adsabs/SciX_UAT_keywords huggingface dataset:
Discovery and Characterization of Two Ultrafaint Dwarfs outside the Halo of the Milky Way: Leo M and Leo K

We report the discovery of two ultrafaint dwarf galaxies, Leo M and Leo K, that lie outside the halo of the Milky Way (MW). Using Hubble Space Telescope imaging of the resolved stars, we create color–magnitude diagrams reaching the oldest main-sequence turnoff of each system and (i) fit for structural parameters of the galaxies; (ii) measure their distances using the luminosity of the horizontal branch stars; (iii) estimate integrated magnitudes and stellar masses; and (iv) reconstruct the star formation histories. Based on their location in the Local Group, neither galaxy is currently within the halo of the MW although Leo K is located ∼26 kpc from the low-mass galaxy Leo T and these two systems may have had a past interaction. Leo M and Leo K have stellar masses of M ⊙ and 1.2 ± 0.2 × 104 M ⊙, and were quenched Gyr and Gyr ago, respectively. Given that the galaxies are at farther distances from the MW, it is unlikely that they were quenched by environmental processing. Instead, given their low stellar masses, their early quenching timescales are consistent with the scenario that a combination of reionization and stellar feedback shut down star formation at early cosmic times.

Keywords: local group; dwarf galaxies; galaxy quenching; reionization; stellar populations; hertzsprung russell diagram; hubble space telescope
Subaru High-z Exploration of Low-luminosity Quasars (SHELLQs). XII. Extended [C II] Structure (Merger or Outflow) in a z = 6.72 Red Quasar

We present Atacama Large Millimeter/submillimeter Array [C II] 158 μm line and far-infrared (FIR) continuum emission observations toward HSC J120505.09-000027.9 (J1205-0000) at z = 6.72 with a beam size of ∼0"8 × 0"5 (or 4.1 kpc × 2.6 kpc), the most distant red quasar known to date. Red quasars are modestly reddened by dust and are thought to be in rapid transition from an obscured starburst to an unobscured normal quasar, driven by powerful active galactic nucleus (AGN) feedback that blows out a cocoon of interstellar medium. The FIR continuum of J1205-0000 is bright, with an estimated luminosity of LFIR ∼ 3 × 1012 L⊙. The [C II] line emission is extended on scales of r ∼ 5 kpc, greater than that of the FIR continuum. The line profiles at the extended regions are complex and broad (FWHM ∼ 630-780 km s-1). Although it is not practical to identify the nature of this extended structure, possible explanations include (i) companion/merging galaxies and (ii) massive AGN-driven outflows. For the case of (i), the companions are modestly star-forming (∼10 M⊙ yr-1) but are not detected by our Subaru optical observations (yAB,5σ = 24.4 mag). For the case of (ii), our lower limit to the cold neutral outflow rate is ∼100 M⊙ yr-1. The outflow kinetic energy and momentum are both much lower than predicted in energy-conserving wind models, suggesting that the AGN feedback in this quasar is not capable of completely suppressing its star formation.

Keywords: agn host galaxies; active galactic nuclei; quasars; submillimeter astronomy; galaxy evolution; galaxy formation; high-redshift galaxies
Insights into Density and Location Diagnostics of Photoionized Outflows in X-Ray Binaries

The population of metastable levels is key to high-precision density diagnostics of astrophysical plasmas. In photoionized plasmas, density is used to infer the distance from the ionizing source, which is otherwise difficult to obtain. Perfecting models that compute these populations is thus crucial. The present paper presents a semianalytic hydrogenic approximation for assessing the relative importance of different processes in populating atomic levels. This approximation shows that in the presence of a radiation source, photoexcitations and collisional excitations are both important over a wide range of plasma temperatures and ionizing spectra, while radiative recombination is orders of magnitude weaker. The interesting case of Fe+21 with a collisional radiative model with photoexcitation demonstrates this effect. The population of the first excited metastable level in Fe+21 is sensitive to the electron number density in the critical range of n e = 1012-1015 cm-3; it was observed to be significantly populated in the X-ray spectrum of the 2005 outburst of the X-ray binary GRO J1655-40. The present model shows that photoexcitation is the predominant process indirectly populating the metastable level. For the photoionized plasma in the GRO J1655-40 outflow, the model indicates a measured value of n e = (2.6 ± 0.5) × 1013 cm-3, implying a distance from the source of r = (4.4 ± 0.4) × 1010 cm. Finally, we show how the computed critical density and distance of Fe+21 yield the correct ionization parameter of the ion, independent of ionization balance calculations.

