Radio Observations of Quasars

Transcription

1 Radio Observations of Quasars Quasars, blazars, Seyfert galaxies, BL Lac objects, active galactic nuclei, QSOs, supermassive black holes. What are these exotic sounding objects, and what do they have in common? What is the role of radio astronomy in untangling the riddle, and what types of radio telescopes are required? This article will give an overview of the disparate objects, their discovery, observation and theory, and finally it will discuss the unified theory of quasars. Seyfert galaxies Before radio astronomy was established as a discipline, astronomers had already identified galaxies which exhibited strange and interesting characteristics. In 1908, Edward Fath noted that galaxy M77 had a distinctive spectrum which included six prominent emission lines which were not characteristic of starlight. Subsequent work by Vesto Slipher and Edwin Hubble found similar anomalous spectra in other galaxies. (SEDSWeb) In 1943, Carl Seyfert identified 12 galaxies with the characteristic spectra. He noted that in each case the galactic nucleus was extremely bright and scarcely distinguishable from stars.(seyfert 1943) Seyfert galaxies became a cause of much speculation over the decades following Seyfert s paper. Numerous theories were advanced to explain the strange emission lines, extremely high core luminosity and the tiny star-like source of the emissions within the galactic core. Figure 1: Seyfert Galaxy NGC4722 Image courtesy NASA & The Hubble Team Radio stars Discrete astronomical radio sources ( radio stars ) where known as early as 1940, and increasing numbers were detected in the late 1940s. In particular John Bolton searched and located a large number of discrete sources. (Bolton 1948)

2 In 1950, the University of Cambridge carried out a comprehensive survey of radio stars, using a small radio interferometer. This survey isolated about 50 discrete sources in the northern hemisphere. (Ryle 1950) The interferometer consisted of two small antenna separated by a distance 100 times the wavelength, giving a resolution equal to a fixed dish of 1km diameter. However this was still not good enough to accurately fix the position of the sources better than ±1 arcminute (right ascension) and ±5 arcminutes (declension). (Ryle 1950) Ryle et al concluded that there was little correlation between the locations of the radio stars and visible sources, except for a few galaxies outside our own. They believed that radio stars were essentially a local phenomenon. Further surveys were carried out using progressively more sophisticated apparatus, and at shorter wavelengths. (Schmidt 1969) Quasars As an increasing number of radio stars was discovered, the nomenclature began to change. It became obvious that these aren t stars as we know them, so the term quasi-stellar radio source was coined. It was also apparent that some of these new objects were not strong radio sources, but still possessed the unusual spectral features. These were termed quasi-stellar objects (QSO). For a while, these clumsy terms were in common use before the word quasar was invented by Hong-Yee Chiu in The red-shift riddle An early problem relating to quasars was the determination of their distance. The high red-shifts in their spectra seemed to indicate that they were at cosmological distances and receding at very high speeds. The red shifts could be explained in various ways, including the cosmological shift caused by the expansion of the universe, indicating extremely great distances, or gravitational red-shift caused by the mass of the object itself. A further consideration was the small apparent size of the objects. In addition, some of these objects showed a variation in their brightness over quite short periods. This implied that the objects were small, but radiating energy at a furious rate in order to be seen at cosmological distances. In 1967, Graham and Margaret Burbage considered some of the potential solutions, including the possibility that the objects had been ejected from our own Galaxy, or an hypothesis by Fred Hoyle that a quasar was a cluster of neutron stars with a consequent steep gravity gradient. They also noted that the quasar spectra bore some similarity to that of Seyfert galaxies. Only a few QSOs had been discovered at that time, all showing a red shift around The Burbages believed that such a consistent red-shift must be an intrinsic property of the object itself, and therefore they could not be receding at high speed. They set an upper limit of 40 megaparsec (about 130 million light years). The paper still makes interesting reading but is, in many key areas, totally wrong! (G&E Burbage 1967) Paul Fisher Radio Observations of Quasars Page 1

3 Figure 2: Radio galaxy J Giant radio plumes surround the jets emitted from the central black hole. 20cm wavelength ATCA image by L. Saripalli, R. Subrahmanyan and Udaya Shankar Rapid development the 1960s During the mid to late 1960s hundreds of papers were published dealing with these strange new objects. Scientific controversy raged about their nature and distance. Were they really at huge cosmological distances (as the red-shift implied) or were they close to home, with the red-shift caused by Doppler or gravitational effects? In the year 1967, Margaret Burbage published a summary of the state of the quasar science at that date. (E. Burbage 1967). She named the first QSO discovered by optical means as 3C48, identified by Standage & Matthews, using the 200 inch Mount Palomar telescope in The radio source had been located by the Owens Valley radio interferometer. Between 1961 and 1963, 3C273 was accurately located by Cyril Hazard by means of a lunar occultation, using the 210-ft Parkes radio telescope. An optical spectrum of this object was subsequently obtained by Schmidt. (Schmidt 1963) (E. Burbage 1967)(SpaceAcademyWeb 1983) 3C273 appeared to have two distinct but small radio sources. Margaret Burbage s paper canvassed and dismissed ideas such as gravitational lensing ( implausible ) and the connection between QSOs and Seyfert galaxies. It appeared, however that the idea of quasars being at cosmological distances was gaining hold. In 1969, Maartin Schmidt published a further synopsis of the state of the art, which had advanced rapidly in the short time since Burbage s paper. In particular, he dismissed the concept that a redshift of 1.95 was somehow an intrinsic characteristic of QSOs. Freed from this restriction, it was possible to seriously consider that the objects were at cosmological distances. Paul Fisher Radio Observations of Quasars Page 2

