Nuclear star clusters (NSCs) are the densest stellar systems in the Universe and are found in the centres of all types of galaxies. They are thought to form via mergers of star clusters such as ancient globular clusters (GCs) that spiral to the centre as a result of dynamical friction or through in-situ star formation directly at the galaxy centre. There is evidence that both paths occur, but the relative contribution of either channel and their correlation with galaxy properties are not yet constrained observationally. We aim to derive the dominant NSC formation channel for a sample of 25 nucleated galaxies, mostly in the Fornax galaxy cluster, with stellar masses between $M_\rm{gal} \sim 10^8$ and $10^{10.5} M_\odot$ and NSC masses between $M_\rm{NSC} \sim 10^5$ and $10^{8.5} M_\odot$. Using Multi-Unit Spectroscopic Explorer (MUSE) data from the Fornax 3D survey and the ESO archive, we derive star formation histories, mean ages and metallicities of NSCs, and compare them to the host galaxies. In many low-mass galaxies, the NSCs are significantly more metal-poor than the hosts with properties similar to GCs. In contrast, in the massive galaxies, we find diverse star formation histories and cases of ongoing or recent in-situ star formation. Massive NSCs ($> 10^7 M_\odot$) occupy a different region in the mass-metallicity diagram than lower mass NSCs and GCs, indicating a different enrichment history. We find a clear transition of the dominant NSC formation channel with both galaxy and NSC mass. We hypothesise that while GC-accretion forms the NSCs of the dwarf galaxies, central star formation is responsible for the efficient mass build up in the most massive NSCs in our sample. At intermediate masses, both channels can contribute. The transition between these formation channels seems to occur at galaxy masses $M_\rm{gal} \sim 10^9 M_\odot$ and NSC masses $M_\rm{NSC} \sim 10^7 M_\odot$.

The outer solar system exhibits an anomalous pattern of orbital clustering, characterized by an approximate alignment of the apsidal lines and angular momentum vectors of distant, long-term stable Kuiper belt objects. One explanation for this dynamical confinement is the existence of a yet-undetected planetary-mass object, "Planet Nine (P9)". Previous work has shown that trans-Neptunian objects, which originate within the scattered disk population of the Kuiper belt, can be corralled into orbital alignment by Planet Nine's gravity over ~Gyr timescales, and characteristic P9 parameters have been derived by matching the properties of a synthetic Kuiper belt generated within numerical simulations to the available observational data. In this work, we show that an additional dynamical process is in play within the framework of the Planet Nine hypothesis, and demonstrate that P9-induced dynamical evolution facilitates orbital variations within the otherwise dynamically frozen inner Oort cloud. As a result of this evolution, inner Oort cloud bodies can acquire orbits characteristic of the distant scattered disk, implying that if Planet Nine exists, the observed census of long-period trans-Neptunian objects is comprised of a mixture of Oort cloud and Kuiper belt objects. Our simulations further show that although inward-injected inner Oort cloud objects exhibit P9-driven orbital confinement, the degree of clustering is weaker than that of objects originating within the Kuiper belt. Cumulatively, our results suggest that a more eccentric Planet Nine is likely necessary to explain the data than previously thought.

An interstellar communication network benefits from relay nodes placed in the gravitational lenses of stars. The signal gains are of order $10^{9}$ with optimal alignment, allowing for GBits connections at kW power levels with meter-sized probes over parsec distances. If such a network exists, there might be a node in our solar system: where is it? With some assumptions on the network topology, candidate sky positions can be calculated. Apparent positions are influenced by the parallax motion from the Earth's orbit around the Sun, and the (slow) drifts caused by proper motions of nearby stars. With Gaia astrometry, instantaneous positions can be determined with arcsec accuracy. These potential node locations can be observed in targeted

