10,197 research outputs found
Peering through the OH Forest: public release of sky-residual subtracted spectra for SDSS DR7
The Sloan Digital Sky Survey (SDSS) automated spectroscopic reduction
pipeline provides >1.5 million intermediate resolution, R~2000, moderate
signal-to-noise ratio (SNR), SNR~15, astronomical spectra of unprecedented
homogeneity that cover the wavelength range 3800-9200AA. However, there remain
significant systematic residuals in many spectra due to the sub-optimal
subtraction of the strong OH sky emission lines longward of 6700AA. The OH sky
lines extend over almost half the wavelength range of the SDSS spectra, and the
SNR over substantial wavelength regions in many spectra is reduced by more than
a factor two over that expected from photon counting statistics. Following the
OH line subtraction procedure presented in Wild & Hewett (2005), we make
available to the community sky-residual subtracted spectra for the Sloan
Digital Sky Survey Data Relase 7. Here we summarise briefly the method,
including minor changes in the implementation of the procedure with respect to
WH05. The spectra are suitable for many science applications but we highlight
some limitations for certain investigations. Details of the data model for the
sky-residual subtracted spectra and instructions on how to access the spectra
are provided.Comment: 11 pages, 5 figures. Spectra may be downloaded from the JHU SDSS
server http://www.sdss.jhu.edu/skypca/spSpec, additionally the QSOs can be
found on the DR7 Value Added Catalogue web page:
http://www.sdss.org/dr7/products/value_added/index.html#quasar
Evidence for dust reddening in DLAs identified through CaII H&K absorption
We present a new sample of 31 CaII(H&K) 3935,3970 absorption line systems
with 0.84<z_abs<1.3 discovered in the spectra of Sloan Digital Sky Survey
(SDSS) Data Release 3 quasars, together with an analysis of their dust content.
The presence of Calcium absorption together with measurements of the MgII 2796,
FeII 2600 and MgI 2853 lines lead to the conclusion that the majority of our
systems are Damped Ly-alpha (DLA) absorbers. The composite spectrum in the rest
frame of the absorber shows clear evidence for reddening. Large and Small
Magellanic Cloud extinction curves provide satisfactory fits, with a best-fit
E(B-V) of 0.06, while the Galactic dust extinction curve provides a poor fit
due to the lack of a strong 2175A feature. A trend of increasing dust content
with equivalent width of CaII is present. Monte Carlo techniques demonstrate
that the detection of reddening is significant at >99.99% confidence. The
discovery of significant amounts of dust in a subsample of DLAs has direct
implications for studies of the metallicity evolution of the universe and the
nature of DLAs in relation to high redshift galaxies. The gas:dust ratio is
discussed. Our results suggest that at least ~40% of the CaII absorption
systems are excluded from the magnitude-limited SDSS quasar sample as a result
of the associated extinction, a fraction similar to the upper limit deduced at
higher redshifts from radio-selected surveys.Comment: 5 pages, 2 figures, accepted MNRAS Letter
Revised time-of-flight calculations for high-latitude geomagnetic pulsations using a realistic magnetospheric magnetic field model
We present a simple time-of-flight analysis of Alfvén pulsations standing on closed terrestrial magnetic field lines. The technique employed in this study in order to calculate the characteristic period of such oscillations builds upon earlier time-of-flight estimates via the implementation of a more recent magnetospheric magnetic field model. In this case the model employed is the Tsyganenko (1996) field model, which includes realistic magnetospheric currents and the consequences of the partial penetration of the interplanetary magnetic field into the dayside magnetopause. By employing a simple description of magnetospheric plasma density, we are therefore able to estimate the period of standing Alfvén waves on geomagnetic field lines over a significantly wider range of latitudes and magnetic local times than in previous studies. Furthermore, we investigate the influence of changing season and upstream interplanetary conditions upon the period of such pulsations. Finally, the eigenfrequencies of magnetic field lines computed by the time-of-flight technique are compared with corresponding numerical solutions to the wave equation and experimentally observed pulsations on geomagnetic field lines
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