[Paper Review] The NGC 1614 Interacting Galaxy: Molecular Gas Feeding a "Ring of Fire"

S. König, S. Aalto+3 more|Research Explorer (The University of Manchester)|Mar 5, 2013

Galaxies: Formation, Evolution, Phenomena59 references18 citations

TL;DR

This study reveals that molecular gas from a tidal dust lane feeds a circumnuclear starburst ring in the minor merger NGC 1614 via a 'cosmic umbilical cord' structure, with high-resolution SMA observations detecting 10 giant molecular associations (GMAs) in a 231 pc ring containing 8.3 × 10⁸ M☉ of gas. The asymmetric ring, fueled by resonant gas inflow, hosts intense star formation, suggesting a combination of cloud-cloud collisions and orbital crowding trigger star formation in the ring's densest regions.

ABSTRACT

Minor mergers frequently occur between giant and gas-rich low mass galaxies and can provide significant amounts of interstellar matter to refuel star formation and power AGN in the giant systems. Major starbursts and/or AGN result when fresh gas is transported and compressed in the central regions of the giant galaxy. This is the situation in NGC1614, whose molecular medium we explore at half arcsecond angular resolution through our observations of 12CO(2-1) emission using the SMA. We compare our maps with optical and Pa alpha, HST and high angular resolution radio continuum images to study the relationships between dense molecular gas and the starburst region. The most intense CO emission occurs in a partial ring with ~230pc radius around the center, with an extension to the north-west into the dust lane that contains diffuse molecular gas. We resolve 10 GMAs in the ring which has an integrated molecular mass of ~8x10^8M_sun. Our observations filter out a large part of the CO(1-0) emission mapped at shorter spacings, indicating that most of the molecular gas is diffuse and that GMAs only exist near and within the circumnuclear ring. The molecular ring is uneven with most of the mass on the western side, which also contains GMAs extending into a pronounced tidal dust lane. The spatial and kinematic patterns suggest that the northwest extension of the ring is a cosmic umbilical cord that is feeding molecular gas associated with the dust lane and tidal debris into the nuclear ring. The astrophysical process for producing a ring structure is not fully understood, but the presence of numerous GMAs suggests an orbit crowding or resonance phenomenon. There is some evidence that star formation is progressing radially outwards within the ring, indicating that a self-triggering mechanism may also affect these processes.

Motivation & Objective

• To investigate the origin and distribution of molecular gas in the starburst minor merger NGC 1614.
• To determine how gas is transported from tidal debris into the central starburst ring.
• To identify the physical mechanisms triggering star formation in the circumnuclear ring.
• To assess the role of resonant structures and cloud-cloud interactions in shaping the molecular gas morphology.

Proposed method

• High-resolution (0.5 arcsec) 12 CO (2–1) observations of NGC 1614 using the Submillimeter Array (SMA).
• Comparison of CO (2–1) data with optical, Paα, Hubble, and high-resolution radio continuum images to trace gas and star formation.
• Identification of giant molecular associations (GMAs) from the 3D data cube using spatial and kinematic criteria.
• Estimation of molecular gas mass from integrated CO line intensities using the X_CO factor.
• Analysis of velocity gradients and kinematic asymmetries to infer gas inflow and resonance-driven dynamics.
• Use of 13 CO (2–1) non-detection to set a lower limit on the 12 CO/13 CO line ratio (≥15).

Experimental results

Research questions

• RQ1 How is molecular gas transported from tidal debris into the central starburst ring in NGC 1614?
• RQ2 What physical mechanisms—resonance, cloud-cloud collisions, or orbital crowding—drive the formation of giant molecular associations (GMAs) in the ring?
• RQ3 Why is star formation concentrated in the ring rather than in the outer tidal structures?
• RQ4 What is the role of the dust lane and its connection to the ring in the gas inflow process?
• RQ5 How does the kinematic structure of the molecular gas support or challenge existing models of circumnuclear ring formation?

Key findings

• The 12 CO (2–1) emission forms an asymmetric ring with a radius of 231 pc, centered on the nucleus, containing a total molecular mass of 8.3 × 10⁸ M☉.
• The ring is most prominent on the western side, with a prominent northwest extension forming a 'cosmic umbilical cord' connecting to the tidal dust lane.
• Ten giant molecular associations (GMAs) are identified, all located within or near the ring, with higher velocity dispersion and mass than GMAs outside the ring.
• 13 CO (2–1) is not detected, implying a 12 CO/13 CO (2–1) line ratio ≥15, consistent with high optical depth or low 13C abundance.
• Molecular gas in the dust lane shows no significant star formation, indicating that star formation is triggered only upon entry into the ring, not during transport.
• The spatial and kinematic patterns support a model in which gas is channeled via tidal structures into a Lindblad resonance, forming the starburst ring through cloud-cloud collisions and crowding.