
TL;DR
This paper proposes a new, observationally based definition of galaxies that does not rely on dark matter models, and explores diagnostics like kinematics and [Fe/H] spreads to classify faint and compact objects.
Contribution
It introduces a galaxy definition independent of dark matter and develops observational diagnostics for classifying faint and compact stellar systems.
Findings
Ultra-faint dwarfs can be classified with kinematics and [Fe/H] spreads.
Globular clusters less luminous than M_V = -10 lack significant [Fe/H] spreads.
Ultra-compact dwarfs may not show dark matter signatures if in similar halos as ultra-faint dwarfs.
Abstract
A growing number of low luminosity and low surface brightness astronomical objects challenge traditional notions of both galaxies and star clusters. To address this, we propose a definition of galaxy independent of the cold dark matter model of the universe: A galaxy is a gravitationally bound set of stars whose properties cannot be explained by a combination of baryons and Newton's laws of gravity. After exploring observational diagnostics of this definition, we examine the classification of ultra-faint dwarfs, globular clusters, ultra-compact dwarfs, and tidal dwarfs. While kinematic studies provide an effective galaxy diagnostic in many regimes, they can be less useful for compact or faint systems. To explore the use of [Fe/H] spread as a complementary diagnostic, we use published spectroscopic [Fe/H] measurements of 16 Milky Way dwarfs and 24 globular clusters to calculate their…
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