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A Tour of the Radio Universe
This composite picture shows the radio sky above an old optical photograph of the NRAO site
in Green Bank, WV. The former 300 Foot Telescope (the large dish standing between the three
85 foot interferometer telescopes on the left and the 140 Foot Telescope on the right) made
this 4.85 GHz radio image, which is about 45 degrees across. Increasing radio brightness is
indicated by lighter shades to indicate how the sky would appear to someone with a "radio
eye" 300 feet in diameter. Image credit
The visible and radio skies reveal quite different "parallel universes" sharing the same space.
Most bright stars are undetectable at radio wavelengths, and most strong radio sources are
optically faint or invisible. Familiar objects like the Sun and planets often look quite different
through the radio and optical windows. The extended radio sources spread along a band from
the lower left to the upper right in this picture lie in the outer Milky Way. The brightest
irregularly shaped sources are clouds of hydrogen ionized by luminous young stars. Such stars
quickly exhaust their nuclear fuel, collapse, and explode as supernovae, whose remnants
appear as faint radio rings. Unlike the nearby (distances < 1 000 light years) stars visible to the
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human eye, almost none of the myriad "radio stars" (unresolved radio sources) scattered
across the sky are actually stars. Most are extremely luminous radio galaxies or quasars, and
their average distance is over 5,000,000,000 light years. Radio waves travel at the speed of
light, so distant extragalactic sources appear today as they actually were billions o f years ago.
Radio galaxies and quasars are beacons of information about galaxies and their enviro ns,
everywhere in the observable universe and ever since the first galaxies were formed.
The brightest discrete radio source is the Sun, but it is much less dominant than it is in visible
light. The radio sky is always dark, even when the Sun is up, because atmospheric dust
doesn't scatter radio waves, whose wavelengths are much longer than the dust particles.
The quiet Sun at · = 4 :6 GHz imaged by the VLA with a resolution of 12 arcsec, or about 8400
km on the surface of the Sun. The brightest features (red) in this false-color image have
brightness temperatures Tb Ù 1 0 6 K and coincide with sunspots. The green features are cooler
and show where the Sun's atmosphere is very dense. At this frequency the radio-emitting
surface of the Sun has an average temperature of 3 Â 1 0 4 K, and the dark blue features are
cooler yet. The blue slash crossing the bottom of the disk is a feature called a filament
channel, where the Sun's atmosphere is very thin: it marks the boundary of the South Pole of
the Sun on this day. The radio Sun is somewhat bigger than the optical Sun: the solar limb
(the edge of the disk) in this image is about 20000 km above the optical limb. Image credit
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The Moon and planets are not detectable by reflected solar radiation at radio wavelengths.
However, they all emit thermal radiation, and Jupiter is a strong nonthermal source as well. If
the Sun were suddenly switched off, the planets would remain radio sources for a long time,
slowly fading as they cooled. At first glance, the Õ = 0 :85 mm radio image of the Moon (below)
looks familiar, but there are differences from the visible Moon.
Thermal emission from the Moon at Õ = 8 50 Öm. Image credit
The darker right edge of the Moon is not being illuminated by the Sun, but it still emits radio
waves because it does not cool to absolute zero during the lunar night. A subtler po int is that
the radio emission is not produced at the visible surface; it emerges from a layer about ten
wavelengths thick. As a result, monthly temperature variations of the Moon decrease with
increasing wavelength. These wavelength-dependent temperature variations encode
information about the conductivity and heat capacity of the rocky and dusty outer layers of
the Moon.
Radio observations of solar-system objects need not be passive. Radar yielded the first
measurement of the rotation period of Venus by penetrating its optically opaque atmosphere,
measured a more accurate value for the astronomical unit (the distance between the Earth
and the Sun), imaged the topography of the solid planets and moons, measured their ro tation
periods, and tracked asteroids and comets. Radar images like the one below were recently
used to search for water ice trapped in cold craters near the lunar poles. For an introduction
to radar astronomy, see the Arecibo radar web page.
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This Arecibo/GBT Õ = 7 0 cm bistatic radar image of the lunar pole did not find any water ice
within a few meters of the lunar surface, even in cold polar craters. Image credit
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This radar image of Venus has a resolution of about 3 km. A mosaic of the Magellan satellite
radar images forms the image base. Gaps in the Magellan coverage were filled with images
from the Earth-based Arecibo radar and with a neutral tone elsewhere (primarily near the
south pole). The composite image was processed to improve contrast and to emphasize small
features, and it was color-coded to represent elevation. Image credit
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This VLA image of Jupiter doesn't look like a planetary disk at all. Most of the radio emission
is synchrotron radiation from electrons in Jupiter's magnetic field. Image credit
The cosmic static discovered by Karl Jansky is dominated by diffuse emission orginating in and
near the disk of our Galaxy. The distribution of 408 MHz continuum emission shown below in
Galactic coordinates is expected since we are located in the disk of a galaxy similar to the
edge-on galaxy NGC 4565 shown below.
