Residual dipole in 3-year WMAP maps
Posted: August 03 2006
Significant gotcha for those working on the 3-year WMAP data, especially for foreground analysis: the 3-year Stokes I maps (both individual DA and band-averaged) contain a residual 'CMB' monopole and dipole. (NB: the single-year maps have large random monopoles, cf Jarosik et al 2006, sec 3.4.3).
The ILC map 'wmap_ilc_3yr_v2.fits' has both the monopole and dipole set to zero. But you can reconstruct them by re-generating the ILC map from the smoothed individual frequency maps using the weights listed by Hinshaw et al (or, with higher precision, in the header of 'wmap_ilc_rgn_defn_3yr_v2.fits').
Hence, subtracting the official ILC map from individual frequency maps leaves a monopole and dipole with the CMB spectrum in the data. Amplitude is about -11 uK for the monopole and approximately (x,y,z) = (-0.6, -8.4, 8.7) uK (Galactic) for the dipole. The latter is nearly parallel to the main dipole as it is essentially a residual calibration error. The effect is quite noticable in V band.
The zero level of the maps is said to be based on a cosec|b| fit to latitudes south of -15deg. In fact (i) this is after the ILC map WITH monopole and dipole has been subtracted and (ii) the fit is not over all pixels; excluding pixels inside Kp2 gives almost but not exactly consistent answers (this also applies to the 1-year analysis).
The ILC map supplied with the likelyhood code (bias_corrected_pass2_3yr_ilc.fits) still contains the residual monopole and dipole; this is the origin of the dipole found by Kate Land in another topic on this board (note that the image she supplied is actually (likelyhood ILC) minus (LAMBDA ILC) not the other way round).
Finally, in working this out I noticed the undocumented feature that in 'wmap_ilc_3yr_v2.fits' the bias correction has only been applied inside the Kp2 mask. Presumably someone felt that the correction is basically noise at high latitude. I can't tell whether the correction has been applied over the full sky in 'bias_corrected_pass2_3yr_ilc.fits' because there are certainly other differences between this and the LAMBDA version (e.g. this does explain the excess differences at the positions of planets).
regards,
Paddy Leahy
The ILC map 'wmap_ilc_3yr_v2.fits' has both the monopole and dipole set to zero. But you can reconstruct them by re-generating the ILC map from the smoothed individual frequency maps using the weights listed by Hinshaw et al (or, with higher precision, in the header of 'wmap_ilc_rgn_defn_3yr_v2.fits').
Hence, subtracting the official ILC map from individual frequency maps leaves a monopole and dipole with the CMB spectrum in the data. Amplitude is about -11 uK for the monopole and approximately (x,y,z) = (-0.6, -8.4, 8.7) uK (Galactic) for the dipole. The latter is nearly parallel to the main dipole as it is essentially a residual calibration error. The effect is quite noticable in V band.
The zero level of the maps is said to be based on a cosec|b| fit to latitudes south of -15deg. In fact (i) this is after the ILC map WITH monopole and dipole has been subtracted and (ii) the fit is not over all pixels; excluding pixels inside Kp2 gives almost but not exactly consistent answers (this also applies to the 1-year analysis).
The ILC map supplied with the likelyhood code (bias_corrected_pass2_3yr_ilc.fits) still contains the residual monopole and dipole; this is the origin of the dipole found by Kate Land in another topic on this board (note that the image she supplied is actually (likelyhood ILC) minus (LAMBDA ILC) not the other way round).
Finally, in working this out I noticed the undocumented feature that in 'wmap_ilc_3yr_v2.fits' the bias correction has only been applied inside the Kp2 mask. Presumably someone felt that the correction is basically noise at high latitude. I can't tell whether the correction has been applied over the full sky in 'bias_corrected_pass2_3yr_ilc.fits' because there are certainly other differences between this and the LAMBDA version (e.g. this does explain the excess differences at the positions of planets).
regards,
Paddy Leahy