Keywords: x-ray binary stars; spectroscopy; excitation rates; radiative processes; electron impact excitation
Arp 220: New Observational Insights into the Structure and Kinematics of the Nuclear Molecular Disks and Surrounding Gas

ALMA cycle 3 observations of 12CO J = 3 → 2, 13CO J = 4 → 3, SiO J = 8 → 7, and HCN J = 5 → 4 are presented. Significant extended emission is detected in 12CO J = 3 → 2 with a morphology that is indicative of m = 2 tidal features, suggesting gas inflow. In addition, outflows for both nuclei are found in 12CO J = 3 → 2. Significant SiO absorption is detected in the western nucleus. HCN that is morphologically distinct from CO is detected in both nuclei. These observations are compared to non-LTE radiative transfer models created using the Line Modeling Engine for simple gas dynamics to gain insight into how physical parameters, such as rotational velocity, turbulent velocity, gas temperature, dust temperature, and gas mass, can reproduce the observed kinematic and spatial features. The eastern nucleus is found to be best modeled with an inclusion of a temperature asymmetry from one side of the disk to the other. It is also found that the western nucleus is optically thick even in the less abundant species of 13CO, absorbing significant amounts of continuum radiation.

Keywords: ultraluminous infrared galaxies; co line emission; radiative transfer
Hidden Companions to Intermediate-mass Stars. III. Discovery of a 1.6M ⊙, 1.3 au Companion to HIP 92041 = ϕ Sagittarii

B-type stars with masses up to about 8 M ⊙ are the progenitors of the heavier white dwarfs yet our knowledge about their multiplicity is still incomplete. The nearby subgiant HIP 92041 = ϕ Sagittarii has had claims of binarity in the past but they have all been refuted as spurious. In this report we show that ϕ Sgr is a binary after all based on a VLTI/GRAVITY interferometric observation. The companion has a K band flux ratio of 6% and a projected separation ρ = 17.7 mas ↔ 1.3 au. The inferred isochrone masses 3.5M ⊙ + 1.6M ⊙ make ϕ Sgr a potential progenitor for a white dwarf binary with a combined mass exceeding the Chandrasekhar limit. *Based on observations collected at the European Southern Observatory, Chile, Program ID 099.D-2020(A).

Keywords: multiple stars; b stars; optical interferometry
OGLE-2017-BLG-0448Lb: A Low Mass–Ratio Wide-orbit Microlensing Planet?

The gravitational microlensing technique is most sensitive to planets in a Jupiter-like orbit and has detected more than 200 planets. However, only a few wide-orbit (s > 2) microlensing planets have been discovered, where s is the planet-to-host separation normalized to the angular Einstein ring radius, θ E. Here, we present the discovery and analysis of a strong candidate wide-orbit microlensing planet in the event OGLE-2017-BLG-0448. The whole light curve exhibits long-term residuals to the static binary-lens single-source model, so we investigate the residuals by adding the microlensing parallax, microlensing xallarap, an additional lens, or an additional source. For the first time, we observe a complex degeneracy between all four effects. The wide-orbit models with s ∼ 2.5 and a planet-to-host mass ratio of q ∼ 10‑4 are significantly preferred, but we cannot rule out the close models with s ∼ 0.35 and q ∼ 10‑3. A Bayesian analysis based on a Galactic model indicates that, despite the complicated degeneracy, the surviving wide-orbit models all contain a super-Earth-mass to Neptune-mass planet at a projected planet-host separation of ∼6 au and the surviving close-orbit models all consist of a Jovian-mass planet at ∼1 au. The host star is probably an M or K dwarf. We discuss the implications of this dimension-degeneracy disaster on microlensing light-curve analysis and its potential impact on statistical studies.

