Table of contents

The growth of black holes (BHs) in disk galaxies lacking classical bulges, which implies an absence of significant mergers, appears to be driven by secular processes. Short bars of sub-kiloparsec radius have been hypothesized to be an important mechanism for driving gas inflows to small scale, feeding central BHs. In order to quantify the maximum BH mass allowed by this mechanism, we examine the robustness of short bars to the dynamical influence of BHs. Large-scale bars are expected to be robust, long-lived structures; extremely massive BHs, which are rare, are needed to completely destroy such bars. However, we find that short bars, which are generally embedded in large-scale outer bars, can be destroyed quickly when BHs of mass ${M}_{\mathrm{bh}}\sim 0.05 \% \mbox{--}0.2 \%$ of the total stellar mass (${M}_{\star }$) are present. In agreement with this prediction, all galaxies observed to host short bars have BHs with a mass fraction less than $0.2 \% \,{M}_{\star }$. Thus, the dissolution of short inner bars is possible, perhaps even frequent, in the universe. An important implication of this result is that inner-bar-driven gas inflows may be terminated when BHs grow to $\sim 0.1 \% \,{M}_{\star }$. We predict that $0.2 \% \,{M}_{\star }$ is the maximum mass of BHs allowed if they are fed predominately via inner bars. This value matches well the maximum ratio of BH-to-host-galaxy stellar mass observed in galaxies with pseudo-bulges and most nearby active galactic nucleus host galaxies. This hypothesis provides a novel explanation for the lower ${M}_{\mathrm{bh}}/{M}_{\star }$ in galaxies that have avoided significant mergers compared with galaxies with classical bulges.

White dwarfs (WDs) have atmospheres that are expected to consist nearly entirely of hydrogen and helium, since heavier elements will sink out of sight on short timescales. However, observations have revealed atmospheric pollution by heavier elements in about a quarter to a half of all WDs. While most of the pollution can be accounted for with asteroidal or dwarf planetary material, recent observations indicate that larger planetary bodies, as well as icy and volatile material from Kuiper belt analog objects, are also viable sources of pollution. The commonly accepted pollution mechanisms, namely scattering interactions between planetary bodies orbiting the WDs, can hardly account for pollution by objects with large masses or long-period orbits. Here we report on a mechanism that naturally leads to the emergence of massive body and icy and volatile material pollution. This mechanism occurs in wide binary stellar systems, where the mass loss of the planets' host stars during post main sequence stellar evolution can trigger the Eccentric Kozai–Lidov mechanism. This mechanism leads to large eccentricity excitations, which can bring massive and long-period objects close enough to the WDs to be accreted. We find that this mechanism readily explains and is consistent with observations.

We report the detection of CO(2–1) emission coincident with the brightest cluster galaxy (BCG) of the high-redshift galaxy cluster SpARCS1049+56, with the Redshift Search Receiver (RSR) on the Large Millimeter Telescope (LMT). We confirm a spectroscopic redshift for the gas of z = 1.7091 ± 0.0004, which is consistent with the systemic redshift of the cluster galaxies of z = 1.709. The line is well fit by a single-component Gaussian with an RSR-resolution-corrected FWHM of 569 ± 63 km s⁻¹. We see no evidence for multiple velocity components in the gas, as might be expected from the multiple image components seen in near-infrared imaging with the Hubble Space Telescope. We measure the integrated flux of the line to be 3.6 ± 0.3 Jy km s⁻¹, and using ${\alpha }_{\mathrm{CO}}$ = 0.8 M_⊙ (K km s⁻¹ pc²)⁻¹, we estimate a total molecular gas mass of 1.1 ± 0.1 × 10¹¹M_⊙ and a M_H2/M_⋆ ∼ 0.4. This is the largest gas reservoir detected in a BCG above z > 1 to date. Given the infrared-estimated star formation rate of 860 ± 130 M_⊙ yr⁻¹, this corresponds to a gas depletion timescale of ∼0.1 Gyr. We discuss several possible mechanisms for depositing such a large gas reservoir to the cluster center—e.g., a cooling flow, a major galaxy–galaxy merger, or the stripping of gas from several galaxies—but conclude that these LMT data are not sufficient to differentiate between them.

