In the previous post I noted how wide-area surveys of the sky like the
Deep Lens Survey serve the dual purposes of finding rare objects and
surveying a representative sample of the universe (to determine its
average density, for example), and I described one of our successes in
the former category while promising to post an example in the second
category. Here it is!
First, we have to understand that the path of light is bent by gravity
and therefore, if we can observe some consequence of this bending, we
can learn about how much mass is between us and the source of light.
I'm not going to explain this in any detail here, but if you wish you
can watch my YouTube video on the subject, or just skip to the part
where I do a demo showing that this bending can lead to magnification.
In that demo I don't specifically point out the magnification, but at
one point you can clearly see that the blue ring on the whiteboard has
been magnified.
If we observe this magnification while looking in one very specific
direction as in the video, we can find how much mass is lurking in the
object which provides the magnification (usually a specific galaxy or
cluster of galaxies). A few galaxies happen to have background
sources of light lined up just right so that we can see the
magnification easily, so we can learn about those specific galaxies.
But are they representative of galaxies in general? Probably not,
because the most massive galaxies provide the most magnification and
are more likely to get noticed this way. Also, having the mass more
concentrated toward the center of the galaxy helps, so if we just
study these galaxies, we will be looking only at the more massive,
concentrated galaxies.
In our wide-field survey, a team led by graduate student Chris
Morrison measured the very small amount of magnification around the
locations of hundreds of thousands of typical galaxies. Their
statistical analysis doesn't measure the magnification caused by each
galaxy (which would be too small to measure), but it measures the
typical magnification caused by the galaxies in aggregate. For this
reason, this type of analysis is called "cosmic magnification" which
sounds mysterious but can be thought of as "magnification caused by
the general distribution of mass in the cosmos rather than by a
specific identifiable lump of mass."
The amount of cosmic magnification tells us not only about the
distribution of mass in the universe, but also about the distances
between us, the magnifying masses, and the sources of light. (Imagine
watching the wineglass demo in my video, but having me move the
wineglass much closer to the whiteboard...you can probably predict
that the magnification will be less.) These are two very fundamental
things about the universe which astronomers are trying to measure,
because they are both affected by the expansion rate of the universe,
and the expansion rate is unexpectedly accelerating. Three
astronomers won the 2011 Nobel Prize in Physics for their role in
discovering this acceleration, and ever since they discovered it
(1998), many astronomers and physicists have focused on figuring out
why. Some attack this question from a theoretical point of view (a
theorist coined the term "dark energy" which has become the popular
term, but be warned that it may not be caused by a new form of energy
at all), and others attack it from an observational point of view: if
we can get better and better measurements of how the expansion is
actually behaving, we can rule out some of the theories which have
been proposed to explain it. Cosmic magnification has a real role to
play in that process, and Morrison's paper is the first one to
measure, even in a crude way, how cosmic magnification increases as we
increase the distance between us and the masses causing the
magnification.
Saturday, August 25, 2012
Tuesday, August 14, 2012
Colliding clusters of galaxies
One of the questions generated by my previous post describing the
Deep Lens Survey is: Why do such a large survey of the sky? What
do you hope to accomplish that the Hubble Space Telescope can't?
HST is great at some things but not others. Expecting HST to be great
at everything in astronomy is like expecting a great criminal-defense
attorney to also be great for cases involving bankruptcy law, probate
law, torts, and tax law. Novices would put all of these things under
the single category of "law," but people closer to the legal system
recognize that these are very different specialties. Similarly, if
you look closer at "astronomy" or "telescopes" you realize that
there's such a wide variety that no one telescope can do it all. And
whether it's attorneys or astronomy, the few performers which become
known outside the field are those with some combination of high
performance in the field and a good public relations machine.
So what is HST great at? It was launched into space primarily because
turbulence in Earth's atmosphere makes images blurry. Above the
atmosphere, HST can take really sharp images. The flip side of
capturing these really fine details is that it can't capture a very
wide panorama. So we need very big, wide surveys from other
telescopes to find things which are interesting enough to follow up with
HST and other specialized telescopes such as X-ray telescopes (which,
like HST, need to be above the atmosphere and are therefore similarly
expensive and rare).
But wide surveys are more than just rare-object finders for HST and
other specialized telescopes. Equally important, they give us a
representative sample of the universe. Just as an anthropologist
could not fully understand how humans live by studying only the "most
interesting" countries (the ones with revolutions underway, for
example), astronomers could not understand the universe in general
just by studying the most interesting objects. I'll give an example
of the rare-object-finding capability of the Deep Lens Survey in this
post, and an example of the understanding-the-universe-in-general
capability in my next post.
