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Star/quasar separation and photo-z's of quasars in narrow-band filter surveys: the role of Bayesian priors

Grant number: 14/21148-9
Support type:Scholarships abroad - Research Internship - Master's degree
Effective date (Start): October 27, 2014
Effective date (End): January 24, 2015
Field of knowledge:Physical Sciences and Mathematics - Astronomy
Principal Investigator:Luis Raul Weber Abramo
Grantee:Carolina Queiroz de Abreu Silva
Supervisor abroad: Silvia Bonoli
Home Institution: Instituto de Física (IF). Universidade de São Paulo (USP). São Paulo , SP, Brazil
Local de pesquisa : Centro de Estudios de Física del Cosmos de Aragón (CEFCA), Spain  
Associated to the scholarship:12/25503-2 - Cosmology with quasars and AGNs, BP.MS


Quasars are the brightest non-transient objects in the Universe. They trace some of the heaviest halos, and are abundant at high redshifts, since their luminosity function peaks around $z~2$. These two facts imply that quasars (AGNs, in a more general sense) have a huge potential to help us map the web of structures of the Universe on its largest scales. This potential has not yet been fully realized with current surveys, which, for the most part, employ broad-band filters (e.g., $ugriz$) to pre-select targets: without their emission lines, broad-line emitting quasars are easily confused with stars, and narrow-line emitting AGNs are very hard to observe. Moreover, given that most quasar spectra have a power-law continuum, their photometric redshifts inferred just on the basis of observations with broad-band filters is basically useless. However, with the advent of narrow-band filter surveys, such as ALHAMBRA (Molino et al. 2014), and, soon, J-PAS and J-PLUS (Ben\'{\i}tez et al. 2014), now there are new prospects to separate quasars from stars, to identify AGNs, and to obtain accurate redshifts for millions of these objects. The goal of this project is to explore the efficiency of star-quasar separation, as well as the accuracy of the photometric redshifts, when narrow-band filters (FWHM $\sim100-200$ \AA) in the optical are employed. The main novelty of our approach is the use of Bayesian priors both for the angular distribution of stars of different types on the sky and for the distribution of quasars and AGNs as a function of redshift (which result from their luminosity functions). The distribution of stars across the Milky Way can be estimated through observations or using empirical models, such as the {\it Besan\c{c}on Model}. The redshift distribution of quasars and AGNs, on the other hand, can be estimated using their luminosity functions -- as it is done in the case of galaxies. The evidence from these priors convolves the angular dependence of stars with the redshift dependence of quasars -- allowing us to control for this critical systematic effect, which plagues all existing quasar surveys. In order to study these issues, we will employ data from ALHAMBRA, as well as simulated data using SDSS and zCOSMOS. Dr. Silvia Bonoli (CEFCA), the local advisor during this BEPE fellowship, is a specialist on the connection of quasars with large-scale structure and with the environments where these objects can be found. She has also been working on ALHAMBRA data in the context of the photo-z code LePhare. The student, Carolina Queiroz A. Silva, already has the Bayesian priors set up and is proficient in both LePhare and BPZ (her priors can be used in both codes). Another critical issue that we will tackle is the enrichment of the sets of templates for stars, quasars and AGNs. We plan to test several combinations of these templates, in order to find the set which maximizes completeness and purity of the quasar and AGN catalog. (AU)