HSIAO-WEN CHEN: Well, good evening.
I'm very glad to be here today.
Traveling around half of the globe to visit--
Delhi.
It's my very first time in India so it's
been really fascinating.
I spent the earlier part of the week attending
a workshop organized by two real experts here from Iutah.
Dr. Shannon and also Dr. Groupdat.
So they are going to pretend they don't
know what I'm talking about.
For the next hour or so.
So I understand that many of you are students from the physics
department and you are here today on a Saturday evening.
You must be very interested in astronomy and astrophysics.
So I trust you guys have been keeping up
to date about the recent development ongoing
in astrophysics research.
And over the last few years, we've
seen a lot of exciting new discoveries,
including many exoplanets habitable zones,
and also, the detection of gravitational waves.
In one case, we even detected the optical counterpart
in the sky.
And just last week, we heard about
a new record-breaking high redshift
quasars further bringing us back in time
within 700 million years after the Big Bang.
So these are all very exciting developments.
And with more newer and more sensitive,
more powerful facilities coming online in the near future--
which I understand India is playing a big role in--
we can only expect there's more to come.
More breakthroughs will come.
So today, I'm going to talk about a research area that
addresses fundamental questions concerning
how the large-scale structures we see in the universe
today formed and evolved.
And in particular, I'm going to focus
on one important component that we know dominates in mass,
but has been largely left out because it's dark.
And because it's difficult, it's challenging
to capture this dark component.
So I will highlight the progress and the future prospects there
are in this particular area.
So you can see in the beginning slide here--
I'm not sure how many of you can recognize this place?
What you see in the background of the entire image,
you can see the beautiful Chilean night sky.
Underneath are the two Magellan telescopes
sitting on top of the Atacama desert in northern Chile.
And you can see the Milky Way, this panoramic image, that
shows you a clear, dark sky--
dark night.
This is what we can see without any help of telescopes.
So while the Milky Way, this beautiful sky
as impressive as it is, it actually
presents a confusing foreground for those of us
who try to study distant objects.
So in order to study the cosmos beyond the Milky Way,
we also have to look between stars.
So if you just focus over the last decade or so,
a lot of the exquisite images by the Hubble Space Telescope.
This is a small area that was taken using the Hubble Space
Telescope.
It's a called a Hubble ultra-deep field.
You can see that--
and this is pointing to supposedly
a blank area of the sky and integrated over a very
long exposure time.
And what you can see is what seemed
to be a blank sky, after you took this long exposure,
you see thousands of galaxies.
And every one of them looks different from the rest.
And actually, I will challenge you to find any two of alike.
So they all look different.
They have different colors.
It looks like not only the visible universe is lumpy,
is blotty, but the galaxies all look very different.
It has a diverse range of morphology.
So if I just zoom in--
So this was actually Hubble.
So Hubble discovered these galaxies beyond the Milky Way
almost a century ago.
And he did his best.
He realized everything looks different from the rest.
And he did his best to classify, to come up with this Hubble
tuning fork diagram.
You can see that he grouped the galaxies into round looking,
elliptical, to spirals, to something
with a central barred structure.
So these days we have tons of astronomical images taken.
So sometimes we see something that looks like a Mexican hat.
Sometimes it looks like a train wreck.
And sometimes this is what people a ring galaxy.
So with all the diverse range, and the seemingly random
phenomenon, the surprising thing is
it appears that there there's actually
some simple principles.
They actually follow some simple correlations.
So to start out, this is so-called Galaxy Bimodality.
What it shows here is the color in the y-axis and mass
in the x-axis.
Roughly speaking, what we find is
galaxies tend to fall into two different categories.
Red tend to be more massive than blue galaxies.
And the red galaxies, they tend to be older, more evolved.
And the blue galaxies appear to be younger.
Many of them are still forming new stars.
So despite-- no matter how weird they look,
they seem to follow this general trend.
And if you look further, there's also a very tight size
and mass correlation.
Again, I want you to remember back
the weird morphologies we saw.
