Determination of the dynamical structure of galaxies using optical spectra

Galaxy spectra are a rich source of kinematical information since the shapes of the absorption lines reflect the movement of stars along the line-of-sight. We present a technique to directly build a dynamical model for a galaxy by fitting model spectra, calculated from a dynamical model, to the observed galaxy spectra. Using synthetic spectra from a known galaxy model we demonstrate that this technique indeed recovers the essential dynamical characteristics of the galaxy model. Moreover, the method allows a statistically meaningful error analysis on the resulting dynamical quantities.Comment: 14 pages, 14 figures, Latexfile, MNRAS, in pres

Spiral eigenmodes triggered by grooves in the phase space of disc galaxies

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.Comment: 18 pages, 17 figures, accepted for publication in MNRA

FCC046: a candidate gaseous polar ring dwarf elliptical galaxy in the Fornax Cluster

FCC046 is a Fornax Cluster dwarf elliptical galaxy. Optical observations have shown that this galaxy, besides an old and metal-poor stellar population, also contains a very young centrally concentrated population and is actively forming stars, albeit at a very low level. Here, we report on 21cm observations of FCC046 with the Australia Telescope Compact Array (ATCA) which we conducted in the course of a small survey of Fornax Cluster early-type dwarf galaxies. We have discovered a ~10^7 Mo HI cloud surrounding FCC046. We show that the presence of this significant gas reservoir offers a concise explanation for this galaxy's optical morphological and kinematical properties. Surprisingly, the HI gas, as evidenced by its morphology and its rotational motion around the galaxy's optical major axis, is kinematically decoupled from the galaxy's stellar body. This is the first time such a ring of gaseous material in minor-axis rotation is discovered around a dwarf galaxy.Comment: 5 pages, 4 figures, published in Astrophysical Journal Letter

How the first stars shaped the faintest gas-dominated dwarf galaxies

Low-mass dwarf galaxies are very sensitive test-beds for theories of cosmic structure formation since their weak gravitational fields allow the effects of the relevant physical processes to clearly stand out. Up to now, no unified account exists of the sometimes seemingly conflicting properties of the faintest isolated dwarfs in and around the Local Group, such as Leo T and the recently discovered Leo P and Pisces A systems. Using new numerical simulations, we show that this serious challenge to our understanding of galaxy formation can be effectively resolved by taking into account the regulating influence of the ultraviolet radiation of the first population of stars on a dwarf's star formation rate while otherwise staying within the standard cosmological paradigm for structure formation. These simulations produce faint, gas-dominated, star-forming dwarf galaxies that lie on the baryonic Tully-Fisher relation and that successfully reproduce a broad range of chemical, kinematical, and structural observables of real late-type dwarf galaxies. Furthermore, we stress the importance of obtaining properties of simulated galaxies in a manner as close as possible to the typically employed observational techniques.Comment: 13 pages, 2 tables, 12 figures. Accepted for publication in Ap

Dwarf Galaxies in Clusters as Probes of Galaxy Formation and Dark Matter

We present the results of a Hubble Space Telescope (HST) ACS and WFPC2 study of dwarf galaxies in the nearby Perseus Cluster, down to M_V = -12, spanning the core and outer regions of this cluster. We examine how properties such as the colour magnitude relation, structure and morphology are affected by environment for the lowest mass galaxies. The low masses of dwarf galaxies allow us to determine their environmentally driven based galaxy evolution, the effects of which are harder to examine in massive galaxies. The structures of our dwarfs in both the core and outer regions of the cluster are quantified using the concentration, asymmetry and clumpiness (CAS) parameters. We find that, on average, dwarfs in the outer regions of Perseus are more disturbed than those in the cluster core, with higher asymmetries and clumpier light distributions. We measure the (V-I)_0 colours of the dEs, and find that dwarfs in both the inner and outer regions of the cluster lie on the same colour magnitude relation. Based on these results, we infer that the disturbed dwarfs in the cluster outskirts are likely "transition dwarfs", with their colours transforming before their structures. Finally, we infer from the smoothness of the cluster core population that dwarfs in the inner regions of the cluster must be highly dark matter dominated to prevent their disruption by the cluster potential. We derive a new method to determine the minimum mass the dwarfs must have to prevent this disruption without the need for resolved spectroscopy, and determine their mass-to-light ratios. At their orbit pericentre, dwarfs in the core of Perseus require mass-to-light ratios between 1 and 120 to prevent their disruption, comparable to those found for the Local Group dSphs.Comment: 6 pages, 5 figures. To appear in the proceedings of "A Universe of dwarf galaxies" (Lyon, June 14-18 2010