The modern search for extraterrestrial intelligence (SETI) began with the seminal publications of Cocconi & Morrison (1959) and Schwartz & Townes (1961), who proposed to search for narrow-band signals in the radio spectrum, and for optical laser pulses. Over the last six decades, more than one hundred dedicated search programs have targeted these wavelengths; all with null results. All of these campaigns searched for classical communications, that is, for a significant number of photons above a noise threshold; with the assumption of a pattern encoded in time and/or frequency space. I argue that future searches should also target quantum communications. They are preferred over classical communications with regards to security and information efficiency, and they would have escaped detection in all previous searches. The measurement of Fock state photons or squeezed light would indicate the artificiality of a signal. I show that quantum coherence is feasible over interstellar distances, and explain for the first time how astronomers can search for quantum transmissions sent by ETI to Earth, using commercially available telescopes and receiver equipment.

In 2017, the Event Horizon Telescope (EHT) Collaboration succeeded in capturing the first direct image of the center of the M87 galaxy. The asymmetric ring morphology and size are consistent with theoretical expectations for a weakly accreting supermassive black hole of mass approximately 6.5 x 10^9 M_solar. The EHTC also partnered with several international facilities in space and on the ground, to arrange an extensive, quasi-simultaneous multi-wavelength campaign. This Letter presents the results and analysis of this campaign, as well as the multi-wavelength data as a legacy data repository. We captured M87 in a historically low state, and the core flux dominates over HST-1 at high energies, making it possible to combine core flux constraints with the more spatially precise very long baseline interferometry data. We present the most complete simultaneous multi-wavelength spectrum of the active nucleus to date, and discuss the complexity and caveats of combining data from different spatial scales into one broadband spectrum. We apply two heuristic, isotropic leptonic single-zone models to provide insight into the basic source properties, but conclude that a structured jet is necessary to explain M87's spectrum. We can exclude that the simultaneous gamma-ray emission is produced via inverse Compton emission in the same region producing the EHT mm-band emission, and further conclude that the gamma-rays can only be produced in the inner jets (inward of HST-1) if there are strongly particle-dominated regions. Direct synchrotron emission from accelerated protons and secondaries cannot yet be excluded.

We present PHANGS-ALMA, the first survey to map CO J=2-1 line emission at ~1" ~ 100pc spatial resolution from a representative sample of 90 nearby (d<~20 Mpc) galaxies that lie on or near the z=0 "main sequence" of star-forming galaxies. CO line emission traces the bulk distribution of molecular gas, which is the cold, star-forming phase of the interstellar medium. At the resolution achieved by PHANGS-ALMA, each beam reaches the size of a typical individual giant molecular cloud (GMC), so that these data can be used to measure the demographics, life-cycle, and physical state of molecular clouds across the population of galaxies where the majority of stars form at z=0. This paper describes the scientific motivation and background for the survey, sample selection, global properties of the targets, ALMA observations, and characteristics of the delivered ALMA data and derived data products. As the ALMA sample serves as the parent sample for parallel surveys with VLT/MUSE, HST, AstroSat, VLA, and other facilities, we include a detailed discussion of the sample selection. We detail the estimation of galaxy mass, size, star formation rate, CO luminosity, and other properties, compare estimates using different systems and provide best-estimate integrated measurements for each target. We also report the design and execution of the ALMA observations, which combine a Cycle~5 Large Program, a series of smaller programs, and archival observations. Finally, we present the first 1" resolution atlas of CO emission from nearby galaxies and describe the properties and contents of the first PHANGS-ALMA public data release.

