Chasing gamma rays in Virgo A

February 2023

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All known astronomical sources of gamma-rays are also sources of radio emission. The relativistic particles that are capable of emitting the very energetic gamma radiation detected with HESS will inevitably emit synchrotron radiation in the radio waveband regime whenever they move through magnetic fields (which are present throughout the entire universe). Extragalactic radio sources exhibit complex morphologies. Most radio sources reveal a compact component ('core'), which is often spatially unresolved, and linear features ('jets') which are mostly either one-sided or two-sided. Images with sensitive instruments typically show the jets to end in or be embedded within more diffuse extended structures (called 'lobes') - see Fig. 1. Since all of these structures radiate synchrotron emission from relativist particles, one may wonder whether these extended structures might also be detectable with gamma-ray instruments.

Fig. 1: Radio emission from M87/Virgo A. The well-known one-sided radio jet resides in the central , orange-colored region. At larger distances from the center, highly asymmetric outflows of relativistic plasma can be traced to distances of many 100.000 light-years. These outflows are connected or fed by the relativistic jet on one side, and an anti-symmetric feature on the other side, but both twist and feed larger-scale radio lobes of relativistic plasma. Credits: VLA (327 MHz by Owen, Eilek & Kassim)

The origin of any gamma ray has to be reconstructed from the shower of secondary particles that are created by the original gamma-photon when entering the atmosphere. This limits the accuracy to which the position can be determined and results in a reduced angular resolution when compared to other fields in astronomy where the incoming photons can be recorded directly. The HESS array with its five telescopes can generate sharper images than other gamma-ray telescopes, but is still outperformed by the human eye in the visible range, with an angular resolution of about one arc-minute, approximately 20% of the resolution of HESS. It is rather challenging, therefore, to actually resolve extended emission in extragalactic objects. Nonetheless, the HESS collaboration had been able to demonstrate that the gamma-ray emission from the well-known radio-galaxy Centaurus A (also known as NGC 5128) is spatially extended (see [1] or SOM 2020-07). Centaurus A at a distance of 'only' 15 million light-years is actually the nearest radio-source.

Fig. 2: Many features contribute to the total emission from relativistic particles in M87 on all linear scales. Apart from indirect inferences on the sizes from variability time scales, direct measurements have been obtained repeatedly. The most tightest constraint on the low-state of emission has now been deduced through measurements with the HESS telescopes (red-marked limit), taken from Ref. 4.

In a new study, the HESS collaboration investigates the extension of the gamma-ray emission of M87, or Virgo A as it is sometimes referred to. It is the brightest galaxy in the local Virgo Galaxy Cluster. Its VHE gamma-ray emission was discovered in the first months of HESS operations in 2004 (SOM 2011-06, [2]) and shown to be rapidly variable. This actually implies that a significant fraction of the gamma-rays are emitted from within a region that is just a few light-days across, not much larger than the Solar System. It is comparable to the size of the supermassive Black Hole in the center of M87. On the other hand, the radio jets of M87 extend out to several kiloparsecs (kpc), about 10.000 light years (see Fig. 2) and the lobes, filled with relativistic particles (i.e. 'Cosmic rays') extend even further to about 30 kpc (see Fig. 1). Acceleration and heating of cosmic rays is expected to happen via a multitude of physical processes, and filamentary X-ray emission (see Fig. 3) may trace out shock waves in the more recent cosmic history.

Fig. 3: Radio emission, as shown in Fig. 1, traces particles, whose relativistic nature is unambiguous. They may, however, have acquired these energies a very long time ago, and may not trace the sites, where particles are currently accelerated. The filamentary X-ray emission, revealed in this observation obtained with the Chandra satellite (taken from [3]).

In the new study, the hypothesis of hadronic emission in the halo of M87 is explored by measuring the angular extent of the gamma-ray emission during those epochs, when the total flux of the object is very low. This approach follows the consideration that bright phases - lasting days to months - are due to a small component (light day to light months across) which outshines a potentially more extended but fainter contribution. Whenever the compact and variable component is faint or switched off entirely, the spatial extent of the remaining emission of the low state can be studied more precisely. The result of the measurement is depicted in Fig. 4. The angular size of the emission in low state has been shown to be compact, no larger than 4.6 kilo-parsec (about 15.000 light years). This is still 5 million times larger than the upper limit on the size of the small variable component that dominates bright phases, but less extended than previously derived upper limits and definitely smaller than the radio lobes of M87 (see Fig. 2).

Fig. 4: The H.E.S.S. excess counts map in the low state is presented in color. Black contours show the 90 cm radio flux obtained with the Very Large Array (VLA). Thin colored circles and crosses in matching colors illustrate the upper limit constraints on the extension of the gamma-ray emission and the center positions from earlier studies taken with the HESS, MAGIC and VERITAS gamma-ray telescope arrays. The new and much more constraining limit derived in the new study is illustrated by the thick, blue-colored circle. The center positions of the different studies are consistent with each other and with the position of the supermassive Black Hole in the center of M87. Figure adapted from [4].

The constraint on the angular size of the VHE gamma-ray emission in a low state is used to infer the role of cosmic-ray nuclei in the halo of this bright radio galaxy. Cosmic Rays accelerated in the core of M87 propagate outwards and exert pressure into the local plasma. It is possible to constrain the magnitude of the pressure that the Cosmic Rays exert, through an estimate of the ratio of cosmic-ray pressure to thermal pressure. This, in turn may be used to derive the total energy in this region. Usually, the ratio of cosmic-ray pressure to thermal pressure declines with the distance from the center of an active galaxy, where particle acceleration takes place. Thermal heating becomes more important than the cosmic-ray pressure at larger radii. For plausible assumptions on the energy spectrum of the proton contribution of Cosmic Rays the amount of cosmic ray pressure was found not to exceed 32% of the thermal pressure at its maximum position. The total energy in cosmic rays within the central 20 kpc of the halo of M87 is found to be less than 5×10⁵⁸ erg. For comparison - the kinetic energy of a supernova is about 10⁵¹ erg. Supernovae are the most powerful explosions seen in the universe.


[1] The H.E.S.S. Collaboration: Nature, 582,356–359, 2020

[2] The H.E.S.S. Collaboration: Science 314, 1424 - 1427, 2006

[3] W. Forman et al., The Astrophysical Journal, 665:1057-1066, 2007

[4] The H.E.S.S. Collaboration: submitted, 2023