Key Insights in Nearby Galaxies: Radio Continuum Survey of the KINGFISH Galaxies
Final Report Abstract
Taking advantage of my radio thermal and nonthermal separation method, dissecting various emissions as a function of spatial scales, and dust modeling method on one hand, and the highly resolved and sensitive observations with Herschel and Spitzer on the other hand, I have performed a detailed study of the ISM properties like the magnetic field strength, cosmic ray electrons, and dust in couple of Nearby galaxies including the KINGFISH galaxy NGC6946. This study presents how the star formation influence each of the mentioned ISM components. These as well as our in-depth study of the radio–FIR correlation led to important results and conclusions as follows: • The nonthermal maps obtained using our separation technique shows strong nonthermal emission close to the complexes of star forming regions. This is not observed in maps separated using the classical separation technique because a fixed steep spectral index is assumed everywhere in that method. Hence, the new method provides a direct way to investigate the nonthermal emission as a star formation tracer in galaxies. • The maps of the cosmic ray electron energy index shows that star formation processes enhance the number of high-energy cosmic rays via acceleration and/or production of new particles. For the first time, we directly detected the energy loss of cosmic ray electrons along the large-scale ordered magnetic field in a galaxy (NGC6946). • We find that the total and turbulent magnetic field strengths are correlated with ΣSFR with a powerlaw index of 0.14 and 0.16, respectively. This indicates production of turbulent magnetic fields with increasing turbulence in actively star forming regions. • In regions where the main heating source of dust is the general interstellar radiation field, the nonthermal emission correlates better with the cold dust than with the warm dust. However, there is no difference between the quality of the correlations for colder/warmer dust in regions of a strong radiation field powered by massive stars. This is expected if warmer dust is mainly heated by massive stars and the nonthermal emission is due to young CREs/turbulent magnetic field, and colder dust by a diffuse ISRF across the disk (where the nonthermal emission produced by old, diffused CREs/large-scale magnetic field). • Star formation is a powerful source of both FIR and radio emission, however, a balance between the CREs/magnetic fields and gas pressures is an unavoidable physical condition for a tight radio–FIR correlation. We found the first observational evidence of the importance of the magnetic fields for this famous correlation. • The cooling scale length of CREs determined using the scale-by-scale analysis of the radio–FIR correlation provides an independent measure of the CRE diffusion coefficient. Our determined value of D0 = 4.6 × 10^28 cm2 s^−1 for the 2.2 GeV CREs agrees with the observed values in the Milky Way. • We also studied the wavelength dependence of the dust emission as a function of position and environment across the disk of M33 using Spitzer and Herschel photometric data. We found that the dust emissivity index β is positively correlated with star formation and with the molecular gas column, as traced by the Hα and CO emission. The lower dust-emissivity index in the outer parts of M33 is most likely related to the reduced metallicity (different grain composition) and possibly to a different size distribution. My methods can be applied or adopted for other galaxies as well.
Publications
- The resolved radio-FIR correlation in nearby galaxies with Herschel and Spitzer, 2012, IAUS, 284, 400
Tabatabaei et al.
- Variation in the dust spectral index across M33, 2012, IAUS, 284, 125
Tabatabaei et al.
- A detailed study of the radio–FIR correlation in NGC6946 with Herschel-PACS/SPIRE from KING- FISH, 2013, A&A, 552, 19
Tabatabaei et al.
(See online at https://doi.org/10.1051/0004-6361/201220249) - Multi-scale radio-infrared correlations in M 31 and M 33: The role of magnetic fields and star formation, 2013, A&A, 557, 129
Tabatabaei et al.
(See online at https://doi.org/10.1051/0004-6361/201218909) - Variation in the dust emmisivity index across M33 with Herschel and Spitzer (HerM 33es), 2014, A&A, 561, 95
Tabatabaei et al.
(See online at https://doi.org/10.1051/0004-6361/201321441)
