So how is it that we know so much more about the galaxy than [UNKNOWN] or
Shapley? main reason that we have more information about the galaxy we live in
is because they were using visible light to make optical observations, and we have
access to the entire electro magnetic spectrum.
And so in this clip we're going to look at the Milky Way through several
different bands, and emphasize what it is that they teach us, each of them about
the Milky Way, and then later we'll put together all that information and see
what we know about the galaxy. So here's the multi-wavelength approach
to the Milky Way, and each of these images is an image of the entire plane of
the milky way, folded around, unfolded, so that the center is Sagittarius, the
direction to the center of the Milky Way, is centered in the images.
And we're seeing all of these except the bottom, are false color,
and we're seeing images of the milky way as it appears in various
frequency bands, and we'll start at the lowest frequency at the top.
These are radio waves, 408Khz, the radiations that these wave lengths is
produced, mostly, by relativistic electrons, propagating in the galactic
magnetic field. So we see that there are a lot of
emissions near the center of the galaxy, and we see some localized sources. The
source on the left is Cassiopeia A, the super nova remnant, we talked about it.
And the emission from there is in fact so brilliant that if you look carefully you
can see a sort of triangle shaped around it, which are the diffraction images of
the legs of the antenna, that is the radio antenna used to produce the image.
And so this gives us a map of where high energy charged particles are found at a
slightly higher frequency and a slightly shorter wave length.
At 1.4GHz, we have an extremely important measurement the wave length corresponding
to 1.4 GHz is about 21 centimeters, and this is an important frequency,
because it allows us to see clouds of neutral atomic hydrogen.
Now neutral atomic hydrogen is difficult to see because, if it is, unless it is
very hot the atoms are mostly in the ground state, and, the frequency of the
wavelength corresponding to excitation from the ground state to excited states
is ultraviolet, so atomic hydro, cold atomic hydrogen can absord, has
absorbtion spectrum in the ultraviolet, but there's not enough necessarily
ultraviolet like to observe it. On the other hand, it can both absorb and
emit, even at the relatively cool temperatures we're talking about, at this
wavelength of 21 centimeters, this is not an atomic transition where the electron
jumps from 1 energy state to the other. In fact, what this is, is the frequency
with which the electron precesses. The electron carries angular momentum,
intrinsic angular momentum in a magnetic moment, and the electron's angular
momentum precesses around, the, direction perpendicular to its orbits.
So, just like the Earth's axis wobbles as it orbits around the sun due to
gravitational tidal effects, in this case, the electron's axis wobbles as it
orbits the proton due to electromagnetic effects, and the frequency with which it
wobbles or precesses is 1.4 GHz. And so this is a low frequency at,
relatively low temperatures that are, these, atoms are excited. So they emit
these radio waves. Two great properties of this, one is it's
emitted by neutral hydrogen, and two, because it is a radio wave, dust and gas
clouds do not obscure it. So, when we look at 21 centimeters at the
galaxy, we can see through the clouds and the dust, and we can see the big gas
clouds themselves. And you see that the 21 centimeter image
of the galaxy shows us that hydrogen is scattered throughout with of course,
increased density near the center. Being able to observe the neutral
hydrogen clouds in, atomic hydrogen clouds in the galaxy is going to be
extremely important. Moving to a higher frequency, a still
shorter wavelength, the 2.7 GHz. notice that the, the wavelength of the 21
centimeter spin transition is rather, sharply defined.
You change wave of frequency, you don't see the atomic hydrogen anymore.
2.7 GHz, we're looking at, emissions by ionized gas and also fast, albeit not
relativistic. Electrons, and again, we see Cassiopeia A
as a source. We see the center of the galaxy as a
source. so these again are energetic, localized
sources producing these. And, there is also a sort of galactic
emission, but that's been subtracted to exhibit, these point sources better in
this picture. The middle pane, is, 115 GHz, and this is
again an important one. In fact, this corresponds to the energy
of a transition in carbon monoxide. Carbon monoxide? Why do, what's, there's,
where would we find carbon monoxide? Turns out that carbon monoxide, while
rare, is a very useful tracer molecule. It's concentrations in various regions of
the galaxy closely trace the concentrations of molecular hydrogen.
Where you find molecular hydrogen clouds, there are small trace concentrations of
carbon monoxide. Following the carbon monoxide, we get a
mapping of molecular hydrogen, and we see, as I said.
that molecular hydrogen is constrained to the plane of the galaxy, and we also see
that in certain places there are a broadening of sort of places where the
hydrogen clouds spill out of the plane, and we'll see those again in a In a
second. At a slightly higher frequency between 3
to 25 times 103 gigahertz is how it's represented.
These are infrared emissions and these far infrared emissions in fact are given
off. We see that there are not that many point
sources, but sort of a. continuous emission from the entire
galaxy, furthermore, we see that these emissions seem to track the H2 emissions
and where there is added H2 there are molecular hydrogen clouds, there's this
infrared emission indeed. At this wave length we are basically
observing the dust. It is extremely important that we can see
the dust in the Milky Way Because then we know how, what we need to subtract,
because we can use this tracing of the dust and of the molecular hydrogen, to
figure out what the extinction is and try to reconstruct the correct luminosity of
objects as seen through the dust, and notice as I said that the dust tracks the
molecular hydrogen rather well The image below that is a higher frequency, shorter
wave length yet. This is the near infrared, and in the near infrared dust
is not hot enough to glow. What we're mostly seeing is the emission
of stars. Which stars emit a lot of infrared
radiation? Red stars.
Which stars emit most of the infrared radiation? Giant red stars, red giants.
And so we're looking at cool giants here. We see in fact that we're seeing point
sources, and only towards the center of the galaxy
we're seeing sort of the far side of the galaxy in the center.
In that directions we see distant stars melded together into this great big glow.
But on the periphery, we're actually able to resolve individual sources.
The important thing is that we can see these bright stars in the infrared,
through the dust, because remember that scattering and absorption and dust is
highly frequency dependent. The high frequencies scatter more, and it turns
out are absorbed more, and so looking in the infrared, we can see through the dust
and distinguish the stars that are behind it.
Stars that produce lots of infrared will of course be more visible in this
wavelength, but we can see through the dust.
Dust. These are all the things that Kapteyn and
Shapley did not have at their disposal. At the bottom we finally see a visible
light image of the Milky Way, this is a familiar image.
We see the dark dust lanes which traverse the Milky Way, where there are clouds of
dust and, molecular hydrogen absorbing the light and we cannot see stars there.
of course in the visible light it's the blue luminous stars that dominate what we
see. But we cannot see them, and it's
interesting to compare the lowest image with say the middle image, and see how
closely the molecular hydrogen clouds track the gaps in visible light.
the gaps in the Milky Way as we see it, are precisely at the positions where
molecular hydrogen and the attendant, clouds of dust are obscuring the light.
This is the, technology from space observatories and X-ray's and gamma rays
that I haven't even included here, 2 radio telescopes that were not available
to [UNKNOWN], and it's thanks to that we have such a, so much better of picture of
how the Milky Way is constructed. Let's see what it has all taught us.