Abstract
Our statistical understanding of galaxy evolution is fundamentally driven by objects that lie above the surface-brightness limits of current wide-area surveys (mu ~ 23 mag arcsec^-2). While both theory and small, deep surveys have hinted at a rich population of low-surface-brightness galaxies (LSBGs) fainter than these limits, their formation remains poorly understood. We use Horizon-AGN, a cosmological hydrodynamical simulation to study how LSBGs, and in particular the population of ultra-diffuse galaxies (UDGs; mu > 24.5 mag arcsec^-2), form and evolve over time. For M* > 10^8 MSun, LSBGs contribute 47, 7 and 6 per cent of the local number, mass and luminosity densities respectively (~85/11/10 per cent for M* > 10^7 MSun). Today's LSBGs have similar dark-matter fractions and angular momenta to high-surface-brightness galaxies (HSBGs; mu < 23 mag arcsec^-2), but larger effective radii (x2.5 for UDGs) and lower fractions of dense, star-forming gas (more than x6 less in UDGs than HSBGs). LSBGs originate from the same progenitors as HSBGs at z > 2. However, LSBG progenitors form stars more rapidly at early epochs. The higher resultant rate of supernova-energy injection flattens their gas-density profiles, which, in turn, creates shallower stellar profiles that are more susceptible to tidal processes. After z ~ 1, tidal perturbations broaden LSBG stellar distributions and heat their cold gas, creating the diffuse, largely gas-poor LSBGs seen today. In clusters, ram-pressure stripping provides an additional mechanism that assists in gas removal in LSBG progenitors. Our results offer insights into the formation of a galaxy population that is central to a complete understanding of galaxy evolution, and which will be a key topic of research using new and forthcoming deep-wide surveys.
Highlights
Our understanding of galaxy evolution is intimately linked to the part of the galaxy population that is visible at the surface-brightness limits of past and current wide-area surveys
Our results offer insights into the formation of a galaxy population that is central to a complete understanding of galaxy evolution, and which will be a key topic of research using new and forthcoming deep-wide surveys
The analysis presented above shows that the formation mechanisms that produce lowsurface-brightness galaxies (LSBGs) act to both increase the effective radii of their progenitors and drive the steady loss of star-forming gas
Summary
Our understanding of galaxy evolution is intimately linked to the part of the galaxy population that is visible at the surface-brightness limits of past and current wide-area surveys. While modern instruments are enabling the study of systems at significantly fainter surface-brightnesses than was previously possible, deep-wide surveys and spectroscopic follow-up of areas large enough to contain significant populations of LSBGs outside dense, cluster environments remain prohibitively expensive. Observational evidence using globular cluster abundances (Beasley & Trujillo 2016; Peng & Lim 2016; Amorisco et al 2018), velocity dispersions (e.g Toloba et al 2018), weak lensing measurements (e.g Sifón et al 2018), stellar populations (e.g Ferré-Mateu et al 2018; Ruiz-Lara et al 2018), and the spatial distributions and abundances of the galaxies themselves (e.g Román & Trujillo 2017a), largely supports the idea that the vast majority of LSBGs are lowmass (i.e. dwarf) galaxies that are hosted by correspondingly low mass dark-matter haloes, except perhaps in a small number of extreme cases (e.g van Dokkum et al 2016; Beasley et al 2016).
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