A Cuspy Dark Matter Halo

Abstract

The cusp–core problem is one of the main challenges of the cold dark matter paradigm on small scales; the density of a dark matter halo is predicted to rise rapidly toward the center as ρ(r) ∝ r α with α between −1 and −1.5, while such a cuspy profile has not been clearly observed. We have carried out the spatially resolved mapping of gas dynamics toward a nearby ultradiffuse galaxy (UDG), AGC 242019. The derived rotation curve of dark matter is well fitted by the cuspy profile as described by the Navarro–Frenk–White model, while the cored profiles including both the pseudo-isothermal and Burkert models are excluded. The halo has α = −(0.90 ± 0.08) at the innermost radius of 0.67 kpc, M halo = (3.5 ± 1.2) × 1010 M ⊙, and a small concentration of 2.0 ± 0.36. The UDG AGC 242019 challenges alternatives of cold dark matter by constraining the particle mass of fuzzy dark matter to be <0.11 × 10−22 or >3.3 × 10−22 eV, the cross section of self-interacting dark matter to be <1.63 cm2 g−1, and the particle mass of warm dark matter to be >0.23 keV, all of which are in tension with other constraints. The modified Newtonian dynamics is also inconsistent with a shallow radial acceleration relationship of AGC 242019. For the feedback scenario that transforms a cusp to a core, AGC 242019 disagrees with the stellar-to-halo mass ratio dependent model but agrees with the star formation threshold dependent model. As a UDG, AGC 242019 is in a dwarf-sized halo with weak stellar feedback, late formation time, normal baryonic spin, and low star formation efficiency (SFR/gas).

A tight angular-momentum plane for disc galaxies

“…ultra-diffuse galaxies (UDGs) AGC 114905 and AGC 242019 (Mancera Piña et al in prep. ;Shi et al 2021) and the giant low surface brightness galaxies (GLSBs) Malin 1 and NGC 7589 (Lelli et al 2010). The UDGs are found to be outliers of the BTFR (Mancera Piña et al 2019, 2020, and both UDGs and GLSBs are extreme galaxies with very extended discs for their M * .…”

A shallow dark matter halo in Ultra Diffuse Galaxy AGC 242019: are UDGs structurally similar to low surface brightness galaxies?

“…AGC 242019 is a field UDG, with a large HI gas content that has allowed detailed rotation curve to be created and mass modelling to be carried out. Shi et al (2021) found that the density profile of this galaxy is best fit by an NFW profile with M halo =3.5×10 10 M and a concentration c = 2.0, the latter being more than 5σ away from theoretical expectations. In this letter we have instead shown that a better fit can be obtained employing a two-power law model, which allows the inner slope of the dark matter halo to be shallower than a cuspy profile.…”

“…Bothun et al 1991Bothun et al , 1997van Dokkum et al 2015;Román & Trujillo 2017) and analysing them (e.g. Ruiz-Lara et al 2018;Forbes et al 2021), but because their diffuse baryonic component allows the underlying dark matter density profiles to be well constrained, particularly when there is sufficient gas within a disk that allows the measurement of rotation velocities (Mancera Piña et al 2020;Shi et al 2021). Various claims have been made about unusual properties of UDGs, such as unusually large (van Dokkum et al 2016) or small (van Dokkum et al 2018) quantities of dark matter (see however Trujillo et al 2019 andMontes et al 2020 for a rebuttal of these claims), and the fact that some UDGs seem to fall off the baryonic Tully-Fisher relation (Mancera Piña et al 2020).…”

A universal relationship between stellar masses and binding energies of galaxies

“…This results in a relationship between halo masses and galaxy stellar masses, but such a correlation shows large scatters at low masses as seen both in observations and simulations (e.g. Miller et al 2014;Oman et al 2016;Beasley et al 2016;van Dokkum et al 2018van Dokkum et al , 2019Read et al 2017;Shi et al 2021). The similar spatial distributions of baryons and dark matters also suggest similar tidal torques over the cosmic history (Peebles 1969), leading to an expectation that the specific angular momenta (angular momenta per mass) of galaxies are similar to those of haloes (Fall & Efstathiou 1980;Mo et al 1998), but observations show that this is correct only to some extent (Kravtsov 2013;Posti et al 2018).…”