| Summary: | 碩士 === 國立臺灣大學 === 天文物理研究所 === 102 === People have discovered more than hundreds of thousands of quasars at a redshift below 6. However, there are only dozens of them discovered above redshift 6. There are three main reasons that cause this situation. First, the higher the redshift, the fainter the quasar. Second, the number density for quasars decreases at higher redshifts. Third, the sensitivity of the instrument especially at longer wavelengths and survey volume there limits detection of those quasars. People have been developing methods to explore higher-redshift quasars, and their color selection to utilize drop in magnitude below the redshifted Lyman alpha emission has been known to be very powerful. This is because the neutral hydrogen along the line of sight absorbs redshifted emission of quasar below its redshifted Lyman-alpha. As the Lyman-alpha is redshifted, the longer wavebands need to be covered to catch such a quasar-specific signature. However, there are some contaminants such as brown dwarf stars to those dropout sources, and improving color-selection criteria for removing those contaminants are highly desirable for efficient and robust detection of presumably very rare high redshift quasars. The goal of this project is to search for z>5.5 quasars while trying to improve the criteria for color selection. The data used in this project are mostly publicly available data. In particular, we utilize data from the Canada-France-Hawaii Telescope Legacy Survey (CFHTLS) for the optical wavebands, Visible and Infrared Survey Telescope for Astronomy (VISTA) Deep Extragalactic Observations (VIDEO) survey for the near infrared wavebands, and the Spitzer Extragalactic Representative Volume Survey (SERVS) and Spitzer Wide-Area Infrared Extragalactic Survey (SWIRE) for the mid-infrared wavebands. Private data from the Infrared Medium-deep Survey (IMS) conducted by United Kingdom Infrared Telescope (UKIRT) are also analyzed. We have worked directly on the deep astronomical array images of IMS and CFHTLS for extracting sources and applying photometric calibrations to create catalogues. We also utilize publicly available catalogues to combine with our own catalogues. The resultant catalogues covering side wavelength ranges from 0.6 to 8.0 micrometers (μm) are examined in combination with colors of known high-redshift quasars and brown dwarfs. By utilizing the wide wavelength coverage, we propose our own color selection criteria for quasar selection. We pay special attention to eliminate brown dwarfs whose spectral energy distribution (SED) are quite different from the power-law like quasar SED. We create our own new catalogues of quasar candidates by utilizing the new criteria. We investigate photometric properties of such color-selected i- and z-dropout quasars, and discuss if expectation from the lower-redshift quasar luminosity function is consistent with our detection rate.
|
|---|
