Monochromatic Globular Clusters as a Critical Test of Formation Models for the Dark Matter–deficient Galaxies NGC 1052-DF2 and NGC 1052-DF4

Abstract

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.

A Candidate Runaway Supermassive Black Hole Identified by Shocks and Star Formation in its Wake

“…RCP 28 was observed 2022 September 5 for one orbit in F606W and one orbit in F814W, in the context of mid-cycle program GO-16912. The individual flc files were combined using DrizzlePac after applying a flat-field correction to account for drifts in the sensitivity of the ACS CCDs (see van Dokkum et al 2022b). Upon reducing the data an almost-straight thin streak was readily apparent in a visual assessment of the data quality (see Figure 1).…”

Confirmation of an Anomalously Low Dark Matter Content for the Galaxy NGC 1052-DF4 from Deep, High-resolution Continuum Spectroscopy

“…No spectroscopic metallicity has so far been measured for the diffuse light of DF4. A photometric measurement and spectroscopic measurements for the GCs and diffuse light in DF2 indicate metallicities of −1.5  [Z/H]  − 1.0 and ages of 7-9 Gyr (van Dokkum et al 2018;Fensch et al 2019;Buzzo et al 2022). We used two templates with metallicity [Z/H] = −1.5 and different ages: 9 and 5 Gyr.…”

“…Its velocity dispersion of -+ 4.2 2.2 4.4 km s −1 was measured from seven globular clusters (GCs) that are associated with DF4. It shares many puzzling properties with the "first galaxy missing dark matter," NGC 1052-DF2 (DF2; van Dokkum et al 2018). Located in the NGC 1052 group (D DF2 = 21.7 Mpc, Appendix; D DF4 = 20.0 Mpc, Danieli et al 2020), both galaxies have low velocity dispersions that imply an extreme deficit of dark matter and host a population of overluminous GCs (Shen et al 2021b).…”

“…Located in the NGC 1052 group (D DF2 = 21.7 Mpc, Appendix; D DF4 = 20.0 Mpc, Danieli et al 2020), both galaxies have low velocity dispersions that imply an extreme deficit of dark matter and host a population of overluminous GCs (Shen et al 2021b). In addition, DF2 and DF4 have the same color in their GCs and the diffuse stellar light (van Dokkum et al 2022b).…”

Testing the Galaxy-collision-induced Formation Scenario for the Trail of Dark-matter-deficient Galaxies with the Susceptibility of Globular Clusters to the Tidal Force

“…The second purpose of this Letter is to test if the DMDG can retain GCs on extended orbits under the influence of the tidal force of NGC 1052. In the mini bullet cluster scenario, the GC formation is expected to happen on a short timescale, ∼100 Myr, immediately following the galaxy collision (Lee et al 2021; see also, e.g., Madau et al 2020), and as a consequence, the homogeneous age and metallicity of GCs in DF2 and DF4 may be explained (Fensch et al 2019b;van Dokkum et al 2022b). The short timescale of the formation of DMDGs and associated GCs suggests that their birthplace virtually corresponds to the place of the galaxy collision.…”