Messier 82

**Messier 82** (also known as **NGC 3034**, **Cigar Galaxy** or **M82**) is a starburst galaxy approximately 12 million light-years away in the constellation Ursa Major. A member of the M81 Group, it is about five times more luminous than the whole Milky Way and has a center one hundred times more luminous than our galaxy's center. The starburst activity is thought to have been triggered by interaction with neighboring galaxy M81. As the closest starburst galaxy to Earth, M82 is the prototypical example of this galaxy type. SN 2014J, a type Ia supernova, was discovered in the galaxy on 21 January 2014. In 2014, in studying M82, scientists discovered the brightest pulsar yet known, designated M82 X-2.

## Discovery

M82 was first discovered by Johann Elert Bode on 31 December 1774 together with M81; he described it as a "nebulous patch", about 0.75 deg away from M81, "very pale and of elongated shape". In 1779, Pierre Méchain independently rediscovered both galaxies and reported them to Charles Messier, who added them to his catalog.

## Structure

M82 was previously believed to be an irregular galaxy. In 2005, however, two symmetric spiral arms were discovered in near-infrared (NIR) images of M82. The arms were detected by subtracting an axisymmetric exponential disk from the NIR images. Even though the arms were detected in NIR images, they are bluer than the disk. The arms were previously missed due to M82's high disk surface brightness, the nearly edge-on view of this galaxy (~80°), and obscuration by a complex network of dusty filaments in its optical images. These arms emanate from the ends of the NIR bar and can be followed for the length of 3 disc scales. Assuming that the northern part of M82 is nearer to us, as most of the literature does, the observed sense of rotation implies trailing arms.

### Starburst region

In 2005, the Hubble Space Telescope revealed 197 young massive clusters in the starburst core. The average mass of these clusters is around 200,000 solar masses, hence the starburst core is a very energetic and high-density environment. Throughout the galaxy's center, young stars are being born 10 times faster than they are inside the entire Milky Way Galaxy.

In the core of M82, the active starburst region spans a diameter of 500 pc. Four high surface brightness regions or clumps (designated A, C, D, and E) are detectable in this region at visible wavelengths. These clumps correspond to known sources at X-ray, infrared, and radio frequencies. Consequently, they are thought to be the least obscured starburst clusters from our vantage point. M82's unique bipolar outflow (or 'superwind') appears to be concentrated on clumps A and C and is fueled by energy released by supernovae within the clumps which occur at a rate of about one every ten years.

The Chandra X-ray Observatory detected fluctuating X-ray emissions from a location approximately 600 light-years away from the center of M82. Astronomers have postulated that this fluctuating emission comes from the first known intermediate-mass black hole, of roughly 200 to 5000 solar masses. M82, like most galaxies, hosts a supermassive black hole at its center with a mass of approximately 3 x 107 solar masses as measured from stellar dynamics.

### Unknown object

In April 2010, radio astronomers working at the Jodrell Bank Observatory of the University of Manchester in the UK reported an object in M82 that had started sending out radio waves, and whose emission did not look like anything seen anywhere in the universe before.

There have been several theories about the nature of this object, but currently no theory entirely fits the observed data. It has been suggested that the object could be an unusual "micro quasar", having very high radio luminosity yet low X-ray luminosity, and being fairly stable, it could be an analogue of the low X-ray luminosity galactic microquasar SS 433. However, all known microquasars produce large quantities of X-rays, whereas the object's X-ray flux is below the measurement threshold. The object is located at several arcseconds from the center of M82 which makes it unlikely to be associated with a supermassive black hole. It has an apparent superluminal motion of four times the speed of light relative to the galaxy center. Apparent superluminal motion is consistent with relativistic jets in massive black holes and does not indicate that the source itself is moving above lightspeed.