Galactic Center Maps
The main data products in this data release are ACT+Planck co-add maps for the galactic center regions at three different frequency bands: f090, f150, f220.
act_planck_sr_gc_1.0_s19_f090_map.fits act_planck_sr_gc_1.0_s19_f150_map.fits act_planck_sr_gc_1.0_s19_f220_map.fits
These are 32-bit float FITS images with shape (960, 480, 3). The first two axes are Galactic longitude and Galactic latitude in the Plate Carreé projection, covering -4 < ι < 4 and -2 < b < 2 at 0.5 arcmin resolution. The last axis represents the three Stokes parameters I, Q and U in the IAU polarization convention. The maps are in units of μK CMB temperature increment. Note that the axes appear in the opposite order when loaded as a pixell enmap, since enmap (like numpy) uses row-major ordering instead of column-major ordering like FITS does.
See below under Known Issues for recommended use of these maps.
The maps can be easily loaded with pixell (attach a link), using
from pixell import enmap imap = enmap.read_map("act_planck_sr_gc_1.0_s19_f090_map.fits") Here imap can be manipulated like a numpy array. The map can be plotted using the enplot module in pixell, with from pixell import enplot enplot.pshow(imap) It can also be plotted easily with plt, fig = plt.figure() ax = fig.add_subplot(111, projection=imap.wcs) ax.imshow(imap) # total intensity
Inverse variance maps
Associated with each of these maps is a noise floor inverse variance map, which has the same shape and contains an estimate of the non-atmospheric inverse variance in 1/μK2 per pixel. These files are labeled with ivar, including:
act_planck_sr_gc_1.0_s19_f090_ivar.fits act_planck_sr_gc_1.0_s19_f150_ivar.fits act_planck_sr_gc_1.0_s19_f220_ivar.fits
We adopted the same bandpass as released in ACT DR5. See DR5 2008-2018 Coadd Maps for more information.
We adopted the same beams as released in ACT DR5. In particular, as we focus on nighttime observations only, the relevant beam transfer functions are:
act_planck_dr5.01_s08s18_f090_night_beam.txt act_planck_dr5.01_s08s18_f150_night_beam.txt act_planck_dr5.01_s08s18_f220_night_beam.txt
- Due to the preliminary nature of the s19 data used in these maps, care should be taken in quantitative analyses with these maps.
- We have assumed the passbands between Planck and ACT are equivalent when coadding. This may lead to additional scale dependence of the effective band-centers.
- The noise model is built on the assumption that difference maps between ACT and Planck contain only noise. This breaks down in regions with strong, variable point sources, and in regions with strong CO(1-0) line emission as it falls within the Planck 100 GHz passband but not the ACT f090 passband. The noise model also assumes that the map is noise dominated, which is not the case in the Galactic center region. This may lead to biases in the map and an overestimation of the noise level. Based on our experience from other high S/N region, we expect the effect to be < 1%, but no attempts have been made to measure or correct for such an effect.
- While the ACT DR4 maps are first calibrated using the Uranus temperature and then calibrated to Planck, the ACT maps used here are calibrated using Uranus temperatures only. This may imply a possible miscalibration error on the order of 1% based on our experience with ACT DR4.
- No null tests have been run with these maps due to the limited amount of data.
- The beam uncertainty and leakage has not been well characterized for these maps. Due to the location of the Galactic center region as seen by ACT, rising and setting scans are poorly crosslinked, leading to a temperature-to-polarization leakage at ~1% level. A preliminary leakage model has been applied to correct for the leakage effect. We estimate that roughly 80% of the leakage effect is corrected. However, the residue leakage effect is still likely to be the dominant systematic error in polarization, which can lead to an error of ~5-10% in polarization measurements, particularly for regions with low polarization fraction. The exact size of effect has not been measured precisely, and will be the subject of follow-up work.
- The Planck CIB monopole model is included in the Planck maps used in the coadded maps.
- The inverse variance maps only describe the part of the noise that is uncorrelated between pixels. This part of the noise dominates on small scales, but on larger scales the noise will be higher (but still much lower than for an act-only map, due to the inclusion of Planck data). The inverse variance maps also ignore the covariance between the I, Q and U stokes components inside each pixel, though these are mostly uncorrelated anyway (<10%).
The recommended use of these maps includes studies of magnetic field morphology, object identification, and analyses that do not require percent-level accuracy in total intensity and can tolerate 5-10% uncertainty in polarization.