7,267 research outputs found
Spectroscopy of i-Dropout Galaxies with an NB921-Band Depression in the Subaru Deep Field
We report new spectroscopy of two star-forming galaxies with strong Ly_alpha
emission at z=6.03 and z=6.04 in the Subaru Deep Field. These two objects are
originally selected as i'-dropouts (i'-z' > 1.5) showing an interesting
photometric property, the ``NB921 depression''. The NB921-band (centered at
9196A) magnitude is significantly depressed with respect to the z'-band
magnitude. The optical spectra of these two objects exhibit asymmetric
emission-lines at lambda_obs ~ 8540A and ~ 8560A, suggesting that these objects
are Ly_alpha emitters at z~6. The rest-frame equivalent widths of the Ly_alpha
emission of the two objects are 94A and 236A; the latter one is the Ly_alpha
emitter with the largest Ly_alpha equivalent width at z > 6 ever
spectroscopically confirmed. The spectroscopically measured Ly_alpha fluxes of
these two objects are consistent with the interpretation that the NB921
depression is caused by the contribution of the strong Ly_alpha emission to the
z'-band flux. Most of the NB921-depressed i'-dropout objects are thought to be
strong Ly_alpha emitters at 6.0 < z < 6.5; Galactic L and T dwarfs and
NB921-dropout galaxies at z > 6.6 do not dominate the NB921-depressed
i'-dropout sample. Thus the NB921-depression method is very useful for finding
high-z Ly_alpha emitters with a large Ly_alpha equivalent width over a large
redshift range, 6.0 < z < 6.5. Although the broadband-selected sample at z ~ 3
contains only a small fraction of objects with a Ly_alpha equivalent width
larger than 100A, the i'-dropout sample of the Subaru Deep Field contains a
much larger fraction of such strong Ly_alpha emitters. This may imply a strong
evolution of the Ly_alpha equivalent width from z > 6 to z ~ 3.Comment: 21 pages, 6 figures, to appear in The Astrophysical Journa
Direct evidence for an early reionization of the Universe?
We examine the possible reionization of the intergalactic medium (IGM) by the
source UDF033238.7-274839.8 (hereafter HUDF-JD2), which was discovered in deep
{\it HST}/VLT/{\it Spitzer} images obtained as part of the Great Observatory
Origins Deep Survey and {\it Hubble} Ultra-Deep Field projects. Mobasher et al
(2005) have identified HUDF-JD2 as a massive ()
post-starburst galaxy at redshift z. We find that HUDF-JD2 may be
capable of reionizing its surrounding region of the Universe, starting the
process at a redshift as high as z.Comment: 6 pages, 2 figures. Accepted for publication in ApJ Letter
Fine structure of -excitons in multilayers of transition metal dichalcogenides
Reflectance and magneto-reflectance experiments together with theoretical
modelling based on the approach have been employed to study
the evolution of direct bandgap excitons in MoS layers with a thickness
ranging from mono- to trilayer. The extra excitonic resonances observed in
MoS multilayers emerge as a result of the hybridization of Bloch states of
each sub-layer due to the interlayer coupling. The properties of such excitons
in bi- and trilayers are classified by the symmetry of corresponding crystals.
The inter- and intralayer character of the reported excitonic resonances is
fingerprinted with the magneto-optical measurements: the excitonic -factors
of opposite sign and of different amplitude are revealed for these two types of
resonances. The parameters describing the strength of the spin-orbit
interaction are estimated for bi- and trilayer MoS.Comment: 14 pages, 10 figure
Electronic structures of CrX (X=S, Te) studied by Cr 2p soft x-ray magnetic circular dichroism
Cr 2p core excited XAS and XMCD spectra of ferromagnetic CrTe
with several concentrations of =0.11-0.33 and ferrimagnetic
CrS have been measured. The observed XMCD lineshapes are found to
very weakly depend on for CrTe. The experimental results
are analyzed by means of a configuration-interaction cluster model calculation
with consideration of hybridization and electron correlation effects. The
obtained values of the spin magnetic moment by the cluster model analyses are
in agreement with the results of the band structure calculation.The calculated
result shows that the doped holes created by the Cr deficiency exist mainly in
the Te 5porbital of CrTe, whereas the holes are likely to be in Cr
3d state for CrS.Comment: 8 pages, 6 figures, accepted for publication in Physical Review
High Redshift Quasars and Star Formation in the Early Universe
In order to derive information on the star formation history in the early
universe we observed 6 high-redshift (z=3.4) quasars in the near-infrared to
measure the relative iron and \mgii emission strengths. A detailed comparison
of the resulting spectra with those of low-redshift quasars show essentially
the same FeII/MgII emission ratios and very similar continuum and line spectral
properties, indicating a lack of evolution of the relative iron to magnesium
abundance of the gas since z=3.4 in bright quasars. On the basis of current
chemical evolution scenarios of galaxies, where magnesium is produced in
massive stars ending in type II SNe, while iron is formed predominantly in SNe
of type Ia with a delay of ~1 Gyr and assuming as cosmological parameters H_o =
72 km/s Mpc, Omega_M = 0.3, and Omega_Lambda = 0.7$, we conclude that major
star formation activity in the host galaxies of our z=3.4 quasars must have
started already at an epoch corresponding to z_f ~= 10, when the age of the
universe was less than 0.5 Gyrs.Comment: 29 pages, 5 figures, ApJ in pres
Effect of Quantum Fluctuations on Magnetic Ordering in CaVO
We present a theoretical model for CaVO: the -depleted square
spin- Heisenberg model which includes both the nearest-neighbor coupling
() and the next-nearest-neighbor coupling (), where and are
antiferromagnetic. Recent experiments of the neutron diffraction by Harashina
et.al. report the magnetic ordering at low temperatures, which may be called as
a stripe phase. It is shown that the observed spin structure is not stable in
the classical theory. By employing the modified spin wave theory, we show that
the stripe phase is stabilized by the quantum fluctuations for .
In CaVO, the coupling constants are estimated as by
comparing the theoretical and experimental results.Comment: submitted to J. Phys. Soc. Jp
- …