2,427 research outputs found
Rapid Environmental Quenching of Satellite Dwarf Galaxies in the Local Group
In the Local Group, nearly all of the dwarf galaxies (M_star < 10^9 M_sun)
that are satellites within 300 kpc (the virial radius) of the Milky Way (MW)
and Andromeda (M31) have quiescent star formation and little-to-no cold gas.
This contrasts strongly with comparatively isolated dwarf galaxies, which are
almost all actively star-forming and gas-rich. This near dichotomy implies a
rapid transformation of satellite dwarf galaxies after falling into the halos
of the MW or M31. We combine the observed quiescent fractions for satellites of
the MW and M31 with the infall times of satellites from the Exploring the Local
Volume in Simulations (ELVIS) suite of cosmological zoom-in simulations to
determine the typical timescales over which environmental processes within the
MW/M31 halos remove gas and quench star formation in low-mass satellite
galaxies. The quenching timescales for satellites with M_star < 10^8 M_sun are
short, < 2 Gyr, and quenching is more rapid at lower M_star. These satellite
quenching timescales can be 1 - 2 Gyr longer if one includes the time that
satellites were environmentally preprocessed by low-mass groups prior to MW/M31
infall. We compare with quenching timescales for more massive satellites from
previous works to synthesize the nature of satellite galaxy quenching across
the observable range of M_star = 10^{3-11} M_sun. The satellite quenching
timescale increases rapidly with satellite M_star, peaking at ~9.5 Gyr for
M_star ~ 10^9 M_sun, and the timescale rapidly decreases at higher M_star to <
5 Gyr at M_star > 5 x 10^9 M_sun. Overall, galaxies with M_star ~ 10^9 M_sun,
similar to the Magellanic Clouds, exhibit the longest quenching timescales,
regardless of environmental or internal mechanisms.Comment: 6 pages, 3 figures. Accepted in ApJ Letters. Matches published
versio
Thermodynamics of Four-Flavour QCD with Improved Staggered Fermions
We have calculated the pressure and energy density in four-flavour QCD using
improved fermion and gauge actions. We observe a strong reduction of finite
cut-off effects in the high temperature regime, similar to what has been noted
before for the SU(3) gauge theory. Calculations have been performed on
and 16^4 lattices for two values of the quark mass,
and 0.1. A calculation of the string tension at zero temperature yields a
critical temperature for the smaller
quark mass value.Comment: 12 pages, LaTeX2e File, 11 encapsulated postscript file
BB Potentials in Quenched Lattice QCD
The potentials between two B-mesons are computed in the heavy-quark limit
using quenched lattice QCD at . Non-zero central
potentials are clearly evident in all four spin-isospin channels, (I,s_l) =
(0,0) , (0,1) , (1,0) , (1,1), where s_l is the total spin of the light degrees
of freedom. At short distance, we find repulsion in the channels and
attraction in the I=s_l channels. Linear combinations of these potentials that
have well-defined spin and isospin in the t-channel are found, in three of the
four cases, to have substantially smaller uncertainties than the potentials
defined with the s-channel (I,s_l), and allow quenching artifacts from single
hairpin exchange to be isolated. The BB*\pi coupling extracted from the
long-distance behavior of the finite-volume t-channel potential is found to be
consistent with quenched calculations of the matrix element of the isovector
axial-current. The tensor potentials in both of the s_l = 1 channels are found
to be consistent with zero within calculational uncertainties.Comment: 30 page
Comparing the Ancient Star Formation Histories of the Magellanic Clouds
We present preliminary results from a new HST archival program aimed at
tightly constraining the ancient (>4 Gyr ago) star formation histories (SFHs)
of the field populations of the SMC and LMC. We demonstrate the quality of the
archival data by constructing HST/WFPC2-based color-magnitude diagrams (CMDs;
M_{F555W} ~ +8) for 7 spatially diverse fields in the SMC and 8 fields in the
LMC. The HST-based CMDs are >2 magnitudes deeper than any from ground based
observations, and are particularly superior in high surface brightness regions,
e.g., the LMC bar, which contain a significant fraction of star formation and
are crowding limited from ground based observations. To minimize systematic
uncertainties, we derive the SFH of each field using an identical maximum
likelihood CMD fitting technique. We then compute an approximate mass weighted
average SFH for each galaxy. We find that both galaxies lack a dominant burst
of early star formation, which suggests either a suppression or an
under-fueling of early star formation. From 10-12 Gyr ago, the LMC experienced
a period of enhanced stellar mass growth relative to the SMC. Similar to some
previous studies, we find two notable peaks in the SFH of the SMC at ~4.5 and 9
Gyr ago, which could be due to repeated close passages with the LMC, implying
an interaction history that has persisted for at least 9 Gyr. We find little
evidence for strong periodic behavior in the lifetime SFHs of both MCs,
suggesting that repeated encounters with the Milky Way are unlikely. Beginning
~3.5 Gyr ago, both galaxies show increases in their SFHs, in agreement with
previous studies, and thereafter, track each other remarkably well. (abridged)Comment: 9 pages, 5 Figures, Accepted for Publication in MNRA
Motivación emprendedora y teorÃa de los stakeholders
Este trabajo desarrolla una justificación para la teorÃa de los stakeholders basada en las motivaciones. La metodologÃa utilizada es diferente y complementaria de la usada en trabajos anteriores, que imponen consideraciones normativas a esta teorÃa. Enfocado al proceso emprendedor, analiza los mecanismos que explican las acciones y el alineamiento de los distintos participantes. El argumento principal es que cada grupo de interés actúa motivado tanto por el resultado de sus acciones como por el aprendizaje que genera el proceso en sà mismo. A su vez, ese proceso, como motivación central, provee una identidad individual y colectiva y, en su último término, explica el alineamiento de los distintos grupos. Este argumento tiene implicaciones tanto a nivel directivo como normativo
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