110 research outputs found
Deep imaging of Eridanus II and its lone star cluster
We present deep imaging of the most distant dwarf discovered by the Dark
Energy Survey, Eridanus II (Eri II). Our Magellan/Megacam stellar photometry
reaches mag deeper than previous work, and allows us to confirm the
presence of a stellar cluster whose position is consistent with Eri II's
center. This makes Eri II, at , the least luminous galaxy known to
host a (possibly central) cluster. The cluster is partially resolved, and at
it accounts for of Eri II's luminosity. We derive
updated structural parameters for Eri II, which has a half-light radius of
pc and is elongated (), at a measured
distance of kpc. The color-magnitude diagram displays a blue,
extended horizontal branch, as well as a less populated red horizontal branch.
A central concentration of stars brighter than the old main sequence turnoff
hints at a possible intermediate-age ( Gyr) population; alternatively,
these sources could be blue straggler stars. A deep Green Bank Telescope
observation of Eri II reveals no associated atomic gas.Comment: 7 pages, 4 figures; ApJL accepte
Antlia B: A faint dwarf galaxy member of the NGC 3109 association
We report the discovery of Antlia B, a faint dwarf galaxy at a projected
distance of 72 kpc from NGC 3109 (15 mag), the primary
galaxy of the NGC 3109 dwarf association at the edge of the Local Group. The
tip of the red giant branch distance to Antlia B is =1.290.10 Mpc,
which is consistent with the distance to NGC 3109. A qualitative analysis
indicates the new dwarf's stellar population has both an old, metal poor red
giant branch (10 Gyr, [Fe/H]2), and a younger blue population
with an age of 200-400 Myr, analogous to the original Antlia dwarf,
another likely satellite of NGC 3109. Antlia B has \ion{H}{1} gas at a velocity
of =376 km s, confirming the association with NGC 3109
(=403 km s). The HI gas mass
(M=2.80.210 M), stellar luminosity
(=9.70.6 mag) and half light radius (=27329 pc) are
all consistent with the properties of dwarf irregular and dwarf spheroidal
galaxies in the Local Volume, and is most similar to the Leo P dwarf galaxy.
The discovery of Antlia B is the initial result from a Dark Energy Camera
survey for halo substructure and faint dwarf companions to NGC 3109 with the
goal of comparing observed substructure with expectations from the
+Cold Dark Matter model in the sub-Milky Way regime.Comment: 7 pages, 3 figures. Submitted to ApJ
Dwarf galaxies beyond our doorstep: the Centaurus A group
The study of dwarf galaxies in groups is a powerful tool for investigating
galaxy evolution, chemical enrichment and environmental effects on these
objects. Here we present results obtained for dwarf galaxies in the Centaurus A
complex, a dense nearby (~4 Mpc) group that contains two giant galaxies and
about 30 dwarf companions of different morphologies and stellar contents. We
use archival optical (HST/ACS) and near-infrared (VLT/ISAAC) data to derive
physical properties and evolutionary histories from the resolved stellar
populations of these dwarf galaxies. In particular, for early-type dwarfs we
are able to construct metallicity distribution functions, find population
gradients and quantify the intermediate-age star formation episodes. For
late-type dwarfs, we compute recent (~1 Gyr) star formation histories and study
their stellar distribution. We then compare these results with properties of
the dwarfs in our Milky Way and in other groups. Our work will ultimately lead
to a better understanding of the evolution of dwarf galaxies.Comment: 6 pages, 5 figures; to appear in the proceedings of the conference "A
Universe of dwarf galaxies" (Lyon, June 14-18, 2010
A close look at the Centaurus A group of galaxies IV. Recent star formation histories of late-type dwarfs around CenA
We study a sample of 5 dwarf irregular galaxies in the CenA/M83 group, which
are companions to the giant elliptical CenA. We aim at deriving their physical
properties over their lifetime and compare them to those of dwarfs located in
different environments. We use archival HST/ACS data and apply synthetic
color-magnitude diagram fitting in order to reconstruct the past star formation
activity of the target galaxies. The average star formation rate for the
studied galaxies ranges from 10^{-3} up to \sim 7x10^{-2} M_odot/yr, and their
mean metallicities correlate with their luminosities (from [Fe/H]\sim -1.4 up
to \sim -1.0). The form of the star formation histories varies across the
sample, with quiescent periods alternating with intermittent enhancements in
the star formation (from a few up to several times the average lifetime value).
The dwarfs in this sample formed ~35% to ~60% of their stellar content prior to
~5 Gyr ago. The resulting star formation histories for the CenA companions are
similar to those found for comparable Local Group and M81 group dwarfs. We
consider this sample of dwarfs together with 5 previously studied M83 dwarf
irregular companions. We find no trend of the average star formation rate with
tidal index or distance from the main galaxy of the group. However, dwarfs with
higher baryonic masses do show higher average star formation rates, underlining
the importance of intrinsic properties in governing the evolution of these
galaxies. On the other hand, there is also a clear trend when looking at the
recent (~0.5-1 Gyr) level of activity. Namely, dwarfs within a denser region of
the group appear to have had their star formation quenched while dwarfs located
in the group outskirts show a wide range of possible star formation rates, thus
indicating that external processes play a fundamental role, complementary to
mass, in shaping the star formation histories of dwarf galaxies.Comment: 16 pages, 10 figures; A&A accepte
Dynamic stability of high speed turboshaft engine with reducer
The dynamic behavior of machines which rotate at extreme high speeds is crucial for stable and long operation. In design process some dynamics calculation must be taken into account to avoid potential operation near resonance areas. This paper considers dynamic analysis of the high speed reducer, designed together with the turbo-shaft engine. Resonances can occur when frequency of rotation and teeth mesh frequency are close to gear pair in mesh natural frequency, to the shafts natural frequencies and to the other parts natural frequencies. Problems can be avoided by translating resonant areas to higher or lower level of frequency by changing some design parameters, mass, stiffness and by variation of the stiffness of bearing supports. The paper presents the approaches to natural frequencies identification for the example of high speed reducer design. Some examples of design solutions, especially for elastic bearing supports and their experimental testing are also presented
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