GOODS-HERSCHEL: IMPACT OF ACTIVE GALACTIC NUCLEI AND STAR FORMATION ACTIVITY ON INFRARED SPECTRAL ENERGY DISTRIBUTIONS AT HIGH REDSHIFT

Abstract

We explore the effects of active galactic nuclei (AGN) and star formation activity on the infrared (0.3 -1000 µm) spectral energy distributions of luminous infrared galaxies from z = 0.5 to 4.0. We have compiled a large sample of 151 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-IR spectrum into contributions from star formation and AGN activity. A significant portion (∼ 25%) of our sample is dominated by an AGN (> 50% of mid-IR luminosity) in the mid-IR. Based on the mid-IR classification, we divide our full sample into four sub-samples: z ∼ 1 starforming (SF) sources; z ∼ 2 SF sources; AGN with clear 9.7 µm silicate absorption; and AGN with featureless mid-IR spectra. From our large spectroscopic sample and wealth of multi-wavelength data, including deep Herschel imaging at 100, 160, 250, 350, and 500 µm, we use 95 galaxies with complete spectral coverage to create a composite spectral energy distribution (SED) for each sub-sample. We then fit a two-temperature component modified blackbody to the SEDs. We find that the IR SEDs have similar cold dust temperatures, regardless of the mid-IR power source, but display a marked difference in the warmer dust temperatures. We calculate the average effective temperature of the dust in each sub-sample and find a significant (∼ 20 K) difference between the SF and AGN systems. We compare our composite SEDs to local templates and find that local templates do not accurately reproduce the mid-IR features and dust temperatures of our high redshift systems. High redshift IR luminous galaxies contain significantly more cool dust than their local counterparts. We find that a full suite of photometry spanning the IR peak is necessary to accurately account for the dominant dust temperature components in high redshift IR luminous galaxies.

Dusty star-forming galaxies at high redshift

“…Dunne & Eales, 2001;Farrah et al, 2003;Galametz et al, 2012). (Kirkpatrick et al, 2012) have argued that for a sample of high-z galaxies, a two component model may fare much better than a single component model. This said, this technique comes at the expense of introducing extra unconstrained parameters to fit (both dust temperatures, both normalizations, both emissivities).…”

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

“…The four IR criteria are those presented by Donley et al (2012), Messias et al (2012), Kirkpatrick et al (2012Kirkpatrick et al ( , 2013. All of these provide low contamination diagnostics (∼ 10 percent) in separating SFGs and AGN up to high redshift (z ∼ 4), by taking into account the redshift evolution of IR colours.…”

“…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.…”