Rare objects are scientifically interesting for many reasons. Some of
them tell us about extremes: knowing the mass of the most massive star
or the luminosity of the most luminous star tells us something about
how stars work. In other cases, what is rare and interesting is not
so much the object itself as the stage it happens to be in right now.
Because the lifetimes of celestial objects are millions or billions of
years, we can't follow a single star, say, over its lifetime to
determine its life stages. Instead we have to piece together their
life cycles from different stars seen in different stages. Imagine an
alien anthropologist who pieces together the human life cycle from one
day's visit to Earth: because a small fraction of humans are babies
right now, that must mean that people spend a small fraction of their
lives as babies. In the same way, a certain star or galaxy may not be
intrinsically special, but if we happen to be seeing it at a special point in
its life cycle, that helps us understand all objects of its type.
Finally, in some cases objects are particularly interesting because we
have a particularly clear view of them. Just as an overhead camera's picture
of the top of a person's head is less informative than a picture of their face,
Earth's view of many celestial objects is not fully informative. Objects
which happen to expose their "faces" to us give us more insight, which
can then be applied even to those objects which do not face us.
Today I want to highlight a collision between two galaxy clusters
which was discovered in the Deep Lens Survey. Imagine observing a
head-on collision between two large trucks. You will observe a lot
more of the details if you are standing by the side of the road (a
"transverse" view) than if you are driving behind one of the trucks.
My student Will Dawson was the first to realize that we have a
transverse view of this collision. This immediately makes it
interesting because if the component parts of the clusters (galaxies,
hot gas, dark matter) become separated, a tranverse view gives us the
best chance of seeing that separation and therefore learning more
about those components.
In particular, separation between the dark matter (which carries most
of the mass) and the hot gas (which is the second-most-massive
component) is important because dark matter has never been observed
very directly. Astronomers infer the existence of dark matter when
orbits (of galaxies in a cluster, for example) are too fast to be
explained by the gravity of all the visible mass (stars and gas).
Therefore, the conclusion goes, there must be some invisible component
with substantial mass: dark matter. Observing a clear separation
between dark matter (ie, the bulk of the mass) and normal matter would
boost our confidence in this conclusion, and help refute competing
hypotheses (for example, that what we understand about gravity and
orbits from studying the solar system may not fully apply to these
other systems). This "direct empirical proof of the existence of dark
matter" was first done for a transversely colliding galaxy cluster
called the Bullet Cluster, which you should definitely read about if
you are interested in this topic. A good place to start for beginners might
be this Nova Science Now video.
Finally, we get to the research paper I wanted to highlight. It's an
examination of the evidence we collected regarding the aforementioned
collision in the Deep Lens Survey, including not only the original DLS
images but also data from HST, the Chandra X-ray telescope, the
10-meter Keck telescopes and other telescopes. The conclusion:
it is indeed a transversely viewed collision of galaxy clusters
with a substantial separation between the dark matter and the hot gas.
My student Will Dawson is the principal author who assembled all these
pieces, with substantial help from many co-authors. This is something
of a teaser paper: it's not an exhaustive analysis, but it's enough of
an analysis to establish that it's an important system worthy of
further study. All further studies of the system (including proposals for more
telescope time) will cite this paper because it lays out the basic facts.
I've tried to keep this post short and relatively free of technical details, so
some readers may want more. A good place to start is Will's research page.
And feel free to ask questions in the comments below! I may give a quick
response in the comments, or I may use them to motivate a future post.
Finally, to bring this back to the question which initially stimulated
this post: this is a special system which we did study with HST, but
we never could have found it without a wide survey like the Deep Lens
Survey.
Deep Lens Survey is: Why do such a large survey of the sky? What
do you hope to accomplish that the Hubble Space Telescope can't?
HST is great at some things but not others. Expecting HST to be great
at everything in astronomy is like expecting a great criminal-defense
attorney to also be great for cases involving bankruptcy law, probate
law, torts, and tax law. Novices would put all of these things under
the single category of "law," but people closer to the legal system
recognize that these are very different specialties. Similarly, if
you look closer at "astronomy" or "telescopes" you realize that
there's such a wide variety that no one telescope can do it all. And
whether it's attorneys or astronomy, the few performers which become
known outside the field are those with some combination of high
performance in the field and a good public relations machine.
So what is HST great at? It was launched into space primarily because
turbulence in Earth's atmosphere makes images blurry. Above the
atmosphere, HST can take really sharp images. The flip side of
capturing these really fine details is that it can't capture a very
wide panorama. So we need very big, wide surveys from other
telescopes to find things which are interesting enough to follow up with
HST and other specialized telescopes such as X-ray telescopes (which,
like HST, need to be above the atmosphere and are therefore similarly
expensive and rare).
