The Redshift Search Receiver Observations of 12CO J=1-->0 in 29 Ultraluminous Infrared Galaxies

TL;DR

This study uses the Redshift Search Receiver to observe CO in 29 ULIRGs, revealing new insights into their molecular gas properties, star formation efficiency, and potential evolution into quasars, with a high detection rate and expanded sample.

Contribution

First large-scale CO survey of ULIRGs using the RSR, increasing the sample size and depth, and providing new data on their molecular gas and star formation characteristics.

Findings

High CO detection rate of 93% in ULIRGs.

Broader range of FIR-to-CO flux ratios observed.

Smoother transition between ULIRGs and QSOs in star formation efficiency.

Abstract

We present 12CO J=1-->0 observations of ultraluminous infrared galaxies (ULIRGs) obtained using the Redshift Search Receiver (RSR) on the 14-m telescope of the Five College Radio Astronomy Observatory. The RSR is a novel, dual-beam, dual-polarization receiver equipped with an ultra-wideband spectrometer backend that is being built as a facility receiver for the Large Millimeter Telescope. Our sample consists of 29 ULIRGs in the redshift range of 0.04-0.11, including 10 objects with no prior 12CO measurements. We have detected 27 systems (a detection rate of 93%), including 9 ULIRGs that are detected in CO for the first time. Our study has increased the number of local ULIRGs with CO measurements by ~15%. The CO line luminosity L'_CO, correlates well with far-infrared luminosity L_FIR, following the general trend of other local ULIRGs. However, compared to previous surveys we probe deeper into the low CO luminosity end of ULIRG population as a single study by including a number of CO faint objects in the sample. As a result, we find 1) a smoother transition between the ULIRG population and local QSOs in L_FIR-L'_CO ("star formation efficiency") space, and 2) a broader range of L_FIR/L'_CO flux ratio (~60--1000 L_sun/[K km/s/pc^2]) than previously reported. In our new survey, we also have found a small number of ULIRGs with extreme L_FIR/L'_CO which had been known to be rare. The mid-IR color and radio-excess of 56 local ULIRGs as a function of FIR-to-CO flux ratio is examined and compared with those of spirals/starburst galaxies and low-z QSOs. In this paper, using a large sample of local ULIRGs we explore the origin of their current power source and potential evolution to QSOs.