Multi-resolution Filtering: An Empirical Method for Isolating Faint, Extended Emission in Dragonfly Data and Other Low Resolution Images

Abstract

We describe an empirical, self-contained method to isolate faint, large-scale emission in imaging data of low spatial resolution. Multi-resolution filtering (MRF) uses independent data of superior spatial resolution and point source depth to create a model for all compact and high surface brightness objects in the field. This model is convolved with an appropriate kernel and subtracted from the low resolution image. The halos of bright stars are removed in a separate step and artifacts are masked. The resulting image only contains extended emission fainter than a pre-defined surface brightness limit. The method was developed for the Dragonfly Telephoto Array, which produces images that have excellent low surface brightness sensitivity but poor spatial resolution. We demonstrate the MRF technique using Dragonfly images of a satellite of the spiral galaxy M101, the tidal debris surrounding M51, two ultra-diffuse galaxies in the Coma cluster, and the galaxy NGC 5907. As part of the analysis we present a newly-identified very faint galaxy in the filtered Dragonfly image of the M101 field. We also discuss variations of the technique for cases when no low resolution data are available (self-MRF and cross-MRF). The method is implemented in mrf, an open-source MIT licensed Python package (https://github.com/AstroJacobLi/mrf).

Improving Physical Cosmology: An Empiricist's Assessment

“…Pure disk galaxies do accrete companions, as must be expected from classical mechanics. A classic example is the pure disk edge-on galaxy NGC 5907 with its stellar stream that we may expect eventually will add to the stellar halo of this galaxy [80]. It happens, but certainly not often among nearby pure disk spirals.…”

Stellar halos from The Dragonfly Edge-on Galaxies Survey

“…The final images used in our analysis were sourcesubtracted images generated by the mrf package (van Dokkum et al 2020). This package takes advantage of secondary high resolution data sets to model closelyspaced compact sources in primary low resolution data sets (i.e.…”

NGC5846-UDG1: A galaxy formed mostly by star formation in massive, extremely dense clumps of gas

“…Instead, we utilized the HST images in a different way. We used the multi-resolution filtering (MRF) software (van Dokkum et al 2020) to remove all compact sources from the DECaLS images and isolated the diffuse component of NGC5846-UDG1. Briefly, SExtractor is run on the high resolution HST images, and the resulting segmentation map was converted to a mask and multiplied by the image to create a flux model of all detected sources.…”