Download Chapter 20. Galaxies

Chapter 20. Galaxies
The study of galaxies – extragalactic astronomy, as it is known – is an enormous one
and in an introductory course like this we can only scratch the surface. Those of you with a
deeper interest in the subject should take our upper level astronomy course. I should also say
that, compared to what we know about stars, including their formation and evolution, we
know very little about galaxies. They are extremely complex and evolving (and interacting!)
systems that are full of surprises. The basic forces acting within them and their basic
constituents are understood, but they are large and have not reached an equilibrium state
and are exceedingly complex and varied as individuals. This, of course, is what makes them
interesting as an area of active research – there is a tremendous amount to learn about them!
1.
Types of galaxies
Galaxies are divided on the basis of their visual appearance into three basic classes
– spirals, ellipticals and irregulars. As the name suggests, spirals show evidence of spiral
structure if viewed face on or show a clear disk if seen edge on. Ellipticals have a much more
uniform distribution of star light and look either spherical or elliptical, depending on their
structure and viewing angle. Irregular is a class for galaxies that do not fit either description
above. Usually they are small galaxies that show evidence of recent star formation but not
in an organized, spiral pattern.
All of these types of galaxies can range in size over many orders of magnitude, with the
ellipticals showing the largest range. Giant ellipticals, often found near the centers of large
clusters of galaxies, can have masses of around 1013 solar masses, while dwarf ellipticals may
be 10 million times less massive. Spirals show a somewhat smaller range of masses from
perhaps 1012 solar masses to 108 solar masses. Irregulars, as just noted, tend to be smaller
galaxies. (See the images on the slides page.)
Galaxies are further classified according to criteria originally established by Hubble and
often referred to as the Hubble “tuning fork” diagram. The ellipticals define a single sequence
that ranges from E0 to about E7 based on how elliptical they appear, with E0’s looking
circular and larger numbers denoting more elliptical looking galaxies. There is a transition
kind of object to spirals, which is called an S0 galaxy. These look a bit like ellipticals since
they do not have spiral arms, but a bit like spirals because they apparently have a disk and a
central bulge of stars. The spirals form two parallel branches, one called normal spirals and
the other called barred spirals and denoted SB. In each case, the classification moves from
a to d as the galaxies move from having large central bulges and tightly wrapped arms (Sa
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or SBa galaxies) to small central bulges and open arms (Sd or SBd galaxies). It is thought
that the Milky Way is an Sb galaxy. The physical significance of this classification scheme
is not fully apparent yet. It certainly does not represent any type of evolutionary sequence.
Galaxies consist of four major components: stars, ISM (gas and dust), central black holes
(perhaps not present in some galaxies, especially very small ones), and dark matter. We
discuss each of these in turn.
2.
Stars
The dominant form of stars in galaxies is low mass (M stars), low luminosity stars that
provide the bulk of the stellar mass of all galaxies. The light of galaxies comes primarily
from the rarer, more luminous stars, supergiants, red giants, and (if present) massive O and
B stars. Within our own galaxy we recognize two “populations” of stars that have a different
distribution within the galaxy, and this seems to extend to other galaxies. Population I stars
are more recently formed stars that have higher metal abundance (having formed after the
explosion of many supernovae generated metals). All young stars, such as O, B and A stars
are of this population, as is the Sun. These stars reside in the disk of spiral galaxies. They
are not found in elliptical galaxies.
Population II stars are the older stellar component – the first stars to form as a galaxy
was forming. They have lower metal abundances, reflecting the fact that less supernovae
had exploded prior to their formation and that there were simply less heavy elements in the
Universe at that time. Since Pop II is an old population we find none of the O, B, A or
even F stars because all of these objects are massive enough to have already evolved into
supernovae. The brightest stars in this population are red giants HB and AGB stars. Within
spirals, these stars are found in the bulge and halo rather than in the disk (although some, of
course, are passing through the disk). This is the only population present in most elliptical
galaxies.
