arXiv subjects

M. Fellhauer

Publications and source records attributed to M. Fellhauer.

69 records · Page 4Linked to original sources

arXiv2006

Enhanced Mass-to-Light Ratios in UCDs through Tidal Interaction with the Centre of the Host Galaxy

A recent study of ultra-compact dwarf galaxies (UCDs) in the Virgo cluster revealed that some of them show faint envelopes and have measured mass-to-light ratios of 5 and larger, which can not be explained by simple population synthesis models. It is believed that this proves that some of the UCDs must possess a dark matter halo and may therefore be stripped nuclei of dwarf ellipticals rather than merged star cluster complexes. Using an efficient N-body method we investigate if a close passage of a UCD through the central region of the host galaxy is able to enhance the measured mass-to-light ratio by tidal forces leaving the satellite slightly out of virial equilibrium and thereby leading to an overestimation of its virial mass. We find this to be possible and discuss the general problem of measuring dynamical masses for objects that are probably interacting with their hosts.

astro-ph↗

arXiv2005

Star Cluster Survival in Star Cluster Complexes under Extreme Residual Gas Expulsion

After the stars of a new, embedded star cluster have formed they blow the remaining gas out of the cluster. Especially winds of massive stars and definitely the on-set of the first supernovae can remove the residual gas from a cluster. This leads to a very violent mass-loss and leaves the cluster out of dynamical equilibrium. Standard models predict that within the cluster volume the star formation efficiency (SFE) has to be about 33 per cent for sudden (within one crossing-time of the cluster) gas expulsion to retain some of the stars in a bound cluster. If the efficiency is lower the stars of the cluster disperse mostly. Recent observations reveal that in strong star bursts star clusters do not form in isolation but in complexes containing dozens and up to several hundred star clusters, i.e. in super-clusters. By carrying out numerical experiments for such objects placed at distances >= 10 kpc from the centre of the galaxy we demonstrate that under these conditions (i.e. the deeper potential of the star cluster complex and the merging process of the star clusters within these super-clusters) the SFEs can be as low as 20 per cent and still leave a gravitationally bound stellar population. Such an object resembles the outer Milky Way globular clusters and the faint fuzzy star clusters recently discovered in NGC 1023.

astro-ph↗

arXiv2005

An Exact Equilibrium Model of an Unbound Stellar System in a Tidal Field

Star clusters and dwarf galaxies gradually dissolve as they move in the potential of their host galaxy. Once their density falls below a certain critical density (which is comparable with the background density of the galaxy) it is often assumed that their evolution is completed. In fact the remnant of such a system forms a distribution of stars which are unbound to each other and which move on similar orbits in their host potential. With this motivation we study the evolution of an idealised unbound system and follow its expansion and dissolution in the tidal field of a model galaxy. Initially the stars are uniformly distributed (with a density below the critical density) within an ellipsoidal volume. The system itself travels on a circular orbit within a galaxy modelled as an isothermal sphere. The initial velocities of the stars are chosen by assuming that they move on (three-dimensional) epicycles with guiding centre at the centre of the ellipsoid, though the usual epicyclic theory is altered to account for the self-gravity of the system. This is believed to be the first exact equilibrium model of a stellar system in a tidal field. Our main task is to study the stability of such configurations and the time-scale of their dissolution, as a function of the initial density and size of the ellipsoid. If the time of dissolution is measured by an increase of the half-mass radius of 50 %, we find that systems of low density (about 1 % of the background density) and small radius (50 pc on an orbit of radius 10 kpc) can survive for about 20 galactic orbits. For small systems we show that the lifetime is approximately proportional to the inverse square root of the density.

astro-ph↗

arXiv2005

A Possible Formation Scenario for the Ultra-Massive Cluster W3 in NGC 7252

The intermediate age star cluster W3 (age ~ 300-500 Myr) in NGC 7252 is the most luminous star cluster known to date with a dynamical mass estimate of 8 +/- 2 x 10^7 M_sun. With an effective radius of about 17.5 pc and a velocity dispersion of 45 km/s this object may be viewed as one of the recently discovered ultra-compact dwarf galaxies (UCDs). Its intermediate age, however, precludes an origin as a stripped nucleated dwarf elliptical galaxy. The galaxy NGC 7252 is a merger remnant from two gas-rich disc galaxies. Interactions between two gas-rich galaxies lead to bursts of intense star formation. The age of the interaction and the age of W3 are in good agreement, suggesting that was W3 probably formed in the starburst. We propose a formation scenario for W3. Observations of interacting galaxies reveal regions of strong star formation forming dozens up to hundreds of star cluster in confined regions of up to several hundred parsec in diameter. The total mass of new stars in these regions can reach 10^7 or even 10^8 M_sun. By means of numerical simulations we have shown that the star clusters in these regions merge on short time-scales of a few Myr up to a few hundred Myr. We apply this scenario to W3 and predict properties which could be observable and the future evolution of this object. This work lends credence to the notion that at least some of the UCDs may be evolved star cluster complexes formed during early hierarchical mergers.

