DOI: 10.7910/dvn/j9jcko
title: DECaPS 3D Dust Map

description: &lt;h1&gt;DECaPS 3D Dust Map&lt;/h1&gt;

 &lt;p&gt;A three-dimensional map of dust reddening, covering the Southern Galactic plane (&lt;code&gt;239&deg; &lt; l &lt; 6&deg;, |b| &lt; 10&deg;&lt;/code&gt;).&lt;/p&gt;

 &lt;p&gt;We use DECaPS optical and near-infrared photometry, in conjunction with complementary photometric and astrometric data, to infer distances and reddenings to 709 million stars. These stars trace the reddening along different lines of sight, allowing us to build up a map of reddening in 3D.&lt;/p&gt;

 &lt;p&gt;The map is structured as a set of sightlines, each of which contains multiple samples of the cumulative dust reddening as a function of distance. Each sightline is identified by a nested pixel index at a HEALPix &lt;code&gt;N&lt;sub&gt;side&lt;/sub&gt; = 8192&lt;/code&gt;. Within each sightline, cumulative reddening is given at discrete distances, spaced evenly in distance modulus. &lt;/p&gt;

 &lt;h2&gt;Quality Assurance Information&lt;/h2&gt;
 &lt;p&gt;Quality assurance information is given for each pixel, including:&lt;/p&gt;
 &lt;ul&gt;
 &lt;li&gt;Whether the fit converged in the pixel&lt;/li&gt;
 &lt;li&gt;Whether the pixel needed to be infilled due to lack of stars&lt;/li&gt;
 &lt;li&gt;The minimum reliable distance modulus in the pixel&lt;/li&gt;
 &lt;li&gt;The maximum reliable distance moduli in the pixel&lt;/li&gt;
 &lt;/ul&gt;

 &lt;p&gt;Alongside the quality assurance information, we also provide the number of stars whose PDF on distance and reddening is used to inform the line of sight fit towards each pixel. Note that this will not sum to the total number of stars used in the construction of the map (709 million) because some stars contribute to multiple pixels based on our Gaussian weighting scheme. &lt;/p&gt;

 &lt;p&gt;Unlike the "Bayestar" 3D dust map from &lt;a href="https://ui.adsabs.harvard.edu/abs/2019ApJ...887...93G/abstract" target="_blank"&gt;
Green et al. 2019&lt;/a&gt; (whose reddening is provided in an arbitrary reddening unit), the DECaPS 3D dust reddening is given in units of &lt;code&gt;E(B-V)&lt;/code&gt; in mags. To convert to &lt;code&gt;A(V)&lt;/code&gt;, assume &lt;code&gt;R(V)=3.32&lt;/code&gt;. When combined with Bayestar, the DECaPS map enables extinction corrections over the entire Galactic plane &lt;code&gt;|b| &lt; 10&deg;&lt;/code&gt;.&lt;/p&gt;

 &lt;p&gt;The 3D map is described in more detail in &lt;strong&gt;&lt;a href="https://ui.adsabs.harvard.edu/abs/2025arXiv250302657Z/abstract" target="_blank"&gt;
 Zucker, Saydjari, and Speagle et al. 2025&lt;/a&gt;.&lt;/p&gt;&lt;/strong&gt;

 &lt;h2&gt;Downloading and Querying the Map&lt;/h2&gt;
&lt;p&gt;This map is included in the Python &lt;strong&gt;&lt;a href="https://dustmaps.readthedocs.io/en/latest/" target="_blank"&gt;dustmaps&lt;/a&gt;&lt;/strong&gt; package, which is available through &lt;code&gt;pip&lt;/code&gt;:&lt;/p&gt;

 &lt;pre&gt;&lt;code&gt;pip install dustmaps&lt;/code&gt;&lt;/pre&gt;

&lt;p&gt;Note that we generate 100 samples of the reddening. In the data product &lt;strong&gt;&lt;code&gt;decaps_mean_and_samples.h5&lt;/code&gt;&lt;/strong&gt;, we calculate the mean from the 100 samples and additionally provide five random samples, all stored as &lt;code&gt;float16&lt;/code&gt; to reduce memory usage (33 GB in size). For those with limited disk space, in &lt;strong&gt;&lt;code&gt;decaps_mean.h5&lt;/code&gt;&lt;/strong&gt;, we provide the option to only download the mean map (no samples), which is significantly smaller in size (8 GB).&lt;/p&gt;

&lt;p&gt;Note that in August 2025, we updated these files to increase the precision of our minimum and maximum reliable distance moduli (from float16 to float32). This change affected less than 1% of pixels (by a very small fractional amount) and should not affect any queries performed prior to August 2025. &lt;/p&gt;