11.3.11 Total Galactic Extinction Maps
Author(s): Ronald Drimmel, Douglas Marshall
All-sky (2D) HEALPix maps of the total Galactic extinction are provided at various resolutions (i.e. HEALPix levels) in two tables for the purpose of supporting extragalactic studies. (For further details on the HEALPix scheme used by Gaia, see Bastian and Portell (2020).) These are the tables total_galactic_extinction_map and total_galactic_extinction_map_opt.
The estimation of the total Galactic extinction in each HEALPix pixel is based on a selected subset of sources in the pixel, referred to as extinction tracers, with extinction estimates provided by GSP-Phot for each tracer (Section 11.3.3). For this data release giants are used as extinction tracers, being intrinsically bright and with a large enough scale height to be found in sufficient numbers outside the dust layer of the Milky Way. The selection is done based on GSP-Phot parameters teff_gspphot (effective temperature K) and mg_gspphot (absolute magnitude ). In addition extinction tracers are required to be at least 300pc above or below the galactic plane (), or with a galactocentric radius kpc. For further details see Delchambre et al. (2023).
The table total_galactic_extinction_map is the primary table, and provides the total Galactic extinction map at four different resolutions (HEALPix levels 6 through 9). The second table, total_galactic_extinction_map_opt, is a reduced version of the primary table, provided for convenience. The total_galactic_extinction_map_opt map is a level 9 HEALPix map where the pixels are filled with parameter values from the subset of pixels in total_galactic_extinction_map that are flagged to be at the ”optimum” resolution. In general a HEALPix pixel is considered as optimum if it is the smallest pixel (highest HEALPix level) with ten or more extinction tracers in a given direction. More precisely, the sub-pixels at the next highest HEALPix level are considered as possibly optimal if all four sub-pixels have 10 or more tracers. With this scheme in any given direction there will be no more than one optimum HEALPix pixel at one of the 4 HEALPix levels. As the base level of the total_galactic_extinction_map is at level 6, these pixels will be flagged as optimal even if there are less than 10 tracers. Any pixels with null values at level 6 are likewise null in the total_galactic_extinction_map_opt table. Optimal pixels in the total_galactic_extinction_map table are flagged as optimal with the boolean flag optimum_hpx_flag, while the parameter optimum_hpx_level in the total_galactic_extinction_map_opt table indicates at which HEALPix level is the optimum pixel.
See the following section for a description of the other parameters.
For a given HEALPix pixel of the total_galactic_extinction_map, if three or more tracers are present in the pixel, the extinction (parameter a0) is taken as the median of the selected extinction tracers, where is the extinction parameter in the adopted Fitzpatrick extinction law (Fitzpatrick 1999), taken as the monochromatic extinction at 541.4nm. This parameter is provided for each tracer by the extinction parameter azero_gspphot, as estimated by GSP-Phot (Section 11.3.3). The uncertainty of (a0_uncertainty) is taken as the uncertainty of the sample mean of the tracers, while the number of tracers in the HEALPix (num_tracers_used), as well as the range of values (a0_min, a0_max) are also provided. To recover the sample standard deviation of the distribution of measures for the selected tracers in a HEALPix, one should multiply the given uncertainty by the square root of the number of tracers.
For further details see Delchambre et al. (2023).
As mentioned above, a HEALPix pixel must have three or more tracers to determine the total galactic extinction assigned to the parameter a0; if a pixel at any HEALPix level should have less than 3 tracers a0 have no value assigned and the status flag will have a value greater than 0. There are 1379 out of 49152 pixels (2.8%) at the base HEALPix level 6 lacking sufficient tracers for a determination of a0. All of these occur at very low galactic latitudes of degrees, as a result of extinction tracers being required to be outside the ISM layer. Indeed, the total Galactic extinction map should not be used at such low galactic latitudes. However, at higher HEALPix levels, the number of HEALPix pixels lacking sufficient tracers increases and occur also at higher latitudes.
