Mid-Infrared Spectral Indicators of Star Formation and Active Galactic Nucleus Activity in Normal Galaxies

Abstract

We investigate the use of mid-infrared (MIR) polycyclic aromatic hydrocarbon (PAH) bands, the continuum, and emission lines as probes of star formation (SF) and active galactic nucleus (AGN) activity in a sample of 100 "normal" and local (z ∼ 0.1) emission-line galaxies. The MIR spectra were obtained with the Spitzer Space Telescope Infrared Spectrograph as part of the Spitzer-SDSS-GALEX Spectroscopic Survey, which includes multiwavelength photometry from the ultraviolet to the far-infrared and optical spectroscopy. The continuum and features were extracted using PAHFIT, a decomposition code which we find to yield PAH equivalent widths (EWs) up to ∼30 times larger than the commonly used spline methods. Despite the lack of extreme objects in our sample (such as strong AGNs, low-metallicity galaxies, or ULIRGs), we find significant variations in PAH, continuum, and emission-line properties, and systematic trends between these MIR properties and optically derived physical properties, such as age, metallicity, and radiation field hardness. We revisit the diagnostic diagram relating PAH EWs and [Ne ii]12.8 μm/[O iv]25.9 μm line ratios and find it to be in much better agreement with the standard optical SF/AGN classification than when spline decompositions are used, while also potentially revealing obscured AGNs. The luminosity of individual PAH components, of the continuum, and, with poorer statistics, of the neon emission lines and molecular hydrogen lines are found to be tightly correlated to the total infrared (TIR) luminosity, making individual MIR components good gauges of the total dust emission in SF galaxies. Like the TIR luminosity, these individual components can be used to estimate dust attenuation in the UV and in Hα lines based on energy balance arguments. We also propose average scaling relations between these components and dust-corrected, Hα-derived SF rates.

Encoding of the infrared excess in theNUVrKcolor diagram for star-forming galaxies

“…2 http://www.cfht.hawaii.edu/~arnouts/lephare.html nosity dependence similar to the SEDs of the libraries proposed by Chary & Elbaz (2001) and Lagache et al (2004). In this scheme, the monochromatic luminosity at any given IR wavelength is linked to L IR by a monotonic relation, similar to the correlations that have been observed between the mid-IR emission and the total IR luminosity of galaxies in the local Universe (e.g., Spinoglio et al 1995;Chary & Elbaz 2001;Takeuchi et al 2003;Treyer et al 2010;Goto et al 2011). In this way, a 24 µm flux density observed at a given redshift is associated to a unique L IR .…”

Exploring the Connection Between Star Formation and Active Galactic Nucleus Activity in the Local Universe

“…The Spitzer-SDSS-GALEX Spectroscopic Survey (SSGSS; PI: D. Schiminovich) contains 101 local (0.03 < z < 0.21) galaxies overlapping the Lockman Hole region (O'Dowd et al 2009;Treyer et al 2010;O'Dowd et al 2011). This sample is augmented by the Spitzer SDSS Spectroscopic Survey (S5; PI: D. Schiminovich), increasing the sample size of local (in this case, 0.05 < z < 0.1) star-forming galaxies to 300 (B. Bertincourt et al, in preparation).…”

“…SFRs have consequently been calibrated on PAH luminosities (e.g., Farrah et al 2007). However, the PAH luminosity may be affected by AGN activity, and the effect may be different for the different PAH features (e.g., O'Dowd et al 2009;Treyer et al 2010;. In this analysis, we consider the sum of the PAH luminosities at 7.7 μm, 11.3 μm, and 17 μm (L ΣPAH 7.7,11.3,17 μm ).…”

“…Disentangling the contribution from the AGN versus star formation then becomes a necessary intermediate step in using continuum IR emission as a tracer of star formation in active galaxies, and such a process can be plagued with uncertainties. Previous studies have indicated that emission from PAHs tends to be suppressed in galaxies hosting an active nucleus, which is sometimes attributed to the harsh radiation field produced by the AGN accretion disk destroying PAHs (e.g., Voit 1992;O'Dowd et al 2009;Treyer et al 2010;LaMassa et al 2010;.…”

“…An anti-correlation between ionization field hardness and the ratio of PAH features at 7.7 μm and 11.3 μm has been reported and is interpreted as due to an increasing contribution to the mid-IR continuum emission by AGN-heated dust. The selective destruction of smaller PAH grains may also play a role (e.g., Smith et al 2007b;O'Dowd et al 2009;Treyer et al 2010;Wu et al 2010), and so the flux ratios of the different PAH features can then also potentially trace the relative contribution of AGNs and star formation in active galaxies.…”