分类： 天文学 >> 天文学 提交时间： 2023-02-19
摘要： It has been suggested that a trail of diffuse galaxies, including two dark matter deficient galaxies (DMDGs), in the vicinity of NGC1052 formed because of a high-speed collision between two gas-rich dwarf galaxies, one bound to NGC1052 and the other one on an unbound orbit. The collision compresses the gas reservoirs of the colliding galaxies, which in turn triggers a burst of star formation. In contrast, the dark matter and pre-existing stars in the progenitor galaxies pass through it. Since the high pressures in the compressed gas are conducive to the formation of massive globular clusters (GCs), this scenario can explain the formation of DMDGs with large populations of massive GCs, consistent with the observations of NGC1052-DF2 (DF2) and NGC1052-DF4. A potential difficulty with this `mini bullet cluster' scenario is that the observed spatial distributions of GCs in DMDGs are extended. GCs experience dynamical friction causing their orbits to decay with time. Consequently, their distribution at formation should have been even more extended than that observed at present. Using a semi-analytic model, we show that the observed positions and velocities of the GCs in DF2 imply that they must have formed at a radial distance of 5-10kpc from the center of DF2. However, as we demonstrate, the scenario is difficult to reconcile with the fact that the strong tidal forces from NGC1052 strip the extendedly distributed GCs from DF2, requiring 33-59 massive GCs to form at the collision to explain observations.
分类： 天文学 >> 天文学 提交时间： 2023-02-19
Stella S. R. Offner
Josh Taylor
Carleen Markey
Hope How-Huan Chen
Jaime E. Pineda
Alyssa A. Goodman
Andreas Burkert
Adam Ginsburg
Spandan Choudhury
摘要： We study the formation, evolution and collapse of dense cores by tracking structures in a magnetohydrodynamic simulation of a star-forming cloud. We identify cores using the dendrogram algorithm and utilize machine learning techniques, including Neural Gas prototype learning and Fuzzy $c$-means clustering, to analyze the density and velocity dispersion profiles of cores together with six bulk properties. We produce a 2-d visualization using a Uniform Manifold Approximation and Projection (UMAP), which facilitates the connection between physical properties and three partially-overlapping phases: i) unbound turbulent structures (Phase I), ii) coherent cores that have low turbulence (Phase II), and iii) bound cores, many of which become protostellar (Phase III). Within Phase II we identify a population of long-lived coherent cores that reach a quasi-equilibrium state. Most prestellar cores form in Phase II and become protostellar after evolving into Phase III. Due to the turbulent cloud environment, the initial core properties do not uniquely predict the eventual evolution, i.e., core evolution is stochastic, and cores follow no one evolutionary path. The phase lifetimes are 1.0$\pm$0.1$\times$10$^5$ yr, 1.3$\pm$0.2$\times$10$^5$ yr, and 1.8$\pm$0.3$\times$10$^5$ yr for Phase I, II, and III, respectively. We compare our results to NH$_3$ observations of dense cores. Known coherent cores predominantly map into Phase II, while most turbulent pressure-confined cores map to Phase I or III. We predict that a significant fraction of observed starless cores have unresolved coherent regions and that $\gtrsim 20$% of observed starless cores will not form stars. Measurements of core radial profiles, in addition to the usual bulk properties, will enable more accurate predictions of core evolution.
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