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<jats:title>Abstract</jats:title> <jats:p>If not properly accounted for, unresolved binary stars can induce a bias in the photometric determination of star cluster masses inferred from star counts and the luminosity function. A correction factor close to 1.15 (for a binary fraction of 0.35) was found in Borodina et al., which needs to be applied to blind photometric mass estimates. This value for the correction factor was found to be smaller than literature values. In an attempt to lift this discrepancy, in this work the focus is on higher order multiple stars with the goal of investigating the effect of triple and quadruple systems adopting the same methodology and data set as in the quoted work. The result is that when triple and quadruple, together with binary, systems are properly accounted for, the actual cluster mass (computed as all stars were single) should be incremented by a factor of 1.18−1.27, depending on the cluster and when the binary fraction <jats:italic>α</jats:italic> is 0.35. Fitting formulae are provided to derive the increment factor for different binary star percentages.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We compile observations of molecular gas contents and infrared-based star formation rates (SFRs) for 112 circumnuclear star-forming regions, in order to reinvestigate the form of the disk-averaged Schmidt surface density star-formation law in starbursts. We then combine these results with total gas and SFR surface densities for 153 nearby nonstarbursting disk galaxies from de los Reyes &amp; Kennicutt (2019), to investigate the properties of the combined star formation law, following Kennicutt (1998). We confirm that the combined Schmidt law can be fitted with a single power law with slope <jats:italic>n</jats:italic> = 1.5 ± 0.05 (including fitting method uncertainties), somewhat steeper than the value <jats:italic>n</jats:italic> = 1.4 ± 0.15 found by Kennicutt. Fitting separate power laws to the nonstarbursting and starburst galaxies, however, produces very different slopes (<jats:italic>n</jats:italic> = 1.34 ± 0.07 and 0.98 ± 0.07, respectively), with a pronounced offset in the zero-point (∼0.6 dex) of the starburst relation to higher SFR surface densities. This offset is seen even when a common conversion factor between CO intensity and molecular hydrogen surface density is applied, and it is confirmed when disk surface densities of interstellar dust are used as proxies for gas measurements. Tests for possible systematic biases in the starburst data fail to uncover any spurious sources for such a large offset. We tentatively conclude that the global Schmidt law in galaxies, at least as it is conventionally measured, is bimodal or possibly multimodal. Possible causes may include changes in the small-scale structure of the molecular interstellar medium or the stellar initial mass function. A single <jats:italic>n</jats:italic> ∼ 1.5 power law still remains as a credible approximation or “recipe” for analytical or numerical models of galaxy formation and evolution.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We report new observations toward the hyperluminous dusty starbursting major merger ADFS-27 (<jats:italic>z</jats:italic> = 5.655), using the Australia Telescope Compact Array (ATCA) and the Atacama Large Millimeter/submillimeter Array (ALMA). We detect CO (<jats:italic>J</jats:italic> = 2 → 1), CO (<jats:italic>J</jats:italic> = 8 → 7), CO (<jats:italic>J</jats:italic> = 9 → 8), CO (<jats:italic>J</jats:italic> = 10 → 9), and H<jats:sub>2</jats:sub>O (3<jats:sub>12</jats:sub> → 2<jats:sub>21</jats:sub>) emission, and a P Cygni−shaped OH<jats:sup>+</jats:sup> (1<jats:sub>1</jats:sub> → 0<jats:sub>1</jats:sub>) absorption/emission feature. We also tentatively detect H<jats:sub>2</jats:sub>O (3<jats:sub>21</jats:sub> → 3<jats:sub>12</jats:sub>) and OH<jats:sup>+</jats:sup> (1<jats:sub>2</jats:sub> → 0<jats:sub>1</jats:sub>) emission and CH<jats:sup>+</jats:sup> (<jats:italic>J</jats:italic> = 1 → 0) absorption. We find a total cold molecular mass of <jats:italic>M</jats:italic> <jats:sub>gas</jats:sub> = (2.1 ± 0.2) × 10<jats:sup>11</jats:sup> (<jats:italic>α</jats:italic> <jats:sub>CO</jats:sub>/1.0) <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub>. We also find that the excitation of the star-forming gas is overall moderate for a <jats:italic>z</jats:italic> &gt; 5 dusty starburst, which is consistent with its moderate dust temperature. A high-density, high kinetic temperature