
First sighting of hot gas sloshing in galaxy cluster - dnetesn
https://phys.org/news/2020-01-sighting-hot-gas-sloshing-galaxy.html
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xioxox
I'm the lead author on this paper, so feel free to ask questions!

EDIT: I should say the analogy for these kinds of events is like wine sloshing
in a glass. The hot X-ray emitting atmosphere of a galaxy cluster would sit
calmly in the gravitational potential well. Likely what is happening is that a
smaller subcluster passes close to the main cluster. This causes the
graviational potential to shift across, the atmosphere is out of equilibrium
and therefore sloshes back and forward for billions of years.

These are fascinating objects. Up to 10^15 more massive than our sun, mostly
made of dark matter (80-90%) and most of the normal baryonic matter is in the
form of a hot plasma, heated up by shocks as the cluster grew by mergers and
accretion of subclusters.

~~~
ConceitedCode
I'm still working through the paper which is fascinating and have only had a
chance to glance it over so forgive me if I ask any questions that are
answered in the paper.

I see lots of large timeframes of the data (20 years), but nothing about how
much data that actually is. I'm not very familiar with this kind of data, but
am curious about the software side and how much data was needed, timeframes
for processing the data, any special hardware required, etc....

How much data did you start with (gigabytes? terabytes?)?

What does this data actually look like? csv, custom binary format, some open
spec maybe?

How much did you end up filtering out for the various reasons in the paper?

Was there anything that surprised you personally while working on this paper?
It seems like most of this is confirming existing theory which is great, but
curious if you had any new take aways.

Does the team want to continue to pursue this? If so, what do they hope to
accomplish or maybe there's some odd data / behavior that you would like to
continue to look at?

~~~
xioxox
Software wise, we use a standard pipeline that reduces the data from the space
observatory into the standard astronomy format (FITS), provided by the
European Space Agency. The output is in the form of events - X-ray photons
which landed on a detector at a particular time. This can then be turned into
spectra with the standard software, extracting from particular spatial
regions. The spectra can be fit with a standard tool in X-ray astronomy
(Xspec), but this also relies on spectral models (some standard, some I made
for this project). However, a lot of the hard work is in the form of Python
code I made for running the pipeline, extracting spectra, collating the
spectra, adding them together, fitting them, collating the results and doing
fits. There are also some scripts in tcl for controlling Xspec. The plots and
things were done with Veusz (which I wrote) and ds9 (a standard astronomy
image GUI).

Yes - we analysed a lot of observations to do the calibration work - that's
the advantage of a big public archive. After processing it takes several
hundred gigabytes. It probably would take a few times more, but I threw away
quite a lot of it which we don't use for this analysis (flared time periods
and low energies). That doesn't included the input raw datasets, which might
be a few TB - I've not checked, as they're on a different system.

The data, as I say above, is in FITS format, which is standard binary table
format. The processed data are these event files (lists of photons), spectra
(tables of energy vs number of photons), and detector responses (matrices to
turn a model spectrum into an observed spectrum). Along the way there are lots
of intermediate text and FITS files. I even used HDF5 for part of the code,
but that's mainly because it's so easy to use from Python.

How much was filtered? Usually we need to filter around 40% of the time
periods for an average observation due to flares caused by soft protons
hitting the detector. In this analysis we also threw away a lot of the data at
lower energies, as we were only interested in the high energy emission lines,
where we can calibrate the detector. I don't know the number there - maybe we
threw away 80% of the total events by filtering the low energies. Finally, we
also throw away half of the events, to retain those with the best energy
resolution (those where a photon hits a single pixel on the detector).

Surprises? For the Perseus cluster, it was nice when I made a map of the
motions and ended up with something that looked like the simulations of
sloshing. For Coma, I was surprised that the gas in the cluster still has the
same velocity as the central galaxies - I would have thought that it should
have slowed down - it will be interesting to discuss this further with
theorists. I was also surprised by the complexity of the detector on the
instrument. It seemed a simple idea when I started, but turned out to be
rather tricky.

We're planning to pursue this further. We have new deep observations of two
other nearby clusters. The aim is study "feedback" by active galactic nuclei -
active black holes affecting their surroundings - in the centre of these
clusters. They should be disturbing the gas/plasma and we hope to measure
that, as that hasn't been done before. There are also some things we could do
to improve the calibration technique if we have time. For example, we could
also use photons which land on multiple pixels.

------
ncmncm
Do astronomers still have the guild requirement to call plasma "hot gas", and
never mention any plasma fluid-dynamic phenomena, or X-radiation that results
from them with no need for extreme temperatures?

I thought that had passed. Is the requirement lifted only in certain domains,
like below some size limit?

