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Goal: To understand voids
Objectives:
1) To examine the Size and
distribution of voids
2) To understand the Properties of
voids
3) To learn about the Formation and
evolution of voids
What is a void?
•
•
•
•
A) A region with no gas
B) A region with no dark matter
C) A region with no galaxies
D) None of the above
Size and distribution of voids
• Voids are regions of space with less
galaxies.
• Voids cover over half of the volume of our
universe!
Quick philosophy
• Everything is connected in someway –
even virtually nothing.
• As we will find, we can still find out a lot
about our universe by looking at areas
where there is very little.
• The moral of the story is to never (in either
life or Astronomy) take anything
forgranted.
Nearest
voids
Universe
And with
time the
voids will
grow!
1)
Clusters will
shrink
2)
Universe
will expand
Properties of voids
• Voids – much like space – is not 100% empty.
• There are still galaxies, but there are only 20% of
them (1/5th normal).
• Voids are about 50-200 million light years across.
• The galaxies seem like normal galaxies.
• They have normal amounts of metals.
• They do tend to have a stronger old population
but have normal amounts of metals.
• The only real difference is the distribution of
galaxy types.
Void galaxies
• Void galaxies are not as you expect though.
• They seem to be brighter than average.
• There seems to be a larger star formation rate
than average (which makes them bluer).
• The only non surprise is a lack of spiral galaxies
(10X lower gas makes it hard to have a spiral
galaxy).
• Spirals tend to reside on the edges of galaxy
clusters or in smaller galaxy groups by
themselves.
• Basically spirals live in the suburbs not the rural
areas.
Problems with observations
• However, these studies only include 1001000 void galaxies (compare that to the
estimated trillions of galaxies in the
universe).
• Could we only be seeing the special
galaxies that have had something strange
happen to them (such as a merger)?
More problems
• Surveys of galaxies (voids and non void) are limited by
some brightness.
• There are usually a lot of galaxies dimmer than can be
observed in any region of space any distance from us.
• So, we don’t get a complete map.
• This affects where we observe the voids to start and stop.
• We can’t measure distances directly. We have to use
redshifts.
• However, galaxies on the edges of voids tend to move
AWAY from the voids. This tends to change our view of
where these galaxies are and what the shape of the void is
(elongates them).
• They are also so big that the Cosmological constant (the
change in the rate that the universe expands) can also
change our viewed shape of the voids in the same fashion.
• Finally, the surveys only go so far. So we can only observe
the properties of the most nearby voids.
Unresolved questions
• Where is the gas coming from for the star
formations?
• We expect the gas to leave the void and
go to the clusters (and voids end up with
only 10% of the normal gas amounts).
• Could the clusters be pumping gas back
into the void?
• Is there more dark matter than expected in
the void?
A NEW issue!
• A group at the University of Minnesota
discovered a void a BILLION light years
across!
• This is something we just cannot explain
currently as we expect them to be 50-200
million light years in size.
So, why bother studying voids?
• What can we learn from voids, why are they
important?
• The size and distribution of the voids eliminates
any possibility that the dark matter in our
universe is warm or hot (it would spread out too
much).
• Therefore the dark matter is cold.
• The sizes and distributions of the voids also
show how much mass our universe contains.
Also learn
• Since the voids form as a result of the
distributions of dark matter in the formation of
the universe therefore the sizes and distributions
tell you something about the distribution of dark
matter when the universe formed!
• Also, since the curvature of the universe (aka
the cosmological constant) changes the shapes
of voids as we see them, we can gain some
understanding of what the curvature of the
universe is (and with less bias than other
methods).
And…
• Some final things we gain are:
• Constraints on galaxy clustering (more
clustering means larger voids, less means
smaller).
• Sheds some insight into galaxy evolution.
• It is amazing how much we can learn from
looking at places with so little.
Conclusion
• Studying voids teaches us a lot about the
universe and the evolution of galaxies.
• Voids are not 100% empty.
• Galaxies, while scarce are teaming with
new stars – which is a great mystery.
• The bottom line though is that most of our
universe (volume wise) is made of voids,
so they have to be important.