An extinction-free AGN selection by 18-band SED fitting in mid-infrared in the AKARI NEP deep field

Abstract

We have developed an efficient active galactic nucleus (AGN) selection method using 18-band spectral energy distribution (SED) fitting in mid-infrared (mid-IR). AGNs are often obscured by gas and dust, and those obscured AGNs tend to be missed in optical, UV and soft X-ray observations. Mid-IR light can help us to recover them in an obscuration-free way using their thermal emission. On the other hand, star-forming galaxies (SFGs) also have strong polycyclic aromatic hydrocarbon emission features in mid-IR. Hence, establishing an accurate method to separate populations of AGNs and SFGs is important. However, in previous mid-IR surveys, only three or four filters were available, and thus the selection was limited. We combined AKARI's continuous nine mid-IR bands with Wide field Infrared Survey Explorer (WISE) and Spitzer data to create 18 mid-IR bands for AGN selection. Among 4682 galaxies in the AKARI north ecliptic pole deep field, 1388 are selected to be AGN hosts, which implies an AGN fraction of 29.6 ± 0.8 per cent (among them 47 per cent are Seyfert 1.8 and 2). Comparing the result from SED fitting into WISE and Spitzer colour-colour diagram reveals that Seyferts are often missed by previous studies. Our result has been tested by stacking median magnitude for each sample. Using X-ray data from Chandra, we compared the result of our SED fitting with WISE's colour box selection.We recovered more X-ray detected AGNs than previous methods by 20 per cent.