Effects of diffuse background emission and source crowding on photometric completeness in Spitzer Space Telescope IRAC surveys: The GLIMPSE Catalogs and Archives

TL;DR

This study assesses how diffuse background emission and source crowding affect the completeness of point source detection in Spitzer IRAC surveys, especially in the Galactic Plane, providing detailed completeness metrics for various conditions.

Contribution

It offers a detailed analysis of completeness dependencies on background brightness and source density, with new quantitative data applicable to GLIMPSE and similar Spitzer IRAC surveys.

Findings

Completeness decreases significantly in high-brightness regions, especially at 5.8 and 8.0 microns.

Diffuse background structure, not photon noise, mainly causes source detection incompleteness.

Provided graphical and tabular data to estimate completeness across different background levels and source densities.

Abstract

We characterize the completeness of point source lists from Spitzer Space Telescope surveys in the four Infrared Array Camera (IRAC) bandpasses, emphasizing the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) programs (GLIMPSE I, II, 3D, 360; Deep GLIMPSE) and their resulting point source Catalogs and Archives. The analysis separately addresses effects of incompleteness resulting from high diffuse background emission and incompleteness resulting from point source confusion (i.e., crowding). An artificial star addition and extraction analysis demonstrates that completeness is strongly dependent on local background brightness and structure, with high-surface-brightness regions suffering up to five magnitudes of reduced sensitivity to point sources. This effect is most pronounced at the IRAC 5.8 and 8.0 microns bands where UV-excited PAH emission produces bright, complex structures (photodissociation regions; PDRs). With regard to diffuse background effects, we provide the completeness as a function of stellar magnitude and diffuse background level in graphical and tabular formats. These data are suitable for estimating completeness in the low-source-density limit in any of the four IRAC bands in GLIMPSE Catalogs and Archives and some other Spitzer IRAC programs that employ similar observational strategies and are processed by the GLIMPSE pipeline. Point source incompleteness is primarily a consequence of structure in the diffuse background emission rather than photon noise. With regard to source confusion in the high-source-density regions of the Galactic Plane, we provide figures illustrating the 90% completeness levels as a function of point source density at each band. (Slightly abridged)