Serendipitous astronomy

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Within the constellation of Ursa Major, about 134 million light years away, an almighty collision is occurring between two galaxies. As the clouds of gas and dust are swirled together an intense burst of star formation is triggered, but is that all that’s been awoken? Has this galactic merger also provided a hidden central black hole with enough matter to accrete to ignite it into an active nucleus (AGN)? This wasn’t the question that Miguel Perez-Torres and his team set out to answer. They were trying to characterise the compact sources within the inner region of Arp 299 (as the merging pair is known), which they were generally expecting to be supernovae and supernova remnants. This is what they found:

The top panel in this picture from their paper shows the inner region they observed, and all the white blobs are the compact sources they were looking for. What caught their interest though was the line of objects toward the top right of this image. This, they reasoned, wasn’t a chance alignment but could either be a chain of supernovae in a super star cluster approximately 500 years old, or a core and jet of a hidden AGN. Normally, if a galaxy (or pair of interacting galaxies) hosts an AGN its output dwarfs the light coming from all the stars in the galaxy combined. However, sometimes (and possibly more often than we realise) the AGN is weak and can only be seen if you really go looking for it.

Super star clusters are known to exist in Arp 299. However, finding four supernovae in one this size and age is very unlikely they reasoned, as it shouldn’t have housed enough massive stars for this to happen. So they had a closer look – the middle and bottom panels of the picture show their close ups of the top right region (indicated by white dotted lines). See how the blobs are linked in the bottom image? That, they concluded is a clear indication of an AGN core with accompanying radio jet. Interestingly, the object labelled A0 isn’t part of the AGN, but has all the characteristics of a young supernova, making it, as they note, “…one of the closest to a central supermassive black hole ever detected.” This could be the reason why Arp 299’s AGN is so weak – nearby massive stars heating their surroundings and dispelling the material that it would normally accrete.

All three of these are radio images, taken at a frequency of 5 GHz (top and middle) and 1.7 GHz (bottom) with an array of linked radio telescopes stretching from Shanghai to Cambridge . Radio is ideal for this investigation as it is not absorbed by the large amounts of dust enshrouding Arp 299’s nuclear region.

Just goes to show what you can find when you don’t go looking for it.

ResearchBlogging.orgPerez-Torres, Miguel A.; Alberdi, Antxon; Romero-Canizales, Cristina; Bondi, Marco (2010). Serendipitous discovery of the long-sought AGN in Arp 299-A Accepted for publication in Letters to Astronomy and Astrophysics : 1008.4466