Search

When star clusters are formed at low star-formation rates (SFRs), their stellar initial mass function (IMF) can hardly be filled continuously with stars at each mass. This lack holds for massive stars and is verified observationally by the correlation between star-cluster mass and its most massive cluster star. Since galaxy evolution is strongly affected by massive stars, numerical models should account for this lack. Because a filled IMF is mostly applied even when only fractions of massive stars form, here we investigate, by means of 3D chemo-dynamical simulations of isolated dwarf galaxies, how deviations from a standard IMF in star clusters affect the evolution. We compare two different IMF recipes, a filled IMF and one truncated at a maximum mass at which a single complete star forms. Attention is given to energetic and chemical feedback by massive stars. Since their energy release is mass-dependent but steeper than the negative IMF slope, the energetic feedback retains a positive mass dependence, so that a filled IMF regulates star formation (SF) more strongly than truncated IMFs, though only stellar number fractions exist. The higher SFR of the truncated IMF in the simulation leads to more Type II supernovae (SNeII), driving galactic winds. Whether this results from the model-inherent larger SFR is questioned and therefore explored analytically. This shows the expected result for the Lyman continuum, but that the total SNII energy release is equal for both IMF modes, while the power is smaller for the truncated IMF. Reasonably, the different IMFs leave fingerprints in the abundance ratios of massive to intermediate-mass star elements. [ABSTRACT FROM AUTHOR]

Ram-pressure stripping (RPS) is a well observed phenomenon of massive spiral galaxies passing through the hot intracluster medium (ICM) of galaxy clusters. For dwarf galaxies (DGs) within a cluster, the transformation from gaseous to gas-poor systems by RPS is not easily observed and must happen in the outskirts of clusters. In a few objects in close by galaxy clusters and the field, RPS has been observed. Since cluster early-type DGs also show a large variety of internal structures (unexpected central gas reservoirs, blue stellar cores, composite radial stellar profiles), we aim in this study to investigate how ram pressure (RP) affects the interstellar gas content and therefore the star formation (SF) activity. Using a series of numerical simulations, we quantify the dependence of the stripped-off gas on the velocity of the infalling DGs and on the ambient ICM density. We demonstrated that SF can be either suppressed or triggered by RP depending on the ICM density and the DGs mass. Under some conditions, RP can compress the gas, so that it is unexpectedly retained in the central DG region and forms stars. When gas clouds are still bound against stripping but lifted from a thin disc and fall back, their new stars form an ellipsoidal (young) stellar population already with a larger velocity dispersion without the necessity of harassment. Most spectacularly, star clusters can form downstream in stripped-off massive gas clouds in the case of strong RP. We compare our results to observations. [ABSTRACT FROM AUTHOR]

We determine abundance ratios of 37 dwarf ellipticals (dEs) in the nearby Virgo cluster. This sample is representative of the early-type population of galaxies in the absolute magnitude range -19.0 < Mr < -16.0. We analyse their absorption line-strength indices by means of index-index diagrams and scaling relations and use the stellar population models to interpret them. We present ages, metallicities, and abundance ratios obtained from these dEs within an aperture size of Re/8.We calculate [Na/Fe] from NaD, [Ca/Fe] from Ca4227, and [Mg/Fe] from Mgb. We find that [Na/Fe] is underabundant with respect to solar, whereas [Mg/Fe] is around solar. This is exactly opposite to what is found for giant ellipticals, but follows the trend with metallicity found previously for the Fornax dwarf NGC 1396. We discuss possible formation scenarios that can result in such elemental abundance patterns, and we speculate that dEs have disc-like star formation history (SFH) favouring them to originate from late-type dwarfs or small spirals. Na-yields appear to be very metal-dependent, in agreement with studies of giant ellipticals, probably due to the large dependence on the neutron-excess in stars. We conclude that dEs have undergone a considerable amount of chemical evolution, they are therefore not uniformly old, but have extended SFH, similar to many of the Local Group galaxies. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Photoionizationmodelling allows us to follow the transport, the emergence, and the absorption of photons taking into account all important processes in nebular plasmas. Such modelling needs the spatial distribution of density, chemical abundances, and temperature, that can be provided by chemodynamical simulations (ChDS) of dwarf galaxies. We perform multicomponent photoionization modelling of the ionized gas using 2D ChDSs of dwarf galaxies. We calculate emissivity maps for important nebular emission lines. Their intensities are used to derive the chemical abundance of oxygen by the so-called Te- and R23-methods. Some disagreements are found between oxygen abundances calculated with these methods and the ones coming from the ChDSs. We investigate the fraction of ionizing radiation emitted in the starforming region which is able to leak out the galaxy. The time- and direction-averaged escape fraction in our simulation is 0.35-0.4. Finally, we have calculated the total Hαluminosity of our model galaxy using Kennicutt's calibration to derive the star formation rate. This value has been compared to the 'true' rate in the ChDSs. The Hα-based star formation rate agrees with the true one only at the beginning of the simulation. Minor deviations arise later on and are due in part to the production of high-energy photons in the warm-hot gas, in part to the leakage of energetic photons out of the galaxy. The effect of artificially introduced thin dense shells (with thicknesses smaller than the ChDSs spatial resolution) is investigated, as well. [ABSTRACT FROM AUTHOR]

