Molecular gas heating mechanisms, and star formation feedback in merger/starbursts: NGC 6240 and Arp 193 as case studies
- School of Physics and Astronomy, Cardiff University, Queen's Buildings, The Parade, Cardiff, CF24 3AA (United Kingdom)
- Purple Mountain Observatory/Key Lab for Radio Astronomy, 2 West Beijing Road, Nanjing 210008 (China)
- Institute of Astronomy, Astrophysics, Space Applications and Remote Sensing, I. Metaxa and Vas. Pavlou str., GR-15236 Athens (Greece)
- Max Planck Institute für Radioastronomie, Auf dem Hügel 69, D-53121 Bonn (Germany)
- Leiden Observatory, Leiden University, P.O. Box 9513, NL-2300 RA Leiden (Netherlands)
- Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT (United Kingdom)
- Research and Scientific Support Department, European Space Agency, Keplerlaan 1, 2200 AG Noordwijk (Netherlands)
We used the SPIRE/FTS instrument aboard the Herschel Space Observatory to obtain the Spectral Line Energy Distributions (SLEDs) of CO from J = 4-3 to J = 13-12 of Arp 193 and NGC 6240, two classical merger/starbursts selected from our molecular line survey of local Luminous Infrared Galaxies (L {sub IR} ≥ 10{sup 11} L {sub ☉}). The high-J CO SLEDs are then combined with ground-based low-J CO, {sup 13}CO, HCN, HCO{sup +}, CS line data and used to probe the thermal and dynamical states of their large molecular gas reservoirs. We find the two CO SLEDs strongly diverging from J = 4-3 onward, with NGC 6240 having a much higher CO line excitation than Arp 193, despite their similar low-J CO SLEDs and L {sub FIR}/L {sub CO,} {sub 1} {sub –0}, L {sub HCN}/L {sub CO} (J = 1-0) ratios (proxies of star formation efficiency and dense gas mass fraction). In Arp 193, one of the three most extreme starbursts in the local universe, the molecular SLEDs indicate a small amount (∼5%-15%) of dense gas (n ≥ 10{sup 4} cm{sup –3}) unlike NGC 6240 where most of the molecular gas (∼60%-70%) is dense (n ∼ (10{sup 4}-10{sup 5}) cm{sup –3}). Strong star-formation feedback can drive this disparity in their dense gas mass fractions, and also induce extreme thermal and dynamical states for the molecular gas. In NGC 6240, and to a lesser degree in Arp 193, we find large molecular gas masses whose thermal states cannot be maintained by FUV photons from Photon-Dominated Regions. We argue that this may happen often in metal-rich merger/starbursts, strongly altering the initial conditions of star formation. ALMA can now directly probe these conditions across cosmic epoch, and even probe their deeply dust-enshrouded outcome, the stellar initial mass function averaged over galactic evolution.
- OSTI ID:
- 22356581
- Journal Information:
- Astrophysical Journal, Vol. 788, Issue 2; Other Information: Country of input: International Atomic Energy Agency (IAEA); ISSN 0004-637X
- Country of Publication:
- United States
- Language:
- English
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