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<jats:title>Abstract</jats:title> <jats:p>We use the optical integral field observations with Multi-Unit Spectroscopic Explorer (MUSE) on the Very Large Telescope, together with CLOUDY photoionization models, to study ionization structure and physical conditions of two luminous H <jats:sc>ii</jats:sc> regions in the N44 star-forming complex of the Large Magellanic Cloud. The spectral maps of various emission lines reveal a stratified ionization geometry in N44 D1. The spatial distribution of [O <jats:sc>i</jats:sc>] <jats:italic>λ</jats:italic>6300 emission in N44 D1 indicates a partially covered ionization front at the outer boundary of the H <jats:sc>ii</jats:sc> region. These observations reveal that N44 D1 is a blister H <jats:sc>ii</jats:sc> region. The [O <jats:sc>i</jats:sc>] <jats:italic>λ</jats:italic>6300 emission in N44 C does not provide a well-defined ionization front at the boundary, while patches of [S <jats:sc>ii</jats:sc>] <jats:italic>λ</jats:italic>6717 and [O <jats:sc>i</jats:sc>] <jats:italic>λ</jats:italic>6300 emission bars are found in the interior. The results of spatially resolved MUSE spectra are tested with the photoionization models for the first time in these H <jats:sc>ii</jats:sc> regions. A spherically symmetric ionization-bounded model with a partial covering factor, which is appropriate for a blister H <jats:sc>ii</jats:sc> region, can well reproduce the observed geometry and most of the diagnostic line ratios in N44 D1. Similarly, in N44 C we apply a low-density and optically thin model based on the observational signatures. Our modeling results show that the ionization structure and physical conditions of N44 D1 are mainly determined by the radiation from an O5 V star. However, local X-rays, possibly from supernovae or stellar wind, play a key role. In N44 C, the main contribution is from three ionizing stars.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Pulsar H<jats:italic>α</jats:italic> bow shocks provide rare opportunities to constrain the energetics and kinematics of the relativistic pulsar wind. We have acquired optical imaging and integral field unit spectroscopy of the bow shock of the millisecond pulsar PSR J1959+2048, measuring the shock symmetry axis at a position angle = 213.2 ± 0.°2 and showing that this slow nonradiative shock has a broad-to-narrow line component ratio <jats:italic>I</jats:italic> <jats:sub> <jats:italic>b</jats:italic> </jats:sub>/<jats:italic>I</jats:italic> <jats:sub> <jats:italic>n</jats:italic> </jats:sub> = 4. The data show that the pulsar’s velocity lies 2.°2 out of the plane of the sky. Coupled with a new fit for its timing proper motion, giving <jats:italic>μ</jats:italic> <jats:sub>tot</jats:sub> = 30.05 mas yr<jats:sup>−1</jats:sup> and a Very Long Baseline Array (VLBA) interferometric parallax measurement giving <jats:inline-formula> <jats:tex-math> <?CDATA $d={2.57}_{-0.77}^{+1.84}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>d</mml:mi> <mml:mo>=</mml:mo> <mml:msubsup> <mml:mrow> <mml:mn>2.57</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.77</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>1.84</mml:mn> </mml:mrow> </mml:msubsup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac6263ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> kpc (90% range), we have unusually complete information on the pulsar kinematics. The bow shock constraints on the wind momentum flux imply that, at the best-fit parallax distance, the pulsar moment of inertia must be very large and/or the H<jats:italic>α</jats:italic> efficiency at its modest shock velocity must be very high.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We searched for massive galaxy population in the known large-scale high-density structure of Ly<jats:italic>α</jats:italic> emitters (LAEs) at <jats:italic>z</jats:italic> = 2.39 near the radio galaxy 53W002 by using imaging data from <jats:inline-formula> <jats:tex-math> <?CDATA $B,\,V,\,i^{\prime} ,\,J,\,H,\,\mathrm{and}\,{K}_{s}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>B</mml:mi> <mml:mo>,</mml:mo> <mml:mspace width="0.50em" /> <mml:mi>V</mml:mi> <mml:mo>,</mml:mo> <mml:mspace width="0.50em" /> <mml:mi>i</mml:mi> <mml:mo accent="false">′</mml:mo> <mml:mo>,</mml:mo> <mml:mspace width="0.50em" /> <mml:mi>J</mml:mi> <mml:mo>,</mml:mo> <mml:mspace width="0.50em" /> <mml:mi>H</mml:mi> <mml:mo>,</mml:mo> <mml:mspace width="0.50em" /> <mml:mi>and</mml:mi> <mml:mspace width="0.50em" /> <mml:msub> <mml:mrow> <mml:mi>K</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>s</mml:mi> </mml:mrow> </mml:msub> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac6257ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> bands taken with Suprime-Cam and MOIRCS on the Subaru telescope. We selected 62 protocluster member candidates by their <jats:italic>JHK</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub>-band colors and spectral energy distribution (SED) fitting analysis (<jats:italic>JHK</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub>-selected galaxies) in our survey field of <jats:inline-formula> <jats:tex-math> <?CDATA $70.2\,{\mathrm{arcmin}}^{2}$?