GMRT 333-MHz observations of six nearby normal galaxies

Basu, Aritra; Mitra, Dipanjan; Wadadekar, Yogesh; Ishwara‐Chandra, C. H.

doi:10.1111/j.1365-2966.2011.19764.x

Cited by 34 publications

(81 citation statements)

References 63 publications

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“…The arm and interarm contrast is highly visible in M 51 at these low frequencies and comparable at first glance to higher frequencies (Fletcher et al 2011). This is in stark contrast to the galaxies observed at 333 MHz with the GMRT where the spiral arms and inter arm regions were often indiscernible from each other (Basu et al 2012a). According to Fletcher et al (2011), this is primarily caused by the low-energy CREs diffusing from star forming regions in the spiral arms without losing much energy compared to higher energy electrons seen at higher frequencies.…”

Section: Total Power Imagecontrasting

confidence: 39%

“…While we see considerable CRE diffusion into the interarm region of M 51, the question arises why we do not observe CREs diffusing the same distances as in the 333 MHz images of other galaxies by Basu et al (2012a). One reason could be a stronger and more turbulent magnetic field in the spiral arms of M 51 compared to galaxies like NGC 6946.…”

Section: Diffusion Of Cres Into the Interarm Regionsmentioning

confidence: 72%

“…Especially in the northern region, the interarm region is becoming squeezed as the CRE diffuse into the interarm region. However, it is not as severe compared to the galaxies observed by Basu et al (2012a).…”

Section: Total Power Imagementioning

confidence: 84%

“…According to observations of several spiral galaxies such as NGC 6946 with the GMRT at 333 MHz, the arm and interarm regions are indiscernible (Basu et al 2012a), while in the case of M 51 at 151 MHz the arm and interarm regions are clearly separated. This is unexpected because at 151 MHz emission emerges from an even older population of CREs that should diffuse further away from star forming regions in the spiral arms.…”

Section: Diffusion Of Cres Into the Interarm Regionsmentioning

confidence: 99%

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The nature of the low-frequency emission of M 51

Mulcahy

Horneffer²,

Beck³

et al. 2014

A&A

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: Total Power Imagecontrasting

confidence: 39%

Section: Diffusion Of Cres Into the Interarm Regionsmentioning

confidence: 72%

Section: Total Power Imagementioning

confidence: 84%

Section: Diffusion Of Cres Into the Interarm Regionsmentioning

confidence: 99%

See 2 more Smart Citations

The nature of the low-frequency emission of M 51

Mulcahy

Horneffer²,

Beck³

et al. 2014

A&A

113

View full text Add to dashboard Cite

show abstract

“…However, deeper observations would be needed to confirm this. The arm and interarm regions are easily seen at 325 MHz, which is in stark contrast to NGC 6946 at the same frequency (Basu et al 2012). M 51, which has stronger magnetic fields than most galaxies owing to its interaction with NGC 5195 causes the CREs to lose their energies on a shorter timescale than in NGC 6946.…”

Section: Gmrt 325 Mhz Observation and Data Reductionmentioning

confidence: 98%

Modelling the cosmic ray electron propagation in M 51

Mulcahy

Fletcher

Beck

et al. 2016

A&A

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Context. Cosmic ray electrons (CREs) are a crucial part of the interstellar medium and are observed via synchrotron emission. While much modelling has been carried out on the CRE distribution and propagation of the Milky Way, little has been done on normal external star-forming galaxies. Recent spectral data from a new generation of radio telescopes enable us to find more robust estimations of the CRE propagation. Aims. To model the synchrotron spectral index of M 51 using the diffusion energy-loss equation and to compare the model results with the observed spectral index determined from recent low-frequency observations with LOFAR. Methods. We solve the time-dependent diffusion energy-loss equation for CREs in M 51. This is the first time that this model for CRE propagation has been solved for a realistic distribution of CRE sources, which we derive from the observed star formation rate, in an external galaxy. The radial variation of the synchrotron spectral index and scale-length produced by the model are compared to recent LOFAR and older VLA observational data and also to new observations of M 51 at 325 MHz obtained with the GMRT. Results. We find that propagation of CREs by diffusion alone is sufficient to reproduce the observed spectral index distribution in M 51. An isotropic diffusion coefficient with a value of 6.6 ± 0.2 × 10 28 cm 2 s −1 is found to fit best and is similar to what is seen in the Milky Way. We estimate an escape time of 11 Myr from the central galaxy to 88 Myr in the extended disk. It is found that an energy dependence of the diffusion coefficient is not important for CRE energies in the range 0.01 GeV-3 GeV. We are able to reproduce the dependence of the observed synchrotron scale-lengths on frequency, with l ∝ ν −1/4 in the outer disk and l ∝ ν −1/8 in the inner disk.

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From 10K to 10 TK: Insights on the Interaction Between Cosmic Rays and Gas in Starbursts

Lacki

2013

Cosmic Rays in Star-Forming Environments

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Recent work has both illuminated and mystified our attempts to understand cosmic rays (CRs) in starburst galaxies. I discuss my new research exploring how CRs interact with the ISM in starbursts. Molecular clouds provide targets for CR protons to produce pionic gamma rays and ionization, but those same losses may shield the cloud interiors. In the densest molecular clouds, gamma rays and 26 Al decay can provide ionization, at rates up to those in Milky Way molecular clouds. I then consider the free-free absorption of low frequency radio emission from starbursts, which I argue arises from many small, discrete H II regions rather than from a "uniform slab" of ionized gas, whereas synchrotron emission arises outside them. Finally, noting that the hot superwind gas phase fills most of the volume of starbursts, I suggest that it has turbulent-driven magnetic fields powered by supernovae, and that this phase is where most synchrotron emission arises. I show how such a scenario could explain the far-infrared radio correlation, in context of my previous work. A big issue is that radio and gamma-ray observations imply CRs also must interact with dense gas. Understanding how this happens requires a more advanced understanding of turbulence and CR propagation.

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GMRT 333-MHz observations of six nearby normal galaxies

Cited by 34 publications

References 63 publications

The nature of the low-frequency emission of M 51

The nature of the low-frequency emission of M 51

Modelling the cosmic ray electron propagation in M 51

From 10K to 10 TK: Insights on the Interaction Between Cosmic Rays and Gas in Starbursts

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