Download Chapter 19. Mapping the Universe from Herschel to Sloan

Chapter 19. Mapping the Universe from Herschel to Sloan
1.
Introduction
Prior to about 1925 most astronomers believed that the primary constituent of the
Universe was stars, not galaxies. Galaxies were believed to be small nebulous things within
the Milky Way or maybe on its outskirts. Beginning with Sir William Herschel in the late
1700’s the goal of astronomers who wished to map the “Universe” was to make a map of the
distribution of the stars. That work culminated in about 1925 with the work of the Dutch
astronomer J.C. Kapteyn. Following Hubble’s discovery that galaxies were actually huge
collections of stars in their own right – “island universes” spread out over huge distances
in space and way beyond the local system of stars (our own Milky Way) that Herschel
and Kapteyn had been mapping – the notion of mapping the Universe changed. Now it is
about mapping the distribution of galaxies, not stars. That proceeds by an entirely different
process!
2.
Mapping the Stars: Herschel and Kapteyn
Making a map requires the ability to plot something (e.g. a star) in three dimensions.
It is easy to get two of those dimensions from the location of the star on the sky. This
can be given, for example, in the celestial equivalent of the Earth’s latitude and longitude
system. Astronomers refer to this as the right ascension (celestial longitude) and declination
(celestial latitude). The hard thing to get is the third dimension – the distance to the star.
We have already referred many times to the basic distance modulus equation, and need not
write it again. Recall that to get the distance to a star, astronomers need to measure its
apparent magnitude, which is relatively easy to do, and its Absolute magnitude, which is
generally difficult. Also, they need to correct for interstellar extinction, although neither
Herschel nor Kapteyn were aware of this.
The basic approach of mapping the stars, then, is to simply survey the sky and determine
the brightnesses of large numbers of stars. Herschel, working in the 1700’s and knowing
nothing of the variety of stellar sizes and luminosities, then made the simplest assumption
he could make – he assumed that all stars were just like the Sun, in terms of their luminosity.
Therefore, their distances depended only their apparent magnitude. Faint stars were distant
and bright stars were nearby. Using this technique he made a map of the distribution of
the stars. About 150 years later, J. C. Kapteyn did pretty much the same thing except
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he took into account that stars have different absolute magnitudes. He had to obtain the
spectral type of each star mapped and use that to determine its absolute magnitude. Again,
he neglected interstellar extinction – a fact which led to serious errors in his map.
Both Herschel and Kapteyn got the same basic result for the distribution of the stars.
They found that it was flattened, like a disk. As they looked in certain directions of space
(along the Milky Way) they saw that there were many faint, distant stars. This, in fact, is
why the Milky Way is visible to the eye – it is the faint glow caused by the accumulation of
millions of faint stars, too faint for the eye to see individually, which collectively cause the
glow. Looking out of the Milky Way, both Kateyn and Herschel found many fewer stars,
suggesting that the system was less extensive in those directions. Today, we know they were
simply measuring the thickness of the disk of our galaxy.
Both Herschel and Kapteyn also had the Sun fairly close to the center of the distribution
of stars. The reason is that we are limited in our view along the Milky Way by interstellar
extinction. Stars don’t really peter out towards the edge as they appear to do. They just
become so extincted that we cannot see to the actual edge of the system. In fact, optically, we
can only see a relatively small fraction of the galaxy – the part right around us. Interstellar
extinction blocks out view to great distances along the plane of the galaxy, although luckily
it is heavily concentrated to the plane of the galaxy so we can still see to the edge of the
Universe by looking out of the plane of the Galaxy.
3.
Globular Clusters and Shapley’s Galaxy
Even while Kapteyn was putting the finishing touches of his map of the “Universe” in
the mid-1920’s some astronomers realized that the true extent of the distribution of stars
might be much different than what Kapteyn was finding. They suspected that his results
might be greatly influenced by interstellar extinction. Also, one of them, Harlow Shapley of
Harvard, focused attention on massive clusters of stars, known as globular clusters. These
could be seen to greater distances than individual stars and were also located primarily out of
the plane of the Milky Way galaxy meaning that they were much less affected by interstellar
dust.
