The importance of warm, AGN-driven outflows in the nuclear regions of nearby ULIRGs

Abstract

We present an optical spectroscopic study of a 90% complete sample of 17\nnearby ULIRGs with optical Seyfert nuclei, with the aim of investigating the\nnature of the nuclear warm gas outflows. A high proportion (94%) of our sample\nshow disturbed emission line kinematics in the form of broad (FWHM > 500 km\ns-1) and/or strongly blueshifted (\\Delta V < -150 km s-1) emission line\ncomponents. This proportion is significantly higher than found in a comparison\nsample of non-Sy ULIRGs (19%). We also find evidence that the [OII]5007,4959\nemission lines in Sy-ULIRGs are broader and more asymmetric that in samples of\nnon-ULIRG Seyferts.\n The Sy-ULIRG sample encompasses a wide diversity of emission line profiles.\nIn most individual objects we are able to fit the profiles of all the emission\nlines with a kinematic model derived from the strong [OIII]4959,5007 lines,\nusing between 2 and 5 Gaussian components. From these fits we derive diagnostic\nline ratios that are used to investigate the ionization mechanisms for the\ndifferent kinematic components. We show that, in general, the line ratios are\nconsistent with gas of super-solar abundance photoionized by a combination of\nAGN and starburst activity, with an increasing contribution from the AGN with\nincreasing FWHM of the individual kinematic components, and the AGN\ncontribution dominating for the broadest components. However, shock ionization\ncannot be ruled out in some cases. Our derived upper limits on the mass\noutflows rates and kinetic powers of the emission line outflows show that they\ncan be as energetically significant as the neutral and molecular outflows in\nULIRGs-consistent with the requirements of the hydrodynamic simulations that\ninclude AGN feedback. However, the uncertainties are large, and more accurate\nestimates of the radii, densities and reddening of the outflows are required to\nput these results on a firmer footing.\n