Self-gravitating astronomical objects often show a central plateau in the density profile (core) whose physical origin is hotly debated. Cores are theoretically expected in N-body systems of maximum entropy, however, they are not present in the canonical N-body numerical simulations of cold dark matter (CDM). Our work shows that despite this apparent contradiction between theory and numerical simulations, they are fully consistent. Simply put, cores are characteristic of systems in thermodynamic equilibrium, but thermalizing collisions are purposely suppressed in CDM simulations. When collisions are allowed, N-body numerical simulations develop cored density profiles, in perfect agreement with the theoretical expectation. We compare theory and two types of numerical simulations: (1) when DM particles are self-interacting (SIDM) with enough cross-section, then the effective two-body relaxation timescale becomes shorter than the Hubble time resulting in cored DM haloes. The haloes thus obtained, with masses from dwarf galaxies to galaxy clusters, collapse to a single shape after normalization, and this shape agrees with the polytropic density profile theoretically expected. (2) The inner radii in canonical N-body numerical simulations are always discarded because the use of finite-mass DM particles artificially increases the two-body collision rate. We show that the discarded radii develop cores that are larger than the employed numerical softening and have polytropic shapes independently of halo mass. Our work suggests that the presence of cores in simulated (or observed) density profiles can used as evidence for systems in thermodynamic equilibrium.

Context: Active M dwarfs frequently exhibit large flares, which can pose an existential threat to the habitability of any planet in orbit in addition to making said planets more difficult to detect. M dwarfs do not lose angular momentum as easily as earlier-type stars, which maintain the high levels of stellar activity for far longer. Studying young, fast-rotating M dwarfs is key to understanding their near stellar environment and the evolution of activity Aims: We study stellar activity on the fast-rotating M dwarf GJ 3270. Methods: We analyzed dedicated high cadence, simultaneous, photometric and high-resolution spectroscopic observations obtained with CARMENES of GJ 3270 over 7.7 h, covering a total of eight flares of which two are strong enough to facilitate a detailed analysis. We consult the TESS data, obtained in the month prior to our own observations, to study rotational modulation and to compare the TESS flares to those observed in our campaign. Results: The TESS data exhibit rotational modulation with a period of 0.37 d. The strongest flare covered by our observing campaign released a total energy of about 3.6e32 erg, putting it close to the superflare regime. This flare is visible in the B,V, r, i, and z photometric bands, which allows us to determine a peak temperature of about 10,000 K. The flare also leaves clear marks in the spectral time series. In particular, we observe an evolving, mainly blue asymmetry in chromospheric lines, which we attribute to a post-flare, corotating feature. To our knowledge this is the first time such a feature has been seen on a star other than our Sun. Conclusions: Our photometric and spectroscopic time series covers the eruption of a strong flare followed up by a corotating feature analogous to a post-flare arcadal loop on the Sun with a possible failed ejection of material.

The Lucy Mission accomplishes its science during a series of five flyby encounters with seven Trojan asteroid targets. This mission architecture drives a concept of operations design that maximizes science return, provides redundancy in observations where possible, features autonomous fault protection and utilizes onboard target tracking near closest approach. These design considerations reduce risk during the relatively short time-critical periods when science data is collected. The payload suite consists of a color camera and infrared imaging spectrometer, a high-resolution panchromatic imager, and a thermal infrared spectrometer. The mission design allows for concurrent observations of all instruments. Additionally, two spacecraft subsystems will also contribute to the science investigations: the Terminal Tracking Cameras will obtain wide field-of-view imaging near closest approach to determine the shape of each of the Trojan targets and the telecommunication subsystem will carry out Doppler tracking of the spacecraft to determine the mass of each of the Trojan targets.

Ultraviolet (UV) and X-ray photons from active galactic nuclei (AGNs) can ionize hydrogen in the intergalactic medium (IGM). We solve radiative transfer around AGNs in high redshift to evaluate the 21-cm line emission from the neutral hydrogen in the IGM and obtain the radial profile of the brightness temperature in the epoch of reionization. The ionization profile extends over 10 [Mpc] comoving distance which can be observed in the order of 10 [arcmin]. From estimation of the radio galaxy number counts with high sensitivity observation through the Square Kilometre Array (SKA), we investigate the capability of parameter constrains for AGN luminosity function with Fisher analysis for three evolution model through cosmic time. We find that the errors for each parameter are restricted to a few percent when AGNs are sufficiently bright at high redshifts. We also investigate the possibility of further parameter constraints with future observation beyond the era of SKA.