“GALAXY,” DEFINED

By B. Willman, J. Strader. 2012

188 citations recorded by the source corpus.

A growing number of low luminosity and low surface brightness astronomical objects challenge traditional notions of both galaxies and star clusters. To address this challenge, we propose a definition of galaxy that does not depend on a cold dark matter model of the universe: a galaxy is a gravitationally bound collection of stars whose properties cannot be explained by a combination of baryons and Newton's laws of gravity. After exploring several possible observational diagnostics of this definition, we critically examine the classification of ultra-faint dwarfs, globular clusters, ultra-compact dwarfs, and tidal dwarfs. While kinematic studies provide an effective diagnostic of the definition in many regimes, they can be less useful for compact or very faint systems. To explore the utility of using the [Fe/H] spread as a complementary diagnostic, we use published spectroscopic [Fe/H] measurements of 16 Milky Way dwarfs and 24 globular clusters to uniformly calculate their [Fe/H] spreads and associated uncertainties. Our principal results are (1) no known, old star cluster less luminous than MV = −10 has a significant (≳0.1 dex) spread in its iron abundance; (2) known ultra-faint dwarf galaxies can be unambiguously classified with a combination of kinematic and [Fe/H] observations; (3) the observed [Fe/H] spreads in massive (≳ 106 M☉) globular clusters do not necessarily imply that they are the stripped nuclei of dwarfs, nor a need for dark matter; and (4) if ultra-compact dwarf galaxies reside in dark matter halos akin to those of ultra-faint dwarfs of the same half-light radii, then they will show no clear dynamical signature of dark matter. We suggest several measurements that may assist the future classification of massive globular clusters, ultra-compact dwarfs, and ultra-faint galaxies. Our galaxy definition is designed to be independent of the details of current observations and models, while our proposed diagnostics can be refined or replaced as our understanding of the universe evolves.

Subfields: Astronomy and Astrophysics

View Paper: https://www.semanticscholar.org/paper/e190ac2d4ef41e8f6f7614f2e043ed81d01576eb

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In Astronomy and Astrophysics

Citations in the corpus

Small-Scale Challenges to the ΛCDM Paradigm (2017, unknown): “(Willman & Strader 2012).”
The birth of a galaxy – III. Propelling reionization with the faintest galaxies (2014, background): “However, 4 We define a galaxy as any metal-enriched stellar system that is gravitationally bound and exists in a dark matter halo (Willman & Strader 2012). the SFRs and UV escape fractions are important to quantify in small volume, high-resolution simulations, so they can guide radiation source…”
A NEW FAINT MILKY WAY SATELLITE DISCOVERED IN THE PAN-STARRS1 3π SURVEY (2015, background): “It has become apparent that the previously clear distinction between the compact globular clusters (GCs) and the brighter, more extended, and dark-matter dominated dwarf galaxies (DGs), blurs out for faint systems (Willman & Strader 2012).”
EIGHT ULTRA-FAINT GALAXY CANDIDATES DISCOVERED IN YEAR TWO OF THE DARK ENERGY SURVEY (2015, background): “…mass that is substantially larger than the mass inferred from the luminous stellar population and/or a significant dispersion in stellar metallicities indicative of multiple generations of star formation and a gravitational potential deep enough to retain supernova ejecta (Willman & Strader 2012).”
EIGHT NEW MILKY WAY COMPANIONS DISCOVERED IN FIRST-YEAR DARK ENERGY SURVEY DATA (2015, background): “…mass that is substantially larger than the mass inferred from the luminous stellar population and/or a significant dispersion in metallicities indicative of multiple generations of star formation and a deep enough gravitational potential to retain supernova ejecta (Willman & Strader 2012).”
The Dark Matter Crisis: Falsification of the Current Standard Model of Cosmology (2012, background): “The definition of a galaxy (Forbes & Kroupa 2011; Willman & Strader 2012) is an important question to consider because depending on it a whole class of objects may be excluded which may forestall further intellectual advance.”
SEARCH FOR GAMMA-RAY EMISSION FROM DES DWARF SPHEROIDAL GALAXY CANDIDATES WITH FERMI-LAT DATA (2015, background): “Globular clusters can be distinguished from dwarf galaxies based on spectroscopic measurements (Willman & Strader 2012).”