Dissecting galaxies: spatial and spectral separation of emission excited by star formation and AGN activity

Abstract

The optical spectra of Seyfert galaxies are often dominated by emission lines\nexcited by both star formation and AGN activity. Standard calibrations (such as\nfor the star formation rate) are not applicable to such composite (mixed)\nspectra. In this paper, we describe how integral field data can be used to\nspectrally and spatially separate emission associated with star formation from\nemission associated with accretion onto an active galactic nucleus (AGN). We\ndemonstrate our method using integral field data for two AGN host galaxies (NGC\n5728 and NGC 7679) from the Siding Spring Southern Seyfert Spectroscopic\nSnapshot Survey (S7). The spectra of NGC 5728 and NGC 7679 form clear sequences\nof AGN fraction on standard emission line ratio diagnostic diagrams. We show\nthat the emission line luminosities of the majority (> 85 per cent) of spectra\nalong each AGN fraction sequence can be reproduced by linear superpositions of\nthe emission line luminosities of one AGN dominated spectrum and one star\nformation dominated spectrum. We separate the Halpha, Hbeta, [N\nII]$\\lambda$6583, [S II]$\\lambda \\lambda$6716, 6731, [O III]$\\lambda$5007 and\n[O II]$\\lambda \\lambda$3726, 3729 luminosities of every spaxel into\ncontributions from star formation and AGN activity. The decomposed emission\nline images are used to derive the star formation rates and AGN bolometric\nluminosities for NGC 5728 and NGC 7679. Our calculated values are mostly\nconsistent with independent estimates from data at other wavelengths. The\nrecovered star forming and AGN components also have distinct spatial\ndistributions which trace structures seen in high resolution imaging of the\ngalaxies, providing independent confirmation that our decomposition has been\nsuccessful.\n