Photometrically distinct nuclear star clusters (NSCs) are common in late-type-disk and spheroidal galaxies. The formation of NSCs is inevitable in the context of normal star formation in which a majority of stars form in clusters. A young, mass-losing cluster embedded in an isolated star-forming galaxy remains gravitationally bound over a period determined by its initial mass and the galactic tidal field. The cluster migrates radially toward the center of the galaxy and becomes integrated in the NSC if it reaches the center. The rate at which the NSC grows by accreting young clusters can be estimated from empirical cluster formation rates and dissolution times. We model cluster migration and dissolution and find that the NSCs in late-type disks and in spheroidals could have assembled from migrating clusters. The resulting stellar nucleus contains a small fraction of the stellar mass of the galaxy; this fraction is sensitive to the high-mass truncation of the initial cluster mass function (ICMF). The resulting NSC masses are consistent with the observed values, but generically, the final NSCs are surrounded by a spatially more extended excess over the inward-extrapolated exponential (or Sérsic) law of the outer galaxy. We suggest that the excess can be related to the pseudobulge phenomenon in disks, though perhaps not all of the pseudobulge mass assembles this way. Comparison with observed NSC masses can be used to constrain the truncation mass scale of the ICMF and the fraction of clusters suffering prompt dissolution. We infer truncation mass scales of 10 6 M ( 10 5 M ) without (with 90%) prompt dissolution. Since the NSC assembly is collisionless and non-dissipative, no relation to the process responsible for central black hole assembly in more massive galaxies is expected.

We investigate the process of metal-free star formation in the first galaxies with a highresolution cosmological simulation. We consider the cosmologically motivated scenario in which a strong molecule-destroying Lyman-Werner (LW) background inhibits effective cooling in low-mass haloes, delaying star formation until the collapse or more massive haloes. Only when molecular hydrogen (H 2 ) can self-shield from LW radiation, which requires a halo capable of cooling by atomic line emission, will star formation be possible. To follow the formation of multiple gravitationally bound objects, at high gas densities we introduce sink particles which accrete gas directly from the computational grid. We find that in a 1 Mpc 3 (comoving) box, runaway collapse first occurs in a 3 × 10 7 M dark matter halo at z ≈ 12 assuming a background intensity of J 21 = 100. Due to a runaway increase in the H 2 abundance and cooling rate, a self-shielding, supersonically turbulent core develops abruptly with ∼10 4 M in cold gas available for star formation. We analyse the formation of this selfshielding core, the character of turbulence and the prospects for star formation. Due to a lack of fragmentation on scales we resolve, we argue that LW-delayed metal-free star formation in atomic cooling haloes is very similar to star formation in primordial minihaloes, although in making this conclusion we ignore internal stellar feedback. Finally, we briefly discuss the detectability of metal-free stellar clusters with the James Webb Space Telescope.

We investigate the process of metal-free star formation in the first galaxies with a high-resolution cosmological simulation. We consider the scenario in which a strong moleculedestroying Lyman-Werner (LW) background inhibits effective cooling in low-mass halos, delaying star formation until the collapse or more massive halos. Only when molecular hydrogen (H 2 ) can self-shield from LW radiation, which requires a halo capable of cooling by atomic line emission, will star formation be possible. To follow the formation of multiple gravitationally bound objects, at high gas densities we introduce sink particles which accrete gas directly from the computational grid. We find that in a 1 Mpc 3 (comoving) box, runaway collapse first occurs in a 3 × 10 7 M dark matter halo at z ≈ 12 assuming a background intensity of J 21 = 100. Due to a runaway increase in the H 2 abundance and cooling rate, a self-shielding, supersonically turbulent core develops abruptly with 10 4 M in cold gas available for star formation. We analyze the formation of this self-shielding core, the character of turbulence, and the prospects for star formation

We present results of a high-resolution cosmological simulation which explores the process of primordial gas collapsing into a high-redshift atomic cooling halo, typically considered to be a 'first galaxy'. We consider a physically motivated scenario where a strong molecule destroying Lyman-Werner (LW) background prevents gaseous collapse, and thus star formation, in small mass halos. Only when molecular hydrogen can shield itself from LW radiation in a sufficiently massive halo will star formation be possible. We find that in a 1 Mpc 3 (comoving) box with a LW background corresponding to J 21 = 100 collapse first occurs in a 3 × 10 7 M dark matter halo at z 12.1. A distinct phase transition occurs in the center of the halo when molecular hydrogen (H 2 ) begins to self-shield, allowing baryons to cool, and to form a supersonically turbulent ∼ 10 pc core. We investigate the character of turbulence in this core, fragmentation properties, and impact on next generation star formation.

SciPy 2020, the 19th annual Python in Science Conference, was held July 6-12, virtually via the conference platform Crowdcast. Due to the COVID-19 pandemic, the SciPy conference was held online. The SciPy Conference brings together a community of researchers, engineers, and programmers dedicated to the advancement of scientific computing through open source Python software.The two main conference themes for 2020 were high performance computing; and, machine learning and data science. Discipline-specific symposia included astronomy and astrophysics; biology and bioinformatics; materials science; earth, ocean, geology, and atmospheric science; and a new symposium dedicated to fostering conversations among maintainers of the open source infrastructure that help power the worlds of scientific discovery and engineering. As was the case in 2019, there were plenary sessions for updates about key scientific software libraries, and three sessions of the ever-popular lightning talks, which this year included SciPy's youngest speaker, Artash Nath, discussing machine learning approaches in exoplanet research.The first conference keynote lecture was delivered by Anne Carpenter, who discussed the history of CellProfiler in the context of developing academic software, current application of the scientific software stack to problems in biology, and future directions for tasks like drug discovery, powered by machine learning. Andrew Chael delivered the second keynote, about the large, interorganizational effort to take the first photograph of black hole M87, and the role of scientific software in that project. This year's diversity plenary was given by Bonny McClain, who delivered an interactive lecture about bias in data, and how to think about measuring what people haven't thought about measuring before.The online format permitted a larger-than-usual number of participants, ultimately attracting 1412 participants from a recordbreaking 57 countries, making this the largest SciPy Conference yet. Participants reported that they enjoyed the broader access to beginner tutorials for popular libraries like PyTorch and xarray -something that would not be possible without having the conference at least partially online. Birds of a Feather (BoF) sessions were organized around the topics of packaging, diversity, Python in education, hardware, and SciPy 2021 with great attendance due to the online format. Sprints that usually gather around tables in conference rooms took the conversation to virtual tables using a ¶ Oracle § Mozilla