Emission lines from clusters: the
legacy of Ariel 5’s discovery
Contents
•  Some personal thoughts on how important was the
discovery of emission lines from clusters.
–  hopefully thought provoking
–  perhaps just provocative
•  Some of the science that has flowed down from this
discovery.
•  I’m really the warm-up act for the two talks that follow.
First, my own personal connection to Ariel 5
•  About 20 years ago, the first paid job I had here at
MSSL was transferring Ariel 5 and other data from old
large-format (12 inch diameter) 800, 1600 and 6250 bpi
tapes to dat casettes.
•  Then the old 12 inch tapes were thrown in a skip.
•  I might be the last person to have handled the original
tapes of Ariel 5 experiment C data* (if those were the
original tapes.. I’ve no idea if they were.)
*if those were the original tapes; I don’t know whether they were.
•  A talk that took place in this room a year or two ago.
•  John Butterworth showed these plots that had come from CERN.
•  The discovery of the Higgs Boson. Nobel prize stuff.
•  These plots look rather like one that I use in my lectures.
Perseus observed in 1975
•  Remarkable visual
similarity between this
and the Higgs plots.
•  But it got me thinking
about how the two results
compare.
If anyone was here from the particle physics
group at UCL, they’d probably cut my head off
for what I’m about to say.
(I’m only joking)
Higgs discovery is undeniably very exciting
•  New particle
•  Confirms a prediction that is decades old
•  Its discovery completes the family of particles that make
up the standard model
•  It is essential for explaining mass in the standard model
Higgs discovery is undeniably very exciting… but
•  As far as I understand, it confirms the
current paradigm
•  i.e. nothing changes
•  The standard model of particle physics
(indeed, quantum mechanics) as a
theory is incomplete. There is as yet
no quantum gravity.
•  The standard model only describes normal matter. It
doesn’t describe (so far) dark matter let alone dark energy.
Whatever dark matter is, it is still missing from the standard
model of particle physics.
•  Normal matter is a tiny fraction of the energy density of
today’s universe.
Lets go back to the Ariel 5 discovery
•  Discovering the emission line meant that the Xray emission was thermal emission from hot
gas.
•  There were several possibilities for the X-ray
emission, so the discovery set the new
paradigm. Clusters contain huge reservoirs of
hot gas.
•  As we heard yesterday, it’s now thought that
more than half of all baryonic matter today is in
the form of hot* gas in clusters and filaments.
•  So arguably, this observation discovered more
than half of the normal matter in the Universe.
*Most of it probably isn’t as hot as in the massive clusters, but pretty hot nonetheless
The hot gas doesn’t just tell us about normal matter
•  It is quite easily worked out that the hot gas wouldn’t stay
put in clusters of galaxies if there wasn’t a huge quantity of
dark matter holding it there.
•  The dark matter density profile can be worked out from the
profile of the emitting gas.
•  The gas wasn’t the first evidence for dark matter, even in
galaxy clusters, but it is a very important tool to
understand it, as we’ve seen already yesterday.
On to dark energy
•  Studies of the hot gas emission in clusters played an
important part in understanding that the Universe’s
gravitational attraction on large scales is dominated by
cold dark matter (CDM)
•  Which was an essential step towards the current ΛCDM
cosmological paradigm.
•  And the hot gas in clusters is still being used to test
cosmology and refine cosmological parameters.
•  So the Ariel 5 discovery was an essential step in
determining the contents, state and shape of our universe
today.
In other words
•  The Higgs is the final particle predicted in an incomplete
theory that describes a tiny fraction of the mass/energy
content of the Universe, and its discovery doesn’t change
anything.
•  The 1975 observation with Ariel 5 set a new paradigm,
effectively discovered more than half of the normal stuff in
the Universe and was an essential step in determining the
contents, state and shape of the Universe today.
Even more extraordinary:
•  The Higgs boson was discovered by a team of thousands,
from tens of nations, working over decades, with a particle
accelerator the size of a small city, with a budget of billions.
•  Emission lines were discovered in clusters of galaxies by 4
UK scientists, working for a few months, using a few days
of data, from a typewriter size instrument, that had been
designed and put together in a few years, in an old house
in England. (Sorry, I don’t know what the budget was).
Let me leave the legacy for a minute to talk
about the paper itself.
•  The first author, R J
Mitchell was Len’s
PhD student.
•  The paper’s not in
Nature, but in Monthly
Notices.
•  What a period of
discovery this was:
read the first line of the
introduction!
The paper itself.
•  Here’s the figure as
presented in the paper.
•  Terrible data by
today’s standard, but
the paper is quite
compelling.
Now (seriously) to the legacy of the Ariel 5
discovery.
•  Scientifically, the legacy is huge.
•  Needless to say:
–  Every scientific paper about X-ray emission from clusters in the
last 40 years has been written within the paradigm that clusters
contain large reservoirs of hot gas.
–  The same for every paper about the Sunyaev Zeldovich effect in
the submillimetre, whereby the microwave background spectrum is
distorted when it passes through the hot gas.
–  And all the research which has used X-ray surveys of clusters for
cosmology.
–  Our current understanding and expectations for the warm/hot
intergalactic medium are rooted in this paradigm too.
and
•  Our understanding of the radio jets and lobes of active
galactic nuclei is also rooted in this paradigm.
•  The jets don’t propagate into a vacuum, but into the
intracluster medium of hot gas.
and
•  The X-ray emission lines themselves in clusters are used
to study cosmic abundances as Jelle told us yesterday.
•  That’s telling us about the history of star formation,
elemental production, supernovae in galaxies.
•  And because the emission lines in the cluster gas come
from heavy elements that can’t be primordial, they tell us
about gas that is leaving galaxies.
Important aside now, to another paradigm-changing
observation of X-ray emission lines in clusters.
•  RGS showed
unambiguously that
there was less cool
material in clusters
of galaxies than
predicted by cooling
flow models.
•  Something is
stopping the gas
from cooling.
Peterson et al. 2003, ApJ, 590, 207
and
•  With AGN mechanical heating being the most plausible
explanation for the lack of (relatively) cool gas, AGN
“feedback” became a key component of models for
galaxy formation.
•  So even the way
galaxies are formed
and how AGN affect
that process is now
rooted in the
paradigm set by the
Ariel 5 discovery.
Bower et al. 2006, MNRAS, 370, 645
The legacy continued
•  If I’ve failed to convince you of the importance of the
scientific legacy of the discovery itself, I’ve one final slide.
•  I don’t know how significantly the Ariel 5 discovery of
emission lines influenced Len. Perhaps he’ll tell us.
•  But 6 years after Mitchell, Culhane, Ives & Davidson 1976,
the following document was submitted to ESA.