Why the Northern Hemisphere Needs a 30-40 m Telescope and the Science at Stake: A Low Surface Brightness Science Case

TL;DR

Extragalactic low-surface-brightness galaxies (mu_V ≳ 27 mag arcsec^-2) represent a crucial frontier for understanding galaxy formation, hierarchical assembly, and dark matter. The paper argues that a $30$–$40$ m class telescope in the Northern Hemisphere with adaptive optics would unlock this regime by resolving stellar populations out to about $15$ Mpc and by probing integrated light in more distant systems. It outlines science cases including chemical tagging of streams, measurement of kinematics to constrain dark-matter density profiles, and mapping of intrahalo light and outer halos across nearby clusters. Strategic Northern Hemisphere placement would enable access to M31, its satellites, and rich clusters like Coma and Perseus, enabling statistically robust tests of assembly scenarios and dark matter in diverse environments.

Abstract

The Extragalactic Low Surface Brightness (LSB, $μ_V\gtrsim 27$ mag/arcsec$^2$) Universe represents a crucial, yet largely unseen, frontier in modern astrophysics. This faint realm holds the keys to completing our understanding of galaxy evolution, hierarchical assembly, and even the fundamental nature of dark matter. Our current theoretical models are inherently incomplete, largely mirroring the properties of the brightest, most easily observed objects. To overcome this critical bias and unlock the secrets of this realm, a transformative leap in observational capability is required. A 30 to 40m class telescope, leveraging unprecedented sensitivity and spatial resolution, especially with adaptive optics, is the essential tool to fundamentally probe these faint, low-density stellar regimes. This white paper details the transformative LSB science that such a facility, strategically positioned in the Northern Hemisphere (NH) to access crucial nearby structures and rich environments, can achieve.