Swinburne Online Education Introductory Radio Astronomy SETI Module
Swinburne Online Education Introductory Radio Astronomy & SETI Module 4: Applications of Radio Astronomy Activity 2: … And Leaving the Milky Way © Swinburne University of Technology
Summary In the last Activity we learned about galactic sources of radio emission. We will now turn our attention to extragalactic sources. In particular, we will look at: 1. HI in other galaxies; 2. giant elliptical and irregular galaxies; 3. quasars; and 4. background radiation left over from the Big Bang.
Extragalactic sources of radio emission Sources of radio emission that lie beyond the Milky Way include: • neutral hydrogen (HI) from other galaxies. • giant elliptical and irregular galaxies that emit more than a millions times as much radio energy as normal spiral galaxies. Astronomers call these radio galaxies. • quasars (or ‘quasi-stellar radio objects’) are the most distant objects known. They are strong, extremely compact, radio sources. • cosmic background radio noise coming from every direction in the sky. This is a remnant of the Big Bang.
Radio emission from neutral hydrogen ep+ HI exists as a single atom of hydrogen with one proton and one electron. HI HI in spin-down state In the last Activity, we learnt that when an atom of hydrogen undergoes a magnetic spin-flip transition, it emits a radio photon with a frequency of 1428 MHz. This corresponds to a wavelength of 21 cm.
Radio emission from neutral hydrogen ep+ HI exists as a single atom of hydrogen with one proton and one electron. HI 21 cm photon HI flips to spin-up state and emits a radio photon In the last Activity, we learnt that when an atom of hydrogen undergoes a magnetic spin-flip transition, it emits a radio photon with a frequency of 1428 MHz. This corresponds to a wavelength of 21 cm.
Radio emission from neutral hydrogen ep+ HI exists as a single atom of hydrogen with one proton and one electron. HI 21 cm photon HI flips to spin-up state and emits a radio photon Astronomers use radio observations of HI emission to study the spiral arm structure of the Milky Way, but they also study HI emission in other galaxies. In particular, we can study the 21 cm emission line from other galaxies to estimate their mass, their distance from Earth, and to work out how fast they are rotating.
HI in other galaxies All galaxies probably contain some neutral hydrogen, but spiral galaxies contain much more than do elliptical galaxies. Astronomers have found that the spectral line profiles of spiral galaxies fall into two groups: double-peaked and singlepeaked. A double-peaked profile HI gas in this indicates a rotating region is moving away from us galaxy that is observed “edge-on” from Earth. The double peak is HI gas in this caused by Doppler shift region is moving of the 21 cm HI emission towards us line. Rotation direction
HI in other galaxies All galaxies probably contain some neutral hydrogen, but spiral galaxies contain much more than do elliptical galaxies. Astronomers have found that the spectral line profiles of spiral galaxies fall into two groups: double-peaked and singlepeaked. Emission from HI A double-peaked profile Emission from moving away HI moving indicates a rotating from us is towards us is Doppler shifted galaxy that is observed towards longer towards shorter “edge-on” from Earth. wavelengths The double peak is caused by Doppler shift of the 21 cm HI emission line. 20 21 22
HI in other galaxies All galaxies probably contain some neutral hydrogen, but spiral galaxies contain much more than do elliptical galaxies. Astronomers have found that the spectral line profiles of spiral galaxies fall into two groups: double-peaked and singlepeaked. A single-peaked profile indicates that we are viewing the galaxy “faceon”. In this case, all the HI emission is observed at approximately the same wavelength.
HI in other galaxies All galaxies probably contain some neutral hydrogen, but spiral galaxies contain much more than do elliptical galaxies. Astronomers have found that the spectral line profiles of spiral galaxies fall into two groups: double-peaked and singlepeaked. Single peak shows A single-peaked profile HI emission indicates that we arriving with approximately the viewing the galaxy “facesame wavelength. on”. In this case, all the HI emission is observed at approximately the same wavelength. 20 21 22
Determining the mass of nearby galaxies The Doppler shift of the 21 cm radio emission line of nearby galaxies can also be used to determine their mass. Astronomers measure the Doppler shift of the 21 cm line at various points along the galaxy’s diameter. Doppler shift measured at each of these points This determines the radial velocity of matter in the galaxy.
Determining the mass of nearby galaxies The Doppler shift of the 21 cm radio emission line of nearby galaxies can also be used to determine their mass. Doppler shift measured at each of these points If the tilt of the galaxy to our line of sight is known, radial velocities can be translated into rotational velocities - a measure of how fast the matter in the galaxy is rotating around the galactic centre. Using Newton’s law of gravitation, astronomers can then determine the mass of the galaxy.
