M. Fellhauer

The cold veil of the Milky Way stellar halo

We build a sample of distant (D > 80 kpc) stellar halo stars with measured radial velocities. Faint (20 < g < 22) candidate blue horizontal branch (BHB) stars were selected using the deep, but wide, multi-epoch Sloan Digital Sky Survey photometry. Follow-up spectroscopy for these A-type stars was performed using the Very Large Telescope (VLT) FOcal Reducer and low dispersion Spectrograph 2 (FORS2) instrument. We classify stars according to their Balmer line profiles, and find that seven are bona fide BHB stars and 31 are blue stragglers (BS). Owing to the magnitude range of our sample, even the intrinsically fainter BS stars can reach out to D ∼ 90 kpc. We complement this sample of A-type stars with intrinsically brighter, intermediate-age, asymptotic giant branch stars. A set of four distant cool carbon stars is compiled from the literature and we perform spectroscopic follow-up on a further four N-type carbon stars using the William Herschel Telescope (WHT) Intermediate dispersion Spectrograph and Imaging System (ISIS) instrument. Altogether, this provides us with the largest sample to date of individual star tracers out to r ∼ 150 kpc. We find that the radial velocity dispersion of these tracers falls rapidly at large distances and is surprisingly cold (σr ≈ 50–60 km s−1) between 100 and 150 kpc. Relating the measured radial velocities to the mass of the Milky Way requires knowledge of the (unknown) tracer density profile and anisotropy at these distances. Nonetheless, by assuming the stellar halo stars between 50 and 150 kpc have a moderate density fall-off (with power-law slope α < 5) and are on radial orbits (graphic), we infer that the mass within 150 kpc is less than 1012 M⊙ and suggest it probably lies in the range (5–10) × 1011 M⊙. We discuss the implications of such a low mass for the Milky Way.

The impact of galaxy harassment on the globular cluster systems of early-type cluster dwarf galaxies

The dynamics of globular cluster systems (GCSs) around galaxies are often used to assess the total enclosed mass, and even to constrain the dark matter distribution. The globular cluster system of a galaxy is typically assumed to be in dynamical equilibrium within the potential of the host galaxy. However cluster galaxies are subjected to a rapidly evolving and, at times, violently destructive tidal field. We investigate the impact of the harassment on the dynamics of GCs surrounding early type cluster dwarfs, using numerical simulations. We find that the dynamical behaviour of the GCS is strongly influenced by the fraction of bound dark matter f_{DM} remaining in the galaxy. Only when f_{DM} falls to ~15%, do stars and GCs begin to be stripped. Still the observed GC velocity dispersion can be used to measure the true enclosed mass to within a factor of 2, even when f_{DM} falls as low as ~3%. This is possible partly because unbound GCs quickly separate from the galaxy body. However even the distribution of {\it{bound}} GCs may spatially expand by a factor of 2-3. Once f_{DM} falls into the <3% regime, the galaxy is close to complete disruption, and GCS dynamics can no longer be used to reliably estimate the enclosed mass. In this regime, the remaining bound GCS may spatially expand by a factor of 4 to 8. It may be possible to test if a galaxy is in this regime by measuring the dynamics of the stellar disk. We demonstrate that if a stellar disk is rotationally supported, it is likely that a galaxy has sufficient dark matter, that the dynamics of the GCS can be used to reliably estimate the enclosed mass.

The sensitivity of harassment to orbit: mass loss from early-type dwarfs in galaxy clusters

We conduct a comprehensive numerical study of the orbital dependence of harassment on early-type dwarfs consisting of 168 different orbits within a realistic, Virgo-like cluster, varying in eccentricity and pericentre distance. We find harassment is only effective at stripping stars or truncating their stellar discs for orbits that enter deep into the cluster core. Comparing to the orbital distribution in cosmological simulations, we find that the majority of the orbits (more than three quarters) result in no stellar mass loss. We also study the effects on the radial profiles of the globular cluster systems of early-type dwarfs. We find these are significantly altered only if harassment is very strong. This suggests that perhaps most early-type dwarfs in clusters such as Virgo have not suffered any tidal stripping of stars or globular clusters due to harassment, as these components are safely embedded deep within their dark matter halo. We demonstrate that this result is actually consistent with an earlier study of harassment of dwarf galaxies, despite the apparent contradiction. Those few dwarf models that do suffer stellar stripping are found out to the virial radius of the cluster at redshift = 0, which mixes them in with less strongly harassed galaxies. However when placed on phase-space diagrams, strongly harassed galaxies are found offset to lower velocities compared to weakly harassed galaxies. This remains true in a cosmological simulation, even when haloes have a wide range of masses and concentrations. Thus phase-space diagrams may be a useful tool for determining the relative likelihood that galaxies have been strongly or weakly harassed.