Cassini discovered a plethora of neutral and ionized molecules in Titan's ionosphere including, surprisingly, anions and negatively charged molecules extending up to 13,800 u q⁻¹. In this Letter, we forward model the Cassini electron spectrometer response function to this unexpected ionospheric component to achieve an increased mass resolving capability for negatively charged species observed at Titan altitudes of 950–1300 km. We report on detections consistently centered between 25.8 and 26.0 u q⁻¹ and between 49.0–50.1 u q⁻¹ which are identified as belonging to the carbon chain anions, CN⁻/C₃N⁻ and/or C₂H⁻/C₄H⁻, in agreement with chemical model predictions. At higher ionospheric altitudes, detections at 73–74 u q⁻¹ could be attributed to the further carbon chain anions C₅N⁻/C₆H⁻ but at lower altitudes and during further encounters extend over a higher mass/charge range. This, as well as further intermediary anions detected at >100 u, provide the first evidence for efficient anion chemistry in space involving structures other than linear chains. Furthermore, at altitudes below <1100 km, the low-mass anions (<150 u q⁻¹) were found to deplete at a rate proportional to the growth of the larger molecules, a correlation that indicates the anions are tightly coupled to the growth process. This study adds Titan to an increasing list of astrophysical environments where chain anions have been observed and shows that anion chemistry plays a role in the formation of complex organics within a planetary atmosphere as well as in the interstellar medium.

Following merger, a neutron star (NS) binary can produce roughly one of three different outcomes: (1) a stable NS, (2) a black hole (BH), or (3) a supramassive, rotationally supported NS, which then collapses to a BH following angular momentum losses. Which of these fates occur and in what proportion has important implications for the electromagnetic transient associated with the mergers and the expected gravitational wave (GW) signatures, which in turn depend on the high density equation of state (EOS). Here we combine relativistic calculations of NS masses using realistic EOSs with Monte Carlo population synthesis based on the mass distribution of NS binaries in our Galaxy to predict the distribution of fates expected. For many EOSs, a significant fraction of the remnants are NSs or supramassive NSs. This lends support to scenarios in which a quickly spinning, highly magnetized NS may be powering an electromagnetic transient. This also indicates that it will be important for future GW observatories to focus on high frequencies to study the post-merger GW emission. Even in cases where individual GW events are too low in signal to noise to study the post merger signature in detail, the statistics of how many mergers produce NSs versus BHs can be compared with our work to constrain the EOS. To match short gamma-ray-burst (SGRB) X-ray afterglow statistics, we find that the stiffest EOSs are ruled out. Furthermore, many popular EOSs require a significant fraction of ∼60%–70% of SGRBs to be from NS–BH mergers rather than just binary NSs.

We investigate the three-dimensional (3D) magnetic structure of a blowout jet originating in the western edge of NOAA active region (AR) 11513 on 2012 July 2 by means of recently developed forced field extrapolation model. The results show that the blowout jet was caused by the eruption of the magnetic flux rope (MFR) consisting of twisted field lines. We further calculate the twist number ${{ \mathcal T }}_{w}$ and squashing factor Q of the reconstructed magnetic field and find that (1) the MFR corresponds well with the high ${{ \mathcal T }}_{w}$ region, and (2) the MFR outer boundary corresponds well with the high Q region, probably interpreting the bright structure at the base of the jet. The twist number of the MFR is estimated to be ${{ \mathcal T }}_{w}=-1.54\pm 0.67$. Thus, the kink instability is regarded as the initiation mechanism of the blowout jet as ${{ \mathcal T }}_{w}$ reaches or even exceeds the threshold value of the kink instability. Our results also indicate that the bright point at the decaying phase is actually composed of some small loops that are heated by the reconnection occurring above. In summary, the blowout jet is mostly consistent with the scenario proposed by Moore et al., except that the kink instability is found to be a possible trigger.

