Fundamental Limitations in deep HST Fields: Surface Brightness, Natural Confusion, & Algorithmic Biases

Details Fundamental Limitations in deep HST Fields: Surface Brightness, Natural Confusion, & Algorithmic Biases Fundamental Limitations in deep HST Fields: Surface Brightness, Natural Confusion, & Algorithmic Biases

Jul 2007

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007hst..prop11287w&link_type=abstract

HST Proposal ID #11287. Cycle 16

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Scientific paper

In the last decade, 1000's of orbits were invested in deep HST surveys. The HUDF, HDF, GOODS, GEMS and COSMOS resulted in many new discoveries, ranging from dwarf galaxies at the end of reionization to the hierarchical formation of galaxies. At the same time, the fundamental limitations in these surveys may not have been explored or quantified well enough to fully understand their impact. For instance, while deep surveys for faint galaxies with HST are obviously limited by surface brightness SB constraints, at the faintest HST limits other effects also begin to play a dominant role. First, the rapid decline in galaxy sizes at fainter fluxes -- reflecting their hierarchical assembly -- may result in a significant fraction being point sources at the HST diffraction limit <0.08" FWHM, hence affecting their selection and preventing reliable size measurements. More seriously, natural confusion may overtake SB-selection as the dominant selection factor at faint fluxes. Natural confusion occurs when there are fewer than a minimum number of "object beams" with area = pi . r_e^2 for each detected extended object, analogous to the role of the telescope FWHM in the classical instrumental confusion limit. Last, various systematics in the parameter determination algorithms can bias the results from deep HST surveys in rather uncharted ways. This Archival project will quantify these fundamental issues as follows: 1 run very high-resolution hierarchical simulations of the expected object sizes for AB<31 mag; 2 simulate effects from the HST-PSF, charge-diffusion, multi-drizzle, and CCD-noise on the measured object sizes; 3 simulate how object finding and deblending algorithms affect measured fluxes and sizes; 4 identify corrections for size overestimation noise and underestimation SB-dimming; and 5 delineate the exact locus of the natural confusion limit, and determine if and how it overtakes SB as the major cause of incompleteness in HST surveys for AB>24-28 mag.;

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