A Gamma-ray view of Radio Galaxies

November 2019

Previous | Index | Next

The exploration of the universe at gamma-ray energies resulted in the discovery of ∼200 sources that emit photons of 1 Terra-Volt (1 000 000 000 000 eV) energy [1]. The largest group among them are so-called blazars, a particular class of Active Galactic Nuclei (AGN). It is now commonly assumed that all galaxies harbor a supermassive black hole in their centers. Some of these black holes are accreting surrounding matter and are thus able to generate an extraordinary luminosity that outshines the entire hosting galaxy with its hundreds of billions of stars. About one percent of these active galactic nuclei eject plasma jets - directed collimated matter outflows - of very high velocities, often reaching and exceeding 99 percent of the speed of light. The plasma of these jets moves with relativistic velocities and thus emits extraordinary amounts of so-called non-thermal radiation. Non-thermal radiation from these sources is often observed throughout the entire electromagnetic spectrum, ranging from low-energy radio waves to very high-energy gamma-rays. The observations of very high-energy gamma-rays now shed new light on this standard model of AGN jets.

Radio, optical and X-ray observations provide detailed images of these jets which often extend more than hundred-thousand light years from the black hole that accelerates the plasma jet. These images also display a large variety of jet morphologies. Some jets can be observed from the side, others are observed nearly end-on. The viewing geometry has a number of important consequences. Since the jets are moving with relativistic velocities, several effects of special relativity become manifest when jets are moving along the line-of-sight towards (or away from) the observer.

During the last decades a paradigm has been established that unifies different classes of AGN by ascribing the multitude of observable characteristics to the viewing geometry, i.e. the relative orientation of the AGN jet with respect to the line-of-sight [2]. This unified scenario suggests that radio galaxies are the 'parent population' of blazars, i.e. it argues that blazars and radio galaxies are identical in principle and appear different observationally only due to their vastly different viewing angle. The larger apparent luminosities of blazars, the apparent absence of detectable jets in images and the rapid variability all result from their jet orientation close to the line-of-sight (Fig. 1).

Fig 1: Illustration of the unified model of Active Galactic Nuclei (AGN). The supermassive black hole in the center of an AGN is surrounded by an accretion disk. Jets extend perpendicular to the disk. According to the unified model, blazars and radio galaxy are intrinsically similar objects but seen from different angles. Blazars are observed under small angles so that only point-like emission from the jet is observed. Their emission is subject to relativistic effects. Instead, radio galaxies are viewed edge-on. Image adapted from Aurore Simonnet (Sonoma State University), MOJAVE program, [2,3].

According to this paradigm, special relativity is responsible for the apparently higher luminosity , faster and more pronounced variability as well as enhanced gamma-ray radiation in blazars. While early discoveries in the very-high-energy (VHE) gamma-ray band, obtained with H.E.S.S. and by our colleagues in the MAGIC and VERITAS collaborations, seemed to confirm this prediction, the more detailed studies carried out over the last few years draw a different picture.

In the VHE gamma-ray range 78 AGN were detected by ground-based gamma-ray instruments so far [1]. While blazars are a majority among those, six radio galaxies have been detected, and they dominate especially in the local universe: Centaurus A (z=0.002) [4], M 87 (z=0.004) [5,6], NGC 1275 (z=0.018) [7], IC 310 (z=0.019) [8], 3C 264 (z=0.021) [9], and PKS 0625-354 (z=0.056) [10]. The entire population of VHE radio galaxies emerged from discoveries with H.E.S.S. (Cen A, M87, PKS 0625-354), MAGIC (NGC 1275, IC 310) and VERITAS (3C 264). The redshift distribution of known radio galaxies detected in the VHE band differs markedly from the distribution of blazars (Fig. 2). This does indeed correspond to the standard model which suggests that blazars are apparently brighter due to their orientation and the resulting relativistic amplification.

