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Register a new userPhenomenological Modeling of the FIR-Radio Correlation within Nearby Galaxies
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E. J. Murphy
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We present an analysis of the far-infrared (FIR)-radio correlation within a group of nearby star-forming galaxy disks observed as part of the {it Spitzer} Infrared Nearby Galaxies Survey (SINGS). In our study we critically test a phenomenological model for the FIR-radio correlation which describes the radio image as a smeared version of the infrared image. The physical basis for this model is that cosmic-ray electrons (CR electrons) will diffuse significant distances from their originating sources before decaying by synchrotron emission. We find that this description generally works well, improving the correlation between the radio and infrared images of our four sample galaxies by an average factor of $sim$1.6. We also find that the best-fit smearing kernels seem to show a dependence on the ongoing star formation activity within each disk. Galaxies having lower star formation activity (NGC 2403 and NGC 3031) are best-fit using larger smearing kernels than galaxies with more active star-forming disks (NGC 5194 and NGC 6946). We attribute this trend to be due to a recent deficit of CR electron injection into the interstellar medium of galaxies with lower star formation activity throughout their disks.
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We find that a preliminary classification of LINERs energetics may be made in terms of the FIR-radio correlation of Wolf-Rayet galaxies. The AGN- or starburst-supported LINERs can be distinguished by their FIR-to-radio ratio, $Qequiv L(1.4{rm GHz})/$ $L(60mu{rm m})>$ or $<0.01$. It is interesting to note that almost all the LINERs with inner rings might be starburst- supported, indicating reduced AGN activities compared with those of the AGN-supported ones. We also find that a shock-heating phase for the warm dust component might be important for some starbursts at the burst age of $ge 10^{7}$ yr, with $Q<0.001$.
(Abridged) We present an initial look at the far infrared-radio correlation within the star-forming disks of four nearby, nearly face-on galaxies (NGC 2403, NGC 3031, NGC 5194, and NGC 6946). Using Spitzer MIPS imaging and WSRT radio continuum data, observed as part of the Spitzer Infrared Nearby Galaxies Survey (SINGS), we are able to probe variations in the logarithmic 24mu/22cm (q_24) and 70mu/22cm (q_70) surface brightness ratios across each disk at sub-kpc scales. We find general trends of decreasing q_24 and q_70 with declining surface brightness and with increasing radius. The residual dispersion around the trend of q_24 and q_70 versus surface brightness is smaller than the residual dispersion around the trend of q_24 and q_70 versus radius, on average by ~0.1 dex, indicating that the distribution of star formation sites is more important in determining the infrared/radio disk appearance than the exponential profiles of disks. We have also performed preliminary phenomenological modeling of cosmic ray electron (CRe^-) diffusion using an image-smearing technique, and find that smoothing the infrared maps improves their correlation with the radio maps. Exponential kernels tend to work better than Gaussian kernels which suggests that additional processes besides simple random-walk diffusion in three dimensions must affect the evolution of CRe^-s. The best fit smoothing kernels for the two less active star-forming galaxies (NGC 2403 and NGC 3031) have much larger scale-lengths than those of the more active star-forming galaxies (NGC 5194 and NGC 6946). This difference may be due to the relative deficit of recent CRe^- injection into the interstellar medium (ISM) for the galaxies having largely quiescent disks.
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