Note that the dominant color of an older population (Pop II) is red, since most of the
light is coming from red giants, whereas Pop I stars may be dominated by the bluer stars
(OBA main sequence stars and/or supergiants). Hence, galaxies such as ellipticals, that
are dominated by Pop II are also redder in their overall color, while galaxies such as spirals
dominated by Pop I stars are bluer (although the reddening effect of dust in the ISM can have
an effect here for some galaxies – e.g. edge-on ones). Irregulars tend to be blue, indicating
they have recent star formation and Pop I stars.
Often stars are concentrated into clusters and one particularly interesting type of cluster
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is called “globular” clusters. These are the largest stellar clusters within a galaxy, sometimes
being as large as one million solar masses. They are always composed of Pop II stars and
are always found in the halo of the galaxy. They seem to represent the first things to form
as a galaxy is forming. The number of globular clusters within a galaxy is generally related
to its mass. The Milky Way has about 150 of them. As noted in the last chapter, Harlow
Shapley used the distribution of globular clusters to determine the location and distance to
the center of the Milky Way.
3.
ISM
Galaxies vary enormously in the amount of interstellar medium (ISM) – gas and dust –
that they contain (see slide from class lecture). It can range from essentially zero (for most
elliptical galaxies) to as much as 90% of the mass of the gas (for some spirals and irregulars).
Typically the ISM is of order 10% of the mass of the stars. It represents the fuel for more
star formation. When a galaxy has used up or lost its ISM, star formation is finished and
the stellar population can only age. Apparently most elliptical galaxies used up all their
ISM very early in their histories, leaving them as now composed of only Pop II stars and
devoid of ISM. A few ellipticals do have evidence of some gas and dust and continuing star
formation but they are a rarity.
Within spiral galaxies the ISM is highly concentrated to the disk of the galaxy and,
within the disk, is concentrated to the spiral arms. This is easy to see visually because of the
effect that the dust component has on obscuring star light. It clearly marks the distribution
of the gas. Edge-on spirals show a dark band along the disk, often cutting it in half. This
shows that the ISM is even more concentrated to the plane of the galaxy than are the Pop
I stars. This is because gas can interact with itself through the E&M force and collides and
dissipates energy, settling to the disk of the galaxy. Once formed, stars are much too small
and widely spread to ever collide with one another so their orbits continue to reflect the
properties they had when they were formed and/or get pumped up to higher distances from
the plane through close encounters with other stars, clusters of stars and/or giant molecular
clouds.
The dark lanes showing high ISM concentrations clearly mark the spiral arms in some
spiral galaxies, showing that the gas gets concentrated (by a spiral density wave in the
underlying low mass stars) into a spiral pattern. As the ISM enters the density wave it gets
compressed, concentrated and giant molecular clouds apparently form. These then give rise
to the O and B stars that light up the spiral arms. This is the basic picture we have of spiral
galaxies, although the details are still quite murky!
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Irregular galaxies often have a great deal of gas and dust in them indicating that star
formation has not (yet?) been very active within them. They tend to be lower mass objects
that may have simply been a bit delayed in contracting, compressing and forming stars. Some
galaxies have obviously been stripped of their ISM, either by collisions with other galaxies,
or passing through the inter cluster gas of a rich cluster of galaxies or having supernovae
within them actually blow their gas and dust right out of the galaxy (especially true for low
mass galaxies where the escape speed may not be that high).
In some cases, the ISM can be kept as a hot halo of gas surrounding the galaxy. This
gas can reach temperatures of millions of K (like the coronal gas within our own galaxy) and
can be detected by its X-ray emission.
4.
Central Black Holes, AGNs and Quasars
The evidence for a central black hole in our own galaxy, based on the rapid orbital
motions of stars at the center of our galaxy, was shown earlier in the class (see links page).
Other nearby galaxies show similarly high velocities for stars and/or ISM that is orbiting
close to its center. This is one indication that central black holes are common in galaxies,
perhaps ubiquitous. Another indication comes from the phenomenon of Active Galactic
Nuclei (AGNs) and quasars.
