adsabs/SciX_UAT_keywords huggingface dataset:
A MeerKAT Polarization Survey of Southern Calibration Sources

We report on full-Stokes L-band observations of 98 MeerKAT calibration sources. Linear polarization is detected in 71 objects above a fractional level of 0.2%. We identify ten sources with strong fractional linear polarization and low Faraday rotation measure that could be suitable for wide-band absolute polarization calibration. We detect significant circular polarization from 24% of the sample down to a detection level of 0.07%. Circularly polarized emission is seen only for flat spectrum sources α > ‑0.5. We compare our polarized intensities and Faraday synthesis results to data from the NVSS at 1400 MHz and the ATCA SPASS survey at 2300 MHz. NVSS data exist for 54 of our sources and SPASS data for 20 sources. The percent polarization and rotation measures from both surveys agree well with our results. The residual instrumental linear polarization for these observations is measured at 0.16%, and the residual instrumental circular polarization is measured at 0.05%. These levels may reflect either instabilities in the relative bandpass between the two polarization channels with either time or antenna orientation, or atmospheric/ionospheric variations with pointing direction. Tracking of the hourly gain solutions on J0408-6545 after transfer of the primary gain solutions suggests a deterioration of the gain stability by a factor of several starting about 2 hr after sunrise. This suggests that observing during the nighttime could dramatically improve the precision of polarization calibration.

Keywords: radio continuum emission; radio interferometry; spectropolarimetry; extragalactic radio sources; active galactic nuclei; radio sources
Deep Low-frequency Radio Observations of A2256. I. The Filamentary Radio Relic

We present deep and high-fidelity images of the merging galaxy cluster A2256 at low frequencies using the upgraded Giant Metrewave Radio Telescope (uGMRT) and LOw-Frequency ARray (LOFAR). This cluster hosts one of the most prominent known relics with a remarkably spectacular network of filamentary substructures. The new uGMRT (300-850 MHz) and LOFAR (120-169 MHz) observations, combined with the archival Karl G. Jansky Very Large Array (VLA; 1-4 GHz) data, allowed us to carry out the first spatially resolved spectral analysis of the exceptional relic emission down to 6″ resolution over a broad range of frequencies. Our new sensitive radio images confirm the presence of complex filaments of magnetized relativistic plasma also at low frequencies. We find that the integrated spectrum of the relic is consistent with a single power law, without any sign of spectral steepening, at least below 3 GHz. Unlike previous claims, the relic shows an integrated spectral index of -1.07 ± 0.02 between 144 MHz and 3 GHz, which is consistent with the (quasi)stationary shock approximation. The spatially resolved spectral analysis suggests that the relic surface very likely traces the complex shock front, with a broad distribution of Mach numbers propagating through a turbulent and dynamically active intracluster medium. Our results show that the northern part of the relic is seen edge-on and the southern part close to face-on. We suggest that the complex filaments are regions where higher Mach numbers dominate the (re)acceleration of electrons that are responsible for the observed radio emission.

Keywords: galaxy clusters; intracluster medium; radio continuum emission; large-scale structure of the universe
A Survey of Herbig Ae/Be Multiplicity

Although the majority of Herbig Ae/Be (HAEBE) stars are likely multiple systems, relatively few multiples have been identified. We aim to create a statistically significant survey of HAEBE multiplicity as a function of stellar mass, to shed light on mixed-mass star formation. Two independent adaptive optics IR imaging surveys were conducted, between 1993 and 2007. A total of 143 stars were observed, including 126 HAEBE stars. These surveys are significantly more sensitive than previous imaging work and, combined with accurate distance and proper-motion data from GAIA Data Release 3, provide a broad survey of intermediate-mass multiplicity. We perform a statistical probability analysis to identify the likely physical companions, and analyze multiepoch observations for common proper motions. We find 250 stellar detections around 88 sample stars. Of these, 70 systems are likely multiple systems, and 42 companions around 29 systems are reported here for the first time. Correcting for completeness, we find a multiplicity fraction of 54%. Multiples are found randomly across all spectral types, but high-order multiples are more common among the hottest stars. Statistically likely companions are found at distances of up to several thousand astronomical units and ΔK magnitudes ranging from <0.1 to ~9, although companions with ΔK < 1.5 are rare. Companions around Herbig Ae stars tend to be closer than those around Herbig Be stars. We confirm 24 proper-motion companions in 21 systems, including roughly half of the statistically very likely companions; however, many proper-motion results are inconclusive.

