Molecular gas in type 2 quasars at z ~ 0.2-0.3

Abstract

We present results of CO(1-0) spectroscopic observations of 10 Sloan Digital Sky Survey type 2 quasars (QSO2) at z ~ 0.2-0.3 observed with the 30-m IRAM radio telescope and the Australia Telescope Compact Array. We report five new confirmed CO(1-0) detections and one tentative detection. They have L'CO ~ several × 109 K km s-1 pc2, while upper limits for the non-detections are L'CO < 3s = several × 109 K km s-1 pc2. This study increases the total number of QSO2 with CO measurements at z 1 to 20, with a 50 per cent detection rate. The vast majority are at z ~ 0.1-0.4. Assuming a conversion factor α = 0.8 M⊙(K km s-1pc2)-1, the implied molecular gas masses are in the range MH2 4 × 108 to ~5 × 109 M⊙. We compare with samples of type 1 quasars (QSO1), luminous and ultraluminous infrared galaxies. We find no difference in the molecular gas content of QSO1 and QSO2 of a given infrared luminosity, although the QSO2 sample is affected by small number statistics. This result, if confirmed, is consistent with the unification model for quasars. QSO2 fall on the L'COversus z, L'COversus LFIR and λ = LFIR LCO versus LFIR correlations defined by quasars at different z. The location of the QSO2 in these diagrams is discussed in comparison with samples of QSO1, luminous and ultraluminous infrared galaxies, and high-z submm sources. CO(1-0) has full width at half-maximum (FWHM) ~ 180-370 km s-1 when detected, with a variety of kinematic profiles (single or double horned). In general, the CO line is narrower than [O III]5007, as observed in low-z QSO1, with FWHM[O III]/FWHMCO ~ 1-2). This probably reveals different spatial sizes and/or geometry of the ionized and molecular phases and a higher sensitivity of the [O III] emission to non-gravitational motions, such as outflows. Considering the z ~ 0.1-0.4 range, where CO measurements for both QSO1 and QSO2 exist, we find no difference in FWHMCO between them, although this result is tentative. In the unification scenario between QSO1 and QSO2, this suggests that the distribution of CO gas is not related to the obscuring torus.