A small fraction of galaxies in our local neighborhood show evidence of violent activity
from their nuclear regions. This can appear as a jet of material, visible optically or at radio
wavelengths, that extends straight out from a galaxy for thousands of parsecs. The staightness of these features and their length indicates ejection of matter at speeds approaching
the speed of light and a temendously efficient and powerful energy source. We understand
this source to be a central black hole which is being “fed” gas – i.e. has an accretion disk
through which matter is moving towards the black hole. The disk can be highly magnetized
and accreting matter is often caught up in the magnetic field lines and accelerated along
them – a phenomenon known as the magneto-centrifugal acceleration. It is often described
as similar to the acceleration of beads on a string that is spun around. This phenomenon
can accelerate gas to close to the speed of light and collimate it into two opposing “jets”
shooting in opposite directions across space.
Besides the jets, AGNS reveal themselves by their X-ray emission associated with the
hot gas in the accretion disk and its vicinity and by optical emission lines coming from the
heated gas in the nucleus. Furthermore, there is often optically visible light generated in the
nuclear region by this black hole engine and accretion disk. The light can even dominate the
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star light from the galaxy. It is further characterized by sometimes extreme variability that
reflects variations in the accretion rate. The time scale for the variations can be as short
as minutes, hours, days or months. When variable galaxies on these time scales were first
discovered it was hard for people to believe, since no object bigger than a few light minutes
could vary on time scales as short as minutes. This meant that the light source at the centers
of some AGNS is no more than, say, 1 AU, but was as powerful as an entire galaxy! Today,
we understand that only a black hole, which can convert gravitational potential energy of
infalling gas to radiation, is capable of such efficient energy generation on such a small scale!
An extreme example of the AGN phenomenon are the so-called Quasars. The term
quasar is short for quasi-stellar object (QSO). These are now recognized as very active
AGNs that are predominantly found at high redshift (i.e. very distant from us). This
is partly because they are a rare phenomenon and partly because they were more common
earlier in the history of the Universe. Quasars occur when the central black holes are massive
and fed large amounts of gas, resulting in an extremely bright AGN which dominates the
light of a galaxy. Viewed from a great distance, one can actually lose sight of the underlying
galaxy, so the objects appear to be stars, although deep photographs show that they have
jets and underlying fuzziness associated with the stars in the galaxies in which they reside.
Quasars are the brightest single things in the Universe and, therefore, important probes
of the Universe out to the greatest distances to which we can look. Their spectra often show
many absorption lines, especially due to hydrogen, from intervening clouds of gas. The most
prominent line is the n=2 to n=1 transition of neutral Hydrogen – the Lyman α line. This
is sometimes referred to as the Lyman forest of lines.
It is by no means clear how the central black holes of galaxies formed. They can be
very massive – up to billions of solar masses, although more typically in the millions. This
is much larger than a stellar-sized black hole, since stars can only be about 100 solar masses
or less. But, it is still only a small percentage of the mass of the galaxy in which they reside.
Interestingly, as shown in the lecture slides, there is a nice correlation between the mass of
a galaxy and the mass of its central black hole. Larger galaxies tend to have larger central
black holes. It is not really known if this correlation extends down to very small galaxies,
some of which may not even have central black holes. How this correlation arose, how the
central black holes formed, how they got to the centers of the galaxies (if they were not born
there) are questions remaining for astronomers of coming generations (you?!) to solve.
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5.
Dark Matter
While it had been suspected even since the 1930’s that there might be a lot more to
galaxies than met the eye (i.e. much of their matter might be invisible to us), this point
was not widely accepted until the 1980’s. The work of Vera Rubin, of Carnegie Institution’s
Dept. of Terrestrial Magnetism was key in convincing astronomers that this must be the
case. She derived rotation curves for galaxies (see figure in slides from this lecture) that
showed that the orbital velocity of their stars and ISM maintained a roughly constant value
even as one moved very far from the center of the galaxy. This was quite surprising since
one expects gravity to weaken with distance from its source and, therefore, orbital velocities
to get lower (as in the solar system). The accepted explanation for this is now that the
mass interior to any point in a galaxy must be continuing to grow substantially as one moves
out, balancing the fact that one is further from the center. This keeps the central force of
gravity higher on any star, even near the periphery of the galaxy, and keeps it moving. Since
the light in galaxies does not behave this way – it is very concentrated to the center of the
galaxy, the mass must not follow the light and is described as “dark matter”.
