1,218 research outputs found
A partnership approach to curriculum design: enabling a holistic curriculum where students can develop their identities and be successful
Time evolution of the fine structure constant
We present a short review of the current quasar (QSO) absorption line
constraints on possible variation of the fine structure constant, alpha =
e^2/(hbar*c). Particular attention is paid to recent optical Keck/HIRES spectra
of 49 absorption systems which indicate a smaller alpha in the past (Murphy et
al. 2001, Webb et al. 2001). Here we present new preliminary results from 128
absorption systems: da/a = (-0.57 +/- 0.10) x 10^{-5} over the redshift range
0.2 < z < 3.7, in agreement with the previous results. Known potential
systematic errors cannot explain these results. We compare them with strong
`local' constraints and discuss other (radio and millimetre-wave) QSO
absorption line constraints on variations in alpha^2 * g_p and alpha^2 * g_p *
m_e/m_p (g_p is the proton g-factor and m_e/m_p is the electron/proton mass
ratio). Finally, we discuss future efforts to rule out or confirm the current
5.7 sigma optical detection.Comment: Invited review at the XXII Physics in Collision Conference (PIC02),
Stanford, CA, USA, June 2002, 11 pages, LaTeX, 7 eps figures. PSN FRA T0
Detection of broad 21-cm absorption at z = 0.656 in the complex sight-line towards 3C336
We report the detection of 21-cm absorption at z = 0.656 towards 1622+238
(3C336). The line is very broad with a Full-Width Half Maximum (FWHM) of 235
km/s, giving a velocity integrated optical depth of 2.2 km/s. The centroid of
the line is offset from that of the known damped Lyman-alpha absorption (DLA)
system by 50 km/s, and if the Lyman-alpha and 21-cm absorption are due to the
same gas, we derive a spin temperature of < 60 K, which would be the lowest yet
in a DLA. The wide profile, which is over four times wider than that of any
other DLA, supports the hypothesis that the hydrogen absorption is occurring
either in the disk of a large underluminous spiral or a group of dim
unidentified galaxies, associated with the single object which has been
optically identified at this redshift.Comment: 5 pages, 3 figures, accepted by MNRAS Letter
An Introduction to the Quasi-Optical Design of the HIFI Instrument for the Herschel Space Observatory
This paper is concerned with the quasi-optical design and analysis of the Heterodyne Instrument for the Far Infrared (HIFI) on board the European Space Agency’s Herschel Space Observatory, which is due for launch in 2007. The paper begins with an introduction to astronomy at submillimetre wavelengths followed by the science that will be carried out by HIFI. The optical layout of HIFI is presented and the quasi-optical techniques used in the analysis of band 5 of the instrument are discussed, in particular, issues associated with the design and performance of the integrated lens antenna for this band. A power coupling efficiency calculation is carried out and the overall performance of the telescope is analysed
The connection between metallicity and metal-line kinematics in (sub-)damped Lyman-alpha systems
A correlation between the metallicity, [M/H], and rest-frame MgII equivalent
width, EW, is found from 49 DLAs and strong sub-DLAs drawn from the literature
over the redshift range 0.2<z_abs<2.6. The correlation is significant at 4.2
sigma and improves to 4.7 sigma when the mild evolution of [M/H] with redshift
is taken into account. Even when including only the 26 DLAs (i.e. excluding
sub-DLAs) which have Zn metallicities and EW>0.7A, the correlation remains at
>3 sigma significance. Since the MgII2796 transition is predominantly saturated
in DLAs (which always have EW greater than 0.3A), EW is far more sensitive to
the kinematic spread of the metal velocity components across the absorption
profile than it is to [M/H]. Thus, the observed [M/H]--EW correlation points to
a strong link between the absorber metallicity and the mechanism for producing
and dispersing the velocity components. We also note that approximately half of
the 13 known molecular hydrogen absorbers have very high EW and very broad
velocity structures which show characteristics usually associated with
outflows. Follow-up ultraviolet- and blue-sensitive high-resolution spectra of
high-EW systems, initially identified in low-resolution spectra, may therefore
yield a large number of new H_2 discoveries.Comment: 9 pages, 2 figures (3 EPS files). Accepted by MNRA
Can hidden correlations mimic a variable fine structure constant?
Murphy et al. (2003a, MNRAS, 345, 609) claim to find evidence of cosmological
variations of the fine structure constant in the spectra of
intervening QSO absorption line systems. We find that this result is affected
by systematic effects. The values estimated in individual line systems
depend on the set of atomic transitions used and therefore the quoted
dependence on the cosmic age may reflect the fact that different sets of
transitions are used at different redshifts. A correlation between line shifts
and relative optical depths of the atomic transitions is also present. This
correlation is very tight for a high-redshift subsample and accounts for the
anomalous dispersion of the values found by Murphy et al. (2003a) in
this subsample. The above correlations are consistent with a scenario in which
gravitational redshift, caused by the gravity of the dark halo, contributes to
the shift in frequency of individual components. Gravitational redshift causes
differential spectral shifts of the same order as magnitude of those measured.
In the presence of line misidentification, these shifts can be interpreted in
terms of a variable . In order to verify the gravitational redshift
hypothesis, a direct access to Murphy et al. (2003a) data, or to a large amount
of new high resolution data, is necessary.Comment: 10 pages, 5 figures, 3 tables, to appear on Astronomy and
Astrophysic
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