## Wednesday, October 12, 2011

### Broderick's Blazars

For today, a brief summary of Avery Broderick's excellent PI talk on TeV blazars. The hypothesis is that complex plasma instabilities, that arise in the interaction of photons, produced $e^{-}e^{+}$ pairs and the CMB background, efficiently thermalise the very high energy gamma rays. The talk discusses the consequences of this hypothesis. Observations of around $50$ nearby extragalactic TeV blazars were made with HESS, Magic and Veritas, and compared to Fermi LAT results in the $< 100$ GeV range.

For the full results see this series of papers. The blazar count peaks at the small redshift of $z = 0.1$, suggesting that TeV gamma rays from further away are absorbed, and the question is, via what process? Supposedly, the TeV photons pair produce, resulting in a so called inverse Compton cascade, where interaction with the CMB results in secondary gamma ray photons. Altogether, we have an intergalactic background plasma, which is studied numerically to obtain parameters such as an inverse Compton cooling rate. Potential consequences include the following.

1. An improved understanding of the intergalactic magnetic field. Fermi has indicated a strong IGMF, but the plasma instabilities could act to randomise the orientation of the $e^{-}e^{+}$ pairs.
2. There should be many more objects at high $z$, that are simply not visible. A successful blazar luminosity function was constructed using $30$ sources and a translation of the quasar luminosity function, suggesting that both object types belong to the same population.
3. A fit for the gamma ray background, which tends to be overproduced in most models.
4. Heating of the intergalactic medium, forcing a rewrite of the cosmic thermal history, whereby there is little heating today in comparison to the past. Details of Hot Void Ly$\alpha$ distributions were considered, producing good fits at low density for $z = 3$. Missing dwarf galaxies are suppressed by blazar heating, which kills around one third of them, in agreement with the observation that many dwarf haloes form at late times. An entropy history suggests that most cluster gas accretes later than $z = 1$, giving two core populations, one hot (late time) and one cold (early).