Determining the distance to galaxies The Doppler shift of the spectral line profile also helps us estimate how far away a galaxy is. This technique was developed by the famous astronomer Edwin Hubble and is one of the most important achievements of modern astronomy.
Hubble discovered that the emission from almost all galaxies is redshifted. This means that their spectral line profiles are Doppler shifted towards longer (‘redder’) wavelengths. red-shifted 21 cm line radio microwave infra-red visible UV X-rays gamma Low frequency High frequency Long wavelength Short wavelength Spectral lines that are shifted towards longer wavelengths are said to be ‘red-shifted’.
21 cm line blue-shifted 21 cm line radio microwave infra-red visible UV X-rays gamma Low frequency High frequency Long wavelength Short wavelength Spectral lines that are shifted towards shorter wavelengths are said to be ‘blue-shifted’.
Introducing redshift Astronomers use the word ‘redshift’ in a very particular way. For a galaxy at a distance, d, from Earth, redshift is defined as, where z is the redshift of the galaxy spectrum, 0 is the wavelength of the spectral line that is emitted by the galaxy, and is the wavelength of the spectral line that we detect (for redshifted galaxies will be greater than 0; for blueshifted galaxies, z will be negative). 0 radio microwave infra-red visible UV X-rays gamma
By observing the redshifts of many, many galaxies, Hubble discovered the empirical law: where z is the redshift, c is the speed of light H is the Hubble constant, and d is the distance to the observed galaxy. H and c are arguably the two most important constants in astronomy. c, the speed of light, equals 3 x 108 ms-1. Determining the value of the Hubble constant (H) is an ongoing project of modern astronomy. At present, we think the value of H is approximately 70 kms-1.
By observing the redshifts of many, many galaxies, Hubble discovered the empirical law: where z is the redshift, c is the speed of light H is the Hubble constant, and d is the distance to the observed galaxy. By rearranging the equation and making a reasonable assumption for the value of H, we get a formula for the distance to a galaxy based on its redshift, ie. ,
By observing the redshifts of many, many galaxies, Hubble discovered the empirical law: where z is the redshift, c is the speed of light H is the Hubble constant, and d is the distance to the observed galaxy. cz is the velocity (v) at which the galaxy is receding. Often astronomers write,
Hubble’s Law and his observations of galactic redshifts had many profound consequences for cosmology. Since nearly all galaxies are redshifted (which means that they are moving away from us), astronomers realised that the universe must be expanding. Moreover, we can use Hubble’s Law to estimate the age of the Universe (t), ie. , Substituting the Newtonian equation for distance d = vt, and cancelling common factors gives The last equation says that the age of the universe is simply the inverse of the Hubble constant! (Note that this doesn’t take into account any change in the rate of the Hubble expansion. )
Determining the rotation of other galaxies You can learn more about Hubble’s Law and further issues in contemporary cosmology in the Unit Theories of Space and Time. For now, let’s look at how we can use Doppler-shifted HI emission from other galaxies to work out how they are rotating. This technique is based on the same principles that astronomers use to determine the rotation of the Milky Way, but now the target area for observation is much smaller and much further away. Let’s work out how the galaxy UGC 9242 is rotating. . .
HI in is ap this are a proa chin g HI in th is re is area cedi ng Optical image of UGC 9242. We will take measurements of HI emission at various points along the narrow window. Doppler-shifted HI emission from UGC 9242.
The Doppler shift data can be translated into velocity measurement for HI in the galaxy. The x-axis measures the distance along the galaxy major axis. Doppler-shifted HI emission from UGC 9242.
Introducing AGNs Until the late 1940 s, astronomers believed that galaxies were simply large collections of dust, stars and gas. One of the major achievements of radio astronomy has been to show that there are many, more complex processes at work within galaxies. In particular, astronomers now separate galaxies into two distinct groups based on their radio emission - so-called ‘normal’ galaxies and the Active Galactic Nuclei group (AGNs). AGNs exhibit very powerful radio emission from a small area within the galactic centre. The AGN group includes objects such as Seyfert galaxies, radio galaxies and quasars.
