Thread by @cjriseley

Paper day! Yesterday in @RAS_Journals #MNRAS, today on @_arXiv_astro_ph! Among other things in the complex cluster Abell 3266, we found a radio halo. This is a discovery almost a decade in the making - buckle up, it's story time... 1/n

(For a link to the paper, see my Tweet yesterday...).
My work on A3266 starts back in the mists of my early PhD. The year was 2013 and I spent a month in Cape Town helping commission KAT-7 (the #MeerKAT precursor) at the @SKA_Africa CT office. 2/n

During that time, I worked on KAT-7 observations of Abell 3667, another cluster that I wrote up as my first first-author paper (10.1093/mnras/stu2591).
Abell 3266 came up as another target of interest - a merging cluster with only shallow radio data... 3/n

but a wealth of X-ray data. It had all the hallmarks of clusters showing a radio halo or relic(s), but none were known. I requested KAT-7 observations, processed the data when they came in, and used archival ATCA data to model the radio galaxy population... 4/n

There was leftover emission in the cluster centre. This could be a halo! But: there were leftovers elsewhere, suggesting the subtraction wasn't perfect... you can see this in my thesis (eprints.soton.ac.uk/405434/ Chapter 3.2). With no other data, I left it there and moved on... 5/n

I had other projects to occupy me, so that paper I started writing in 2014 or so never saw publication (by me anyway - Gianni Bernardi and collaborators used the same KAT-7 data and came to much the same conclusions around the same time: dx.doi.org/10.1093/mnras/…)... 6/n

Time went by, I wrapped up my thesis, and in late 2016 I joined @CSIRO_ATNF in Perth, where I started working with #ASKAP. The @mwatelescope GLEAM survey images were released not long later (huge kudos to @ColourfulCosmos and the GLEAM team)... 7/n

Comparing the GLEAM images of A3266 (panels 1-4) and my KAT-7 images (panels 5 & 6), two sources immediately stood out:
- the ultra-bright source to the north-west, invisible with KAT-7
- the extended source to the south-east, 1 Mpc from the cluster centre (dashed circle) ...
8/n

The booming bright source had to have an ultra-steep spectrum to disappear by the time you get to GHz frequencies. We now know that this is source D2: the ultra-steep spectrum fossil. A fading remnant of a no-longer-active galactic nucleus... 9/n

But we didn't jump straight from GLEAM & KAT-7 to what you see above... we got our hands on some #ASKAP-16 commissioning data (processed and studied by Riley Keel and @stay_irradiated), which was extremely exciting... 10/n

For the first time, we saw the fossil and the relic resolved. The relic was arc-shaped - we confirmed it was definitely a relic! The fossil was barely detected, but it was clearly ultra-faint!
Tantalising results, but the data quality wasn't good enough... 11/n

However... it acted as a springboard for securing the deeper #ATCA and #ASKAP-36 observations that we published in the last 24 hours. A long journey, with stalls, thrills, spills and excitement all along the way. So what do we now know? Bear with me a little more... 12/n

Here's Figure 1 of our paper in all its glory again. Blue traces the hot plasma "soup" of A3266. Yellow/orange/red traces our ATCA and ASKAP radio data. The redder the emission, the more ASKAP dominates the brightness - it's lower-energy, older emission... 13/n

D1 is the "wrong-way" relic (credit to @kilokilok9 for the catchy moniker). It's powered by a merger shock, but the concave shape is unusual. The spectrum is odd: too flat at low frequency, way too steep at high frequency. It breaks what we know about shock acceleration... 14/n.

D2 is the fossil. Ultra-steep at low frequencies, giga-steep at high frequencies (I feel like "giga-steep" is apt for a spectral index of *-7.9*). The break frequency is probably below the MWA band, meaning it is ancient. Our best models don't work... 15/n.

D3 is the brightest part of the Mpc-scale radio halo we found. The halo shows substructure - different components - which seem to have different connection between the radio and X-ray emission. Is this physics or environment? We can't tell yet, more evidence is needed... 16/n

Props to Larry Rudnick for running his multi-resolution spatial filtering algorithm and enabling the halo detection 🙌 with all the excitement about the relic, the fossil, and all the radio galaxies, I'd forgotten to look for a larger-scale halo... 17/n

And speaking of radio galaxies... RG1, RG2 and RG5 are all weird and wonderful in their own right, deserving of follow-up. I'll briefly draw your attention to RG5 in particular, as the others are spectacular and so more well-explored... 18/n

RG5 is pretty spectacular too. It's part of one of the groups in the cluster outskirts, but the shape of the tail is... unusual. The sudden bend and curl is odd. As is the faint and (very) red blob further SE 🤔 we didn't explore it in the paper, but one totally could... 19/n

So... to wrap it all up, we found: (i) a "wrong-way" relic that breaks our models; (ii) an ultra-steep (giga-steep?) spectrum fossil that breaks our models; (iii) a radio halo that... is puzzling; (iv) heaps of spectacular, weird and wonderful radio galaxies... 20/n

This project answers a question I posed during my PhD: "does A3266 host a radio halo?". The answer is yes! But in finding that answer, we've raised yet more questions. This is a hugely messy cluster, and I for one cannot wait to use @SKAO to unpack more. /FIN