Image of the Day: ARP 147 Colliding Galaxies –Fuel Growth of Supermassive Black Holes



The above image of Arp galaxies 147 are a perfect example galaxy mergers or even close interactions that trigger bursts of star formation. But exactly how a collision produces new stars is not clear: sometimes star formation activity is seen all across an interacting galaxy, but in other cases it is localized to a region around the galactic nucleus.  

Galaxy mergers and close interactions are also thought to drive the growth of central supermassive black holes and their high-energy activity. The collision prompts material to fall into the massive central region, stimulating the black hole to become what is called an active galactic nucleus (AGN). Sorting out these two effects of a merger is a key issue in modern astronomy, and is associated with determining if and how star burst activity is connected to the growth of the central black hole.

Harvard-Smithsonian Center for Astrophysics astronomers Xin Liu and Yue Shen, together with a colleague, studied a set of 1286 pairs of AGN, presumably interacting with one another, that were observed optically in a large, systematic survey of galaxies.

They used the strength of ionized oxygen and hydrogen emission lines to quantify the black hole and/or star formation activity in these sources, and then compared the results to strengths seen in single AGN that are not interacting. They report that tidal interactions enhance both black hole accretion and star formation, especially in closer pairs.

They also find that visible tidal features can be a misleading measure of an interaction – some AGN pairs may be interacting yet show no tidal features, perhaps because of the galaxy composition or details of the collision.

One of the best examples in the Universe is Arp 147 (image at top and bottow of page), a pair of interacting galaxies located about 430 million light years from Earth. The iamge  shows X-rays from the NASA's Chandra X-ray Observatory (pink) and optical data from the Hubble Space Telescope (red, green, blue) produced by the Space Telescope Science Institute.

Arp 147 contains the remnant of a spiral galaxy (right) that collided with the elliptical galaxy on the left. This collision has produced an expanding wave of star formation that shows up as a blue ring containing in abundance of massive young stars. These stars race through their evolution in a few million years or less and explode as supernovas, leaving behind neutron stars and black holes.

A fraction of the neutron stars and black holes will have companion stars, and may become bright X-ray sources as they pull in matter from their companions. The nine X-ray sources scattered around the ring in Arp 147 are so bright that they must be black holes, with masses that are likely ten to twenty times that of the Sun.

An X-ray source is also detected in the nucleus of the red galaxy on the left and may be powered by a poorly-fed supermassive black hole. This source is not obvious in the composite image but can easily be seen in the X-ray image. Other objects unrelated to Arp 147 are also visible: a foreground star in the lower left of the image and a background quasar as the pink source above and to the left of the red galaxy.

Infrared observations with NASA's Spitzer Space Telescope and ultraviolet observations with NASA's Galaxy Evolution Explorer (GALEX) have allowed estimates of the rate of star formation in the ring. These estimates, combined with the use of models for the evolution of binary stars have allowed the authors to conclude that the most intense star formation may have ended some 15 million years ago, in Earth's time frame.



The Daily Galaxy via Harvard-Smithsonian Center for Astrophysics and chandra.harvard

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