CO (J = 4 --> 3) and 650 Micron Continuum Observations of z=0.93 Hyperluminous Infrared Galaxy FSC 15307+3252

Abstract

We report the results of our CO J = 4 --> 3 line and rest frame 650 micron continuum observations of the z=0.93 hyperluminous infrared galaxy FSC 15307+3252 using the Owens Valley Millimeter Array. No line or continuum emission was detected, but the derived limits provide a useful constraint on the temperature, emissivity, and mass of the cold dust associated with FSC 15307+3252 and its molecular gas content. The 3 sigma upper limit on the velocity integrated CO (4-3) line flux is 1.6 Jy km/s (for 300 km/s line width). This corresponds to a surprisingly small total molecular gas mass limit of 5 x 10^9 h^{-2} solar mass for this galaxy with infrared luminosity L(FIR) > 10^{13} solar luminosity. Combined with existing photometry data, our 3 sigma upper limit of 5.1 mJy for the 239 GHz (650 micron rest wavelength) continuum flux yields a total dust mass of 0.4-1.5 times 10^8 solar mass. The CO luminosity (thus molecular gas content) and the resulting gas-to-dust ratio are lower than the values typical for the more gas-rich infrared galaxies, but they are within the observed ranges. On the other hand, FSC 15307+3252 has a dust content and infrared luminosity 40 and 200 times larger than the infrared bright elliptical-like galaxies NGC 1275 and Cygnus A. The FIR luminosity to dust mass ratios, L(FIR)/M(dust), for all three galaxies hosting a powerful AGN (FSC 10214+4724, FSC 15307+3252, Cygnus A) are larger than reasonably expected for a galaxy dominated by a starburst and four times larger than Arp 220. Therefore the bulk of the observe FIR luminosity in these galaxies is likely powered by their luminous active nucleus.