3 research outputs found
Draco, a flawless dwarf galaxy
The Draco dwarf spheroidal galaxy (dSph), with its apparent immense mass to
light ratio and compact size, holds many clues to the nature of the enigmatic
dark matter. Here we present deep photometric studies of this dwarf galaxy,
undertaken with the MegaCam Camera at the Canada-France-hawaii Telescope, the
Wide Field Camera at the Isaac Newton Telescope and the Wide-Field and
Planetary Camera on board the Hubble Space Telescope. The new photometric data
cover the entirety of the galaxy, and reach i=24.5 at 50% completeness,
significantly deeper than previous panoramic studies, allowing searches for
tidal disturbances of much lower surface brightness than has been possible
before. With these improved statistics, we find no evidence for asymmetric
disturbances or tidal tails that possess more than 3% of the stars found within
the centre of the galaxy. We find that the central stellar density, as probed
by the HST data, rises into the central 0.5'. Uncertainties in the position of
the centroid of the galaxy do not allow us to determine whether the apparent
flattening of the profile interior to 0.5' is reliable or not. Draco is
therefore a flawless dwarf galaxy, featureless and apparently unaffected by
Galactic tides.Comment: 13 pages, 16 figures, accepted for publication in MNRA
Blue straggler stars in dwarf spheroidal galaxies
Blue straggler star (BSS) candidates have been observed in all old dwarf
spheroidal galaxies (dSphs), however whether or not they are authentic BSSs or
young stars has been a point of debate. To both address this issue and obtain a
better understanding of the formation of BSSs in different environments we have
analysed a sample of BSS candidates in two nearby Galactic dSphs, Draco and
Ursa Minor. We have determined their radial and luminosity distributions from
wide field multicolour imaging data extending beyond the tidal radii of both
galaxies.
BSS candidates are uniformly distributed through the host galaxy, whereas a
young population is expected to show a more clumpy distribution. Furthermore,
the observed radial distribution of BSSs, normalized to both red giant branch
(RGB) and horizontal branch (HB) stars, is almost flat, with a slight decrease
towards the centre. Such a distribution is at odds with the predictions for a
young stellar population, which should be more concentrated. Instead, it is
consistent with model predictions for BSS formation by mass transfer in
binaries (MT-BSSs). Such results, although not decisive, suggest that these
candidates are indeed BSSs and that MT-BSSs form in the same way in Draco and
Ursa Minor as in globular clusters. This favours the conclusion that Draco and
Ursa Minor are truly 'fossil' galaxies, where star formation ceased completely
more than 8 billion years ago.Comment: 16 pages, 10 figures, MNRAS, in pres
Feedback and the Formation of Dwarf Galaxy Stellar Halos
Stellar population studies show that low mass galaxies in all environments
exhibit stellar halos that are older and more spherically distributed than the
main body of the galaxy. In some cases, there is a significant intermediate age
component that extends beyond the young disk. We examine a suite of Smoothed
Particle Hydrodynamic (SPH) simulations and find that elevated early star
formation activity combined with supernova feedback can produce an extended
stellar distribution that resembles these halos for model galaxies ranging from
= 15 km s to 35 km s, without the need for accretion of
subhalos.Comment: 15 pages, 15 figures, accepted MNRA