The Piston Driving the Shock
M87 hosts a supermassive black hole whose mass has been dynamically measured to be about 3 billion solar masses concentrated in a volume about the size of our solar system.
Matter surrounding the black hole forms a disk as it nears the black hole. The disk provides a mechanism for the matter to dissipate its angular momentum and ultimatey fall into the black hole. The accretion power provides light, at all wavelengths, but also drives an outflow of matter that we observe as the famous jet pointing to the northwest. The jet is seen from the optical to the X-ray. The black hole, through its jet, is the driving force that produces the shocks we observe in the gas surrounding M87.
Because M87 (and nearly all other massive elliptical galaxies) are surrounded by thermal gas which is visible only in X-rays (because of its high temperature), X-ray images provide an historical record of supermassive black hole outbursts over cosmic time. Thus, when we observe M87 with Chandra, we have a unique opportunity to study its outburst history.
The left panel shows the 0.5-2.5 keV band full resolution (1 pix = $0.492''$) of M87 after background subtraction and ``flat fielding'' of the center of M87. We discuss the many features below. The central panel shows the 6 cm VLA radio image from Hines et al. (1989) showing the radio jet, and the synchrotron emission from the "cocoon". The right panel shows the IR view of the center of M87 from the Spitzer telescope IRAC detector. The 4.5 micron image is divided by an azimuthally averaged model to remove the strong gradient of emission from the galaxy light.
Prominent X-ray features of the central region show the counterjet cavity surrounded by a very fine rim of gas and cavities to the west and southwest of the jet after the jet passes the sonic point and the radio emitting plasma bends clockwise. The innermost buoyant bubble (X-ray cavity, labeled ``bud'' in left panel) coincides with the radio synchrotron emission extending south from the cocoon (center panel). The IRAC image shows the emission from the nucleus and the jet. The IR jet emission ends just before the feature ``Jet Cavity'' in the X-ray image. On the counter jet side of the nucleus, two bright IR patches (labeled with arrows in the IRAC image) lie within a ``C'' shaped region. The two bright IR patches coincide with brighter regions of 6~cm emission (also marked with arrows in the center panel) and associated with structures $\theta$ and $\eta$ in Hines et al. (1989). The IR emission (and the coincident radio emission) lie at nearly 90\dg~ from the direction of the jet (in projection) and arise from unbeamed emission.
The cocoon of relativistic plasma is the ``piston'' that mediates outbursts from the central SMBH and drives shocks into the surrounding X-ray emitting, thermal gas. The cocoon cavity "captures" the initially beamed emission from the jet and acts as a rapidly inflating balloon, a spherical piston (actually the cocoon is an elliptical shaped region) that drives shocks into the surrounding gas whenever the supermassive black hole at the center of M87 undergoes a significant outburst.
The left panel shows the hard X-ray band image and the right panel is the 2 cm VLA image with the two circles drawn to scale. The X-ray image shows the clear shock ring that was driven by the relativistic particles in the radio piston. The outburst that drove the X-ray shock occurred about 14 million years ago (see Next: Modelling the Shock for details). Hence, the cocoon we see now is no longer the one that powered the older outburst, but instead reflects more recent outflows from the supermassive black hole in M87.
The relationship between the radio cocoon, the overpressured region seen in the 3.5-7.5 keV energy band (circle in the figure above), and the cooler gas is best seen in the 1.2-2.5 keV band Chandra X-ray image below. The two inner contours (magenta) show the 6 cm VLA synchrotron emission which represents the inner cocoon (the ``piston'' of relativistic plasma that mediates the energy output of the SMBH and drives the shocks into the atmosphere of M87). The outer contour (cyan) is derived from the $0.6'$ high pressure region seen in the hard band image (circle in the previous image and the region of enhanced pressure marked with the black arrow in Fig. 2 in the discussion of the classical shock) and represents the over-pressurized gas, that is being driven by the ``piston'' during the current outburst. The figure shows the cool rim (labeled ``Cool Rim'') that almost completely surrounds the over-pressurized region. The cool rim is gas that has been displaced by the piston as it has expanded into M87's atmosphere. Also marked are the bud (arrow denotes its outer cool shell) and the base of the southwestern arm, part of the filamentary structure.
The figure also shows the cool gas that surrounds the piston. Cool rims, surrounding plasma filled, radio emitting cavities, are commonly seen in the X-ray images. As we discuss in the section where we model the X-ray shock , the cool rims imply that the outbursts must last a few million years.