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Herschel-SPIRE Fourier transform spectrometer observations of excited CO and [CI] in the antennae (NGC 4038/39): warm and cold molecular gas

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Abstract
We present Herschel Spectral and Photometric Imaging Receiver (SPIRE) Fourier Transform Spectrometer (FTS) observations of the Antennae (NGC 4038/39), a well-studied, nearby (22 Mpc), ongoing merger between two gas-rich spiral galaxies. The SPIRE-FTS is a low spatial (FWHM similar to 19"-43") and spectral (1.2 GHz) resolution mapping spectrometer covering a large spectral range (194-671 mu m, 450-1545 GHz). We detect five CO transitions (J = 4-3 to J = 8-7), both [CI] transitions, and the [N I] 205 mu m transition across the entire system, which we supplement with ground-based observations of the CO J = 1-0, J = 2-1, and J = 3-2 transitions and Herschel Photodetecting Array Camera and Spectrometer (PACS) observations of [C II] and [O I] 63 Am. Using the CO and [CI] transitions, we perform both a local thermodynamic equilibrium (LTE) analysis of [CI] and a non-LTE radiative transfer analysis of CO and [C I] using the radiative transfer code RADEX along with a Bayesian likelihood analysis. We find that there are two components to the molecular gas: a cold (T-kin similar to 10-30 K) and a warm (T-kin greater than or similar to 100 K) component. By comparing the warm gas mass to previously observed values, we determine a CO abundance in the warm gas of x(CO) similar to 5 x 10(-5). If the CO abundance is the same in the warm and cold gas phases, this abundance corresponds to a CO J = 1-0 luminosity-to-mass conversion factor of alpha(co) similar to 7 M circle dot pc(-2) (K km s(-1))(-1) in the cold component, similar to the value for normal spiral galaxies. We estimate the cooling from H-2, [C II], CO, and [O 1] 63 Am to be similar to 0.01 L circle dot/M circle dot. We compare photon-dominated region models to the ratio of the flux of various CO transitions, along with the ratio of the CO flux to the far-infrared flux in NGC 4038, NGC 4039, and the overlap region. We find that the densities recovered from our non-LTE analysis are consistent with a background far-ultraviolet field of strength G(0) similar to 1000. Finally, we find that a combination of turbulent heating, due to the ongoing merger, and supernova and stellar winds are sufficient to heat the molecular gas.
Keywords
galaxies: interactions, infrared: galaxies, NGC 4039), galaxies: individual (NGC 4038, ISM: molecules, submillimeter: galaxies, SPECTRAL ENERGY-DISTRIBUTION, STAR-FORMING GALAXIES, INTERSTELLAR-MEDIUM, NEARBY GALAXIES, SUPERNOVA-REMNANTS, CIRCUMNUCLEAR DISK, INFRARED GALAXIES, ARP 220, EMISSION, M82

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Citation

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Chicago
Schirm, Maximilien RP, Christine D Wilson, Tara J Parkin, Julia Kamenetzky, Jason Glenn, Naseem Rangwala, Luigi Spinoglio, et al. 2014. “Herschel-SPIRE Fourier Transform Spectrometer Observations of Excited CO and [CI] in the Antennae (NGC 4038/39): Warm and Cold Molecular Gas.” Astrophysical Journal 781 (2).
APA
Schirm, M. R., Wilson, C. D., Parkin, T. J., Kamenetzky, J., Glenn, J., Rangwala, N., Spinoglio, L., et al. (2014). Herschel-SPIRE Fourier transform spectrometer observations of excited CO and [CI] in the antennae (NGC 4038/39): warm and cold molecular gas. ASTROPHYSICAL JOURNAL, 781(2).
Vancouver
1.
Schirm MR, Wilson CD, Parkin TJ, Kamenetzky J, Glenn J, Rangwala N, et al. Herschel-SPIRE Fourier transform spectrometer observations of excited CO and [CI] in the antennae (NGC 4038/39): warm and cold molecular gas. ASTROPHYSICAL JOURNAL. 2014;781(2).
MLA
Schirm, Maximilien RP, Christine D Wilson, Tara J Parkin, et al. “Herschel-SPIRE Fourier Transform Spectrometer Observations of Excited CO and [CI] in the Antennae (NGC 4038/39): Warm and Cold Molecular Gas.” ASTROPHYSICAL JOURNAL 781.2 (2014): n. pag. Print.
@article{4385142,
  abstract     = {We present Herschel Spectral and Photometric Imaging Receiver (SPIRE) Fourier Transform Spectrometer (FTS) observations of the Antennae (NGC 4038/39), a well-studied, nearby (22 Mpc), ongoing merger between two gas-rich spiral galaxies. The SPIRE-FTS is a low spatial (FWHM similar to 19{\textacutedbl}-43{\textacutedbl}) and spectral (1.2 GHz) resolution mapping spectrometer covering a large spectral range (194-671 mu m, 450-1545 GHz). We detect five CO transitions (J = 4-3 to J = 8-7), both [CI] transitions, and the [N I] 205 mu m transition across the entire system, which we supplement with ground-based observations of the CO J = 1-0, J = 2-1, and J = 3-2 transitions and Herschel Photodetecting Array Camera and Spectrometer (PACS) observations of [C II] and [O I] 63 Am. Using the CO and [CI] transitions, we perform both a local thermodynamic equilibrium (LTE) analysis of [CI] and a non-LTE radiative transfer analysis of CO and [C I] using the radiative transfer code RADEX along with a Bayesian likelihood analysis. We find that there are two components to the molecular gas: a cold (T-kin similar to 10-30 K) and a warm (T-kin greater than or similar to 100 K) component. By comparing the warm gas mass to previously observed values, we determine a CO abundance in the warm gas of x(CO) similar to 5 x 10(-5). If the CO abundance is the same in the warm and cold gas phases, this abundance corresponds to a CO J = 1-0 luminosity-to-mass conversion factor of alpha(co) similar to 7 M circle dot pc(-2) (K km s(-1))(-1) in the cold component, similar to the value for normal spiral galaxies. We estimate the cooling from H-2, [C II], CO, and [O 1] 63 Am to be similar to 0.01 L circle dot/M circle dot. We compare photon-dominated region models to the ratio of the flux of various CO transitions, along with the ratio of the CO flux to the far-infrared flux in NGC 4038, NGC 4039, and the overlap region. We find that the densities recovered from our non-LTE analysis are consistent with a background far-ultraviolet field of strength G(0) similar to 1000. Finally, we find that a combination of turbulent heating, due to the ongoing merger, and supernova and stellar winds are sufficient to heat the molecular gas.},
  articleno    = {101},
  author       = {Schirm, Maximilien RP and Wilson, Christine D and Parkin, Tara J and Kamenetzky, Julia and Glenn, Jason and Rangwala, Naseem and Spinoglio, Luigi and Pereira-Santaella, Miguel and Baes, Maarten and Barlow, Michael J and Clements, Dave L and Cooray, Asantha and De Looze, Ilse and Karczewski, Oskar \unmatched{0141} and Madden, Suzanne C and R{\'e}my-Ruyer, Aur{\'e}lie and Wu, Ronin},
  issn         = {0004-637X},
  journal      = {ASTROPHYSICAL JOURNAL},
  language     = {eng},
  number       = {2},
  pages        = {24},
  title        = {Herschel-SPIRE Fourier transform spectrometer observations of excited CO and [CI] in the antennae (NGC 4038/39): warm and cold molecular gas},
  url          = {http://dx.doi.org/10.1088/0004-637X/781/2/101},
  volume       = {781},
  year         = {2014},
}

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