6 research outputs found
Suppression of AGN-driven Turbulence by Magnetic Fields in a Magnetohydrodynamic Model of the Intracluster Medium
We investigate the role of AGN feedback in turbulent heating of galaxy clusters. Specifically, we analyze the production of turbulence by g-modes generated by the supersonic expansion and buoyant rise of AGN-driven bubbles. Previous work which neglects magnetic fields has shown that this process is inefficient, with less than 1% of the injected energy ending up in turbulence. This inefficiency is primarily due to the fact that the bubbles are shredded apart by hydrodynamic instabilities before they can excite sufficiently strong g-modes. Using a plane-parallel model of the ICM and 3D ideal MHD simulations, we examine the role of a large-scale magnetic field which is able to drape around these rising bubbles, preserving them from hydrodynamic instabilities. We find that, while magnetic draping appears better able to preserve AGN-driven bubbles, the driving of g-modes and the resulting production of turbulence is still inefficient. The magnetic tension force prevents g-modes from tran- sitioning into the nonlinear regime, suppressing turbulence in our model ICM. Our work highlights the ways in which ideal MHD is an insufficient description for the cluster feedback process, and we discuss future work such as the inclusion of anisotropic viscosity as a means of simulating high β plasma kinetic effects. These results suggest the hypothesis that other mechanisms of heating the ICM plasma such as sound waves or cosmic rays may be responsible for observed feedback in galaxy clusters
Evolution of active galactic nuclei
[Abriged] Supermassive black holes (SMBH) lurk in the nuclei of most massive
galaxies, perhaps in all of them. The tight observed scaling relations between
SMBH masses and structural properties of their host spheroids likely indicate
that the processes fostering the growth of both components are physically
linked, despite the many orders of magnitude difference in their physical size.
This chapter discusses how we constrain the evolution of SMBH, probed by their
actively growing phases, when they shine as active galactic nuclei (AGN) with
luminosities often in excess of that of the entire stellar population of their
host galaxies. Following loosely the chronological developments of the field,
we begin by discussing early evolutionary studies, when AGN represented beacons
of light probing the most distant reaches of the universe and were used as
tracers of the large scale structure. This early study turned into AGN
"Demography", once it was realized that the strong evolution (in luminosity,
number density) of the AGN population hindered any attempt to derive
cosmological parameters from AGN observations directly. Following a discussion
of the state of the art in the study of AGN luminosity functions, we move on to
discuss the "modern" view of AGN evolution, one in which a bigger emphasis is
given to the physical relationships between the population of growing black
holes and their environment. This includes observational and theoretical
efforts aimed at constraining and understanding the evolution of scaling
relations, as well as the resulting limits on the evolution of the SMBH mass
function. Physical models of AGN feedback and the ongoing efforts to isolate
them observationally are discussed next. Finally, we touch upon the problem of
when and how the first black holes formed and the role of black holes in the
high-redshift universe.Comment: 75 pages, 35 figures. Modified version of the chapter accepted to
appear in "Planets, Stars and Stellar Systems", vol 6, ed W. Keel
(www.springer.com/astronomy/book/978-90-481-8818-5). The number of references
is limited upon request of the editors. Original submission to Springer: June
201
The X-ray counterpart to the gravitational wave event GW 170817
A long-standing paradigm in astrophysics is that collisions- or mergers- of
two neutron stars (NSs) form highly relativistic and collimated outflows (jets)
powering gamma-ray bursts (GRBs) of short (< 2 s) duration. However, the
observational support for this model is only indirect. A hitherto outstanding
prediction is that gravitational wave (GW) events from such mergers should be
associated with GRBs, and that a majority of these GRBs should be off-axis,
that is, they should point away from the Earth. Here we report the discovery of
the X-ray counterpart associated with the GW event GW170817. While the
electromagnetic counterpart at optical and infrared frequencies is dominated by
the radioactive glow from freshly synthesized r-process material in the merger
ejecta, known as kilonova, observations at X-ray and, later, radio frequencies
exhibit the behavior of a short GRB viewed off-axis. Our detection of X-ray
emission at a location coincident with the kilonova transient provides the
missing observational link between short GRBs and GWs from NS mergers, and
gives independent confirmation of the collimated nature of the GRB emission.Comment: 38 pages, 10 figures, Nature, in pres