A parametric study on the formation of extended star clusters and ultra-compact dwarf galaxies

Context. In the last decade, very extended old stellar clusters with m asses in the range from a few 10 4 to 10 8 Mhave been found in various types of galaxies in different environments. Objects with masses comparable to normal globular clusters (GCs) are called extended clusters (ECs), while objects with masses in the dwarf galaxy regime are called ultra-compact dwarf galaxies (UCDs). In heavily interacting galaxies star clusters tend to form in l arger conglomerations called star cluster complexes (CCs). The individual star clusters in a CC can merge and form a variety of spheroidal stellar objects. Aims. The parametric study aims to analyze how the structural parameters of the final merger objects correlate with the underly ing CC parameter space. Methods. In this work we systematically scan a suitable parameter space for CCs and perform numerical simulations to study their further fate. The varied sizes and masses of the CCs cover a matrix of 5x6 values with CC Plummer radii between 10 - 160 pc and CC masses between 10 5.5 - 10 8 M� , which are consistent with observed CC parameters. The CCs of the parametric study are on orbits with galactocentric distances between 20 kpc and 60 kpc. In addition, we studied also the evolution of CCs on a circular orbit at a galactocentric distance of 60 kpc to verify that also extremely extended ECs and UCDs can be explained by our formation scenario. Results. All 54 simulations end up with stable merger objects, wherein 26 to 97% of the initial CC mass is bound. The objects show a general trend of increasing effective radii with increasing mass. Despite the large range of input Plummer radii of the CCs (10 to 160 pc) the effective radii of the merger objects are constrained to values between 10 and 20 pc at the low mass end and to values between 15 and 55 pc at the high mass end. The structural parameters of the models are comparable to those of the observed ECs and UCDs. The results of the circular orbits demonstrate that even ver y extended objects like the M31 ECs found by Huxor in 2005 and the very extended (reff > 80 pc), high-mass UCDs can be explained by merged cluster complexes in regions with low gravitational fields at large galactocentric radii. Conclusions. We conclude that the observed ECs and UCDs can be well explained as evolved star cluster complexes.

Surviving infant mortality in the hierarchical merging scenario

We examine the effects of gas expulsion on initially substructured and out-of-equilibrium star clusters. We perform N-body simulations of the evolution of star clusters in a static background potential before removing that potential to model gas expulsion. We find that the initial star formation efficiency is not a good measure of the survivability of star clusters. This is because the stellar distribution can change significantly, causing a large change in the relative importance of the stellar and gas potentials. We find that the initial stellar distribution and velocity dispersion are far more important parameters than the initial star formation efficiency and that clusters with very low star formation efficiencies can survive gas expulsion. We suggest that it is variations in cluster initial conditions rather than in their star formation efficiencies that cause some clusters to be destroyed while a few survive.

Mass loss and expansion of ultra compact dwarf galaxies through gas expulsion and stellar evolution for top-heavy stellar initial mass functions

The dynamical V-band mass-to-light ratios of ultra compact dwarf galaxies (UCDs) are higher than predicted by simple stellar population models with the canonical stellar initial mass function (IMF). One way to explain this finding is a top-heavy IMF so that the unseen mass is provided by additional remnants of high-mass stars. A possible explanation for why the IMF in UCDs could be top-heavy while this is not the case in less massive stellar systems is that encounters between protostars and stars become probable in forming massive systems. However, the required number of additional stellar remnants proves to be rather high, which raises the question of how their progenitors would affect the early evolution of a UCD. We have therefore calculated the first 200 Myr of the evolution of the UCDs, using the particle-mesh code SUPERBOX. It is assumed that the stellar populations of UCDs were created in an initial starburst, which implies heavy mass loss during the following ≈40 Myr due to primordial gas expulsion and supernova explosions. This mass loss is modelled by reducing the mass of the particles according to tabulated mass-loss histories which account for different IMFs, star formation efficiencies (SFEs), heating efficiencies (HEs), initial masses and initial extensions of the computed UCDs. For each combination of SFE and HE, we find objects that roughly resemble UCDs at the end of the simulation. For low SFEs, the IMF would have to be steeper than in the case of very high SFEs for the models not to expand too much. However, the main conclusion is that the existence of UCDs does not contradict the notion that their stellar populations formed rapidly and with a top-heavy IMF. We find tentative evidence that the UCDs may have had densities as high as 10 8 M ⊙ pc ―3 at birth. This will have to be confirmed by follow-up modelling.

