The variability of the black hole image in M87 at the dynamical timescale

The black hole images obtained with the Event Horizon Telescope (EHT) are expected to be variable at the dynamical timescale near their horizons. For the black hole at the center of the M87 galaxy, this timescale (5–61 days) is comparable to the 6 day extent of the 2017 EHT observations. Closure phases along baseline triangles are robust interferometric observables that are sensitive to the expected structural changes of the images but are free of station-based atmospheric and instrumental errors. We explored the day-to-day variability in closure-phase measurements on all six linearly independent nontrivial baseline triangles that can be formed from the 2017 observations. We showed that three triangles exhibit very low day-to-day variability, with a dispersion of ∼3°–5°. The only triangles that exhibit substantially higher variability (∼90°–180°) are the ones with baselines that cross the visibility amplitude minima on the u–v plane, as expected from theoretical modeling. We used two sets of general relativistic magnetohydrodynamic simulations to explore the dependence of the predicted variability on various black hole and accretion-flow parameters. We found that changing the magnetic field configuration, electron temperature model, or black hole spin has a marginal effect on the model consistency with the observed level of variability. On the other hand, the most discriminating image characteristic of models is the fractional width of the bright ring of emission. Models that best reproduce the observed small level of variability are characterized by thin ring-like images with structures dominated by gravitational lensing effects and thus least affected by turbulence in the accreting plasmas.https://iopscience.iop.org/article/10.3847/1538-4357/ac332e/pdfPublished versio

Nonlinear color-metallicity relations of globular clusters. IV. testing the nonlinearity scenario for color bimodality via HST/WFC3 u-band photometry of M84 (NGC 4374)

Color distributions of globular clusters (GCs) in most massive galaxies are bimodal. Assuming linear color-to-metallicity conversions, bimodality is viewed as the presence of merely two GC subsystems with distinct metallicities, which serves as a critical backbone of various galaxy formation theories. Recent studies, however, revealed that the color-metallicity relations (CMRs) often used to derive GC metallicities (e.g., CMRs of g-z, V-I, and C-T1) are in fact inflected. Such inflection can create bimodal color distributions if the underlying GC metallicity spread is simply broad as expected from the hierarchical merging paradigm of galaxy formation. In order to test the nonlinear-CMR scenario for GC color bimodality, the u-band photometry is proposed because the u-related CMRs (e.g., CMRs of u-g and u-z) are theoretically predicted to be least inflected and most distinctive among commonly used optical CMRs. Here, we present Hubble Space Telescope (HST)/WFC3 F336W (u-band) photometry of the GC system in M84, a giant elliptical in the Virgo galaxy cluster. Combining the u data with the existing HST ACS/WFC g and z data, we find that the u-z and u-g color distributions are different from the g-z distribution in a very systematic manner and remarkably consistent with our model predictions based on the nonlinear-CMR hypothesis. The results lend further confidence to the validity of the nonlinear-CMR scenario as an explanation for GC color bimodality. There are some GC systems showing bimodal spectroscopic metallicity, and in such systems the inflected CMRs often create stronger bimodality in the color domain. \ua9 2013. The American Astronomical Society. All rights reserved.Peer reviewed: YesNRC publication: Ye

Isotropic at the Break? 3D Kinematics of Milky Way Halo Stars in the Foreground of M31

We present the line-of-sight (LOS) velocities for 13 distant main sequence Milky Way halo stars with published proper motions. The proper motions were measured using long baseline (5-7 years) multi-epoch HST/ACS photometry, and the LOS velocities were extracted from deep (5-6 hour integrations) Keck II/DEIMOS spectra. We estimate the parameters of the velocity ellipsoid of the stellar halo using a Markov chain Monte Carlo ensembler sampler method. The velocity second moments in the directions of the Galactic (l,b, LOS) coordinate system are ⟨v2l⟩1/2=138+43−26 km/s, ⟨v2b⟩1/2=88+28−17 km/s, and ⟨v2LOS⟩1/2=91+27−14 km/s. We use these ellipsoid parameters to constrain the velocity anisotropy of the stellar halo. Ours is the first measurement of the anisotropy parameter β using 3D kinematics outside of the solar neighborhood. We find β=−0.3+0.4−0.9, consistent with isotropy and lower than solar neighborhood β measurements by 2σ (βSN∼0.5−0.7). We identify two stars in our sample that are likely members of the known TriAnd substructure, and excluding these objects from our sample increases our estimate of the anisotropy to β=0.1+0.4−1.0, which is still lower than solar neighborhood measurements by 1σ. The potential decrease in β with Galactocentric radius is inconsistent with theoretical predictions, though consistent with recent observational studies, and may indicate the presence of large, shell-type structure (or structures) at r∼25 kpc. The methods described in this paper will be applied to a much larger sample of stars with 3D kinematics observed through the ongoing HALO7D program

A Kinematic View of NGC 1261: Structural Parameters, Internal Dispersion, Absolute Proper Motion, and Blue Straggler Stars

We constructed a Hubble Space Telescope (HST) astrophotometric catalog of the central region of the Galactic globular cluster NGC 1261. This catalog, complemented with Gaia DR2 data sampling the external regions, has been used to estimate the structural parameters of the system (i.e., core, half-mass, tidal radii, and concentration) from its resolved star density profile. We computed high-precision proper motions thanks to multi-epoch HST data and derived the cluster velocity dispersion profile in the plane of the sky for the innermost region, finding that the system is isotropic. The combination with the line-of-sight information collected from spectroscopy in the external regions provided us with the cluster velocity dispersion profile along the entire radial extension. We also measured the absolute proper motion of NGC 1261 using a few background galaxies as a reference. The radial distribution of the Blue Straggler Star population shows that the cluster is in a low/intermediate phase of dynamical evolution

Prognostic Impact of Microsatellite Instability in Asian Gastric Cancer Patients Enrolled in the ARTIST Trial

Background: Caucasian patients with microsatellite instability (MSI)-high gastric cancer (GC) may have better prognosis but worse outcomes. Objective: Here we explored the prognostic role of MSI in Asian patients. Methods: This post hoc analysis comprehended radically resected GC patients randomized to XP (capecitabine/cisplatin) or XPRT. MSI status was assessed by combining immunohistochemistry with multiplex polymerase chain reaction. The MSI prognostic effect on disease-free survival (DFS) and overall survival (OS) was evaluated. Results: 393 tissue samples were analyzed and 35 (9%) were MSI-high. This subgroup was characterized by: older age, Borrmann classification 1-2, antral localization, T3-4 stage, and intestinal type. At univariable analysis, the microsatellite-stable subgroup showed a trend toward a worse prognosis as compared to the MSI-high group: 3-year DFS was 76.3 versus 85.4% (p = 0.122); 3-year OS was 81.7 versus 91.4% (p = 0.046). Multivariable analyses confirmed it in both DFS (hazard ratio, HR = 2.32 [95% CI 0.91, 5.88]; p = 0.077) and OS (HR = 3.17 [95% CI 0.97, 10.43]; p = 0.057). Conclusions: MSI-high status was associated with specific clinical-pathological features and a trend toward better outcomes of Asian GC patients