DENSE GAS PROPERTIES IN ARP 220
Studying dense gas is of fundamental importance for understanding star formation and galaxy evolution. We present data from the Atacama Large Millimeter/Submillimeter Array (ALMA) for the dense molecular gas tracers HNC (hydrogen isocyanide) with transition J = 2-1, CS (carbon monosulfide) with transition J = 4-3, and CO (carbon monoxide) with transition J = 6-5. We studied some properties of dense gases to find their effect on star formation in Arp 220 galaxy. We identify Arp 220 into two regions in which we reveal emission from every one of the three molecules, including the east and west nuclei. We calculate the ratios of LHNC/LCO, LCS/LCO, and LCS/LHNC at a sub-arcsecond scale. The ratios of LHNC / LCO and LCS / LCO represent fractions of the dense molecular gas. The values of these ratios are found equal to (0.31, 0.67) and (0.15, 0.31) in the west and east nuclei, respectively. We detect that these ratios are greater in the east nucleus than in the west nucleus. We also calculated the star formation efficiency (SFEdense), which is the rate of star formation per unit dense gas mass at a sub-arcsecond scale. We found that this galaxy has a long depletion time and low SFEdense compared to the global scale.
Keywords: Galaxy, Arp 220, Dense Gas, ALMA.