GOODS-HERSCHEL: SEPARATING HIGH-REDSHIFT ACTIVE GALACTIC NUCLEI AND STAR-FORMING GALAXIES USING INFRARED COLOR DIAGNOSTICS

Abstract

We have compiled a large sample of 151 high redshift (z = 0.5 − 4) galaxies selected at 24 µm (S 24 > 100 µJy) in the GOODS-N and ECDFS fields for which we have deep Spitzer IRS spectroscopy, allowing us to decompose the mid-infrared spectrum into contributions from star formation and activity in the galactic nuclei. In addition, we have a wealth of photometric data from Spitzer IRAC/MIPS and Herschel PACS/SPIRE. We explore how effective different infrared color combinations are at separating our mid-IR spectroscopically determined active galactic nuclei from our star forming galaxies. We look in depth at existing IRAC color diagnostics, and we explore new colorcolor diagnostics combining mid-IR, far-IR, and near-IR photometry, since these combinations provide the most detail about the shape of a source's IR spectrum. An added benefit of using a color that combines far-IR and mid-IR photometry is that it is indicative of the power source driving the IR luminosity. For our data set, the optimal color selections are S 250 /S 24 vs. S 8 /S 3.6 and S 100 /S 24 vs. S 8 /S 3.6 ; both diagnostics have ∼10% contamination rate in the regions occupied primarily by star forming galaxies and active galactic nuclei, respectively. Based on the low contamination rate, these two new IR color-color diagnostics are ideal for estimating both the mid-IR power source of a galaxy when spectroscopy is unavailable and the dominant power source contributing to the IR luminosity. In the absence of far-IR data, we present color diagnostics using the WISE mid-IR bands which can efficiently select out high z (z ∼ 2) star forming galaxies.

Red but not dead: unveiling the star-forming far-infrared spectral energy distribution of SpARCS brightest cluster galaxies at 0 < z < 1.8

“…This selection criterion yields a contribution of 10 luminous AGN to our sub-sample of 167 24-µm-bright BCGs, with all but one lying within the 0.78 < z < 1.36 redshift range. We note, however, that these numbers reflect only luminous AGN, as the Donley et al (2012) criterion misses 39% of spectroscopically confirmed mid-infrared-bright AGN (Kirkpatrick et al 2013), i.e. low-luminosity and/or host-dominated AGN, and those with a strong 9.7-µm silicate absorption feature.…”

Dusty star-forming galaxies at high redshift

“…They determine an AGN fraction of just 9% at S 24 = 350µJy increasing up to 74±20% at S 24 ≈ 3 mJy. Also see Kirkpatrick et al (2013) for a detailed discussion of how AGN fraction changes with 24µm flux density limit.…”

The eMERGE Survey – I: Very Large Array 5.5 GHz observations of the GOODS-North Field

“…Donley et al (2012) redefined, in a more restricted way, the IRAC colours-based AGN selection criteria previously developed by Lacy et al (2004Lacy et al ( , 2007 obtaining a highly-reliable classification for deep IRAC surveys (referred to as the "Donley wedge" hereafter). Messias et al (2012) developed their own IR colour-colour diagram, using Ks and IRAC bands (the KI diagram) and, finally, Kirkpatrick et al (2012Kirkpatrick et al ( , 2013 presented two different combinations of colours that combine MIR and far-IR (FIR) photometry (Spitzer IRAC/MIPS and Herschel PACS/SPIRE) to classify high redshift (z = 0.5 − 4) galaxies selected at 24 µm with Spitzer IRS spectroscopy. These criteria will be briefly presented in the following.…”

“…The line separate RE-AGN (above the line) from star-forming or passive galaxies (below the line), according to Kirkpatrick et al (2012Kirkpatrick et al ( , 2013 Bottom-Right: IR colour-colour plot showing log(S 8.0 /S 3.6 ) versus log(S 100 /S 24 ). The line separates RE-AGN (above the line) from star-forming or passive galaxies (below the line), according to Kirkpatrick et al (2012Kirkpatrick et al ( , 2013. In the two lower plots, empty symbols refer to 100 µm and 250 µm upper limits.…”

“…Before reviewing the results from the diagnostics using FIR photometry (Kirkpatrick et al 2012(Kirkpatrick et al , 2013, we note that this is possible for a smaller number of sources (66 compared to 77 objects). Of the 11 sources without a FIR counterpart, 9 are associated with RI-AGN hosted by red passive galaxies which are typically faint in the MIR, and therefore mostly undetected at 24 µm.…”