Hintze Centre for Astrophysical Surveys

Abstract:

Observations of binary stars containing an accreting black hole or neutron star often show x-ray emission extending to high energies (>10 kilo--electron volts), which is ascribed to an accretion disk corona of energetic particles akin to those seen in the solar corona. Despite their ubiquity, the physical conditions in accretion disk coronae remain poorly constrained. Using simultaneous infrared, optical, x-ray, and radio observations of the Galactic black hole system V404 Cygni, showing a rapid synchrotron cooling event in its 2015 outburst, we present a precise 461 ± 12 gauss magnetic field measurement in the corona. This measurement is substantially lower than previous estimates for such systems, providing constraints on physical models of accretion physics in black hole and neutron star binary systems.

Abstract:

© 2017. The American Astronomical Society. All rights reserved. We present data for 16 galaxies in the overdensity JKCS 041 at z ≃ 1.80 as part of the K-band Multi-Object Spectrograph (KMOS) Cluster Survey (KCS). With 20 hr integrations, we have obtained deep absorption-line spectra from which we derived velocity dispersions for seven quiescent galaxies. We combined photometric parameters derived from Hubble Space Telescope images with the dispersions to construct a fundamental plane (FP) for quiescent galaxies in JKCS 041. From the zero-point evolution of the FP, we derived a formation redshift for the galaxies of z form = 3.0 ± 0.3, corresponding to a mean age of 1.4 ± 0.2 Gyr. We tested the effect of structural and velocity dispersion evolution on our FP zero-point and found a negligible contribution when using dynamical mass-normalized parameters (∼3%) but a significant contribution from stellar-mass-normalized parameters (∼42%). From the relative velocities of the galaxies, we probed the 3D structure of these 16 confirmed members of JKCS 041 and found that a group of galaxies in the southwest of the overdensity had systematically higher velocities. We derived ages for the galaxies in the different groups from the FP. We found that the eastextending group had typically older galaxies (2.1 +0.3 0.2 Gyr) than those in the southwest group (0.3 ± 0.2 Gyr). Although based on small numbers, the overdensity dynamics, morphology, and age results could indicate that JKCS 041 is in formation and may comprise two merging groups of galaxies. This result could link large-scale structure to ages of galaxies for the first time at this redshift.