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

Tabatabaei, F. S.; Berkhuijsen, E. M.; Frick, Peter; Beck, Rainer; Schinnerer, E.

doi:10.1051/0004-6361/201218909

Cited by 81 publications

(119 citation statements)

References 60 publications

(149 reference statements)

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“…(8) and assumed l dif = 1 kpc at 5 GHz which seems too high compared to our results. Tabatabaei et al (2013) estimated l dif > 0.5 kpc in M 51 at 1.4 GHz where τ syn 1.5 × 10 7 years, which yields D > 1.3 × 10 27 cm 2 s −1 , consistent with our result. On the other hand, the values of D ∼ (2−4) × 10 28 cm 2 s −1 used in the models by Moskalenko & Strong (1998) for the Milky Way are higher.…”

Section: Cre Diffusion Coefficients In M 51supporting

confidence: 91%

“…Our value for l dif in M 51 between 1.4 GHz and 70 μm is smaller than that for M 31 (l dif = 3 ± 1 kpc for our definition of the break scale) and also for NGC 6946 (l dif = 5 ± 1 kpc) (Tabatabaei et al 2013). This further supports our result from Sect.…”

Section: Wavelet Cross-correlation and Cre Propagation Lengthcontrasting

confidence: 68%

“…M 51 and M 31 have a similar degrees of magnetic field order, so that the relation between l dif and the degree of field order proposed by Tabatabaei et al (2013) predicts similar values of l dif . However, we find a value only half as large in M 51.…”

Section: Wavelet Cross-correlation and Cre Propagation Lengthmentioning

confidence: 75%

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The nature of the low-frequency emission of M 51

Mulcahy

Horneffer²,

Beck³

et al. 2014

A&A

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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: Cre Diffusion Coefficients In M 51supporting

confidence: 91%

Section: Wavelet Cross-correlation and Cre Propagation Lengthcontrasting

confidence: 68%

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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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“…The flux density of T H and f th may be overestimated by 20-25% because we used a constant value of α n , which is too large for star-forming regions (Tabatabaei et al 2007a(Tabatabaei et al , 2013a. In this case, NT H (p n ) is underestimated (overestimated) by nearly 20%.…”

Section: Thermal and Non-thermal Emissionmentioning

confidence: 97%

Radio polarization and magnetic field structure in M 101

Berkhuijsen

Urbanik

Beck

et al. 2016

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We observed total and polarized radio continuum emission from the spiral galaxy M 101 at λλ 6.2 cm and 11.1 cm with the Effelsberg telescope. The angular resolutions are 2. 5 (=5.4 kpc) and 4. 4 (=9.5 kpc), respectively. We use these data to study various emission components in M 101 and properties of the magnetic field. Separation of thermal and non-thermal emission shows that the thermal emission is closely correlated with the spiral arms, while the non-thermal emission is more smoothly distributed indicating diffusion of cosmic ray electrons away from their places of origin. The radial distribution of both emissions has a break near R = 16 kpc (=7. 4), where it steepens to an exponential scale length of L 5 kpc, which is about 2.5 times smaller than at R < 16 kpc. The distribution of the polarized emission has a broad maximum near R = 12 kpc and beyond R = 16 kpc also decreases with L 5 kpc. It seems that near R = 16 kpc a major change in the structure of M 101 takes place, which also affects the distributions of the strength of the random and ordered magnetic field. Beyond R = 16 kpc the radial scale length of both fields is about 20 kpc, which implies that they decrease to about 0.3 μG at R = 70 kpc, which is the largest optical extent. The equipartition strength of the total field ranges from nearly 10 μG at R < 2 kpc to 4 μG at R = 22−24 kpc. As the random field dominates in M 101 (B ran /B ord 2.4), wavelength-independent polarization is the main polarization mechanism. We show that energetic events causing H i shells of mean diameter <625 pc could partly be responsible for this. At radii <24 kpc, the random magnetic field depends on the star formation rate/area, Σ SFR , with a powerlaw exponent of b = 0.28 ± 0.02. The ordered magnetic field is generally aligned with the spiral arms with pitch angles that are about 8 • larger than those of H i filaments.

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“…It is particularly remarkable that the correlation holds even for small galaxies like the Magellanic Clouds (LMC, SMC; Xu et al 1992;Hughes et al 2006;Leverenz & Filipović 2013). A more recent study shows the role of the magnetic fields in controlling the radio-IR correlation in nearby galaxies (Tabatabaei et al 2013a).…”

Section: Introductionmentioning

confidence: 99%

A new interpretation of the far-infrared – radio correlation and the expected breakdown at high redshift

Schleicher

Beck

2013

A&A

115

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Context. Observations of galaxies up to z ∼ 2 show a tight correlation between far-infrared and radio continuum emission, suggesting a relation between star formation activity and magnetic fields in the presence of cosmic rays. Aims. We explain the far-infrared -radio continuum correlation by relating star formation and magnetic field strength in terms of turbulent magnetic field amplification, where turbulence is injected by supernova explosions from massive stars. We assess the potential evolution of this relation at high redshift, and explore the impact on the far-infrared -radio correlation. Methods. We calculate the expected amount of turbulence in galaxies based on their star formation rates, and infer the expected magnetic field strength due to turbulent dynamo amplification. We calculate the timescales for cosmic ray energy losses via synchrotron emission, inverse Compton scattering, ionization and bremsstrahlung emission, probing up to which redshift strong synchrotron emission can be maintained. Results. We find that the correlation between star formation rate and magnetic field strength in the local Universe can be understood as a result of turbulent magnetic field amplification. The ratio of radio to far-infrared surface brightness is expected to increase with total field strength. A continuation of the correlation is expected towards high redshifts. If the typical gas density in the interstellar medium increases at high z, we expect an increase of the magnetic field strength and the radio emission, as indicated by current observations. Such an increase would imply a modification, but not a breakdown of the far-infrared -radio correlation. We expect a breakdown at the redshift when inverse Compton losses start dominating over synchrotron emission. For a given star formation surface density, we calculate the redshift where the far-infrared -radio correlation will break down, yielding z ∼ (Σ SFR /0.0045 M kpc −2 yr −1 ) 1/(6−α/2) . In this relation, the parameter α describes the evolution of the characteristic ISM density in galaxies as (1 + z) α . We note that observed frequencies of 1−10 GHz are particularly well-suited to explore this relation, as bremsstrahlung losses could potentially dominate at low frequencies. Conclusions. Both the possible raise of the radio emission at high redshift and the final breakdown of the far-infrared -radio correlation at a critical redshift will be probed by the Square Kilometre Array (SKA) and its pathfinders, while the typical ISM density in galaxies will be probed with ALMA. The combined measurements will thus allow a verification of the model proposed here.

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

Cited by 81 publications

References 60 publications

The nature of the low-frequency emission of M 51

The nature of the low-frequency emission of M 51

Radio polarization and magnetic field structure in M 101

A new interpretation of the far-infrared – radio correlation and the expected breakdown at high redshift

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