The Seyfert 2 galaxy NGC 5252 contains a recently identified ultra-luminous X-ray (ULX) source that has been suggested to be a possible candidate off-nuclear low-mass active galactic nucleus. We present follow-up optical integral-field unit observations obtained using Gemini Multi-Object Spectrographs on the Gemini-North telescope. In addition to confirming that the ionized gas in the vicinity of the ULX is kinematically associated with NGC 5252, the new observations reveal ordered motions consistent with rotation around the ULX. The close coincidence of the excitation source of the line-emitting gas with the position of the ULX further suggests that ULX itself is directly responsible for the ionization of the gas. The spatially resolved measurements of [N ii] λ6584/Hα surrounding the ULX indicate a low gas-phase metallicity, consistent with those of other known low-mass active galaxies but not that of its more massive host galaxy. These findings strengthen the proposition that the ULX is not a background source but rather that it is the nucleus of a small, low-mass galaxy accreted by NGC 5252.

Neutron star–black hole (NS–BH) coalescences are widely believed to be promising gravitational-wave sources in the era of advanced detectors of LIGO/Virgo, but the rate of this population is highly uncertain due to the lack of direct detection of such binaries. There is growing evidence for the connection between the observed three luminous macronova (also known as kilonova) events and NS–BH mergers. In this work, we propose, for the first time based on such a link, a fiducial lower limit of NS–BH coalescence rate density ${{ \mathcal R }}_{\mathrm{nsbh}}\,\approx {18.8}_{-8.6}^{+12.5}\,{\mathrm{Gpc}}^{-3}\,{\mathrm{yr}}^{-1}\,{({\theta }_{{\rm{j}}}/0.1\mathrm{rad})}^{-2}$, where ${\theta }_{{\rm{j}}}$ is the typical half-opening angle of the GRB ejecta. After marginalizing over distributions of black hole masses and spins, we find a rate density ${{ \mathcal R }}_{\mathrm{nsbh}}\geqslant {10}^{2}\,{\mathrm{Gpc}}^{-3}\,{\mathrm{yr}}^{-1}$, depending upon the equation of state (EoS) of NS material and the properties of the NS–BH system. With the O1 non-observation by advanced LIGO, we show a preference for NS–BH systems with a stiffer NS EoS and a larger contribution from low-mass/high-spin BHs. Our estimate predicts the first detection of an NS–BH system can be as early as the late O2 run or the early O3 run. We expect that future multi-messenger observations can much better constrain NS–BH systems' properties.

Present-day clusters are massive halos containing mostly quiescent galaxies, while distant protoclusters are extended structures containing numerous star-forming galaxies. We investigate the implications of this fundamental change in a cosmological context using a set of N-body simulations and semi-analytic models. We find that the fraction of the cosmic volume occupied by all (proto)clusters increases by nearly three orders of magnitude from z = 0 to z = 7. We show that (proto)cluster galaxies are an important and even dominant population at high redshift, as their expected contribution to the cosmic star formation rate density rises (from 1% at z = 0) to 20% at z = 2 and 50% at z = 10. Protoclusters thus provide a significant fraction of the cosmic ionizing photons, and may have been crucial in driving the timing and topology of cosmic reionization. Internally, the average history of cluster formation can be described by three distinct phases: at z ∼ 10–5, galaxy growth in protoclusters proceeded in an inside-out manner, with centrally dominant halos that are among the most active regions in the universe; at z ∼ 5–1.5, rapid star formation occurred within the entire 10–20 Mpc structures, forming most of their present-day stellar mass; at z ≲ 1.5, violent gravitational collapse drove these stellar contents into single cluster halos, largely erasing the details of cluster galaxy formation due to relaxation and virialization. Our results motivate observations of distant protoclusters in order to understand the rapid, extended stellar growth during cosmic noon, and their connection to reionization during cosmic dawn.

