ALBERT

Description: We use the horizontal branch (HB) morphology of 48 Galactic globular clusters (GCs) to study the radial distributions of the different stellar populations known to exist in GCs. Assuming that the (extremely) blue HB stars correspond to stars enriched in helium and light elements, we compare the radial distributions of stars selected according to colour on the HB to trace the distribution of the secondary stellar populations in GCs. Unlike other cases, our data show that the populations are well mixed in 80 per cent of the cases studied. This provides some constraints on the mechanisms proposed to pollute the interstellar medium in young GCs.

Description: We use our Galactic Globular Cluster Catalog (G2C2) photometry for 111 Galactic globular clusters (GCs) in g and z , as well as r and i photometry for a subset of 60 GCs and u photometry for 22 GCs, to determine the structural parameters assuming King models. In general, the resulting core radii are in good comparison with the current literature values. However, our half-light radii are slightly lower than the literature. The concentrations (and therefore also the tidal radii) are poorly constrained mostly because of the limited radial extent of our imaging. Therefore, we extensively discuss the effects of a limited field of view on the derived parameters using mosaicked Sloan Digital Sky Survey data, which do not suffer from this restriction. We also illustrate how red giant branch (RGB) stars in cluster cores can stochastically induce artificial peaks in the surface brightness profiles. The issues related to these bright stars are scrutinized based on both our photometry and simulated clusters. We also examine colour gradients and find that the strongest central colour gradients are caused by central RGB stars and thus not representative for the cluster light or colour distribution. We recover the known relation between the half-light radius and the Galactocentric distance in the g band, but find a lower slope for redder filters. We did not find a correlation between the scatter on this relation and other cluster properties. We find tentative evidence for a correlation between the half-light radii and the [Fe/H], with metal-poor GCs being larger than metal-rich GCs. However, we conclude that this trend is caused by the position of the clusters in the Galaxy, with metal-rich clusters being more centrally located.

Description: We use linear perturbation theory to investigate how a groove in the phase space of a disc galaxy changes the stellar disc's stability properties. Such a groove is a narrow trough around a fixed angular momentum from which most stars have been removed, rendering part of the disc unresponsive to spiral waves. We find that a groove can dramatically alter a disc's eigenmode spectrum by giving rise to a set of vigorously growing eigenmodes. These eigenmodes are particular to the grooved disc and are absent from the original ungrooved disc's mode spectrum. We discuss the properties and possible origin of the different families of new modes. By the very nature of our technique, we prove that a narrow phase-space groove can be a source of rapidly growing spiral patterns that are true eigenmodes of the grooved disc and that no non-linear processes need to be invoked to explain their presence in N -body simulations of disc galaxies. Our results lend support to the idea that spiral structure can be a recurrent phenomenon, in which one generation of spiral modes alters a disc galaxy's phase space in such a way that a following generation of modes is destabilized.

Description: Blue compact dwarf galaxies (BCDs) form stars at, for their sizes, extraordinarily high rates. In this paper, we study what triggers this starburst and what is the fate of the galaxy once its gas fuel is exhausted. We select four BCDs with smooth outer regions, indicating them as possible progenitors of dwarf elliptical galaxies. We have obtained photometric and spectroscopic data with the FORS and ISAAC instruments on the VLT. We analyse their infrared spectra using a full spectrum fitting technique, which yields the kinematics of their stars and ionized gas together with their stellar population characteristics. We find that the stellar velocity to velocity dispersion ratio (( v /) * ) of our BCDs is of the order of 1.5, similar to that of dwarf elliptical galaxies. Thus, those objects do not require significant (if any) loss of angular momentum to fade into early-type dwarfs. This finding is in discordance with previous studies, which however compared the stellar kinematics of dwarf elliptical galaxies with the gaseous kinematics of star-forming dwarfs. The stellar velocity fields of our objects are very disturbed and the star formation regions are often kinematically decoupled from the rest of the galaxy. These regions can be more or less metal rich with respect to the galactic body and sometimes they are long lived. These characteristics prevent us from pinpointing a unique trigger of the star formation, even within the same galaxy. Gas impacts, mergers, and in-spiraling gas clumps are all possible star formation igniters for our targets.

