Pinpointing the molecular gas within an lyα blob at z 2.7

Abstract

We present IRAM Plateau de Bure Interferometer observations of the CO(3-2) and CO(5-4) line transitions from an Lyα blob at z 2.7 in order to investigate the gas kinematics, determine the location of the dominant energy source, and study the physical conditions of the molecular gas. CO line and dust continuum emissions are detected at the location of a strong MIPS source that is offset by 1.″5 from the Lyα peak. Neither of these emission components is resolved with the 1.″7 beam, showing that the gas and dust are confined to within 7 kpc from this galaxy. No millimeter source is found at the location of the Lyα peak, ruling out a central compact source of star formation as the power source for the Lyα emission. Combined with a spatially resolved spectrum of Lyα and He II, we constrain the kinematics of the extended gas using the CO emission as a tracer of the systemic redshift. Near the MIPS source, the Lyα profile is symmetric, and its line center agrees with that of the CO line, implying that there are no significant bulk flows and that the photo-ionization from the MIPS source might be the dominant source of the Lyα emission. In the region near the Lyα peak, the gas is slowly receding (100 km s-1) with respect to the MIPS source, thus making the hyper-/superwind hypothesis unlikely. We find a sub-thermal line ratio between two CO transitions, I CO(5-4)/ICO(3-2) = 0.97 ± 0.21. This line ratio is lower than the average values found in high-z submillimeter galaxies and QSOs but is consistent with the value found in the Galactic center, suggesting that there is a large reservoir of low-density molecular gas that is spread over the MIPS source and its vicinity. © 2014. The American Astronomical Society. All rights reserved..