Keywords: gravitational microlensing exoplanet detection
The Galaxies Missed by Hubble and ALMA: The Contribution of Extremely Red Galaxies to the Cosmic Census at 3 < z < 8

Using deep JWST imaging from JADES, JEMS, and SMILES, we characterize optically faint and extremely red galaxies at z > 3 that were previously missing from galaxy census estimates. The data indicate the existence of abundant, dusty, and poststarburst-like galaxies down to 108 M ⊙, below the sensitivity limit of Spitzer and the Atacama Large Millimeter/submillimeter Array (ALMA). Modeling the NIRCam and Hubble Space Telescope (HST) photometry of these red sources can result in extremely high values for both stellar mass and star formation rate (SFR); however, including seven MIRI filters out to 21 μm results in decreased masses (median 0.6 dex for > 10) and SFRs (median 10× for SFR > 100 M ⊙ yr‑1). At z > 6, our sample includes a high fraction of "little red dots" (LRDs; NIRCam-selected dust-reddened active galactic nucleus (AGN) candidates). We significantly measure older stellar populations in the LRDs out to rest-frame 3 μm (the stellar bump) and rule out a dominant contribution from hot dust emission, a signature of AGN contamination to stellar population measurements. This allows us to measure their contribution to the cosmic census at z > 3, below the typical detection limits of ALMA (L IR < 1012 L ⊙). We find that these sources, which are overwhelmingly missed by HST and ALMA, could effectively double the obscured fraction of the star formation rate density at 4 < z < 6 compared to some estimates, showing that prior to JWST, the obscured contribution from fainter sources could be underestimated. Finally, we identify five sources with evidence for Balmer breaks and high stellar masses at 5.5 < z < 7.7. While spectroscopy is required to determine their nature, we discuss possible measurement systematics to explore with future data.

Keywords: high-redshift galaxies; active galaxies; agn host galaxies
K-band Luminosity Functions of IllustrisTNG300-1 Galaxies

This Research Note summarizes preliminary results, presented at the 238th AAS meeting, for the K-band luminosity functions (LFs) of IllustrisTNG300-1 galaxies. For MK ≥ -22, the simulation reproduces both the shape and amplitude of the LF for observed field galaxies. However, it significantly overproduces field galaxies with MK < -22. For MK ≤ -22, the simulation reproduces the shape of the LF for observed cluster galaxies, however at fainter magnitudes the LF falls below a simple extrapolation of recent observations. For MK ≤ -22, the shapes of the LFs for red cluster galaxies and blue cluster galaxies agree reasonably well with observations, but neither is well-fitted by a Schechter function at fainter magnitudes. In addition, the faint-end slope of the LF in the inner regions of the clusters is significantly flatter than it is in the outer regions.

Keywords: galaxy luminosities; galaxy clusters
Brown Dwarf Candidates in the JADES and CEERS Extragalactic Surveys

By combining the James Webb Space Telescope (JWST)/NIRCam JADES and CEERS extragalactic data sets, we have uncovered a sample of 21 T and Y brown dwarf candidates at best-fit distances between 0.1 and 4.2 kpc. These sources were selected by targeting the blue 1–2.5 μm colors and red 3–4.5 μm colors that arise from molecular absorption in the atmospheres of T eff < 1300 K brown dwarfs. We fit these sources using multiple models of substellar atmospheres and present the resulting fluxes, sizes, effective temperatures, and other derived properties for the sample. If confirmed, these fits place the majority of the sources in the Milky Way thick disk and halo. We observe proper motions for seven of the candidate brown dwarfs, with directions in agreement with the plane of our Galaxy, providing evidence that they are not extragalactic in nature. We demonstrate how the colors of these sources differ from selected high-redshift galaxies, and explore the selection of these sources in planned large-area JWST NIRCam surveys. Deep imaging with JWST/NIRCam presents an an excellent opportunity for finding and understanding these ultracool dwarfs at kiloparsec distances.

Keywords: brown dwarfs; t dwarfs; t subdwarfs; y dwarfs; halo stars; james webb space telescope
Center-to-limb Variation of the Inverse Evershed Flow

We present the properties of the inverse Evershed flow (IEF) based on the center-to-limb variation of the plasma speed and loop geometry of chromospheric superpenumbral fibrils in eleven sunspots that were located at a wide range of heliocentric angles from 12° to 79°. The observations were acquired at the Dunn Solar Telescope in the spectral line of Hα at 656 nm to determine chromospheric flows and the photospheric Si I line at 1082.7 nm to estimate the photospheric umbral magnetic field strength. All sunspots display opposite line-of-sight (LOS) velocities on the limb and center side with a distinct shock signature near the outer penumbral edge. We developed a simplified flexible sunspot model assuming axisymmetry and prescribing the radial flow speed profile at a known loop geometry to replicate the observed two-dimensional IEF patterns under different viewing angles. The simulated flow maps match the observations for chromospheric loops with 10-20 Mm length starting at 0.8-1.1 sunspot radii, an apex height of 1-3 Mm, and a flow speed of 2-9 km s-1. We find on average a good agreement of the simulated velocities and the observations on elliptical annuli around the sunspot. Individual IEF channels show a significant range of variation in their properties and reach maximal LOS speeds of up to 12 km s-1. Upwards or downwards directed flows do not show a change of sign in the LOS velocities for heliocentric angles above 30°. Our results are consistent with the IEF being caused by a siphon flow mechanism driving a flow at about sonic speed along elevated loops with a flattened top in the chromosphere.