Shapley plotted the distribution of globular clusters and found that they are not uniformly distributed around the Sun, as would be expected if the Sun were near the center
of the galaxy, but they were mostly on one side of the Sun and had an average distance of
about 8000 pc from the Sun. This indicated that, unlike the view that Kapteyn had, the
Sun was nowhere near the center of the galaxy (assuming that these massive clusters orbited
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the center of the galaxy, as the laws of gravity would suggest). The center of the galaxy had
to be about 8 kpc from the Sun in the direction of the constellation Sagittarius. Shapley
correctly inferred that Kapteyn’s mistake was due to his neglect of interstellar extinction.
Shapley’s work helped us understand the true size of the Galaxy and the place of the Sun
in it (near the edge, not near the center). However, the large breakthrough that occurred
around that time (1925) was to come from the work of Henrietta Leavitt and Edwin Hubble
on Cepheid variables and the nature of the galaxies.
4.
Cepheid Variables and the Distance to M31 and M33
The key to understanding the nature of galaxies was to get their distances. In the mid1920’s telescopes were able to resolve individual stars in the closest galaxies, such as M31
(Andromeda Galaxy) and M33, but the star were too faint to obtain spectra which could be
classified. Therefore, we did not know if these stars were ordinary main sequence objects in
a rather nearby cluster of stars, within the boundaries of the Milky Way Galaxy or whether
they were supergiants in a distant stellar system. If distant, then the size of these systems
of stars would rival or exceed the size of our own galaxy!
The key to getting the distance to these systems came from the work of Henrietta
Leavitt of Harvard Observatory who was working on a type of variable star known as Cepheid
variables. These objects undergo brightness variations by a factor of two or more that are
periodic on time scales of a few days to more than 100 days. The cause of these variations
is pulsation of the star. Importantly, this type of pulsation only occurs in supergiant stars,
so the mere existence of the Cepheid phenomenon in a star marks it as extremely luminous.
Furthermore, Leavitt found an interesting relationship among the Cepheids in the Large and
Small Magellanic Clouds, two satellite galaxies of the Milky Way that are only visible from
the southern hemisphere. The period of pulsation is directly related to the luminosity (or
absolute magnitude) of the star (see slides for this lecture for a figure showing this).
This discovery gave Edwin Hubble the tool he needed to determine the distances to other
galaxies. The LMC and SMC are so close to the Milky Way that getting there distances
did not really prove the nature of the other galaxies. However, Andromeda and M33 are
about 30 times more distant than the size of the galaxy, even as estimated by Shapley.
When Hubble was able to use Cepheid variables in these systems to get their distance (by
measuring periods and using the Period-Luminosity relation derived for the SMC and LMC),
he proved that galaxies were huge, separate systems of stars comparable to the Milky Way.
This changed the size, structure and nature of the way we viewed the Universe immensely.
It was now recognized that to map the Universe, we had to map out the galaxies, not the
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stars. Galaxies were giant collections of stars, gas and dust and to understand how matter
was distributed through space, we had to focus on their distribution. Since faint galaxies
could be seen right to the limit of the largest telescopes, it became clear that the Universe
was much larger than we had thought and we had not yet come even close to seeing to its
edge, if it had one!
5.
The Hubble Law
While Cepheids are useful for getting distances to nearby galaxies it is also a painfully
slow process because you need to get many images of a galaxy spread out over more than
a year and discover all the Cepheids, measure their brightnesses as a function of time, etc.
Fortunately, a second discovery by Hubble has made this process much easier. While studying
the velocities of galaxies (using their radial velocities as measured by the Doppler shift)
Hubble stumbled on perhaps the most important scientific discovery of the 20th Century.