Examples of Active Galactic Nuclei The Seyfert galaxy NGC 7742 Seyfert galaxies are named after Carl Seyfert, who first identified them in 1943. They have very bright nuclei and are usually spiral galaxies. Approximately 1% of all spiral galaxies are Seyferts. Like most AGNs, Seyfert galaxies have broad emission lines and their luminosity varies over short timescales (ie. a few days to a few months). For this reason, Seyferts are identified with AGNs even though their radio emission is not particularly strong.
Examples of Active Galactic Nuclei The radio galaxy NGC 4261 Unlike Seyferts, radio galaxies are all elliptical or irregular. Radio galaxies are one of the most fascinating objects in the Universe often exhibiting strange emission structures, and perhaps the home of massive black holes! We will be discussing radio galaxies in more detail later in this Activity.
Examples of Active Galactic Nuclei High redshift quasars Quasars are bright, compact radio sources, several thousand times more luminous than the Milky Way. In size, however, they may be small as our Solar System. Quasars are some of the most distant (and oldest) objects that we can see in the universe. The study of quasars is very important for cosmology.
The discovery of radio galaxies Following the Second World War, advances in radio technology were turned to more peaceful, astronomical purposes. Several distant, massive galaxies were soon identified as the sources of strong radio emission, sometimes 100 million times brighter than the radio emission from the Milky Way. Astronomers named these objects radio galaxies. Radio emission contours superimposed onto an optical image of the radio galaxy 3 C 368.
Extended and compact radio galaxies Today we use the term radio galaxy to denote a subset of AGNs that produce more than 1033 watts of radio power. Radio galaxies are typically elliptical, and fall into two main types - extended and compact. Extended radio galaxies exhibit radio emission that extends beyond the optical image of the galaxy. Compact radio galaxies exhibit radio emission that is contained within the optical image of the galaxy. extended radio galaxy compact radio galaxy
Cygnus A, a classic radio galaxy The radiative mechanism of radio galaxies is still not wellunderstood, although astronomers believe that the radio emission is caused by synchrotron radiation. Cygnus A was one of the earliest radio galaxies to be discovered. It is the second brightest radio source in the sky, dimmer only than the Cas A supernova remnant.
The double-lobed emission region What is most interesting about radio galaxies like Cygnus -A, however, is that the strongest emission does not seem to come from the galactic centre. Rather, radio emission is strongest in two giant lobes that are ejected on either side of the galaxy. This structure is not visible at optical frequencies.
Radio image of Cygnus A. l. y 0 0 0 , 430 . Size of Milky Way Cygnus A Same-scale optical image of Cygnus A. It lies near the dusty, star-strewn plane of the Milky Way, and is difficult to see clearly. Many of the blobs are nearby galaxies.
A supermassive black hole? The lobes of radio galaxies are one the richest areas of astronomical inquiry. Observations show that they are continuously supplied with high energy particles and magnetic fields by jets that originate in the galactic nucleus. Astronomers suspect that the extraordinarily energetic processes that occur in the nuclei of radio galaxies may be powered by supermassive blackholes.
A supermassive black hole? The lobes of radio galaxies are one the richest areas of astronomical inquiry. Observations show that they are continuously supplied with high energy particles and magnetic fields by jets that originate in the galactic nucleus. The jet from the radio galaxy M 87 is created by energetic gas swirling around a massive black hole at the galaxy's centre. Electrons are ejected at almost lightspeed from the region surrounding the black hole’s event horizon, emitting visible light as they spiral in the magnetic field.
Introducing quasars The discovery of radio galaxies was quickly followed by that of quasars. Quasars are bright, compact radio sources that were initially thought to be stars. They are several thousand times more luminous than the entire Milky Way galaxy, but may be as small as our Solar System. In the 1950 s, quasars had astronomers bamboozled they showed distinct spectral line emission, but the emission did not correspond to the emission from transitions or collisions of any common elements.
The most distant objects in the Universe Finally, Maarten Schmidt realised that the line emission profiles looked strange simply because they were extremely redshifted. The redshift of quasars is quite remarkable - quasars have been discovered with redshifts as high as 5. 8! Do you remember our equation? Since c and H are both constants, an object with a high redshift (z) must also be very far away.
Quasars are AGNs In fact, quasars are amongst the most distant objects known, and offer a unique perspective on the cosmology of the young universe. At such distances, it is clear that quasars cannot be stars. Astronomers now believe that they are extremely bright active galactic nuclei that outshine the emission from the rest of their host galaxies. A yet-unnamed quasar, the most distant object known in the universe (June 00) with a redshift of 5. 8.
