Contamination by field late-M, L and T dwarfs in deep surveys

TL;DR

This paper models and quantifies the contamination of deep astronomical surveys by late-type field dwarfs, providing tools to improve the accuracy of studies on substellar objects and high-redshift quasars.

Contribution

It offers a detailed calculation method for foreground and background dwarf contamination and supplies practical tools for survey analysis.

Findings

Contamination increases at faint magnitudes, affecting substellar studies.

Derived a simplified density expression for thin disc dwarfs.

Applied model to surveys of the sigma Orionis cluster.

Abstract

Context: Deep photometric surveys for substellar objects in young clusters and for high-redshift quasars are affected by contaminant sources at different heliocentric distances.If not correctly taken into account, the contamination may have a strong effect on the Initial Mass Function determination and on the identification of quasars. Aims: We calculate in detail the back- and foreground contamination by field dwarfs of very late spectral types (intermediate and late M, L and T) in deep surveys and provide the data and tools for the computation. Methods: Up-to-date models and data from the literature have been used: (i) a model of the Galactic thin disc by an exponential law; (ii) the length and height scales for late-type dwarfs; (iii) the local spatial densities, absolute magnitudes and colours of dwarfs for each spectral type. Results: We derive a simplified expression for the spatial density in the thin disc that depends on the heliocentric distance and the galactic coordinates (l, b) and integrate it in the truncated cone screened in the survey. As a practical application, we compute the numbers of L- and T-type field dwarfs in very deep (I = 21-29 mag) surveys in the direction of the young sigma Orionis cluster. The increasing number of contaminants at the faintest magnitudes could inhibit the study of the opacity mass limit at M <~ 0.003 M_sol in the cluster.