Using 3-dimensional hydrodynamical simulations of isolated dwarf spheroidal galaxies (dSphs), we undertake an analysis of the chemical properties of their inner regions, identifying the respective roles played by Type Ia (SNe Ia) and Type II (SNe II) supernovae. The effect of inhomogeneous pollution from SNe Ia is shown to be prominent within two core radii, with the stars forming therein amounting to ~20% of the total. These stars are relatively iron-rich and alpha-element-depleted compared to the stars forming in the rest of the galaxy. At odds with the projected stellar velocity dispersion radial profile, the actual 3-dimensional one shows a depression in the central region, where the most metal-rich (ie. [Fe/H]-rich) stars are partly segregated. This naturally results in two different stellar populations, with an anti-correlation between [Fe/H] and velocity dispersion, in the same sense as that observed in the Sculptor and Fornax dSphs. Because the most iron-rich stars in our model are also the most alpha-depleted, a natural prediction and test of our model is that the same radial segregation effects should exist between [alpha/Fe] and velocity dispersion.
The very inner structure of massive YSOs is difficult to trace. With conventional observational methods we often identify structures still several hundreds of AU in size. But we also need information about the innermost regions where the actual mass transfer onto the forming high-mass star occurs. An innovative way to probe these scales is to utilise mid-infrared interferometry. Here, we present first results of our MIDI GTO programme at the VLTI. We observed 10 well-known massive YSOs down to scales of 20 mas. We clearly resolve these objects which results in low visibilities and sizes in the…
A Bayesian likelihood analysis of the stellar density profile shapes of several dark matter dominated Galactic dwarf spheroidal galaxies (dSphs) has been completed. This analysis indicates that the inner power law, ρ\propto r^{-γ}, of each of these profiles satisfies γ< 1.21 at greater than 99% confidence with a slight preference for γ\sim 0.7 . Speculating that this bound on the stellar profiles may be related to the inner power law of the underlying dark matter distribution reveals a statistical discrepancy with the values favoured by recent N-body simulations of cold dark matter (CDM), γ_{C…
The possibility that the Sagittarius dwarf spheroidal might have formed as a Searle-Zinn fragment in the outer halo of the Galaxy is discussed. Arguments in favor of this hypothesis are: (1) The luminosity distribution of globular clusters in both Sagittarius and in the outer halo (Rgc > 80 kpc), appear to be bimodal with peaks near MV ~ -5 and MV ~ -10, and (2) the globular clusters in both Sgr, and in the outer halo, have a significantly larger age spread than do the globulars in the inner halo of the Galaxy. However, a counter argument is that only one of the four globulars associated with …
Most of the globular clusters in the main body of the Galactic halo were formed almost simultaneously. However, globular cluster formation in dwarf spheroidal galaxies appears to have extended over a significant fraction of a Hubble time. This suggests that the factors which suppressed late-time formation of globulars in the main body of the Galactic halo were not operative in dwarf spheroidal galaxies. Possibly the presence of significant numbers of ``young'' globulars at R_{GC} > 15 kpc can be accounted for by the assumption that many of these objects were formed in Sagittarius-like (but not…
We propose a simple model for the formation of dwarf spheroidal galaxies, in which stars are assumed to have formed from isothermal gas in hydrostatic equilibrium inside extended dark matter halos. After expelling the leftover gas, the stellar system undergoes a dynamical relaxation inside the dark matter halo. These models can adequately describe the observed properties of three (Draco, Sculptor, and Carina) out of four Galactic dwarf spheroidal satellites studied in this paper. We suggest that the fourth galaxy (Fornax), which cannot be fitted well with our model, is observed all the way to …
The dark matter dominated Fornax dwarf spheroidal has five globular clusters orbiting at ~1 kpc from its centre. In a cuspy CDM halo the globulars would sink to the centre from their current positions within a few Gyrs, presenting a puzzle as to why they survive undigested at the present epoch. We show that a solution to this timing problem is to adopt a cored dark matter halo. We use numerical simulations and analytic calculations to show that, under these conditions, the sinking time becomes many Hubble times; the globulars effectively stall at the dark matter core radius. We conclude that t…
We present a detailed mid-infrared study of the nearby, face-on spiral galaxy M83 based on ISOCAM data. M83 is a unique case study, since a wide variety of MIR broad-band filters as well as spectra, covering the wavelength range of 4 to 18μm, were observed and are presented here. Emission maxima trace the nuclear and bulge area, star-formation regions at the end of the bar, as well as the inner spiral arms. The fainter outer spiral arms and interarm regions are also evident in the MIR map. Spectral imaging of the central 3'x3' (4 kpc x 4 kpc) field allows us to investigate five regions of diff…
