Spectral Energy Distribution and Radio Halo of NGC 253 at Low Radio Frequencies

Kapińska, A. D.; Staveley‐Smith, L.; Crocker, Roland M.; Meurer, G. R.; Bhandari, Shivani; Hurley‐Walker, N.; Offringa, A. R.; Hanish, D. J.; Seymour, N.; Ekers, R. D.; Bell, M. E.; Callingham, J. R.; Dwarakanath, K. S.; For, Bi-Qing; Gaensler, B. M.; Hancock, P. J.; Hindson, L.; Johnston‐Hollitt, M.; Lenc, E.; McKinley, B.; Morgan, John S.; Procopio, P.; Wayth, R. B.; Wu, Chen; Zheng, Qian; Barry, N.; Beardsley, Adam P.; Bowman, Judd D.; Briggs, F.; Carroll, P.; Dillon, Joshua S.; Ewall‐Wice, Aaron; Lü, Feng; Greenhill, L. J.; Hazelton, B. J.; Hewitt, Jacqueline N.; Jacobs, Daniel; Kim, H. S.; Kittiwisit, Piyanat; Line, J.; Loeb, Abraham; Mitchell, D. A.; Morales, M. F.; Neben, Abraham R.; Paul, S.; Pindor, B.; Pober, Jonathan C.; Riding, J.; Sethi, Shiv K.; Shankar, N. Udaya; Subrahmanyan, R.; Sullivan, M.; Tegmark, Max; Thyagarajan, Nithyanandan; Tingay, S. J.; Trott, Cathryn M.; Webster, R. L.; Wyithe, J. Stuart B.; Cappallo, R. J.; Deshpande, A. A.; Kaplan, David L.; Lonsdale, Colin J.; McWhirter, S. R.; Morgan, E.; Oberoi, D.; Ord, S. M.; Prabu, T.; Srivani, K. S.; Williams, Anne; Williams, C. L.

doi:10.3847/1538-4357/aa5f5d

Cited by 29 publications

(27 citation statements)

References 81 publications

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“…With the full extents of disk and the halo of about 32 kpc along the plane and 14.5 kpc perpendicular to the plane, respectively, the halo-to-disk extent ratio at 146 MHz is ≈ 0.45. This is similar to the ratio for NGC 253 at 200 MHz (Kapińska et al 2017). However, we caution that the halo-to-disk extent ratio is strongly dependent on sensitivity and angular resolution; a better way to quantify the halo emission is via the scale height (see Section 8).…”

Section: Resultssupporting

confidence: 86%

“…Observations with the VLA at 330 MHz and 74 MHz suggested a flattening of the spectrum along the disk of the Milky Way (Kassim, private communication). Spectral turnovers due to free-free absorption have been observed in the integrated spectra of nearby starburst galaxies such M 82 at ≈ 500 MHz (Adebahr et al 2013), NGC 253 at 230 MHz (Kapińska et al 2017), and IC 10 at 322 MHz (Basu et al 2017). Spatially resolved free-free absorption has been observed in M 82 with Merlin at 408 MHz (Wills et al 1997) and with LOFAR at 154 MHz (Varenius et al 2015).…”

Section: Thermal Absorption In the Disk Of Ngc 891mentioning

confidence: 99%

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Investigation of the cosmic ray population and magnetic field strength in the halo of NGC 891