4 Technology Before going on to discuss what we now know about quasars, let s briefly digress and examine some of the technology used in the search. Single dish radio telescopes Single dish telescopes have been used to study radio sources by the method of lunar occultation. Some of the most important early single dish telescopes include the 250ft Lovell telescope at Jodrell Bank near Manchester, the 140ft Greenbank telescope in the US and the 210ft Parkes radio telescope in Australia. (Cohen 1969) Figure 3: The Dish - Parkes radio telescope. (Image copyright J Smith, CSIRO) The Green Bank Telescope in West Virginia is currently (2011) the largest fully steerable radio telescope, with a dish of 110 x 100m. It is unique in having an active surface with actuators constantly adjusting the surface shape to ensure a clean focus. (NRAO 2011) Despite the strengths of telescope arrays, large single dish telescopes still have advantages due to their larger spectral bandwidths enabling the accurate measurement of molecular transition emissions in quasars with high red-shifts. (Riechers 2006) Interferometer arrays Interferometers are instruments which superimpose electromagnetic waves and measure the resultant fringing to determine characteristics of the source. Radio interferometers consist of two or more antennae which observe a source simultaneously. By combining the signals from the antennae a more detailed picture emerges at a resolution equivalent to a single antenna many times of the size of the interferometer elements. The resolution of a radio interferometer is given by λ/d, where λ is the wavelength and d is the baseline length. (Cohen 1969) Long baselines and high frequency / short wavelength observations can result in very high resolutions. Paul Fisher Radio Observations of Quasars Page 3

5 Figure 4: Australia Telescope Compact Array (Image copyright CSIRO) Radio interferometer arrays were in use as early as (Cohen 1969) The first radio interferometer was the Cliff Antenna in Sydney, which carried out solar observations using the reflection of the Sun s radiation off the ocean. By 1952, Bernie Mills was using a two-element interferometer (linked by radio) with a baseline of up to 10km to more accurately locate radio sources. (Mills 1953) By the late 1960, radio interferometers were producing maps in sufficient detail to detect the jets emitted by some radio objects. (Cohen 1969) In the current era, radio interferometers have grown in size, power and sophistication. The Very Large Array in New Mexico consists of twenty-seven 25m antennae. The resolution of the array is equivalent to an antenna 36km across. (VLAWeb 2011) The Australia Telescope Compact Array (ACTA) comprises six 22m antennae, which can be arranged on a baseline up to 6km in length. (CSIRO_Web 2011) Very long baseline interferometers Very long baseline interferometers (VLBI) have played an important part in determining the nature of quasars. As early as the late 1960s, experiments had been carried out with a baseline up to 10,000km. (Cohen 1969) The Australian Long Baseline Array (LBA) links telescopes across the continent. (CSIRO_Web3 2011). The LBA can link with telescopes throughout the Asia-Pacific region to form the Australia Pacific Telescope. And to further stretch the technology, the LBA is part of the VLBI Space Observatory Program, which makes use of a Japanese satellite carrying an antenna in an orbit with an apogee of 21,000km. The baseline is three times longer than can be achieved on Earth. (VSOP n.d.) Paul Fisher Radio Observations of Quasars Page 4

6 Figure 5: Quasar showing the core and jet. Source: VSOP Current knowledge Coming up to date, many of the uncertainties of the past have been cleared away. In particular, it is now considered that Seyfert galaxies, other forms of active galactic nucleus, quasars and blazars are all manifestations of a single phenomenon, and all driven by supermassive black holes. Material spirals in to the black hole s accretion disk, and the intense magnetic fields power the jets of particles and energy. The appearance of active galaxy, quasar or blazer is primarily a function of the angle of the object to the observer. We see Seyfert galaxies nearly face-on, but with quasars and blazers we are in almost direct line with the jets. (NASA_Web 2010) Figure 6: 5000 light year jet streaming from M87. Image courtesy NASA and The Hubble Heritage Team (STScI/AURA) The unification of the theory of these strange objects is due in great part to the improved radio astronomical techniques, combined with optical observations by the Hubble Space Telescope and the new generation of very large telescopes. The use of VLBI radio telescopes can map quasars and their immediate surroundings in some considerable detail. (Figure 7) (Antonucci 1993) Paul Fisher Radio Observations of Quasars Page 5

7 Figure 7: High definition radio map of blazar 3C371. Source: Wrobel & Lind (1990) cited in Antonucci (1993) Paul Fisher Radio Observations of Quasars Page 6

8 Works Cited Bolton, J. 1948, Nature 162, 141 Burbage, E. 1967, ARA&A 5, 399 Burbage, G & E. 1967, ApJ 148, 107. Cohen, M. 1969, ARA&A 7, 619. CSIRO_Web (accessed March 28, 2011). CSIRO_Web (accessed March 28, 2011). CSIRO_Web (accessed March 28, 2011). Mills, B. 1953, AuPhJ 6, 452. NRAO (accessed March 28, 2011). Riechers, et al ASP. Ryle, Smith & Elsmore. 1950, MNRAS 110, 508 Schmidt, M. 1969, ARA&A 7, 527 Schmidt, M. 1963, Nature 197, 1040 SEDSWeb. (accessed 03 27, 2011). Seyfert, CK. 1943, ApJ 97, 28 SpaceAcademyWeb (accessed March 27, 2011). University of California, San Diego. A Bestiary of Active Galaxies. Gene Smith's Astronomy Tutorial. 29 Sep (accessed March 27, 2011). VLAWeb (accessed March 28, 2011). VSOP. (accessed March 28, 2011). Paul Fisher Radio Observations of Quasars Page 7