Color-magnitude diagrams from deep, wide-field CCD photometry of the Sculptor dwarf spheroidal galaxy reveal that the red horizonal branch (RHB) stars are strongly concentrated towards the center of the galaxy relative to the dominant old population in Sculptor, confirming an earlier claim of such a gradient (Da Costa et al. 1996). Since we find no radial gradients of the age or metallicity distribution within Sculptor, neither age nor metallicity can individually account for the internal `second parameter' problem observed within the galaxy. We have also identified an unusual `spur' of stars …
We have used UVES on VLT-UT2 to take spectra of 15 individual red giant stars in the centers of four nearby dwarf spheroidal galaxies: Sculptor, Fornax, Carina and Leo I. We measure the abundance variations of numerous elements in these low mass stars with a range of ages (1-15Gyr old). This means that we can effectively measure the chemical evolution of these galaxies WITH TIME. Our results show a significant spread in metallicity with age, but an overall trend consistent with what might be expected from a closed (or perhaps leaky) box chemical evolution scenario over the last 10-15Gyr. We no…
We describe the first results from two observational projects aimed at measuring the amount and spatial distribution of dark matter in distant early-type galaxies (E/S0s) and clusters of galaxies. At the galaxy scale, the Lenses Structure and Dynamics (LSD) Survey is gathering kinematic data for distant (up to $z\sim1$) E/S0s that are gravitational lenses. A joint lensing and dynamical analysis constrains the fraction of dark matter within the Einstein radius, the mass-to-light ratio of the stellar component, and the total slope of the mass density profile. These properties and their evolution…
It is now established that several of the Local Group dwarf Spheroidal galaxies (dSphs) have large mass-to-light ratios. We consider the possibility that the dark matter in the halos of dSphs is composed of massive black holes with masses in the range 10^5 to 10^7 solar masses. We use direct N-body simulations to determine the long term evolution of a two-component dSph composed of a pressure-supported stellar population in a black hole halo. The black holes are initially distributed according to a Navarro, Frenk & White profile. For black hole masses between 10^5 and 10^6 solar masses, the da…
We present (V,I) photometry of two wide (~25 X 25 arcmin^2) fields centered on the low surface brightness dwarf spheroidal galaxies Draco and Ursa Minor. New estimates of the distance to these galaxies are provided ((m-M)_0(UMi)=19.41 \pm 0.12 and (m-M)_0(Dra)=19.84 \pm 0.14) and a comparative study of their evolved stellar population is presented. We detect for the first time the RGB-bump in the Luminosity Function of UMi (V_{RGB}^{Bump}=19.40\pm 0.06) while the feature is not detected in Draco. Photometric metallicity distributions are obtained for the two galaxies and an accurate analysis t…
Several recent studies find that 10 - 50% of morphologically selected field early-type galaxies at redshifts z <= 1 have blue colors indicative of recent star formation. Such ``blue spheroids''might be massive early-type galaxies with active star formation, perhaps induced by recent merger events. Alternatively, they could be starbursting, low-mass spheroids. To distinguish between these two choices, we have selected 10 ``Blue Spheroid Candidates'' (hereafter, BSCs) from a quantitatively selected E/S0 sample and studied their properties, including kinematics from Keck spectra obtained as part …
JHK images obtained with the Canada-France-Hawaii Telescope (CFHT) Adaptive Optics Bonnette (AOB) are used to investigate the near-infrared photometric properties of red giant branch (RGB) and horizontal branch (HB) stars in eight metal-poor globular clusters with R_GC < 2 kpc. The slope of the RGB on the (K, J-K) CMDs confirms the metal-poor nature of these clusters, four of which appear to have metallicities comparable to M92. The luminosity functions of RGB stars in inner spheroid and outer halo clusters have similar slopes, although there is a tendency for core-collapsed clusters to have s…
In this work we build a detailed dynamic model for a S0 galaxy possibly hosting a central massive dark object (MDO). We show that the photometric profiles and the kinematics along the major and minor axes, including the h3 and h4 profiles, imply the presence of a central MDO of mass M = 1.5 - 9.7 10^8 solar masses, i.e. 0.3-2.8% of the mass derived for the stellar spheroidal component. Models without MDO are unable to reproduce the kinematic properties of the inner stars and of the rapidly rotating nuclear gas. The stellar population comprise of an exponential disc (27% of the light) and a dif…
