Resolving stellar populations with crowded field 3D spectroscopy

TL;DR

This paper introduces a novel method for extracting stellar spectra from crowded field 3D spectroscopic data, leveraging existing source catalogs and modeling PSF variations to improve spectral recovery in dense stellar environments.

Contribution

The paper presents a new algorithm that extends crowded field photometry techniques to 3D spectroscopy, enabling efficient spectral extraction without requiring isolated PSF calibrators.

Findings

Spectral S/N decreases with crowding, limiting useful spectra to about 0.2 stars per resolution element.

The method accurately estimates PSF variations across spectral layers using simple models.

Application to simulated globular cluster data demonstrates the algorithm's effectiveness in crowded fields.

Abstract

(Abridged) We describe a new method to extract spectra of stars from observations of crowded stellar fields with integral field spectroscopy (IFS). Our approach extends the well-established concept of crowded field photometry in images into the domain of 3-dimensional spectroscopic datacubes. The main features of our algorithm are: (1) We assume that a high-fidelity input source catalogue already exists and that it is not needed to perform sophisticated source detection in the IFS data. (2) Source positions and properties of the point spread function (PSF) vary smoothly between spectral layers of the datacube, and these variations can be described by simple fitting functions. (3) The shape of the PSF can be adequately described by an analytical function. Even without isolated PSF calibrator stars we can therefore estimate the PSF by a model fit to the full ensemble of stars visible within the field of view. (4) By using sparse matrices to describe the sources, the problem of extracting the spectra of many stars simultaneously becomes computationally tractable. We present extensive performance and validation tests of our algorithm using realistic simulated datacubes that closely reproduce actual IFS observations of the central regions of Galactic globular clusters. We investigate the quality of the extracted spectra under the effects of crowding. The main effect of blending between two nearby stars is a decrease in the S/N in their spectra. The effect increases with the crowding in the field in a way that the maximum number of stars with useful spectra is always ~0.2 per spatial resolution element. This balance breaks down when exceeding a total source density of ~1 significantly detected star per resolution element. We close with an outlook by applying our method to a simulated globular cluster observation with the upcoming MUSE instrument at the ESO-VLT.