So here it shows the so-called half mass radius.
Within this radius, within this size
here, we see half of the total mass versus the total mass that
shows at the top axis.
Over more than four orders of magnitude,
we see that more massive galaxies systematically
show bigger, are more extended.
Again, irrespective of what they look,
there seems to be this tight correlation.
Just one more, so there's this so-called mass metallicity
relation.
So for those who are new in astronomy,
we have a funny way of calling our heavy elements.
We call them metal.
So the metallicity is a measure of how many heavy elements are
there per hydrogen particle.
So it's a relative ratio.
If there's more heavy elements, the metallicity is higher.
And as it turns out, just like the size mass correlation,
there is also a very tight correlation
between mass in the x-axis here and the amount
of heavy elements that have been produced over time.
So somehow bigger galaxies, on average,
are older, and more massive, and also more
enriched with heavy elements irrespective of what they look.
So there is this order in chaos that
seems to tell us there are some simple, underlying driving
principles that govern the galaxy formation and evolution.
So to astrophysicists, this is good news
because there are ways-- there must
be ways to be able to reproduce the observed, large-scale
structure based on some simple principles.
And this is a struggle over two decades.
And it's a success.
So just in the last couple years,
all the numerical simulations-- most
of the numerical simulations are able to produce realistic
looking galaxies.
So here, I have to mention, these are not real.
These are simulated galaxies.
At the top are the disk spiral types
look top down, so go face on.
You can see really detailed spiral arms and also
dust lens, the dark part, and the star forming regions.
And when you see the disk spiral edge on,
the thin disk is really reproducing
the observed central bulge to the disk ratio.
Amazing.
So like I said, most simulation groups are able to do that now.
So this is a separate group that shows
that they can reproduce the Hubble tuning fork diagram
in their simulation volume.
Again, starting from elliptical galaxies evolving to the right
to spiral looking.
Even bar spirals you can see.
I also tried to capture some train wreck examples.
So it's a clear success, victory.
Are we done?
Since I'm asking, that means the answer is no.
So what is-- what is left out as I said in the beginning?
So these days, we know very well what the universe is made of.
We know about 22% of the entire mass density
resides in dark matter, whatever that is.
And 74% are in the so-called dark energy--
again, the second fairytale.
So a lot of people are pursuing the nature
of the two dark forces.
So the tiny, little sliver of 4% are what we are made of,
regular, ordinary matter.
So we know exactly what they are.
This is so-called barium.
So in principle, we should know where to find it.
Agreed?
So when you actually look into the composition--
cosmic composition trying to identify
the 4% of the total mass budget in regular matter,
we could only identify--
so the known matter, like stars or gas,
together make up just about 10% of all regular matter
that we know exists.
The rest is still to be found.
And considering this is 90% of all the regular matter,
this is a serious problem.
So now, going back to the success in simulations.
So they can reproduce realistic looking galaxies.
What about the remaining 90%?
So if we look back to, again, one
of the numerical simulations, this
is showing you the expected spatial distribution
of dark matter, over about 15 megaparsecs across.
So I'm going to use megaparsec--
I'm going to use parsec.
So this is an astronomical distance measure
unit that corresponds to about three light years.
So again, astronomical number, the universe
is so huge, vast space we have to invent a new unit.
And even after we invented three light years, parsec,
we still need to go up to million parsecs
to describe a tiny, little volume in the cosmos.
So this square here, this panel here,
is about 15 megaparsecs across.
And by the way, the solar system,
we are about 8,000 parsecs to the galactic center.
That gives you a sense of the size of the Milky Way disk.
So in the simulations, even though we
don't know what made--
what the dark matter is made of, but it follows very simple
physical principles.
So we can predict from the first initial density fluctuation
what the dark matter distribution is going
to be like in the current time.
And you can see that it's really rich in structure
on all scales.
Yes?
AUDIENCE: What is this image looking at?
Is is the galactic center?
HSIAO-WEN CHEN: No, this is just a simulation volume that covers
about 15 megaparsecs across.