Mulcahy

Horneffer

Beck

et al. 2018

A&A

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Context. Cosmic rays and magnetic fields play an important role for the formation and dynamics of gaseous halos of galaxies. Aims. Low-frequency radio continuum observations of edge-on galaxies are ideal to study cosmic-ray electrons (CREs) in halos via radio synchrotron emission and to measure magnetic field strengths. Spectral information can be used to test models of CRE propagation. Free-free absorption by ionized gas at low frequencies allows us to investigate the properties of the warm ionized medium in the disk. Methods. We obtained new observations of the edge-on spiral galaxy NGC 891 at 129-163 MHz with the LOw Frequency ARray (LOFAR) and at 13-18 GHz with the Arcminute Microkelvin Imager (AMI) and combine them with recent high-resolution Very Large Array (VLA) observations at 1-2 GHz, enabling us to study the radio continuum emission over two orders of magnitude in frequency.Results. The spectrum of the integrated nonthermal flux density can be fitted by a power law with a spectral steepening towards higher frequencies or by a curved polynomial. Spectral flattening at low frequencies due to free-free absorption is detected in star-forming regions of the disk. The mean magnetic field strength in the halo is 7 ± 2 µG. The scale heights of the nonthermal halo emission at 146 MHz are larger than those at 1.5 GHz everywhere, with a mean ratio of 1.7 ± 0.3, indicating that spectral ageing of CREs is important and that diffusive propagation dominates. The halo scale heights at 146 MHz decrease with increasing magnetic field strengths which is a signature of dominating synchrotron losses of CREs. On the other hand, the spectral index between 146 MHz and 1.5 GHz linearly steepens from the disk to the halo, indicating that advection rather than diffusion is the dominating CRE transport process. This issue calls for refined modelling of CRE propagation. Conclusions. Free-free absorption is probably important at and below about 150 MHz in the disks of edge-on galaxies. To reliably separate the thermal and nonthermal emission components, to investigate spectral steepening due to CRE energy losses, and to measure magnetic field strengths in the disk and halo, wide frequency coverage and high spatial resolution are indispensable.

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Section: Resultssupporting

confidence: 86%

Section: Thermal Absorption In the Disk Of Ngc 891mentioning

confidence: 99%

Investigation of the cosmic ray population and magnetic field strength in the halo of NGC 891

Mulcahy

Horneffer

Beck

et al. 2018

A&A

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“…Adebahr et al (2013) found a spectral turnover for the integrated spectrum of M 82 at 300 MHz, and even higher, at 600 MHz, for the starburst nucleus. Similarly, Kapińska et al (2017) found the 500 pc nuclear starburst region in NGC 253 best described by an internally free-free absorbed synchrotron spectrum with a turnover frequency of 230 MHz. Clearly, these two galaxies represent the more extreme cases in the local Universe and their starburst nuclei have SFR surface densities well in excess of 1 M yr −1 kpc −2 , hence are factor of 10 or higher than what is usually referred to as starburst galaxies.…”

Section: Introductionmentioning

confidence: 72%

Calibrating the relation of low-frequency radio continuum to star formation rate at 1 kpc scale with LOFAR

Heesen

Buie

Huff

et al. 2019

A&A

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Context. Radio continuum (RC) emission in galaxies allows us to measure star formation rates (SFRs) unaffected by extinction due to dust, of which the low-frequency part is uncontaminated from thermal (free-free) emission. Aims. We calibrate the conversion from the spatially resolved 140 MHz RC emission to the SFR surface density (Σ SFR ) at 1 kpc scale. Radio spectral indices give us, by means of spectral ageing, a handle on the transport of cosmic rays using the electrons as a proxy for GeV nuclei. Methods. We used recent observations of three galaxies (NGC 3184, 4736, and 5055) from the LOFAR Two-metre Sky Survey (LoTSS), and archival LOw-Frequency ARray (LOFAR) data of NGC 5194. Maps were created with the facet calibration technique and converted to radio Σ SFR maps using the Condon relation. We compared these maps with hybrid Σ SFR maps from a combination of GALEX far-ultraviolet and Spitzer 24 µm data using plots tracing the relation at the highest angular resolution allowed by our data at 1.2 × 1.2-kpc 2 resolution. Results. The RC emission is smoothed with respect to the hybrid Σ SFR owing to the transport of cosmic-ray electrons (CREs) away from star formation sites. This results in a sublinear relation (Σ SFR ) RC ∝ [(Σ SFR ) hyb ] a , where a = 0.59 ± 0.13 (140 MHz) and a = 0.75 ± 0.10 (1365 MHz). Both relations have a scatter of σ = 0.3 dex. If we restrict ourselves to areas of young CREs (α > −0.65; I ν ∝ ν α ), the relation becomes almost linear at both frequencies with a ≈ 0.9 and a reduced scatter of σ = 0.2 dex. We then simulate the effect of CRE transport by convolving the hybrid Σ SFR maps with a Gaussian kernel until the RC-SFR relation is linearised; CRE transport lengths are l = 1-5 kpc. Solving the CRE diffusion equation, assuming dominance of the synchrotron and inverse-Compton losses, we find diffusion coefficients of D = (0.13-1.5) × 10 28 cm 2 s −1 at 1 GeV. Conclusions. A RC-SFR relation at 1.4 GHz can be exploited to measure SFRs at redshift z ≈ 10 using 140 MHz observations.

