Studying the halos of nearby galaxies in the HERON Survey through galaxy decomposition using GALFIT

Laura DeGroot

The halos of nearby galaxies can provide important insight into the formation and evolution of galaxies. Current cosmology assumes that hierarchical structure formation of galaxies is due to the accretion of smaller, dwarf-size dark matter clumps. The dark matter and stellar content are well constrained and can be predicted based on galaxy mass. By studying the observed stellar content of faint haloes of galaxies, we can compare this to theoretical predictions to test modern-day cosmology. The HERON program is using a dedicated telescope to image nearby galaxies to low surface brightness, so that the halo can be studied.

To complete this study, we use GALFIT (Peng et al. 2002, 2010) to decompose nearby elliptical and spiral galaxies into their components, including the bulge, disk, and halo, so that we can separate and study the low surface brightness halo component of each galaxy. The difficulties of completing multi-component modeling includes determining reasonable estimates of input parameters, such as size, magnitude, and Sérsic index, as well as understanding the uncertainty in the resulting measured morphological parameters. Also, due to the extreme low surface brightness of the halo, the galaxy model would also need to take into account any light contamination from other objects in the image, which is done by masking or simultaneous modeling of the light of surrounding objects. 

Once the halo light of the galaxy is accurately modeled, we can determine the fraction of the galaxy light that is in the halo as well as its shape, which can be compared to theoretical predictions from the HERON Survey team. For elliptical galaxies, a primary focus of the project, we can also measure the boxiness or diskyness of the halo isophote, which can help to constrain galaxy formation and evolution scenarios.