Deep radio observations of the radio halo of the bullet cluster 1E 0657-55.8

We present deep 1.1-3.1 GHz Australia Telescope Compact Array observations of the radio halo of the bullet cluster, 1E 0657-55.8. In comparison to existing images of this radio halo the detection in our images is at higher significance. The radio halo is as extended as the X-ray emission in the direction of cluster merger but is significantly less extended than the X-ray emission in the perpendicular direction. At low significance we detect a faint second peak in the radio halo close to the X-ray centroid of the smaller sub-cluster (the bullet) suggesting that, similarly to the X-ray emission, the radio halo may consist of two components. Finally, we find that the distinctive shape of the western edge of the radio halo traces out the X-ray detected bow shock. The radio halo morphology and the lack of strong point-to-point correlations between radio, X-ray and weak-lensing properties suggests that the radio halo is still being formed. The colocation of the X-ray shock with a distinctive radio brightness edge illustrates that the shock is influencing the structure of the radio halo. These observations support the theory that shocks and turbulence influence the formation and evolution of radio halo synchrotron emission.

A new twist to an old story: HE 0450-2958, and the ULIRG$to $(optically bright QSO) transition hypothesis

We report on interferometric imaging of the CO J=1--0 and J=3--2 line emission from the controversial QSO/galaxy pair HE 0450--2958. {it The detected CO J=1--0 line emission is found associated with the disturbed companion galaxy not the luminous QSO,} and implies $rm M_{gal}(H_2)sim (1-2)times 10^{10} M_{odot}$, which is $ga 30% $ of the dynamical mass in its CO-luminous region. Fueled by this large gas reservoir this galaxy is the site of an intense starburst with $rm SFRsim 370 M_{odot} yr^{-1}$, placing it firmly on the upper gas-rich/star-forming end of Ultra Luminous Infrared Galaxies (ULIRGs, $rm L_{IR}>10^{12} L_{odot}$). This makes HE 0450--2958 the first case of extreme starburst and powerful QSO activity, intimately linked (triggered by a strong interaction) but not coincident. The lack of CO emission towards the QSO itself renews the controversy regarding its host galaxy by making a gas-rich spiral (the typical host of Narrow Line Seyfert~1 AGNs) less likely. Finally, given that HE 0450--2958 and similar IR-warm QSOs are considered typical ULIRG$to $(optically bright QSO) transition candidates, our results raise the possibility that some may simply be {it gas-rich/gas-poor (e.g. spiral/elliptical) galaxy interactions} which ``activate an optically bright unobscured QSO in the gas-poor galaxy, and a starburst in the gas-rich one. We argue that such interactions may have gone largely unnoticed even in the local Universe because the combination of tools necessary to disentagle the progenitors (high resolution and S/N optical {it and} CO imaging) became available only recently.