He found that there is an exceeding good, essentially linear correlation between the radial
velocity of a galaxy and its distance from us. This is shown in one or two figures on the slides
associated with the lecture. It means that every galaxy in the Universe (with the exception
of Andromeda and a couple of other extremely close ones that are actually orbiting the Milky
Way) is moving away from us and that the further away it is, the faster it is moving. The
interpretation of the Hubble Law (as it is now known) is at the heart of modern cosmology,
and we will return to it in a later lecture. Here we simply exploit this fact of nature to
determine distances to galaxies.
The Hubble Law makes it relatively easy to map the Universe because once it is calibrated (which requires distances by means such as Cepheid variables and/or other methods)
it can be employed to get the distance to any galaxy for which we can determine a radial
velocity from its red shift. Note that we use the term red shift here to denote the shift in
the spectral features of a galaxy to the red due to the Doppler effect. All galaxies (except
Andromeda and one or two other close ones) are red shifted because they are all moving
away from us. Motion towards us would produce a blue shift and this is never observed,
with the few exceptions noted.
To map the Universe, all needs to do is obtain a spectrum of each galaxy out there, and
measure its red shift. From the amount of shift, we get the radial velocity relative to the
galaxy (it is necessary, of course, to correct for the orbital and spin motions of the Earth
around the Sun and for the orbital motion of the Sun around the Galaxy). From the velocity,
the distance follows from Hubble’s Law. Note that the law can be written as
v = H0 d
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where v is the radial velocity, usually expressed in km/sec and d is the distance, usually
expressed in Mpc (Megaparsecs). H0 is called the “Hubble constant”. It is given the subscript
0 to indicate that it is the value of the constant “today”. If we go back in time a great distance
(corresponding to looking out in space a great distance), then the Hubble constant may have
been different. This means that the relationship between v and d is not exactly a straight
line when extended to very large values of v and d. For the local Universe, a straight line is
a perfectly good representation, however.
6.
Distribution of Galaxies on the Sky
Even before the true nature of galaxies was known, it was understood that they were
not uniformly distributed around the sky. A plot of where galaxies lie on the sky shows a
clear “Zone of Avoidance” (as it is called) that corresponds to the Milky Way. Today we
understand that this is because local dust in our own Galaxy is concentrated to the plane of
the Milky Way and blocks our view of galaxies in those directions. Looking perpendicular
to the galactic plane we can get a good sense of how galaxies are distributed through space.
Obviously we can only see the closer ones, the more distant ones being too faint to image.
As the slide on the lecture page shows, the galaxies are clearly not uniformly spread even
over the clear regions of space where we can see them. They tend to cluster together into
small groups and/or large clusters. There is an additional level of clustering even above
that – so-called “super clusters” (clusters of clusters). On the largest scale we begin to see
connections between the clusters, forming walls of galaxies on the edges of voids, where there
are fewer or no galaxies. However, the full exposition of this structure requires that we bring
in the third dimension – distance. To do that required obtaining spectra (and redshifts) for
hundreds of thousands of galaxies – a massive undertaking. This is precisely what the Sloan
Digital Sky Survey has done.
7.
The Sloan Digital Sky Survey (SDSS)
With financial backing from the Sloan Foundation, a group of astronomers recently took
on the task of creating the most extensive map of our Universe ever done using a robotic
telescope in New Mexico. Visit the SDSS Web site to get all the details. Basically, the
instrument has surveyed the part of the sky visible from New Mexico that is not within the
Zone of Avoidance and mapped the three dimensional distribution of hundreds of thousands
of galaxies, by obtaining their redshifts.
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While the full map is 3-d and hard to display, one cut through it is shown on the
class slides and others can be found at the SDSS Web site. One now clearly sees that the
distribution of galaxies can be described as “frothy” or bubbly. The galaxies are found
primarily on the periphery of large holes or voids. There are long walls of galaxies that
connect the concentrations (clusters and super clusters). There are huge empty spaces. We
will come back to this observed structure in a couple of chapters when we look at simulations
of how the Universe formed and how galaxies formed.