To quantify this a bit, consider the case of the Solar System which gives rise to so-called
“Keplerian” orbital velocity behavior, in honor of Kepler, who first discovered it. Using
Newton’s laws we can write:
F = ma
or
from which it follows that
GMm
mv 2
=
r2
r
r
GM
.
v=
r
Hence, in the solar system we find that orbital velocity, v, falls off as r−0.5 as one moves away
from the Sun.
In galaxies, we expected to find this but Rubin’s measurements showed otherwise.
Rougly, v remains constant. To understand this, recall that the force of gravity felt by
an object within a sphere of matter comes only from the matter closer to the center of the
sphere than the object itself. This is referred to as the mass interior to the object and is
often written as Mr . Recall that we used this concept in developing the equations of stellar
structure. Now, in the solar system, the mass interior to any location does not change – it
is just the mass of the Sun – because the planets have negligible mass compared to the Sun.
Therefore, we can represent Mr as a constant equal to the mass of the Sun. Adopting it as
a constant is what gives rise to the Keplerian velocity behavior of v ∝ r −0.5 .
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As you worked out in one of your problem set questions (and review the answer sheet
if you didn’t get it right!), if we observe v = constant, instead of a falling v, we can infer
(assuming that laws of gravity still hold even over the great distances of a galaxy’s radius)
that Mr must be growing within galaxies and we can derive its dependence on r and the
total amount of mass required. In this way, we have determined that galaxies are, in fact,
MOSTLY dark matter. The dark matter component is typically many times larger than the
amount of mass in the other components (stars, ISM and central black holes). Galaxies are
mostly dark matter!
Additional evidence for dark matter and the fact that it is somewhat concentrated
towards the centers of galaxies, but not extremely so, comes the motions of galaxies within
clusters. These are generally too large to be explained by just the visible (stars and ISM
and central black holes) components of galaxies. Furthermore, some galaxies are enveloped
in extremely hot, coronal gas, that could only be kept confined to that galaxy if its mass
were much larger than met the eye. These X-ray detected halos of hot gas are an additional
argument for the fact that dark matter dominates galaxies.
The question remains, of course – what is the nature of the dark matter. There are two
basic hypotheses: it could be normal, baryonic matter (remember that baryons are protons
or neutrons – normal atoms, composed of quarks) in a particularly non-luminous form (e.g.
white dwarf, neutron star, black hole, very low mass star, brown dwarf, planet, etc.) or it
could be something exotic – a massive particle not yet known to physicists. All evidence
favors the latter!
If it were normal matter in non-luminous form it would be in compact objects that must
inhabit the halo of our galaxy. Some of these objects would be passing through the plane
of the galaxy and some would be in our vicinity. While very underluminous and hard to
detect by their light, it still would be possible to detect them by their gravitational lensing
effect on any stars with which they happen to align. Surveys for such alignments have been
carried out extensively. Such objects have been found and are referred to as MACHO’s,
which stands for Massive Compact Halo Objects (see link on Links page). However, there
are not nearly enough of them for them to be an important mass component in our galaxy.
They can certainly not account for the required amount of dark matter.
Another argument against normal matter in compact, non-luminous form, being the
dark matter comes from the abundance of isotopes of light elements like Helium, Lithium
and Beryllium in the Universe. We will come to that argument in a later chapter. Here we
simply note that the strong weight of evidence in now in the corner that the main constituent
of galaxies is dark matter and that the dark matter is composed of some exotic particle not
yet known to physicists. The race is on to discover what this stuff is and to assimilate it into
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our understanding of the Universe. Again – there is much work here for the generations of
astronomers (and physicists, particularly, in this case) to come. The term WIMPs (meaning
Weakly Interacting Massive Particles) and contrasting with MACHOs is sometimes used to
describe exotic particles which comprise the dark matter. (We should always keep in mind
that the dark matter may turn out to be more than just one thing – it may have its own
complexity, and probably does!)