The Magellanic Stream and the Leading Arm form a massive, filamentary system of gas clouds surrounding the Large and Small Magellanic Clouds. Here we present a new component-level analysis of their ultraviolet (UV) kinematic properties using a sample of 31 sightlines through the Magellanic System observed with the Hubble Space Telescope/Cosmic Origins Spectrograph. Using Voigt-profile fits to UV metal-line absorption, we quantify the kinematic differences between the low-ion ( and ), intermediate-ion ( ), and high-ion ( and ) absorption lines and compare the kinematics between the Stream and Leading Arm. We find that the Stream shows generally simple, single-phase kinematics, with statistically indistinguishable b-value distributions for the low-, intermediate-, and high-ion components, all dominated by narrow ( km s−1) components that are well aligned in velocity. In contrast, we find tentative evidence that the Leading Arm shows complex, multi-phase kinematics, with broader high ions than low ions. These results suggest that the Stream is photoionized up to by a hard ionizing radiation field. This can be naturally explained by the Seyfert-flare model of Bland-Hawthorn, in which a burst of ionizing radiation from the Galactic Center photoionized the Stream as it passed below the south Galactic pole. The Seyfert flare is the only known source of radiation that is both powerful enough to explain the Hα intensity of the Stream and hard enough to photoionize and to the observed levels. The flare’s timescale of a few Myr suggests it is the same event that created the giant X-ray/γ-ray Fermi Bubbles at the Galactic Center.

We present the first Data Release (DR1) from the Hubble Space Telescope (HST) Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) program, a Director’s Discretionary program devoting approximately 1000 HST orbits to the production of an ultraviolet spectroscopic library of young high- and low-mass stars in the local universe. The science products in this release are combined from individual, extracted and calibrated spectra obtained with the COS and STIS instruments aboard HST. Products are made using both archival HST data and new HST observations obtained through the ULLYSES program. DR1 acts as the first step toward completing a goal of generating a fully public data set for the purpose of enabling, supporting, and stimulating a broad range of transformative astrophysical research.

We identify and characterize compact dwarf starburst (CDS) galaxies in the RE-SOLVE survey, a volume-limited census of galaxies in the local universe, to probe whether this population contains any residual "blue nuggets," a class of intensely starforming compact galaxies first identified at high redshift z. Our 50 low-z CDS galaxies are defined by dwarf masses (stellar mass M * < 10 9.5 M ), compact bulged-disk or spheroid-dominated morphologies (using a quantitative criterion, µ ∆ > 8.6), and specific star formation rates above the defining threshold for high-z blue nuggets (log SSFR [Gyr −1 ] > −0.5). Across redshifts, blue nuggets exhibit three defining properties: compactness relative to contemporaneous galaxies, abundant cold gas, and formation via compaction in mergers or colliding streams. Those with halo mass below M halo ∼ 10 11.5 M may in theory evade permanent quenching and cyclically refuel until the present day. Selected only for compactness and starburst activity, our CDS galaxies generally have M halo 10 11.5 M and gas-to-stellar mass ratio 1. Moreover, analysis of archival DECaLS photometry and new 3D spectroscopic observations for CDS galaxies reveals a high rate of photometric and kinematic disturbances suggestive of dwarf mergers. The SSFRs, surface mass densities, and number counts of CDS galaxies are compatible with theoretical and observational expectations for redshift evolution in blue nuggets. We argue that CDS galaxies represent a maximally-starbursting subset of traditional compact dwarf classes such as blue compact dwarfs and blue E/S0s. We conclude that CDS galaxies represent a low-z tail of the blue nugget phenomenon formed via a moderated compaction channel that leaves open the possibility of disk regrowth and evolution into normal disk galaxies.

The Hubble Space Telescope (HST) Ultraviolet Legacy Library of Young Stars as Essential Standards (ULLYSES) program is a HST Director's Discretionary program that is in the process of obtaining a large library of ultraviolet spectroscopic observations of young stars of both high and low masses. We present information on close companions to the T Tauri stars CVSO 109 and CVSO 165 in Orion that were observed with the HST as part of this program. CVSO 109 has a fainter companion at an angular separation ρ = 0.″64 near PA = 218° and Δ(F28X50LP) ≈ 0.6  mag, while the CVSO 165 companion is at ρ = 0.″30 near PA = 326° with Δ(F28X50LP) ≈ 1.7  mag. Both components of CVSO 165 appear to be active stars with strong emission features, while the CVSO 109 companion shows only modest indications of stellar activity. Extracted spectra for each of these components will be included in ULLYSES Data Release 2.

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