Dark matter-deficient dwarf galaxies form via tidal stripping of dark matter in interactions with massive companions

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.

The tidal evolution of dark matter substructure – II. The impact of artificial disruption on subhalo mass functions and radial profiles

“…Carlsten et al 2020;Meneghetti et al 2020) and/or the DM deficiency of associated satellite galaxies (e.g. Ogiya 2018;Jackson et al 2021).…”

Galaxy lacking dark matter in the standard cosmological paradigm

“…mass fractions as low as ours: three members of our sample have dark-matter mass fractions below 10% and one has only 1% of its mass in dark matter -rendering them almost dark-matter free galaxies. See the Methods Section for work by other groups 5,[12][13][14][15][16][17][18][19][20][21] . One possible explanation is that our simulation has the distinct advantage in being able to model the small-scale (∼ 20 pc) interstellar medium of individual galaxies within a cosmological region (∼ 20 Mpc) large enough to contain several massive groups of galaxies.…”

“…Simulations by other groups. The identification of galaxies lacking dark matter in cosmological simulations has been a challenge [12][13][14][17][18][19][20] . Whilst many of these studies have shown that galaxies can be produced with dark matter fractions lower than typical, the great majority of them do not produce galaxies below the 1 : 1 line on the M dm − M ⋆ plane (Figure 2).…”

“…First, as mentioned in the main text, our simulations are relatively unique in that they explicitly track dense, molecular gas in low-mass galaxies within a volume that is large enough to contain several massive group-size halos. There are published simulations 19,21 with similar formal spatial resolution to our own within comparable cosmological volumes, but these utilize different star-formation and galaxy-formation physics. In particular, our star-formation threshold is > times higher than used in these comparably-sized simulations and this allows us to track the internal baryonic structure of galaxies with higher precision.…”

“…For this reason, the detection of DF2 and DF4, two low-mass galaxies devoid of dark matter, was not anticipated 3,4 . This intriguing discovery immediately sparked several searches of numerical analogs in cosmological simulations 5,[12][13][14][15][16][17][18][19][20][21] , with limited success. Unsurprisingly, the absence of dark-matter deficient galaxies in these simulations also raised doubts on the validity of the standard paradigm itself 5,11 .…”