[Paper Review] On the prospect of discovering `galaxy groups' through radio observations

Surajit Paul, Prateek Gupta+2 more|arXiv (Cornell University)|Mar 27, 2018

Astrophysics and Cosmic Phenomena5 references3 citations

TL;DR

This study presents a novel radio emission model for galaxy groups and clusters using cosmological hydrodynamic simulations (ENZO), incorporating both diffusive shock acceleration (DSA) and turbulent re-acceleration (TRA) to compute synchrotron emissions. It reveals a flatter mass-radio power scaling ($M_{500} \propto P_{1.4\text{GHz}}^{2.17\pm0.08}$) and strong $L_X$–$P_{1.4\text{GHz}}$ correlation, predicting detectable radio flux ($\sim$10–100 $\mu$Jy/beam) in groups below $10^{14}\,M_\odot$, enabling detection with current and future telescopes like SKA and uGMRT.

ABSTRACT

Observed steep mass scaling of radio power from the available high mass clusters has ruled out the prospect of detection of 'galaxy groups'. But, the available simulations and observations of thermal emissions show that the groups are merger prone, thus non-virialised, indicating better visibility in the radio waves. Detection of radio emissions from them would help us to understand the scale-dependent particle acceleration mechanisms also groups can be a unique laboratory to test the models of cosmic magnetism and canbe the potential source of WHIMs. So, we have modelled radio emissions from the simulated structures using {\sc{ENZO}}. We present a model for computing magnetic field and for the first time, used the electron energy spectrum from both the Fermi I (DSA) and Fermi II (TRA) mechanisms to compute radio emissions. Computed radio power from more than 200 simulated objects, mass ranging $\geq 10^{13}$ to $2 imes 10^{15} M_{\odot}$ show a new mass scaling of $M_{500} \propto P_{1.4\;GHz}^{2.17\pm 0.08}$ and a strong correlation scale of $L_X \propto P_{1.4\;GHz}^{1.08\pm 0.05}$. Both magnetic field and radio power are shown to have adequately replicated the available observations at high mass, allowing us to extend the results to further smaller masses. We report that groups below $10^{14}\; m{M_{\odot}}$ show the existence of 10s of nano to a sub-$μ$G magnetic field and about 10$^{19-23}$ W Hz$^{-1}$ of radio power, much higher than what existing mass scaling predicts. We found that the combined radio power from TRA and DSA electrons can only fit very well to all the observed `radio halos'. Finally, we have implemented this model on a real data set obtained from the Sloan Digital Sky Survey (SDSS). It predicts about 10s to 100s $μ$Jy/(10$\arcsec$ beam) of radio flux in groups indicating their detectability with existing and aplenty with the future radio telescopes.

Motivation & Objective

• To address the lack of observational and theoretical understanding of non-thermal radio emissions in low-mass galaxy groups.
• To test whether existing mass–radio power scaling laws derived from massive clusters can be extrapolated to galaxy groups.
• To model magnetic fields and radio synchrotron emissions from intergalactic medium electrons using both DSA and TRA mechanisms in cosmological simulations.
• To assess the detectability of radio emissions from groups with current and upcoming radio telescopes.
• To explore the potential of galaxy groups as probes of cosmic magnetism and the missing baryon problem (WHIM).

Proposed method

• Simulated galaxy groups and clusters using cosmological hydrodynamics in the ENZO code with $\geq 10^{13}$ to $2\times10^{15}\,M_\odot$ mass range.
• Model magnetic field strength using turbulence-driven saturation, assuming $B \propto \rho^{1/2} v_{\text{turb}}^{1/2}$.
• Compute electron energy spectra from both DSA and TRA mechanisms to estimate synchrotron radio emission at 1.4 GHz.
• Use a time-frozen simulation snapshot approach to compute radio power and magnetic fields without radiation transport or spectral ageing.
• Apply the model to real SDSS group catalog data using scaling relations for velocity dispersion and density to estimate radio flux.
• Validate the model against observed $P_{1.4\text{GHz}}$–$L_X$ and $M_{500}$–$P_{1.4\text{GHz}}$ scaling relations at high masses.

Experimental results

Research questions

• RQ1 Can galaxy groups below $10^{14}\,M_\odot$ produce detectable radio emissions despite the steep mass–radio power scaling observed in massive clusters?
• RQ2 Do combined DSA and TRA mechanisms better explain observed radio halo emissions in clusters than TRA alone?
• RQ3 What is the predicted radio flux level in low-mass galaxy groups, and is it detectable with existing or upcoming radio telescopes?
• RQ4 How do the $M_{500}$–$P_{1.4\text{GHz}}$ and $L_X$–$P_{1.4\text{GHz}}$ scaling relations behave in the low-mass regime?
• RQ5 Can galaxy groups serve as viable probes of cosmic magnetism and the origin of the WHIM?

Key findings

• The model predicts a flatter mass–radio power scaling of $M_{500} \propto P_{1.4\text{GHz}}^{2.17\pm0.08}$ for groups and clusters, deviating from the steep $\sim4$ slope observed in massive clusters.
• A strong correlation is found between X-ray luminosity and radio power: $L_X \propto P_{1.4\text{GHz}}^{1.08\pm0.05}$, consistent with observations.
• Galaxy groups below $10^{14}\,M_\odot$ host magnetic fields of 10s of nG to sub-\textmu G, providing a testbed for turbulent dynamo theories.
• Radio power in groups ranges from $10^{19}$ to $10^{23}$ W Hz$^{-1}$, significantly higher than predictions from cluster-only scaling laws.
• The combined DSA+TRA model fits observed radio halo emissions in clusters better than TRA alone, indicating a significant role of merger shocks.
• SDSS-based implementation predicts $\sim$10–100 $\mu$Jy/beam flux in many groups, with ~3% detectable with current telescopes and up to 20% with SKA.