> </jats:tex-math> <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>70.2</mml:mn> <mml:mspace width="0.50em" /> <mml:msup> <mml:mrow> <mml:mi>arcmin</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> </mml:math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjac6257ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> and compared their physical properties estimated from the SED fitting with a comparison sample in the COSMOS field. We found significant number density excesses for the <jats:italic>JHK</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub>-selected galaxies in the 53W002 field at <jats:italic>K</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> &lt; 22.25, <jats:italic>J</jats:italic> − <jats:italic>K</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> &gt; 2, or <jats:italic>V</jats:italic> − <jats:italic>K</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> &gt; 4. In particular, the number density of the <jats:italic>JHK</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub>-selected galaxies with <jats:italic>K</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> &lt; 22.25 and <jats:italic>J</jats:italic> − <jats:italic>K</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> &gt; 2 in the 53W002 field is nine times higher than the comparison sample. Most of those with <jats:italic>K</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> &lt; 22.25 and <jats:italic>J</jats:italic> − <jats:italic>K</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> &gt; 2 are massive galaxies with <jats:italic>M</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> &gt; 10<jats:sup>11</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, and their specific star formation rates (sSFRs) of 10<jats:sup>−11</jats:sup>–10<jats:sup>−10</jats:sup> yr<jats:sup>−1</jats:sup> suggest that the star formation has not yet stopped completely. We also found a density excess of quiescent galaxies with <jats:italic>M</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> = 5 × 10<jats:sup>10</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> to 10<jats:sup>11</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and sSFR &lt; 10<jats:sup>−11</jats:sup> yr<jats:sup>−1</jats:sup>, as well as that of low-mass galaxies with <jats:italic>M</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> = 10<jats:sup>9.75</jats:sup>–10<jats:sup>10</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> and various sSFRs. The massive galaxies with <jats:italic>M</jats:italic> <jats:sub> <jats:italic>s</jats:italic> </jats:sub> &gt; 10<jats:sup>11</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> are not located at the density peaks of LAEs but widely distributed along a similar direction to the structure of LAEs over ∼15–20 comoving Mpc. On the other hand, the quiescent galaxies with sSFR &lt; 10<jats:sup>−11</jats:sup> yr<jats:sup>−1</jats:sup> clearly avoid the structure of LAEs. Our results suggest that massive galaxies also exist in this protocluster discovered by the moderate overdensity of LAEs and their star formation activities depend on location in the protocluster.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We combine the power of two stream-searching tools, <jats:monospace>STREAMFINDER</jats:monospace> and <jats:monospace>StarGO</jats:monospace> applied to the Gaia EDR3 data, to detect stellar debris belonging to the Cetus stream system that forms a complex, nearly polar structure around the Milky Way. In this work, we find the southern extensions of the northern Cetus stream as the Palca stream and a new southern stream, which overlap on the sky but have different distances. These two stream wraps extend over more than ∼100° on the sky (−60° &lt; <jats:italic>δ</jats:italic> &lt; +40°). The current <jats:italic>N</jats:italic>-body model of the system reproduces both as two wraps in the trailing arm. We also show that the Cetus system is confidently associated with the Triangulum/Pisces, Willka Yaku, and the recently discovered C-20 streams. The association with the ATLAS-Aliqa Uma stream is much weaker. All of these stellar debris are very metal-poor, comparable to the average metallicity of the southern Cetus stream with [Fe/H] = −2.17 ± 0.20. The estimated stellar mass of the Cetus progenitor is at least 10<jats:sup>5.6</jats:sup> <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>, compatible with Ursa Minor or Draco dwarf galaxies. The associated globular cluster with similar stellar mass, NGC 5824 very possibly was accreted in the same group infall. The multi-wrap Cetus stream is a perfect example of a dwarf galaxy that has undergone several periods of stripping, leaving behind debris at multiple locations in the halo. The full characterization of such systems is crucial to unravel the history of the assembly of the Milky Way, and importantly, to provide nearby fossils to study ancient low-mass dwarf galaxies.