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<jats:title>Abstract</jats:title> <jats:p>We present subarcsecond optical images and low- to moderate-resolution spectra of the compact, X-ray, and optically bright supernova remnant known as S8 in the nearby dwarf galaxy IC 1613. Deep H<jats:italic>α</jats:italic> images of the remnant show a sharply defined crescent-shaped nebula, while narrow passband images reveal a coincident and unexpectedly bright continuum nebulosity exhibiting a size and morphology like that seen for the remnant’s line emissions. Low-dispersion spectra covering 3600–9000 Å show numerous low-ionization line emissions such as [O <jats:sc>i</jats:sc>] and [Fe <jats:sc>ii</jats:sc>], along with higher-ionization emission lines including He <jats:sc>ii</jats:sc> and optical coronal lines [Fe <jats:sc>vii</jats:sc>], [Fe <jats:sc>x</jats:sc>], [Fe <jats:sc>xi</jats:sc>], and [Fe <jats:sc>xiv</jats:sc>]. This suggests the presence of a wide range of shock velocities from ∼50 to over 350 km s<jats:sup>−1</jats:sup>, corresponding to preshock densities of ∼1–30 cm<jats:sup>−3</jats:sup>. Higher-resolution spectra indicate an expansion velocity around 180 km s<jats:sup>−1</jats:sup> with an ∼45 km s<jats:sup>−1</jats:sup> wide central cavity. The H<jats:italic>α</jats:italic> emission spans rest-frame velocities of +120 to −240 km s<jats:sup>−1</jats:sup>, and we estimate a total nebula mass of 119 ± 34 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. We conclude that S8 is a relatively young supernova remnant (≃2700–4400 yr) exhibiting properties remarkably like those seen in the young LMC remnant N49, including age, physical size, shock velocities, filament densities, optical line strengths, X-ray and optical luminosities, and coronal line and continuum emissions.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The current understanding of the characteristics of solar and inner heliospheric electron events is inferred almost entirely from observations made by spacecraft located at 1 astronomical unit (au). Previous observations within 1 au of the Sun, by the Helios spacecraft at ∼0.3–1 au, indicate the presence of electron events that are not detected at 1 au or may have merged during transport from the Sun. Parker Solar Probe’s close proximity to the Sun at perihelion provides an opportunity to make the closest measurements yet of energetic electron events. We present an overview of measurements of electrons with energies between ∼17 keV and ∼1 MeV made by the Parker Solar Probe Integrated Science Investigation of the Sun (IS⊙IS) instrument suite during Encounter 2 (2019 March 31–April 10 with perihelion of ∼0.17 au), including several small electron events. We examine these events in the context of the electromagnetic and solar wind environment measured by the FIELDS and SWEAP instruments on Parker Solar Probe. We find most of these electron enhancements to be associated with type III radio emissions that reach the local plasma frequency and one enhancement that appears to be primarily associated with abrupt changes in the local magnetic field. Together, these associations suggest that these are indeed the first measurements of energetic electron events within 0.2 au.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Recent studies have shown that magnetic fields in the solar corona are strong enough to significantly mix the two 3<jats:italic>p</jats:italic> <jats:sup>4</jats:sup>3<jats:italic>d</jats:italic> <jats:sup>4</jats:sup>D<jats:sub>5/2,7/2</jats:sub> levels in Cl-like Fe <jats:sc>x</jats:sc>. This mixing gives rise to a magnetically induced transition (MIT) component in the bright Fe <jats:sc>x</jats:sc> 257.3 Å line, commonly observed by current instrumentation, that can be used for coronal magnetic field diagnostics. This line, commonly observed by the still operational EIS spectrometer on board the Hinode satellite since 2007, opens a new window into the coronal magnetic field. However, the strength of this MIT transition depends on the square of the energy difference Δ<jats:italic>E</jats:italic> of the two <jats:inline-formula> <jats:tex-math> <?CDATA ${}^{4}{{\rm{D}}}_{5/\mathrm{2,7}/2}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabb2a6ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> levels, so that an accurate determination of Δ<jats:italic>E</jats:italic> is of critical importance to accurately measure coronal magnetic field strengths. In the present work we present a new measurement of Δ<jats:italic>E</jats:italic> obtained determining the separation of the two component of the Fe <jats:sc>x</jats:sc> doublet close to 1603.3 Å from deep-exposure spectra of a quiescent streamer at the solar limb taken with the SUMER instrument on board SoHO. Our measurement of Δ<jats:italic>E</jats:italic> = 2.29 ± 0.50 cm<jats:sup>−1</jats:sup> agrees with, and improves upon, an earlier measurements by Judge et al. by decreasing its uncertainty from 80% to approximately 20%, improving the attainable accuracy of magnetic field strength measurements obtainable with the Fe <jats:sc>x</jats:sc> 257.26 Å line.