The Mystery of Filamentary Structures in the Radio Galaxy ESO 137-000 

Threads or filamentary kinds of structures are one of the recently discovered phenomena in the field of Astrophysics. We find one galaxy named ESO 137-006 has a similar kind of feature. Like thin woven layers of threads are connecting somehow the bipolar jets and hence lobes of its AGN activities. Some properties of the galaxy and threads in terms of its mathematical numbers are mentioned. With the Hubble parameter, H_0 = 70 km/s/Mpc with lambda cold dark matter cosmology model having omega_m=0.3 and lambda_omega=0.7, it is located at a distance of 70 Mpc (z=0.0162) having radio flux of 8 mJy at 1030 MHz and the threads have spectral index moreover -2. The largest collimated synchrotron thread (hereby referred to as CST1) is about 150 kpc long and 1 kpc thick.

Synchrotron emissions are nothing but radiation from freshly produced electrons generated from the hot luminous AGN (for our case) and its central object got accelerated in the intracluster medium (hereby referred as ICM) magnetic field and followed a path along it. It also includes self-absorption of plasma and high energy electrons losing energy following some spectral index.

From comparing with the cosmic sound speed (~ 200-300 km/s) the distance it made within that time, synchrotron ageing calculation the possibility of them generating from a forward shockfront is much more feasible. Because turbulence makes any feebly structures like thread or filament destroyed and in our case it should take much less (5 Myr) than the age of the CSTs (150 Myr). Though other possibilities of key players like the magnetic field, cosmic ray particles can play a significant role in shaping them. The exact ratio of thermal energy distributed in cosmic ray particles hence cosmic electrons and in magnetic field is unknown but the product of them can be yielded by comparing the theoretical synchrotron emissivity with the observed radio flux. We found that a thermally confined filament is consistent with the observed brightness for the CST1 (Please refer to Toushif et. al. 2025 submitted for more details). But as we already discussed such structures can easily be torn out by the turbulence in ICM.

We have done simulation of the shockfront generated in the central galaxy governed by the total bubble energy it contains and evolved it with time. The initial shockfront can be taken as a surface of various forms depending on the nature of the AGN and the filaments it produced. The ambient density also plays a crucial role. We validated our simulation for spherical blastwave like supernova explosion (likely symmetric in nature) and compared with analytical solution of Sedov-Taylor blast wave equation which presents a strong framework for propagation of shocks that only depends on the total input energy, the backdrop medium density and time and obtained an error margin lesser than 1%. Then in the same numerical process of our simulation work we evolved an elliptical blastwave with time upto 150 Myr. The forward shockfront at 70-100 Myr quietly resembles the observed structure of the CST1 and also the similarity of the dynamical time (∼ 70 Myr) of the shock generated by the AGN cocoon and the synchrotron cooling timescale (Tsync ∼ 150 Myr), the possibility of the main radio filament CST1 connecting the two radio lobes in the galaxy is most probably represent the forward shock of the AGN cocoon is established.

We also discussed the accretion rate of the central supermassive black hole (hereby called as SMBH) from the total energy of the AGN cocoon (~ 10⁶¹ erg) and its age (~ 150 Myr). The mechanical luminosity is calculated to be 0.01 L_edd where L_edd is the eddington luminosity for this kind of cD galaxy. So the central SMBH seems to be accreting at sub-eddington rate. 

We also obtained the X-ray photon count rate (Brightness) and radio brightness along the thread to get relative increase or decrease of flux from the background. As expected radio brightness is more on the thread regions but no subsequent X-ray dip or excess is found. Hence inference from the X-ray data remains inconclusive.

Now we are getting more and more filamentary kinds of structures, some as a result of ram stripped galaxy, some as the forward cocoon materials and more importantly we see gigantic bubbles in our very own galaxy, the milky way and also so many structures in the central molecular zone (CMZ). It's a fascinating world (universe) and these are just their fossil signatures.