A massive reservoir of low-excitation molecular gas at high redshift

Abstract

Molecular hydrogen (H 2 ) is an important component of galaxies because it fuels star formation and accretion onto active galactic nuclei (AGN), the two processes that generate the large infrared luminosities of gas-rich galaxies 1,2 .Observations of spectral-line emission from the tracer molecule carbon monoxide (CO) are used to probe the properties of this gas. But the lines that have been studied in the local Universe -mostly the lower rotational transitions of J = 1 → 0 and J = 2 → 1 -have hitherto been unobservable in highredshift galaxies. Instead, higher transitions have been used, although the densities and temperatures required to excite these higher transitions may not be reached by much of the gas. As a result, past observations may have underestimated the total amount of molecular gas by a substantial amount. Here we report the discovery of large amounts of low-excitation molecular gas around -1 -the infrared-luminous quasar, APM 08279+5255 at z = 3.91, using the two lowest excitation lines of 12 CO (J = 1 → 0 and J = 2 → 1). The maps confirm the presence of hot and dense gas near the nucleus 3 , and reveal an extended reservoir of molecular gas with low excitation that is 10 to 100 times more massive than the gas traced by higher-excitation observations. This raises the possibility that significant amounts of low-excitation molecular gas may lurk in the environments of high-redshift (z > 3) galaxies.APM 08279+5255 was discovered serendiptously during a search for Galactic halo carbon stars 4 . It comprises a strongly lensed broad absorption line quasar at z = 3.9 with a complex optical spectrum and a bolometric luminosity of ∼ 10 14 L ⊙ (where L ⊙ is the luminosity of the Sun) after correcting for lensing magnification; it displays extremely strong rest-frame far-infrared emission 5 suggestive of dust-rich gas undergoing rapid star formation. The CO(4→3) and CO(9→8) lines have been detected 3 suggesting that the molecular gas must be warm, extended on a scale of up to 1 ′′ in the image plane (only 160-270 pc in the source plane), with a mass of (1 − 6) × 10 9 M ⊙ .We have used the National Radio Astronomy Observatory's Very Large Array (VLA) to detect and image CO J=1→0 and J=2→1 emission from APM 08279+5255. Emission in the CO(1→0) line ( Fig. 1) extends over scales of ∼ 7 ′′ (∼ 30 kpc at z = 3.9 for H 0 = 75 km s −1 Mpc −1 , q 0 = 0.5), well beyond the gravitationally lensed nucleus 6 where the emission from the CO(4→3) and CO(9→8) transitions is confined 3 . Maps of CO(2→1) detect the northern part of the extended emission and show it to consist of two kinematically and spatially distinct features (Fig. 2). The continuum emission detected at -2 -23 GHz emanates from the lensed nucleus and is unresolved (≤ 0.6 ′′ ) with a flux density of 0.30 ± 0.05 mJy. At 43 GHz, no continuum emission is detected at a level of σ = 0.30 mJy, and the emission expected from the hot dust in the nucleus 5 is negligible (≤ 0.04 mJy). No appreciable dust continuum is expected underlying the extra-nuclear CO(2→1) emissi...

Observing Cold Gas in Submillimeter Galaxies: Detection of CO (1→0) Emission in SMM J13120+4242 with the Green Bank Telescope

“…With a low threshold for excitation, requiring n(H 2 ) $ 10 2 cm À3 and ÁE/k $ 5 K, CO (1!0) emission ( rest ¼ 115:271 GHz) is the most representative tracer of the metal-enriched molecular gas mass in galaxies because it is sensitive to the cold, diffuse gas that may dominate. Papadopoulos & Ivison (2002) and Greve et al (2003) presented evidence for large amounts of cold, low-excitation gas at high redshift, as first suggested by Papadopoulos et al (2001), in the galaxy HR 10. In this submillimeter-bright extremely red object (ERO) that lacks obvious AGN signatures, at z CO ¼ 1:439, the CO (5!4) line luminosity underestimates the cold molecular gas mass by nearly an order of magnitude.…”

“…Previous studies of CO (1!0) emission from high-z galaxies have been carried out with the Very Large Array ( VLA; e.g., Papadopoulos et al 2001;Carilli et al 2002b;Greve et al 2003Greve et al , 2004. The Robert C. Byrd Green Bank Telescope (GBT) far exceeds the current capabilities of the VLA in instantaneous spectral bandwidth (800 MHz maximum per spectrometer bank, compared with 50 MHz at the VLA) with a comparable effective collecting area.…”

“…In contrast, previous CO (1!0) detections of other z > 3 active galaxies have implied lower limits on the cold gas density nearly an order of magnitude larger. PSS 2322+1944, APM 08279+5255, and 4C 60.07 have cold gas densities derived from r 43 and LVG models in the range n(H 2 ) > 10 3 Y10 4 cm À3 (Carilli et al 2002b;Papadopoulos et al 2001;Greve et al 2004).…”

Genesis and evolution of dust in galaxies in the early Universe

“…A higher amount of M H 2 as denoted by the higher value of α in these massive galaxies might be possible. For example, the presence of large amounts of cold and low-excited molecular gas have been suggested by Papadopoulos et al (2001) for the QSO APM 08279+5255 at z = 3.91.…”

The first spectral line surveys searching for signals from the dark ages

“…In addition to the high excitation CO observations at z = 6.4 (Bertoldi et al 2003) this molecule has also been observed in for example a damped Lyman-α system at z = 2.4 (Srianand et al 2008) and in a field containing an over-density of Lyman break galaxies at z = 5.1 (Stanway et al 2008). A massive CO reservoir has also been detected at z = 3.9 (Papadopoulos et al 2001). An unusually high amount of neutral hydrogen was found by Frye et al (2008) in a 14 Mpc region surrounding a young galaxy at z = 4.9.…”