astro-ph↗

arXiv2004

How star clusters could survive low star formation efficiencies

After the stars of a new, embedded star cluster have formed they blow the remaining gas out of the cluster. Especially winds of high mass stars and definitely the on-set of the first super novae can remove the residual gas from a cluster. This leads to a very violent mass-loss and leaves the cluster out of virial equilibrium. Standard models predict that the star formation efficiency (SFE) has to be about 33 per cent for sudden (within one crossing-time of the cluster) gas expulsion to retain some of the stars in a bound cluster. If the efficiency is lower the stars of the cluster disperse completely. Recent observations reveal that in strong star bursts star clusters do not form in isolation but in complexes containing dozens and up to several hundred star clusters (super-clusters). By carrying out numerical experiments we demonstrate that in these environments (i.e. the deeper potential of the star cluster complex and the merging process of the star clusters within these super-clusters) the SFEs could be as low as 20 per cent, leaving a gravitationally bound stellar population. We demonstrate that the merging of the first clusters happens faster than the dissolution time therefore enabling more stars to stay bound within the merger object. Such an object resembles the outer Milky Way globular clusters and the faint fuzzy star clusters recently discovered in NGC 1023.

astro-ph↗

arXiv2004

A Formation Scenario for the Heavy-Weight Cluster W3 in NGC 7252

The young star cluster W3 (age 300-500 Myr) in NGC 7252 is the most luminous star cluster known to date. Dynamical mass estimates result in 8 +/- 2 * 10^7 M_sun. With an effective radius of about 18 pc and a velocity dispersion of 45 km/s this object is rather one of the recently discovered ultra-compact dwarf (UCD) galaxies than a star cluster. In our model we propose a formation scenario for W3. Observations of interacting galaxies reveal regions of strong star formation forming dozens up to hundreds of star clusters in confined regions of several hundred parsec in diameter. The total mass of new stars in these regions can reach 10^7 or even 10^8 solar masses. By means of numerical simulations we have shown that the star clusters in these regions merge on short time-scales forming objects like UCD galaxies or W3.

astro-ph↗

arXiv2004

omega-Centauri - an Ultra Compact Dwarf Galaxy ?

A new formation scenario for omega-Cen is presented, in which omega-Cen has formed from a star cluster complex (= super-cluster). The star-burst which has led to the formation of this cluster of star clusters was triggered by the infall and destruction of a gas-rich dwarf galaxy during the early build-up phase of the Milky Way. While the star clusters in the super-cluster merge on a few crossing times scale, which is always longer than the individual cluster formation time-scale, they capture stars from the destroyed dwarf galaxy very efficiently. This process makes it possible to explain the age-spread, the spatial and the kinematical differences of the populations observed in omega-Cen. This formation scenario places omega-Cen into the same class of objects as the ultra-compact dwarf galaxies (UCDs) and the faint fuzzy star clusters (FFs).

astro-ph↗

arXiv2003

The White Dwarf Deficit in Open Clusters: Dynamical Processes

In Galactic open clusters, there is an apparent paucity of white dwarfs compared to the number expected assuming a reasonable initial mass function and that main-sequence stars with initial mass <= 8 M_sun become white dwarfs. We suggest that this lack of white dwarfs is due at least in part to dynamical processes. Non-spherically symmetric mass loss during the post-main-sequence evolution would lead to a few km/s isotropic recoil speed for the white dwarf remnant. This recoil speed can cause a substantial fraction of the white dwarfs formed in a cluster to leave the system. We investigate this dynamical process by carrying out high-precision N-body simulations of intermediate-mass open clusters, where we apply an isotropic recoil speed to the white dwarf remnants. Our models suggest that almost all white dwarfs would be lost from the cluster if the average recoil speed exceeds twice the velocity dispersion of the cluster.

astro-ph↗

arXiv2002

omega-Cen - an Ultra Compact Dwarf Galaxy ?

We study the merging of star clusters out of cluster aggregates similar to Knot S in the Antennae on orbits close to the one of omega-Cen by carrying out high resolution numerical N-body simulations. We want to constrain the parameter space which is able to produce merger objects with similar properties as omega-Cen.

astro-ph↗

arXiv2002

The possible origin of the faint fuzzy star clusters in NGC 1023

In the lenticular galaxy NGC 1023 a new population of star clusters (``faint fuzzies'') was recently discovered by Larsen & Brodie. These clusters are found inside the disc and are faint (23 < V < 24 mag) and extended with effective radii of r_eff approx. 7 to 15 pc. We present here N-body calculations of a likely formation-scenario through merging star clusters in clusters of star clusters (super-clusters). Such super-clusters are observed to form in interacting galaxies. The resulting merger objects have masses comparable to the ``faint fuzzies'' and show large effective radii (r_eff > 7 pc). Even though these objects are suffering from strong tidal forces they are able to survive and reach the estimated ages of the extended star clusters in NGC 1023.

astro-ph↗

arXiv2001

Merging Massive Star Clusters as Building Blocks of Dwarf Galaxies ?

