A Deep Search For Co J = 2–1 Emission From A Lyα Blob At Z ∼ 6.595

Abstract

We have used the Green Bank Telescope to carry out a deep search for redshifted CO J = 2−1 line emission from an extended (>17 kpc) Lyα blob (LAB), "Himiko," at z ~ 6.595. Our non-detection of CO J = 2−1 emission places the strong 3σ upper limit of L'CO < 1.8 × 10¹⁰ × (ΔV/250)1/2 K km s^-1 pc2 on the CO line luminosity. This is comparable to the best current limits on the CO line luminosity in LABs at z ~ 3 and lower-luminosity Lyα emitters at z gsim 6.5. High-z LABs appear to have lower CO line luminosities than the host galaxies of luminous quasars and submillimeter galaxies at similar redshifts, despite their high stellar mass. Although the CO-to-H2 conversion factor is uncertain for galaxies in the early universe, we assume XCO = 0.8 M_⊙ (K km s^-1 pc²)^-1 to obtain the limit M(H2) <1.4 × 10¹⁰  M_⊙ on Himiko's molecular gas mass; this is a factor of gsim 2.5 lower than the stellar mass in the z ~ 6.595 LAB.