GOODS: The Great Observatories Origins Deep Survey
April 28, 2007 on 11:51 pm | In Uncategorized | 219 CommentsGOODS unites extremely deep observations from NASA’s Great Observatories, the Spitzer Space Telescope, Hubble, and Chandra, ESA’s XMM-Newton, and from the most powerful ground-based facilities, to survey the distant universe to the faintest flux limits across the broadest range of wavelengths. GOODS incorporates a Spitzer Space Telescope Legacy Program to carry out the deepest observations with that facility at 3.6 to 24 microns, and a Hubble Space Telescope Treasury Program for deep high-resolution optical imaging. GOODS covers a total of roughly 320 square arcminutes in two fields centered on the Hubble Deep Field North and the Chandra Deep Field South. The space-based observations are complemented by ground-based imaging and spectroscopy, including an extensive commitment of ESO and NOAO observing time.
http://www.stsci.edu/science/goods/
numerica
April 25, 2007 on 2:35 am | In Uncategorized | 459 Comments*马铁尤《计算流体动力学》
*C.-W. Shu, High order ENO and WENO schemes for computational fluid dynamics, in “High-Order Methods for Computational Physics”, T.J. Barth and H. Deconinck, editors, Lecture Notes in Computational Science and Engineering, volume 9, Springer, (1999), 439-582.
negative…
# Essentially Non-Oscillatory, ENO; Weighted…, WENO
# fixed stencil interpolation: one always looks one cell to the left, one cell to the right,
plus the center cell itself, regardless of where in the domain one is situated. This works well
for globally smooth problems. The resulting scheme is linear for linear PDEs, hence stability
can be easily analyzed by Fourier transforms. (for uniform grid periodic case) However, ~ of
second or higher order accuracy is necessarily oscillatory near a discontinuity. Such
oscillations, which are called the Gibbs phenomena in spectral methods, do not decay in
magnitude when the mesh is refined. It is a nuisance to say the least for practical
calculations, and often leads to numerical instabilities in nonlinear problems containing
discontinuities.
*NumComp @ smth
done…
*Lax-Wendroff
negative…
# Discretize the function on a regular grid $ f(x) \rightarrow (x_j, f_j)$ , $ j=1,N$
# Expand the differential operators in time using a Taylor series
$\displaystyle f_{x_j}^{t+\Delta t} = f_{x_j}^t +\left.\Delta t\frac{\partial f}... ...2}\frac{\partial^2 f}{\partial t^2}\right\vert _{x_j} + \mathcal{O}(\Delta t^3)$
# Substitute the time derivatives from the master equation; for the case of advection , this yields
$\displaystyle f_{x_j}^{t+\Delta t} = f_{x_j}^t -\left.u\Delta t\frac{\partial f... ...{2}\frac{\partial^2 f}{\partial x^2}\right\vert _{x_j} +\mathcal{O}(\Delta t^3)$
# Use centered differences for spatial operators
$\displaystyle f_j^{t+\Delta t} = f_j^t -\frac{\beta }{2}\left(f_{j+1}^t-f_{j-1}... ...{2}\left(f_{j+1}^t-2f_{j}^t+f_{j-1}^t\right) +\mathcal{O}(\Delta x^3\Delta t^3)$
GSO notes
April 22, 2007 on 3:52 pm | In Uncategorized | 105 Comments1. Loeb & Barkana 2001, ARA&A 39 19
Once a significant volume of the ISM has become ionized, we expect
ionized (and thus transparent) volumes around individual radiation sources.
These would appear as holes in the absorption when a line of sight passes
through such a volume, in a more powerful version of the proximity effect.
This effect appears as a decrease in the number of Lyman alpha forest features
near the redshifts of QSOs, whether the background QSO whose spectrum is
being measured or fortuitously located foreground objects within a Mpc or
so of the line of sight. As long as the differential Hubble expansion across an
ionized volume is greater than the span of absorption wings from adjacent
material, an ionized region will appear as a local deficit in absorption.
—by W. C. Keel, 2004
2. 3C 273
a quasar located in the constellation Virgo. the first quasar discovered, 1963.
optically-brightest quasar in our sky (m ~ 12.9), and one of the closest with a redshift, z, of 0.158. A luminosity distance of DL = 2.44 Gly (749 Mpc) may be calculated from z. It is also one of the most luminous quasars known, with an absolute magnitude of -26.7.
has a large-scale visible jet, which measures ~62 kpc long having an apparent size of 23″. In 1995 Optical imaging using the HST of the jet revealed a structured morphology evidenced by repeated bright knots interlaced by areas of weak emission
the most remote object normally accessible to serious amateurs. Right ascension 12h 29m 06.7s Declination +02° 03′ 09″.
http://en.wikipedia.org/wiki/3C273
3.
Type 1 objects exhibit the straight physics of AGNs with no absorption, and type 2 objects arise when the view is obscured by the torus.
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