Local Group dSphs Research Articles

The Local Group Dwarf Spheroidal Galaxies: A Key to Building Blocks in the Universe

Recent studies have been revealing the properties of dwarf spheroidal galaxies (dSphs). Their low mass indicates that the dSphs may provide a clue to physical properties of the building blocks in the hierarchical structure formation. We select the Local Group dSphs as a sample. To obtain the information on the star formation history of dSphs, we investigate the relation between their metallicity and virial mass. According to our scenario, the star formation efficiency of the dSphs is low because of strong regulation. This is consistent with their high mass-to-light ratios. We also comment on the environmental effects on the dSphs.

Mass loss from dwarf spheroidal galaxies: the origins of shallow dark matter cores and exponential surface brightness profiles

Dwarf spheroidal galaxies have shallow central dark matter density profiles, low angular momentum and approximately exponential surface brightness distributions. Through N-body simulations and analytic calculations we investigate the extent to which these properties can be generated from ``typical'' $\Lambda$CDM galaxies, which differ in all of these properties, by the dynamical consequences of feedback. We find that, for a wide range of initial conditions, one impulsive mass loss event will naturally produce a surface brightness profile in the remaining stellar component of a dwarf spheroidal galaxy (dSph) which is well fit over many scale lengths by an exponential, in good qualitative agreement with observations of Local Group dSphs. Furthermore, two impulsive mass loss phases, punctuated by significant gas re-accretion, are found to be sufficient to transform a central density cusp in the dark matter profile into a near-constant density core. This may then provide the missing link between current cosmological simulations, which predict a central cusp in the dark matter density profile, and current observations, which find much shallower central density profiles. We also look at the angular momentum history of dSphs and demonstrate that if these galaxies have spent most of their lifetime in tidal isolation from massive galaxies then they cannot have formed from high angular momentum gas discs.

HI Observations of Nearby Dwarf Galaxies

We present HI detections towards several Local Group dSphs and dIrr/dSphs. The possibility that the detected emission can be due to High Velocity Clouds (HVC) is ruled out although it appears that most of our targets are in HVC rich regions.

The Metamorphosis of Tidally Stirred Dwarf Galaxies

We present results from high-resolution N-body/SPH (smoothed particle hydrodynamic) simulations of rotationally supported dwarf irregular galaxies moving on bound orbits in the massive dark matter halo of the Milky Way. The dwarf models span a range in disk surface density and the masses and sizes of their dark halos are consistent with the predictions of cold dark matter cosmogonies. We show that the strong tidal field of the Milky Way determines severe mass loss in their halos and disks and induces bar and bending instabilities that transform low surface brightness dwarfs (LSBs) into dwarf spheroidals (dSphs) and high surface brightness dwarfs (HSBs) into dwarf ellipticals (dEs) in less than 10 Gyr. The final central velocity dispersions of the remnants are in the range 8-30 km s-1 and their final v/σ falls to values less than 0.5, matching well the kinematics of early-type dwarfs. The transformation requires the orbital time of the dwarf to be ≲3-4 Gyr, which implies a halo as massive and extended as predicted by hierarchical models of galaxy formation to explain the origin of even the farthest dSph satellites of the Milky Way, Leo I, and Leo II. We show that only dwarfs with central dark matter densities as high as those of Draco and Ursa Minor can survive for 10 Gyr in the proximity of the Milky Way. A correlation between the central density and the distance of the dwarfs from the primary galaxy is indeed expected in hierarchical models, in which the densest objects should have small orbital times because of their early formation epochs. Part of the gas is stripped and part is funneled to the center because of the bar, generating one strong burst of star formation in HSBs and smaller, multiple bursts in LSBs. Therefore, the large variety of star formation histories observed in Local Group dSphs arises because different types of dIrr progenitors respond differently to the external perturbation of the Milky Way. Our evolutionary model naturally explains the morphology-density relation observed in the Local Group and in other nearby loose groups. Extended low surface brightness stellar and gaseous streams originate from LSBs and follow the orbit of the dwarfs for several gigayears. Because of their high velocities, unbound stars projected along the line of sight can lead to overestimating the mass-to-light ratio of the bound remnant by a factor ≲2, but this does not eliminate the need of extremely high dark matter contents in some of the dSphs.

Tidal Stirring and the Origin of Dwarf Spheroidals in the Local Group

N-Body/SPH simulations are used to study the evolution of dwarf irregular galaxies (dIrrs) entering the dark matter halo of the Milky Way or M31 on plunging orbits. We propose a new dynamical mechanism driving the evolution of gas rich, rotationally supported dIrrs, mostly found at the outskirts of the Local Group (LG), into gas free, pressure supported dwarf spheroidals (dSphs) or dwarf ellipticals (dEs), observed to cluster around the two giant spirals. The initial model galaxies are exponential disks embedded in massive dark matter halos and reproduce nearby dIrrs. Repeated tidal shocks at the pericenter of their orbit partially strip their halo and disk and trigger dynamical instabilities that dramatically reshape their stellar component. After only 2-3 orbits low surface brightness (LSB) dIrrs are transformed into dSphs, while high surface brightness (HSB) dIrrs evolve into dEs. This evolutionary mechanism naturally leads to the morphology-density relation observed for LG dwarfs. Dwarfs surrounded by very dense dark matter halos, like the archetypical dIrr GR8, are turned into Draco or Ursa Minor, the faintest and most dark matter dominated among LG dSphs. If disks include a gaseous component, this is both tidally stripped and consumed in periodic bursts of star formation. The resulting star formation histories are in good qualitative agreement with those derived using HST color-magnitude diagrams for local dSphs.

Gas‐Rich Dwarf Spheroidals

We present evidence that nearly half of the dwarf spheroidal galaxies (dSph and dSph/dIrr) in the Local Group are associated with large reservoirs of atomic gas, in some cases larger than the stellar mass. The gas is sometimes found at large distance (~10 kpc) from the center of a galaxy and is not necessarily centered on it. Similarly large quantities of ionized gas could be hidden in these systems as well. The properties of some of the gas reservoirs are similar to the median properties of the High-Velocity Clouds (HVCs); two of the HI reservoirs are catalogued HVCs. The association of the HI with the dwarf spheroidals might thus provide a link between the HVCs and stars. We show that the HI content of the Local Group dSphs and dIrrs exhibits a sharp decline if the galaxy is within 250 kpc of either the Milky Way or M31. This can be explained if both galaxies have a sufficiently massive x-ray emitting halo that produces ram-pressure stripping if a dwarf ventures too close to either giant spiral. We also investigate tidal stripping of the dwarf galaxies and find that although it may play a role, it cannot explain the apparent total absence of neutral gas in most dSph galaxies at distances less than 250 kpc. For the derived mean density of the hot gas, n_0 = 2.5e-5 cm^-2, ram-pressure stripping is found to be more than an order of magnitude more effective in removing the gas from the dSph galaxies. The hot halo, with an inferred mass of 1e10 solar masses, may represent a reservoir of ~1000 destroyed dwarf systems, either HVCs or true dwarf galaxies similar to those we observe now.

Constraints on Interactions and Mergers from dSph Galaxies and Globular Clusters

Observations at high redshifts are revealing numerous interactions and ongoing mergers. Our own Milky Way is currently merging with the Sagittarius dwarf spheroidal (dSph) galaxy. Past mergers with dwarf galaxies may have contributed significantly to the Galactic halo and possibly to the thick disk. The properties of Local Group dSphs and halo globular clusters impose constraints on the merger history of the Milky Way.