Statistics – Applications
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995phdt.........1e&link_type=abstract
Thesis (PH.D.)--MASSACHUSETTS INSTITUTE OF TECHNOLOGY, 1995.Source: Dissertation Abstracts International, Volume: 56-06, Secti
Statistics
Applications
6
Gravitational Lens, Lens Galaxies
Scientific paper
Gravitational lenses provide unique opportunities to probe distant galaxies and to examine models of the universe. We focus our attention on the gravitational lens MG 0414+0534. The bright four-image geometry and source variability make this system an excellent candidate for study. The first step in addressing astrophysical applications is the accurate determination of the lens matter distribution. In this thesis, we focused on improving the lens inversion algorithms and obtaining high-resolution observations of MG 0414+0534 to yield a reliable reconstruction of the lens. We then use this information to measure astrophysical properties of the lens and universe. Multiple imaging by the lens provides strong constraints in the observed images allowing reconstruction of both the total matter distribution in the lens and the light distribution in the source. Although the lens inversion algorithm, LensClean, has been applied successfully, we find systematic errors which bias the results. Systems with compact images are affected to a greater extent, though the effects are also significant in systems dominated by diffuse emission. The primary errors result from the inclusion of negative clean components and the assumption that the standard image reconstruction procedures do not affect the lens modeling. We develop the Visibility LensClean algorithm which removes the systematic errors by requiring positive clean components and by operating directly on the complex visibility data. We find the Visibility LensClean algorithm dramatically reduces the systematic errors and provides more reliable lens inversion results. This algorithm also yields a better reconstruction than the standard image reconstruction techniques once a lens model is found which fits the system reasonably well. Using 15 GHz (lambda2cm) VLA observations, we find that a simple monopole plus quadrupole structure is insufficient in describing the lens in MG 0414+0534. The angular structure of the lens models does not produce the observed image geometry, resulting in a position error of ~0_sp {.}{''}1. Since the nature of LensClean requires that the nonlinear equation for the images be solved efficiently, we can not explore a more general class of lens models within the LensClean framework at present. Instead, using a point source inversion procedure, we find the lens is fit satisfactorily by a singular isothermal sphere plus the two lowest order terms in the multipole expansion due to a mass distribution outside the ring of images, {1over2}gamma r^2 cos 2(theta-theta _gamma) and beta r^3 cos 3(theta-theta_ beta). We observed MC 0414+0534 at 5 GHz (X(2 6cm) using Very Long Baseline Interferometry and detected all the components except C. We find resolved structures in A1, A2, and B at the 10 milliarcsecond level. We modeled the lens using the two strongest components in each VLBI image and find the cos 38 model does fit the VLBI data and the best fit lens parameters are consistent with those found with the 15 GHz VLA data. (Copies available exclusively from MIT Libraries, Rm. 14-0551, Cambridge, MA 02139-4307. Ph. 617-253-5668; Fax 617-253-1690.) (Abstract shortened by UMI.).
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