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The Fermi Large Area Telescope (Fermi-LAT) 3FHL catalog is the latest catalog of &gt;10 GeV sources and will remain an important resource for the high-energy community for the foreseeable future. Therefore, it is crucial that this catalog is made complete by providing associations for most sources. In this paper, we present the results of the X-ray analysis of 38 3FHL sources. We found a single bright X-ray source in 20 fields, two sources each in two fields, and none for the remaining 16. The analysis of the properties of the 22 3FHL fields with X-ray sources led us to believe that most (∼19/22) are of extragalactic origin. A machine-learning algorithm was used to determine the source type and we find that 15 potential blazars are likely BL Lacertae objects (BL Lac objects). This is consistent with the fact that BL Lac objects are by far the most numerous population detected above &gt;10 GeV in the 3FHL.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We derive a distance of 15.8 ± 0.4 Mpc to the archetypal Seyfert 1 galaxy NGC 4151 based on the near-infrared Cepheid period–luminosity relation and new Hubble Space Telescope multiband imaging. This distance determination, based on measurements of 35 long-period (<jats:italic>P</jats:italic> &gt; 25 days) Cepheids, will support the absolute calibration of the supermassive black hole mass in this system, as well as studies of the dynamics of the feedback or feeding of its active galactic nucleus.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Bursts of localized star formation in galaxies can levitate material from the midplane. Spiral galaxies that are edge-on allow a clear distinction of material that is levitated off the galaxies’ midplanes. We used SOFIA to measure the vertical distribution of [C <jats:sc>ii</jats:sc>] 157.7 <jats:italic>μ</jats:italic>m line emission for two nearby, edge-on galaxies, NGC 891 and NGC 5907. We find that for the central region and actively star-forming regions in the northern portion of NGC 891, and for NGC 5907, a thin (0.3 kpc) disk is supplemented by a thick disk with an exponential scale height of ∼2 kpc. The [C <jats:sc>ii</jats:sc>] is far more extended than mid-infrared emission (0.1 kpc, tracing present-day massive star formation) but not as extended as the H <jats:sc>i</jats:sc> (100 kpc, tracing low-metallicity circum/intergalactic matter). The extraplanar [C <jats:sc>ii</jats:sc>] may arise in walls of chimneys that connect the disk to the halo.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The VVV microlensing search has recently surveyed the region of the Galactic plane at <jats:italic>b</jats:italic> = 0 within −10.00 ≤ <jats:italic>l</jats:italic> ≤ 10.44 deg. in the near-infrared (IR) discovering hundreds of microlensing events. In this paper we explore the microlensing events with background sources that might be located in the far disk of the Galaxy, beyond the bulge. We discuss the possible configurations for the microlensing lenses and sources located at different places within the Galactic plane. Then we search for these events using the local red clump centroids of the VVV near-IR color–magnitude diagrams. According to the estimated distances and proper motions, <jats:italic>N</jats:italic> = 20 events may have sources located in the far disk. The candidates for far-disk sources show on average longer timescales (<jats:italic>t</jats:italic> <jats:sub> <jats:italic>E</jats:italic> </jats:sub> = 49.3 ± 7.9 days) than the mean of the timescale distribution for the bulge red clump sources (<jats:italic>t</jats:italic> <jats:sub> <jats:italic>E</jats:italic> </jats:sub> = 36.4 ± 1.1 days). We conclude that the population of microlensing events in the region −10.00 ≤ <jats:italic>l</jats:italic> ≤ 10.44, −0.46 ≤ <jats:italic>b</jats:italic> ≤ 0.65 deg. contains a non-negligible number of events with candidate far-disk sources (∼11%). Our results are relevant in view of future microlensing plans with the Roman Space Telescope (formerly WFIRST) in the near-IR.