8,469 research outputs found
Mid-infrared colour gradients and the colour-magnitude relation in Virgo early-type galaxies
We make use of Spitzer imaging between 4 and 16 micron and near-infrared data
at 2.2 micron to investigate the nature and distribution of the mid-infrared
emission in a sample of early-type galaxies in the Virgo cluster. These data
allow us to conclude, with some confidence, that the emission at 16 micron in
passive ETGs is stellar in origin, consistent with previous work concluding
that the excess mid-infrared emission comes from the dusty envelopes around
evolved AGB stars. There is little evidence for the mid-infrared emission of an
unresolved central component, as might arise in the presence of a dusty torus
associated with a low-luminosity AGN. We nonetheless find that the 16 micron
emission is more centrally peaked than the near-infrared emission, implying a
radial stellar population gradient. By comparing with independent evidence from
studies at optical wavelengths, we conclude that a metallicity that falls with
increasing radius is the principal driver of the observed gradient. We also
plot the mid-infrared colour-magnitude diagram and combine with similar work on
the Coma cluster to define the colour-magnitude relation for absolute K-band
magnitudes from -26 to -19. Because a correlation between mass and age would
produce a relation with a gradient in the opposite sense to that observed, we
conclude that the relation reflects the fact that passive ETGs of lower mass
also have a lower average metallicity. The colour-magnitude relation is thus
driven by metallicity effects. In contrast to what is found in Coma, we do not
find any objects with anomalously bright 16 micron emission relative to the
colour-magnitude relation. Although there is little overlap in the mass ranges
probed in the two clusters, this may suggest that observable ``rejuvenation''
episodes are limited to intermediate mass objects.Comment: 8 pages, 4 figure
Ion mixing to produce amorphous Mo-Ru superconducting films
Amorphous Mo55Ru45 alloy films were formed by ion mixing of multilayered samples. The ion mixed films, which contain no metalloid element, show excellent superconducting properties. The measured properties are correlated with the microstructure obtained by both x-ray diffraction and transmission electron microscopy
Photon Statistics of a Single Atom Laser
We consider a laser model consisting of a single four-level or three-level atom, an optical cavity, and an incoherent pump. Results for photon statistics for varying pump levels are obtained using a quantum trajectory algorithm. In particular, we calculate the mean photon number, Fano factor (which is the variance over the mean). We examine that the behavior of the single-atom device as β, the fraction of spontaneous emission into the lasing mode, is varied. Typical values considered for β are 0.01\u3cβ\u3c1.0. We find that for large enough β, lasing action, with properties similar to those predicted by semiclassical theories that factorize atom-field correlations and use a small-noise approximation, can occur. Squeezing can occur as β is increased. There is no evidence of a sharp phase transition from weakly excited thermal light to coherent light at a particular pump power. This is consistent with work on many-atom lasers with β values in the range considered here. As β is increased, the output goes from quasithermal light to coherent and finally to squeezed light, progressing into a fully quantum-mechanical regime. We also consider the effects of cavity damping and spontaneous emission rates on these results
Enhanced Spontaneous Emission Into The Mode Of A Cavity QED System
We study the light generated by spontaneous emission into a mode of a cavity
QED system under weak excitation of the orthogonally polarized mode. Operating
in the intermediate regime of cavity QED with comparable coherent and
decoherent coupling constants, we find an enhancement of the emission into the
undriven cavity mode by more than a factor of 18.5 over that expected by the
solid angle subtended by the mode. A model that incorporates three atomic
levels and two polarization modes quantitatively explains the observations.Comment: 9 pages, 2 figures, to appear in May 2007 Optics Letter
Where do "red and dead" early-type void galaxies come from?
Void regions of the Universe offer a special environment for studying
cosmology and galaxy formation, which may expose weaknesses in our
understanding of these phenomena. Although galaxies in voids are observed to be
predominately gas rich, star forming and blue, a sub-population of bright red
void galaxies can also be found, whose star formation was shut down long ago.
Are the same processes that quench star formation in denser regions of the
Universe also at work in voids?
We compare the luminosity function of void galaxies in the 2dF Galaxy
Redshift Survey, to those from a galaxy formation model built on the Millennium
Simulation. We show that a global star formation suppression mechanism in the
form of low luminosity "radio mode" AGN heating is sufficient to reproduce the
observed population of void early-types. Radio mode heating is environment
independent other than its dependence on dark matter halo mass, where, above a
critical mass threshold of approximately M_vir~10^12.5 M_sun, gas cooling onto
the galaxy is suppressed and star formation subsequently fades. In the
Millennium Simulation, the void halo mass function is shifted with respect to
denser environments, but still maintains a high mass tail above this critical
threshold. In such void halos, radio mode heating remains efficient and red
galaxies are found; collectively these galaxies match the observed space
density without any modification to the model. Consequently, galaxies living in
vastly different large-scale environments but hosted by halos of similar mass
are predicted to have similar properties, consistent with observations.Comment: 6 pages, 3 figures, accepted MNRA
On the HI-Hole and AGB Stellar Population of the Sagittarius Dwarf Irregular Galaxy
Using two HST/ACS data-sets that are separated by ~2 years has allowed us to
derive the relative proper-motion for the Sagittarius dwarf irregular (SagDIG)
and reduce the heavy foreground Galactic contamination. The proper-motion
decontaminated SagDIG catalog provides a much clearer view of the young
red-supergiant and intermediate-age asymptotic giant branch populations. We
report the identification of 3 Milky Way carbon-rich dwarf stars, probably
belonging to the thin disk, and pointing to the high incidence of this class at
low Galactic latitudes. A sub-group of 4 oxygen-rich candidate stars depicts a
faint, red extension of the well-defined SagDIG carbon-rich sequence. The
origin of these oxygen-rich candidate stars remains unclear, reflecting the
uncertainty in the ratio of carbon/oxygen rich stars. SagDIG is also a gas-rich
galaxy characterized by a single large cavity in the gas disk (HI-hole), which
is offset by ~360 pc from the optical centre of the galaxy. We nonetheless
investigate the stellar feedback hypothesis by comparing the proper-motion
cleaned stellar populations within the HI-hole with appropriately selected
comparison regions, having higher HI densities external to the hole. The
comparison shows no significant differences. In particular, the centre of the
HI-hole (and the comparison regions) lack stellar populations younger than ~400
Myr, which are otherwise abundant in the inner body of the galaxy. We conclude
that there is no convincing evidence that the SagDIG HI-hole is the result of
stellar feedback, and that gravitational and thermal instabilities in the gas
are the most likely mechanism for its formation.Comment: Accepted for publication in A&A, 11 pages, 6 jpeg figure
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