The fraction of star-forming to quiescent dwarf galaxies varies from almost infinity in the field to zero in the centers of rich galaxy clusters. What is causing this pronounced morphology-density relation? What do quiescent dwarf galaxies look like when studied in detail, and what conclusions can be drawn about their formation mechanism? Here we study a nearly magnitude-complete sample (–19 < Mr < –16 mag) of 121 Virgo cluster early types with deep near-infrared images from the SMAKCED project. We fit two-dimensional models with optional inner and outer components, as well as bar and lens components (in ∼15% of the galaxies), to the galaxy images. While a single Sérsic function may approximate the overall galaxy structure, it does not entirely capture the light distribution of two-thirds of our galaxies, for which multicomponent models provide a better fit. This fraction of complex galaxies shows a strong dependence on luminosity, being larger for brighter objects. We analyze the global and component-specific photometric scaling relations of early-type dwarf galaxies and discuss similarities with bright early and late types. The dwarfs' global galaxy parameters show scaling relations that are similar to those of bright disk galaxies. The inner components are mostly fitted with Sérsic n values close to 1. At a given magnitude, they are systematically larger than the bulges of spirals, suggesting that they are not ordinary bulges. We argue that the multicomponent structures in early-type dwarfs are mostly a phenomenon inherent to the disks and may indeed stem from environmental processing. [ABSTRACT FROM AUTHOR]

Context. Energetic feedback from supernovae and stellar winds can drive galactic winds. Dwarf galaxies, due to their shallower potential wells, are assumed to be more vulnerable to this phenomenon. Metal loss through galactic winds is also commonly invoked to explain the low metal content of dwarf galaxies. Aims. Our main aim in this paper is to show that galactic mass cannot be the only parameter determining the fraction of metals lost by a galaxy. In particular, the distribution of gas must play an equally important role. Methods. We performed 2D chemo-dynamical simulations of galaxies characterized by different gas distributions, masses, and gas fractions. Results. The gas distribution can change the fraction of lost metals through galactic winds by up to one order of magnitude. In particular, disk-like galaxies tend to lose metals more easily than roundish ones. Consequently, the final metallicities attained by models with the same mass but with different gas distributions can also vary by up to one dex. Confirming previous studies, we also show that the fate of gas and freshly produced metals strongly depends on the mass of the galaxy. Smaller galaxies (with shallower potential wells) more easily develop large-scale outflows, so that the fraction of lost metals tends to be higher. [ABSTRACT FROM AUTHOR]

ABSTRACT Several scenarios have been suggested to explain the phase-space distribution of the Milky Way (MW) satellite galaxies in a disc of satellites (DoS). To quantitatively compare these different possibilities, a new method analysing angular momentum directions in modelled data is presented. It determines how likely it is to find sets of angular momenta as concentrated and as close to a polar orientation as is observed for the MW satellite orbital poles. The method can be easily applied to orbital pole data from different models. The observed distribution of satellite orbital poles is compared to published angular momentum directions of subhaloes derived from six cosmological state-of-the-art simulations in the Aquarius project. This tests the possibility that filamentary accretion might be able to naturally explain the satellite orbits within the DoS. For the most likely alignment of main halo and MW disc spin, the probability to reproduce the MW satellite orbital pole properties turns out to be less than 0.5 per cent in the Aquarius models. Even an isotropic distribution of angular momenta has a higher likelihood to produce the observed distribution. The two Via Lactea cosmological simulations give results similar to the Aquarius simulations. Comparing instead with numerical models of galaxy interactions gives a probability of up to 90 per cent for some models to draw the observed distribution of orbital poles from the angular momenta of tidal debris. This indicates that the formation as tidal dwarf galaxies in a single encounter is a viable, if not the only, process to explain the phase-space distribution of the MW satellite galaxies. [ABSTRACT FROM AUTHOR]

