GMRT Discovers an Exotic Galaxy Revealing Tantalizing Tale

A galaxy with a combination of characteristics never seen before is giving astronomers a tantalizing peek at processes they believe played key roles in the growth of galaxies and clusters of galaxies early in the history of the Universe. 

Giant Meterwave Radio Telescope (GMRT) is the largest and the most powerful radio telescope in the world in the radio frequency range of 150-1500 MHz. Using GMRT and various other telescopes, an international team of astronomers, lead by Ananda Hota, have discovered an enigmatic object in the sky which 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 and evolved into what we see today," said Dr. Ananda Hota, currently working in the Academia Sinica Institute of Astronomy and Astrophysics (ASIAA), Taiwan, who discovered this exotic galaxy.  

Dr. Hota first surmised one of the possible peculiarity of this exotic new galaxy, dubbed Speca (an acronym for Spiral-host Episodic radio galaxy tracing Cluster Accretion) in an image that combined data from the visible-light Sloan Digital Sky Survey (SDSS) and the FIRST survey done with the Very Large Array (VLA) radio telescope, both in the USA. Ultraviolet data from NASA's GALEX space telescope confirmed that the galaxy has unusually high rate of forming young stars. It is only the second spiral, as opposed to elliptical, galaxy known to produce large, powerful jets of subatomic particles moving at nearly the speed of light. These several million light year long jets, produced by massive black holes located at the centre of galaxies, are prominently visible only to radio telescopes.   

Looking at the low resolution image from the NRAO VLA Sky Survey (NVSS) radio data it was noticed two nearby regions of emission, unseen in the FIRST data. It was suspected that it could be the left-over plasma from an earlier episode of black holes jet activity. For this deeper 'archeological excavation' of black holes past jet activity, low frequency radio observation is needed, and GMRT is the best. Indeed, the high resolution GMRT images at radio waves of wavelength nearly one meter, confirmed that these two outermost regions of radio emission are irregular in shape and are relic radio plasma. This plasma is several hundred million years old, since when it has been ejected by the black hole, at the centre of the galaxy. The GMRT images showed yet another tiny pair of radio lobes just outside the stars of the galaxy. This plasma, in contrast, is only a few million years old. Thus, it turned out to be only the second case that such jet-lobe activity occurred in three separate episodes.   

Dr. Sandeep K. Sirothia of NCRA says, ``The ongoing TIFR GMRT Sky Survey (TGSS available on the web at http://tgss.ncra.tifr.res.in) is going to discover more Speca-like galaxies and Dr. Hota's discovery is a potential demonstrator."   

The real reason behind this on-off jetting out act of the black holes or why they stop acreting gas and stars around them for several million years and start again, is still unknown. Dr. Chiranjib Konar, of ASIAA, states, ``This kind of reborn radio galaxies are the laboratories to understand many puzzling aspects of black holes."  

Dr. Hota got the biggest surprise, when the brightness of the outer relic lobes in GMRT and NVSS images were compared. Such old plasma, nearly 1 million light year in size, should have been very cool, but it has been heated up (re-accelerated) by some cosmic shock wave. That how cold it is would have helped determine how old is it, since its creation. But it has been re-heated, clock has been re-set or rotated back making it difficult to estimate how old it is. While hunting for a clue to what has happened, in the SDSS data, a cluster of about sixty galaxies around Speca was found. But no evidence of two clusters of galaxies colliding to produce the required shock wave. ``The only other option, is cosmological cluster accretion shock" explains Dr. Hota. When clusters of galaxies grow by gravitational infall of matter from huge filaments of the cosmic-web, supersonic infall motion would cause shock waves, predicted by cosmologists over two decades ago but almost no evidence yet. Speca shows the first such evidence in the 're-heated' property of the relic radio lobes. Black holes control the matter acretion, by a so called ``feedback process" pushing away infalling gas by jets. This jet-driven feedback process is responsible for transforming gas-rich spiral galaxies into gas-poor elliptical galaxies when two spirals merge. Speca is now an excellent new laboratory for studying galaxy evolution. Its odd natures are like a 'missing-link', not seen in the present day Universe but expected to be normal in the early Universe.   

Padmashri Prof. Govind Swarup, an FRS, who is considered the 'father' of Indian Radio astronomy and the chief architect of GMRT, described the finding as ``an outstanding discovery that is very important for cluster formation models and highlights the importance of sensitive observations at meter wavelengths provided by the GMRT." Finding signatures of more such cases of cosmological shock-waves of cluster merger and accretion from the cosmic-web by evolving clusters, at higher redshift, will be like finding 'proto-clusters'.   

Dr. Hota has also been following up the target galaxy with Dr. C. S. Stalin from Indian Institute of Astrophysics, Bangalore. They are using the Himalayan Chandra Telescope to locate galaxy filaments of the cosmic-web.  

Ananda Hota is an alumini of Indian Institute of Science (IISc), Bagnalore and worked with the GMRT at the National Centre for Radio Astrophysics of the Tata Institute of Fundamental Research (NCRA-TIFR), Pune. In addition to Dr. Hota, Dr. Sirothia and Dr. Konar, the discovery team includes: Dr. Youichi Ohyama, and Dr. Satoki Matsushita of ASIAA; Mr. Suk Kim and Prof. Soo-Chang Rey of Chungnam National University in Korea; Prof. D.J. Saikia of NCRA and Dr. Judith H. Croston of the University of Southampton in England. The scientists published their findings in the Letters of the Monthly Notices of the Royal Astronomical Society, England.   

More details of this tantalizing discovery was first press-released by NRAO, USA at www.nrao.edu/pr/2011/spiralradio/ and then by Royal Astronomical Society, UK, at www.ras.org.uk/news-and-press/217-news2011/2003-exotic-galaxy-reveals-tantalising-tale , and various other websites searchable by Google.   

The GMRT is built and operated by National Centre for Radio Astrophysics, Tata Institute of Fundamental Research. The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc., USA.   

Contact: Dr. Ananda Hota (hotaananda@gmail.com, +886-98737-9694)