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WEAVE, the new wide-field, massively multiplexed spectroscopic survey facility for the William Herschel Telescope, saw first light in late 2022. WEAVE comprises a new 2-deg field-of-view prime-focus corrector system, a nearly 1000-multiplex fibre positioner, 20 individually deployable 'mini' integral field units (IFUs), and a single large IFU. These fibre systems feed a dual-beam spectrograph covering the wavelength range 366–959 nm at R ∼ 5000, or two shorter ranges at |$R\sim 20\, 000$|⁠. After summarizing the design and implementation of WEAVE and its data systems, we present the organization, science drivers, and design of a five- to seven-year programme of eight individual surveys to: (i) study our Galaxy's origins by completing Gaia 's phase-space information, providing metallicities to its limiting magnitude for ∼3 million stars and detailed abundances for ∼1.5 million brighter field and open-cluster stars; (ii) survey ∼0.4 million Galactic-plane OBA stars, young stellar objects, and nearby gas to understand the evolution of young stars and their environments; (iii) perform an extensive spectral survey of white dwarfs; (iv) survey ∼400 neutral-hydrogen-selected galaxies with the IFUs; (v) study properties and kinematics of stellar populations and ionized gas in z < 0.5 cluster galaxies; (vi) survey stellar populations and kinematics in |${\sim} 25\, 000$| field galaxies at 0.3 ≲ z ≲ 0.7; (vii) study the cosmic evolution of accretion and star formation using >1 million spectra of LOFAR-selected radio sources; and (viii) trace structures using intergalactic/circumgalactic gas at z > 2. Finally, we describe the WEAVE Operational Rehearsals using the WEAVE Simulator. [ABSTRACT FROM AUTHOR]

We show that a string of H i clouds that form part of the high-velocity cloud complex known as GCN is a probable gaseous stream extending over more than in the Galactic halo. The radial velocity gradient along the stream is used to deduce transverse velocities as a function of distance, enabling a family of orbits to be computed. We find that a direction of motion towards the Galactic disc coupled with a mid-stream distance of provides a good match to the observed sky positions and radial velocities of the H i clouds comprising the stream. With an estimated mass of , its progenitor is likely to be a dwarf galaxy. However, no stellar counterpart has been found amongst the currently known Galactic dwarf spheroidal galaxies or stellar streams and the exact origin of the stream is therefore currently unknown. [ABSTRACT FROM AUTHOR]

We present a geometrodynamical method for determining distances to orbital streams of H i gas in the Galaxy. The method makes use of our offset from the Galactic Centre and assumes that the gas comprising the stream nearly follows a planar orbit about the Galactic Centre. We apply this technique to the Magellanic Stream and determine the distances to all points along it; a consistency check shows that the angular momentum is approximately constant. Applying this technique to the Large Magellanic Cloud itself gives an independent distance which agrees within its accuracy of around 10 per cent. Relaxing the demand for exact conservation of energy and angular momentum at all points along the stream allows for an increase in orbital period between the lagging end and the front end led by the Magellanic Clouds. Similar methods are applicable to other long streams of high-velocity clouds, provided they also nearly follow planar orbits; these would allow otherwise unknown distances to be determined. [ABSTRACT FROM AUTHOR]

It is now established that several of the Local Group dwarf spheroidal galaxies (dSphs) have large mass-to-light ratios. We consider the possibility that the dark matter in the haloes of dSphs is composed of massive black holes with masses in the range. We use directN-body simulations to determine the long-term evolution of a two-component dSph composed of a pressure-supported stellar population within a black hole dominated halo. The black holes are initially distributed according to a Navarro–Frenk–White profile. For black hole masses between 105 and, the dark matter halo evolves towards a shallower inner profile in less than a Hubble time. This suggests that black holes in this mass range might provide an explanation for the origin of the dark matter cores inferred from observations of low surface brightness galaxy rotation curves. We compare the simulated evolution of the stellar population with observed data for the Draco dSph, and we find that dynamical heating generally leads to the rapid dispersal of the stellar population to large radii. The dependence of the heating rate on the black hole mass is determined, and an upper limit ofis placed on the individual black holes comprising the dark matter halo of Draco, if its present stellar distribution is representative of the initial one. We also present a simple scaling argument, which demonstrates that the dynamical heating of an initially compact, although not self-gravitating, stellar distribution might produce a remnant distribution similar in extent to Draco after 10 Gyr, even for black hole masses somewhat in excess of. [ABSTRACT FROM AUTHOR]

We consider the possibility that the Galactic neutral hydrogen stream Complex A and the stellar Orphan stream are related, and use this hypothesis to determine possible distances to Complex A and the Orphan stream, and line-of-sight velocities for the latter. The method presented uses our current knowledge of the projected positions of the streams, as well as line-of-sight velocities for Complex A, and we show that a solution exists in which the two streams share the same orbit. If Complex A and the Orphan stream are on this orbit, our calculations suggest the Orphan stream to be at an average distance of 9 kpc, with heliocentric radial velocities of approximately . Complex A would be ahead of the Orphan stream in the same wrap of the orbit, with an average distance of 4 kpc, which is consistent with the distance constraints determined through interstellar absorption line techniques. [ABSTRACT FROM AUTHOR]