I review photo-polarimetric and spectropolarimetric observations of V838 Mon, which revealed that it had an asymmetrical inner circumstellar envelope following its 2nd photometric outburst. Electron scattering, modified by pre- or post-scattering H absorption, is the polarizing mechanism in V838 Mon's envelope. The simplest geometry implied by these observations is that of a spheroidal shell, flattened by at least 10% and having a projected position angle on the sky of $\sim37^{\circ}$. Analysis of V838 Mon's polarized flux reveals that this electron scattering shell lies interior to the envel…
In this paper, we study the dynamical stability and time evolution of the central dark matter cores of low-mass (about 10^8-10^9 M_solar) galactic haloes found in recent cold dark matter simulations at high redshift. From those simulations we extract three haloes, assembled by hierarchical merging, that at redshift z>10 display a core and we evolve them without further merging to low redshift using direct N-body integration. The central core in the dark matter profile is found to be dynamically stable: it survives for many crossing times without evolution into a cusp. This result supports the …
We have obtained deep BVI CCD photometry of Phoenix, a galaxy considered a transition case between dwarf spheroidal and dwarf irregular galaxies. A comparison of our data with the RGBs of Galactic globular clusters gives a mean metal abundance [Fe/H] = -1.81+/-0.10 dex. The presence of an intrinsic color dispersion in the upper red RGB suggests an abundance range of about 0.5 dex, although a range in age may also affect the RGB width. For the first time, a HB has been revealed at V~23.8. The HB is predominantly red yet moderately extended to the blue, which indicates the presence of a signific…
Recent observations have revealed that high surface-brightness, metal-rich debris is present over large regions of the Andromeda (M31) stellar halo. We use a set of numerical models to determine whether extended metal-rich debris is expected to exist in galaxy halos formed in a hierarchical LambdaCDM universe. We identify tidal debris in the simulations according to the current best surface brightness detection limits in M31 and demonstrate that bright features in the outer halo regions usually originate from single satellites, with very little contamination from other sources due to the low p…
We present the discovery of a kinematically-cold stellar population along the SE minor axis of the Andromeda galaxy (M31) that is likely the forward continuation of M31's giant southern stream. This discovery was made in the course of an on-going spectroscopic survey of red giant branch (RGB) stars in M31 using the DEIMOS instrument on the Keck II 10-m telescope. Stellar kinematics are investigated in eight fields located 9-30 kpc from M31's center (in projection). A likelihood method based on photometric and spectroscopic diagnostics is used to isolate confirmed M31 RGB stars from foreground …
Kinematical decoupling between two components of a galaxy suggests the occurrence of a second event. In disk galaxies it is usually observed as a counterrotation of the stellar disk with respect to the gaseous component and/or with respect to a second stellar disk. We discuss here three cases of kinematical orthogonal decoupling of the innermost region of the spheroidal component with respect to the disk (or ring) component in the two Sa spirals NGC 4698 and NGC 4672, and in the elliptical galaxy with a polar ring AM 2020-504. Moreover the major axis of the disk (or ring) component forms an an…
In a previous paper we have proposed a non--Newtonian, phenomenological description of the effective gravitational force acting between exotic dark matter and baryons, which was shown to fit well the kinematics in the inner regions of low surface brightness, gas--rich galaxies. The Gravitational Suppression (GraS) scheme is, admittedly, an ad-hoc model; it does however successfully address interesting cosmological issues. Here, we test the potential generality of the GraS concept. GraS predictions on motions on scales measurable in dwarf spheroidal galaxies and in the solar neighborhood show s…
We probe the evolution of globular clusters that could form in giant molecular clouds within high-redshift galaxies. Numerical simulations demonstrate that the large and dense enough gas clouds assemble naturally in current hierarchical models of galaxy formation. These clouds are enriched with heavy elements from earlier stars and could produce star clusters in a similar way to nearby molecular clouds. The masses and sizes of the model clusters are in excellent agreement with the observations of young massive clusters. Do these model clusters evolve into globular clusters that we see in our a…
We fit axisymmetric 3-integral dynamical models to NGC3379 using the line-of-sight velocity distribution obtained from HST/FOS spectra of the galaxy center and ground-based long-slit spectroscopy along four position angles, with the light distribution constrained by WFPC2 and ground-based images. We have fitted models with inclinations from 29 (intrinsic galaxy type E5) to 90 degrees (intrinsic E1) and black hole masses from 0 to 1e9 M_solar. The best-fit black hole masses range from 6e7 to 2e8 M_solar, depending on inclination. The velocity ellipsoid of the best model is not consistent with e…