Infant mortality in the hierarchical merging scenario: dependence on gas expulsion time-scales

We examine the effects of gas expulsion on initially substructured and out-of-equilibrium star clusters. We perform N-body simulations of the evolution of star clusters in a static background potential before adjusting that potential to model gas expulsion. We investigate the impact of varying the rate at which the gas is removed, and the instant at which gas removal begins. Reducing the rate at which the gas is expelled results in an increase in cluster survival. Quantitatively, this dependence is approximately in agreement with previous studies, despite their use of smooth and virialized initial stellar distributions. However, the instant at which gas expulsion occurs is found to have a strong effect on cluster response to gas removal. We find if gas expulsion occurs prior to one crossing time, cluster response is poorly described by any global parameters. Furthermore, in real clusters the instant of gas expulsion is poorly constrained. Therefore, our results emphasize the highly stochastic and variable response of star clusters to gas expulsion.

Formation rates of star clusters in the hierarchical merging scenario

Stars form with a complex and highly structured distribution. For a smooth star cluster to form from these initial conditions, the star cluster must erase this substructure. We study how substructure is removed using N-body simulations that realistically handle two-body relaxation. In contrast to previous studies, we find that hierarchical cluster formation occurs chiefly as a result of scattering of stars out of clumps, and not through clump merging. Two-body relaxation, in particular within the body of a clump, can significantly increase the rate at which substructure is erased beyond that of clump merging alone. Hence the relaxation time of individual clumps is a key parameter controlling the rate at which smooth, spherical star clusters can form. The initial virial ratio of the clumps is an additional key parameter controlling the formation rate of a cluster. Reducing the initial virial ratio causes a star cluster to lose its substructure more rapidly.

Ram pressure drag – the effects of ram pressure on dark matter and stellar disc dynamics

We investigate the effects of ram pressure stripping on gas-rich disc galaxies in the cluster environment. Ram pressure stripping principally affects the atomic gas in disc galaxies, stripping away outer disc gas to a truncation radius. We demonstrate that the drag force exerted on truncated gas discs is passed to the stellar disc, and surrounding dark matter through their mutual gravity. Using a toy model of ram pressure stripping, we show that this can drag a stellar disc and dark matter cusp off centre within its dark matter halo by several kiloparsecs. We present a simple analytical description of this process that predicts the drag force strength and its dependency on ram pressures and disc galaxy properties to first order. The motion of the disc can result in temporary deformation of the stellar disc. However, we demonstrate that the key source of stellar disc heating is the removal of the gas potential from within the disc. This can result in disc thickening by approximately a factor of 2 in gas-rich discs.

RAyMOND: an N-body and hydrodynamics code for MOND

The LCDM concordance cosmological model is supported by a wealth of observational evidence, particularly on large scales. At galactic scales, however, the model is poorly constrained and recent observations suggest a more complex behaviour in the dark sector than may be accommodated by a single cold dark matter component. Furthermore, a modification of the gravitational force in the very weak field regime may account for at least some of the phenomenology of dark matter. A well-known example of such an approach is MOdified Newtonian Dynamics (MOND). While this idea has proven remarkably successful in the context of stellar dynamics in individual galaxies, the effects of such a modification of gravity on galaxy interactions and environmental processes deserves further study. To explore this arena we modify the parallel adaptive mesh refinement code RAMSES to use two formulations of MOND. We implement both the fully non-linear aquadratic Lagrangian (AQUAL) formulation as well as the simpler quasi-linear formulation (QUMOND). The relevant modifications necessary for the Poisson solver in RAMSES are discussed in detail. Using idealised tests, in both serial and parallel runs, we demonstrate the effectiveness of the code.