CO (4–3) and Dust Emission in Two Powerful High‐zRadio Galaxies, and CO Lines at High Redshifts

Abstract

We report the detection of sub-mm emission from dust at 850 microns and of the 12CO J=4-3 line in the two distant powerful radio galaxies 4C 60.07 (z=3.79) and 6C 1909+722 (z=3.53). In the case of 4C 60.07 the dust emission is also detected at 1.25 mm. The estimated molecular gas masses are large, of the order of ~(0.5-1)x10^{11} Solar. The large FIR luminosities (L_fir ~ 10^{13} Solar) suggest that we are witnessing two major starburst phenomena, while the observed large velocity widths (FWHM > 500 km/sec) are characteristic of mergers. In the case of 4C 60.07 the CO emission extends over ~30 kpc and spans a velocity range of >1000 km/sec. It consists of two distinct features with FWHM of >= 550 km/sec and ~150 km/sec and line centers separated by >=700 km/sec The least massive of these components is probably very gas-rich with potentially >=60% of its dynamical mass in the form of molecular gas. The extraordinary morphology of the CO emission in this object suggests that it is not just a scaled-up version of a local Ultra Luminous Infrared Galaxy, and it may be a formative stage of the elliptical host of the residing radio-loud AGN. Finally we briefly explore the effects of the wide range of gas excitation conditions expected for starburst environments on the luminosity of high-J CO lines. We conclude that in unlensed objects, CO (J+1-->J), J+1>3 lines can be significantly weak with respect to CO J=1-0 and this can hinder their detection even in the presence of substantial molecular gas masses.