Keywords: multiple stars; herbig ae/be stars; surveys
Measurement of the Gamma-Ray Energy Spectrum beyond 100 TeV from the HESS J1843-033 Region

HESS J1843-033 is a very high energy gamma-ray source whose origin remains unidentified. This work presents, for the first time, the energy spectrum of gamma rays beyond 100 TeV from the HESS J1843-033 region using the data recorded by the Tibet air shower array and its underground muon detector array. A gamma-ray source with an extension of 0.°34 ± 0.°12 is successfully detected above 25 TeV at (α, δ) = (281.°09 ± 0.°10, -3.°76 ± 0.°09) near HESS J1843-033 with a statistical significance of 6.2σ, and the source is named TASG J1844-038. The position of TASG J1844-038 is consistent with those of HESS J1843-033, eHWC J1842-035, and LHAASO J1843-0338. The measured gamma-ray energy spectrum in 25 TeV < E < 130 TeV is described with ${dN}/{dE}=(9.70\pm 1.89)\times {10}^{-16}$ (E/40 TeV)-3.26±0.30 TeV-1 cm-2 s-1, and the spectral fit to the combined spectra of HESS J1843-033, LHAASO J1843-0338, and TASG J1844-038 implies the existence of a cutoff at 49.5 ± 9.0 TeV. Associations of TASG J1844-038 with SNR G28.6-0.1 and PSR J1844-0346 are also discussed in detail for the first time.

Keywords: galactic cosmic rays; gamma-ray astronomy; gamma-ray sources; gamma-ray observatories; cosmic ray sources
Joint Survey Processing. I. Compact Oddballs in the COSMOS Field-Low-luminosity Quasars at z > 6?

The faint-end slope of the quasar luminosity function at z ~ 6 and its implication on the role of quasars in reionizing the intergalactic medium at early times has been an outstanding problem for some time. The identification of faint high-redshift quasars with luminosities of <1044.5 erg s-1 is challenging. They are rare (few per square degree), and the separation of these unresolved quasars from late-type stars and compact star-forming galaxies is difficult from ground-based observations alone. In addition, source confusion becomes significant at >25 mag, with ~30% of sources having their flux contaminated by foreground objects when the seeing resolution is ~0″.7. We mitigate these issues by performing a pixel-level joint processing of ground and space-based data from Subaru/Hyper-SuprimeCam (HSC) and Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS). We create a deconfused catalog over the 1.64 deg2 of the COSMOS field, after accounting for spatial varying point-spread functions and astrometric differences between the two data sets. We identify twelve low-luminosity (M UV ~ -21 mag) z > 6 quasar candidates through (i) their red color measured between ACS/F814W and HSC/i band and (ii) their compactness in the space-based data. Nondetections of our candidates in Hubble DASH data argues against contamination from late-type stars. Our constraints on the faint end of the quasar luminosity function at z ~ 6.4 suggest a negligibly small contribution to reionization compared to the star-forming galaxy population. The confirmation of our candidates and the evolution of number density with redshift could provide better insights into how supermassive galaxies grew in the first billion years of cosmic time.

Keywords: surveys; quasars; high-redshift galaxies; photometry; astronomical techniques
Understanding the Puzzling Acceleration of Jets of Active Galactic Nuclei

As relativistic plasma launches from a compact object at the center of a galaxy, the corresponding outflow should slow down with the increase of separation from the core due to energy dissipation along the path. However, some long-baseline observations of active galactic nucleus (AGN) jets show that the velocity of jets increases rather than decreases at a larger and larger separation from the core. The mechanism of such an acceleration of astrophysical jets has not been well understood so far, although much progress has been achieved on theoretical and observational perspectives. This paper illustrates the phenomenon of jet acceleration that emerged in some AGNs by the nonballistic model in which some nonconsecutive knots are produced by a continuous outflow at different distances from the central black hole; such knots appear to rotate along different radii at the same precession cone in the case of a precessing jet. The projection of the trajectories of such knots on the plane of the sky leads us to expect that jet components further from the core move at larger apparent velocities. The investigation provides a very simple scenario to the puzzling phenomena of astrophysical jets.