You can imagine-- so the galactic center may
be one of these red dots here.
For this to be this big, it's going to be a cluster.
So since you asked where's the galactic center--
so this is what the simulation predicts
spatial distribution for dark matter, how it looks like.
So what do you think the galaxies--
the distribution for galaxies should look like?
Can you find any galaxies here?
AUDIENCE: No.
HSIAO-WEN CHEN: You need to look hard.
Maybe we need to turn the lights off.
But you can see that they are tiny, small yellow dots
in this vast volume.
This is what we expect galaxies--
how sparsely distributed galaxies are
relative to dark matter.
So remember, this is 10% of all regular matter.
What about the rest of the 90%?
So this is what the simulations predict
that the rest of the 90% should exist,
should fill up the cosmos along the so-called cosmic web,
filamentary structure.
And the filaments are the crucial fuel
that feed the formation of galaxies and continued
growth of these galaxies.
At least this is what theory expects.
So to some extent, we can actually
test whether the idea is generally correct.
So we can start looking into our own backyard.
This is an old sky map of the Milky Way.
So again, this is unwrapped.
Remember the old sky image from Chile.
It shows the central disk here.
And then, there's a bulge.
That's the galactic center.
So this is the optical view.
This is what we call optical view of the Milky Way.
When we look into 21 centimeter in radial
targeting hydrogen atom, we do see
a lot of clouds floating in the Milky Way sky.
The only caveat is we don't have the distance.
We don't know where they are really
because we are fundamentally blindsided
because of the proximity.
So if we move beyond the Milky Way to our next door neighbor
Andromeda--
so I don't know if you guys can see Andromeda from here.
You can?
Again, this is the optical image of--
also called M31.
When we look into, again, the radial,
we see, again, these clouds floating around the disk.
But the difference is this time because we
see from the outside, we know right away
that they go all the way out to as far as 50 kiloparsecs.
So 50,000 times of the parsec.
So what about further away?
So this is a galaxy group--
that means there's more than one galaxy
in this small volume that come from similar proximity.
And in the optical image-- this is a Sloan Digital Sky Survey
image--
you can see a giant spiral surrounded
by a couple of small disk galaxies and this prototype
starburst galaxy.
Just to zoom into--
to impress you the kind of detail we know these days.
So when we look into radial, what does it look like?
The space between galaxy is filled with this hydrogen gas,
again, out, extend beyond 50 KPC.
So qualitatively, at least in terms
of the comparison on the surface,
it does look like there is extended--
there's the tenuous gas around galaxies
as expected from theory.
But how consistent are the observations and simulations?
Naturally, we want to make a direct comparison
over a large volume.
But this kind of observation, radial observation,
trying to detect this tenuous gas quickly runs out of steam.
You just move slightly beyond the local volume,
it became very difficult to detect this kind of gas.
So that's why we need to have a new tool, a new way,
of probing this kind of tenuous gas a much larger distance.
So what do we do?
So we go back to physics.
So this is just to show you, again,
simulation box, the filamentary structure.
And that's filled with mostly hydrogen, 99.9% hydrogen.
So what do we think could happen when a photon hits
one of the hydrogen particles?
So physics tells us if the photon is at the right energy,
it's going to interact with the electron in hydrogen.
And it's going to accept the electron the photon gas last.
So if you have a background source,
if you have a source that serves as a backlight shine
through these filamentary, this cosmic web,
you came imagine that every time the light
hits a filament it's going to create a dip in the backlight.
And remember that the universe continues to expand.
So the light will be red shifted as things move away from us.
So along a finite path slowly you
build up this map of filaments at the right location
just by studying where the dip comes in.
So this is absorption, absorbing the background light.
So just to help you visualize what's happening,
this is a very nice movie made by Andrew Pontzen
at the University of the College of London.
As the light from the background source
traveled through the cosmos to an observer on the right,
you'll see that every time--
and this is the spectrum.
This is the light from the background source.
