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The ultra-diffuse galaxies DF2 and DF4 in the NGC 1052 group share several unusual properties: they both have large sizes 1 , rich populations of overluminous and large globular clusters 2 – 6 , and very low velocity dispersions that indicate little or no dark matter 7 – 10 . It has been suggested that these galaxies were formed in the aftermath of high-velocity collisions of gas-rich galaxies 11 – 13 , events that resemble the collision that created the bullet cluster 14 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 galaxies 12 . 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 expected 11 at the leading edges of the trail. The dark-matter-free dwarf galaxies DF2 and DF4 in the NGC 1052 group probably formed together in the aftermath of a single bullet-dwarf collision around eight billion years ago.

Fuzzy dark matter (FDM), consisting of ultralight bosons, is an intriguing alternative to cold dark matter. Numerical simulations solving the Schrödinger–Poisson (SP) equation, which governs FDM dynamics, show that FDM halos consist of a central solitonic core (representing the ground state of the SP equation), surrounded by a large envelope of excited states. Wave interference gives rise to density fluctuations of order unity throughout the envelope and causes the soliton to undergo density oscillations and execute a confined random walk in the central region of the halo. The resulting gravitational potential perturbations are an efficient source of dynamical heating. Using high-resolution numerical simulations of a 6.6 × 109 M ⊙ FDM halo with boson mass m b = 8 × 10−23 eV, we investigate the impact of this dynamical heating on the structure and kinematics of spheroidal dwarf galaxies of a fixed mass but different initial sizes and ellipticities. The galaxies are set up in equilibrium in the time-and-azimuthally averaged halo potential and evolved for 10 Gyr in the live FDM halo. We find that they continuously increase their sizes and central velocity dispersions. In addition, their kinematic structures become strongly radially anisotropic, especially in the outskirts. Dynamical heating also causes initially ellipsoidal galaxies to become more spherical over time from the inside out and gives rise to distorted, nonconcentric isodensity contours. These telltale characteristics of dynamical heating of dwarf galaxies in FDM halos can potentially be used to constrain the boson mass.

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 σintr=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.

We study the radial transport of cold gas within simulated disk galaxies at cosmic noon, aiming at distinguishing between disk instability and accretion along cold streams from the cosmic web as its driving mechanism. Disks are selected based on kinematics and flattening from the VELA zoom-in hydro-cosmological simulations. The radial velocity fields in the disks are mapped, their averages are computed as a function of radius and over the whole disk, and the radial mass flux in each disk as a function of radius is obtained. The transport directly associated with fresh incoming streams is identified by selecting cold gas cells that are either on incoming streamlines or have low metallicity. The radial velocity fields in VELA disks are found to be highly non-axisymmetric, showing both inflows and outflows. However, in most cases, the average radial velocities, both as a function of radius and over the whole disk, are directed inwards, with the disk-averaged radial velocities typically amounting to a few percent of the disk-averaged rotational velocities. This is significantly lower than the expectations from various models that analytically predict the inward mass transport as driven by torques associated with disk instability. Under certain simplifying assumptions, the latter typically predict average inflows of more than \\(10\\%\\) of the rotational velocities. Analyzing the radial motions of streams and off-stream material, we find that the radial inflow in VELA disks is dominated by the stream inflows themselves, especially in the outer disks. The high inward radial velocities inferred in observed disks at cosmic noon, at the level of \\( \\! 20\\%\\) of the rotational velocities, may reflect inflowing streams from the cosmic web rather than being generated by disk instability.

The scenario of feedback-free starbursts (FFB), which predicts excessively bright galaxies at cosmic dawn as observed using JWST, may provide a natural setting for black hole (BH) growth. This involves the formation of intermediate-mass seed BHs and their runaway mergers into super-massive BHs with high BH-to-stellar mass ratios and low AGN luminosities. We present a scenario of merger-driven BH growth in FFB galaxies and study its feasibility. BH seeds form within the building blocks of the FFB galaxies, namely, thousands of compact star clusters, each starbursting in a free-fall time of a few Myr before the onset of stellar and supernova feedback. The BH seeds form by rapid core collapse in the FFB clusters, in a few free-fall times, sped up by the migration of massive stars due to the young, broad stellar mass function and stimulated by a `gravo-gyro' instability due to internal cluster rotation and flattening. BHs of \\(10^4 M_\\) are expected in \\(10^6 M_\\) FFB clusters within sub-kpc galactic disks at \\(z 10\\). The BHs then migrate to the galaxy center by dynamical friction, hastened by the compact FFB stellar galactic disk configuration. Efficient mergers of the BH seeds will produce \\(10^6-8 M_\\) BHs with a BH-to-stellar mass ratio \\( 0.01\\) by \\(z 4-7\\), as observed. The growth of the central BH by mergers can overcome the bottleneck introduced by gravitational wave recoils if the BHs inspiral within a relatively cold disk or if the escape velocity from the galaxy is boosted by a wet compaction event. Such events, common in massive galaxies at high redshifts, can also help by speeding up the inward BH migration and by providing central gas to assist with the final parsec problem. The cold disk version of the FFB scenario provides a feasible route for the formation of supermassive BHs.

