The ALHAMBRA survey: accurate merger fractions derived by PDF analysis of photometrically close pairs^⋆,⋆⋆

¹ Centro de Estudios de Física del Cosmos de Aragón, Plaza San Juan 1, 44001 Teruel, Spain
e-mail: clsj@cefca.es
² Instituto de Astrofísica de Andalucía (IAA-CSIC), Glorieta de la astronomía s/n, 18008 Granada, Spain
³ Institute for Computational Cosmology, Department of Physics, Durham University, South Road, Durham DH1 3LE, UK
⁴ GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 61 Avenue de l’Observatoire, 75014 Paris, France
⁵ Instituto de Física de Cantabria, Avenida de los Castros s/n, 39005 Santander, Spain
⁶ Observatori Astronòmic, Universitat de València, C/ Catedrático José Beltrán 2, 46980 Paterna, Spain
⁷ Observatório Nacional, COAA, Rua General José Cristino 77, 20921-400 Rio de Janeiro, Brazil
⁸ Instituto de Astrofísica de Canarias, vía Láctea s/n, La Laguna, 38200 Tenerife, Spain
⁹ Departamento de Astrofísica, Facultad de Física, Universidad de la Laguna, 38200 La Laguna, Spain
¹⁰ Department of Theoretical Physics, University of the Basque Country UPV/EHU, 48040 Bilbao, Spain
¹¹ Departamento de Física Atómica, Molecular y Nuclear, Facultad de Física, Universidad de Sevilla, 41080 Sevilla, Spain
¹² Institut de Ciències de l’Espai (IEEC-CSIC), Facultat de Ciéncies, Campus UAB, 08193 Bellaterra, Spain
¹³ Departamento de Astronomía, Pontificia Universidad Católica, Santiago, 782-0436 Macul, Chile
¹⁴ Departament d’Astronomia i Astrofísica, Universitat de València, 46100 Burjassot, Spain

Received: 3 September 2014
Accepted: 10 December 2014

Abstract

Aims. Our goal is to develop and test a novel methodology to compute accurate close-pair fractions with photometric redshifts.

Methods. We improved the currently used methodologies to estimate the merger fraction f_m from photometric redshifts by (i) using the full probability distribution functions (PDFs) of the sources in redshift space; (ii) including the variation in the luminosity of the sources with z in both the sample selection and the luminosity ratio constrain; and (iii) splitting individual PDFs into red and blue spectral templates to reliably work with colour selections. We tested the performance of our new methodology with the PDFs provided by the ALHAMBRA photometric survey.

Results. The merger fractions and rates from the ALHAMBRA survey agree excellently well with those from spectroscopic work for both the general population and red and blue galaxies. With the merger rate of bright (M_B ≤ −20−1.1z) galaxies evolving as (1 + z)ⁿ, the power-law index n is higher for blue galaxies (n = 2.7 ± 0.5) than for red galaxies (n = 1.3 ± 0.4), confirming previous results. Integrating the merger rate over cosmic time, we find that the average number of mergers per galaxy since z = 1 is N_m^red = 0.57 ± 0.05 for red galaxies and N_m^blue = 0.26 ± 0.02 for blue galaxies.

Conclusions. Our new methodology statistically exploits all the available information provided by photometric redshift codes and yields accurate measurements of the merger fraction by close pairs from using photometric redshifts alone. Current and future photometric surveys will benefit from this new methodology.