A Radio Survey of the Southern Milky Way at a Frequency of 1440 Mc/s. II. The Continuum Emission from the Galactic Disk

Abstract

SummaryThe 1440 Mcls survey (Part I of this series) has been used in conjunction with the 85·5Mc/s survey of Hill, Slee, and Mills (1958) to delineate the distribution of the thermal and nonthermal radiation from the disk component of the Southern Milky Way and so complete an investigation commenced by the Northern Hemisphere observers Westerhout (Leiden) and Large, Mathewson, and Haslam (Jodrell Bank). Results of the analysis show an intense concent,ration of ionized hydrogen in an irregular spiral structure in the inner regions of the Galaxy. From lII=256° to 88°, good agreement was obtained between the longitudes at which concentrations of neutral hydrogen were found to occur from H-line studies and the longitudes at which the ionized hydrogen was concentrated. The steps in the longitude distribution of the 85·5 Mcls radiation which Mills used to delineate the spiral arms of the Galaxy were not all visible in the longitude distribution of the nonthermal component obtained from this present analysis. It is believed that three of Mills's steps are thermal in origin.

AHerschel[C II] Galactic plane survey

“…We corrected for the difference in frequencies between the two data sets and reduced the emission derived for WMAP by 50% as suggested by Alves et al (2012). We find that the longitudinal distribution of the ELDWIM free-free emission derived for WMAP and those derived by Westerhout (1958) and Mathewson et al (1962) are in good agreement, with their intensities agreeing within 20-30%. We assumed uncertainties in the free-free distribution of 30%.…”

“…The frequency and angular resolution A121, page 2 of 13 of these data sets are (1.44 GHz, 50 ) for the Mathewson et al (1962) and (1.39 GHz, 34 ) for the Westerhout (1958) data sets 2 . To isolate the contribution from free-free emission in the observed radio continuum distribution, Westerhout (1958) and Mathewson et al (1962) subtracted the contribution from nonthermal synchrotron emission using the 85.5 MHz survey by Hill et al (1958) (which is dominated by non-thermal emission) assuming a flux density spectral index (I ∝ ν β ) for this emission mechanism of β = −2.6, between 85.5 MHz and 1.4 GHz. The uncertainties in the assumed spectral index of the non-thermal emission dominates the uncertainties in the separation between thermal and non-thermal contributions to the 1.4 GHz emission.…”

“…The uncertainties in the assumed spectral index of the non-thermal emission dominates the uncertainties in the separation between thermal and non-thermal contributions to the 1.4 GHz emission. To compare the free-free distribution from Westerhout (1958) and Mathewson et al (1962) with modern-day data, we compared it with that derived for WMAP by Gold et al (2011) at 23 GHz using the Maximum Entropy Method with 1…”

“…The free-free intensities are taken from Guesten & Mezger (1982) and were derived by Mathewson et al (1962) and Westerhout (1958) for the southern and northern parts of the Galaxy, respectively. The frequency and angular resolution A121, page 2 of 13 of these data sets are (1.44 GHz, 50 ) for the Mathewson et al (1962) and (1.39 GHz, 34 ) for the Westerhout (1958) data sets 2 .…”

“…The free-free intensities are taken from Guesten & Mezger (1982) and were derived by Mathewson et al (1962) and Westerhout (1958) for the southern and northern parts of the Galaxy, respectively. The frequency and angular resolution A121, page 2 of 13 of these data sets are (1.44 GHz, 50 ) for the Mathewson et al (1962) and (1.39 GHz, 34 ) for the Westerhout (1958) data sets 2 . To isolate the contribution from free-free emission in the observed radio continuum distribution, Westerhout (1958) and Mathewson et al (1962) subtracted the contribution from nonthermal synchrotron emission using the 85.5 MHz survey by Hill et al (1958) (which is dominated by non-thermal emission) assuming a flux density spectral index (I ∝ ν β ) for this emission mechanism of β = −2.6, between 85.5 MHz and 1.4 GHz.…”