Abstract
ABSTRACT In the standard ΛCDM (Lambda cold dark matter) paradigm, dwarf galaxies are expected to be dark matter-rich, as baryonic feedback is thought to quickly drive gas out of their shallow potential wells and quench star formation at early epochs. Recent observations of local dwarfs with extremely low dark matter content appear to contradict this picture, potentially bringing the validity of the standard model into question. We use NewHorizon, a high-resolution cosmological simulation, to demonstrate that sustained stripping of dark matter, in tidal interactions between a massive galaxy and a dwarf satellite, naturally produces dwarfs that are dark matter-deficient, even though their initial dark matter fractions are normal. The process of dark matter stripping is responsible for the large scatter in the halo-to-stellar mass relation in the dwarf regime. The degree of stripping is driven by the closeness of the orbit of the dwarf around its massive companion and, in extreme cases, produces dwarfs with halo-to-stellar mass ratios as low as unity, consistent with the findings of recent observational studies. ∼30 per cent of dwarfs show some deviation from normal dark matter fractions due to dark matter stripping, with 10 per cent showing high levels of dark matter deficiency (Mhalo/M⋆ < 10). Given their close orbits, a significant fraction of dark matter-deficient dwarfs merge with their massive companions (e.g. ∼70 per cent merge over time-scales of ∼3.5 Gyr), with the dark matter-deficient population being constantly replenished by new interactions between dwarfs and massive companions. The creation of these galaxies is therefore a natural by-product of galaxy evolution and their existence is not in tension with the standard paradigm.
Highlights
In the standard ΛCDM paradigm, dwarf galaxies (M★ < 109.5 M ) are expected to be dark matter (DM) rich, because their shallow potential wells make it easier for processes like stellar and supernova feedback to drive gas out from their central regions at early epochs
The degree of stripping is driven by the closeness of the orbit of the dwarf around its massive companion and, in extreme cases, produces dwarfs with halo-to-stellar mass ratios as low as unity, consistent with the findings of recent observational studies. ∼30 per cent of dwarfs show some deviation from normal dark matter fractions due to dark matter stripping, with 10 per cent showing high levels of dark matter deficiency (Mhalo/M★
A well-known channel for forming such systems are tidal dwarfs (Wetzstein et al 2007; Bournaud et al 2008a,b; Kaviraj et al 2012; Kroupa 2012; Ploeckinger et al 2015; Haslbauer et al 2019). These dwarf galaxies are formed in the tidal tails that emerge as a result of gas-rich major mergers of massive galaxies, either through Jeans instabilities within the gas which lead to gravitational collapse and the formation of self-bound objects (Elmegreen et al 1993), or a large fraction of the stellar material in the progenitor disk being ejected and providing a local potential well into which gas condenses and fuels star formation (e.g. Barnes & Hernquist 1992; Duc et al 2004; Hancock et al 2009)
Summary
In the standard ΛCDM paradigm, dwarf galaxies (M★ < 109.5 M ) are expected to be dark matter (DM) rich, because their shallow potential wells make it easier for processes like stellar and supernova feedback to drive gas out from their central regions at early epochs This reduces their star formation rates and produces systems that. The contribution of tidal dwarfs to the galaxy population, at the stellar masses of the DM deficient galaxies found by recent observational studies (∼109 M ), is extremely small (e.g. Kaviraj et al 2012). A comprehensive analysis of whether DM deficient systems (dwarfs in particular) can form naturally as a by-product of the process of galaxy evolution demands a hydrodynamical simulation in a cosmological volume which has both high mass and spatial resolution.
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