Studying Galactic interstellar turbulence through fluctuations in synchrotron emission

Abstract

Aims. The characteristic outer scale of turbulence (i.e. the scale at which the dominant source of turbulence injects energy to the interstellar medium) and the ratio of the random to ordered components of the magnetic field are key parameters to characterise magnetic turbulence in the interstellar gas, which affects the propagation of cosmic rays within the Galaxy. We provide new constraints to those two parameters. Methods. We use the LOw Frequency ARray (LOFAR) to image the diffuse continuum emission in the Fan region at (l, b) ∼ (137.0 • , +7.0 • ) at 80 × 70 resolution in the range [146, 174] MHz. We detect multi-scale fluctuations in the Galactic synchrotron emission and compute their power spectrum. Applying theoretical estimates and derivations from the literature for the first time, we derive the outer scale of turbulence and the ratio of random to ordered magnetic field from the characteristics of these fluctuations. Results. We obtain the deepest image of the Fan region to date and find diffuse continuum emission within the primary beam. The power spectrum displays a power law behaviour for scales between 100 and 8 arcmin with a slope α = −1.84 ± 0.19. We find an upper limit of ∼20 pc for the outer scale of the magnetic interstellar turbulence toward the Fan region, which is in agreement with previous estimates in literature. We also find a variation of the ratio of random to ordered field as a function of Galactic coordinates, supporting different turbulent regimes. Conclusions. We present the first LOFAR detection and imaging of the Galactic diffuse synchrotron emission around 160 MHz from the highly polarized Fan region. The power spectrum of the foreground synchrotron fluctuations is approximately a power law with a slope α ≈ −1.84 up to angular multipoles of 1300, corresponding to an angular scale of ∼8 arcmin. We use power spectra fluctuations from LOFAR as well as earlier GMRT and WSRT observations to constrain the outer scale of turbulence (L out ) of the Galactic synchrotron foreground, finding a range of plausible values of 10−20 pc. Then, we use this information to deduce lower limits of the ratio of ordered to random magnetic field strength. These are found to be 0.3, 0.3, and 0.5 for the LOFAR, WSRT and GMRT fields considered respectively. Both these constraints are in agreement with previous estimates.

All-sky angular power spectrum – I. Estimating brightness temperature fluctuations using the 150-MHz TGSS survey

“…The TGSS observations can probe the angular scales in the range 0.045 • to 1.2 • , however due to convolution of the primary beam and the residual point sources, we are limited in the range 0.3 • to 0.8 • for the DGSE measurements. As discussed in several previous studies (Bernardi et al 2009;Ghosh et al 2012;Iacobelli et al 2013;Choudhuri et al 2017a), it is possible to fit C with a power-law A×(1000/ ) β in this range. We present the results for the power-law fitting in Figure 1 for the four representative fields.…”

“…Parsons et al (2010) have presented the all-sky synthesized map and estimated the C between 139 MHz and 174 MHz. It has also been measured in only a few small fields at low Galactic latitude |b| < 14 • in the frequency range 150 -160 MHz (Bernardi et al 2009(Bernardi et al , 2010Ghosh et al 2012;Iacobelli et al 2013;Choudhuri et al 2017a). Bernardi et al (2009) and Ghosh et al (2012) have, respectively, analysed 150 MHz WSRT and GMRT observations where they respectively found β = 2.2 ± 0.3 and β = 2.34 ± 0.28 up to = 900.…”

“…The extra-galactic point sources are the most dominant foreground components at that angular scales which are relevant for telescopes like LOFAR and SKA (Ali, Bharadwaj & Chengalur 2008;Ghosh et al 2012). The DGSE dominates at large angular scale > 10 arcmin after point sources are subtracted at ∼10-20 mJy level (Bernardi et al 2009;Ghosh et al 2012;Iacobelli et al 2013;Choudhuri et al 2017a).…”

“…A precise characterization and a detailed understanding of the DGSE are needed to remove foregrounds in 21-cm experiments reliably. Also, the angular fluctuations of the DGSE are directly related to the fluctuations in the magnetic field and the fluctuations in the cosmic ray electron density of the turbulent interstellar medium (ISM) of our Galaxy (Cho & Lazarian 2008;Waelkens et al 2009;Regis 2011;Lazarian & Pogosyan 2012;Iacobelli et al 2013), a subject that is not very well understood at present. Lazarian & Pogosyan (2012) have shown avenues for quantitative studies of magnetic turbulence in our Galaxy and beyond using observations of the synchrotron emission, and it also outlined the directions of how synchrotron foreground emission can be separated from the cosmological signal, i.e., from cosmic microwave background or highly redshifted HI 21-cm emission.…”

“…Bernardi et al (2009) and Ghosh et al (2012) have, respectively, analysed 150 MHz WSRT and GMRT observations where they respectively found β = 2.2 ± 0.3 and β = 2.34 ± 0.28 up to = 900. Iacobelli et al (2013) have measured the C of the DGSE at 160 MHz using LOFAR data and reported that the angular power spectrum has a slope β ≈ 1.8 down to the angular multipoles of 1300. In an earlier paper (Choudhuri et al 2017a), we have analysed two fields from the TIFR GMRT Sky Survey (TGSS)-ADR1 survey at 150 MHz (Intema et al 2017) and measured the C of the DGSE across the range 240 ≤ ≤ 500 and found that the values of β are 2.8 ± 0.3 and 2.2 ± 0.4 respectively in the two fields.…”