Fuzzy Dark Matter (FDM), consisting of ultralight bosons, is an intriguing alternative to Cold Dark Matter. Numerical simulations solving the Schrödinger-Poisson (SP) equation, which governs FDM dynamics, show that FDM halos consist of a central solitonic core (representing the ground state of the SP equation), surrounded by a large envelope of excited states. Wave interference gives rise to order unity density fluctuations throughout the envelope and causes the soliton to undergo density oscillations and execute a confined random walk in the central region of the halo. The resulting gravitational potential perturbations are an efficient source of dynamical heating. Using high-resolution numerical simulations of a \\(6.6 10^9 M_\\) FDM halo with boson mass, \\(m_ b=8 10^-23 \\ eV\\), we investigate the impact of this dynamical heating on the structure and kinematics of spheroidal dwarf galaxies of a fixed mass but different initial sizes and ellipticities. The galaxies are set up in equilibrium in the time-and-azimuthally averaged halo potential and evolved for \\(10 \\ Gyr\\) in the live FDM halo. We find that they continuously increase their sizes and central velocity dispersions. In addition, their kinematic structures become strongly radially anisotropic, especially in the outskirts. Dynamical heating also causes initially ellipsoidal galaxies to become more spherical over time from the inside out and gives rise to distorted, non-concentric isodensity contours. These tell-tale characteristics of dynamical heating of dwarf galaxies in FDM halos can potentially be used to constrain the boson mass.

It was recently proposed that the dark matter-deficient ultra-diffuse galaxies DF2 and DF4 in the NGC1052 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 F606W-F814W 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 \\(-0.003 0.005\\) mag and the observed scatter for the combined sample of 18 clusters with \\(M_V<-8.6\\) in both galaxies is \\(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 \\(0.008^+0.005_-0.006\\) 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.

About 84% of the total mass budget of the Universe consists of dark matter. While it does not interact electromagnetically and, therefore, cannot be “seen”, one can hope to infer its properties from its gravitational imprints on the structure and dynamics of various astrophysical objects (e.g., star clusters, black holes, galaxies). While cold dark matter, where dark matter is collisionless and most likely consists of weakly interacting massive particles (WIMPs), is the most widely studied dark matter model, WIMPs remain undetected. An intriguing alternative is Fuzzy Dark Matter, where dark matter is made up of ultralight bosons with masses in the range 10-23 eV ≲ mb ≲ 10−19 eV. Unlike in CDM, FDM halos consist of a central core, surrounded by an envelope of order unity density fluctuations. The central core is the ground state of the Schrödinger-Poisson (SP) equation that governs FDM dynamics, and the envelope consists of the excited states. The excited states also interfere with the soliton, causing it to undergo temporal oscillations and a confined random walk within the central region of the halo. Using novel, high-resolution numerical simulations of an FDM halo corresponding to a particular boson mass, this dissertation demonstrates that the gravitational potential fluctuations associated with the soliton's random walk, its oscillations, and the envelope density fluctuations dynamically heat nuclear objects (e.g., central star clusters and supermassive black holes) and dwarf galaxies. As a result, nuclear objects, initially located at rest at the soliton center, migrate outwards over time until the outward motion is counteracted by dynamical friction, and an equilibrium is reached. Similarly, dwarf galaxies continue to increase their sizes and central velocity dispersions. In addition, their kinematic structures become strongly radially anisotropic, especially in the outskirts. Dynamical heating also causes initially ellipsoidal galaxies to become more spherical over time from the inside out and gives rise to distorted, non-concentric isodensity contours. Generalizing these results for other halo and boson masses and comparing them with observations (such as galaxy size-age relation, measured offsets of supermassive black holes and nuclear star clusters from the centers of their host galaxies) can potentially constrain the boson mass. In addition to studying the dynamics of collisionless particles in FDM, this dissertation also investigates the dynamics of the GC system in the recently discovered, dark matter deficient galaxy NGC 1052-DF2. By studying the dynamical friction-induced orbital decay of its GCs in different mass models that are allowed within the constraints from stellar and GC kinematics, it is demonstrated that a stars-only model (or a low mass dark matter core) is favored over a low mass dark matter cusp, as the presence of a dark matter cusp would lead to rapid orbital decay of the inner GCs, forming a nuclear cluster, which is not supported by observations. Finally, the probability of GC-GC mergers in this galaxy is also investigated, and it is shown that such events are rare and, therefore, cannot explain the existence of the overly luminous GCs.

The ultra-diffuse galaxies DF2 and DF4 in the NGC1052 group share several unusual properties: they both have large sizes, rich populations of overluminous and large globular clusters, and very low velocity dispersions indicating little or no dark matter. It has been suggested that these galaxies were formed in the aftermath of high velocity encounters of gas rich galaxies, events that resemble the collision that created the bullet cluster 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 galaxies. 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 7-11 large, low-luminosity objects. We propose that these all originated in the same event, forming a trail of dark matter-free galaxies that is more than 2 Mpc long and angled 7 +- 2 degrees from the line of sight. We also tentatively identify the highly dark matter-dominated remnants of the two progenitor galaxies that are expected at the leading edges of the trail.

Several analytic and numerical studies have indicated that the interstellar medium of a quasar host galaxy heated by feedback can contribute to a substantial secondary signal in the cosmic microwave background (CMB) through the thermal Sunyaev-Zel'dovich (SZ) effect. Recently, many groups have tried to detect this signal by cross-correlating CMB maps with quasar catalogs. Using a self-similar model for the gas in the intra-cluster medium and a realistic halo occupation distribution (HOD) prescription for quasars we estimate the level of SZ signal from gravitational heating of quasar hosts. The bias in the host halo signal estimation due to unconstrained high mass HOD tail and yet unknown redshift dependence of the quasar HOD restricts us from drawing any robust conclusions at low redshift (z<1.5) from our analysis. However, at higher redshifts (z>2.5), we find an excess signal in recent observations than what is predicted from our model. The excess signal could be potentially generated from additional heating due to quasar feedback.