Keywords: active galactic nuclei
Probing the Mass Relation between Supermassive Black Holes and Dark Matter Halos at High Redshifts by Gravitational Wave Experiments

Numerous observations have shown that almost all galaxies in our Universe host supermassive black holes (SMBHs), but there is still much debate about their formation and evolutionary processes. Recently, gravitational waves (GWs) have been expected to be a new and important informative observation, in particular, in the low-frequency region by making use of the Laser Interferometer Space Antenna (LISA) and Pulsar Timing Arrays (PTAs). As an evolutionary process of the SMBHs, we revisit a dark matter (DM) halo-SMBH coevolution model based on the halo merger tree employing an ansatz for the mass relation between the DM halos and the SMBHs at z = 6. In this model, the mass of SMBHs grows through their mergers associated with the halo mergers, and hence, the evolutionary information must be stored in the GWs emitted at the mergers. We investigate the stochastic gravitational background from the coalescing SMBH binaries, which the PTAs can detect, and also the GW bursts emitted at the mergers, which can be detected by the mHz band observations such as LISA. We also discuss the possibility of probing the mass relation between the DM halos and the SMBHs at high redshift by future GW observations.

Keywords: supermassive black holes
APOGEE Data and Spectral Analysis from SDSS Data Release 16: Seven Years of Observations Including First Results from APOGEE-South

The spectral analysis and data products in Data Release 16 (DR16; 2019 December) from the high-resolution near-infrared Apache Point Observatory Galactic Evolution Experiment (APOGEE)-2/Sloan Digital Sky Survey (SDSS)-IV survey are described. Compared to the previous APOGEE data release (DR14; 2017 July), APOGEE DR16 includes about 200,000 new stellar spectra, of which 100,000 are from a new southern APOGEE instrument mounted on the 2.5 m du Pont telescope at Las Campanas Observatory in Chile. DR16 includes all data taken up to 2018 August, including data released in previous data releases. All of the data have been re-reduced and re-analyzed using the latest pipelines, resulting in a total of 473,307 spectra of 437,445 stars. Changes to the analysis methods for this release include, but are not limited to, the use of MARCS model atmospheres for calculation of the entire main grid of synthetic spectra used in the analysis, a new method for filling "holes" in the grids due to unconverged model atmospheres, and a new scheme for continuum normalization. Abundances of the neutron-capture element Ce are included for the first time. A new scheme for estimating uncertainties of the derived quantities using stars with multiple observations has been applied, and calibrated values of surface gravities for dwarf stars are now supplied. Compared to DR14, the radial velocities derived for this release more closely match those in the Gaia DR2 database, and a clear improvement in the spectral analysis of the coolest giants can be seen. The reduced spectra as well as the result of the analysis can be downloaded using links provided on the SDSS DR16 web page.

Keywords: galactic abundances; galaxy radial velocities; stellar abundances; near infrared astronomy; high resolution spectroscopy
COMAP Early Science. III. CO Data Processing

We describe the first-season CO Mapping Array Project (COMAP) analysis pipeline that converts raw detector readouts to calibrated sky maps. This pipeline implements four main steps: gain calibration, filtering, data selection, and mapmaking. Absolute gain calibration relies on a combination of instrumental and astrophysical sources, while relative gain calibration exploits real-time total-power variations. High-efficiency filtering is achieved through spectroscopic common-mode rejection within and across receivers, resulting in nearly uncorrelated white noise within single-frequency channels. Consequently, near-optimal but biased maps are produced by binning the filtered time stream into pixelized maps; the corresponding signal bias transfer function is estimated through simulations. Data selection is performed automatically through a series of goodness-of-fit statistics, including χ 2 and multiscale correlation tests. Applying this pipeline to the first-season COMAP data, we produce a data set with very low levels of correlated noise. We find that one of our two scanning strategies (the Lissajous type) is sensitive to residual instrumental systematics. As a result, we no longer use this type of scan and exclude data taken this way from our Season 1 power spectrum estimates. We perform a careful analysis of our data processing and observing efficiencies and take account of planned improvements to estimate our future performance. Power spectrum results derived from the first-season COMAP maps are presented and discussed in companion papers.

