High Resolution Multi-transition Study Of NGC 1614 With ALMA

In the local Universe,the(Ultra-)Luminosity Infrared Galaxies-(U)LIRGs are mostly gas-rich galaxy mergers,rich in molecular gas and having active star formation in the central galaxies.But is it driven by AGN(Active Galatic Nucleus)or SB(Star Bursts)or both is still one of hot topics.(U)LIRGs that observed with high spatial resolution can study the star-forming activities of small-scale structures in galaxies,NGC 1614 is a nearby bright LIRG with significant merging characteristics.We investigate the molecular gas properties,especially the dense molecular gas in NGC 1614,using the ALMA high resolution archival data 12CO(1-0),13CO(1-0),12CO(2-1),13CO(2-1),12CO(3-2),12CO(6-5),HCN(3-2),HCO+(3-2),HCN(4-3),and HCO+(4-3).With these high-resolution molecular line data,we have made integrated intensity maps,and integrated intensity ratio maps to explore the physical properties of different regions of the galaxy.From these integrated intensity maps,NGC 1614 is clearly spatially resolved in all line emission,with a ring-like structure.The line emission is strong in the ring structure and the nuclear region have little molecular line emission,which indicates that the molecular gas is mainly distributed in the ring and the nucleus has little molecular gas.The 12CO(1-0)emission show extended structure to the southern,northern and southeastern region,while the high-J CO J=3-2,6-5 and HCN and HCO+line emission show that the dense molecular gas is main concentrated on the ring.From the ratio maps,both the 12CO/13CO line ratio at J=1-0 and J=2-1 transitions show a decrease tendency,from the ring to the outer region of the galaxy,which is likely due to lower gas density in the dust lane,which leads to a increase in the strength of the molecular line of 12CO J=1-0,2-1,increasing the values of 12CO/13CO.The HCN(4-3)/12CO(1-0)and HCO+(4-3)/12CO(1-0)line ratio maps show the dense molecular gas is concentrated on the ring.The HCN/HCO+ratios,show that the different regions of the starburst ring might have different excitation conditions.From our analysis,the variations of HCN/HCO+values in different region of central region may be due to the gas density and temperature.There may be other mechanisms that may cause the variation of HCN/HCO+ values.For HCN/HCO+(4-3)ratio map,the higher intensity values were obtained in the eastern and western parts of ring may due to the increasing HCN emission intensity caused by the UV radiation of the short lived O-star in young star forming regions,and this effect is true for the western region of HCN/HCO+(3-2)ratio map.While the increased HCO+emission intensity in the PDRs(Photo Dissociation Regions)of northern and southern parts of ring leads to a lower values,and this effect can be seen in the northern and southern region of HCN/HCO+(4-3),especially obvious in the northern region of HCN/HCO+(3-2).The HCN/HCO+flux ratios in different regions are all less than 1,indicating that NGC 1614 may be the starburst-dominated galaxy.