Connecting Far‐Infrared and Radio Morphologies of Disk Galaxies: Cosmic‐Ray Electron Diffusion After Star Formation Episodes

Abstract

We present results on the ISM properties of 29 galaxies based on a comparison of Spitzer far-infrared and Westerbork Synthesis Radio Telescope radio continuum imagery. Of these 29 galaxies, 18 are close enough to resolve at P1 kpc scales at 70 m and 22 cm. We extend the approach of our earlier work of smoothing infrared images to approximate cosmic-ray (CR) electron spreading and thus largely reproduce the appearance of radio images. Using a wavelet analysis, we decompose each 70 m image into one component containing the star-forming structures and a second one for the diffuse disk. The components are smoothed separately, and their combination compared to a free-free corrected 22 cm radio image; the scale lengths are then varied to best match the radio and smoothed infrared images. We find that late-type spirals having high amounts of ongoing star formation benefit most from the two-component method. We also find that the disk component dominates for galaxies having low star formation activity, whereas the structure component dominates at high star formation activity. We propose that this result arises from an age effect rather than from differences in CR electron diffusion due to varying ISM parameters. The bulk of the CR electron population in actively star-forming galaxies is significantly younger than that in less active galaxies due to recent episodes of enhanced star formation; these galaxies are observed within $10 8 yr since the onset of the most recent star formation episode. The sample irregulars have anomalously low best-fit scale lengths for their surface brightnesses compared to the rest of the sample spirals, which we attribute to enhanced CR electron escape.

Multi-scale radio-infrared correlations in M 31 and M 33: The role of magnetic fields and star formation

“…By separately smoothing these components with exponential kernels, until their sum matched the corrected radio image, they determined a structure scale length and a disk scale length, which they interpreted as diffusion lengths of two populations of CREs. There are some major differences between the approach of Murphy et al (2008) and our work, making a comparison of results difficult.…”

“…It seems unlikely that the "diffuse source component" exists. Murphy et al (2008) also derived global scale lengths, l glob , after smoothing of the unseparated, observed 70 μm image, and it is interesting to compare their values for M 51 and NGC 6946 with our l dif . For M 51, they find l glob = 500 ± 100 pc which is consistent with our lower limit of l dif > 500 pc.…”

“…Hence, although the presence of massive star formation influences the field structure, it is the ratio B ord /B tur that is crucial for the propagation of the CREs. Murphy et al (2008) determined propagation lengths of CREs in 18 well-resolved galaxies (<1 kpc resolution) by smoothing 70 μm Spitzer images until they matched the 22 cm Westerbork maps, corrected for thermal emission. Using a wavelet technique, they separated the 70 μm images in a "structure component" of scales <1 kpc containing the star formation regions and a diffuse "disk component" of scales >1 kpc, heated by older stars.…”

“…Our main concern about the work of Murphy et al (2008) is the reality of the disk component as a CRE source distribution. We believe that their scale l disk is the difference between the exponential radial scale lengths of the diffuse dust emission powered by old stars and the extended synchrotron emission (see their example for NGC 6946).…”

Modelling the cosmic ray electron propagation in M 51

“…Some studies, when investigating the FIR-Radio correlation (Murphy et al 2008;Tabatabaei et al 2013) routinely use this energy value. Others claim that the energy dependence in diffusivity does not become relevant until 4 GeV (van der Walt 1997).…”