Radio galaxies

B.E.J. Pagel The G-Dwarf Problem and Radio-Active Cosmochronology . In Evolutionary Phenomena in Galaxies, Summer School at Puerto de la Cruz, Spain, July i 15, 1988, ed. by J.E. Beckman, B.E.J. Pagel (Cambridge University Press, Cambridge, New York 1989), pp. 201-223... 

Radioastronomers first learned of 3He in 1955 at the fourth I.A.U. Symposium in Jodrell Bank, when the frequency of the hyperfine 3He+ line at 8.666 GHz (3.46 cm) was included by Charles Townes in a list of radio-frequency lines of interest to astronomy (Townes 1957). The line was (probably) detected for the first time only twenty years later, by Rood, Wilson Steigman (1979) in W51, opening the way to the determination of the 3He abundance in the interstellar gas of our Galaxy via direct (although technically challenging) radioastronomical observations. In the last two decades, a considerable collection of 3He+ abundance determinations has been assembled in Hi I regions and planetary nebulae. The relevance of these results will be discussed in Sect. 4 and 5 respectively. 

Dense molecular clouds are too dusty to be studied by optical means, but radio and mm wave observations reveal numerous molecules such as CO, CH2O, HCN, NH3 etc. which give valuable information on isotope ratios across the Galaxy and... 

The IRAS galaxies provided some of the earliest evidence from redshift surveys, and from source counts as a function of observed flux, that the spiral galaxy population has undergone evolution (ORS see Fig. 12.2). This result is analogous to similar evidence from source counts of radio galaxies and quasars, as well as quasar redshifts, and a correlation that has been observed between radio and infrared luminosity suggests that the evolution could be similar in both cases. Typical simple models for such evolution include luminosity evolution according to... 

Sub-mm galaxies are another class of star-forming galaxies at typical redshifts around 2 found using the SCUBA detector5 and pin-pointed from their radio... 

Discovery of deuterium (Urey), and of radio waves from the Galaxy (Jansky). 

Anticipated next supernova in our Galaxy may be undetectable by the optical instrument due to the Galactic extinction. However, supernovae are now known to be intense radio sources after a year or so of the explosion. Even if the positions are beyond the Galactic center, the radio supernova could be observed using middle size radio telescope. 

Calculations of weak radiosources show that they have undergone strong cosmological evolution their number has decreased relative to galaxies by thousands of times between the moment corresponding to a red shift of Z 2-3 and the present. In his only paper on this subject, Ya.B. (with A. G. Doroshkevich and M. S. Longair, 1970) [60] introduces a now commonly accepted function of the evolution of radio sources. 

In a talk, very important even today, J. H. Oort discussed the Distribution of Galaxies and the Density of the Universe. Lovell presented Radio Astronomical Observations Which May Give Information on the Structure of the Universe. ... 

Around about 1980, William Tifft, a radio astronomer at the University of Arizona in Tucson, had the wild idea that, perhaps, the cosmological redshifts of galaxies had preferences for multiples of some basic unit. Subsequently, he looked and made two claims [1,2] ... 

Figure 5.3. Left. The gamma-ray emission from XX annihilation in a rich, Coma-like, nearby galaxy cluster is shown Mx = 70 — 500 GeV (from top down). The integral flux is compared to the sensitivity of ongoing and planned gamma-ray experiments, as labelled. Right. The diffuse synchrotron emission spectrum of secondary electrons produced in XX annihilation is shown to fit the Coma radio-halo spectrum the green area represent the prediction of a model in which the x annihilates predominantly into fermions, while the blue area represent the gauge-boson dominated x annihilation (from Colafrancesco Mele 2001).

Several galaxy clusters show also an emission of extreme UV (Lieu et al. 1996, Durret et al. 2002) and soft X-ray (Bonamente et al. 2002, Kaastra et al. 2002) radiation in excess w.r.t. the thermal bremsstrahlung emission. This EUV emission excess may be consistent with both ICS of CMB photons off a non-thermal electron population (e.g., Lieu et al. 1999, Bowyer 2000) with Ee = 608.5 MeV (hv/keV)1/2 149 MeV for hv 60 eV, and with thermal emission from a warm gas at ksTe V 1 keV (Bonamente et al. 2002). In the case of Coma, the simple extrapolation of the ICS spectrum which fits the HXR excess down to energies 0.25 keV does not fit the EUV excess measured in Coma because it is too steep and yields a too high flux compared to the measured flux by the EUV satellite in the 0.065 — 0.245 keV band (Ensslin Biermann 1998). Thus, under the assumption that the HXR and the EUV emission of Coma is produced by ICS of CMB photons, the minimal requirement is that a break in the electron spectrum should be present in the range 0.3 — 2.8 GeV in order to avoid an excessive EUV contribution by the ICS emission and to be consistent with the radio halo spectrum. 

There is not yet, however, a definite detection of diffuse gamma-ray emission from galaxy clusters. While there is a preliminary evidence of gamma-ray emission from a dozen bright, radio-active clusters which host powerful radio galaxies and Blazars and are associated to unidentified EGRET sources (Co-lafrancesco 2002), many of the quiet, X-ray selected clusters only have upper limits for their emission at E > 100 MeV. 

Figure 6.3. Left. The overall observed spectrum of the Coma cluster from radio to gamma-ray frequencies (see labels). Right. The constraints to the spectrum of relativistic electrons in Coma as obtained from different observation (see labels) the radio halo data (blue dashed lines) for different values of the IC magnetic field the HXR data (red solid fine with arrows), the EUV data (green solid line with arrows) and the EGRET upper limit (magenta arrow). The arrows indicate that the spectra should be considered as upper limits, because we cannot exclude that a fraction of the HXR and EUV flux is provided by active galaxies or warm gas, respectively.

Cosmic rays residing in galaxy clusters produce several astrophysical signatures among which there are diffuse synchrotron radio emission, ICS of CMB (and other background) photons which are then moved to higher frequencies... 

Evidence for the presence of a black hole at the center of our galaxy comes from studies of the motion of stars in orbit around the center. The speeds of these stars decrease from the center as the inverse square root of the radius, which is the primary indication for the existence of a point mass at the center. The mass of the central object is measured to be 4 x 106 solar masses, which are contained within a sphere of less than 0.05 pc Eckart Genzel (1997) Ghez, Klein, Morris Becklin (1998) Ghez et al.(2003). No stellar or gas system can be so dense, indicating that the central object is most probably a black hole. The position of the black hole happens to coincide with the position of a strong radio source called Sagittarius A, which is thus identified with the central black hole. 

The radio emission from Sgr A is easily explained by thermal emission of hot matter falling into the black hole. However, contrary to many of the similar black holes observed at the center of external galaxies, our galactic black hole does not emit intensely in the X-ray band, and it is controversial if it emits gamma-rays. Models for such quiet black holes do exist, however, such as those involving advection-dominated accretion flows (ADAFs). 

Our fastest spaceships can travel about one six-thousandth the speed of light. Our fastest ships would require 25,000 years to reach Proxima Centauri, the closest star. Radio messages would take decades to reach our neighbors and thousands of years to cross the Galaxy. 

Among the most exciting evidence for black holes comes from 10 radio telescopes (collectively known as the Very Long Baseline Array, or VLBA, allowing scientists in 1995 to peer into the spiral galaxy NGC 4258. Researchers mea-... 

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