Document Details

Document Type : Article In Journal 
Document Title :
Ammonia as a temperature tracer in the ultraluminous galaxy merger Arp220
Ammonia as a temperature tracer in the ultraluminous galaxy merger Arp220
 
Subject : Astronomyِ 
Document Language : English 
Abstract : We present Australia Telescope Compact Array (ATCA) and Robert C. Byrd Green Bank Telescope (GBT) observations of ammonia (NH3) and the 1.2cm radio continuum toward the ultraluminous infrared galaxy merger Arp220. We detect the NH3(1,1), (2,2), (3,3), (4,4), (5,5), and (6,6) inversion lines in absorption against the unresolved, (62 ± 9) mJy continuum source at 1.2cm. The peak apparent optical depths of the ammonia lines range from 0.05 to 0.18. The absorption lines are well described by single-component Gaussians with central velocities in between the velocities of the eastern and western cores of Arp220. Therefore, the ammonia likely traces gas that encompasses both cores. The absorption depth of the NH3(1,1) line is significantly shallower than expected based on the depths of the other transitions. The shallow (1,1) profile may be caused by contamination from emission by a hypothetical, cold (≲ 20 K) gas layer with an estimated column density of ≲ 2 × 1014cm-2. This layer would have to be located behind or away from the radio continuum sources to produce the contaminating emission. The widths of the ammonia absorption lines are 120-430kms-1, in agreement with those of other molecular tracers. We cannot confirm the extremely large line widths of up to 1800kms-1 previously reported for this galaxy. Using all of the ATCA detections except for the shallow (1,1) line, we determine a rotational temperature of (124 ± 19) K, corresponding to a kinetic temperature of T kin = (186 ± 55) K. Ammonia column densities depend on the excitation temperature. For excitation temperatures of 10K and 50K, we estimate N(NH 3) = (1.7 ± 0.1) × 1016cm-2 and (8.4 ± 0.5) × 1016cm-2, respectively. The relation scales linearly for possible higher excitation temperatures. Our observations are consistent with an ortho-to-para-ammonia ratio of unity, implying that the ammonia formation temperature exceeds 30 K. In the context of a model with a molecular ring that connects the two nuclei in Arp220, we estimate the H2 gas density to be f -0.5V × (1-4) × 103, where f V is the volume filling factor of the molecular gas. In addition to ammonia, our ATCA data show an absorption feature adjacent in frequency to the NH3(3,3) line. The line does not appear in the GBT spectrum. If we interpret the line to be from the OH 2Π3/2 J = 9/2 F = 4-4 transition, it would have a line width, systemic velocity, and apparent optical depth similar to what we detect in the ammonia lines. Comparing the new line to the previously detected 6GHz OH 2Π3/2 J = 5/2 F = 2-2 transition, we determine a rotational OH temperature of 245 K, about two times the rotational temperature of ammonia. If this association with OH is correct, it marks the first detection of the highly excited (511 K above ground state) 2Π 3/2 J = 9/2 F = 4-4 OH line in an extragalactic object. 
ISSN : 0004-637X 
Journal Name : Astrophysical Journal 
Volume : 742 
Issue Number : 2 
Publishing Year : 1432 AH
2011 AD 
Article Type : Article 
Added Date : Monday, March 12, 2012 

Researchers

Researcher Name (Arabic)Researcher Name (English)Researcher TypeDr GradeEmail
Jürgen OttOtt, Jürgen ResearcherDoctoratejott@nrao.edu
Christian HenkelHenkel, Christian ResearcherDoctorate 
James A BraatzBraatz, James AResearcherDoctorate 
Axel WeißWeiß, Axel ResearcherDoctorate 

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