The large and diffuse galaxies NGC 1052–DF2 and NGC 1052–DF4 have been found to have very low dark matter content and a population of luminous globular clusters (GCs). Accurate distance measurements are key to interpreting these observations. Recently, the distance to NGC 1052–DF4 was found to be 20.0 ± 1.6 Mpc by identifying the tip of the red giant branch (TRGB) in 12 orbits of Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) imaging. Here we present 40 orbits of HST ACS data for NGC 1052–DF2 and use these data to measure its TRGB. The TRGB is readily apparent in the color–magnitude diagram. Using a forward model that incorporates photometric uncertainties, we find a TRGB magnitude of m F814W,TRGB = 27.67 ± 0.10 mag. The inferred distance is D TRGB = 22.1 ± 1.2 Mpc, consistent with the previous surface brightness fluctuation distances to the bright elliptical galaxy NGC 1052. The new HST distance rules out the idea that some of NGC 1052–DF2's unusual properties can be explained if it were at ∼13 Mpc; instead, it implies that the galaxy’s GCs are even more luminous than had been derived using the previous distance of 20 Mpc. The distance from NGC 1052–DF2 to NGC 1052–DF4 is well-determined at 2.1 ± 0.5 Mpc, significantly larger than the virial diameter of NGC 1052. We discuss the implications for formation scenarios of the galaxies and for the external field effect, which has been invoked to explain the intrinsic dynamics of these objects in the context of modified Newtonian dynamics.
The ultra-diffuse galaxies DF2 and DF4 in the NGC 1052 group share several unusual properties: they both have large sizes1, rich populations of overluminous and large globular clusters2–6, and very low velocity dispersions that indicate little or no dark matter7–10. It has been suggested that these galaxies were formed in the aftermath of high-velocity collisions of gas-rich galaxies11–13, events that resemble the collision that created the bullet cluster14 but on much smaller scales. The gas separates from the dark matter in the collision and subsequent star formation leads to the formation of one or more dark-matter-free galaxies12. Here we show that the present-day line-of-sight distances and radial velocities of DF2 and DF4 are consistent with their joint formation in the aftermath of a single bullet-dwarf collision, around eight billion years ago. Moreover, we find that DF2 and DF4 are part of an apparent linear substructure of seven to eleven large, low-luminosity objects. We propose that these all originated in the same event, forming a trail of dark-matter-free galaxies that is roughly more than two megaparsecs long and angled 7° ± 2° from the line of sight. We also tentatively identify the highly dark-matter-dominated remnants of the two progenitor galaxies that are expected11 at the leading edges of the trail.
The dark matter content of the ultra-diffuse galaxy NGC 1052-DF2, as inferred from globular cluster (GC) and stellar kinematics, carries a considerable amount of uncertainty, with current constraints also allowing for the complete absence of dark matter. We test the viability of such a scenario by examining whether in a “baryon-only” mass model the observed GC population experiences rapid orbital decay due to dynamical friction. Using a suite of 50 multi-GC N-body simulations that match observational constraints on both the stellar component of NGC 1052-DF2 and its GC population but differ in the initial line-of-sight positions and the tangential velocities of the GCs, we show that there is a substantial amount of realization-to-realization variance in the evolution of the GCs. Nevertheless, over ∼10 Gyr, some of the GCs experience significant orbital evolution. Others evolve less. A combination of reduced dynamical friction in the galaxy core and GC–GC scattering keeps the GCs afloat, preventing them from sinking all the way to the galaxy center. While the current phase-space coordinates of the GCs are not unlikely for a baryon-only mass model, the GC system does evolve over time. Therefore, if NGC 1052-DF2 has no dark matter, some of its GCs must have formed farther out, and the GC system must have been somewhat more extended in the past. The presence of a low-mass cuspy halo, while allowed by the kinematics, seems improbable, as significantly shorter inspiral timescales in the central region would quickly lead to the formation of a nuclear star cluster.
Fuzzy dark matter (FDM), consisting of ultralight bosons ( m b ∼ 10 − 22 eV ), is an intriguing alternative to cold dark matter. Numerical simulations that solve the Schrödinger–Poisson (SP) equation show that FDM halos consist of a central solitonic core, which is the ground state of the SP equation, surrounded by an envelope of interfering excited states. These excited states also interfere with the soliton, causing it to oscillate and execute a confined random walk with respect to the halo center of mass. Using high-resolution numerical simulations of a 6.6 × 10 9 M ⊙ FDM halo with m b = 8 × 10 − 23 eV in isolation, we demonstrate that the wobbling, oscillating soliton gravitationally perturbs nuclear objects, such as supermassive black holes or dense star clusters, causing them to diffuse outwards. In particular, we show that, on average, objects with mass ≲0.3% of the soliton mass ( M sol ) are expelled from the soliton in ∼3 Gyr , after which they continue their outward diffusion due to gravitational interactions with the soliton and the halo granules. More massive objects (≳ 1 % M sol ), while executing a random walk, remain largely confined to the soliton due to dynamical friction. We also present an effective treatment of the diffusion, based on kinetic theory, that accurately reproduces the outward motion of low-mass objects and briefly discuss how the observed displacements of star clusters and active galactic nuclei from the centers of their host galaxies can be used to constrain FDM.
It was recently proposed that the dark matter–deficient ultradiffuse galaxies DF2 and DF4 in the NGC 1052 group could be the products of a “bullet dwarf” collision between two gas-rich progenitor galaxies. In this model, DF2 and DF4 formed at the same time in the immediate aftermath of the collision, and a strong prediction is that their globular clusters should have nearly identical stellar populations. Here we test this prediction by measuring accurate V 606 − I 814 colors from deep HST/ACS imaging. We find that the clusters are extremely homogeneous. The mean color difference between the globular clusters in DF2 and DF4 is ΔDF2−DF4 = −0.003 ± 0.005 mag, and the observed scatter for the combined sample of 18 clusters with M 606 < −8.6 in both galaxies is σ obs = 0.015 ± 0.002 mag. After accounting for observational uncertainties and stochastic cluster-to-cluster variation in the number of red giants, the remaining scatter is σ intr = 0.008 − 0.006 + 0.005 mag. Both the color difference and the scatter are an order of magnitude smaller than in other dwarf galaxies, and we infer that the bullet scenario passes an important test that could have falsified it. No other formation models have predicted this extreme uniformity of the globular clusters in the two galaxies. We find that the galaxies themselves are slightly redder than the clusters, consistent with a previously measured metallicity difference. Numerical simulations have shown that such differences are expected in the bullet scenario, as the galaxies continued to self-enrich after the formation of the globular clusters.
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