Every time it hits a filament, a dense filament,
it's going to create a dip.
So let's see if this works.
So you can see starting from the source location, every time it
hits a dense light it's going to create
a deeper absorption absorbing a larger fraction of the light.
And you see here, this is super dense
and it completely absorbed the light below this energy.
Yes?
AUDIENCE: What are the peaks above?
The density peaks are also the--
HSIAO-WEN CHEN: This peak?
AUDIENCE: Yeah.
HSIAO-WEN CHEN: This is from the background light.
It's like the fingerprint that's associated
with the nature of the region of the object itself.
So it has nothing to do with the space in between.
I want you to focus on the absorption forest.
This is what's called Lyman-alpha forest.
By studying the Lyman-alpha forest,
we can construct a one-dimensional map, basically
just a map of density fluctuation
along one line of sight.
So this is what we call core-sampling if you wish.
This is equivalent to core sampling of the dark universe
using the so-called absorption spectroscopy.
So here I just put this cartoon.
Again, remember this is the M81 group
with the abundant extended gas.
Imagine there's a background source
shines through the space.
And then it's going to produce this so-called Lyman-alpha
forest.
And the good thing about absorption spectroscopy is we
don't just study hydrogen. So it turns out the Lyman-alpha
forest-- a large fraction of them--
come with associated features produced by heavy elements.
So in the cosmos, in addition to hydrogen,
there's also a small fraction of heavy elements.
So again, if the condition is right,
the photon hits the heavy elements.
It will also be absorbed.
And through comparing the relative strength,
we can also constrain the chemical enrichment--
the fraction of heavy elements that have been produced.
So this provides a very sensitive empirical probe
of tenuous gas in and around galaxies.
So this is a technique that people have been using actually
for over two decades.
The advantage, as I said, is because it's very sensitive.
The disadvantage is it's a painstaking process because you
can only have 1-D probe.
And when it comes to probing gas around galaxies, it's a 3-D--
it's an extended structure.
So we need to--
we need to have a 2-D map.
So how do we do it?
Essentially, what we end up doing
is to identify a large sample of galaxies
that happen to sit in front of a background source.
So here, I just show six examples
of-- every fuzzy, yellow blob is a galaxy.
And you notice they all have a blue dot next to it
at different distances.
So this is the background light.
By taking, again, the spectrum of thee background source, we
can identify associated absorption features
that coexisted in location with the galaxy.
But as they move, you can see the strength of this absorption
actually changes as you go further out, further
away from the galaxy.
And what we can do next is to form the so-called ensemble
average.
We combine the entire galaxy together
forming some sort of cosmic average,
essentially, with quasars--
the background source showing up at different locations
in the sky.
And by examining how the signal changes with the distance
to the galaxy, we can build up a cosmic mean,
a 1-D average of the density profile
from the galaxy for the extended gas.
So just to visualize, imagine if this galaxy is surrounded
by this giant filament that we saw in simulations, presumably
every background source that hits
the filament should produce a strong absorption feature.
And by examining the strength, how that changes with distance,
we could-- that forms as the first order test of the model.
And this shows you that we can do this for hydrogen.
And this shows you how strong the signal
is as a function of distance from the galaxy.
Not surprisingly, just like what we
see in a visible optical disk how stars are distributed,
we see a very fast declining trend
as we move further away from the galaxy indicating
that the gas is thinning out as you go away
from the central mass.
And likewise, we're going to do that for heavy elements.
It also shows very similar trend,
but it stops much earlier than hydrogen gas.
It indicates that while heavy elements may exist around
galaxies, it doesn't extend as far out.
So that gives us another hint about how
the heavy elements we see away from galaxies are generated.
So now with this in mind, we can do this to distance as high--
as far away as the background light can be.
Unlike in the radial 21 centimeters,
we are limited to very nearby universe
because of the sensitivity of the instrument.
But for this, so long as you can find the background quasars,
which as I said the latest one is within 700 million years
after the Big Bang, then you can probe all the way out--
the gas all the way out into the early epoch.
