Dark cloud discovery:  Overlay of MeerKAT 21cm neutral hydrogen gas on a deep optical image.

An international team of astronomers led by Gyula Józsa, and Michelle Cluver and Professor Thomas Jarrett from the Department of Astronomy at UCT, has utilized the South African MeerKAT telescope to discover a mysterious chain of hydrogen gas clouds the size of a massive galaxy. Indeed, the accumulation of so much elemental hydrogen without associated stellar components is the largest yet discovered. Appearing at the edge of a relatively massive group of galaxies, there is the possibility that the cloud chain is gas stripped from group-member galaxies, but it may also be primordial and gravitationally drawn into the group through a cosmic filament pathway. Whatever the case, MeerKAT is proving to be a ground-breaking telescope, and this "dark" cloud discovery should soon be followed by many such discoveries in the exciting days ahead.

Where are the stars?

Professor Tom Jarrett comments on the discovery: " It is very puzzling to see such a massive and large gas cloud without a "host" galaxy, or simply, where are the stars?   The hydrogen is the fuel for star formation, and so where you have a lot of hydrogen, amassed into a dense object, you have stars (or better, a galaxy of stars).   So it is quite a discovery.   Other "dark" clouds have been seen of course, going back decades, but nothing this large or concentrated like a big galaxy.   What I think is that these things exist, perhaps in large numbers, but it required a new and amazing telescope -- MeerKAT -- to finally see them.

What is next for us is to figure out how this cloud came to be, and where it is going, its fate if you will. It could be the detritus from a titanic collision between two galaxies, stripping and separating the gas from the stars.  But we really don't see the progenitors, the two (or however many) galaxies that did this.  They could be there, just hiding somehow.    Alternatively, it could be more pristine (primordial) gas that has been flowing through the filament of the cosmic web, into the "attractor" that it appears to be aimed.  This gravitational attractor is a massive galaxy group (many galaxies, bound together to form a group).  We need deeper MeerKAT observations, and a deeper optical imaging to dig down into the fainter stuff to see if we can discern any gas or star "trails" that point to a past tidal disturbance."

How long have they been working on this:  The project started 5 years ago, in 2016, with investigation of a particular region of the sky called G23 (GAMA Field 23),  in which we have optical redshifts and infrared imaging. We have been working on understanding the galaxy distribution and evolution (papers published).   We then proposed MeerKAT observations (in 2019) to go after a cosmic filament in our field, a very interesting large scale object in its own right, worthy of additional study. These observations were proposed by Prof. Michelle Cluver (Swinburne University), who is a proud and awesome graduate of UCT (both undergrad, and PhD).   Once the data were reduced, we started the analysis.  

Strange Looking object  - blobby - observed

With regard to the discovery, Prof Jarrett describes, "One day I was looking at the data - the data cube that is - using our new VR system developed by the IDIA/UCT-Astro Visualisation Laboratory (of which Prof Jarrett is the co-director), and by golly I noticed this strange looking object in the hydrogen gas emission.   One thing led to another as we dived into deep imaging from our data repository arsonal (note G23 project, we are well equipped to study this area), and we realized this thing (we called it the "fuzzy blob" or just "blobby"), did not have a counterpart in the UV, optical and infrared.  That is, no host galaxy.   It was free floating !!  Well my first worry was that it was an artifact, a bogey. But we tracked everything down, ruled out all sorts of issues that could be in play.   We convinced ourselves. Real.

And then one day we were looking at another data archive, of a survey done 20 years ago using a single dish antenna (the famous "The Dish" telescope in Parkes, Australia),  and we could see our fuzzy blob in their data (only a a spectrum, not an object with structure).  It's in our paper;  this was a great thing because now we are 100% sure it is REAL!  As Prof Jarrett comments: "this is the good stuff - Research is fun!  However, we are not done, we have to do deeper observations to really see what may be going on."

MeerKAT - a game-changer for Astronomy

MeerKAT is just incredible, what a game changer for radio astronomy.  It is sensitive, it is wide, and it has great angular resolution. All three are BIG improvements over the previous generation.  And MeerKAT is the precursor to the SKA-Mid, which is coming in the near future.  What a huge success MeerKAT is proving to be.  Well done by the engineering and science (commissioning) staff that has made this possible.  Incredible stuff really.  Prof Jarrett is a big fan of MeerKAT.

The surprise of the serendipitous discovery

Prof Jarrett says, "Our original plan was to study the galaxy population in G23 and the specificial filament (large scale structure).  We expected to see lots of galaxies, and we did.  This project is going well.  We have two students at UCT working on it - one has just finished his PhD using some of this data, and the other is working on another mysterious object in our data !!  But in the course of doing something routine, with a new and sensitive telescope, we made this serendipitous discovery.  It was not something we were looking for, but in hindsight, with the new phase space (from the MeerKAT sensitivity and angular resolution), this sort of thing can happen.  So the surprise of finding such a strange object, but also at the stage of not knowing the origin, evolution and fate of this object, was fascinating to me. I guess I'm surprised that we still don't know what it is - we have so much data and brilliant people looking at it, including colleagues all over the world (this is a BIG international project, one only needs to look at the author list of the paper that has just been published),   yet we are in the early stages of understanding this thing, and class of objects it may belong to. 

Implications for future findings

This discovery has huge implications because we found this thing relatively easily and we think these must be more common.  It may be a completely new type of object, and that always leads to new understanding of nature, or it might be a new class of object, common but not seen (much, if at all) until now, that will also lead to new insights with galaxy evolution.  "Hydrogen is the fuel with which to light the fire of star formation" says Prof Jarrett.  Galaxies are built from hydrogen and we have found a lot of hydrogen in this mysterious object.

Watch this space....

To read the full paper just published click here 

Story:  Professor Tom Jarrett