The ALHAMBRA survey: Accurate merger fractions by PDF analysis of photometric close pairs

TL;DR

This paper introduces a new method using full photometric redshift PDFs to accurately measure galaxy merger fractions from photometric data, aligning well with spectroscopic results and enabling better analysis of galaxy evolution.

Contribution

The paper presents a novel methodology that leverages full photometric redshift PDFs, including spectral template information, to improve merger fraction estimates from photometric surveys.

Findings

Merger rates evolve as (1+z)^n with n=2.7 for blue and 1.3 for red galaxies.

Average mergers per galaxy since z=1 are 0.57 for red and 0.26 for blue galaxies.

Method achieves accuracy comparable to spectroscopic studies using only photometric data.

Abstract

Our goal is to develop and test a novel methodology to compute accurate close pair fractions with photometric redshifts. We improve the current 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 selection of the samples and in the luminosity ratio constrain, and (iii) splitting individual PDFs into red and blue spectral templates to deal robustly with colour selections. We test the performance of our new methodology with the PDFs provided by the ALHAMBRA photometric survey. The merger fractions and rates from the ALHAMBRA survey are in excellent agreement with those from spectroscopic work, both for the general population and for 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 larger 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 = 0.57 +- 0.05 for red galaxies and N_m = 0.26 +- 0.02 for blue galaxies. Our new methodology exploits statistically all the available information provided by photometric redshift codes and provides accurate measurements of the merger fraction by close pairs only using photometric redshifts. Current and future photometric surveys will benefit of this new methodology.