Recent spectroscopic observations of galaxies in the Fornax-Cluster reveal nearly unresolved `star-like' objects with red-shifts appropriate to the Fornax-Cluster. These objects have intrinsic sizes of about 100 pc and absolute B-band magnitudes in the range -14 < M_B < -11.5 mag and lower limits for the central surface brightness mu_B >= 23 mag/arcsec^2 (Phillipps et al. 2001, Hilker et al. 1999), and so appear to constitute a new population of ultra-compact dwarf galaxies (UCDs). Such compact dwarfs were predicted to form from the amalgamation of stellar super-clusters (= clusters of star clusters; not to confuse with super stellar clusters (SSC)) by P. Kroupa (1998), which are rich aggregates of young massive star clusters (YMCs) that can form in collisions between gas-rich galaxies. Here we present the evolution of super-clusters in a tidal field. The YMCs merge on a few super-cluster crossing times. Super-clusters that are initially as concentrated and massive as Knot S in the interacting Antennae galaxies (Whitmore et al. 1999) evolve to merger objects that are long-lived and show properties comparable to the newly discovered UCDs.

astro-ph↗

arXiv2001

The Formation of Ultra-Compact Dwarf Galaxies

Recent spectroscopic observations of galaxies in the Fornax-Cluster reveal nearly unresolved `star-like' objects with red-shifts appropriate to the Fornax-Cluster. These objects have intrinsic sizes of about 100 pc and absolute B-band magnitudes in the range -14 < M_B < -11.5 mag and lower limits for the central surface brightness mu_B > 23 mag/arcsec^2 (Phillipps et al. 2001), and so appear to constitute a new population of ultra-compact dwarf galaxies (UCDs). Such compact dwarfs were predicted to form from the amalgamation of stellar super-clusters (Kroupa 1998), which are rich aggregates of young massive star clusters (YMCs) that can form in collisions between gas-rich galaxies. Here we present the evolution of super-clusters in a tidal field. The YMCs merge on a few super-cluster crossing times. Super-clusters that are initially as concentrated and massive as knot~S in the interacting Antennae galaxies (Whitmore et al. 1999) evolve to merger objects that are long-lived and show properties comparable to the newly discovered UCDs. Less massive super-clusters resembling knot 430 in the Antennae may evolve to omega-Cen-type systems. Low-concentration super-clusters are disrupted by the tidal field, dispersing their surviving star clusters while the remaining merger objects rapidly evolve into the mu_B-M_B region populated by low-mass Milky-Way dSph satellites.

astro-ph↗

arXiv2001

Merging Timescales and Merger Rates of Star Clusters in Dense Star Cluster Complexes

Interacting galaxies like the famous Antennae (NGC 4038/4039) or Stephan's Quintet (HCG 92) show considerable star forming activity in their tidal arms. High resolution images (e.g. from HST-observations) indicate that these regions consist of up to hundreds of massive stellar clusters or tidal dwarf galaxies (TDG). In this paper we want to investigate the future fate of these clusters of massive star clusters (in this work called super-clusters). We simulate compact super-clusters in the tidal field of a host-galaxy and investigate the influence of orbital and internal parameters on the rate and timescale of the merging process. We show that it is possible that such configurations merge and build a dwarf galaxy, which could be an important mechanism of how long-lived dwarf satellite galaxies form. A detailed study of the merger object will appear in a follow-up paper.

astro-ph↗

arXiv2001

Building dwarf galaxies out of merged young star clusters

Young star clusters in interacting galaxies are often found in groups or clusters of star clusters containing up to 100 single clusters. In our project we study the future fate of these clusters of star clusters. We find that the star clusters merge on time scales of a few dynamical crossing times of the super-cluster. The resulting merger object has similarities with observed dwarf ellipticals (dE). Furthermore, if destructive processes like tidal heating, dynamical friction or interaction with disc or bulge of the parent galaxy are taken into account our merger objects may evolve into objects resembling dwarf spheroidal galaxies (dSph), without the need of a high dark matter content.

astro-ph↗

arXiv2000

Superbox - An Efficient Code for Collisionless Galactic Dynamics

We present Superbox, a particle-mesh code with high resolution sub-grids and an NGP (nearest grid point) force-calculation scheme based on the second derivatives of the potential. Superbox implements a fast low-storage FFT-algorithm, giving the possibility to work with millions of particles on desk-top computers. Test calculations show energy and angular momentum conservation to one part in 10^5 per crossing-time. The effects of grid and numerical relaxation remain negligible, even when these calculations cover a Hubble-time of evolution. As the sub-grids follow the trajectories of individual galaxies, the code allows a highly resolved treatment of interactions in clusters of galaxies, such as high-velocity encounters between elliptical galaxies and the tidal disruption of dwarf galaxies. Excellent agreement is obtained in a comparison with a direct-summation N-body code running on special-purpose Grape3 hardware. The orbital decay of satellite galaxies due to dynamical friction obtained with Superbox agrees with Chandrasekhar's treatment when the Coulomb logarithm is approximately 1.5.

astro-ph↗