# 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 many strong radio sources are optically faint or invisible.  Familiar objects like the Sun and planets can 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 $< 1000$ light years) stars visible to the 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 of years ago. Radio galaxies and quasars are beacons carrying information about galaxies and their environs, 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 $\nu = 4.6$ 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 $T_{\rm b} \approx 10^6$ 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 $3\times10^4$ 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.  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

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 $\lambda = 0.85$ mm radio image of the Moon (below) looks familiar, but there are differences from the visible Moon.

Thermal emission from the Moon at $\lambda = 850~\mu$m. 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 point 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 studies of solar-system objects are active experiments, not just passive observations.  Planetary radar correctly determined 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, 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.

This Arecibo/GBT $\lambda = 70$ 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

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

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 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 $\lambda \approx 21$ cm hyperfine line at $\nu \approx 1420.4$ 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 throughout our Galaxy and nearby external galaxies.

Red indicates directions of high HI column 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.

The 21 cm HI line traces cold hydrogen tidally torn from the galaxies in the M81 group. Image credit

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.

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 composite image is based on VLA data 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

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

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 about 90% of the $\nu \approx 1$ 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.

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

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.

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 $z \approx 0.057$, corresponding to a distance $d \sim 240$ 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 $\approx 10^{45}$ erg s$^{-1} = 10^{38}$ W) could be detected almost anywhere in the universe.  The 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 not stars.  Gravitational energy released by matter accreting onto a supermassive ($M \sim 10^9\,M_\odot$) 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, $z \approx 0.16$. 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.

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 s$^{-1}$) 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.

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 $z \approx 1100$, so the photons we see today were emitted when the universe was only about $4 \times 10^5$ years old.   The CMBR is very nearly isotropic and very nearly a perfect blackbody with $T \approx 2.73$ 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.

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. 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.

The first massive stars and quasars re-ionized the universe during the first few hundred million years.  Observations of the changing HI signal at redshifts may tell us about the end of the "dark ages" and the epoch of reionization.