Solving the Mystery of Extreme Galaxies


Arp 220, located 250 million light-years away in the constellation Serpens, is the closest galaxy to the Milky Way with an extreme luminosity, defined as being more than about 300 times that of our own galaxy. Some dramatic galaxies have values of luminosity ten times brighter still. Astronomers are still piecing together the reasons for these huge energy outputs, while sorting out why our own galaxy is so modest.

The Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) has uncovered a collision between two spiral galaxies in the heart of Arp 220. The collision has provided the spark for a burst of star formation.

The NICMOS image above and below capture bright knots of stars forming in the heart of the galaxy. The bright, crescent moon-shaped object is a remnant core of one of the colliding galaxies. The core is a cluster of 1 billion stars. 

The core's half-moon shape suggests that its bottom half is obscured by a disk of dust about 300 light-years across. This disk is embedded in the core and may be swirling around a black hole. The core of the other colliding galaxy is the bright round object to the left of the crescent moon-shaped object. Both cores are about 1,200 light-years apart and are orbiting each other.

The two primary suspects for the energetics are bursts of star formation that produce many hot young stars, and processes associated with accretion of material onto a supermassive black hole at a galaxy's nucleus. Arp 220 is the closest example, and one of the best places to probe these scenarios.

A team of astronomers have used the Submillimeter Array (SMA) to obtain the first unbiased galaxy survey of molecular and atomic lines using a telescope array. They covered a complete, large wavelength interval in the millimeter regime that is accessible through Earth's atmosphere. 

The team reports finding seventy-three spectral features from fifteen molecular species in this survey band. A remarkable 28% of the total flux from this galaxy in this band is emitted by these molecules. The SMA also obtains images of the galaxy at each of the many wavelengths.

The results are consistent with Arp 220's luminosity being driven primarily by star-formation. The chemistry of the galaxy derived from the observations also leads to this conclusion, with species normally enhanced by star formation clearly detected. Moreover, it appears one such burst of activity is currently underway. 

The astronomers estimate that several million regions of activity are localized within a relatively small volume (a few thousand light-years) around the nucleus. The new results are an important improvement in our understanding of what powers extreme galaxies, and how they differ from the Milky Way.

           Arp220 (1)

The Daily Galaxy via Harvard-Smithsonian Center for Astrophysics 

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