Keywords: solar chromosphere; sunspots; solar photosphere
Unveiling Properties of the Nonthermal X-Ray Production in the Gamma-Ray Binary LS 5039 Using the Long-term Pattern of Its Fast X-Ray Variability

Gamma-ray binary systems, a subclass of high-mass X-ray binaries, show nonthermal emissions from radio to TeV. While efficient electron acceleration is considered to take place in them, the nature of the acceleration mechanism and the physical environments in these systems have been a long-standing question. In this work, we report on long-term recurrent patterns in the short-term variability of the soft X-ray emission of LS 5039, one of the brightest gamma-ray binary systems. The Neutron star Interior Composition Explorer (NICER) observed LS 5039 four times from 2018 to 2021. By comparing them with the previous Suzaku and NuSTAR long-exposure observations, we studied the long-term evolution of the orbital light curve in the soft X-ray band. Although the observations by NICER and Suzaku are separated by ~14 yr, i.e., more than 103 orbits, the orbital light curves show remarkable consistency after calculating their running averages with a window width ≳70 ks. Furthermore, all of the light curves show short-term variability with a timescale of ~10 ks. Since the column density did not vary when the flux changed abruptly, such a short-term variability seems to be an intrinsic feature of the X-ray emission. We propose that the short-term variability is caused by clumps (or inhomogeneities) of the companion star wind impacting the X-ray production site. The observed timescale matches well with the lifetime of the clumps interacting with the pulsar wind and the dynamical timescale of the relativistic intrabinary shock in the pulsar wind scenario.

Keywords: compact binary stars; x-ray binary stars; gamma-ray sources; x-ray sources; high mass x-ray binary stars
Tearing-mediated Reconnection in Magnetohydrodynamic Poorly Ionized Plasmas. I. Onset and Linear Evolution

In high-Lundquist-number plasmas, reconnection proceeds via the onset of tearing, followed by a nonlinear phase during which plasmoids continuously form, merge, and are ejected from the current sheet (CS). This process is understood in fully ionized, magnetohydrodynamic plasmas. However, many plasma environments, such as star-forming molecular clouds and the solar chromosphere, are poorly ionized. We use theory and computation to study tearing-mediated reconnection in such poorly ionized systems. In this paper, we focus on the onset and linear evolution of this process. In poorly ionized plasmas, magnetic nulls on scales below v A,n0/ν ni0, with v A,n0 the neutral Alfvén speed and ν ni0 the neutral–ion collision frequency, will self-sharpen via ambipolar diffusion. This sharpening occurs at an increasing rate, inhibiting the onset of reconnection. Once the CS becomes thin enough, however, ions decouple from neutrals and the thinning of the CS slows, allowing the onset of tearing in a time of order . We find that the wavelength and growth rate of the mode that first disrupts the forming sheet can be predicted from a poorly ionized tearing dispersion relation; as the plasma recombination rate increases and ionization fraction decreases, the growth rate becomes an increasing multiple of ν ni0 and the wavelength becomes a decreasing fraction of v A,n0/ν ni0.

Keywords: interstellar plasma; plasma astrophysics; solar magnetic reconnection; molecular clouds; solar chromosphere; plasma physics
Prospects for the Detection of the Sgr A* Photon Ring with Next-generation Event Horizon Telescope Polarimetry