gas component embedded in the gas reservoir is required to fully explain the CO line ladder. This component is likely associated with the “maximum starburst” nuclei in the two merging galaxies, which are separated by only 140 ± 13 km s<jats:sup>−1</jats:sup> along the line of sight and 9.0 kpc in projection. The kinematic structure of both components is consistent with galaxy disks, but this interpretation remains limited by the spatial resolution of the current data. The OH<jats:sup>+</jats:sup> features are only detected toward the northern component, which is also the one that is more enshrouded in dust and thus remains undetected up to 1.6 <jats:italic>μ</jats:italic>m even in our sensitive new Hubble Space Telescope Wide Field Camera 3 imaging. The absorption component of the OH<jats:sup>+</jats:sup> line is blueshifted and peaks near the CO and continuum emission peak, while the emission is redshifted and peaks offset by 1.7 kpc from the CO and continuum emission peak, suggesting that the gas is associated with a massive molecular outflow from the intensely star-forming nucleus that supplies 125 <jats:italic>M</jats:italic> <jats:sub>⊙</jats:sub> yr<jats:sup>−1</jats:sup> of enriched gas to its halo.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present the first spectroscopically resolved H<jats:italic>α</jats:italic> emission map of the Large Magellanic Cloud’s (LMC) galactic wind. By combining new Wisconsin H-alpha Mapper observations (<jats:italic>I</jats:italic> <jats:sub>H<jats:italic>α</jats:italic> </jats:sub> ≳ 10 mR) with existing H <jats:sc>i</jats:sc> 21 cm emission observations, we (1) mapped the LMC’s nearside galactic wind over a local standard of rest (LSR) velocity range of +50 ≤ <jats:italic>v</jats:italic> <jats:sub>LSR</jats:sub> ≤ +250 km s<jats:sup>−1</jats:sup>, (2) determined its morphology and extent, and (3) estimated its mass, outflow rate, and mass-loading factor. We observe H<jats:italic>α</jats:italic> emission from this wind to typically 1° off the LMC’s H <jats:sc>i</jats:sc> disk. Kinematically, we find that the diffuse gas in the warm-ionized phase of this wind persists at both low (≲100 km s<jats:sup>−1</jats:sup>) and high (≳100 km s<jats:sup>−1</jats:sup>) velocities, relative to the LMC’s H <jats:sc>i</jats:sc> disk. Furthermore, we find that the high-velocity component spatially aligns with the most intense star-forming region, 30 Doradus. We, therefore, conclude that this high-velocity material traces an active outflow. We estimate the mass of the warm (<jats:italic>T</jats:italic> <jats:sub> <jats:italic>e</jats:italic> </jats:sub> ≈ 10<jats:sup>4</jats:sup> K) ionized phase of the nearside LMC outflow to be <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}\left({M}_{\mathrm{ionized}}/{M}_{\odot }\right)=7.51\pm 0.15$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd320ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> for the combined low and high-velocity components. Assuming an ionization fraction of 75% and that the wind is symmetrical about the LMC disk, we estimate that its total (neutral and ionized) mass is <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}\left({M}_{\mathrm{total}}/{M}_{\odot }\right)=7.93$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd320ieqn2.gif" xlink:type="simple" /> </jats:inline-formula>, its mass-flow rate is <jats:inline-formula> <jats:tex-math> <?CDATA ${\dot{M}}_{\mathrm{outflow}}\approx 1.43\,{M}_{\odot }\,{\mathrm{yr}}^{-1}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd320ieqn3.gif" xlink:type="simple" /> </jats:inline-formula>, and its mass-loading factor is <jats:italic>η</jats:italic> ≈ 4.54. Our average mass-loading factor results are roughly a factor of 2.5 larger than previous H<jats:italic>α</jats:italic> imaging and UV absorption line studies, suggesting that those studies are missing nearly half the gas in the outflows.