So then we can compare-- with a large sample,
we can compare with simulations.
It turns out there's a problem.
So what this is showing here is from the simulations
that can successfully reproduce realistic looking galaxies--
at the top either it is red or spiral looking galaxies--
they underestimate the amount of signal
we see out into galactic halos.
So while they can--
they appear to be able to model the 4%--
10% of the regular matter, they fail
in capturing the physics that's driving the rest of the 90%.
So what can we do to help improve our understanding
of this subject?
So if we go back--
so this brings us to the ongoing research project
and the future development.
So when we come back to this image,
looking at this extended gas around M81 again,
if you look carefully, just by morphology again,
an image can tell us many different things.
If you look at the image, you see different structures.
From something as simple as a rotating disk
in the middle to--
there's the gas being stripped out of a galaxy
here, one of the group members.
To something like tidal tail--
due to tidal force gas--
material gets stripped out of the galaxy.
To some sort of free floating clouds.
And they all have different motion.
Rotating disk by design, it should be just spinning.
But stripped gas has very distinct directional motion.
And these clouds, they will just be randomly floating
around the galaxy.
So not only the spatial distribution
is distinct between these different components,
their motion is also different from
organized, coherent spinning to something that is more random.
So that gives us a hint.
To really bridge the gap between observations and simulations,
we really need to have a map.
1-D probe just doesn't seem to be enough.
I mean, it brought us closer.
It gave us new insight, but now we need more.
So what can we do?
So down here, you can see the hint of filaments coming in,
just to tie back with the simulation
result. One more component, that's
the starburst driven outflow.
So this is really energetic wings
that's being brought up by intense star
formation in the galaxy.
Contradictory to the spinning this structure,
the distinct morphology is this conical outflow.
Again, the directional, very orthogonal,
completely independent of what we see here.
Just to highlight the prospect of sorting
through these complex physical processes,
if we have a spatially resolved map of the gas--
so essentially, we want to go back
to making radial more powerful.
But we don't have to.
So one of the new approaches is to utilize
multipally lensed background source due to the foreground
mass concentration.
This is the so-called gravitational lensing.
Going back to Einstein's general relativity,
space time gets distorted when there's
a huge amount of mass present.
And through gravitational lensing,
single background source gets split into multiples.
So by observing the multipally lensed images
we immediately expand the number of probes from one to four,
in this case.
So that's a factor of four gain.
That's pretty good.
So just to give you an idea about what might be possible
with the multiple probes, in a very simplified world
imagine if there's a spiral spinning spiral galaxy.
In this case, we're seeing the galaxy from top down.
But if there's an observer that's
viewing the galaxy from sideways--
so we put the observer down here looking up through the disk.
And remember, the disk is spinning, so I'm going to use--
resort to Doppler shift again.
Different motion will move the features different according
to how fast the disk is moving.
And also, I should add that one thing you can see
is the light is brighter-- much brighter at the center
and gradually declined to the outskirts.
So you can infer that the density is dropping
as you go to the outskirts.
So coupled with the motion, what do we
expect to see in the spectral feature
due to the spinning disk?
So if I just highlight two points--
one, closer to the center of the galaxy, the motion of the disk
almost goes in parallel with the line of sight.
That means just by observing the velocity offset, we capture
most of the rotation motion.
The projection is nearly 100%.
In contrast to this point, a point in the outskirt,
further away, the density is lower.
And it's moving perpendicular to the line of sight.
That means the projection will be near zero.
I hope that's clear.
So on the left you, can see that near the center almost
parallel to the line of sight, there's a strong signal--
and most red-shifted.
It's moving away from the observer.
The strongest component is also most red-shifted.
As you go further away, the density declines.
Absorption strength declines.
And you see that the weakest component
appears as something that's closer to the systemic velocity
of the galaxy.
So if you understand this, now I'm
going to put in a second sight line opposite side from--
of the center.
So now, how does the disk move?
Also, the observer stands at the same place.