Constraining the subgrid physics in simulations of isolated dwarf galaxies

Simulating dwarf galaxy halos in a reionizing Universe puts severe constraints on the sub-grid model employed in the simulations. Using the same sub-grid model that works for simulations without a UV-background (UVB) results in gas poor galaxies that stop forming stars very early on, except for halos 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 sub-grid 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.Comment: Accepted for publication in MNRAS. 25 pages, 2 tables and 36 figures. Interactive plots can be found on http://www.dwarfs.ugent.be/btfr

Simulations of the formation and evolution of isolated dwarf galaxies

We present new fully self-consistent models of the formation and evolution of isolated dwarf galaxies. We have used the publicly available N-body/SPH code HYDRA, to which we have added a set of star formation criteria, and prescriptions for chemical enrichment (taking into account contributions from both SNIa and SNII), supernova feedback, and gas cooling. The models follow the evolution of an initially homogeneous gas cloud collapsing in a pre-existing dark-matter halo. These simplified initial conditions are supported by the merger trees of isolated dwarf galaxies extracted from the milli-Millennium Simulation. The star-formation histories of the model galaxies exhibit burst-like behaviour. These bursts are a consequence of the blow-out and subsequent in-fall of gas. The amount of gas that leaves the galaxy for good is found to be small, in absolute numbers, ranging between 3x10^7 Msol and 6x10^7 Msol . For the least massive models, however, this is over 80 per cent of their initial gas mass. The local fluctuations in gas density are strong enough to trigger star-bursts in the massive models, or to inhibit anything more than small residual star formation for the less massive models. Between these star-bursts there can be time intervals of several Gyrs. We have compared model predictions with available data for the relations between luminosity and surface brightness profile, half-light radius, central velocity dispersion, broad band colour (B-V) and metallicity, as well as the location relative to the fundamental plane. The properties of the model dwarf galaxies agree quite well with those of observed dwarf galaxies.Comment: 16 pages, 20 figures, accepted for publication in MNRA

The dynamics of general relativistic isotropic stellar cluster models -- Do relativistic extensions of the Plummer model exist?

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)^alpha, with E binding energy and E_0 a constant that determines the model's outer radius. The slope alpha 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 alpha3.5, 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 alpha=2.75 and a central redshift z_c~0.55.Comment: 10 pages, 5 figures, accepted for publication by MNRA

A genetic algorithm for the non-parametric inversion of strong lensing systems

We present a non-parametric technique to infer the projected-mass distribution of a gravitational lens system with multiple strong-lensed images. The technique involves a dynamic grid in the lens plane on which the mass distribution of the lens is approximated by a sum of basis functions, one per grid cell. We used the projected mass densities of Plummer spheres as basis functions. A genetic algorithm then determines the mass distribution of the lens by forcing images of a single source, projected back onto the source plane, to coincide as well as possible. Averaging several tens of solutions removes the random fluctuations that are introduced by the reproduction process of genomes in the genetic algorithm and highlights those features common to all solutions. Given the positions of the images and the redshifts of the sources and the lens, we show that the mass of a gravitational lens can be retrieved with an accuracy of a few percent and that, if the sources sufficiently cover the caustics, the mass distribution of the gravitational lens can also be reliably retrieved. A major advantage of the algorithm is that it makes full use of the information contained in the radial images, unlike methods that minimise the residuals of the lens equation, and is thus able to accurately reconstruct also the inner parts of the lens.Comment: 11 pages, accepted for publication by MNRA

N-body/SPH study of the evolution of dwarf galaxies in a cluster environment

Using an N-body/SPH code, we explore the scenario in which a dwarf elliptical galaxy (dE) is subjected to ram-pressure stripping due to the intracluster medium (ICM). Our simulations show that while (i) smaller dEs lose their ISM almost immediately after entering the cluster, (ii) more massive dEs are able to retain their gas for considerable timespans.Comment: 1 page, no figures, poster contribution to the Splinter Meeting "Galaxies in interaction" at the joint meeting of the Czech Astronomical Society and the Astronomische Gesellschaft (20-25 Sept. 2004, Prague, Czech Republic