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E(V-I) Reddening Map in the Magellanic System from OGLE-IV Red Clump stars

Skowron et al. 2021, ApJS, 252, 23
arXiv:2006.02448, OGLE-ing the Magellanic System: Optical Reddening Maps of the Large and Small Magellanic Cloud from Red Clump Stars D. M. Skowron et al. 2021 (Received 2020 June 4, Accepted 2020 November 9, Published 2021 February 1) We present the most extensive and detailed reddening maps of the Magellanic Clouds (MCs) derived from the color properties of Red Clump (RC) stars. The analysis is based on the deep photometric maps from the fourth phase of the Optical Gravitational Lensing Experiment (OGLE-IV), covering approximately 670 deg2 of the sky in the Magellanic System region. The resulting maps provide reddening information for 180 deg2 in the Large Magellanic Cloud (LMC) and 75 deg2 in the Small Magellanic Cloud (SMC), with a resolution of 1.7x1.7 arcmin in the central parts of the MCs, decreasing to approximately 27x27 arcmin in the outskirts. The mean reddening is E(V-I) = 0.100 +- 0.043 mag in the LMC and E(V-I) = 0.049 +- 0.024 mag in the SMC. We refine methods of calculating the RC color to obtain the highest possible accuracy in determining the intrinsic color of RC stars. We show that the intrinsic RC color in the LMC is $(V-I)_0 = 0.876 +- 0.013 mag and there is no color gradient in the verifiable range, i.e. from 3 up to 9 deg from the galaxy center. In the case of the SMC (V-I)_0 = 0.856 +- 0.016 mag and is constant between 1 and 4 deg from the galaxy center, followed by a slight decline beyond 4 deg from the center of the SMC. The reddening map of the MCs is available on-line both in the downloadable form and as an interactive interface.abstract?
(Received 2020 June 4, Accepted 2020 November 9, Published 2021 February 1)

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Coordinates may be entered in decimal (H.MMMMM D.DDDDD), sexagesimal (HH:MM:SS.SS or HH MM SS.SS) format or any of 5.45 -71.42 5.45, -71.42 5:26:34.1, -71:25:04.2 5 26 34.1 -71 25 04.2 5h26m34.1s, -71d25m04.2s 5h 26m 34.1s, -71d 25m 04.2s J052634.10-712504.2 J052700-712512 052700+712512 5 27 -71 25.2 5:27, -71:25.2 5h27m, -71d25.2m 5h 27m, -71d 25.2m 05 -71 the following ?.

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The uploaded file should have coordinates in columns 1 and 2. Use drop-down menu above to choose RA/Dec in (hrs  deg) or (deg  deg), or galactic coordinates.

You can also click on the map below. More download options are provided at the bottom of the page.

Result:

                                                       |----------------- OGLE  Red Clump stars ---------------|
       RA           Dec            RA         Dec        E(V-I) -sigma1 +sigma2   (V-I)_RC  (V-I)_0  E(V-I)peak E(V-I)sfd  box     sep
      [hr]         [deg]          [hr]       [deg]       median  [-34%]  [+34%]    median    [mag]      mode    from SFD  arcmin  arcmin

Column 'box' informs about the size of the area (in arcmin) the measurement was estimated from. Value of 'sep' provides the distance (in arcmin) to the grid point, from which the measurement was taken.

Extinction can be estimated using   A(I) ~= 1.5 E(V-I), where the coefficient can vary wildly, approximately between 1.1 and 1.7, depending on particular dust characteristics. Typically, higher coefficient values are observed in highly reddened fields. In the southern part of the LMC, were the reddening is dominated by the Galactic dust, the coefficient is approximately in the 1.1 – 1.3 range, which is closer to the standard extinction law.

The Schlegel, Finkbeiner & Davis 1998 (ApJ 500, 525, hereafter SFD) reddening map is provided for comparison. Using the calibrations of Schlafly & Finkbeiner 2011 (ApJ 737, 103): A(V) = 2.742 E(B-V)SFD,original, A(I) = 1.505 E(B-V)SFD,original, we obtain the reddening E(V-I)SFD = 1.237 E(B-V)SFD,original. This value is provided in the last column for convenience. Note that E(V-I)SFD values are grossly overestimated in the central parts of the Magellanic Clouds, and therefore should not be used. However in the outskirts, both maps are consistent (with a scatter of only +- 0.013 mag) and either can be used to estimate the reddening. In particular, in the furthest areas, where the spatial resolution of the SFD map is higher, it could be used instead of the OGLE map. This is true for radii larger than about 5.5-6.5 deg from the LMC and 2-3 deg from the SMC center. The rule of thumb could be, that if the box size > 7 arcmin, use the SFD, otherwise use the OGLE reddening map (note, that the adjacent box centers are closer to each other than their size would suggest).

Download

The full reddening map is also available as:

  1. FITS file (10 MB) only E(V-I) — 2-dimensional (V-I) extinction map with WCS Equatorial coordinates
  2. FITS file (80 MB) all data (extensions: E(V-I), SIGMA_1, SIGMA_2, (V-I)_MEDIAN, (V-I)_0, E(V-I)_MODE, BOX_SIZE) — to view all extensions use: "ds9 -multiframe -zscale -zoom 0.3 skowron2020_all.fits"
  3. ASCII .txt tables — in four spatial resolutions and box sizes (3.4, 6.9, 13.8 and 27.5 arcmin).

DSS & IRAS images acknowledgment

Send comments to this address. Please cite the following paper when using the data or referring to these OGLE results: Skowron et al. 2021, ApJS, 252, 23 (arXiv:2006.02448).

Data updated: 2020 Sep 25