College of Liberal Arts & Social Sciences Theses and Dissertations

Graduation Date

8-2011

Document Type

Thesis

Department/Program Conferring Degree

Physics

Keywords

Arp 220, EVLA, hydrogen cyanide, methanimine, radio astronomy

Abstract

We present observations of the 5289.8MHz$1{10}\rightarrow1_{11}$ transition of the pre-biotic molecule methanimine(\meth) and the 6731.9 MHz J=5 direct $|$-type transition in the vibrationally excited($\nu_2=1$) state of hydrogen cyanide(HCN) toward the nearest Ultraluminous Infrared Galaxy, Arp 220, taken with the Expanded Very Large Array (EVLA) in A configuration. The 5289.8 MHz \meth\ line was detected in emission toward the western nucleus, but not toward the eastern nucleus. The 6731.9 MHz HCN line was detected in absorption toward the western nucleus, but not toward the eastern nucleus. Both of these are new results from the observations reported in this thesis, and are in contrast with rotational HCN transitions that trace cold and dense molecular gas and have been observed in emission toward both nuclei. We have also confirmed that the\meth\line is most like a maser; this represents the first independent confirmation of the only \meth\ maser ever detected. This \meth\ maser was found to have an isotropic luminosity of 5$L_\odot$, close to the luminosity (12$L_odot$) of a formaldehyde maser in Arp 220, and similar to the luminosities of \h\ kilomasers, most of which are believed to be excited in active star forming regions in external galaxies. The western nucleus is known to have undergone an intense starburst, and we conclude that the \meth\ maser is being amplified in the foreground gas along the line of sight to the western nucleus by the background continuum of the nucleus. The absorbing HCN gas toward the western nucleus was found to have a column density $\sim 1.04\times 10^{19}$cm$^{-2}$ corresponding to an excitation temperature near 550 K; the column density may be an order of magnitude higher at $\sim 10^{20}$ cm$^{-2}S if the excitation temperature has a lower value of 150K. If we assume that the HCN abundance relative to molecular hydrogen is the same as in our Galaxy ($10^{-5}$), this gives about $1 \times 10^{24}$cm$^{-2}$(or$10^{25}$ cm$^{-2}$) for the molecular hydrogen column density, which is in reasonable agreement with the values reported by other observers toward the western nucleus. We conclude that the observed HCN absorption is likely taking place in a column of highly excited molecular gas in the foreground of the western nucleus. The observations reported in this thesis provide a self consistent picture of highly excited gas in the foreground of the western nucleus, and a convincing demonstration that the line of sight toward the western nucleus has different physical conditions than that toward the eastern nucleus. These observations have demonstrated conclusively that a new range of molecular transitions can now be detected and imaged with cm-wavelength telescopes like the EVLA, thus opening up new opportunities for the study of the physics and chemistry of other galaxies.

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