Theoretical lower limits on sizes of ultra faint dwarf galaxies from dynamical friction

Abstract

Dwarf spheroidal galaxies are the smallest known stellar systems where under Newtonian interpretations, a significant amount of dark matter is required to explain observed kinematics. In fact, they are in this sense the most heavily dark matter dominated objects known. That, plus the increasingly small sizes of the newly discovered ultra faint dwarfs, puts these systems in the regime where dynamical friction on individual stars starts to become relevant. We calculate the dynamical friction timescales for pressure supported isotropic spherical dark matter dominated stellar systems, yielding ${\tau }_{DF}=0.93(r_h/10pc{)}^2(\sigma /km{s}^{-1})Gyr$, { where $r_h$ is the half-light radius}. For a stellar velocity dispersion value of $3km/s$, as typical for the smallest of the recently detected ultra faint dwarf spheroidals, dynamical friction timescales becomes smaller than the $10Gyr$ typical of the stellar ages for these systems, for $r_h<19pc$. Thus, this becomes a theoretical lower limit below which dark matter dominated stellar systems become unstable to dynamical friction. We present a comparison with structural parameters of the smallest ultra faint dwarf spheroidals known, showing that these are already close to the stability limit derived, any future detection of yet smaller such systems would be inconsistent with a particle dark matter hypothesis.

6 pages , 1 figure. Extended, corrected version matches accepted manuscript. MNRAS in press

References in corpus (9)

• Cats and Dogs, Hair and A Hero: A Quintet of New Milky Way Companions
• Beasts of the Southern Wild: Discovery of nine Ultra Faint satellites in the vicinity of the Magellanic Clouds
• The Observed properties of Dark Matter on small spatial scales
• Does the Fornax dwarf spheroidal have a central cusp or core?
• Dynamical friction in constant density cores: a failure of the Chandrasekhar formula
• Planet-disc evolution and the formation of Kozai-Lidov planets
• A census of the expected properties of classical Milky Way dwarfs in Milgromian dynamics
• Probing the boundary between star clusters and dwarf galaxies: A MUSE view on the dynamics of Crater/Laevens I
• The gap of stellar mass in galaxy groups: another perspective of the Too-big-To-Fail problem in the Milky Way

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