Context. We constructed a series of model galaxies in rotational equilibrium consisting of gas, stars, and a fixed dark matter (DM) halo and studied how these equilibrium systems depend on the mass and form of the DM halo, gas temperature, non-thermal and rotation support against gravity, and also on the redshift of galaxy formation. For every model galaxy we found the minimum gas mass Mgmin required to achieve a state in which star formation (SF) is allowed according to contemporary SF criteria. The obtained Mgmin-MDM relations were compared against the baryon-to-DM mass relation Mb-MDM inferred from the λCDM theory and WMAP4 data. Aims. Our aim is to construct realistic initial models of dwarf galaxies (DGs), which take into account the gas self-gravity and can be used as a basis to study the dynamical and chemical evolution of DGs. Methods. Rotating equilibria are found by solving numerically the steady-state momentum equation for the gas component in the combined gravitational potential of gas, stars, and DM halo using a forward substitution procedure. Results. We find that for a given MDM the value of Mgmin depends crucially on the gas temperature Tg, gas spin parameter α, degree of non-thermal support σeff, and somewhat on the redshift for galaxy formation zgf. Depending on the actual values of Tg, α, σeff, and zgf, model galaxies may have Mgmin that are either greater or smaller than Mb. Galaxies with MDM ≳ 109 M⊙ are usually characterized by Mgmin ≤ Mb, implying that SF in such objects is a natural outcome because the required gas mass is consistent with what is available according to the λCDM theory. On the other hand, models with MDM ≤ 109 M⊙ are often characterized by Mgmin » Mb, implying that they need much more gas than available to achieve a state in which SF is allowed. Conclusions. Our modeling suggests that a star-formation-allowed state is more difficult to achieve in DM halos with mass ≤ 109 M⊙ than in their upper-mass counterparts, because the required gas mass often exceeds both Mb and MDM. In the framework of the λCDM theory, this implies the existence of a critical DM halo mass below which the likelihood of star formation and hence the total stellar mass may drop substantially, in accordance with the stellar versus DM halo mass relations recently derived from the SDSS survey and millennium simulations. On the other hand, DGs that do not follow the λCDM trend are feasible and have recently been identified, which raises questions about the universality of the λCDM paradigm. [ABSTRACT FROM AUTHOR]

Dwarf galaxies (DGs) serve as extremely challenging objects in extragalactic astrophysics. Their origin is expected to be set as the first units in CDM cosmology. Nevertheless they are the galaxy type most sensitive to environmental influences and their division into multiple types with various properties have invoked the picture of their variant morphological transformations. Detailed observations reveal characteristics which allow to deduce the evolutionary paths and to witness how the environment has affected the evolution. Here we refer to general morphological DG types and review some general processes, most of which deplete gas-rich irregular DGs. Moreover, the variety of pecularities is briefly refered, but cannot be comprehensively analyzed because of limited paper space. [ABSTRACT FROM AUTHOR]

Here we investigate an exemplary chemodynamical evolutionary simulation of a dwarf irregular galaxy. By means of this model we demonstrate the existence of three gas mixing cycles: 1) An inner local cycle mixing the metals produced in stars locally, and 2) an outer galactic cycle on which hot gas is driven out of the galaxy by multiple supernovae type II and mixes on a short timescale with the available cold gas. 3) Only a small fraction of the metals leaves the galactic gravitational field and follows the global cycle with the intergalactic matter. The large-scale mixing results in a temporary depletion of supernova ejected metals. We will discuss this delayed recycling and its influence on the chemical evolution, especially on the nitrogen over oxygen ratio which is increased temporarily. The results presented here are also relevant for less sophisticated analytical approaches and chemical evolutionary models of galaxies which have to parameterize the metal loss through outflow. [ABSTRACT FROM AUTHOR]

Within the Cold Dark Matter scenario the hierarchical merging paradigm is the natural result to form massive galactic halos by the minor mergers of sub-halos and, by this, inherently their stellar halo. Although this must be also invoked for the Milky Way, the context of chemical and kinematic coherence of halo stars and dwarf spheroidal galaxies is yet unsolved a focus of present-day research. To examine this issue we model the chemo-dynamical evolution of the system of satellites selected from the cosmological Via Lactea II simulations to be similar for the Milky Way environment but at an early epoch. [ABSTRACT FROM AUTHOR]

We analyze the stellar kinematics of 39 dwarf early-type galaxies (dEs) in the Virgo Cluster. Based on the specific stellar angular momentum λRe and the ellipticity, we find 11 slow rotators and 28 fast rotators. The fast rotators in the outer parts of the Virgo Cluster rotate significantly faster than fast rotators in the inner parts of the cluster. Moreover, 10 out of the 11 slow rotators are located in the inner 3° (D < 1 Mpc) of the cluster. The fast rotators contain subtle disk-like structures that are visible in high-pass filtered optical images, while the slow rotators do not exhibit these structures. In addition, two of the dEs have kinematically decoupled cores and four more have emission partially filling in the Balmer absorption lines. These properties suggest that Virgo Cluster dEs may have originated from late-type star-forming galaxies that were transformed by the environment after their infall into the cluster. The correlation between λRe and the clustercentric distance can be explained by a scenario where low luminosity star-forming galaxies fall into the cluster, their gas is rapidly removed by ram-pressure stripping, although some of it can be retained in their core, their star formation is quenched but their stellar kinematics are preserved. After a long time in the cluster and several passes through its center, the galaxies are heated up and transformed into slow rotating dEs. [ABSTRACT FROM AUTHOR]