Description: We show that the general relativistic theory of the dynamics of isotropic stellar clusters can be developed essentially along the same lines as the Newtonian theory. We prove that the distribution function can be derived from any isotropic momentum moment and that every higher order moment of the distribution can be written as an integral over a zeroth-order moment. We propose a mathematically simple expression for the distribution function of a family of isotropic general relativistic cluster models and investigate their dynamical properties. In the Newtonian limit, these models obtain a distribution function of the form F ( E ) ( E  –  E 0 ) α , with E binding energy and E 0 a constant that determines the model's outer radius. The slope α sets the steepness of the distribution function and the corresponding radial density and pressure profiles. We show that the field equations only yield solutions with finite mass for α ≤ 3.5. Moreover, in the limit α -〉 3.5, only Newtonian models exist. In other words: within the context of this family of models, no general relativistic version of the Plummer model exists. The most strongly bound model within the family is characterized by α = 2.75 and a central redshift z c 0.55.

Description: We investigate the evolution of dwarf galaxies using N -body/smoothed particle hydrodynamics simulations that incorporate their formation histories through merger trees constructed using the extended Press–Schechter formalism. The simulations are computationally cheap and have high spatial resolution. We compare the properties of galaxies with equal final mass but with different merger histories with each other and with those of observed dwarf spheroidals and irregulars. We show that the merger history influences many observable dwarf galaxy properties. We identify two extreme cases that make this influence stand out most clearly: (i) merger trees with one massive progenitor that grows through relatively few mergers and (ii) merger trees with many small progenitors that merge only quite late. At a fixed halo mass, a type (i) tree tends to produce galaxies with larger stellar masses, larger half-light radii, lower central surface brightness and, since fewer potentially angular momentum cancelling mergers are required to build up the final galaxy, a higher specific angular momentum, compared with a type (ii) tree. We do not perform full-fledged cosmological simulations and therefore cannot hope to reproduce all observed properties of dwarf galaxies. However, we show that the simulated dwarfs are similar to real ones.

Description: Using computer simulations, we explored gaseous infall as a possible explanation for the starburst phase in Blue Compact Dwarf galaxies. We simulate a cloud impact by merging a spherical gas cloud into an isolated dwarf galaxy. We investigated which conditions were favourable for triggering a burst and found that the orbit and the mass of the gas cloud play an important role. We discuss the metallicity, the kinematical properties, the internal dynamics and the gas, stellar and dark matter distribution of the simulations during a starburst. We find that these are in good agreement with observations and depending on the set-up (e.g. rotation of the host galaxy, radius of the gas cloud), our bursting galaxies can have qualitatively very different properties. Our simulations offer insight in how starbursts start and evolve. Based on this, we propose what postburst dwarf galaxies will look like.

Description: Simulating dwarf galaxy haloes in a reionizing Universe puts severe constraints on the subgrid model employed in the simulations. Using the same subgrid model that works for simulations without a UV-background (UVB) results in gas-poor galaxies that stop forming stars very early on, except for haloes with high masses. This is in strong disagreement with observed galaxies, which are gas rich and star forming down to a much lower mass range. To resolve this discrepancy, we ran a large suite of isolated dwarf galaxy simulations to explore a wide variety of subgrid models and parameters, including timing and strength of the UVB, strength of the stellar feedback and metallicity-dependent Pop III feedback. We compared these simulations to observed dwarf galaxies by means of the baryonic Tully–Fisher relation (BTFR), which links the baryonic content of a galaxy to the observationally determined strength of its gravitational potential. We found that the results are robust to changes in the UVB. The strength of the stellar feedback shifts the results on the BTFR, but does not help to form gas-rich galaxies at late redshifts. Only by including Pop III feedback are we able to produce galaxies that lie on the observational BTFR and that have neutral gas and ongoing star formation at redshift zero.