The disk here is moving--
moving toward the observer on this side of the disk.
So what we see is closer to the center
the strongest component is going to be most blue-shifted.
And the weakest component will be closer
to zero relative velocity.
So do you see the difference?
Opposite side, mirror image.
So you can call this a definitive identification
of a rotation disk, if you have more than one sight line.
A second example would be the starburst outflows
that I said before, there's this bi-conical feature.
So again, in the idealized experiment
if we put the observer on the left looking at the disk face
on, in this case now.
And again, two sight lines on the opposite side of the disk.
Instead of seeing this kind of mirror image,
or asymmetric profile, we see something
that's more or less symmetric, one blue-shifted, one
red-shifted, on both sides.
So if we see these features, we can
identify the process, the physical process,
that's driving the absorption--
the absorbing gas that produces this feature here.
And now, with a lot of all sky surveys--
so we started with Sloane Digital Sky Survey.
And now we are actively building the large synoptic telescope
in Chile that is going to do an all Southern sky
survey to very deep level.
The survey combined is going to produce thousands
of those gravitationally lensed objects that will
help with this kind of study.
So the future is really bright.
But we can actually do better than that.
So I talked about absorption.
So absorption is when a photon comes
is and interacts with an electron
and then boosts the electron charge state.
The photon gets lost.
But the electron is not going to stay out there forever.
There's the reverse process.
In very short time, especially for hydrogen, the electron
is going to come down.
It's going to lose energy.
And it will emit a photon.
So this is very obvious.
So why did we not do this in the first place?
The challenge here is when the photon--
I mean, when the electron comes down,
the photon is emitted in any random direction.
So every absorption, the signal when it gets emitted,
is diluted by the entire solid angle, the entire sky.
So you can imagine the signal is significantly weaker.
But in principle, you can do it.
And it's actually just being feasible now given
the advancement in instrumentation,
in astronomical instrumentation and also detector technology.
And here is an example of a recent detection
of this large scale low luminosity emission around very
distant objects.
So to facilitate this kind of study,
this was made possible by this idea of an image slicer.
So what is an image slicer?
Literally, we slice-- given a two-dimensional image,
essentially we put a slit along the entire frame.
We cut it up into a thin slit and re-align them
into a pseudo long slit, and send that
to a disperser, a prism, to disperse the light.
And after the light is dispersed,
we re-package the pseudo slit back to its original form.
It's like playing the Lego blocks.
But in this case, we have an image
for every photon, every photon energy.
So it's trying to highlight here, for blue photons,
we can build an image.
For green photons we can build another image.
And we can look at how different things are.
Why do we want to do that?
Because the emission I said earlier
while it's being diluted by the entire sky,
it's only coming from very narrow--
energy range.
The wavelength is narrow.
Doing this image slicer allows us
to identify the slice we want that targets
the energy range we want, that exact photon we want.
And I'm going to show you this very nice example that
was just obtained recently.
So what you see here is actually an HST image
of the field around a luminous quasar,
this supermassive black hole being powered
by a supermassive black hole.
A red-shift 0.6.
So I used another word, red-shift.
So essentially, this is about four or five billion years
from now, four or five billion years ago.
And that's the quasar in the middle.
It looks just like a star.
And surrounding the quasar you see many galaxies.
I hope you can see in the back that there
is spiral looking galaxy.
There are things that look like interacting, like in the stage
of making a train wreck.
And then there's nothing.
The rest is just dark space.
So recently, there's a new set of data obtained using
the image slicer technology.
And that allows us to build, to extract,
the spectrum of every single pixel in this image.
So what I did here is I just collect
all the pixels that coincide with the quasar
here and build the quasar spectrum down here.
These are all typical emission line features,
that you asked earlier, that's associated
with the central engine.
But because we know what they are,
we use these features to identify the exact location
of the quasar.
And because this is pretty much noiseless,
we can determine the location very precisely,
very accurately.
So now, I can-- again, going back to the image slicing idea.
