Structural parameters of star clusters: relations among light, mass and star-count radial profiles and the dependence on photometric depth

TL;DR

This study examines how different observational methods and photometric depths affect the measurement of structural parameters in star clusters, revealing that some profiles are more robust against observational limitations.

Contribution

It introduces a comprehensive analysis of radial profile measurements across various photometric conditions, highlighting the dependence of structural parameters on observational depth and cluster properties.

Findings

Surface-brightness profiles are insensitive to photometric depth.

Number-density and mass-density radii vary significantly with depth.

Deep photometry yields larger radii than shallow observations.

Abstract

Structural parameters of model star clusters are measured in radial profiles built from number-density, mass-density and surface-brightness distributions, assuming as well different photometric conditions. Star clusters of different ages, structure and mass functions are modelled by assuming that the radial distribution of stars follows a pre-defined analytical form. Near-infrared surface brightness and mass-density profiles result from mass-luminosity relations taken from a set of isochrones. Core, tidal and half-light, half-mass and half-star count radii, together with the concentration parameter, are measured in the three types of profiles, which are built under different photometric depths. While surface-brightness profiles are almost insensitive to photometric depth, radii measured in number-density and mass-density profiles change significantly with it. Compared to radii derived with deep photometry, shallow profiles result in lower values. This effect increases for younger ages. Radial profiles of clusters with a spatially-uniform mass function produce radii that do not depend on depth. With deep photometry, number-density profiles yield radii systematically larger than those derived from surface-brightness ones. In general, low-noise surface-brightness profiles result in uniform structural parameters that are essentially independent of photometric depth. For less-populous star clusters, those projected against dense fields and/or distant ones, which result in noisy surface-brightness profiles, this work provides a quantitative way to estimate the intrinsic radii by means of number-density profiles built with depth-limited photometry.