Keywords: cosmological evolution; co line emission; high-redshift galaxies; molecular gas; radio astronomy
Synchrotron Pair Production Equilibrium in Relativistic Magnetic Reconnection

Magnetic reconnection is ubiquitous in astrophysical systems, and in many such systems the plasma suffers from significant cooling due to synchrotron radiation. We study relativistic magnetic reconnection in the presence of strong synchrotron cooling, where the ambient magnetization, σ, is high and the magnetic compactness, ℓ B , of the system is of order unity. In this regime, e± pair production from synchrotron photons is inevitable, and this process can regulate the magnetization σ surrounding the current sheet. We investigate this self-regulation analytically and find a self-consistent steady state for a given magnetic compactness of the system and initial magnetization. This result helps estimate the self-consistent upstream magnetization in systems where plasma density is poorly constrained, and can be useful for a variety of astrophysical systems. As illustrative examples, we apply it to study the properties of reconnecting current sheets near the supermassive black hole of M87, as well as the equatorial current sheet outside the light cylinder of the Crab pulsar.

Keywords: plasma astrophysics; gamma-ray sources; pulsars; black holes; compact objects
Measuring the Variability in K2 Optical Light Curves of 3C 273 and Other Fermi Active Galactic Nuclei in 2015-2017

We characterize the variability in nearly continuous optical observations of the bright radio-loud quasar 3C 273 and nine additional active galactic nuclei detected with the Fermi Gamma-ray Space Telescope Large Area Telescope (Fermi-LAT). Optical observations were obtained during the K2 mission with the Kepler spacecraft for periods of 49 to 83 days conducted with ≃1 minute (short) or ≃30 minutes (long) cadences in 2015-2017. 3C 273 was quiescent during the course of the observations, varying by only a factor of 1.02. Three objects, PKS 0047+023, PKS 1216-10, and PKS B2320-035, were active, varying by factors of 1.8-3.4. Six other objects were comparatively quiet, varying by factors of less than 1.4. Power spectral densities (PSDs) were calculated for each object. Overall, the slopes of most PSDs, as well as those we reported in an earlier paper, were in the range -2.0 to -2.7 and are consistent with those produced by turbulence in the relativistic jet, and not by "hot spots" in the disk emission. Mechanisms operating in the jet other than turbulence, such as "mini-jets" or "jet-in-jets", may also produce the observed range of PSD slopes. Both accretion disk and jet models are plausible origins for the 3C 273 optical variability during the K2 observations.

Keywords: active galactic nuclei; active galaxies; jets
CEERS Key Paper. VII. JWST/MIRI Reveals a Faint Population of Galaxies at Cosmic Noon Unseen by Spitzer

The Cosmic Evolution Early Release Science program observed the Extended Groth Strip (EGS) with the Mid-Infrared Instrument (MIRI) on the James Webb Space Telescope (JWST) in 2022. In this paper, we discuss the four MIRI pointings that observed with longer-wavelength filters, including F770W, F1000W, F1280W, F1500W, F1800W, and F2100W. We compare the MIRI galaxies with the Spitzer/MIPS 24 μm population in the EGS field. We find that MIRI can observe an order of magnitude deeper than MIPS in significantly shorter integration times, attributable to JWST's much larger aperture and MIRI's improved sensitivity. MIRI is exceptionally good at finding faint (L IR < 1010 L ⊙) galaxies at z ~ 1-2. We find that a significant portion of MIRI galaxies are "mid-IR weak"-they have strong near-IR emission and relatively weaker mid-IR emission, and most of the star formation is unobscured. We present new IR templates that capture how the mid-to-near-IR emission changes with increasing infrared luminosity. We present two color-color diagrams to separate mid-IR weak galaxies and active galactic nuclei (AGN) from dusty star-forming galaxies and find that these color diagrams are most effective when used in conjunction with each other. We present the first number counts of 10 μm sources and find that there are ≲10 IR AGN per MIRI pointing, possibly due to the difficulty of distinguishing AGN from intrinsically mid-IR weak galaxies (due to low metallicities or dust content). We conclude that MIRI is most effective at observing moderate-luminosity (L IR = 109-1010 L ⊙) galaxies at z = 1-2, and that photometry alone is not effective at identifying AGN within this faint population.

