Point spread function tails and the measurements of diffuse stellar halo light around edge-on disc galaxies

TL;DR

This paper highlights that extended PSF tails significantly affect measurements of diffuse stellar halo light around edge-on galaxies, emphasizing the need for careful PSF modeling to accurately interpret halo properties.

Contribution

It demonstrates that scattered galaxy light due to PSF tails can account for a large fraction of observed halo light and colors, challenging previous interpretations of stellar halos.

Findings

Scattered light explains 20-80% of halo light in SDSS images.

PSF tails can mimic halo features along the minor axis.

Careful PSF measurement is essential for accurate halo analysis.

Abstract

Measuring the integrated stellar halo light around galaxies is very challenging. The surface brightness of these haloes are expected to be many magnitudes below dark sky and the central brightness of the galaxy. Here I show that in some of the recent literature the effect of very extended Point Spread Function (PSF) tails on the measurements of halo light has been underestimated; especially in the case of edge-on disc galaxies. The detection of a halo along the minor axis of an edge-on galaxy in the Hubble Ultra Deep Field can largely be explained by scattered galaxy light. Similarly, depending on filter and the shape one assumes for the uncertain extended PSF, 20 to 80 per cent of the halo light found along the minor axis of scaled and stacked Sloan Digital Sky Survey (SDSS) edge-on galaxy images can be explained by scattered galaxy light. Scattered light also significantly contributes to the anomalous halo colours of stacked SDSS images. The scattered light fraction decreases when looking in the quadrants away from the minor axis. The remaining excess light is well modelled with a Sersic profile halo with shape parameters based on star count halo detections of nearby galaxies. Even though the contribution from PSF scattered light does not fully remove the need for extended components around these edge-on galaxies, it will be very challenging to make accurate halo light shape and colour measurements from integrated light without very careful PSF measurements and scattered light modelling.