The Active Galactic Nuclei Contribution to the Mid‐Infrared Emission of Luminous Infrared Galaxies

Abstract

We determine the contribution of AGN to the mid-IR emission of luminous infrared galaxies (LIRGs) at z > 0.6 by measuring the mid-IR dust continuum slope of 20,039 mid-IR sources. The 24 µm sources are selected from a Spitzer/MIPS survey of the NOAO Deep Wide-Field Survey Boötes field and have corresponding 8 µm data from the IRAC Shallow Survey. There is a clear bimodal distribution in the 24 µm to 8 µm flux ratio. The X-ray detected sources fall within the peak corresponding to a flat spectrum in νf ν , implying that it is populated by AGN-dominated LIRGs, whereas the peak corresponding to a higher 24 µm to 8 µm flux ratio is likely due to LIRGs whose infrared emission is powered by starbursts. The 24 µm emission is increasingly dominated by AGN at higher 24 µm flux densities (f 24 ): the AGN fraction of the z > 0.6 sources increases from 9% at f 24 ≈0.35 mJy to 74±20% at f 24 ≈ 3 mJy in good agreement with model predictions. Deep 24 µm, small area surveys, like GOODS, will be strongly dominated by starburst galaxies. AGN are responsible for ∼3-7% of the total 24 µm background.

AGN counts at 15$\mu{\rm m}$

“…These results recently have been confirmed and extended by the analysis of joint Spitzer and X-ray data, where a contribution of 3-11% is derived (see e.g. Treister et al 2006;Barmby et al 2006;Brand et al 2006).…”

Population synthesis modelling of luminous infrared galaxies at intermediate redshift

“…We overplot the histogram of the objects previously identified as AGNs by Stern et al (2005). We find that they are not necessarily identified by Brand et al (2006). CIGALE allows estimating the fraction of L dust due to an AGN.…”

“…Synthetic models connecting the X-ray and IR SED of AGN indicate that the emission arising from pure AGNs should be negligible (i.e., < ∼ 10%) in highredshift sources detected at 24 μm (Silva et al 2004). To have more precision about the possible fraction of AGN in our sample, we use a diagnostics colour-colour given by Stern et al (2005) and the diagnostics of the mid-IR (MIR) slope of Brand et al (2006) (see Sect. 3.3); we also use a specific output of the code CIGALE (see Sect.…”

“…We also use the diagnostics of Brand et al (2006) who examine the MIR slope ζ of the SEDs for their sample. Here, ζ corresponds to the observed flux ratio log 10 (ν f ν (24 μm)/ν f ν (8 μm)).…”

“…The MIR slope is supposed to be steeper for starburst-dominated sources. Brand et al (2006) assume that sources (z > 0.6) with ζ peaking at ≈0.0 and ≈0.5 are AGN-and starburst-dominated, respectively. Figure 3 shows the flux ratio ζ for the total sample.…”