Comparing quasars and radio galaxies Like radio galaxies, the radio emission from quasars is caused by synchrotron radiation. Quasars, however, are much more compact sources than radio galaxies and many quasars have been identified with spiral, rather than elliptical, galaxies. Of the AGNs with strong radio emission and optical counterparts, 1/3 have been identified as quasars, and the other 2/3 are radio galaxies.
Radio emission from the Big Bang The final source of radio emission we will consider in this Activity is cosmic background radiation. From every direction in the sky, there is a low level of radio noise that is left over from the Big Bang. Astronomers have measured the cosmic background radiation across the entire electromagnetic spectrum and found that the universe is an almost perfect blackbody! intensity Expected radiation from a perfect blackbody Observed background radiation of the universe wavelength
Blackbody theory shows that there is a correlation between the temperature of a blackbody and the peak of the radiation spectrum. For the cosmic background, the spectrum peaks at 1 cm, corresponding to a temperature of about 2. 73 K. This radiation is therefore usually called cosmic microwave background radiation. intensity 1 cm, T=2. 73 K Expected radiation from a perfect blackbody Observed background radiation of the universe wavelength
Observing the cosmic background Our effort to map the cosmic background radiation have been mainly conducted by the COBE (The COsmic Background Explorer) satellite. Here are some real data images from the COBE satellite. The Milky Way in the far infrared spectrum
Observing the cosmic background Our effort to map the cosmic background radiation have been mainly conducted by the COBE (The COsmic Background Explorer) satellite. Here are some real data images from the COBE satellite. The Milky Way in the near infra-red spectrum
Conclusions In this Activity, we have examined some common sources of extragalactic radio emission. These include: • HI radio emission from other galaxies; • giant elliptical and irregular galaxies; • quasars; and • the cosmic microwave background. Radio astronomy has provided us with a unique and valuable view of the universe. Without radio observations, there are many exciting objects and processes in the universe that we would know nothing about!
Image Credits Title Slide: X-Ray Jet From Centaurus A X-Ray image: NASA/ CXC/ SAO, Optical image: AURA/ NOAO/ NSF http: //antwrp. gsfc. nasa. gov/apod/ap 991028. html Sombrero Galaxy http: //antwrp. gsfc. nasa. gov/apod/ap 000228. html Sunflower Galaxy http: //antwrp. gsfc. nasa. gov/apod/ap 000627. html M 81 by G. Bothun (U. Oregon), courtesy W. Keel (U. Alabama) http: //antwrp. gsfc. nasa. gov/apod/ap 970726. html Rotation curve of UGC 9242 http: //astrosun. tn. cornell. edu/courses/astro 201/rotcurve. htm Seyfert Galaxy NGC 7742 by Hubble Heritage Team (AURA/ STSc. I/ NASA) http: //antwrp. gsfc. nasa. gov/apod/ap 981023. html
Image Credits Elliptical NGC 4261 by H. Ford (Johns Hopkins), L. Ferrarese (UCLA), W. Jaffe (Leiden), NASA http: //antwrp. gsfc. nasa. gov/apod/ap 991107. html High Redshift Quasars by Sloan Digital Sky Survey http: //antwrp. gsfc. nasa. gov/apod/ap 981211. html 3 C 368: NASA, NRAO, VLA, HST, WFPC 2, M. Longair (U. Cambridge) http: //antwrp. gsfc. nasa. gov/apod/ap 950930. html Coma elliptical galaxy - W. Baum & Hubble Space Telescope WFPC Team http: //oposite. stsci. edu/pubinfo/pr/1995/07. html Cygnus A: Radio Image, Courtesy NRAO/AUI, Copyright (c) NRAO/AVI R. A. Perley, J. W. Dreher (observers) http: //www. ncsa. uiuc. edu/ Radio images Cygnus A - NRAO/AUI http: //www. nrao. edu
Image Credits DSS optical image of Cygnus A, Palomar Observatory Sky Survey (funded by the National Geographic Society) using Oschin Schmidt Telescope compression and distribution by STSc. I data via Sky. View service of NASA Goddard Spaceflight Centre; © AURA http: //skyview. gsfc. nasa. gov Jet from M 87 by J. A. Biretta et al. , Hubble Heritage Team (STSc. I /AURA), NASA http: //antwrp. gsfc. nasa. gov/apod/ap 000706. html Redshift 5. 8 by Stephen Kent (FNAL), SDSS Collaboration http: //antwrp. gsfc. nasa. gov/apod/ap 000419. html COBE images http: //space. gsfc. nasa. gov/astro/cobe/
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