Resolving the Steep-Spectrum Emission in the Central Radio Source in ZwCl 0735.7+7421

It has long been know that an extremely steep spectrum radio source lies at the center of ZwCl 0735.7+7421, a galaxy cluster with high X-ray luminosity and a cooling core. In this Letter, we present VLA observations of this radio source at both 1425 and 325 MHz. With a resolution below 21'' (75 kpc) for both 1425 and 325 MHz, we show the morphology of the central source, and find that it is most likely a large (400 kpc) radio galaxy rather than diffuse cluster emission. We estimate a steep spectral index of $\alpha_{325}^{1400} = -1.54$ for the core (although it may be contaminated by lobe emission), while the outer lobes are extremely steep objects, both with $\alpha_{325}^{1400} < -3.1$. We also find evidence for restarted core activity in the form of a set of inner lobes oriented at a somewhat different angle from the outer lobes. A spectral analysis extending the frequency range down to 74 MHz appears to show a turnover at very low frequencies. Comparison of the minimum energy radio pressures with the average thermal pressure surrounding the radio cavities from McNamara et al.(2005) shows that the radio lobes appear to be roughly in pressure balance with the thermal gas.Comment: Accepted for publication in the Astrophysical Journal Letters; 12 pages, 5 figures, 2 table