334 research outputs found
Impact of Reionization on the Stellar Populations of Nearby Dwarf Galaxies
Cold dark matter models for galaxy formation predict that low-mass systems
will be the first sites of star formation. As these objects have shallow
gravitational potential wells, the subsequent growth of their stellar
populations may be halted by heating and gas loss due to reionization. This
effect has been suggested to have profoundly influenced properties of
present-day dwarf galaxies, including their stellar populations and even
survival as visible galaxies. In this Letter we draw on results from
quantitative studies of Local Group dwarf galaxy star formation histories,
especially for Milky Way satellites, to show that no clear signature exists for
a widespread evolutionary impact from reionization. All nearby dwarf galaxies
studied in sufficient detail contain ancient populations indistinguishable in
age from the oldest Galactic globular clusters. Ancient star formation activity
proceeded over several Gyr, and some dwarf spheroidal galaxies even experienced
fairly continuous star formation until just a few Gyr ago. Despite their
uniformly low masses, their star formation histories differ considerably. The
evolutionary histories of nearby dwarf galaxies appear to reflect influences
from a variety of local processes rather than a dominant effect from
reionization.Comment: Accepted by The Astrophysical Journal Letters. 5 pages, one figur
On the Energy Required to Eject Processed Matter from Galaxies
We evaluate the minimum energy input rate that starbursts require for
expelling their newly processed matter from their host galaxies. Special
attention is given to the pressure caused by the environment in which a galaxy
is situated, as well as to the intrinsic rotation of the gaseous component. We
account for these factors and for a massive dark matter distribution, and
develop a self-consistent solution for the interstellar matter gas
distribution. Our results are in excellent agreement with the results of Mac
Low & Ferrara (1999) for galaxies with a flattened disk-like ISM density
distribution and a low intergalactic gas pressure ( 1
cm K). However, our solution also requires a much larger energy input
rate threshold when one takes into consideration both a larger intergalactic
pressure and the possible existence of a low-density, non-rotating, extended
gaseous halo component.Comment: 7 pages, 4 figures, 1 table, Accepted for publication in Ap
Evolution of Interstellar Clouds in Local Group Dwarf Spheroidal Galaxies in the Context of Their Star Formation Histories
We consider evolution of interstellar clouds in Local Group dwarf spheroidal
galaxies (dSphs) in the context of their observed star formation histories. The
Local Group dSphs generally experienced initial bursts of star formations in
their formation epochs ( Gyr ago), when hot gas originating from the
supernovae can make the cold interstellar clouds evaporate. We find that the
maximum size of evaporating cloud is 10 pc. Thus, clouds larger than 10 pc can
survive during the initial star formation. These surviving clouds can
contribute to the second star formation to produce ``intermediate-age (
3--10 Gyr ago) stellar populations.'' Assuming that collisions between clouds
induce star formation and that the timescale of the second star formation is a
few Gyr, we estimate the total mass of the clouds. The total mass is about
, which is 1--3 orders of magnitude smaller than the typical
stellar mass of a present dSph. This implies that the initial star formation is
dominant over the second star formation, which is broadly consistent with the
observed star formation histories. However, the variety of the dSphs in their
star formation histories suggests that the effects of environments on the dSphs
may be important.Comment: 14 pages LaTeX, no figures, to appear in Ap
The Star-Forming Dwarf Galaxy Populations of two z ~ 0.4 Clusters: MS1512.4+3647 and Abell 851
We present the results of a deep narrow-band [OII] 3727 \AA emission-line
search for faint ( 27), star-forming galaxies in the field of the
MS1512.4+3647 cluster. We find no evidence for an over-density of emission-line
sources relative to the field at 0.4 (Hogg et al. 1998), and therefore
conclude that the MS1512.4+3647 sample is dominated by field [OII]
emission-line galaxies which lie along the 180 Mpc line of sight
immediately in front and behind the cluster. This is surprising, given that the
previously surveyed cluster Abell 851 has 3-4 times the field
emission-line galaxy density (Martin et al. 2000). We find that the
MS1512.4+3647 sample is deficient in galaxies with intermediate colors (1.0 2.0) and implied star-formation exponential decay timescales
