The nature of co emission from z ∼ 6 quasars

Desika Narayanan, Yuexing Cindy Li, Thomas J. Cox, Lars Hernquist, Philip Hopkins, Sukanya Chakrabarti, Romeel Davé, Tiziana Di Matteo, Liang Gao, Craig Kulesa, Brant Robertson, Christopher K. Walker

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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 A 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.

Original languageEnglish (US)
Pages (from-to)13-30
Number of pages18
JournalAstrophysical Journal, Supplement Series
Volume174
Issue number1
DOIs
StatePublished - Jan 1 2008

Fingerprint

carbon monoxide
quasars
halos
gas
visual perception
molecular gases
galaxies
temporal evolution
line of sight
merger
radiative transfer
luminosity
tracer
history
tracers
dark matter
simulation
flux density

All Science Journal Classification (ASJC) codes

  • Astronomy and Astrophysics
  • Space and Planetary Science

Cite this

Narayanan, D., Li, Y. C., Cox, T. J., Hernquist, L., Hopkins, P., Chakrabarti, S., ... Walker, C. K. (2008). The nature of co emission from z ∼ 6 quasars. Astrophysical Journal, Supplement Series, 174(1), 13-30. https://doi.org/10.1086/521776
Narayanan, Desika ; Li, Yuexing Cindy ; Cox, Thomas J. ; Hernquist, Lars ; Hopkins, Philip ; Chakrabarti, Sukanya ; Davé, Romeel ; Di Matteo, Tiziana ; Gao, Liang ; Kulesa, Craig ; Robertson, Brant ; Walker, Christopher K. / The nature of co emission from z ∼ 6 quasars. In: Astrophysical Journal, Supplement Series. 2008 ; Vol. 174, No. 1. pp. 13-30.
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author = "Desika Narayanan and Li, {Yuexing Cindy} and Cox, {Thomas J.} and Lars Hernquist and Philip Hopkins and Sukanya Chakrabarti and Romeel Dav{\'e} and {Di Matteo}, Tiziana and Liang Gao and Craig Kulesa and Brant Robertson and Walker, {Christopher K.}",
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Narayanan, D, Li, YC, Cox, TJ, Hernquist, L, Hopkins, P, Chakrabarti, S, Davé, R, Di Matteo, T, Gao, L, Kulesa, C, Robertson, B & Walker, CK 2008, 'The nature of co emission from z ∼ 6 quasars', Astrophysical Journal, Supplement Series, vol. 174, no. 1, pp. 13-30. https://doi.org/10.1086/521776

The nature of co emission from z ∼ 6 quasars. / Narayanan, Desika; Li, Yuexing Cindy; Cox, Thomas J.; Hernquist, Lars; Hopkins, Philip; Chakrabarti, Sukanya; Davé, Romeel; Di Matteo, Tiziana; Gao, Liang; Kulesa, Craig; Robertson, Brant; Walker, Christopher K.

In: Astrophysical Journal, Supplement Series, Vol. 174, No. 1, 01.01.2008, p. 13-30.

Research output: Contribution to journalArticle

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AU - Hopkins, Philip

AU - Chakrabarti, Sukanya

AU - Davé, Romeel

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N2 - 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 A 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.

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Narayanan D, Li YC, Cox TJ, Hernquist L, Hopkins P, Chakrabarti S et al. The nature of co emission from z ∼ 6 quasars. Astrophysical Journal, Supplement Series. 2008 Jan 1;174(1):13-30. https://doi.org/10.1086/521776