This all-sky 408 MHz continuum image (Haslam et al. 1982, A&AS, 47, 1) is shown in Galactic
coordinates, with the galactic center in the middle and the galactic disk extending horizontally
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from it.
The edge-on galaxy NGC 4565. We are located in the disk of a galaxy like this one. Image
credit
Interstellar gas in our Galaxy emits spectral lines as well as continuum noise. Neutral hydrogen
(HI) gas is ubiquitous in the disk. The brightness of the Õ Ù 2 1 cm hyperfine line at · Ù 1 420:4
MHz is proportional to the column density of HI along the line of sight and is nearly independent
of the gas temperature. It is not affected by dust absorption, so we can see the HI
everywhere in our Galaxy and in nearby external galaxies.
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Red indicates directions of high hydrogen density, while blue and black show areas with little
hydrogen. The figure is centered on the Galactic center and Galactic longitude increases to the
left. Some of the hydrogen loops outline old supernova remnants.
Image credit
The 21 cm HI line traces cold hydrogen tidally torn from the galaxies in the M81 group. Image
credit
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This false-color image of CO (J = 2-1) emission from the face-on spiral galaxy M51 was
made with the Smithsonian Submillimeter Array (SMA). It reveals regions containing dense
molecular gas, dust, and star formation that are optically obscured. Image credit
Some of the diffuse continuum emission from our Galaxy can be resolved into discrete
sources.
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Cassiopeia A (Cas A) is the remnant of a supernova explosion that occured over 300 years
ago in our Galaxy, at a distance of about 11,000 light years from us. Its name is derived from
the constellation in which it is seen: Cassiopeia, the Queen. A radio supernova is the explosion
that occurs at the end of a massive star's life, and Cas A is the expanding shell of material
that remains from such an explosion. This image was made by the VLA at three different
frequencies: 1.4, 5.0, and 8.4 GHz. The material that was ejected from the supernova
explosion can be seen in this image as bright filaments. Image credit
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This multiwavelength composite image of the Crab Nebula shows its X-ray (blue), optical
(green), and radio (red) emission. The pulsar is the bright point source at the center. Image
credit
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M80 is a dense globular cluster of several hundred thousand stars, most of which are very
old. The density of stars in such globular clusters is so high that stellar collisions are
common. Globular clusters "recycle" old pulsars to produce new pulsars with millisecond
periods. Image credit
Supernova remnants and the relativistic electrons accelerated in them account for abo ut 90%
of the · Ù 1 GHz continuum emission from our Galaxy. Most of the remaining continuum
emission at 1 GHz is thermal emission from HII regions, hydrogen clouds ionized by UV
radiation from extremely massive stars.
The nearest large HII region is the Orion Nebula.
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The Orion Nebula is a picture book of star formation, from the massive, young stars that are
shaping the nebula to the pillars of dense gas that may be the homes of budding stars. The
bright central region is the home of the four heftiest stars in the nebula. The stars are called
the Trapezium because they are arranged in a trapezoid pattern. Ultraviolet light unleashed by
these stars is carving a cavity in the nebula and disrupting the growth of hundreds of smaller
stars. Located near the Trapezium stars are stars still young enough to have disks of material
encircling them. These disks are called protoplanetary disks or "proplyds" and are too small to
see clearly in this image. The disks are the building blocks of solar systems.
The bright glow at upper left is from M43, a small region being shaped by a massive, young
star's ultraviolet light. Astronomers call the region a miniature Orion Nebula because only one
star is sculpting the landscape. The Orion Nebula has four such stars. Next to M43 are dense,
dark pillars of dust and gas that point toward the Trapezium. These pillars are resisting erosion
from the Trapezium's intense ultraviolet light. The glowing region on the right reveals arcs and
bubbles formed when stellar winds—streams of charged particles ejected from the Trapezium
stars—collide with material.
The faint red stars near the bottom are the myriad brown dwarfs that Hubble spied for the
first time in the nebula in visible light. Sometimes called "failed stars," brown dwarfs are cool
objects that are too small to be ordinary stars because they cannot sustain nuclear fusion in
their cores the way our Sun does. The dark red column, below, left, shows an illuminated edge
of the cavity wall. Image credit
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Orion's radio continuum is free-free thermal emission from the hot ionized hydrogen. The
dusty nebula is transparent at high radio frequencies, so all of the ionized hydrogen contributes
to the image below.
Thermal emission from the Orion nebula. Image credit
Thus massive, short-lived stars are responsible for nearly all of the radio continuum from our
Galaxy.
The radio luminosities of most spiral galaxies are proportional to their recent star-formation
rates. The nearby "starburst" galaxy M82 has a star-formation rate about ten times that of
our Galaxy and is a correspondingly brighter radio source. Most galaxies with little or no recent
star formation (e.g., elliptical galaxies) are radio quiet.
