The Dense Molecular Gas in the Luminous Infrared Galaxy NGC 1614

Abstract

We investigate the molecular gas properties, especially the dense molecular gas in NGC 1614, using the ALMA (Atacama Large Millimeter/ submillimeter Array) high resolution archival data of ${}^{12}$CO(1-0), ${}^{13}$CO(1-0), ${}^{12}$CO(3-2), ${}^{12}$CO(6-5), HCN(3-2), HCO${}^{+}$(3-2), HCN(4-3), and HCO${}^{+}$(4-3). From the high-resolution integrated intensity maps, a ring structure was detected in the central region ($<$ 1 kpc), and the molecular gas mainly distributes in the central region while there is little gas in the nucleus. The ${}^{12}$CO(1-0) shows an extended structure to the south, north, and southeast, and CO ($J⩾3$, $J$ is the quantum number of rotational vibration levels), HCN, and HCO${}^{+}$ show that the dense gas mainly concentrated in the central region. Similarly, HCN(4-3)/${}^{12}$CO(1-0) and HCO${}^{+}$(4-3)/${}^{12}$CO(1-0) ratio maps show that the dense molecular gas is mainly concentrated in the ring of the central region. The HCN/HCO${}^{+}$ ratios show that the different regions of the starburst ring might have different excitation conditions. The variations of HCN/HCO${}^{+}$ values in different region of central region may be due to the gas density and temperature. For HCN/HCO${}^{+}$(4-3) ratio map, the higher intensity values ($\sim$0.44 $\pm$ 0.04) were obtained in the eastern and western region of the ring, compared to the northern and southern region ($\sim$0.35 $\pm$ 0.03). For HCN/HCO${}^{+}$(3-2) ratio map, the higher values ($\sim$0.38 $\pm$ 0.04) are distributed at peak location of HCN(3-2) emission, while the lower values ($\sim$0.3 $\pm$ 0.03) are distributed at the northwestern and southeastern region of the ring. The mechanism of variations of HCN/HCO${}^{+}$ values in the central region of the galaxy has been discussed in the article.