We report the discovery of Pegasus IV, an ultra-faint dwarf galaxy found in archival data from the Dark Energy Camera processed by the DECam Local Volume Exploration Survey. Pegasus IV is a compact, ultra-faint stellar system ( = -r(1/2) 41(-6)(+8) pc; M-V = -4.25 +/- 0.2 mag) located at a heliocentric distance of 90(-6)(+4) kpc. Based on spectra of seven nonvariable member stars observed with Magellan/IMACS, we confidently resolve Pegasus IV's velocity dispersion, measuring s = sigma(v) 3.3(-1.1)(+1.7) km s(-1) (after excluding three velocity outliers); this implies a mass-to-light ratio of M1/2LV ,(1/2)=167(-99)(+224) M-?/L-? for the system. From the five stars with the highest signal-to-noise spectra, we also measure a systemic metallicity of [Fe/H] = -2.63(-0.30)(+0.26) dex, making Pegasus IV one of the most metal-poor ultra-faint dwarfs. We tentatively resolve a nonzero metallicity dispersion for the system. These measurements provide strong evidence that Pegasus IV is a dark-matter-dominated dwarf galaxy, rather than a star cluster. We measure Pegasus IV's proper motion using data from Gaia Early Data Release 3, finding (mu(alpha*, mu delta)) = (0.33 +/- 0.07, -0.21 +/- 0.08) mas yr(-1). When combined with our measured systemic velocity, this proper motion suggests that Pegasus IV is on an elliptical, retrograde orbit, and is currently near its orbital apocenter. Lastly, we identify three potential RR Lyrae variable stars within Pegasus IV, including one candidate member located more than 10 half-light radii away from the system's centroid. The discovery of yet another ultra-faint dwarf galaxy strongly suggests that the census of Milky Way satellites is still incomplete, even within 100 kpc.

Fe2P-type dioxides are significant both for geoscience and condensed-matter physics. For example, Fe2P-type SiO2 has been proposed to be one of the dominant components in the mantles of super-Earths and Fe2P-type TiO2 has been shown to have a large visible absorbance. Here we report the discovery of an Fe2P-type phase in a typical transition-metal dichalcogenide (TMD), TiTe2, using crystal structure prediction and first-principles calculations. Ambient layered TiTe2 will first transform to a monoclinic C2/m phase and then finally to the hexagonal Fe2P-type phase above 33 GPa. Fe2P-type TiTe2 is predicted to be metallic, contrasting with the semiconductivity of Fe2P-type TiO2. The same high-pressure phase (Fe2P type) appears both in transition-metal dioxides and TMDs, indicating that the stacking patterns of anions and cations play an increasingly important role in determining the high-pressure phase.

The intestines of animals are colonized by commensal microbes, which impact host development, health, and behavior. Precise quantification of colonization is essential for studying the complex interactions between host and microbe both to validate the microbial composition and study its effects. Drosophila melanogaster,which has a low native microbial diversity and is economical to rear with defined microbial composition, has emerged as a model organism for studying the gut microbiome. Analyzing the microbiome of an individual organism requires identification of which microbial species are present and quantification of their absolute abundance. This article presents a method for the analysis of a large number of individual fly microbiomes. The flies are prepared in 96-well plates, enabling the handling of a large number of samples at once. Microbial abundance is quantified by plating up to 96 whole fly homogenates on a single agar plate in an array of spots and then counting the colony forming units (CFUs) that grow in each spot. This plating system is paired with an automated CFU quantification platform, which incorporates photography of the plates, differentiation of fluorescent colonies, and automated counting of the colonies using an ImageJ plugin. Advantages are that (i) this method is sensitive enough to detect differences between treatments, (ii) the spot plating method is as accurate as traditional plating methods, and (iii) the automated counting process is accurate and faster than manual counting. The workflow presented here enables high-throughput quantification of CFUs in a large number of replicates and can be applied to other microbiology study systems including in vitro and other small animal models.