CO (4-3) and dust emission in two powerful high-z radio galaxies, and CO lines at high redshifts

We report the detection of submillimeter emission from dust at 850 μm 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 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) × 10¹¹ M⊙. The large far-infrared (FIR) luminosities (L_FIR ∼ 10¹³ L⊙) suggest that we are witnessing two major starburst phenomena, while the observed large velocity widths (ΔV_FWHM ≳ 500 km s^-1) 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 s^-1. It consists of two distinct features with FWHM of ≳550 km s^-1 and ∼150 km s^-1, and line centers separated by ≳700 km s^-1. 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 ultraluminous infrared galaxy, and it may be a formative stage of the elliptical host of the residing radio-loud active galactic nucleus (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 and 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.