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Galaxies
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Early galaxies, seen in the infrared by the Hubble Space Telescope.
Herschel is expected to see even earlier galaxies at longer wavelengths.
(Image: Hubble Space Telescope)
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Some time around its one billionth birthday, the universe emerged
from its Dark Age into an era full of brightly burning galaxies. How
that happened, and how the universe of that time led to the one in
which we live today, are among the most intriguing and compelling
puzzles facing science.
Most of the light reaching us from primeval galaxies is in the
far-infrared and submillimeter range, and remains largely
undetectable until Herschel begins operation.
What we have been able to see so far tells us that the story of
the early universe involves supermassive black holes, dwarf galaxies
colliding into each other, starbursts, and lots of gas and dust.
Herschel's ability to see through and derive information from
that gas and dust will enable it to add greatly to our understanding
of early-universe residents and how they are related to each other
and to the modern universe. It will also enable us to learn much
more about parts of our local universe - such as the core of the
Milky Way - that are concealed by dust from other kinds of
telescopes.
Herschel may help to answer some fundamental questions: Did
galaxies all form around the same time in the early universe, or is
galaxy-formation an ongoing process that continues today? Did stars
lead to galaxies, or galaxies to stars? What kind of objects existed
in the early universe? And how did they evolve over the eons into
the galaxies of today?
Active Galactic Nuclei
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The center of galaxy M87 emits a near-light speed jet of electrons that is
5000 light years long. It's also firing a similar jet in the opposite direction.
(Image: Hubble Space Telescope)
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Among the stranger objects that appear to have populated the
early universe are active galactic nuclei (AGNs). As the name
implies, these are galaxies in which the cores emit vast amounts of
energy, far outshining the rest of the galaxy.
AGNs are classified as quasars, blazars, and Seyfert galaxies.
Quasars are found mostly at tremendous distances, indicating the
very early universe. Blazars, which appear even brighter, are
probably quasars that happen to have a radio-emitting jet pointed
straight at us. Seyfert galaxies appear to be closer (and so less
ancient) and less energetic. They may actually represent a later,
quieter stage of quasar evolution.
AGNs shoot vast amounts of energy out into space in two
enormously powerful jets, one at each pole. In the vast majority of
observed cases, a donut-shaped ring, or torus, of orbiting gas and
dust lies perpendicular to the jets.
It is generally held that all three types of AGN are really the
same kind of object. The perceived differences result from actual
differences in their energy output - which could be as much as
several billion suns - and also from the angle at which we view
them. If an AGN happens to be situated so that one of its jets faces
Earth head-on, we get the full impact and it looks extremely bright.
If instead we're positioned so that we see it through the edge of
its dusty torus, with the jets angled away from us, it appears
dimmer.
What makes AGNs so powerful? Supermassive black holes almost
certainly play an important role.
Monster in the Middle
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Artist's conception of a spinning black hole. Click to see animation.
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Black holes are objects in which so much mass is concentrated in
such small volume that nothing - not even light - can escape its
gravitational pull. A massive black hole has recently been detected
at the center of our galaxy, the Milky Way, and astronomers think
these ultradense objects may be typical features of galaxies. Black
holes can be so gravitationally strong that they twist space along
with them as they rotate!
AGNs are thought to contain supermassive black holes at their
centers, which suck in material from an "accretion disk," a ring of
dust and gas that may include matter drawn from nearby stars or even
captured galaxies! As the material spirals into the black hole, it
heats up to enormous temperatures and emits radiation. Magnetic
fields channel the radiation into polar jets, perpendicular to the
disk.
The brightness of an AGN tends to vary over time, perhaps
reflecting changes in the amount of its available fuel supply.
While AGNs have been detected by telescopes sensitive to optical
and higher frequencies, it is thought that those observations tend
to be of the later stages, when the objects have exhausted or blown
away most of their dusty envelopes. Looking at far-infrared and
submillimeter wavelengths, Herschel should be able to observe
younger AGNs, still enshrouded in the dust that feeds them.
Herschel may be able to help scientists answer some perplexing
questions:
Which came first - black holes or the galaxies in which they
reside? Does a black hole somehow form, and then draw the makings of
a galaxy around it? Or does a galaxy form first and then have its
center collapse into a massive black hole?
Did AGN-type galaxies evolve into ones like the Milky Way and its
neighbors? Is the Milky Way's core black hole a potential AGN?
Starbursts
Black holes may not be the only kind of engine that drives AGNs.
Starbursts may also play a role. Herschel should help scientists
sort out how much each kind of phenomenon contributes to the
enormous energy of an AGN.
Starbursts are intense bursts of star formation. They are thought
to occur when shock waves compress vast clouds of gas and dust to
the point where they collapse and form stars at rates up to hundreds
of times greater than "normal" galaxies.
