NGC 4565 is one of the most impressive of all edge-on spirals. Its symmetry and graceful form is striking and has granted it an iconic status among galaxies. At an inclination of 86 degrees, the impressive edge-on disk is one of the largest and most massive of the relatively nearby spiral galaxies. It has been extensively investigated at different wavelengths and its many similarities to the Milky Way are well established. Its size (over 100,000 light years), mass (200 billion suns), number of globular clusters (200) and its rotational velocity are all remarkably similar to the Milky Way. Although the nature of its prominent peanut shaped central bulge is uncertain, evidence points to a barred structure with the long axis pointing in our line of sight, another similarity with the Milky Way, which is believed to also possess a barred nucleus.
A peculiar feature of NGC 4565 is the apparent warping of its outer disk edges particularly the northwest arm which varies from the equatorial disk plane by some 7800 light years at its edge. Warping is a feature more easily observed with edge-on galaxies but surprisingly present in about 50% of all spirals including the Milky Way. Although its cause is uncertain, galactic disk warping may be related to gravitational interactions with nearby companions. The warping of the disk of NGC 4565 may be due to an encounter with its companion NGC 4562 some 300 million years ago.
Recent observations at X-ray wavelengths revealed two powerful x-ray sources arising from the core region of NGC 4565. One of the sources appears to coincide with the galactic nucleus and is consistent with a low luminosity active galactic nucleus (AGN). An AGN produces x-rays and other forms of energy through an accreting supermassive black hole at the center of the galaxy. The other source is located in a faint globular cluster at the outer edge of NGC 4565's bulge.
The edge on orientation of NGC 4565 allows astronomers a good look at the nature of its disk structure. The "thin" disk is 3300 light years thick and constitutes the dust, gas, and young stellar populations of the spiral arms. A brighter "thick" disk is noted to surround the thin disk and is believed to be populated by an older subset of population 1 type stars which have migrated from the thin disk and extend out some 10,000 light years from the galactic plane. Even further out in the perpendicular plane is a dim structure called the galactic corona. The nature of coronal light is uncertain but some have questioned whether it represents a dim optical counterpart to the galactic halo. The halo of a spiral galaxy is a spherical structure that surrounds the disk and central bulge. The galactic halo consists of older population II stars including almost all of the globular clusters belonging to the galaxy. The nature of the halo is an enigma for the following reason. The rotational velocity of most spiral galaxies is unchanged out to the furthest radii although the luminosity falls off dramatically. Because of the physics of rotation, an unexplained, and unseen dark form of matter must exist in the galactic halo to support the rapid velocity of the outer disk components. Without such an entity the disk could not support such a high velocity in its outer regions and the galaxy would literally fly apart. The mysterious "dark" matter" is believed to constitute up to 90% of the total matter of the galaxy. Its nature is unknown and constitutes a vexing problem in astronomy today.