But wide surveys are more than just rare-object finders for HST and
other specialized telescopes. Equally important, they give us a
representative sample of the universe. Just as an anthropologist
could not fully understand how humans live by studying only the "most
interesting" countries (the ones with revolutions underway, for
example), astronomers could not understand the universe in general
just by studying the most interesting objects. I'll give an example
of the rare-object-finding capability of the Deep Lens Survey in this
post, and an example of the understanding-the-universe-in-general
capability in my next post.
Rare objects are scientifically interesting for many reasons. Some of
them tell us about extremes: knowing the mass of the most massive star
or the luminosity of the most luminous star tells us something about
how stars work. In other cases, what is rare and interesting is not
so much the object itself as the stage it happens to be in right now.
Because the lifetimes of celestial objects are millions or billions of
years, we can't follow a single star, say, over its lifetime to
determine its life stages. Instead we have to piece together their
life cycles from different stars seen in different stages. Imagine an
alien anthropologist who pieces together the human life cycle from one
day's visit to Earth: because a small fraction of humans are babies
right now, that must mean that people spend a small fraction of their
lives as babies. In the same way, a certain star or galaxy may not be
intrinsically special, but if we happen to be seeing it at a special point in
its life cycle, that helps us understand all objects of its type.
Finally, in some cases objects are particularly interesting because we
have a particularly clear view of them. Just as an overhead camera's picture
of the top of a person's head is less informative than a picture of their face,
Earth's view of many celestial objects is not fully informative. Objects
which happen to expose their "faces" to us give us more insight, which
can then be applied even to those objects which do not face us.
Today I want to highlight a collision between two galaxy clusters
which was discovered in the Deep Lens Survey. Imagine observing a
head-on collision between two large trucks. You will observe a lot
more of the details if you are standing by the side of the road (a
"transverse" view) than if you are driving behind one of the trucks.
My student Will Dawson was the first to realize that we have a
transverse view of this collision. This immediately makes it
interesting because if the component parts of the clusters (galaxies,
hot gas, dark matter) become separated, a tranverse view gives us the
best chance of seeing that separation and therefore learning more
about those components.
In particular, separation between the dark matter (which carries most
of the mass) and the hot gas (which is the second-most-massive
component) is important because dark matter has never been observed
very directly. Astronomers infer the existence of dark matter when
orbits (of galaxies in a cluster, for example) are too fast to be
explained by the gravity of all the visible mass (stars and gas).
Therefore, the conclusion goes, there must be some invisible component
with substantial mass: dark matter. Observing a clear separation
between dark matter (ie, the bulk of the mass) and normal matter would
boost our confidence in this conclusion, and help refute competing
hypotheses (for example, that what we understand about gravity and
orbits from studying the solar system may not fully apply to these
other systems). This "direct empirical proof of the existence of dark
matter" was first done for a transversely colliding galaxy cluster
called the Bullet Cluster, which you should definitely read about if
you are interested in this topic. A good place to start for beginners might
be this Nova Science Now video.
Finally, we get to the research paper I wanted to highlight. It's an
examination of the evidence we collected regarding the aforementioned
collision in the Deep Lens Survey, including not only the original DLS
images but also data from HST, the Chandra X-ray telescope, the
10-meter Keck telescopes and other telescopes. The conclusion:
it is indeed a transversely viewed collision of galaxy clusters
with a substantial separation between the dark matter and the hot gas.
My student Will Dawson is the principal author who assembled all these
pieces, with substantial help from many co-authors. This is something
of a teaser paper: it's not an exhaustive analysis, but it's enough of
an analysis to establish that it's an important system worthy of
further study. All further studies of the system (including proposals for more
telescope time) will cite this paper because it lays out the basic facts.
 |
| Closeup of our colliding clusters, with the location of the mass (mostly dark matter) painted on in blue and the location of the hot gas painted on in red. If you look closely you can see that there are many galaxies colocated with the mass, but not with the hot gas. This (temporary) ejection of the hot gas allows us to study dark matter more clearly. |
I've tried to keep this post short and relatively free of technical details, so
some readers may want more. A good place to start is Will's research page.
And feel free to ask questions in the comments below! I may give a quick
response in the comments, or I may use them to motivate a future post.
Finally, to bring this back to the question which initially stimulated
this post: this is a special system which we did study with HST, but
we never could have found it without a wide survey like the Deep Lens
Survey.
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