M61 is a grand design spiral and one of the brightest and most intensively studied barred galaxies in the Virgo cluster. It is located along the outer edge of the Virgo galaxy cluster. Star formation occurrs at a high level in M61 indicated by the numerous HII regions and high number of supernovae (4) observed in the last century. Barred galaxies comprise more than 50% of all disk galaxies and the percentage rises when near infrared images are used for classification. Like other very bright spirals M61 shows considerable energy outflow from its central region. The energy output is particularly abundant at X-ray wavelengths with 83% of its total x-ray emission arising from its nuclear region. The origin of this X-ray emission has two likely sources. The dominant source is a nuclear starburst ring composed of young stellar populations arranged in a ring some 730 light years from the nucleus. The stars are massive hot stars with an age range between 5 and 25 million years old. The starburst ring shows a particularly dense concentration of star formation near the ends of the central bar which has driven the formation of numerous HII regions. The numerous HII regions located at the tips of the bar are easily visible in the image. Central bars are known to cause an inflow of gas from the disk into the more central regions triggering the formation of starburst rings. Astronomers also have discovered that the corotation of a bar and disk together creates forces which drive gas accumulation to the tips of the bar where bursts of star formation preferentially occur.
The starburst ring does not explain the
entire radiation output from the nucleus of M61. M61 is a luminous
galaxy and like other highly luminous galaxies an active galactic
nucleus (AGN) is the central engine which produces the light and
radiation output. The energy source of active galactic nuclei
is believed to originate in mass accretion by a supermassive black
hole within the nucleus of the galaxy. Supermassive black holes
are now believed to be ubiquitous in the nuclei of normal galaxies.
M61 is a unique galaxy because it is both a starburst and a low
luminosity AGN or Seyfert 2 type. These two phenomena may not
just coexist in galaxies like M61 but may actually be mutually
related and dependent. M61 has two companion galaxies, NGC 4292
and NGC 4303A that most likely have interacted with it during
its immense lifetime.