Laura Trouille (CIERA)

X-ray/IR Stacking Analysis of BPT-comp: AGN Activity Dominates

Approximately 20% of SDSS emission-line galaxies lie in the BPT-comp regime, between the Kauffmann et al. (2003) empirically determined SF-dominated regime and the Kewley et al. (2001) theoretically predicted AGN-dominated regime. Whether to include this significant number of sources in samples of AGN or samples of star-forming galaxies is an open question and has important implications for galaxy evolution studies, metallicity studies, etc. Using a large spectroscopic sample of GOODS-N and LH galaxies with deep Chandra imaging, we perform an X-ray stacking analysis of X-ray undetected BPT-comp. We find the stacked signal to be X-ray hard and that the majority of X-ray detected BPT-comp are also X-ray hard. This X-ray hardness can be indicative of obscured AGN activity or the presence of HMXBs associated with ongoing star formation. In order to distinguish between these scenarios, we perform an IR stacking analysis using Spitzer 24?m data. The stacked BPT-comp lies well above the expected value for L_x/L_IR for pure star-forming galaxies; similarly for the X-ray detected BPT-comp. We also find that the BPT-comp lie in the AGN-dominated regime of our new TBT diagnostic, which uses [NeIII]/[OII] versus rest-frame g-z color to identify AGN and star forming galaxies out to z=1.4. [NeIII], which has a higher ionization potential than other commonly used forbidden emission lines, appears to foster a more reliable selection of AGN-dominated galaxies. These findings suggest that both the X-ray and optical signal in BPT-comp are dominated by obscured or low accretion rate AGN activity rather than star formation. This is in contrast to claims by previous optical emission-line studies that the signal in BPT-comp is dominated by star-formation activity. Therefore, we recommend that groups carefully consider the impact of excluding or including BPT-comp on the interpretation of their results. For example, for studies involving determining the bolometric contribution from AGN activity or the role of AGN activity in galaxy evolution, we advise maximal inclusiveness. Since BPT-comp comprise a significant percentage of the overall emission-line galaxy population, inclusion of the BPT-comp would provide a more comprehensive picture of the true impact of AGN activity in these studies.