We present axisymmetric numerical simulations of radiatively inefficient accretion flows onto black holes combining general relativity, magnetohydrodynamics, self-consistent electron thermodynamics, and frequency-dependent radiation transport. We investigate a range of accretion rates up to ${10}^{-5}\,{\dot{M}}_{\mathrm{Edd}}$ onto a ${10}^{8}\,{M}_{\odot }$ black hole with spin ${a}_{\star }=0.5$. We report on averaged flow thermodynamics as a function of accretion rate. We present the spectra of outgoing radiation and find that it varies strongly with accretion rate, from synchrotron-dominated in the radio at low $\dot{M}$ to inverse-Compton-dominated at our highest $\dot{M}$. In contrast to canonical analytic models, we find that by $\dot{M}\approx {10}^{-5}\,{\dot{M}}_{\mathrm{Edd}}$, the flow approaches $\sim 1 \%$ radiative efficiency, with much of the radiation due to inverse-Compton scattering off Coulomb-heated electrons far from the black hole. These results have broad implications for modeling of accreting black holes across a large fraction of the accretion rates realized in observed systems.

We report the first map of large-scale (10 pc in length) emission of millimeter-wavelength hydrogen recombination lines (mm-RRLs) toward the giant H ii region around the W43-Main young massive star cluster (YMC). Our mm-RRL data come from the IRAM 30 m telescope and are analyzed together with radio continuum and cm-RRL data from the Karl G. Jansky Very Large Array and HCO⁺ 1–0 line emission data from the IRAM 30 m. The mm-RRLs reveal an expanding wind-blown ionized gas shell with an electron density ∼70–1500 cm⁻³ driven by the WR/OB cluster, which produces a total Lyα photon flux of $1.5\times {10}^{50}$ s⁻¹. This shell is interacting with the dense neutral molecular gas in the W43-Main dense cloud. Combining the high spectral and angular resolution mm-RRL and cm-RRL cubes, we derive the two-dimensional relative distributions of dynamical and pressure broadening of the ionized gas emission and find that the RRL line shapes are dominated by pressure broadening (4–55 $\mathrm{km}\,{{\rm{s}}}^{-1}$) near the YMC and by dynamical broadening (8–36 $\mathrm{km}\,{{\rm{s}}}^{-1}$) near the shell's edge. Ionized gas clumps hosting ultra-compact H ii regions found at the edge of the shell suggest that large-scale ionized gas motion triggers the formation of new star generation near the periphery of the shell.

We compute the energy spectra of antideuterons ($\overline{{\rm{d}}}$) and antihelium ($\overline{\mathrm{He}}$) in cosmic rays (CRs) in a scenario where hadronic interactions inside supernova remnants (SNRs) can produce a diffusively shock-accelerated "source component" of secondary antinuclei. The key parameters that specify the SNR environment and the interstellar CR transport are tightly constrained with the new measurements provided by the AMS experiment on the B/C ratio and on the $\bar{p}$/$p$ ratio. The best-fit models obtained from the two ratios are found to be inconsistent with each other, as the $\bar{p}$/$p$ data require enhanced secondary production. Thus, we derive conservative (i.e., B/C-driven) and speculative ($\bar{p}$/$p$-driven) upper limits to the SNR flux contributions for the $\overline{{\rm{d}}}$ and $\overline{\mathrm{He}}$ spectra in CRs, along with their standard secondary component expected from CR collisions in the interstellar gas. We find that the source component of antinuclei can be appreciable at kinetic energies above a few ∼10 GeV n⁻¹, but it is always sub-dominant below a few GeV n⁻¹, that is the energy window where dark matter (DM) annihilation signatures are expected to exceed the level of secondary production. We also find that the total (standard + SNR) flux of secondary $\overline{{\rm{d}}}$ and $\overline{\mathrm{He}}$ is tightly constrained by the data. Thus, the presence of interaction processes in SNRs does not critically affect the total background for DM searches.