* Astronomy

J170902+641728 is a highly unusual galaxy. In ground-based optical images, it is unresolved (less than 1\arcsec) and thus appears to be a fairly faint star. However, sensitive radio continuum observations of the galaxy cluster Abell 2255 associated it with a radio source. Although this would suggest the object is a quasar, its redshift (z=0.0896) derived from its optical spectrum reveals it to lie at the high end of A2255's velocity distribution. Decomposition of the spectrum reveals the presence of a broad line region and an implied black hole mass of ~5 x 105 solar masses. Similarly, higher resolution radio observations made with the VLA+PT indicate the source is unresolved down to a linear size scale of under 100 pc. The presence of a moderately strong AGN in a compact and low mass galaxy makes J170902+641728 an intriguing object.


Astronomers using the National Science Foundation's Very Large Array (VLA) radio telescope and optical telescopes at the Apache Point Observatory have found a relatively small galaxy, dubbed J170902+641728, whose central black-hole pouring out energy at a rate equal to that of much larger galaxies. J170902+641728 is part of a cluster of galaxies that is more than a billion light-years from Earth.

In visible-light images, the galaxy is swamped by the glare from the bright central region, but those images place strong limits on the galaxy's size.

"This thing looks like a quasar in VLA images, but quasars come in big galaxies, not little ones like this" - Neal Miller, astronomer with the National Radio Astronomy Observatory.

Miller and Kurt Anderson of New Mexico State University presented their findings to the American Astronomical Society's meeting in Washington, DC.
Most galaxies have black holes at their centres. The black hole, a concentration of mass whose gravity is so strong that not even light can escape it, can draw material into itself from the surrounding galaxy. If the black hole has gas or stars to "eat," that process generates large amounts of energy as the infalling gas is compressed and heated to high temperatures. This usually is seen in young galaxies, massive galaxies, or in galaxies that have experienced close encounters with companions, stirring up the material and sending it close enough to the black hole to be gobbled up.
The black hole in J170902+641728 is about a million times more massive than the Sun. Images show that the galaxy can be no larger than about 2,000 light-years across. Our Milky Way Galaxy is about 100,000 light-years across.

"There are other galaxies that are likely to be the same size as this one that have black holes of similar mass. However, their black holes are quiet -- they're not putting out the large amounts of energy we see in this one. We're left to wonder just why this one is so active" - Neal Miller.

Answering that question may help astronomers better understand how galaxies and their central black holes are formed.


Position(2000): RA: 17:12:30.30, Dec: 64:01:49.80
Abell 2255

"This galaxy is a rare find -- a tiny galaxy that is still building up the mass of its black hole. It's exciting to find an object that can help us understand this important aspect of galaxy evolution" - Neal Miller.

J170902+641728 was observed with the VLA, with the 3.5-meter telescope at Apache Point Observatory, and with the Sloan Digital Sky Survey telescope at Apache Point. All these telescopes are in New Mexico.

Adapted from source