86,205 research outputs found
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
Dirac Equation Studies in the Tunnelling Energy Zone
We investigate the tunnelling zone V0 < E < V0+m for a one-dimensional
potential within the Dirac equation. We find the appearance of superluminal
transit times akin to the Hartman effect.Comment: 12 pages, 4 figure
A near-infrared study of AGB and red giant stars in the Leo I dSph galaxy
A near-infrared imaging study of the evolved stellar populations in the dwarf
spheroidal galaxy Leo I is presented. Based on JHK observations obtained with
the WFCAM wide-field array at the UKIRT telescope, we build a near-infrared
photometric catalogue of red giant branch (RGB) and asymptotic giant branch
(AGB) stars in Leo I over a 13.5 arcmin square area. The V-K colours of RGB
stars, obtained by combining the new data with existing optical observations,
allow us to derive a distribution of global metallicity [M/H] with average
[M/H] = -1.51 (uncorrected) or [M/H] = -1.24 +/- 0.05 (int) +/- 0.15 (syst)
after correction for the mean age of Leo I stars. This is consistent with the
results from spectroscopy once stellar ages are taken into account. Using a
near-infrared two-colour diagram, we discriminate between carbon- and
oxygen-rich AGB stars and obtain a clean separation from Milky Way foreground
stars. We reveal a concentration of C-type AGB stars relative to the red giant
stars in the inner region of the galaxy, which implies a radial gradient in the
intermediate-age (1-3 Gyr) stellar populations. The numbers and luminosities of
the observed carbon- and oxygen-rich AGB stars are compared with those
predicted by evolutionary models including the thermally-pulsing AGB phase, to
provide new constraints to the models for low-metallicity stars. We find an
excess in the predicted number of C stars fainter than the RGB tip, associated
to a paucity of brighter ones. The number of O-rich AGB stars is roughly
consistent with the models, yet their predicted luminosity function is extended
to brighter luminosity. It appears likely that the adopted evolutionary models
overestimate the C star lifetime and underestimate their K-band luminosity.Comment: MNRAS, accepte
Metallicity Distribution Functions of Four Local Group dwarf galaxies
We present stellar metallicities in Leo I, Leo II, IC 1613, and Phoenix dwarf
galaxies derived from medium (F390M) and broad (F555W, F814W) band photometry
using the Wide Field Camera 3 (WFC3) instrument aboard the Hubble Space
Telescope. We measured metallicity distribution functions (MDFs) in two ways,
1) matching stars to isochrones in color-color diagrams, and 2) solving for the
best linear combination of synthetic populations to match the observed
color-color diagram. The synthetic technique reduces the effect of photometric
scatter, and produces MDFs 30-50 % narrower than the MDFs produced from
individually matched stars. We fit the synthetic and individual MDFs to
analytical chemical evolution models (CEM) to quantify the enrichment and the
effect of gas flows within the galaxies. Additionally, we measure stellar
metallicity gradients in Leo I and II. For IC 1613 and Phoenix our data do not
have the radial extent to confirm a metallicity gradient for either galaxy.
We find the MDF of Leo I (dwarf spheroidal) to be very peaked with a steep
metal rich cutoff and an extended metal poor tail, while Leo II (dwarf
spheroidal), Phoenix (dwarf transition) and IC 1613 (dwarf irregular) have
wider, less peaked MDFs than Leo I. A simple CEM is not the best fit for any of
our galaxies, therefore we also fit the `Best Accretion Model' of Lynden-Bell
1975. For Leo II, IC 1613 and Phoenix we find similar accretion parameters for
the CEM, even though they all have different effective yields, masses, star
formation histories and morphologies. We suggest that the dynamical history of
a galaxy is reflected in the MDF, where broad MDFs are seen in galaxies that
have chemically evolved in relative isolation and narrowly peaked MDFs are seen
in galaxies that have experienced more complicated dynamical interactions
concurrent with their chemical evolution.Comment: 15 pages, 8 figures, accepted in A
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