dieses Szenario für die Entstehung von dEs ist jedoch eine Herausforderung für das zur Zeit vorherrschende kosmologische Modell

Faint dwarf galaxies in nearby clusters

Besides giant elliptical galaxies, a number of low-mass stellar systems inhabit the cores of galaxy clusters, such as dwarf elliptical galaxies (dEs/dSphs), ultra-compact dwarf galaxies (UCDs), and globular clusters. The detailed morphological examination of faint dwarf galaxies has, until recently, been limited to the Local Group (LG) and the two very nearby galaxy clusters Virgo and Fornax. Here, we compare the structural parameters of a large number of dEs/dSphs in the more distant clusters Hydra I and Centaurus to other dynamically hot stellar systems.Comment: 2 pages, 1 figure; to appear in "A Universe of Dwarf Galaxies: Observations, Theories, Simulations", held in Lyon, France (June 14-18, 2010), eds. M. Koleva, P. Prugniel & I. Vauglin, EAS Series (Paris: EDP

The formation of UCDs and massive GCs: Quasar-like objects for testing for a variable stellar initial mass function (IMF)

The stellar initial mas function (IMF) has been described as being invariant, bottom heavy or top-heavy in extremely dense star burst conditions. To provide usable observable diagnostic we calculate redshift dependent spectral energy distributions of stellar populations in extreme star burst clusters which are likely to have been the precursors of present day massive globular clusters (GCs) and of ultra compact dwarf galaxies (UCDs). The retention fraction of stellar remnants is taken into account to asses the mass to light ratios of the ageing star-burst. Their redshift dependent photometric properties are calculated as predictions for James Webb Space Telescope (JWST) observations. While the present day GCs and UCDs are largely degenerate concerning bottom-heavy or top-heavy IMFs, a metallicity- and density-dependent top-heavy IMF implies the most massive UCDs, at ages <100 Myr, to appear as objects with quasar-like luminosities with a 0.1-10% variability on a monthly time scale due to core collapse supernovae.Comment: Accepted for publication in A&A, 12 pages, 10 figures + appendix, version 2: language corrections adde

The integrated galaxy-wide stellar initial mass function over the radial acceleration range of early-type galaxies

The observed radial accelerations of 462 Early-type galaxies (ETGs) at their half-mass radii are discussed. They are compared to the baryonic masses of the same galaxies, which are derived from theoretical expectations for their stellar populations and cover a range from $\approx 10^4 \, {\rm M}_{\odot}$ to $\approx 10^{11} \, {\rm M}_{\odot}$. Both quantities are plotted against each other, and it is tested whether they lie (within errors) along theoretical radial acceleration relations (RARs). We choose the Newtonian RAR and two Milgromian, or MONDian RARs. At low radial accelerations (corresponding to low masses), the Newtonian RAR fails without non-baryonic dark matter, but the two MONDian ones may work, provided moderate out-of-equilibrium dynamics in some of the low-mass ETGs. However all three RARs fail at high accelerations (corresponding to high masses) if all ETGs have formed their stellar populations with the canonical stellar initial mass function (IMF). A much better agreement with the observations can however be accomplished, if the theory of the integrated galaxy-wide stellar initial mass functions (IGIMFs) is used instead. This is because the IGIMF-theory predicts the formation of an overabundance of stellar remnants during the lifetime of the massive ETGs. Thus their baryonic masses today are higher than they would be if the ETGs had formed with a canonical IMF. Also the masses of the stellar-mass black holes should be rather high, which would mean that most of them probably formed when the massive ETGs were not as metal-enriched as they are today. The IGIMF-approach confirms downsizing.Comment: 25 pages, 12 figures, accepted to MNRA

Gas expulsion in highly substructured embedded star clusters

We investigate the response of initially substructured, young, embedded star clusters to instantaneous gas expulsion of their natal gas. We introduce primordial substructure to the stars and the gas by simplistically modelling the star formation process so as to obtain a variety of substructure distributed within our modelled star forming regions. We show that, by measuring the virial ratio of the stars alone (disregarding the gas completely), we can estimate how much mass a star cluster will retain after gas expulsion to within 10% accuracy, no matter how complex the background structure of the gas is, and we present a simple analytical recipe describing this behaviour. We show that the evolution of the star cluster while still embedded in the natal gas, and the behavior of the gas before being expelled, are crucial processes that affect the timescale on which the cluster can evolve into a virialized spherical system. Embedded star clusters that have high levels of substructure are subvirial for longer times, enabling them to survive gas expulsion better than a virialized and spherical system. By using a more realistic treatment for the background gas than our previous studies, we find it very difficult to destroy the young clusters with instantaneous gas expulsion. We conclude that gas removal may not be the main culprit for the dissolution of young star clusters.Comment: 19 pages, 8 figures, 2 tables. Accepted for publication in MNRA

Differences between the globular cluster systems of the Virgo and Fornax Galaxy Clusters

It is well known that Globular cluster systems are different among galaxies. Here we test to which degree these differences remain on the scale of galaxy clusters by comparing the globular clusters (GCs) in optical surveys of the Virgo galaxy cluster (ACSVCS) and the Fornax galaxy cluster (ACSFCS) in Kolmogorov-Smirnoff Tests. Both surveys were obtained with the Advanced Camera for Surveys (ACS) on board the Hubble Space Telescope, and contain thousands of GCs in dozens of galaxies each. Also well over 100 point sources in the Chandra X-ray Observatory source catalogue were attributed to the GCs in both optical catalogues, and interpreted as low-mass X-ray binaries (LMXBs). Thus, the optical and X-ray data are as uniform as possible. Our main findings are as follows: (1) The spread in luminosities and half-light radii is larger in the ACSVCS than in the ACSFCS. (2) The ratio between the half-light radii for the F475W-passband and the F850LP-passband is on average smaller in the ACSVCS. (3) The distribution of the LMXBs with the luminosity of the GCs is different between both surveys. These findings are significant. The first finding could be a consequence of a wider spread in the distances of the GCs in the ACSVCS, but the others must have internal reasons in the GCs. Thus, the GC systems are also different on a galaxy cluster scale.Comment: 24 pages, 13 figures, accepted to MNRA