Herschel-SPIRE Fourier Transform Spectrometer Observations of Excited CO and [C I] in the Antennae (NGC 4038/39): Warm and Cold Molecular Gas
Wilson, Christine D.; De Looze, Ilse; Baes, Maarten; Madden, Suzanne C.; Barlow, Michael J.; Glenn, Jason; Cooray, Asantha; Spinoglio, Luigi; Pereira-Santaella, Miguel; Rangwala, Naseem; Kamenetzky, Julia; Schirm, Maximilien R. P.; Clements, Dave L.; Karczewski, Oskar Ł.; Parkin, Tara J.; Rémy-Ruyer, Aurélie; Wu, Ronin
Canada, United States, Italy, Belgium, United Kingdom, France
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 ~ 19''-43'') and spectral (~1.2 GHz) resolution mapping spectrometer covering a large spectral range (194-671 μm, 450-1545 GHz). We detect five CO transitions (J = 4-3 to J = 8-7), both [C I] transitions, and the [N II] 205 μm 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 μm. Using the CO and [C I] transitions, we perform both a local thermodynamic equilibrium (LTE) analysis of [C I] 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 ~ 10-30 K) and a warm (T kin >~ 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 ~ 5 × 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 αCO ~ 7 M ⊙ pc-2 (K km s-1)-1 in the cold component, similar to the value for normal spiral galaxies. We estimate the cooling from H2, [C II], CO, and [O I] 63 μm to be ~0.01 L ⊙/M ⊙. 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 ~ 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.
Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.