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This mosaic image is the sharpest wide-angle view ever obtained of M82. The galaxy is
remarkable for its bright blue disk, webs of shredded clouds, and fiery-looking plumes of
glowing hydrogen blasting out of its central regions. Throughout the galaxy's center, young
stars are being born 10 times faster than they are inside our entire Milky Way Galaxy. The
resulting huge concentration of young stars carved into the gas and dust at the galaxy's
center. The fierce galactic superwind generated from these stars compresses enough gas to
make millions of more stars. In M82, young stars are crammed into tiny but massive star
clusters. These, in turn, congregate by the dozens to make the bright patches, or "starburst
clumps," in the central parts of M82. The clusters in the clumps can only be distinguished in
the sharp Hubble images. Most of the pale, white objects sprinkled around the body of M82
that look like fuzzy stars are actually individual star clusters about 20 light-years across and
contain up to a million stars. The rapid rate of star formation in this galaxy eventually will be
self-limiting. When star formation becomes too vigorous, it will consume or destroy the
material needed to make more stars. The starburst then will subside, probably in a few tens of
millions of years. Image credit
Star-forming galaxies are very common, but their radio sources are not especially luminous, so
they account for less than 1% of the strongest extragalactic radio sources and somewhat less
than half of the cosmic radio-source background.
The strongest extragalactic radio source in the sky is the radio galaxy Cygnus A. The 1954
identification of this source with an extremely distant (redshift z Ù 0 :057, corresponding to a
distance d Ø 2 40 Mpc and a lookback time of about 700 million years) galaxy stunned radio
astronomers, who immediately recognized that such a luminous radio source (total radio
luminosity Ù 1 0 45 erg sÀ1 = 1 0 38 W) could be detected almost anywhere in the universe. The
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angular extent of this source, about 100 arcsec, implies a linear extent of about 100 kpc,
which is much larger than the host galaxy of stars. The energy source is clearly no t stars.
Gravitational energy released by matter accreting onto a supermassive ( M Ø 1 0 9 MÌ) black
hole in the center of the host galaxy powers this and other luminous extragalactic radio
sources.
A high-resolution VLA image of the radio source Cygnus A. The bright central component is
thought to coincide with a supermassive black hole that accelerates the relativistic electrons
along two jets terminating in lobes well outside the host galaxy. Image credit
The bright radio source 3C 273 was identified with the first quasar at an even higher redshift,
z Ù 0 :16. Such quasars appear to be radio galaxies in an especially active state, when visible
light from the region near the black hole overwhelms the starlight from the host galaxy and
makes the quasar look like a bright star.
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This HST gray-scale image of the quasar 3C 273 includes radio contours superimposed on the
optical jet emission. Image credit
Some exotic phenomena are radio sources but were discovered in other wavelength ranges.
Gamma-ray bursts (GRBs) are briefly the most luminous (up to 10 53 erg sÀ1 ) discrete sources
in the universe, so bright that they were discovered in the 1960s by the VELA nuclear-test
monitoring satellites. (For a good history, see the NASA/Swift GRB page). Their faint radio
afterglows have proven very useful in constraining the energetics and parent populations of
GRBs.
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Artist's conception of a gamma-ray burst. Radio observations made with the Very Large
Array, as well as the Australia Telescope Compact Array and the Ryle Telescope, have been
combined with optical and X-ray data to show that this cosmic explosion had a nested jet
structure as shown here. The thin core of the jet produced weak gamma-rays while the thicker
envelope produced copious radio waves. This information reveals that different types of
cosmic explosions (gamma-ray bursts, X-ray flashes, and some types of supernovae) have
the same amount of total energy and therefore share a common origin. Image credit
The final stop on any tour of the radio universe is the cosmic microwave background radiation
(CMBR), which is thermal radiation from the hot big bang. It fills the universe and is the
energetically dominant component of all electromagnetic radiation. We see the surface of last
scattering beyond which the universe was ionized and opaque. No radio sources, even if any
exist, could be seen beyond this point. The surface of last scattering is at redshift z Ù 1 100, so
the photons we see today were emitted when the universe was only about 4 Â 1 0 5 years old.
The CMBR is very nearly isotropic and very nearly a perfect blackbody with T Ù 2 :73 K.
The Wilkinson Microwave Anisotropy Probe (WMAP) satellite, in orbit near the L2 Lagrange
point, has made all-sky images of the tiny fluctuations in CMBR brightness.
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The WMAP spacecraft near L2 beyond the Moon. Image credit
Small fluctuations in the brightness of the CMBR, greatly accentuated in this false-color image.
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Image credit
The angular power spectrum of CMBR brightness fluctuations. Image credit
The angular power spectrum of these fluctuations constrains a host of fundamental
cosmological parameters. See the WMAP web site http://map.gsfc.nasa.gov/ for the latest
results.
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