There are 49 HEALPix pixels in table total_galactic_extinction_map with 0.0 given as the a0_uncertainty. This occurs for pixels with very few tracers, all having the same, or very nearly the same, extinction values. This happens most often as a consequence of a maximum azero_gspphot value of 10 magnitudes being imposed by GSP-Phot, the module estimating the stellar APs. When the number of tracers (num_tracers_used) is very small, the a0 values and associated uncertainties should be used with caution. Unfortunately this occurs for one base level 6 HEALPix (having only 5 extinction tracers), whose values are inherited by 64 level 9 pixels in the total_galactic_extinction_map_opt map.
As mentioned above, total_galactic_extinction_map contains the all-sky maps of the total galactic extinction at four different HEALPix levels (levels 6 through 9). (See Figure 11.48) At the base level 6, with the largest HEALPix pixels, the map covers 100% of the sky at galactic latitudes , with only 2.8% of the sky (1379 out of 49152 pixels) having no a0 values due to an insufficient number of tracers. The fraction of pixels with an insufficient number of tracers increases at the higher HEALPix levels as the pixels become smaller. The total_galactic_extinction_map_opt has the same sky coverage as the level 6 map, but conveniently provides extinction estimates at higher resolution when the number density of tracers are sufficient. The HEALPix level that contributes to the most pixels of the optimum map is level 6 (42%), though this also includes 2.8% of the sky at low galactic latitudes with null values (less than 3 extinction tracers), and another 1.4% have less than 10 extinction tracers.
At level 6 the mean a0_uncertainty of the good HEALPix pixels (status = 0) is 0.034 magnitudes (median 0.023 magnitudes). Uncertainties are generally higher for the higher HEALPix level maps, as expected, since the uncertainty of the sample mean of the tracers is used here, and so is strongly dependent on the number of tracers used, which necessarily diminishes with smaller pixels: The mean number of tracers used per HEALPix at level 6 (for the pixels with status = 0) is 268, while at level 9 it is just over 10. For the subset of optimum pixels for which the mean number of tracers used is 30.3, the mean uncertainty is 0.05 magnitudes.
Figure 11.49 shows the uncertainty maps at each HEALPix level.
For further details see Delchambre et al. (2023).
The total Galactic extinction maps are meant to describe the total foreground extinction from the Milky Way to extragalactic sources. The extinction map is given for the monochromatic extinction parameter , from which extinction in standard photometric bands can be derived following the procedure outlined in Section 11.2.3. The user is recommended to make use of the status flag to exclude null values and to remember that HEALPix pixels whose estimates are based on a small num_tracers_used will have less reliable estimates and uncertainties. The optimum_hpx_flag is offered as an attempt to tag which HEALPix level map gives the ”best” extinction estimate in any given direction, albeit using somewhat arbitrary criteria to strike a balance between resolution and reliable statistics. The total_galactic_extinction_map_opt map is at HEALPix level 9, but in fact captures the intrinsic variable resolution that a map based on background extinction tracers must have due to large variation across the sky of available tracers.
As selected extinction tracers were only required to be beyond a certain distance to assure that they were outside the ISM layer of the Milky Way’s disk, sources in nearby galaxies were also selected as tracers. This means that the total Galactic extinction maps include not only the foreground extinction of the Milky Way’s disk, but also the extinction in these nearby galaxies. Indeed, the Large and Small Magellanic Clouds are quite prominent in the extinction maps. We have decided to make no attempt to remove the additional extinction toward these galaxies, in part because the original intent of the map was to assist in the study of (unresolved) extragalactic sources, and in part because of the potential scientific value of the (unintended) mapping of the extinction in these Milky Way neighbours. However, we warn the user that, due in part to the GSP-Phot prior which does not account for the LMC/SMC, there is a significant bias on the order of 0.6mag in the extinction estimates in these directions. See Delchambre et al. (2023) for further details.
For additional caveats and peculiarities, see the Section 11.3.11 above.