What might our Milky Way look like across the vastness of 50 million light years? Because of striking similarities to our own galaxy, NGC 7331 offers us an unusual outside perspective of our galaxy. Recent information gained by the Spitzer Infrared Galaxy Survey revealed great similarities in the distribution of stars, overall mass, spiral pattern, and the likely presence of a similar monstrous black hole at the core of the two galaxies. A curious ring of molecular gas coupled with newly formed stars extends outward some 20,000 light years from the core. The ring is not visible in optical images but is strongly apparent at infrared wavelengths which show the myriad of young stars. The huge reservoir of gas and dust associated with the ring-like structure contains the raw materials to produce another 4 billion suns. Curiously gas appears to be depleted inside the molecular ring. One possibility is that the gas was channeled into an accretion disk feeding an immense black hole at the galaxy's center. Alternatively the gas may have been consumed during a period of very active star formation in the nuclear region of the galaxy.
NGC 7331 has been an intriguing subject for learning about galactic structure. Although knowledge of its complex structure has become more complete, consensus among astronomers has been elusive. It has been proposed that the galaxy harbors a black hole of some 500 million solar masses related to a Seyfert type AGN but this has not been proven beyond doubt. A peculiar feature of the galaxy is a counter-rotating inner stellar disk component with a radius of 1300 to 5000 light years. To make matters more complicated the galaxy possesses a decoupled inner disk-bulge component within the central 650 light years where ionized gas is rotating faster than the rest of the surrounding central bulge. The stellar population of the circumnuclear stellar-gaseous disk is also relatively young compared to the normal older bulge population with stars that are about 2 billion years old. Counter-rotating stellar systems have been detected in other galaxies such as M104 and NGC 2841. The most likely scenario accounting for the retrograde disk component and the decoupled gas is a large scale merger in the remote past where the aberrantly rotating system is genetically related to the merged galaxy.
NGC 7331 is steeply inclined at 77 degrees.
The galaxy shows a distinct warp both optically and in it gaseous
infrastructure at radio wavelengths. Although NGC 7331 seems to
be an isolated galaxy without companions at present it does belong
to the same group as NGC 7320 which is projected among the much
more distant grouping known as Stephan's Quintet. At a projected
separation of over 300,000 light years the two galaxies most likely
have had no previous encounters. Visible in the image are three
distant background galaxies (NGC7336, 7335, 7337). They are approximately
10 times more distant than NGC 7331 and lie fortuitously in the
same line of site.