Description: Transition-type dwarf (TTD) galaxies share characteristics of early- and late-type dwarfs. Thus, they are suspected to be the thread that connects them. We selected 19 TTD galaxies in the nearby Universe ( cz  〈 2900 km s –1 ) from the Sloan Digital Sky Survey. They span the luminosity range from ~– 14.5 to –19.0 mag in the B band, and are located in different environments. We derive their single stellar population parameters and star formation histories, using the full spectrum fitting technique with two independent population synthesis models. Irrespective of the synthesis models, we find that these dwarfs have a relatively young mean age (around 1–2 Gyr) and low metallicities (~– 0.7 dex). Moreover, they had approximately constant star formation rates until a few Gyr ago, associated with strong metal enrichment during the first few Gyr of their evolution. We compare these results with the results from Koleva et al., who studied dwarf elliptical (dE) galaxies in the same luminosity range. We find that (1) both samples occupy the same region in the luminosity–metallicity relation, (2) the build-up of the stellar mass in both types of galaxies is very similar, with most of the stars already formed 5 Gyr ago and (3) contrary to the dEs, TTDs are forming stars at present, but after 1 Gyr of passive evolution, their star formation histories would appear identical to that of dEs. As far as the stellar population is concerned, the transformation of TTDs into dEs is definitely possible. A star-forming dwarf galaxy can be stripped of at least a fraction of its gas, and its star formation rate can be reduced to that of the TTDs of the present sample. Continued gas removal may drive a galaxy to the state of a gas-depleted bona fide dE. However, we cannot exclude a scenario where a star-forming galaxy is rapidly transformed into an early type without passing through a noticeable ‘transition’ phase, as suggested by the relatively small fraction of observed dEs with an interstellar medium. We cannot exclude swinging back and forth between a late-type dwarf and a TTD (in the case of episodic star formation) or an early-type dwarf and a TTD (in the case of gas infall).

Description: We present a detailed analysis of the formation, evolution and possible longevity of metallicity gradients in simulated dwarf galaxies. Specifically, we investigate the role of potentially orbit-changing processes such as radial stellar migration and dynamical heating in shaping or destroying these gradients. We also consider the influence of the star formation scheme, investigating both the low-density star formation threshold of 0.1 a.m.u. cm –3 , which has been in general use in the field, and the much higher threshold of 100 cm –3 , which, together with an extension of the cooling curves below 10 4  K and increase of the feedback efficiency, has been argued to represent a much more realistic description of star-forming regions. The N -body–SPH models that we use to self-consistently form and evolve dwarf galaxies in isolation show that, in the absence of significant angular momentum, metallicity gradients are gradually built up during the evolution of the dwarf galaxy, by ever more centrally concentrated star formation adding to the overall gradient. Once formed, they are robust and can easily survive in the absence of external disturbances, with their strength hardly declining over several Gyr, and they agree well with observed metallicity gradients of dwarf galaxies in the Local Group. The underlying orbital displacement of stars is quite limited in our models, being of the order of only fractions of the half-light radius over time-spans of 5–10 Gyr in all star formation schemes. This is contrary to the strong radial migration found in massive disc galaxies, which is caused by scattering of stars off the corotation resonance of large-scale spiral structures. In the dwarf regime, the stellar body only seems to undergo mild dynamical heating, due to the lack of long-lived spiral structures and/or discs. The density threshold, while having profound influences on the star formation mode of the models, has only an minor influence on the evolution of metallicity gradients. Increasing the threshold 1000-fold causes comparatively stronger dynamical heating of the stellar body due to the increased turbulent gas motions and the scattering of stars off dense gas clouds, but the effect remains very limited in absolute terms.