The Event Horizon Telescope (EHT) has imaged two supermassive black holes, Messier 87* (M87*) and Sagittarius A* (Sgr A*), using very-long-baseline interferometry (VLBI). The theoretical analyses of each source suggest magnetically arrested disk (MAD) accretion viewed at modest inclination. These MADs exhibit rotationally symmetric polarization of synchrotron emission caused by symmetries of their ordered magnetic fields. We leverage these symmetries to study the detectability of the black hole photon ring, which imposes known antisymmetries in polarization. In this Letter, we propose a novel observational strategy based on coherent baseline averaging of polarization ratios On a rotating basis to detect the photon ring with 345 GHz VLBI from the Earth's surface. Using synthetic observations from a likely future EHT, we find a reversal in polarimetric phases on long baselines that reveals the presence of the Sgr A* photon ring in a MAD system at 345 GHz, a critical frequency for lengthening baselines and overcoming interstellar scattering. We use our synthetic data and analysis pipeline to estimate requirements for the EHT using a new metric: SNRPR, the signal-to-noise ratio of this polarimetric reversal signal. We identify long, coherent integrations using frequency phase transfer as a critical enabling technique for the detection of the photon ring and predict a SNRPR ∼ 2‑3 detection using proposed next-generation Event Horizon Telescope parameters and currently favored models for the Sgr A* accretion flow. We find that higher sensitivity, rather than denser Fourier sampling, is the most critical requirement for polarimetric detection of the photon ring.

Keywords: supermassive black holes; very long baseline interferometry; polarimetry
A Dearth of Close-in Planets around Rapidly Rotating Stars or a Dearth of Data?

A dearth of close-in planets orbiting rapid rotators was reported almost a decade ago. According to this view, only slowly spinning stars with rotation periods longer than 5-10 days would host planets with orbital periods shorter than 2 or 3 days. This Letter brings an enlarged and more detailed analysis that led us to the question: Is there really a dearth in that distribution or is it a dearth of data? For this new analysis, we combined different samples of Kepler and TESS stars with confirmed planets or planet candidates with measured stellar rotation periods, using Gaia data to perform an in-depth selection of 1013 planet-hosting main-sequence stars. With the newer, enlarged, and more refined data, the reported dearth of close-in planets orbiting rapid rotators tends to disappear, thus suggesting that it may reflect a scarcity of data in the prior analysis. A two-sample statistical test strongly supports our results, showing that the distribution of close-in planets orbiting rapid rotators is almost indistinguishable from that for close-in planets orbiting slow rotators.

Keywords: main sequence stars; stellar rotation; exoplanets; planetary science
SPROUT: A Moving-mesh Hydro Code Using a Uniformly Expanding Cartesian Grid

We present the publicly available moving-mesh hydrodynamics code Sprout. Sprout solves the equations of ideal hydrodynamics on an expanding Cartesian mesh. The expanding mesh can follow fluid outflows for several orders of magnitude with very little numerical diffusion, thereby capturing shocks and fine structures accurately. Following the bulk flow accurately also allows for longer time steps in general. This makes Sprout particularly suitable for studying expanding outflows such as supernova remnants and active galactic nuclei. Relative to other moving-mesh codes, the simple mesh structure in Sprout is also convenient for implementing additional physics or algorithms. Many code tests are performed to test the accuracy and performance of the numerical scheme.

Keywords: hydrodynamics; astrophysical fluid dynamics; computational methods
A Multiwavelength View of IC 860: What Is in Action inside Quenching Galaxies

We present a multiwavelength study of IC 860, a nearby post-starburst galaxy at the early stage of transitioning from blue and star forming to red and quiescent. Optical images reveal a galaxy-wide, dusty outflow originating from a compact core. We find evidence for a multiphase outflow in the molecular and neutral gas phase from the CO position-velocity diagram and NaD absorption features. We constrain the neutral mass outflow rate to be ~0.5 M ⊙ yr-1, and the total hydrogen mass outflow rate to be ~12 M ⊙ yr-1. Neither outflow component seems able to escape the galaxy. We also find evidence for a recent merger in the optical images, CO spatial distribution, and kinematics, and evidence for a buried active galactic nucleus in the optical emission line ratios, mid-IR properties, and radio spectral shape. The depletion time of the molecular gas reservoir under the current star formation rate is ~7 Gyr, indicating that the galaxy could stay at the intermediate stage between the blue and red sequence for a long time. Thus the timescales for a significant decline in star formation rate (quenching) and gas depletion are not necessarily the same. Our analysis supports the quenching picture where outflows help suppress star formation by disturbing rather than expelling the gas and shed light on possible ongoing activities in similar quenching galaxies. * Herschel is an ESA space observatory with science instruments provided by European-led principal investigator consortia and with important participation from NASA.

Keywords: galaxy evolution; active galactic nuclei; galaxies; post-starburst galaxies; galaxy quenching; galaxy physics; galactic and extragalactic astronomy