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“…Strong thermal absorption seems to be the only reasonable explanation for the distinct drop of low-frequency radio emission in the central part of M 82 (Wills et al 1997;Varenius et al 2015) and in other starburst galaxies (e.g. NGC 253 Kapińska et al 2017). Also, the turnovers found in the spectra of some regions in the centre of the Milky Way (Roy & Pramesh Rao 2006) are well accounted for by absorption effects.…”

Section: Discussionmentioning

confidence: 96%

LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby galaxies

Chyży

Jurusik

Piotrowska

et al. 2018

A&A

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Aims. The shape of low-frequency radio continuum spectra of normal galaxies is not well understood, the key question being the role of physical processes such as thermal absorption in shaping them. In this work we take advantage of the LOFAR Multifrequency Snapshot Sky Survey (MSSS) to investigate such spectra for a large sample of nearby star-forming galaxies. Methods. Using the measured 150 MHz flux densities from the LOFAR MSSS survey and literature flux densities at various frequencies we have obtained integrated radio spectra for 106 galaxies characterised by different morphology and star formation rate. The spectra are explained through the use of a three-dimensional model of galaxy radio emission, and radiation transfer dependent on the galaxy viewing angle and absorption processes. Results. Our galaxies' spectra are generally flatter at lower compared to higher frequencies: the median spectral index α low measured between ≈50 MHz and 1.5 GHz is −0.57 ± 0.01 while the high-frequency one α high , calculated between 1.3 GHz and 5 GHz, is −0.77 ± 0.03. As there is no tendency for the highly inclined galaxies to have more flattened low-frequency spectra, we argue that the observed flattening is not due to thermal absorption, contradicting the suggestion of Israel & Mahoney (1990). According to our modelled radio maps for M 51-like galaxies, the free-free absorption effects can be seen only below 30 MHz and in the global spectra just below 20 MHz, while in the spectra of starburst galaxies, like M 82, the flattening due to absorption is instead visible up to higher frequencies of about 150 MHz. Starbursts are however scarce in the local Universe, in accordance with the weak spectral curvature seen in the galaxies of our sample. Locally, within galactic disks, the absorption effects are distinctly visible in M 51-like galaxies as spectral flattening around 100-200 MHz in the face-on objects, and as turnovers in the edge-on ones, while in M 82-like galaxies there are strong turnovers at frequencies above 700 MHz, regardless of viewing angle. Conclusions. Our modelling of galaxy spectra suggests that the weak spectral flattening observed in the nearby galaxies studied here results principally from synchrotron spectral curvature due to cosmic ray energy losses and propagation effects. We predict much stronger effects of thermal absorption in more distant galaxies with high star formation rates. Some influence exerted by the Milky Way's foreground on the spectra of all external galaxies is also expected at very low frequencies.

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Spectral Energy Distribution and Radio Halo of NGC 253 at Low Radio Frequencies

Cited by 29 publications

References 81 publications

Investigation of the cosmic ray population and magnetic field strength in the halo of NGC 891

Investigation of the cosmic ray population and magnetic field strength in the halo of NGC 891

Calibrating the relation of low-frequency radio continuum to star formation rate at 1 kpc scale with LOFAR

LOFAR MSSS: Flattening low-frequency radio continuum spectra of nearby galaxies

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