In the early 1960s, astronomers were puzzled by quasars — sources of intense radio emission that seemed to be stars, but had unintelligible optical spectra. In 1963, Maarten Schmidt solved the puzzle by recognizing the Balmer lines of hydrogen, strongly redshifted, in the spectrum of the quasar 3C 273. Schmidt reached the "most direct and least objectionable" conclusion, that 3C 273 was no star, but the enormously bright nucleus of a distant galaxy.

3C 273: A Star-like Object with Large Red-Shift

Energy transport in radio galaxies and quasars

Several low-power kiloparsec-scale jets in nearby radio galaxies are known for their synchrotron radiation extending up to optical and X-ray photon energies. Here we comment on high-energy γ-ray emission of one particular object of this kind, i.e., the kiloparsec-scale jet of the M87 radio galaxy, resulting from Comptonization of the starlight photon field of the host galaxy by the synchrotron-emitting jet electrons. In our analysis, we include the relativistic bulk velocity of the jet, as well as the Klein-Nishina effects. We show that upper limits to the kiloparsec-scale jet inverse Compton radiation imposed by the HESS and HEGRA Cerenkov Telescopes—which detected a variable source of very high energy γ-ray emission within 01 (~30 kpc) of the M87 central region—give us an important constraint on the magnetic field strength in this object, namely, that the magnetic field cannot be smaller than the equipartition value (referring solely to the radiating electrons) in the brightest knot of the jet, and most likely, is even stronger. In this context, we point out a need for the amplification of the magnetic energy flux along the M87 jet from the subparsec to kiloparsec scales, suggesting the turbulent dynamo as a plausible process responsible for the aforementioned amplification.

https://iopscience.iop.org/article/10.1086/430087/pdf

On the Magnetic Field in the Kiloparsec-Scale Jet of Radio Galaxy M87

We report X-ray imaging of the powerful FR II radio galaxy 3C 353 using the Chandra X-Ray Observatory Due to 3C 353's two -->4' ' wide and -->2' long jets we are able to study in detail the internal structure of the large-scale relativistic outflows at both radio and X-ray photon energies with the subarcsecond spatial resolution provided by the VLA and Chandra instruments In a 90 ks Chandra observation, we have detected X-ray emission from most radio structures in 3C 353, including the nucleus, the jet and the counterjet, the terminal jet regions (hot spots), and one radio lobe We show that the detection of the X-ray emission associated with the radio knots and counterknots, which is most likely nonthermal in origin, puts several crucial constraints on the X-ray emission mechanisms in powerful large-scale jets of quasars and FR II sources In particular, we show that this detection is inconsistent with the inverse-Compton model proposed in the literature and instead implies a synchrotron origin of the X-ray jet photons We also find that the width of the X-ray counterjet is possibly narrower than that measured in radio bands, that the radio to X-ray flux ratio decreases systematically downstream along the jets, and that there are substantial (kpc-scale) offsets between the positions of the X-ray and radio intensity maxima within each knot, whose magnitudes increase away from the nucleus We discuss all these findings in the wider context of the physics of extragalactic jets, proposing some particular although not definitive solutions or interpretations for each problem In general, we find that the synchrotron X-ray emission of extragalactic large-scale jets is not only shaped by the global hydrodynamical configuration of the outflows, but is also likely to be very sensitive to the microscopic parameters of the jet plasma A complete, self-consistent model for the X-ray emission of extragalactic jets still remains elusive

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Chandra Reveals Twin X-Ray Jets in the Powerful FR II Radio Galaxy 3C 353

We present the results of an HST and ground-based imaging study of a complete 3C sample of z ~ 1 sources, including 5 quasars and 5 radio galaxies. We have resolved continuum structure around all of our quasars in the WFPC2 images and in four of the five ground-based K' images. All of the quasars have some optical continuum structure that is aligned with the radio axis. In 3 of these cases, some of this optical structure is most likely due to optical synchrotron radiation, including optical counterparts to two radio jets and one radio lobe. Two quasars have aligned continuum and emission-line structures that are probably not due to beamed optical synchrotron emission. In another quasar, we see a red aligned object that lies 3 arcsec beyond the radio lobe, and may be an unassociated foreground galaxy, but has a remarkable morphological resemblance to the radio lobe itself. The radio galaxies and the quasars in this small sample have similar incidence of alignment, and the optical and K' flux densities are consistent within the high dispersion. The average quasar host galaxy luminosity is equivalent to, or a little fainter than, L*. All components around the quasars have optical-infrared colors that are redder than or similar to the colors of their respective nuclei; this is generally more consistent with a stellar rather than scattered origin for the emission. This study provides qualitative support for the unification of FRII quasars and galaxies.

Deep WFPC2 and Ground-based Imaging of a Complete Sample of 3C Quasars and Galaxies

Large-scale extragalactic jets, observed to extend from a few to a few hundred kiloparsecs from active galactic nuclei, are now studied over many decades in frequency of electromagnetic spectrum, from radio until (possibly) TeV γ rays. For hundreds of known radio jets, only about 30 are observed at optical frequencies. Most of them are relatively short and faint, with only a few exceptions, like 3C 273 or M 87, allowing for detailed spectroscopic and morphological studies. Somewhat surprisingly, the large-scale jets can be very prominent in X-rays. Up to now, about 30 jets were detected within the 1 - 10 keV energy range, although the nature of this emission is still under debate. In general, both optical and X-ray jet observations present serious problems for standard radiation models for the considered objects. Recent TeV observations of M 87 suggest the possibility of generating large photon fluxes at these high energies by its extended jet. In this paper we summarize information about multiwavelength emission of the large-scale jets, and we point out several modifications of the standard jet radiation models (connected with relativistic bulk velocities, jet radial stratification and particle energization all the way along the jet), which can possibly explain some of the mentioned puzzling observations. We also comment on γ-ray emission of the discussed objects.

https://iopscience.iop.org/article/10.1088/1009-9271/3/S1/383/pdf

Łukasz Stawarz

Papers

Multifrequency Radiation of Extragalactic Large-Scale Jets