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Lurking in ULIRGs: Molecular Gas in local Merging Galaxies


Manohar, Swarnima (2016) Lurking in ULIRGs: Molecular Gas in local Merging Galaxies. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/Z9T72FDK.


Mergers and interacting galaxies are pivotal to the evolution of galaxies in the universe. They are the sites of prodigious star formation and key to understanding the starburst processes: the physical and chemical properties and the dynamics of the molecular gas. ULIRGs or Ultraluminous Infrared Galaxies are a result of many of these mergers. They host extreme starbursts, AGNs, and mergers. They are the perfect laboratory to probe the connection between starbursts, black hole accretion and mergers and to further our understanding of star formation and merging.

NGC 6240 and Arp 220 can be considered the founding members of this very active class of objects. They are in different stages of merging and hence are excellent case studies to further our understanding about the merging process. We have imaged the dense star-forming regions of these galaxies at sub-arcsec resolution with CARMA C and B Configurations (2" and 0.5 - 0.8"). Multi-band imaging allows excitation analysis of HCN, HCO+, HNC, and CS along with CO transitions to constrain the properties of the gas. Our dataset is unique in that we have observed these lines at similar resolutions and high sensitivity which can be used to derive line ratios of faint high excitation lines.

Arp 220 has not had confirmed X-ray AGN detections for either nuclei. However, our observations indicate HCN/HNC ratios consistent with the chemistry of X-ray Dominated Regions (XDRs) -- a likely symptom of AGN. We calculated the molecular Hydrogen densities using each of the molecular species and conclude that assuming abundances of HNC and HCO+ similar to those in galactic sources are incorrect in the case of ULIRGs. The physical conditions in the dense molecular gas in ULIRGs alter these abundances. The derived H2 volume densities are ~ 5 x 104 cm-3 in both Arp 220 nuclei and ~ 104 cm-3 in NGC 6240.

Item Type:Thesis (Dissertation (Ph.D.))
Subject Keywords:Galaxies: individual (Arp 220, NGC 6240); Galaxies: Starbursts
Degree Grantor:California Institute of Technology
Division:Physics, Mathematics and Astronomy
Major Option:Astrophysics
Thesis Availability:Public (worldwide access)
Research Advisor(s):
  • Scoville, Nicholas Zabriskie
Group:Owens Valley Radio Observatory (OVRO), Astronomy Department
Thesis Committee:
  • Sargent, Anneila Isabel (chair)
  • Readhead, Anthony C. S.
  • Hopkins, Philip F.
  • Mawet, Dimitri
  • Scoville, Nicholas Zabriskie
Defense Date:12 January 2016
Non-Caltech Author Email:swarnim (AT)
Record Number:CaltechTHESIS:03042016-144127692
Persistent URL:
Manohar, Swarnima0000-0002-3506-566X
Default Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:9604
Deposited By: Swarnima Manohar
Deposited On:08 Mar 2016 18:56
Last Modified:02 Dec 2022 19:59

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