A comparison of the local spiral structure from Gaia DR2 and VLBI maser parallaxes

Context. The Gaia mission has released the second data set (Gaia DR2), which contains parallaxes and proper motions for a large number of massive, young stars. Aims. We investigate the spiral structure in the solar neighborhood revealed by Gaia DR2 and compare it with that depicted by VLBI maser parallaxes. Methods. We examined three samples with different constraints on parallax uncertainty and distance errors and stellar spectral types: (1) all OB stars with parallax errors of less than 10%; (2) only O-type stars with 0.1 mas errors imposed and with parallax distance errors of less than 0.2 kpc; and (3) only O-type stars with 0.05 mas errors imposed and with parallax distance errors of less than 0.3 kpc. Results. In spite of the significant distance uncertainties for stars in DR2 beyond 1.4 kpc, the spiral structure in the solar neighborhood demonstrated by Gaia agrees well with that illustrated by VLBI maser results. The O-type stars available from DR2 extend the spiral arm models determined from VLBI maser parallaxes into the fourth Galactic quadrant, and suggest the existence of a new spur between the Local and Sagittarius arms.Comment: 4 pages, 3 figures, 1 table, accepted for publication in A&

SDSS J143030.22-001115.1: A misclassified narrow-line Seyfert 1 galaxy with flat X-ray spectrum

We used multi-component profiles to model H$\beta$ and [O III]$\lambda \lambda$4959,5007 lines for SDSS J143030.22-001115.1, a narrow-line Seyfert 1 galaxy (NLS1) in a sample of 150 NLS1s candidates selected from the Sloan Digital Sky Survey (SDSS) Early Data Release (EDR). After subtracting the H$\beta$ contribution from narrow line regions (NLRs), we found that its full width half maximum (FWHM) of broad H$\beta$ line is nearly 2900 \kms, significantly larger than the customarily adopted criterion of 2000 \kms. With its weak Fe II multiples, we think that SDSS J143030.22-001115.1 can't be classified as a genuine NLS1. When we calculate the virial black hole masses of NLS1s, we should use the H$\beta$ linewidth after subtracting the H$\beta$ contribution from NLRs.Comment: 7 pages, 1 table, accepted by ChJA

Oscillatory surface dichroism of an insulating topological insulator Bi2Te2Se

Using circular dichroism-angle resolved photoemission spectroscopy (CD-ARPES), we report a study of the effect of angular momentum transfer between polarized photons and topological surface states on the surface of highly bulk insulating topological insulator Bi2Te2Se. The photoelectron dichroism is found to be strongly modulated by the frequency of the helical photons including a dramatic sign-flip. Our results suggest that the observed dichroism and its sign-flip are consequences of strong coupling between the photon field and the spin-orbit nature of the Dirac modes on the surface. Our studies reveal the intrinsic dichroic behavior of topological surface states and point toward the potential utility of bulk insulating topological insulators in device applications.Comment: 5 pages, 4 figure

Suppressed star formation in circumnuclear regions in Seyfert galaxies

Feedback from black hole activity is widely believed to play a key role in regulating star formation and black hole growth. A long-standing issue is the relation between the star formation and fueling the supermassive black holes in active galactic nuclei (AGNs). We compile a sample of 57 Seyfert galaxies to tackle this issue. We estimate the surface densities of gas and star formation rates in circumnuclear regions (CNRs). Comparing with the well-known Kennicutt-Schmidt (K-S) law, we find that the star formation rates in CNRs of most Seyfert galaxies are suppressed in this sample. Feedback is suggested to explain the suppressed star formation rates.Comment: 1 color figure and 1 table. ApJ Letters in pres

Synthesis of Silicate-Bridged Heterojunctional SnO2/BiVO4 Nanoplates as Efficient Photocatalysts to Convert CO2 and Degrade 2,4-Dichlorophenol

