Physics & Astronomy ETDs


Paul Thorman

Publication Date



The Deep Lens Survey, a 20 deg2 optical survey, was used to search for two types of rare, very red objects: faint high-redshift (z > 5.4) quasars, which trace structure formation in the early Universe and can be used to study reionization; and ultracool dwarf stars and brown dwarfs, the most numerous constituents of our own Galaxy, which are seldom detected beyond the solar neighborhood due to their low luminosities. To distinguish between these two populations, we added near-infrared imaging to a portion of the Deep Lens Survey area, covering 1.12 deg2 with survey J-band (1.2 μm) imaging, and successfully targeting 181 out of a planned 280 very red targets over an additional 2.4 deg2. These observations were reduced and cataloged, producing colors or color limits for 1626 very red objects in the field. The colors were corrected using a combination of outside photometric standards and internal checks, and a model for the colors of the target populations was created from existing measurements in the literature. The relative classification probabilities were estimated for each object by using a color model, and those probabilities were used to generate Monte Carlo realizations of the populations, either for comparison to a simulated Galaxy (for ultracool dwarfs) or for the calculation of a luminosity function (for high-redshift QSOs). For QSOs, the results were consistent with the most recent work on faint QSOs: we measured a bright-end power-law slope of for the luminosity function. Citing the similarity to earlier results for brighter QSOs, we rule out QSOs as the dominant source of ionizing photons at the time of reionization (although they may have contributed at the 10-20% level). For ultracool dwarfs, the model that best reproduced our measured magnitudes was a sech2 vertical disk profile with for the earliest spectral type range (M9-L2), with hints of a lower scale height for later spectral types. We integrate a range of best-fit models to predict the number of ultracool dwarfs to be found in future surveys, and estimate the errors introduced into current surveys by over-simplified models of the Galaxy. Spectroscopic observations were obtained for a few of our very red objects; these are displayed and analyzed. Although none of the targeted objects was definitively identified as a QSO, they showed interesting and unusual spectral features nonetheless. The methods demonstrated here for probabilistic classification of galactic and extragalactic objects will be essential to exploit the full potential of future wide deep surveys, such as LSST. These surveys will reach such faint objects that spectroscopy will be difficult or impossible for many years to come, yet we have shown that the imaging alone, augmented when necessary with follow-up photometry in specific bands, can provide the classifications needed to explore these unknown populations.

Degree Name


Level of Degree


Department Name

Physics & Astronomy

First Committee Member (Chair)

Jones, Rhian

Second Committee Member

McGraw, John

Third Committee Member

Rand, Richard

Project Sponsors

New Mexico Space Grant




Quasars, Dwarf stars, Brown dwarf stars, Cool stars, Low mass stars, Stars--Classification.

Document Type