100 Myr - 1 Gyr that dominate the Abell 851 emission-line galaxy population.
Instead, the majority of [OII] emission-line galaxies surrounding the
MS1512.4+3647 cluster are blue () and forming stars in bursts
with 100 Myr. In both samples, galaxies with the shortest
star-formation timescales are preferentially among the faintest star-forming
objects. Their i luminosities are consistent with young stellar populations
\sim 10^8 - 10^9 \Msun, although an additional factor of ten in stellar mass
could be hiding in underlying old stellar populations. We discuss the
implications for the star-formation histories of dwarf galaxies in the field
and rich clusters.Comment: 26 pages, including 5 tables and 13 figures; accepted for publication
in the Astrophysical Journa
Evolution of the Luminosity Density in the Universe: Implications for the Nonzero Cosmological Constant
We show that evolution of the luminosity density of galaxies in the universe
provides a powerful test for the geometry of the universe. Using reasonable
galaxy evolution models of population synthesis which reproduce the colors of
local galaxies of various morphological types, we have calculated the
luminosity density of galaxies as a function of redshift . Comparison of the
result with recent measurements by the Canada-France Redshift Survey in three
wavebands of 2800{\AA}, 4400{\AA}, and 1 micron at z<1 indicates that the
\Lambda-dominated flat universe with \lambda_0 \sim 0.8 is favored, and the
lower limit on \lambda_0 yields 0.37 (99% C.L.) or 0.53 (95% C.L.) if
\Omega_0+\lambda_0=1. The Einstein-de Sitter universe with (\Omega_0,
\lambda_0)=(1, 0) and the low-density open universe with (0.2, 0) are however
ruled out with 99.86% C.L. and 98.6% C.L., respectively. The confidence levels
quoted apply unless the standard assumptions on galaxy evolution are
drastically violated. We have also calculated a global star formation rate in
the universe to be compared with the observed rate beyond z \sim 2. We find
from this comparison that spiral galaxies are formed from material accretion
over an extended period of a few Gyrs, while elliptical galaxies are formed
from initial star burst at z >~ 5 supplying enough amount of metals and
ionizing photons in the intergalactic medium.Comment: 11 pages including 3 figures, LaTeX, uses AASTeX. To Appear in ApJ
Letter
Application of fuzzy logic in multi-mode driving for a battery electric vehicle energy management
Energy management system is an area of emerging interest in a full electric vehicle research. With the increasing moves to a more sustainable vehicle, there is a need to extend the battery range that simultaneously satisfying the conflicting demand between battery capacity and vehicle weight or volume. This paper presents a research conducted in the Universiti Putra Malaysia, focusing on the energy management strategy of a battery-powered electric vehicle. Three vehicle driving modes; sport, comfort, and eco have been individually modelled. Each mode is capable to dominate different driving environments; highway, suburban, and urban. In European driving cycle simulation test, comfort and eco modes have shown large extension in driving range with the maximum of 7.33% and 19.70% respectively. However the speeds have been confined by certain specific limits. The proposed of integrated multimode driving using fuzzy logic has enabled an adaptive driving by automatically select the driving parameters based on the speed conditions. The results have proven its ability in reducing the energy consumption as much as 32.25%, and increasing the driving range of 4.21% without downgrading the speed performance
Passive Evolution: Are the Faint Blue Galaxy Counts Produced by a Population of Eternally Young Galaxies?
A constant age population of blue galaxies, postulated in the model of
Gronwall & Koo (1995), seems to provide an attractive explanation of the excess
of very blue galaxies in the deep galaxy counts. Such a population may be
generated by a set of galaxies with cycling star formation rates, or at the
other extreme, be maintained by the continual formation of new galaxies which
fade after they reach the age specified in the Gronwall and Koo model. For both
of these hypotheses, we have calculated the luminosity functions including the
respective selection criteria, the redshift distributions, and the number
counts in the B_J and K bands. We find a substantial excess in the number of
galaxies at low redshift (0 < z < 0.05) over that observed in the CFH redshift
survey (Lilly et al. 1995) and at the faint end of the Las Campanas luminosity
function (Lin et al. 1996). Passive or mild evolution fails to account for the
deep galaxy counts because of the implications for low redshift determinations
of the I-selected redshift distribution and the r-selected luminosity function
in samples where the faded counterparts of the star-forming galaxies would be
detectable.Comment: 11 pages, LaTeX type (aaspp4.sty), 3 Postscript figures, submitted to
ApJ Letter
Accretion powered spherical wind in general relativity
Using full general relativistic calculations, we investigate the possibility
of generation of mass outflow from spherical accretion onto non-rotating black
holes. Introducing a relativistic hadronic-pressure-supported steady, standing,
spherically-symmetric shock surface around a Schwarzschild black hole as the
effective physical barrier that may be responsible for the generation of
spherical wind, we calculate the mass outflow rate in terms of
three accretion parameters and one outflow parameter by simultaneously solving
the set of general relativistic hydrodynamic equations describing spherically
symmetric, transonic, polytropic accretion and wind around a Schwarzschild
black hole. Not only do we provide a sufficiently plausible estimation of
, we also successfully study the dependence and variation of this
rate on various physical parameters governing the flow. Our calculation
indicates that independent of initial boundary conditions, the baryonic matter
content of this shock-generated wind always correlates with post-shock flow
temperature.Comment: 14 single column pages. 7 black and white encapsulated post-script
figures. Published in A &
The Star Formation History of NGC 6822
Images of five fields in the Local Group dwarf irregular galaxy NGC 6822
obtained with the {\it Hubble Space Telescope} in the F555W and F814W filters
are presented. Photometry for the stars in these images was extracted using the
Point-Spread-Function fitting program HSTPHOT/MULTIPHOT. The resulting
color-magnitude diagrams reach down to , a level well below the red
clump, and were used to solve quantitatively for the star formation history of
NGC 6822. Assuming that stars began forming in this galaxy from low-metallicity
gas and that there is little variation in the metallicity at each age, the
distribution of stars along the red giant branch is best fit with star
formation beginning in NGC 6822 12-15 Gyr ago. The best-fitting star formation
histories for the old and intermediate age stars are similar among the five
fields and show a constant or somewhat increasing star formation rate from 15
Gyr ago to the present except for a possible dip in the star formation rate
from 3 to 5 Gyr ago. The main differences among the five fields are in the
higher overall star formation rate per area in the bar fields as well as in the
ratio of the recent star formation rate to the average past rate. These
variations in the recent star formation rate imply that stars formed within the
past 0.6 Gyr are not spatially very well mixed throughout the galaxy.Comment: 47 pages, 28 Figures, accepted for publication in the Astronomical
Journa
Cylindrical thin-shell wormholes
A general formalism for the dynamics of non rotating cylindrical thin-shell
wormholes is developed. The time evolution of the throat is explicitly obtained
for thin-shell wormholes whose metric has the form associated to local cosmic
strings. It is found that the throat collapses to zero radius, remains static
or expands forever, depending only on the sign of its initial velocity.Comment: 10 page
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