Now, using the same data set, I can
focus on one specific wavelength and look at the field, what
it looks like.
And I hope you can see a very good one to one comparison.
The only difference here is this is a space telescope image.
It looks much sharper without the atmosphere.
This is obtained from the ground.
So the atmosphere, you go out to see the stars, they twinkle.
The atmosphere really smears the image quite a bit.
But aside from that, you can see there's pretty much a very good
one to one comparison.
There's all the galaxies you find
in the Hubble Space Telescope image in the ground based image
here.
And if I move to a narrow line region,
again we can look at the image side by side.
They still look pretty similar, except for this part.
I hope you can see this does not have a counterpart in the space
image.
So something different is happening.
What is this?
So what I'm going to do next is to show you
a movie of this field stepping through the wavelengths
centered around this narrow line, the entire field, though.
So the narrow line is set by the location of the quasar,
but I'm going to show you what the image, the field,
looks like as we step through different wavelengths.
And I can convert that to relative velocity.
In doing so, I will remove-- let's see.
This is not going to come.
I'm going to remove--
I removed the quasar light in this frame
just so you can see the surrounding much better.
So I'm going to start the movie at about 1,400 kilometers
per second blue-shifted in front--
well, moving toward us from the quasar position.
Does that make sense?
From negative 1,400 kilometers per second to plus,
moving toward--
further away to plus 300 kilometers per second.
What you're going to see--
I'm just going to tell you what you're going to see.
At this location, that bright stream does not--
it doesn't exist.
Disappeared.
So starting from about 1,370 kilometers per second
you will see gas signal being extended out
of this interacting pair here.
You're going to see signal from here and the track wreck mass.
And the signal is going to move to this side, the left side,
where you see one, two, three, four, five,
six, seven galaxies there.
It turns out that they are all interacting like crazy.
They are all pulling each other like crazy.
And then, you're going to see the signal
move to that streak you saw earlier,
and pointing directly to where the quasar is.
So I'm going to start the movie now.
So you see starting from here, there's
gas being pulled out of the interacting gas--
the interacting galaxy.
Then, it moves here, at the bottom,
minus 500 kilometers second.
These seemingly isolated galaxies
are interacting like crazy.
And then, there's the streak going--
feeding into the center of the quasar.
Should I do this again?
Maybe I'll do this again.
So it's starting.
Again, gas being totally stripped out of the galaxy.
And then, here, you can see the streak.
And then it's going to come in here.
There may have been a dwarf being destroyed some time ago.
And then gas started to show up right near--
right at where the quasar is.
So this is what I would say, the so-called co-evolution
between super massive black hole feeding and the halo gas
caught in action for the first time.
So again, just to summarize, this is a snapshot of that.
It's just the image highlights where the galaxies are.
And this is a different panel of the gas in different wavelength
slide, velocity slice.
So just to compare with what we saw earlier in radial,
so remember the idea is to have a spatially resolved data using
both the morphology and velocity field
to identify the process that's driving--
that's dictating the content of the halo gas.
So we are very close.
And like I said, this instrument was only commissioned
a couple of years ago.
There's a lot more to come.
So now, I'm toward the end.
This is the end of my talk.
So I just close--
I hope you are convinced that we are really--
in terms of astrophysics research,
this is really exciting time for us.
A lot has happened, but a lot more are going to happen.
And for those who are interested in pursuing this,
I want to end with this quote from Dr. Vera Rubin, who
sadly passed away last year.
Does anybody know who she is?
Yes.
AUDIENCE: She found out the [INAUDIBLE] curve [INAUDIBLE]
in the galaxies.
HSIAO-WEN CHEN: Yes.
She found the first--
she discovered the first empirical piece of evidence
for dark matter through observation of the rotation
velocity is flattened.
So she had a lot of wise words, but this one stuck with me.
So naturally, given what she did, what she accomplished,
"don't shoot for the stars.
We already know what's there.
Shoot for the space in between because that's
where the real mystery lies."
I will just end here.
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