The Main Attraction

M106 (NGC 4258), at a distance of 3.5 million lightyears is a barred spiral galaxy and it is the largest galaxy in the Canes Venatici II group of galaxies. It also is the host of the nearest extragalactic astrophysical jet, sprouting from a supermassive black hole. The jet gives rise to two plume like gas streams that are very visible in Ha light (zoom to the full size image to inspect the core region). There is an underlying spiral form of hot young stars, which are visible underneath the Ha filaments in the core region and especially as a blue ring in the outer parts of the galaxy.

The black hole inside

In the vicinity of the core of the galaxy, a number of water masers are present (masers show very high surface brightness in a very concentrated area. The emission from a maser is due to stimulation of atoms by radiation and the subsequent re-radiation whose frequency corresponds to the energy difference between two quantum mechanical energy levels, resulting in monochromatic radiation in all directions. This makes it possible to measure both their position and velocities). These masers have been studied with Very Long Baseline Interferometry for a number of years. From these VLBI position and velocity measurements an application of Kepler law for rotation in the gravitational field of a point mass infers a central mass of 35*million solar mass within a radius of not more than 0,8 lightyear, indicating a supermassive black hole.

The jet

M106 hosts the nearest extragalactic astrophysical jet. The physics of jets is not yet fully understood but they likely arise from dynamic interactions possibly as a result of entanglement of magnetic fields within the rapidly spinning accretion disks (in this case around the massive black hole in the center), creating two diametrically opposing beams away from the central source.

The anomalous arms

Two prominent arms emanate from the bright nucleus and spiral outward. These arms are dominated by young, bright stars, which light up the gas within the arms. However, on top of that two additional faint anomalous arms (filaments of gas) are visible. These arms are hardly visible in RGB images, but do become visible in Ha (that is how they were first discovered) and are dominant in radio and x-ray images. These anomalous arms consist not of stars, but of gas. The existence of these arms and the radiation coming from them is thought to be resulting from shock waves caused by the interaction of the gas in the central part of the galaxy with the jets, heating up the gas. Gas further out is not more loosely gravitationally bound and is blown from the plane of the disk of the galaxy as can be seen in this image. There is a nice artist impression animation of this phenomenon here: Spacetelescope.org

Sources:

Extra Galactic Astronomy and Cosmology, an introduction 2nd edition, by Peter Schneider

Annals of the Deep Sky, Vol 3, by Jeff Kanipe and Denis Webb