Molecular Gas in Redshift 6 Quasar Host Galaxies

Source: arXiv

ABSTRACT We report our new observations of redshifted carbon monoxide emission from six z~6 quasars, using the PdBI. CO (6-5) or (5-4) line emission was detected in all six sources. Together with two other previous CO detections, these observations provide unique constraints on the molecular gas emission properties in these quasar systems close to the end of the cosmic reionization. Complementary results are also presented for low-J CO lines observed at the GBT and the VLA, and dust continuum from five of these sources with the SHARC-II bolometer camera at the CSO. We then present a study of the molecular gas properties in our combined sample of eight CO-detected quasars at z~6. The detections of high-order CO line emission in these objects indicates the presence of highly excited molecular gas, with estimated masses on the order of 10^10 M_sun within the quasar host galaxies. No significant difference is found in the gas mass and CO line width distributions between our z~6 quasars and samples of CO-detected $1.4\leq z\leq5$ quasars and submillimeter galaxies. Most of the CO-detected quasars at z~6 follow the far infrared-CO luminosity relationship defined by actively star-forming galaxies at low and high redshifts. This suggests that ongoing star formation in their hosts contributes significantly to the dust heating at FIR wavelengths. The result is consistent with the picture of galaxy formation co-eval with supermassive black hole (SMBH) accretion in the earliest quasar-host systems. We investigate the black hole--bulge relationships of our quasar sample, using the CO dynamics as a tracer for the dynamical mass of the quasar host. The results place important constraints on the formation and evolution of the most massive SMBH-spheroidal host systems at the highest redshift. Comment: 34 pages, 8 figures, accepted for publication in ApJ

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    ABSTRACT: We present an analysis of the excitation conditions of the molecular gas in the streamers and the outflow of M82 based on observations obtained at the IRAM 30m telescope. Our analysis of J=1-0 and J=2-1 transitions of CO and 13CO and the CO(3-2) line in 13 regions outside the central starburst disk shows that the gas density within the streamer/outflow system is about an orderof magnitude lower (n(H2) ~ 10^3 cm^-3) than in the central molecular disk. We have used an LVG model and data from the literature to constrain the flux density in each CO transition (the `CO line SED') arising from the streamer/outflow system and the central starburst disk itself. Globally, we find that the CO flux density up to the J=3-2 line is dominated by the diffuse outer regions while lines above the J=5-4 transition are almost exclusively emitted by the central starburst disk. We compare the CO line SED of M82 to CO observations of galaxies at high redshift and suggest that small high-J/low-J CO flux density ratios (observed in some of these sources) are not necessarily caused by a different excitation of the central molecular gas concentration, but may result from an additional, more extended and diffuse gas reservoir around these systems, reminiscent of the situation in M82. Comment: 12 pages, 7 figures, accepted by A&A
    Astronomy and Astrophysics 04/2005; · 5.08 Impact Factor
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    ABSTRACT: We report the detection of the CO 4-3, 6-5, 9-8, 10-9, and 11-10 lines in the Broad Absorption Line quasar APM 08279+5255 at z=3.9 using the IRAM 30 m telescope. We also present IRAM PdBI high spatial resolution observations of the CO 4-3 and 9-8 lines, and of the 1.4 mm dust radiation as well as an improved spectrum of the HCN(5-4) line. Unlike CO in other QSO host galaxies, the CO line SED of APM 08279+5255 rises up to the CO(10-9) transition. The line fluxes in the CO ladder and the dust continuum fluxes are best fit by a two component model, a “cold” component at ~ 65 K with a high density of n(H2) = 1×105 cm-3, and a “warm”, ~ 220 K component with a density of 1×104 cm-3. We show that IR pumping via the 14 μm bending mode of HCN is the most likely channel for the HCN excitation. From our models we find, that the CO(1-0) emission is dominated by the dense gas component which implies that the CO conversion factor is higher than usually assumed for high-z galaxies with α ≈ 5 M⊙ (K km s-1 pc^2)-1. Using brightness temperature arguments, the results from our high-resolution mapping, and lens models from the literature, we argue that the molecular lines and the dust continuum emission arise from a very compact (r ≈ 100-300 pc), highly gravitationally magnified ( m = 60-110) region surrounding the central AGN. Part of the difference relative to other high-z QSOs may therefore be due to the configuration of the gravitational lens, which gives us a high-magnification zoom right into the central 200-pc radius of APM 08279+5255 where IR pumping plays a significant role for the excitation of the molecular lines.
    Astronomy and Astrophysics 06/2007; 467(3):955-969. · 5.08 Impact Factor
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    ABSTRACT: We investigate the nature of molecular gas emission from z ~ 6 quasars via the commonly observed tracer of H2, carbon monoxide (CO). We achieve this by combining non-LTE radiative transfer calculations with merger-driven models of z ~ 6 quasar formation that arise naturally in Λ cold dark matter structure formation simulations. Motivated by observational constraints, we consider four representative z ~ 6 quasars formed in the halo mass range ~1012-1013 M☉ from different merging histories. Our main results are as follows. We find that, owing to massive starbursts and funneling of dense gas into the nuclear regions of merging galaxies, the CO is highly excited during both the hierarchical buildup of the host galaxy and the quasar phase, and the CO flux density peaks between J = 5 and 8. The CO morphology of z ~ 6 quasars often exhibits multiple CO emission peaks which arise from molecular gas concentrations which have not yet fully coalesced. Both of these results are found to be consistent with the sole CO detection at z ~ 6, in quasar J1148+5251. Quasars which form at z ~ 6 display a large range of sight line-dependent line widths. The sight line dependencies are such that the narrowest line widths are when the rotating molecular gas associated with the quasar is viewed face-on (when the LB is largest) and broadest when the quasar is seen edge-on (and the LB is lowest). Thus, we find that for all models selection effects exist such that quasars selected for optical luminosity are preferentially seen to be face-on which may result in CO detections of optically luminous quasars at z ~ 6 having line widths narrower than the median. The mean sight line-averaged line width is found to be reflective of the circular velocity of the host halo and thus scales with halo mass. For example, the mean line width for the ~1012 M☉ halo is σ ~ 300 km s−1, while the median for the ~1013 M☉ quasar host is σ ~ 650 km s−1. Depending on the host halo mass, approximately 2%-10% of sight lines in our modeled quasars are found to have narrow line widths compatible with observations of J1148+5251. When considering the aforementioned selection effects, these percentages increase to 10%-25% for quasars selected for optical luminosity. When accounting for both temporal evolution of CO line widths in galaxies, as well as the redshift evolution of halo circular velocities, these models can self-consistently account for the observed line widths of both submillimeter galaxies and quasars at z ~ 2. Finally, we find that the dynamical mass derived from the mean sight line-averaged line widths provide a good estimate of the total mass and allow for a massive molecular reservoir, supermassive black hole, and stellar bulge, consistent with the local MBH-Mbul relation.
    The Astrophysical Journal Supplement Series 12/2008; 174(1):13. · 16.24 Impact Factor


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Dominik A. Riechers