No abstract

The inner regions of barred galaxies contain substructures such as off-axis shocks, nuclear rings, and nuclear spirals. These substructure may affect star formation, and control the activity of a central black hole (BH) by determining the mass inflow rate. We investigate the formation and properties of such substructures using high-resolution, grid-based hydrodynamic simulations. The gaseous medium is assumed to be infinitesimallythin, isothermal, and non-self-gravitating. The stars and dark matter are represented by a static gravitational potential with four components: a stellar disk, the bulge, a central BH, and the bar. To investigate various galactic environments, we vary the gas sound speed c s as well as the mass of the central BH M BH . Once the flow has reached a quasi-steady state, off-axis shocks tend to move closer to the bar major axis as c s increases. Nuclear rings shrink in size with increasing c s , but are independent of M BH , suggesting that ring position is not determined by the Lindblad resonances. Rings in low-c s models are narrow since they are occupied largely by gas on x 2 -orbits and well decoupled from nuclear spirals, while they become broad because of large thermal perturbations in high-c s models. Nuclear spirals persist only when either c s is small or M BH is large; they would otherwise be destroyed completely by the ring material on eccentric orbits. The shape and strength of nuclear spirals depend sensitively on c s and M BH such that they are leading if both c s and M BH are small, weak trailing if c s is small and M BH is large, and strong trailing if both c s and M BH are large. While the mass inflow rate toward the nucleus is quite small in low-c s models because of the presence of a narrow nuclear ring, it becomes larger than 0.01 M ⊙ yr −1 when c s is large, providing a potential explanation of nuclear activity in Seyfert galaxies.

In 2017 April, the Event Horizon Telescope (EHT) observed the near-horizon region around the supermassive black hole at the core of the M87 galaxy. These 1.3 mm wavelength observations revealed a compact asymmetric ring-like source morphology. This structure originates from synchrotron emission produced by relativistic plasma located in the immediate vicinity of the black hole. Here we present the corresponding linear-polarimetric EHT images of the center of M87. We find that only a part of the ring is significantly polarized. The resolved fractional linear polarization has a maximum located in the southwest part of the ring, where it rises to the level of ∼15%. The polarization position angles are arranged in a nearly azimuthal pattern. We perform quantitative measurements of relevant polarimetric properties of the compact emission and find evidence for the temporal evolution of the polarized source structure over one week of EHT observations. The details of the polarimetric data reduction and calibration methodology are provided. We carry out the data analysis using multiple independent imaging and modeling techniques, each of which is validated against a suite of synthetic data sets. The gross polarimetric structure and its apparent evolution with time are insensitive to the method used to reconstruct the image. These polarimetric images carry information about the structure of the magnetic fields responsible for the synchrotron emission. Their physical interpretation is discussed in an accompanying publication.