The molecular gas, H$_2$, that fuels star formation in galaxies is difficult to observe directly. As such, the ratio of $L_{rm IR}$ to $L^prime_{rm CO}$ is an observational estimation of the star formation rate compared with the amount of molecular g
as available to form stars, which is related to the star formation efficiency and the inverse of the gas consumption timescale. We test what effect an IR luminous AGN has on the ratio $L_{rm IR}/L^prime_{rm CO}$ in a sample of 24 intermediate redshift galaxies from the 5 mJy Unbiased Spitzer Extragalactic Survey (5MUSES). We obtain new CO(1-0) observations with the Redshift Search Receiver on the Large Millimeter Telescope. We diagnose the presence and strength of an AGN using Spitzer IRS spectroscopy. We find that removing the AGN contribution to $L_{rm IR}^{rm tot}$ results in a mean $L_{rm IR}^{rm SF}/L^prime_{rm CO}$ for our entire sample consistent with the mean $L_{rm IR}/L^prime_{rm CO}$ derived for a large sample of star forming galaxies from $zsim0-3$. We also include in our comparison the relative amount of polycyclic aromatic hydrocarbon emission for our sample and a literature sample of local and high redshift Ultra Luminous Infrared Galaxies and find a consistent trend between $L_{6.2}/L_{rm IR}^{rm SF}$ and $L_{rm IR}^{rm SF}/L^prime_{rm CO}$, such that small dust grain emission decreases with increasing $L_{rm IR}^{rm SF}/L^prime_{rm CO}$ for both local and high redshift dusty galaxies.
We investigate the far-infrared (IR) dust emission for 20 local star forming galaxies from the Key Insights on Nearby Galaxies: A Far-IR Survey with Herschel (KINGFISH) sample. We model the far-IR/submillimeter spectral energy distribution (SED) usin
g images from Spitzer Space Telescope and Herschel Space Observatory. We calculate the cold dust temperature (T(cold)) and emissivity (beta) on a pixel by pixel basis (where each pixel ranges from 0.1-3 kpc^2) using a two temperature modified blackbody fitting routine. Our fitting method allows us to investigate the resolved nature of temperature and emissivity variations by modeling from the galaxy centers to the outskirts (physical scales of ~15-50 kpc, depending on the size of the galaxy). We fit each SED in two ways: (1) fit T(cold) and beta simultaneously, (2) hold beta constant and fit T(cold). We compare T(cold) and beta with star formation rates (calculated from L(Halpha) and L(24)), the luminosity of the old stellar population (traced through L(3.6), and the dust mass surface density (traced by 500 micron luminosity, L(500)). We find a significant trend between SFR/L(500) and T(cold), implying that the flux of hard UV photons relative to the amount of dust is significantly contributing to the heating of the cold, or diffuse, dust component. We also see a trend between L(3.6)/L(500) and beta, indicating that the old stellar population contributes to the heating at far-IR/submillimeter wavelengths. Finally, we find that when beta is held constant, T(cold) exhibits a strongly decreasing radial trend, illustrating that the shape of the far-IR SED is changing radially through a galaxy, thus confirming on a sample almost double in size the trends observed in Galametz et al. (2012).
Submillimeter excess emission has been reported at 500 microns in a handful of local galaxies, and previous studies suggest that it could be correlated with metal abundance. We investigate the presence of an excess submillimeter emission at 500 micro
ns for a sample of 20 galaxies from the Key Insights on Nearby Galaxies: a Far Infrared Survey with Herschel (KINGFISH) that span a range of morphologies and metallicities (12+log(O/H)=7.8-8.7). We probe the far-infrared (IR) emission using images from the Spitzer Space Telescope and Herschel Space Observatory in the wavelength range 24-500 microns. We model the far-IR peak of the dust emission with a two-temperature modified blackbody and measure excess of the 500 micron photometry relative to that predicted by our model. We compare the submillimeter excess, where present, with global galaxy metallicity and, where available, resolved metallicity measurements. We do not find any correlation between the 500 micron excess and metallicity. A few individual sources do show excess (10-20%) at 500 microns; conversely, for other sources, the model overpredicts the measured 500 micron flux density by as much as 20%, creating a 500 micron deficit. None of our sources has an excess larger than the calculated 1-sigma uncertainty, leading us to conclude that there is no substantial excess at submillimeter wavelengths at or shorter than 500 microns in our sample. Our results differ from previous studies detecting 500 micron excess in KINGFISH galaxies largely due to new, improved photometry used in this study.
We have compiled a large sample of 151 high redshift (z=0.5-4) galaxies selected at 24 microns (S24>100 uJy) in the GOODS-N and ECDFS fields for which we have deep Spitzer IRS spectroscopy, allowing us to decompose the mid-infrared spectrum into cont
ributions 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 color-color diagnostics combining mid-IR, far-IR, and near-IR photometry, since these combinations provide the most detail about the shape of a sources 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 S250/S24 vs. S8.0/S3.6 and S100/S24 vs. S8.0/S3.6; 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.
