[HN Gopher] First sighting of hot gas sloshing in galaxy cluster
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First sighting of hot gas sloshing in galaxy cluster
Author : dnetesn
Score : 21 points
Date : 2020-01-12 12:21 UTC (10 hours ago)
(HTM) web link (phys.org)
(TXT) w3m dump (phys.org)
| xioxox wrote:
| 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 wrote:
| 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 wrote:
| 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.
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