A New, Large-scale Map of Interstellar Reddening Derived from H i Emission

Abstract We present a new map of interstellar reddening, covering the 39% of the sky with low H i column densities ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>N</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">I</mml:mi> </mml:mrow> </mml:msub> <mml:mo>&lt;</mml:mo> <mml:mn>4</mml:mn> <mml:mo>×</mml:mo> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>20</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> </mml:math> cm −2 or <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>E</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>B</mml:mi> <mml:mo>−</mml:mo> <mml:mi>V</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>≈</mml:mo> <mml:mn>45</mml:mn> </mml:math> mmag) at <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>16</mml:mn> <mml:mo>.′</mml:mo> <mml:mn>1</mml:mn> </mml:math> resolution, based on all-sky observations of Galactic H i emission by the HI4PI Survey. In this low-column-density regime, we derive a characteristic value of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msub> <mml:mrow> <mml:mi>N</mml:mi> </mml:mrow> <mml:mrow> <mml:mi mathvariant="normal">H</mml:mi> <mml:mspace width="0.25em"/> <mml:mi mathvariant="normal">I</mml:mi> </mml:mrow> </mml:msub> <mml:mrow> <mml:mo stretchy="true">/</mml:mo> </mml:mrow> <mml:mi>E</mml:mi> <mml:mo stretchy="false">(</mml:mo> <mml:mi>B</mml:mi> <mml:mo>−</mml:mo> <mml:mi>V</mml:mi> <mml:mo stretchy="false">)</mml:mo> <mml:mo>=</mml:mo> <mml:mn>8.8</mml:mn> <mml:mspace width="0.25em"/> <mml:mo>×</mml:mo> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mn>10</mml:mn> </mml:mrow> <mml:mrow> <mml:mn>21</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi>cm</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:mspace width="0.25em"/> <mml:msup> <mml:mrow> <mml:mi>mag</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> for gas with <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo stretchy="false">∣</mml:mo> <mml:msub> <mml:mrow> <mml:mi>v</mml:mi> </mml:mrow> <mml:mrow> <mml:mi>LSR</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">∣</mml:mo> <mml:mo>&lt;</mml:mo> <mml:mn>90</mml:mn> </mml:math> km s −1 and find no significant reddening associated with gas at higher velocities. We compare our H i -based reddening map with the Schlegel et al. (SFD) reddening map and find them consistent to within a scatter of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mo>≃</mml:mo> <mml:mn>5</mml:mn> </mml:math> mmag. Further, the differences between our map and the SFD map are in excellent agreement with the low-resolution ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mn>4</mml:mn> <mml:mo>.°</mml:mo> <mml:mn>5</mml:mn> </mml:math> ) corrections to the SFD map derived by Peek and Graves based on observed reddening toward passive galaxies. We therefore argue that our H i -based map provides the most accurate interstellar reddening estimates in the low-column-density regime to date. Our reddening map is made publicly available at doi.org/10.7910/DVN/AFJNWJ .

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