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>The multi-messenger discovery of gravitational waves (GWs) and light from the binary neutron star (NS) merger GW170817, associated with gamma-ray burst (GRB) 170817A and kilonova AT2017gfo, has marked the start of a new era in astrophysics. GW170817 has confirmed that binary NS mergers are progenitors of at least some short GRBs. The peculiar properties of the GRB 170817A radio afterglow, characterized by a delayed onset related to the off-axis geometry, have also demonstrated how some nearby short GRBs may not be identified as such with standard short-timescale electromagnetic follow-up observations. Building upon this new information, we performed late-time radio observations of a sample of four short GRBs with unknown redshift and no previously detected afterglow in the Swift/BAT sample in order to identify nearby (<jats:inline-formula> <jats:tex-math> <?CDATA ${d}_{L}\lesssim 200$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd315ieqn1.gif" xlink:type="simple" /> </jats:inline-formula> Mpc) off-axis GRB candidates via their potential late-time radio signatures. We find a previously uncatalogued radio source within the error region of GRB 130626 with a <jats:inline-formula> <jats:tex-math> <?CDATA $3\mbox{--}6\,\mathrm{GHz}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd315ieqn2.gif" xlink:type="simple" /> </jats:inline-formula> flux density consistent with an NS radio flare at a distance of ∼100 Mpc. An origin related to a persistent radio source unrelated to the GRB cannot be excluded nor confirmed given the high chance of false positives in error regions as large as those considered here, and the limited time baseline of our observations. Further radio (and X-ray) follow-up observations are needed to better understand the origin of this source.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Most of the baryonic mass in the circumgalactic medium (CGM) of a spiral galaxy is believed to be warm-hot, with temperature around 10<jats:sup>6</jats:sup> K. The narrow O <jats:sc>vi</jats:sc> absorption lines probe a somewhat cooler component at <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}\,T({\rm{K}})=5.5$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd03fieqn1.gif" xlink:type="simple" /> </jats:inline-formula>, but broad O <jats:sc>vi</jats:sc> absorbers have the potential to probe the hotter CGM. Here we present 376 ks Chandra LETG observations of a carefully selected galaxy in which the presence of broad O <jats:sc>vi</jats:sc> together with the non-detection of Ly<jats:italic>α</jats:italic> was indicative of hot gas. The strongest line expected to be present at ≈10<jats:sup>6</jats:sup> K is O <jats:sc>vii</jats:sc> <jats:italic>λ</jats:italic>21.602. There is a hint of an absorption line at the redshifted wavelength, but the line is not detected with better than 2<jats:italic>σ</jats:italic> significance. A physical model, taking into account strengths of several other lines, provides better constraints. Our best-fit absorber model has <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}\,T({\rm{K}})=6.3\pm 0.2$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd03fieqn2.gif" xlink:type="simple" /> </jats:inline-formula> and <jats:inline-formula> <jats:tex-math> <?CDATA $\mathrm{log}{N}_{{\rm{H}}}({\mathrm{cm}}^{-2})={20.7}_{-0.5}^{+0.3}$?> </jats:tex-math> <jats:inline-graphic xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="apjabd03fieqn3.gif" xlink:type="simple" /> </jats:inline-formula>. These parameters are consistent with the hot plasma model based on UV observations; other O <jats:sc>vi</jats:sc> models of cooler gas phases are ruled out at better than 99% confidence. Thus we have suggestive, but not conclusive evidence for the broad O <jats:sc>vi</jats:sc> absorber probing the hot gas from the shallow observations of this pilot program. About 800 ks of XMM-Newton observations will detect the expected absorption lines of O <jats:sc>vii</jats:sc> and O <jats:sc>viii</jats:sc> unequivocally. Future missions like XRISM, Arcus, and Athena will revolutionize the CGM science.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Nova Cas 2020 was a second example of a classical nova forming both C<jats:sub>2</jats:sub> and CN molecules during its near-maximum phase. The formation C<jats:sub>2</jats:sub> and CN is indicative of the carbon-rich (C/O &gt; 1) gas envelope of the nova. Our low-resolution spectroscopic observations in the optical from UT 2020 July 31 to August 19 revealed the appearance and the disappearance of the molecular absorption bands of C<jats:sub>2</jats:sub> and CN in Nova Cas 2020. These molecules were present during ∼3 days only. Based on analysis of the C<jats:sub>2</jats:sub> Swan band profiles, C<jats:sub>2</jats:sub> (and probably CN also) formed at ∼5000 K in the nova envelope (see ∼8000 K for typical novae at their visual brightness maxima). Spectral evolution and the formation conditions of the molecules are similar to those of V2676 Oph, which is the first example of a C<jats:sub>2</jats:sub>/CN-forming nova. We predict that a late-phase grain formation episode similar to that seen in V2676 Oph will occur in Nova Cas 2020.