Keywords: agn host galaxies; active galaxies; infrared galaxies
New Bistable Solutions in Molecular Cloud Chemistry: Nitrogen and Carbon Autocatalysis

We have investigated the chemistry of dense interstellar clouds and found new bistable solutions in the nitrogen and carbon chemistries. We identify the autocatalytic processes that are present in the pure, reduced, chemical networks and find, as previously found for oxygen chemistry, that He+ plays an important role. The applicability of these results to astronomical environments is briefly discussed. The bistable solutions found for carbon chemistry occur for low densities and high ionization fractions that are not compatible with those found in cold, dense clouds. Bistability in the pure nitrogen chemistry occurs for conditions that are relevant for prestellar cores in which significant CO depletion has taken place. We conclude that several autocatalyses are embedded in gas-phase interstellar chemistry and that many more are potentially present.

Keywords: astrochemistry; dark interstellar clouds; interstellar abundances
Advection by Large-scale Spiral Flows in Galaxy Clusters

The intracluster medium of a galaxy cluster often shows an extended quasi-spiral structure, accentuated by tangential discontinuities known as cold fronts (CFs). These discontinuities are thought to isolate between low-entropy, high-metallicity gas inside (I.e., below) the CF that was lifted from the center of the cluster by some radial factor fI and high-entropy, low-metallicity gas outside the CF that was pushed inward by a factor fo. We find broad support for such a picture, by comparing the entropy and metallicity discontinuities with the respective azimuthal averages, using newly deprojected thermal profiles in clusters A2029, A2142, A2204, and Centaurus, supplemented by deprojected CFs from the literature. In particular, the mean advection factors fK and fZ, inferred from entropy and metallicity, respectively, strongly correlate ( ${ \mathcal R }={0.7}_{-0.3}^{+0.2}$ ) with each other, consistent with large-scale advection. However, unlike sloshing simulations, in which the inside/outside phases are an inflow/outflow settling back to equilibrium after a violent perturbation, our results are more consistent with an outflow/inflow, with the fast, Mach ${{ \mathcal M }}_{I}\sim 0.8$ gas inside the CF being a rapidly heated or mixed outflow, probably originating from the cD galaxy, and gas outside the CF being an ${{ \mathcal M }}_{o}\sim 0.03$ , slowly cooling inflow. In particular, entropy indicates an outside advection factor $1.3\,\lesssim $ fKo $\lesssim 1.5$ that is approximately constant in all CFs, gauging the distance traversed by inflowing gas within a cooling time. In contrast, $1.1\lesssim {f}_{{Ki}}\lesssim 2.5$ and $1\lesssim {f}_{Z}\lesssim 17$ vary considerably among clusters and strongly correlate ( $3.1\sigma {\rm{\mbox{--}}}4.2\sigma $ ) with the virial mass, ${f}_{{Ki}}\propto {M}_{200}^{0.14\pm 0.07}$ and ${f}_{Z}\propto {M}_{200}^{1.4\pm 0.4}$ , suggesting that each cluster sustains a quasi-steady spiral flow.

Keywords: galaxy clusters; intracluster medium; cooling flows; x-ray astronomy
Constraining the Planet Occurrence Rate around Halo Stars of Potentially Extragalactic Origin

The search for planets orbiting other stars has recently expanded to include stars from galaxies outside the Milky Way. With the Transiting Exoplanet Survey Satellite (TESS) and Gaia surveys, photometric and kinematic information can be combined to identify transiting planet candidates of extragalactic origin. Here, 1080 low-luminosity red-giant branch stars observed by Gaia and TESS with kinematics suggesting a high likelihood of extragalactic origin were searched for planet transits. Transit injection-recovery tests were performed to measure the sensitivity of the TESS data and completeness of the transit search. Injected signals of planets larger than Jupiter with orbital periods of 10 days or less were recovered in ≈44% of cases. Although no planet transits were detected in this sample, we find an upper limit on planet occurrence of 0.52% for hot Jupiters, consistent with previous studies of planet occurrence around similar host stars. As stars in the halo tend to be lower metallicity, and short-period giant planet occurrence tends to be strongly correlated with stellar metallicity, we predict that relative to the Galactic disk population, a smaller fraction of halo stars will host planets detectable by transit surveys. Thus, applying the known planet occurrence trends to potential planet detection around halo stars, we predict ≳13,000 stars must be searched with similar cadence and precision as the stars studied here before a detection of a planet of extragalactic origin is likely. This may be possible with future data releases from the TESS and Gaia missions.

Keywords: exoplanets; red giant stars; halo stars