Bismuth vanadate (BiVO4) is a promising visible‐light responsive photocatalyst, whose photocatalytic activity can be significantly improved by increasing its surface area and utilizing its high‐energy‐level photogenerated electrons effectively. In this work, 2D BiVO4 nanoplates with large specific surface area are successfully fabricated by hydrothermal conversion with the pre‐prepared BiOCl nanosheets as precursors. To improve the photogenerated charge separation, resulted BiVO4 nanoplates are further coupled with nanocrystalline SnO2 to construct heterojunctions, then silicate bridges are introduced between the interfaces of BiVO4 and SnO2. The amount‐optimized silicate‐bridged SnO2/BiVO4 nanocomposite exhibit exceptional visible‐light photocatalytic activities, by ≈7‐time and 4‐time enhancements for CO2 conversion to CH4 and for 2,4‐dichlorophenol degradation, respectively, compared to bare BiVO4 nanoparticles. The significantly enhanced charge separation is verified by steady‐state and time‐resolved surface photovoltage responses and produced hydroxyl radical amounts. Moreover, it is deduced through designed photo‐electrochemical experiments that the introduced SnO2 acts as a proper‐energy platform capable of accepting the photogenerated electrons of BiVO4 nanoplates, and the constructed silicate bridges further facilitate the electron transfer between BiVO4 and SnO2. This work opens up a feasible route to synthesize visible‐light‐driven 2D bismuth‐based nano‐photocatalysts with high photocatalytic activities for efficient fuel production and environmental remediation

Understanding AGN-Host Connection in Partially Obscured Active Galactic Nuclei. Part I: The Nature of AGN+HII Composites

The goal of our serial papers is to examine the evolutionary connection between AGN and star formation in its host galaxy in the partially obscured AGNs (i.e., Seyfert 1.8 and 1.9 galaxies). Taking advantage of these galaxies, the properties of both components can be studied together by direct measurements. In this paper, we focus on the broad-line composite galaxies (composite AGNs) which are located between the theoretical and empirical separation lines in the [NII]/Ha vs. [OIII]/Hb diagram. These galaxies are searched for from the composite galaxies provided by the SDSS DR4 MPA/JHU catalogs. After re-analyze the spectra, we perform a fine classification for the 85 composite AGNs in terms of the BPT diagrams. All the objects located below the three theoretical separation lines are associated with a young stellar population (<1Gyrs), while either a young or old stellar population is identified in the individual multiply-classified object. The multiply-classified objects with a very old stellar population are located in the LINER region in the [OI]/Ha vs. [OIII]/Hb diagram. We then consider the connection between AGN and star formation to derive the key results. The Eddington ratio inferred from the broad Ha emission, the age of the stellar population of AGN's host as assessed by D_n(4000), and the line ratio [OI]/Ha are found to be related with each other. These relations strongly support the evolutionary scenario in which AGNs evolve from high L/L_Edd state with soft spectrum to low L/L_Edd state with hard spectrum as young stellar population ages and fades. The significant correlation between the line ratio [OI]/Ha and D_n(4000) leads us to suggest that the line ratio could be used to trace the age of stellar population in type I AGNs.Comment: 39 pages, 11 figures, 1 table, accepted by Ap

The SAMI Galaxy Survey: energy sources of the turbulent velocity dispersion in spatially-resolved local star-forming galaxies

We investigate the energy sources of random turbulent motions of ionised gas from H$\alpha$ emission in eight local star-forming galaxies from the Sydney-AAO Multi-object Integral field spectrograph (SAMI) Galaxy Survey. These galaxies satisfy strict pure star-forming selection criteria to avoid contamination from active galactic nuclei (AGN) or strong shocks/outflows. Using the relatively high spatial and spectral resolution of SAMI, we find that -- on sub-kpc scales our galaxies display a flat distribution of ionised gas velocity dispersion as a function of star formation rate (SFR) surface density. A major fraction of our SAMI galaxies shows higher velocity dispersion than predictions by feedback-driven models, especially at the low SFR surface density end. Our results suggest that additional sources beyond star formation feedback contribute to driving random motions of the interstellar medium (ISM) in star-forming galaxies. We speculate that gravity, galactic shear, and/or magnetorotational instability (MRI) may be additional driving sources of turbulence in these galaxies.Comment: 11 pages, 5 figures, 3 tables. Accepted by MNRA

Ductile bulk metallic glass by controlling structural heterogeneities

A prerequisite to utilize the full potential of structural heterogeneities for improving the room-temperature plastic deformation of bulk metallic glasses (BMGs) is to understand their interaction with the mechanism of shear band formation and propagation. This task requires the ability to artificially create heterogeneous microstructures with controlled morphology and orientation. Here, we analyze the effect of the designed heterogeneities generated by imprinting on the tensile mechanical behavior of the Zr52.5Ti5Cu18Ni14.5Al10 BMG by using experimental and computational methods. The imprinted material is elastically heterogeneous and displays anisotropic mechanical properties: strength and ductility increase with increasing the loading angle between imprints and tensile direction. This behavior occurs through shear band branching and their progressive rotation. Molecular dynamics and finite element simulations indicate that shear band branching and rotation originates at the interface between the heterogeneities, where the characteristic atomistic mechanism responsible for shear banding in a homogeneous glass is perturbed