Formation of Dark Matter-Deficient Galaxies (DMDGs) during High-velocity Galaxy Collisions: Mini-Bullet (Bullet Dwarf) Collision

Along with their luminous member globular clusters (GCs), the recent discovery of diffuse dwarf galaxies that are deficient in dark matter — such as NGC1052-DF2 and NGC1052-DF4 — appears to challenge the current paradigm of structure formation in our Universe.

In the first paper (Shin et al. 2020), we describe the numerical experiments using the adaptive mesh refinement code (Enzo) and particle-based gravito-hydrodynamics code (Gadget-2) to determine if the so-called dark matter-deficient galaxies (DMDGs) could be produced when two gas-rich, dwarf-sized galaxies collide with a high relative velocity of \(\gtrsim 300\) km/s. Also, using a large simulated universe IllustrisTNG, we discover a number of high-velocity galaxy collision events in which DMDGs are expected to form. However, we did not find evidence that these types of collisions actually produced DMDGs in the IllustrisTNG100-1 run. We argue that the resolution of the numerical experiment is critical to realize the “collision-induced” DMDG formation scenario. Our results demonstrate one of many routes in which galaxies could form with unconventional dark matter fractions.

In the second paper (Lee et al. 2021), using idealized high-resolution simulations, we demonstrate the simultaneous formation of DMDGs and their star clusters (SCs) in high-velocity galaxy collisions that separate dark matter from the warm disk gas which subsequently is compressed by shock and tidal interaction to form stars efficiently. In particular, we show that the galaxy collision spawns multiple massive SCs (\(\gtrsim 10^{6} \, {\rm M}_{\odot}\)) within 150 Myr after the collision. At the end of our \(\sim 800\) Myr fiducial run, the resulting DMDG of \(M_{\star} \simeq 3.5 \times 10^{8} \, {\rm M}_{\odot}\) hosts 10 luminous (\(M_{V} \lesssim -8.5 \, {\rm {mag}}\)), gravitational bound SCs with a line-of-sight velocity dispersion 11.2 km/s. Our study suggests that DMDGs and their luminous member SCs could form simultaneously in high-velocity galaxy collisions while being in line with the key observed properties of NGC1052-DF2 and NGC1052-DF4.

In the third paper (Lee et al. 2023), inspired by recent observations of an aligned trail of \(7−11\) UDGs near NGC1052, including DMDGs DF2 and DF4, suggesting a common formation site and time, \(\sim 8.9 \pm 1.5\) Gyr ago, we start from initial conditions for colliding satellites tailored to match the observed UDGs in the NGC1052 group and simulate the Mini-Bullet satellite-satellite galaxy collision with the hydro/N-body code Enzo, supplemented by galaxy orbit integrations. We demonstrate the formation of a trail of \(\sim 10\) DMDGs, including two massive ones that replicate the observed motions of DF2 and DF4. We present the resultant properties of the produced aligned DMDGs, the alignment and deviation from that, the linear relation between positions and velocities, their stellar properties, and their sizes. We also show that a \(\Lambda{\rm CDM}\) cosmological simulation (IllustrisTNG100-1) in a \((100\;{\rm cMpc})^{3}\) box finds our required initial conditions \(\sim 10\) times at \(z<3\). Our results indicate current observations of NGC1052 group galaxies are consistent with the Mini-Bullet scenario.

The Inhomogeneous Rise of Metallicity During the Epoch of Reionization

When galaxies and stars began to form, they released ionizing radiation into the intergalactic medium which resulted in its reionization over the course of the first billion years. This ionizing radiation was dominated by massive stars. Reionization was inhomogeneous in space and time, reflecting the clustering of galaxies, and the inhomogeneous density field into which their radiation caused ionization fronts to propagate, resulting in different arrival times of those ionization fronts at different locations. The same massive stars that released this ionizing radiation also formed and released heavy elements when they exploded as supernovae, enriching the metal-free primordial gas both inside galaxies and outside them, by driving winds into the surrounding IGM. Just as reionization was inhomogeneous, so must the rise of metallicity during the Epoch of Reionization (EoR) have been. The theory of this inhomogeneous rise of metallicity is, therefore, inseparable from the theory of reionization, and predicting its observable consequences requires us to model both processes, together, self-consistently. As JWST pushes the observational frontier, to observe galaxies and detect absorption lines in the intergalactic medium during and after the epoch of reionization, we must predict and interpret what it sees. Towards this end, we have analyzed the results of the latest state-of-the-art radiation-hydro simulation of fully-coupled galaxy formation and reionization by The Cosmic Dawn (“CoDa”) Project, CoDa III, including its self-consistent tracking of the inhomogeneous rise of metallicity thru the end of the EoR and beyond, down to z = 4.6. CoDa III is the first trillion-element radiation-hydro simulation of the EoR, with enough dynamic range to resolve the formation of every star-forming galaxy believed to be responsible for reionization (and, hence, metallicity) in its simulation volume of 94.4 cMpc across. We will present these CoDa III results for the inhomogeneous evolution of metallicity in the universe and its observable consequences.