Fig 2: Cumulative number densities of active galaxies emitting TeV radiation within one gigaparsec distance to the Sun. While blazars (blue histogram) are more numerous, radio galaxies (red histogram) are the dominant population in the local universe. This illustrates that blazars are more rare and (apparently) brighter. Both characteristics are consistent with the standard paradigm of unification.

Interestingly, however, radio galaxies and blazars display similar variability timescales (examples are shown in Fig. 3). In fact, the very first radio galaxy discovered in the VHE band with H.E.S.S. in 2005 (M87 in the constellation Virgo) [5] found more pronounced and faster variations in the VHE band than had ever been detected in any radio galaxy in any part of the electromagnetic spectrum at that date. In the meantime, variability has been repeatedly confirmed and found to display times-scales as short as seen in the most rapidly varying blazars. The statistical properties of the samples are indistinguishable.

Fig 3: A comparison of TeV light curves of an archetypical blazar (PKS 2155-304) and a typical radio galaxy (M87), both detected with the H.E.S.S. telescopes and shown with nightly averaged flux measurements. The amplitudes and variability time scales in the TeV light curves are comparable. This similarity extends to statistical properties of all radio galaxies and blazars and illustrates that variability properties of both classes of objects are indistinguishable. This disagrees with the standard paradigm of unification. Left: Flux variations of the blazar PKS 2155-304 observed above 200 GeV in mid of 2009. Right: Daily measurements above 350 GeV of the radio galaxy M 87 in early 2010. Data taken from [6] and [11].

Similar variability properties in these two classes are at odds with the standard paradigm. This is because special relativity not only causes an apparent amplification of blazar luminosities. It also results in a compression of time-scales and leads to apparently faster variations in jets moving towards the observer. If radio galaxies and blazars intrinsically only differ in orientation with a resulting modifications of their apparent luminosities, then also the apparent variability time-scales are expected to differ.

VHE studies of the entire class of radio galaxies and blazars hence suggest that the standard paradigm of unification in Active Galactic Nuclei is insufficient and proves that well established assumptions, based upon extensive research in the radio through X-ray bands are challenged by the results of gamma-ray studies.


[1]  http://tevcat.uchicago.edu/
[2]  M. Urry & P. Padovani, 1995, Unified Schemes for Radio-Loud Active Galactic Nuclei, PASP, 107, 803
[3]  J. O. Burns & R. M. Price, 1983, Centaurus A: the nearest active galaxy, SciAm, 249, 50
[4]  H.E.S.S. Collaboration: H. Abdalla et al., 2018, The gamma-ray spectrum of the core of Centaurus A as observed with H.E.S.S. and Fermi-LAT, A&A, 619, A71
[5]  H.E.S.S. Collaboration: F. Aharonian et al.,2006, Fast Variability of Tera-Electron Volt gamma-rays from the Radio Galaxy M87, Science 314, 1424
[6]  A. Abramowski et al., 2012, The 2010 Very High Energy γ-Ray Flare and 10 Years of Multi-wavelength Observations of M 87, ApJ, 746, 151
[7]  MAGIC Collaboration: J. Aleksic et al., 2014, Black hole lightning due to particle acceleration at subhorizon scales, Science, 346, 1080
[8]  MAGIC Collaboration: S. Ansoldi et al., 2018, Gamma-ray flaring activity of NGC1275 in 2016-2017 measured by MAGIC, A&A, 617, A91
[9]  W. Benbow for the VERITAS Collaboration, 2019, Highlights from the VERITAS AGN ObservationProgram, PoS (ICRC2019), 632
[10]  H.E.S.S. Collaboration: H. Abdalla et al., 2018, H.E.S.S. discovery of very high energy gamma-ray emission from PKS 0625-354, MNRAS, 476, 4187
[11]  H.E.S.S. Collaboration: A. Abramowski et al., 2014, Long-term monitoring of PKS 2155−304 with ATOM and H.E.S.S.: investigation of optical/gamma-ray correlations in different spectral states, A&A, 571, A39