ALMA [C i]<sup>3</sup> P <inf>1</inf>-<sup>3</sup> P <inf>0</inf> Observations of NGC 6240: A Puzzling Molecular Outflow, and the Role of Outflows in the Global α <inf>CO</inf> Factor of (U)LIRGs
Authors
Severgnini, P
Papadopoulos, PP
Feruglio, C
Zhang, ZY
Ceca, RD
Fiore, F
Wagg, J
Publication Date
2018-08-20Journal Title
Astrophysical Journal
ISSN
0004-637X
Publisher
American Astronomical Society
Volume
863
Issue
2
Pages
1-18
Type
Article
Metadata
Show full item recordCitation
Cicone, C., Severgnini, P., Papadopoulos, P., Maiolino, R., Feruglio, C., Treister, E., Privon, G., et al. (2018). ALMA [C i]<sup>3</sup> P <inf>1</inf>-<sup>3</sup> P <inf>0</inf> Observations of NGC 6240: A Puzzling Molecular Outflow, and the Role of Outflows in the Global α <inf>CO</inf> Factor of (U)LIRGs. Astrophysical Journal, 863 (2), 1-18. https://doi.org/10.3847/1538-4357/aad32a
Abstract
We present ALMA and ACA [CI]$^{3}P_{1}-^{3}P_{0}$ ([CI](1-0)) observations of
NGC6240, which we combine with ALMA CO(2-1) and IRAM PdBI CO(1-0) data to study
the physical properties of the massive molecular (H$_2$) outflow. We discover
that the receding and approaching sides of the H$_2$ outflow, aligned
east-west, exceed 10 kpc in their total extent. High resolution ($0.24"$)
[CI](1-0) line images surprisingly reveal that the outflow emission peaks
between the two AGNs, rather than on either of the two, and that it dominates
the velocity field in this nuclear region. We combine the [CI](1-0) and CO(1-0)
data to constrain the CO-to-H$_2$ conversion factor ($\alpha_{\rm CO}$) in the
outflow, which is on average $2.1\pm1.2~\rm M_{\odot} (K~km~s^{-1}~pc^2)^{-1}$.
We estimate that $60\pm20$ % of the total H$_2$ gas reservoir of NGC6240 is
entrained in the outflow, for a resulting mass-loss rate of $\dot{M}_{\rm
out}=2500\pm1200~M_{\odot}~yr^{-1}$ $\equiv 50\pm30$ SFR. This energetics rules
out a solely star formation-driven wind, but the puzzling morphology challenges
a classic radiative-mode AGN feedback scenario. For the quiescent gas we
compute $\langle\alpha_{\rm CO}\rangle = 3.2\pm1.8~\rm M_{\odot}
(K~km~s^{-1}~pc^2)^{-1}$, which is at least twice the value commonly employed
for (U)LIRGs. We observe a tentative trend of increasing $r_{21}\equiv
L^{\prime}_{\rm CO(2-1)}/L^{\prime}_{\rm CO(1-0)}$ ratios with velocity
dispersion and measure $r_{21}>1$ in the outflow, whereas $r_{21}\simeq1$ in
the quiescent gas. We propose that molecular outflows are the location of the
warmer, strongly unbound phase that partially reduces the opacity of the CO
lines in (U)LIRGs, hence driving down their global $\alpha_{\rm CO}$ and
increasing their $r_{21}$ values.
Sponsorship
ERC Advanced Grant 695671 "QUENCH"
STFC ST/M001172/1
Funder references
Science and Technology Facilities Council (ST/M001172/1)
European Research Council (695671)
Embargo Lift Date
2100-01-01
Identifiers
External DOI: https://doi.org/10.3847/1538-4357/aad32a
This record's URL: https://www.repository.cam.ac.uk/handle/1810/283044
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