These new stars burn fast and bright, then the larger ones
explode as supernovas, generating more shock waves and renewing the
star-formation process. This chain reaction probably lasts ten
million years or so - a relatively brief time in the life of a
galaxy - until most of the gas and dust is spent.
IRAS, an infrared telescope satellite, discovered thousands of
starburst galaxies. Herschel will study them and possibly discover
more, taking advantage of its sensitivity to the infrared emissions
of the dust clouds.
When Galaxies Collide!
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What causes the shock waves that trigger starbursts and,
possibly, massive black holes? One probable cause is galactic
collision.
We know that galaxies collide today. Our own Milky Way shows
signs of having swallowed another galaxy in the distant past, and is
headed for a run-in with the Andromeda galaxy in three billion
years!
In the early universe, galaxies must have bumped into each other
much more frequently than they do now. Since the universe is
expanding, the early universe must have been considerably smaller.
Galaxies would have been closer together and able to feel each
other's gravitational pull more strongly. They were likely drawn to
each other much more often than is the case today.
Though it may sound like the title of a science-fiction disaster
movie, what really happens when galaxies collide is a gravitational
dance lasting billions of years, which produces beautiful swirls of
stars, intense bursts of star formation and often - we think - an
elliptical galaxy at the end.
Galaxies are not solid objects, of course. They consist of stars
separated by vast stretches of empty space. So when galaxies
"collide," they spiral around each other for a few eons, and then
may pass right through each other before they settle down as one
new, larger galaxy.
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Colliding galaxies known as
"The Mice".
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You might say that what actually collides are the galaxies'
gravitational fields. Galaxies pack a lot of gravity, the sum total
of all their stars, interstellar gas and dust, and whatever "dark
matter" they may include. Two or more galaxies passing close to or
through each other can drastically alter each other's structures.
Stars may be whipped out of their home galaxies on long tails, drawn
into a bridge to the other galaxy, or driven together into hot
clusters.
There is a profound effect, too, on their huge interstellar
clouds of gas and dust. Shock waves can compress them to the point
where they trigger starbursts. And something in the process - maybe
the shock wave itself, maybe the starburst activity - might help
massive black holes to form or at least to grow.
The process of spiraling into each other and ultimately merging
can take a billion years or more. So when astronomers observe
galaxies in the act of merging, they don't see the motion of the
galaxies falling in toward each other. Rather, they see the galaxies
apparently motionless, at whatever phase of the interaction they
happen to be. They may be moving at breakneck speeds, but the
distances are so vast that from our point of view, the motion is
imperceptible.
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Hoag's Object, a ring of stars around a yellow nucleus, may be
the result of a galactic collision.
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So astronomers scanning the skies are apt to find galaxies that
appear to be frozen at all different stages of interaction and
merger (one of which is thought to be responsible for ultraluminous
infrared galaxies.) Herschel will be used to study all the stages of
galaxy interactions.
By studying galactic mergers in the local universe, which can be
seen in detail, Herschel can help scientists interpret what it will
uncover in the early universe. And by observing mergers at a range
of ages, Herschel will help to determine whether merging can lead to
present day elliptical galaxies.
For more on galactic collisions,
click here.
Building Blocks
Galactic mergers are likely the means by which smaller "building
blocks" grew into larger galaxies like those of today, each of which
may have as much mass as a billion or more suns.
It is commonly believed that there are many "small" galaxies of
only a few million solar masses in the distant, early universe,
which are yet to be discovered. Infrared background radiation
detected by the COBE satellite, launched in November 1989, supports
that idea.
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Arrows point to an ancient galaxy seen as dual images due to
a gravitational lens.
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The first sighting of one of these possible building blocks was
made in October, 2001 by the Hubble and Keck telescopes, with the
aid of a gravitational lens.
A gravitational lens is an effect in which light from a distant
source is bent by the gravitational pull of an intervening galaxy
cluster in such a way that the image is magnified when it reaches
us. It can make it possible to see objects that would otherwise be
too dim to detect.
The Hubble-Keck sighting was made at optical wavelengths, the
light frequencies visible to the human eye. Scientists expect to see
as many as 1000 times more gravitationally-lensed primeval galaxies
at the sub-millimeter wavelengths that Herschel will detect than in
any other waveband.
The galactic building blocks would presumably contain little or
no heavy elements, since their component stars would not have had
time to manufacture them. Herschel will study local blue compact
dwarf galaxies, which have similar compositions, as models for their
primeval counterparts.
Local News
Herschel will also contribute greatly to knowledge about galaxies
in our local part of the universe.
It will enable scientists to get a really good look at the Milky
Way's dusty core, including its recently-discovered black hole.
It will study the evolution of neighboring galaxies to shed new
light on the development of elliptical galaxies and spirals like
ours.
Herschel will survey nearby bright galaxies to determine how the
spectra of gas and dust is affected by galactic rotation, density
waves, tidal forces, winds and turbulence. This will provide
valuable insights that can be applied both to our local universe and
to observations of the dim, distant past.
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