1,216 research outputs found
Are polar rings indeed polar?
We have considered polar ring galaxy candidates, the images of which can be
found in the SDSS. The sample of 78 galaxies includes the most reliable
candidates from the SPRC and PRC catalogs, some of which already have kinematic
confirmations. We analyze the distributions of studied objects by the angle
between the polar ring and the central disk, and by the optical diameter of the
outer ring structures. In the vast majority of cases, the outer structures lie
in the plane close to polar (within 10-20 deg) which indicates the stability of
the corresponding orbits in the gravitational potential of the halo. Moderately
inclined outer structures are observed only in about 6% of objects which
probably indicates their short lifetime. In such an unstable configuration, the
polar ring would often cross the disk of the galaxy, being smaller than it in
the diameter. We show that the inner polar structures and outer large-scale
polar rings form a single family in the distribution of diameters normalized to
the optical size of the galaxy. At the same time, this distribution is bimodal,
as the number of objects with d_ring= (0.4-0.7)*d_disk is negligible. Such a
shape of size distribution is most likely due to the fact that the stability of
polar orbits in the inner regions of galaxies is maintained by the bulge or the
bar, while in the outer regions it is provided by the spheroidal (or triaxial)
halo.Comment: Corrected version accepted in Astrophysical Bulletin, 8 pages, 5 EPS
figure
Photon storage in Lambda-type optically dense atomic media. I. Cavity model
In a recent paper [Gorshkov et al., Phys. Rev. Lett. 98, 123601 (2007)], we
used a universal physical picture to optimize and demonstrate equivalence
between a wide range of techniques for storage and retrieval of photon wave
packets in Lambda-type atomic media in free space, including the adiabatic
reduction of the photon group velocity, pulse-propagation control via
off-resonant Raman techniques, and photon-echo-based techniques. In the present
paper, we perform the same analysis for the cavity model. In particular, we
show that the retrieval efficiency is equal to C/(1+C) independent of the
retrieval technique, where C is the cooperativity parameter. We also derive the
optimal strategy for storage and, in particular, demonstrate that at any
detuning one can store, with the optimal efficiency of C/(1+C), any smooth
input mode satisfying T C gamma >> 1 and a certain class of resonant input
modes satisfying T C gamma ~ 1, where T is the duration of the input mode and 2
gamma is the transition linewidth. In the two subsequent papers of the series,
we present the full analysis of the free-space model and discuss the effects of
inhomogeneous broadening on photon storage.Comment: 16 pages, 2 figures. V2: significant changes in presentation, new
references, higher resolution of figure
Photon storage in Lambda-type optically dense atomic media. III. Effects of inhomogeneous broadening
In a recent paper [Gorshkov et al., Phys. Rev. Lett. 98, 123601 (2007)] and
in the two preceding papers [Gorshkov et al., Phys. Rev. A 76, 033804 (2007);
76, 033805 (2007)], we used a universal physical picture to optimize and
demonstrate equivalence between a wide range of techniques for storage and
retrieval of photon wave packets in homogeneously broadened Lambda-type atomic
media, including the adiabatic reduction of the photon group velocity,
pulse-propagation control via off-resonant Raman techniques, and
photon-echo-based techniques. In the present paper, we generalize this
treatment to include inhomogeneous broadening. In particular, we consider the
case of Doppler-broadened atoms and assume that there is a negligible
difference between the Doppler shifts of the two optical transitions. In this
situation, we show that, at high enough optical depth, all atoms contribute
coherently to the storage process as if the medium were homogeneously
broadened. We also discuss the effects of inhomogeneous broadening in solid
state samples. In this context, we discuss the advantages and limitations of
reversing the inhomogeneous broadening during the storage time, as well as
suggest a way for achieving high efficiencies with a nonreversible
inhomogeneous profile.Comment: 15 pages, 8 figures. V2: minor changes in presentation, new
references, higher resolution of figure
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