BACKGROUND INFORMATION: GETTING TO THE HEART OF A GALAXY SMASHUP
Probing the nucleus of the peculiar galaxy called Arp 220 the Hubble Space Telescope's Near Infrared Camera and Multi-Object Spectrometer (NICMOS) has discovered a 300-light-year diameter dust disk and other remarkably complex structure. The infrared camera also clearly shows the galaxy's unique "double nucleus" of two bright compact star clusters about 1,200 light-years apart and each containing a billion stars. These gigantic clusters may be the remnant cores of the two spiral galaxies before they collided.
Scientists were intrigued to find a vast disk of entrapped dust which appears in the NICMOS images silhouetted against one of the twin star clusters at the nucleus. The clue is that the cluster appears as a quarter moon, rather than spherical, due to obscuration of the bottom half of the cluster by the opaque disk. The new images clearly indicate that this disk is embedded within the cluster.
The gas in this disk probably fuels the formation of young, very bright stars, thereby accounting for the observed high star formation rate, or "starburst" within the galaxy. The star formation rate in the heart of Arp 220 is probably 100 times that of the entire Milky Way, yet concentrated at 100 times smaller a radius.
The infrared camera also pinpoints the dramatic results of this starburst – at least 12, possibly young globular star clusters which are dense collections of a few hundred thousand to a few million stars into a spherical region of space a few dozen light-years across.
Arp 220 is an ideal target for NICMOS because the galaxy is so heavily enshrouded in dust that only 5% of the energy escapes as visible light, the other 95% is in infrared where the galaxy nucleus shines brighter than 100 Milky Ways.
During this particularly spectacular galactic collision which probably occurred several hundred million years ago, the interstellar matter (gas and dust) normally distributed over a disk 30,000 light-years in radius got 'kicked' up and dumped into the nucleus of the galaxy. As a result, the nucleus has become heavily obscured by dust.
With the infrared images obtained using NICMOS, astronomers can see through most of the dust, into the heart of the merging galaxy system and uncover the cores of the original two galaxies and the firestorm of starbirth activity resulting from the great accumulation of interstellar gas in the central regions.
One of the nearest of a special class of galaxies called ultraluminous infrared galaxies, Arp 220 offers a unique laboratory for studying the details of galaxy collisions and galaxy growth through merging – processes which were more common in the early, formative years of galaxy evolution. The galaxy is the 220th object in Halton Arp's Atlas of Peculiar Galaxies and is 250 million light-years away in the constellation Serpens.
These ultraluminous infrared galaxies have attracted much interest over the last decade due to the growing recognition that they probably are objects along an evolutionary path to forming quasars (the most luminous objects in the universe). Within the quasar galaxy nuclei, it is believed that the energy release is due to gas falling into a central black hole with mass approximately 100 million times the mass of the sun. The accretion disk now seen for the first time in Arp 220 may be the first stage in building such a massive black hole.
Disks have long been hypothesized as a mechanism for feeding matter from the larger galactic disk into a black hole having a radius of less than one light-year. In Arp 220, astronomers may well be witnessing the growth of such a black hole. This conclusion is bolstered by radio observations showing several billion solar masses of interstellar gas and dust are within the central 1,000 light-years of Arp 220.
