Erratum: Galactic synchrotron emissivity measurements between 250° < l < 355° from the GLEAM survey with the MWA Free

This is an erratum to the paper ‘Galactic synchrotron emissivity measurements between 250° < l < 355° from the GLEAM survey with the MWA’, published in MNRAS, 465, 3163 (2017).

The conversion factor (5652.1) from Jy beam⁻¹ to Kelvin was overestimated by a factor of 4 in the original publication because we used the semi-major and semi-minor axis lengths rather than the major and minor axis lengths while calculating this conversion factor. This leads to an overestimate of the emissivities by a factor of 1.1–2.7, thus resulting in overestimated parameters in our models.

The correct conversion factor is 1413.8. Our corrected emissivities are listed in Table 1, corrected comparisons with literature are presented in Table 2, and corrected model parameters are shown in Table 3. Figs 1–5 replaces the original Figs 4–8, respectively.

Figure 1.

Distribution of emissivities (ε_b) on lines of sight and the Two-circle model. Each colour represents a path over which the emissivities are averaged. The colours of the lines indicate the value of emissivities along different paths. The Galactic non-thermal emission is assumed to lie in a disk with a Galactocentric radius of 20  kpc shown by the grey circle segment. The two circles near the Galactic Centre divide the Galactic plane into three regions with emissivities of ε₁, ε₂, and ε₃ respectively. The Galactocentric radius of the Sun is 8.5  kpc. The Sun and Galactic Centre (GC) are marked as a black dot and plus respectively. This figure replaces Fig. 4 in the original paper.

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Figure 2.

Distributions of emissivities (ε_b) with respect to Galactic longitude (left) and latitude (right). Some measurements have large errors due to the uncertainty of the distances of H ii regions. This figure replaces Fig. 5 in the original paper.

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Figure 3.

Distributions of emissivity from the Sun to the Galactic edge over the line of sight with respect to the Galactic longitude in the range 250° < l < 355°, |b| < 3°: this includes the contributions from point sources and supernova remnants. The bin size in Galactic longitude is 3|${^{\prime}_{.}}$|75. There is a trend of increasing emissivity with longitude. The high emissivity near the Galactic longitude 264° is contributed by the Vela supernova remnant. Note that we do not recover the total power of the Galaxy due to the interferometric nature of the measurements. This figure replaces Fig. 6 in the original paper.

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Figure 4.

Triangle plot showing the correlations between the two-circle model parameters: the grey-scale traces the distributions of chi-square with darker regions indicating lower values. The 2σ boundaries of chi-square are shown by white contours. The solid black lines show the relative likelihood distributions with dashed lines showing the best values and 2σ limits of the parameters. This figure replaces Fig. 7 in the original paper.

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Figure 5.

Distributions of emissivities (ε_b) from our measurements and models with the path length from H ii regions to the Galactic edge along the line of sight. This figure replaces Fig. 8 in the original paper.

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After correction, all average emissivities between the H ii regions and the Galactic edge along the line of sight (ε_b) are in the range of 0.24  ∼  0.70  K  pc⁻¹ with an average of 0.40  K  pc⁻¹ and a variance of 0.10  K  pc⁻¹. The average emissivities between the H ii region regions and the Sun (ε_f) have a large range of −12.72  ∼  0.50  K  pc⁻¹ with a mean of −1.56  K  pc⁻¹ and a variance of 2.2  K  pc⁻¹, although these are subject to an uncertainty in total power affecting ε_f. These numbers replace the corresponding numbers in our original abstract and the first paragraph of the original Section 3.1. Most of the emissivities between the H ii regions and the Sun are negative now because the corrected brightness temperatures of H ii regions are lower than their electron temperatures, fundamentally, because some flux of the largest scales is missed in our observations.

In the second paragraph in the original Section 2.1, 1 Jy beam⁻¹ = 1413.8 K.

In the note of the original page 2, 1 K pc⁻¹ is equivalent to 7.71  ×  10⁻⁴¹  W  m⁻³  Hz⁻¹  sr⁻¹ at 88  MHz in our observations.

The effect of the extragalactic emission (∼0.03  K  pc⁻¹) on our measurements is smaller than our minimum error (0.05  K  pc⁻¹) and smaller than our average error (0.06  K  pc⁻¹). This replaces the last sentence in the third paragraph in the original Secion 3.1.

According to the brightness temperature spectrum T(ν) ∝ ν^−2.3 (Guzmán et al. 2011) and the typical emissivity of 0.01  K  pc⁻¹ (Beuermann, Kanbach & Berkhuijsen 1985) at 408  MHz (Haslam et al. 1982), the typical emissivity at 88  MHz is expected to be 0.34  K  pc⁻¹. The mean of our measured emissivity (ε_b) of 0.40  K  pc⁻¹ with a variance of 0.10  K  pc⁻¹ agrees within 1σ with this estimation. To check for any systematic differences between the samples, we compare the measurements for two H ii regions that appear both in our sample and N06, in Table 2. Using the same distances and electron temperatures as listed in N06, and scaling our measurements from 88 to 74 MHz using a brightness temperature spectral index of 2.3 (T_ν ∝ ν^−2.3, Guzmán et al. 2011), our measurements agree within 1.5σ, 0.1σ, and 5σ with those in N06 for the three matched H ii regions respectively. The difference in the last pair may be due to the low brightness temperature in the surrounding region in our observations. This paragraph replaces the first and second ones in the original Section 3.2.

The results of modelling are shown in Table 3. The Uniform model shows an average emissivity of |$0.38^{+0.04}_{-0.02}$| K pc⁻¹ with a poor reduced chi-square (⁠|$\chi ^2_{\text{red}} = 6.51$|⁠). The Gaussian and Exponential models have similarly poor fits (⁠|$\chi ^2_{\text{red}}\sim$| 6.03 and 5.77). The Two-circle model has the best performance compared with the other three models (⁠|$\chi ^2_{\text{red}}=3.74$|⁠). It shows a high emissivity (2.0|$^{+1.8}_{-1.7}\,\,$|K  pc⁻¹) region near the Galactic Centre within a radius of 1.7|$^{+0.1}_{-0.4}\,\,$|kpc, a low emissivity (0.30|$^{+0.13}_{-0.17}\,\,$|K  pc⁻¹) region outside of a radius of 7.5|$^{+5.5}_{-2.9}\,\,$|kpc, a medium emissivity (0.59|$^{+0.32}_{-0.49}\,\,$|K  pc⁻¹) region between the two. This replaces the second to the fifth sentences in the ninth paragraph in the original Section 4. The 10th sentence in that paragraph is redundant and should be deleted.

The number in the last sentence in the last fourth paragraph in the original Section 4 should be 2.0|$^{+1.8}_{-1.7}\,\,$|K  pc⁻¹.

Acknowledgements

We thank Irene Polderman for identifying the mistake that caused this erratum.

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