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present neutral hydrogen (H <jats:sc>i</jats:sc>) observations using the Robert C. Byrd Green Bank Telescope (GBT) of 70 optically detected UDG candidates in the Coma region from the Systematically Measuring Ultra-Diffuse Galaxies survey (SMUDGes). We detect H <jats:sc>i</jats:sc> in 18 targets, confirming nine to be gas-rich UDGs and the remainder to be foreground dwarfs. None of our H <jats:sc>i</jats:sc>-detected UDGs are Coma Cluster members and all but one are in low-density environments. The H <jats:sc>i</jats:sc>-detected UDGs are bluer and have more irregular morphologies than the redder, smoother candidates not detected in H <jats:sc>i</jats:sc>, with the combination of optical color and morphology being a better predictor of gas richness than either parameter alone. There is little visual difference between the gas-rich UDGs and the foreground dwarfs in the SMUDGes imaging, and distances are needed to distinguish between them. We find that the gas richnesses of our H <jats:sc>i</jats:sc>-confirmed UDGs and those from other samples scale with their effective radii in two stellar mass bins, possibly providing clues to their formation. We attempt to place our UDGs on the baryonic Tully–Fisher relation (BTFR) using optical ellipticities and turbulence-corrected H <jats:sc>i</jats:sc> line widths to estimate rotation velocities, but the potential systematics associated with fitting smooth Sérsic profiles to clumpy, low-inclination disks of low surface brightness precludes a meaningful analysis of potential BTFR offsets. These observations are a pilot for a large campaign now under way at the GBT to use the H <jats:sc>i</jats:sc> properties of gas-rich UDGs to quantitatively constrain how these galaxies form and evolve.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We increase the sample of ultradiffuse galaxies (UDGs) in lower-density environments with characterized globular cluster (GC) populations using new Hubble Space Telescope observations of nine UDGs in group environments. While the bulk of our UDGs have GC abundances consistent with normal dwarf galaxies, two of these UDGs have excess GC populations. These two UDGs both have GC luminosity functions consistent with higher surface brightness galaxies and cluster UDGs. We then combine our nine objects with previous studies to create a catalog of UDGs with analyzed GC populations that spans a uniquely diverse range of environments. We use this catalog to examine broader trends in the GC populations of low stellar mass galaxies. The highest GC abundances are found in cluster UDGs, but whether cluster UDGs are actually more extreme requires the study of many more UDGs in groups. We find a possible positive correlation between GC abundance and stellar mass, and between GC abundance and galaxy size at fixed stellar mass. However, we see no significant relation between stellar mass and galaxy size, over our limited stellar mass range. We consider possible origins of the correlation between GC abundance and galaxy size, including the possibility that these two galaxy properties are both dependent on the galaxy dark matter halo, or that they are related through baryonic processes like internal feedback.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The nondetection of companion stars in SN Ia progenitor systems lends support to the notion of double-degenerate systems and explosions triggered by the merging of two white dwarfs. This very asymmetric process should lead to a conspicuous polarimetric signature. By contrast, observations consistently find very low continuum polarization as the signatures from the explosion process largely dominate over the pre-explosion configuration within several days. Critical information about the interaction of the ejecta with a companion and any circumstellar matter is encoded in the early polarization spectra. In this study, we obtain spectropolarimetry of SN 2018gv with the ESO Very Large Telescope at −13.6 days relative to the <jats:italic>B</jats:italic>-band maximum light, or ∼5 days after the estimated explosion—the earliest spectropolarimetric observations to date of any SN Ia. These early observations still show a low continuum polarization (≲0.2%) and moderate line polarization (0.30% ± 0.04% for the prominent Si <jats:sc>ii</jats:sc> <jats:italic>λ</jats:italic>6355 feature and 0.85% ± 0.04% for the high-velocity Ca component). The high degree of spherical symmetry implied by the low-line and continuum polarization at this early epoch is consistent with explosion models of delayed detonations and is inconsistent with the merger-induced explosion scenario. The dense UV and optical photometry and optical spectroscopy within the first ∼100 days after the maximum light indicate that SN 2018gv is a normal SN Ia with similar spectrophotometric behavior to SN 2011fe.</jats:p>