Scaling and correlation analysis of galactic images

Frick, Peter; Beck, Rainer; Berkhuijsen, E. M.; Patrickeyev, Igor

doi:10.1046/j.1365-8711.2001.04812.x

Cited by 84 publications

(148 citation statements)

References 18 publications

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“…The wavelet cross-correlation is a useful method to compare different images as a function of spatial scales (Frick et al 2001;Dumas et al 2011;and Tabatabaei et al 2007a). Whilst normal cross-correlation analysis such as pixel to pixel correlation can be dominated by bright extended regions or large scale structure, the wavelet cross-correlation allows the analysis of a scale-dependent correlation between two images.…”

Section: Wavelet Cross-correlation and Cre Propagation Lengthmentioning

confidence: 99%

The nature of the low-frequency emission of M 51

Mulcahy

Horneffer²,

Beck³

et al. 2014

A&A

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113

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Context. Low-frequency radio continuum observations (<300 MHz) can provide valuable information on the propagation of low-energy cosmic ray electrons (CRE). Nearby spiral galaxies have hardly been studied in this frequency range because of the technical challenges of low-frequency radio interferometry. This is now changing with the start of operations of LOFAR. Aims. We aim to study the propagation of low-energy CRE in the interarm regions and the extended disk of the nearly face-on spiral galaxy Messier 51. We also search for polarisation in M 51 and other extragalactic sources in the field. Methods. The grand-design spiral galaxy M 51 was observed with the LOFAR High Frequency Antennas (HBA) and imaged in total intensity and polarisation. This observation covered the frequencies between 115 MHz and 175 MHz with 244 subbands of 8 channels each, resulting in 1952 channels. This allowed us to use RM synthesis to search for polarisation. Results. We produced an image of total emission of M 51 at the mean frequency of 151 MHz with 20 resolution and 0.3 mJy rms noise, which is the most sensitive image of a galaxy at frequencies below 300 MHz so far. The integrated spectrum of total radio emission is described well by a power law, while flat spectral indices in the central region indicate thermal absorption. We observe that the disk extends out to 16 kpc and see a break in the radial profile near the optical radius of the disk. The radial scale lengths in the inner and outer disks are greater at 151 MHz, and the break is smoother at 151 MHz than those observed at 1.4 GHz. The arm-interarm contrast is lower at 151 MHz than at 1400 MHz, indicating propagation of CRE from spiral arms into interarm regions. The correlations between the images of radio emission at 151 MHz and 1400 MHz and the FIR emission at 70 μm reveal breaks on scales of 1.4 and 0.7 kpc, respectively. The total (equipartition) magnetic field strength decreases from about 28 μG in the central region to about 10 μG at 10 kpc radius. No significant polarisation was detected from M 51, owing to severe Faraday depolarisation. Six extragalactic sources are detected in polarisation in the M 51 field of 4.1 • × 4.1 • size. Two sources show complex structures in Faraday space. Conclusions. Our main results, the scale lengths of the inner and outer disks at 151 MHz and 1.4 GHz, arm-interarm contrast, and the break scales of the radio-FIR correlations, can be explained consistently by CRE diffusion, leading to a longer propagation length of CRE of lower energy. The distribution of CRE sources drops sharply at about 10 kpc radius, where the star formation rate also decreases sharply. We find evidence that thermal absorption is primarily caused by H ii regions. The non-detection of polarisation from M 51 at 151 MHz is consistent with the estimates of Faraday depolarisation. Future searches for polarised emission in this frequency range should concentrate on regions with low star formation rates.Key words. polarization -cosmic rays -galaxies: ISM -galaxies: ...

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Section: Wavelet Cross-correlation and Cre Propagation Lengthmentioning

confidence: 99%

The nature of the low-frequency emission of M 51

Mulcahy

Horneffer²,

Beck³

et al. 2014

A&A

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113

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“…We use κ = 2, which provides the same power law for the wavelet spectrum as for the conventional second-order structure function (Frick et al 2001). This selection also ensures a consistent comparison with previous studies (e.g.…”

Section: Algorithmsmentioning

confidence: 99%

“…Hughes et al 2006;Tabatabaei et al 2007a;Dumas et al 2011). By decomposing the images into maps containing the structures of a given scale we can analyze the cross-correlation of the analyzed maps scale-by-scale (Nesme-Ribes et al 1995;Frick et al 2001). The wavelet cross-correlation coefficient at scale a is defined for 2D maps as…”

Section: Algorithmsmentioning

confidence: 99%

“…To separate the contribution from different spatial scales in the global correlation of two maps Frick et al (2001) introduced the "wavelet cross-correlation". The wavelet transform, similar to the Fourier transform, decomposes the images scale-by-scale, but, in contrast to the Fourier decomposition, wavelets keep the information about the distribution of structures of a given scale in the physical space and therefore allow one to analyse the correlation of two fields scale-by-scale.…”

Section: Introductionmentioning

confidence: 99%

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

Tabatabaei

Berkhuijsen

Frick

et al. 2013

A&A

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Interstellar magnetic fields and the propagation of cosmic ray electrons have an important impact on the radio-infrared (IR) correlation in galaxies. This becomes evident when studying different spatial scales within galaxies. We investigate the correlation between the IR and free-free/synchrotron radio continuum emission at 20 cm from the two local group galaxies M 31 and M 33 on spatial scales between 0.4 and 10 kpc. The multi-scale radio-IR correlations have been carried out using a wavelet analysis. The free-free and IR emission are correlated on all scales, but on some scales the synchrotron emission is only marginally correlated with the IR emission. The synchrotron-IR correlation is stronger in M 33 than in M 31 on small scales (<1 kpc), but it is weaker than in M 31 on larger scales. Taking the smallest scale on which the synchrotron-IR correlation exists as the propagation length of cosmic ray electrons, we show that the difference on small scales can be explained by the smaller propagation length in M 33 than in M 31. On large scales, the difference is due to the thick disk/halo in M 33, which is absent in M 31. A comparison of our data with data on NGC 6946, the LMC and M 51 suggests that the propagation length is determined by the ratio of ordered-to-turbulent magnetic field strength, which is consistent with diffusion of CR electrons in the ISM. As the diffusion length of CR electrons influences the radio-IR correlation, this dependence is a direct observational evidence of the importance of magnetic fields for the radio-IR correlation within galaxies. The star-formation rate per surface area only indirectly influences the diffusion length as it increases the strength of the turbulent magnetic field.

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“…This property of the wavelet transform has led to the development of the wavelet-based Multiscale Vision Model (MVM; Bijaoui & Rué 1995;Rué & Bijaoui 1997), a procedure useful for identifying morphological features in astronomical images (e.g., Starck et al 2000;Adami et al 2005). Wavelet analyses have also been used to study the FIR-radio correlation within individual systems such as NGC 6946 (Frick et al 2001), the LMC ( Hughes et al 2006), and M33 (Tabatabaei et al 2007); in each of these studies a cross-correlation analysis was performed on the wavelet power spectra for images acquired at various wavelengths.…”

Section: Wavelet-based Image Decompositionmentioning

confidence: 99%

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

Murphy

Hélou

Kenney

et al. 2008

ApJ

113

182

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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.

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Scaling and correlation analysis of galactic images

Cited by 84 publications

References 18 publications

The nature of the low-frequency emission of M 51

The nature of the low-frequency emission of M 51

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

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

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