Large-Scale Galactic Stokes-I Continuum Science Chris Salter (NAIC/Arecibo Observatory)

General Points: 1)Arecibo can cover about 32% of the full sky. 2)The 305-m telescope offers essentially full spatial-frequency coverage up to > 200m. 3)This allows it to provide the missing short spacings for the VLA up to the B-array configuration. 4)Compact-source subtraction from the GALFACTS images should be possible to high accuracy using the NVSS images, (though beware variability!), revealing weak large-scale emission. 5)The majority of the GALFACTS sky lies at intermediate and high Galactic latitudes, so considerable attention should be given to what can be said re-Galactic structure in these directions. 6) At high Galactic latitudes there are almost no full-spatial frequency studies of the wider distribution of the continuum emission with a resolution of < 0.5 o. Introduction

GALFACTS Stokes-I Requirements The wide dynamic range in Stokes-I Galactic background (from the confusion level of Tb ~20 mK to the peak brightness on the Galactic plane of ~20 K) demands the best possible calibration, and the best practical CLEANing of the sidelobe responses of the ALFA beams. Stockert 1.4 GHz survey (Reich & Reich, 1986)

The Galactic Background: State of Play The Situation Circa )What is the mechanism for the non-thermal emission? 2)What contribution is made by discrete sources? 3)What fraction of the Galactic continuum is thermal? 4)What is the relation of extended regions (e.g. Cygnus X) to the rest of the Galactic radio background? 5)How is the radio background attenuated at low frequencies? 6)Does a radio halo extend about the Galaxy? 7)What is the North Polar Spur (NPS) and is it unique? Well, 1) and 2) can probably be considered solved, but the rest still pose a number of unsolved problems each. GALFACTS cannot say much about 5), but can contribute to many aspects of the other questions. In respect of 7), we now know that the NPS is not unique, but we can ask whether all the large-scale “Loops” have a similar origin? Of course the existence on linear polarization to the Galactic non-thermal emission was only revealed in 1962, this proving to be a vital tool for investigating the Galactic background.

Components of the Galactic Continuum Background a) The Galactic Center Region b) The Galactic Disc c) Discrete Galactic sources d) The high-latitude loops & spurs e) A possible Galactic radio halo We cannot observe Component (a) from Arecibo. However, all other of the components lie, at least in part, within the Arecibo sky. Arecibo Sky

The Disc Emission. I Possible Investigations: 1)Thermal-nonthermal separation of the low-latitude emission. If we assume low optical depth: T b ( ) =  ( ) T e + T N , where  =  (Although, if the optical depth is not low, the measured value of T b depends on the relative distributions of the thermal and non-thermal material.) If only images at two frequencies are available to make the thermal/non-thermal separation, one would have to make the best possible guesses for T e and , and solve for  0 and T N. If there are more frequencies, less assumptions need to be made. If a thermal/non-thermal separation could be made, then the brightness of the derived thermal component is invaluable input for interpretation of hydrogen recombination line data for the diffuse ionized component of the ISM. Note: To make a thermal/nonthermal separation, the best possible absolute calibration is needed for the surveys to be used.

The Disc Emission. II Investigation of spiral structure for the thermal and non-thermal components. a) Look for “steps” or “bumps” in longitude identifying lines of sight tangential to spiral arms. b) “Unfold” the low-latitude continuum emission for an assumed spiral model. From such an investigation, Beuermann et al. (1985) derived “thin” and “thick” disc components, with the latter having ten times the scale height of the former, and both of which increase in thickness with galactocentric distance and display spiral structure. c) To measure the latitude dependence of the peak disc emission as a function of longitude, and investigate the deviations from b = 0 o ---- systematic global deviations, evidence for “warps”, etc.

The Galactic Loops: Introduction The Galactic Loops are very large-diameter (> 40 o ) radio continuum emission arcs tracing significant fractions of celestial small circles. The existence of six such Loops has been proposed. The largest and brightest is Loop 1, the brightest segment of which is known as the North Polar Spur (NPS). The characteristics of the loops are;  No known associated optical emission.  Steep non-thermal spectra (S  , with  ~ between 0.4 and 1.4 GHz; Borka 2007).  High polarization percentages ( % at 1.4 GHz for NPS).  At least Loop I has associated HI and X-ray emission.  Loop I has “internal ridging”, a sharp outer gradient and a higher brightness internal to the Loop, all suggesting that it is a shell. Loops I -- IV

The Galactic Loops and GALFACTS From Arecibo it is possible to map considerable segments of five of these six Galactic Loops; Loop III alone lies completely outside of the Arecibo sky. Only the North Polar Spur has been studied with a resolution of > 0.5 o (and then only very incompletely.) With a resolution of 10 arcmin, Sofue and Reich (1979) mapped the NPS down into the Galactic plane, tracing it to b = +3 o ; its width there is no wider than 18 arcmin. Only Loop III has been traced across the galactic plane, although there have been claims that Loop I can be seen below the plane in HI and X-ray emission. Loop IV is entirely contained in the northern Galactic hemisphere. Sofue & Reich (1979)

The Galactic Loops and HI The occurrence of HI associated with Loop I (and lying a few degrees outside of the continuum loop) is an example of the exciting possibilities that can come from comparing Stokes-I continuum structures with the HI images that will come from the TOGS2 commensal project. A comparison of the HI data with the Stokes-I images of the five spurs that cross the Arecibo sky should help resolve many of the conflicting conclusions concerning the association of the two. Many of these previous comparisons have been made between data of differing resolutions, with many databases being under-sampled.

The Galactic Loops: Fine Scale Structure Morphological details of the NPS contain an amount of relatively fine scale structure. Apart from the “narrow neck” near the plane, Holden (1969) found a series of “steps” in the outer gradient of the continuum NPS (and its internal ridging) whose widths were at least as narrow as 20 arcmin despite having lengths of some 6 o. In addition, a circular feature of diameter 14 o has been found superimposed on the NPS ridge peak at b ~ 35 o, this containing clear sub-structure.

The Zone of Influence of the NPS There is a considerable amount of evidence that the zone of influence of the NPS extends for some distance outside of its main continuum ridge. Both the existence of exterior continuum ridges parallel to the main ridge, And the presence of the exterior, concentric HI arc are cases in point. In 1979, Rickard & Cronyn noted an excess of IPS non-scintillators at 81.5 MHz in a band lying parallel to but outside the NPS. This they put down to scatter broadening in a turbulence shell outside the NPS. Their result was confirmed by Purvis (private comm.) via a later Cambridge 81.5 MHz IPS survey. Ooty IPS measurements at 327 MHz found no lack of scintillators in this band, supporting the scatter-broadening hypothesis. In view of the above, it will be extremely interesting to see the distribution of the linearly polarized component within this band of longitude.

The Nature of the Galactic Loops Among the present preferred theories of the origin of the Galactic Loops are that they they represent either/or; a)Nearby, very old supernova remnants (SNR). b)For Loop I, a stellar wind bubble associated with the Sco-Cent OB association, and containing embedded SNRs. c)It has been suggested (Sofue 1994) that the NPS is the result of propagation of a shock in the halo of the Galaxy caused by an explosion and/or starburst at the Galactic Center some 15  10 6 yr ago. While the first two of the above have most support, the jury will remain out until further observational evidence is obtained. GALFACTS and its commensal partner, TOGS2, can provide just the evidence needed. Until then, it is not even clear that all The Galactic Loops have originated via the same process.

Existing Wide-Area Continuum Surveys