Extremely inverted peaked spectrum radio sources

Abstract

We report our ongoing search for extremely inverted spectrum compact radio galaxies, for which the defining feature in the radio spectrum is not the spectral peak, but instead the slope of the spectrum (𝛼) in the high-opacity (i.e., lower frequency) part of the radio spectrum. Specifically, our focus is on the spectral regime with a spectral index, 𝛼 thick > +2.5. The motivation for our study is, first, extragalactic sources with such extreme spectral index are extremely rare, because of the unavailability of the right combination of sensitivity and resolution over a range of low frequencies. The second reason is more physically motivated since α = +2.5 is the maximum slope theoretically possible for a standard radio source emitting synchrotron radiation. Therefore such sources could be the test-bed for some already proposed alternative scenarios for synchrotron self-absorption (SSA), like the free-free absorption (FFA) highlighting the importance of jet-ISM interaction in the radio galaxy evolution.