AI Insight
This study analyzes thirty Hoag-type galaxies—rare systems with a smooth central core surrounded by a detached outer ring—using imaging data from the DESI Legacy Survey. The researchers found that the cores typically lie on the red sequence and resemble early-type galaxies but with slightly bluer colors, smaller sizes, and higher stellar-mass surface densities, while the rings show more diverse colors extending toward blue. Most systems (87.5%) are central galaxies in their groups rather than satellites, which may favor gas retention and ring survival by reducing environmental disruption from nearby galaxies.
Why it matters
Understanding these unusual galaxy structures helps astronomers refine models of galaxy formation and evolution, particularly regarding how dense cores form while maintaining detached outer rings. The high fraction of central galaxies among Hoag-type systems provides insights into environmental factors that enable such rare morphologies to persist.
Understand the Science
⚠️ Preprint – Noch nicht peer-reviewed
Dieser Artikel wurde noch nicht von unabhängigen Experten begutachtet. Die Ergebnisse sind vorläufig und sollten mit Vorsicht interpretiert werden.
Abstract: Hoag-type galaxies contain a smooth central core surrounded by a detached outer ring, but their rarity has limited previous studies mainly to individual systems. We identify thirty galaxies with apparent Hoag-type morphology in DESI imaging by visually inspecting two machine-learning ring-galaxy catalogues and analyse their core and ring properties using DESI Legacy Imaging Surveys $g$, $r$, and $z$ images. We measure the rings after subtracting two-dimensional core models and compare the cores with matched SDSS galaxies. The cores lie predominantly on the red sequence, whereas the rings span a broader colour range extending toward the blue cloud. The cores broadly follow early-type mass scaling relations, but their mass-matched distributions tend towards slightly bluer colours, smaller sizes, higher stellar-mass surface densities, and lower concentrations and S’ersic indices. Of the 16 systems with SDSS group information, 14 (87.5 per cent) are centrals, compared with $sim64%$ in the mass-matched early-type control sample. The halo-mass and large-scale overdensity distributions become much more similar after central–satellite status is also matched. Ring stellar-mass surface density and normalized width decrease with increasing core-plus-inner-halo to ring mass ratio. Most core $R_{90}$ values lie inside the outermost stable circular orbit of a simplified core-plus-ring model, while the rings frequently extend across its unstable interval. The slightly bluer core colours, lower concentrations, and lower S’ersic indices suggest a possible secular contribution, but the compact, dense cores imply more complex structural assembly than a simple diffuse-pseudobulge origin. The high central fraction may favour gas retention and ring survival by reducing satellite-specific environmental processing. The diversity of ring properties nevertheless leaves room for multiple formation pathways.