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present results from our X-ray analysis of a systematic search for triple active galactic nuclei (AGNs) in nearby (<jats:italic>z</jats:italic> &lt; 0.077) triple galaxy mergers. We analyze archival Chandra observations of seven triple galaxy mergers with <jats:sc>baymax</jats:sc> (Bayesian Analysis of Multiple AGNs in X-rays), fitting each observation with single, dual, and triple X-ray point-source models. In doing so, we conclude that one triple merger has one X-ray point source (SDSS J0858+1822, although it is unlikely to be an AGN), five triple mergers are likely composed of two X-ray point sources (NGC 3341, SDSS J1027+1749, SDSS J1631+2352, SDSS J1708+2153, and SDSS J2356−1016), and one system is composed of three X-ray point sources (SDSS J0849+1114). By fitting the individual X-ray spectra of each point source, we analyze the 2−7 keV luminosities, as well as the levels of obscuration associated with each potential AGN. We find that 4/5 dual X-ray point-source systems have primary and secondary point sources with bright X-ray luminosities (<jats:italic>L</jats:italic> <jats:sub>2−7kev</jats:sub> &gt; 10<jats:sup>40</jats:sup> erg s<jats:sup>−1</jats:sup>), possibly associated with four new undetected dual AGNs. The dual and triple-point-source systems are found to have physical separations between 3 and 9 kpc and flux ratios between 2 × 10<jats:sup>−3</jats:sup> and  0.84. A multiwavelength analysis to determine the origin of the X-ray point sources discovered in this work is presented in our companion paper (Foord et al. 2020c).</jats:p>

<jats:title>Abstract</jats:title> <jats:p>We present results from a multiwavelength analysis searching for multiple active galactic nucleus (AGN) systems in nearby (<jats:italic>z</jats:italic> &lt; 0.077) triple galaxy mergers. Combining archival Chandra, Sloan Digital Data Survey (SDSS), Wide-field Infrared Survey Explorer (WISE), and Very Large Array observations, we quantify the rate of nearby triple AGNs, as well as investigate possible connections between supermassive black hole accretion and merger environments. Analyzing the multiwavelength observations of seven triple galaxy mergers, we find that one triple merger has a single AGN (NGC 3341); we discover, for the first time, four likely dual AGNs (SDSS J1027+1749, SDSS J1631+2352, SDSS J1708+2153, and SDSS J2356−1016); we confirm one triple-AGN system, SDSS J0849+1114; and one triple merger in our sample remains ambiguous (SDSS J0858+1822). Analyzing the WISE data, we find a trend of increasing <jats:italic>N</jats:italic> <jats:sub>H</jats:sub> (associated with the primary AGN) as a function of increasing W1 – W2 color, reflecting that the motions of gas and dust are coupled in merging environments, where large amounts of both can be funneled into the active central region during mergers. Additionally, we find that the one triple-AGN system in our sample has the highest levels of <jats:italic>N</jats:italic> <jats:sub>H</jats:sub> and W1 – W2 color, while the dual-AGN candidates all have lower levels; these results are consistent with theoretical merger simulations that suggest that higher levels of nuclear gas are more likely to activate AGNs in mergers.</jats:p>

<jats:title>Abstract</jats:title> <jats:p>Observationally, electron beams with power-law energy spectra are commonly associated with solar flares. Previous studies have found that during magnetic reconnection with a guide field <jats:italic>B</jats:italic> <jats:sub> <jats:italic>g</jats:italic> </jats:sub> larger than 0.1 times the asymptotic field <jats:italic>B</jats:italic> <jats:sub>0</jats:sub>, electron beams are unable to develop due to the strong deflection caused by the guide field. Using particle-in-cell simulations we show that in force-free reconnection, the development of an electron Kelvin–Helmholz instability can suppress the Hall effect and produce a flute-like outflow exhaust, in which both electrons and ions are nearly frozen-in with the magnetic field. The coupling of a continuously growing electron velocity shear and <jats:bold> <jats:italic>E</jats:italic> </jats:bold> × <jats:bold> <jats:italic>B</jats:italic> </jats:bold> drift drive the electrons out of magnetic vortices and results in collimated jets with a power-law energy spectrum in the elongated exhaust. The spatial density of electron jets is comparable to the background and is highly inhomogeneous, signifying on asymmetric density structure in guide field reconnection.</jats:p>