Oral Presentation

ALMA Deep Field in SSA22 : Cosmic Molecular Gas Mass Density at z = 0.7 and 6.2

Author(s): Natsuki H. Hayatsu (University of Tokyo) and ADF22 Team

Presenter: Natsuki H. Hayatsu (University of Tokyo)

Cosmic molecular mass density describes redshift evolution of fuel for star formation. Recent ALMA spectroscopic survey in the Hubble ultra deep field provides a deep understanding of evolution of cosmic molecular gas mass density at z < 4. In order to reduce observational bias, measurement by using different survey fields is essential. At high redshift, cosmic molecular gas mass density can also be estimated by [CII] emission which is known as a strong line emission.
We report measurements of cosmic molecular gas mass densities at z = 0.7 and, for the first time, z = 6.2 by using the result of spectroscopic survey in the ALMA deep field in SSA22 (ADF22). ADF22 is a deep, contiguous 1.1mm survey field taken in ALMA Cycle2. The observed area of our data is 2’x 3’, angular resolution is 0’’.7 and RMS is 0.8 mJy/beam at 36 km/s velocity resolution. In this blind survey, we successfully detected a CO(4-3) emitter at z = 0.7 and [CII] emitter candidates at z = 6.0 and 6.5.
The line luminosities of the emitters are ~ 5 x 10^6 Lsun and 3-8 x 10^9 Lsun, for the CO(4-3) emitter and the [CII] emitter candidates, respectively. Comparing their luminosities and number densities with previous observational/theoretical results, the emitters are likely normal population at their redshifts. The estimated value of molecular gas mass of the CO(4-3) emitter is ~ 2 x 10^10 Msun, which is comparable to known galaxies in the same redshift range. We also estimate the molecular gas masses of the [CII] emitter candidates by assuming local luminosity ratio between CO(1-0) and [CII] line emissions. From the blind detection of the emitters, we derive lower limits of molecular gas mass densities at z = 0.7 and z = 6.2. We found that the estimated molecular gas mass density at z = 0.7 is consistent with previous studies, and at z > 3, it is likely to decrease with increasing redshift.
ADF22 is now under observation by band 3 and will also be observed in Cycle 4. Future milli/sub-millimeter deep line surveys enable us to detect normal CO/[CII] emitters that contribute cosmic molecular gas mass density.