Molecular Gas Heating Mechanisms, and Star Formation Feedback in Merger/Starbursts: NGC 6240 and Arp 193 as Case Studies
Abstract
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 IR >= 1011 L ⊙). The high-J CO SLEDs are then combined with ground-based low-J CO, 13CO, HCN, HCO+, 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 FIR/L CO, 1 - 0, L HCN/L 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 >= 104 cm-3) unlike NGC 6240 where most of the molecular gas (~60%-70%) is dense (n ~ (104-105) cm-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.
- Publication:
-
The Astrophysical Journal
- Pub Date:
- June 2014
- DOI:
- 10.1088/0004-637X/788/2/153
- arXiv:
- arXiv:1404.6090
- Bibcode:
- 2014ApJ...788..153P
- Keywords:
-
- galaxies: active;
- galaxies: ISM;
- galaxies: starburst;
- ISM: abundances;
- ISM: kinematics and dynamics;
- ISM: molecules;
- techniques: spectroscopic;
- Astrophysics - Astrophysics of Galaxies
- E-Print:
- 54 pages, 19 figures, accepted for publication in The Astrophysical Journal