A unique galaxy, which holds clues to the evolution of galaxies billions of years ago, has now been discovered by an Indian-led international team of astronomers. The discovery, which will enable scientists to unearth new aspects about the formation of galaxies in the early universe, has been made using the Giant Meterwave Radio Telescope (GMRT) of the National Centre for Radio Astrophysics, Tata Institute of Fundamental Research (NCRA-TIFR). CREDIT: Hota et al., SDSS, NCRA-TIFR, NRAO/AUI/NSF
Its catalog number is NGC 3801, but its name is SPECA - a Spiral-host Episodic radio galaxy tracing Cluster Accretion. That's certainly a mouthful of words for this unusual galaxy, but there's a lot more going on here than just its name. “This is probably the most exotic galaxy with a black hole, ever seen. It has the potential to teach us new lessons about how galaxies and clusters of galaxies formed in the early Universe,” said Ananda Hota, of the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), in Taiwan and who discovered this exotic galaxy. "We find this merger-remnant early-type galaxy to have an intriguing spiral-wisp of young star forming regions."
Located about 1.7 billion light-years from Earth, NGC 3801 is a radio source that contains a central supermassive black hole. As we have learned, galaxies of this type produce relativistic "jets" which are responsible for the radio frequencies, but that's not all they create. While radio galaxies are generally elliptical, Speca is a spiral - one which was probably created in a merger event. As the relativistic jets surge with time, they create lobes of sub-atomic material at the outer edges which fan out as the material slows down... and Speca is one of only two galaxies so far discovered to show this type of recurrent jet activity. Normally it might occur twice within formation, but here it has happened three times! Are we looking at the beginning phase of a black hole jet?
“Both elliptical and spiral galaxies have black holes, but Speca and another galaxy have been seen to produce large jets. It is also one of only two galaxies to show that such activity occurred in three separate episodes." explains Sandeep Sirothia of NCRA-TIFR. "The reason behind this on-off activity of the black hole to produce jets is unknown. Such activities have not been reported earlier in spiral galaxies, which makes this new galaxy unique. It will help us learn new theories or change existing ones. We are now following the object and trying to analyse the activities."
Dr. Hota and an international team of scientists reached their first conclusions while studying combined data from the visible-light Sloan Digital Sky Survey (SDSS) and the FIRST survey done with the Very Large Array (VLA) radio telescope. Here they discovered an unusually high rate of star formation where there should be none and they then confirmed their findings with ultraviolet data from NASA's GALEX space telescope. Then the team dug even deeper with radio information obtained from the NRAO VLA Sky Survey (NVSS). Was it possible that left-over plasma was the culprit responsible for these active regions where there should be none? At several hundred million years old, these outer lobes should be beyond their reproductive years... Yet, that wasn't all. GMRT images displayed yet another, tiny lobe located just outside the stars at the edge of NGC 3801 - in plasma that is just a few million years old.
"We think these old, relic lobes have been 're-lighted' by shock waves from rapidly-moving material falling into the cluster of galaxies as the cluster continues to accrete matter," said Ananda. "These jets are supposed to remove a large fraction of gas from a galaxy and stop further star formation. If the galaxy is gas-rich in the central region, and as the jet direction changes with time, it can have an adverse effect on the star formation history of a galaxy. All these phenomena combined in one galaxy make Speca and its neighbors a valuable laboratory for studying how galaxies and clusters evolved billions of years ago."
As you watch the above galaxy merger simulation created by Tiziana Di Matteo, Volker Springel, and Lars Hernquist, you are taking part in a visualization of two galaxies combining which both have central supermassive black holes and the gas distribution only. As they merge, you time travel over two billion years where the brightest hues indicate density while color denotes temperature. Speca may have once been part of such a scenario, as it is a member of a cluster of around sixty galaxies. In all our many studies, there have only been four recorded cases where x-ray observations have uncovered jet driven shocks heating the interstellar medium.
"Taking clues from dust/PAH, Hi and CO emission images we interpret NGC 3801 to have a kinamatically decoupled core or an extremely warped gas disk. From the HST data we also show evidence of ionised gas outflow similar to that observed in Hi and molecular gas (CO) data, which may have caused the decline of star formation leading to the red optical colour of the galaxy. However, from these panchromatic data we interpret that the expanding shock shells from the young radio jets are yet to reach the outer gaseous regions of the galaxy." explains Dr. Hota. "It seems, we observe this galaxy at a rare stage of its evolutionary sequence where post-merger star formation has already declined and new powerful jet feedback is about to affect the gaseous star forming outer disk within the next 10 million years to further transform it into a red-and-dead early-type galaxy."
The one million year old slice of plasma pizza has been re-heated and, in this case, the left-overs apparently taste as good as the original recipe. What should have been cold is hot - and there isn't much evidence to help explain exactly why. It could have been the handiwork of a cosmic shockwave when two galaxies combined, but there just isn't any evidence of that happening within the cluster region that harbors Speca. "The only other option, is cosmological cluster accretion shock." explains Dr. Hota. "After 10 years of the first discovery, this is the second spiral-host large radio galaxy. The cause behind why present day radio galaxies do not contain young star forming disk is not clear. This is likely to serve as a missing link, what is rare in the present day Universe and what may be a common phenomena in the distant past "
Written by Tammy Plotner. Original Research Paper: NGC 3801 caught in the act: A post-merger starforming early-type galaxy with AGN-jet feedback. For Further Reading: Various press releases and news on the discovery of Speca.