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the results from a spectroscopic survey using the MOSFIRE near-infrared spectrograph on the 10 m Keck telescope to search for Ly<jats:italic>α</jats:italic> emission from candidate galaxies at <jats:italic>z</jats:italic> ∼ 9–10 in four of the CANDELS fields (GOODS-N, EGS, UDS, and COSMOS). We observed 11 target galaxies, detecting Ly<jats:italic>α</jats:italic> from one object in ∼8.1 hr of integration, at <jats:italic>z</jats:italic> = 8.665 ± 0.001 with an integrated signal-to-noise ratio &gt; 7. This galaxy is in the CANDELS Extended Groth Strip (EGS) field and lies physically close (3.5 physical Mpc [pMpc]) to another confirmed galaxy in this field with Ly<jats:italic>α</jats:italic> detected at <jats:italic>z</jats:italic> = 8.683. The detection of Ly<jats:italic>α</jats:italic> suggests the existence of large (∼1 pMpc) ionized bubbles fairly early in the reionization process. We explore the ionizing output needed to create bubbles of this size at this epoch and find that such a bubble requires more than the ionizing power provided by the full expected population of galaxies (by integrating the UV luminosity function down to <jats:italic>M</jats:italic> <jats:sub>UV</jats:sub> = −13). The Ly<jats:italic>α</jats:italic> we detect would be able to escape the predominantly neutral intergalactic medium at this epoch if our detected galaxy is inhabiting an overdensity, which would be consistent with the photometric overdensity previously identified in this region by Finkelstein et al. This implies that the CANDELS EGS field is hosting an overdensity at <jats:italic>z</jats:italic> = 8.7 that is powering one or more ionized bubbles, a hypothesis that will be imminently testable with forthcoming James Webb Space Telescope observations in this field.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Metal-poor nearby galaxies hosting massive stars have a fundamental role to play in our understanding of both high-redshift galaxies and low-metallicity stellar populations. But while much attention has been focused on their bright nebular gas emission, the massive stars that power it remain challenging to constrain. Here we present exceptionally deep Hubble Space Telescope ultraviolet spectra targeting six local (<jats:italic>z</jats:italic> &lt; 0.02) galaxies that power strong nebular C <jats:sc>iv</jats:sc> emission approaching that encountered at <jats:italic>z</jats:italic> &gt; 6. We find that the strength and spectral profile of the nebular C <jats:sc>iv</jats:sc> in these new spectra follow a sequence evocative of resonant scattering models, indicating that the hot circumgalactic medium likely plays a key role in regulating C <jats:sc>iv</jats:sc> escape locally. We constrain the metallicity of the massive stars in each galaxy by fitting the forest of photospheric absorption lines, reporting measurements driven by iron that lie uniformly below 10% solar. Comparison with the gas-phase oxygen abundances reveals evidence for enhancement in O/Fe 2–4 times above solar across the sample, robust to assumptions about the absolute gas-phase metallicity scale. This supports the idea that these local systems are more chemically similar to their primordial high-redshift counterparts than to the bulk of nearby galaxies. Finally, we find significant tension between the strong stellar wind profiles observed and our population synthesis models constrained by the photospheric forest in our highest-quality spectra. This reinforces the need for caution in interpreting wind lines in isolation at high redshift, but also suggests a unique path toward validating fundamental massive star physics at extremely low metallicity with integrated ultraviolet spectra.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>High brightness-temperature radiation is observed in various astrophysical sources: active galactic nuclei, pulsars, interstellar masers, and flaring stars; the discovery of fast radio bursts renewed interest in the nonlinear interaction of intense radiation with plasma. In astronomical systems, the radiation frequency is typically well above the plasma frequency and its spectrum is broad, so nonlinear processes differ considerably from those typically studied in laboratory plasma. This paper is the first in a series devoted to the numerical study of nonlinear interactions of electromagnetic waves with plasma. We start with nonmagnetized pair plasmas, where the primary processes are induced (Compton) scattering and filamentation instability. In this paper, we consider waves in which electron oscillations are nonrelativistic. Here, the numerical results can be compared to analytical theory, facilitating the development of appropriate numerical tools and framework. We distill the analytic theory, reconciling the plasma and radiative transfer pictures of induced scattering and developing in detail the kinetic theory of modulation/filamentation instability. We carry out homogeneous numerical simulations using the particle-in-cell codes EPOCH and Tristan-MP for both monochromatic waves and wave packets. We show that simulations of both processes are consistent with theoretical predictions, setting the stage for analyzing the highly nonlinear regime.</jats:p>