Galactic Dust Reddening and Extinction Service Gives the foreground (Milky Way) reddening for a line of sight, returning a reddening map, the corresponding 100 micron intensity, dust temperature, statistics, and Galactic extinctions using two methods.

All-sky view of the IRAS 100 micron imaging data, representing a MONTAGE-generated combination of the individual images created by Schlegel, Finkbeiner & Davis (1998). The Galactic Aitoff projection is shown in false color (blue is low intensity, red/white is high intensity).

Background

D.J. Schlegel, D.P. Finkbeiner, & M. Davis (1998, ApJ, 500, 525) combined the strengths of IRAS and COBE/DIRBE to create a relatively high resolution (~few arc-minute) 100-micron intensity map of the sky that is free of striping and accurately recalibrated to the absolute photometry of COBE/DIRBE. The maps can be used, in conjunction with a spectral template for the background, to derive the dust temperature & opacity, and hence, extinction, along the line of sight (assuming a standard extinction law). These Schlegel et al. maps have proven to be a popular product for astronomers, though users of this service should read the cautionary Notes below.

NEW (July 2013): Schlafly and Finkbeiner (2011, ApJ 737, 103) provide new estimates of Galactic dust extinction from an analysis of the Sloan Digital Sky Survey. Their re-calibration is presented here alongside the Schlegel et al. results, though values rely on the Schlegel et al. E(B-V) maps as a starting point.

Technical Summary

The Technical Summary from the Schlegel et al web site is repeated here:

"We present a full sky 100 micron map that is a reprocessed composite of the COBE/DIRBE and IRAS/ISSA maps, with the zodiacal foreground and confirmed point sources removed. Before using the ISSA maps, we remove the remaining artifacts from the IRAS scan pattern. Using the DIRBE 100 micron and 240 micron data, we have constructed a map of the dust temperature, so that the 100 micron map can be converted to a map proportional to dust column density. The dust temperature varies from 17 K to 21 K, which is modest but does modify the estimate of the dust column by a factor of 5. The result of these manipulations is a map with DIRBE-quality calibration and IRAS resolution. A wealth of filamentary detail is apparent on many different scales at all Galactic latitudes. In high latitude regions, the dust map correlates well with maps of HI emission, but deviations are coherent in the sky and are especially conspicuous in regions of saturation of HI emission toward denser clouds or the formation of H2 in molecular clouds. In contrast, high-velocity HI clouds are deficient in dust emission, also as expected.

To generate the full sky dust maps, we must first remove zodiacal light contamination as well as a possible cosmic infrared background (CIB). This is done via a regression analysis of the 100 micron DIRBE map against the Leiden-Dwingeloo map of HI emission, with corrections for the zodiacal light via a suitable expansion of the DIRBE 25 micron flux. This procedure removes virtually all traces of the zodiacal foreground. For the 100 micron map no signficant CIB is detected, but in the 140 micron and 240 micron maps, where the zodiacal contamination is weaker, we detect the CIB at surprisingly high flux levels of 30 +/- 8 nW/m2/sr at 140\micron, and 16 +/- 3.4 nW/m2/sr at 240 micron (95% confidence), which is an integrated flux approximately twice as large as measured for the Hubble Deep Field.

The primary use of these maps is likely to be as a new estimator of Galactic extinction. To calibrate our maps, we assume a standard reddening law, and use the colors of elliptical galaxies to measure the reddening per flux density of 100 micron emission. We find consistent calibration using the (B-R) color distribution of a sample of 106 brightest cluster ellipticals, as well as a sample of 389 ellipticals with (B-V) and Mg2 index measurements. For the latter sample, we use the correlation of intrinsic B-V versus Mg linestrength to greatly tighten the power of the test. We demonstrate that the new maps are twice as accurate as the older Burstein-Heiles reddening estimates in regions of low and moderate reddening. The maps are expected to be significantly more accurate in regions of high reddening. These dust maps will also be useful for estimating millimeter emission that contaminates CMBR experiments and for estimating soft X-ray absorption."

Cautionary Notes

• After photometrically calibrating the IRAS 100 micron intensity images using COBE/DIRBE, Schlegel et al. (1998) used the far-infrared intensity ratio of 100 um and 240 um to measure the dust temperature and thereby trace the 100 um dust column density (the resulting spatial resolution is a ~few arcminutes). This assumes that one dust temperature describes the bulk of the dust that is blocking optical/near-infrared starlight. The dust column density is in turn converted to the equivalent B-V reddening (E(B-V)) using a calibration that employs the apparent B-V color of elliptical galaxies and their Mg2 line strength (which tightly correlates with intrinsic B-V color). Finally, the extinction is derived from the reddening using a standard extinction law and assuming a constant total-to-selective extinction parameter (Rv = 3.1). Hence, there are several instances in which systematics may creep into the resultant extinction maps. In particular, some studies have suggested that the Schlegel et al. maps overestimate the extinction (or reddening) when the visual extinction is appreciable (cf. Arce & Goodman 1999).

• The method is most accurate when a single dust temperature adequately describes the bulk of the dust that is absorbing/scattering background starlight, conditions typically met in low to moderate reddening regions of the sky (i.e., above and below the Galactic Plane). Within the plane of the Milky Way itself, there is a distinct possibility of either multiple dust temperature distributions or variable grain sizes (most notably, large "fluffy" grains) and the extinction results may be less accurate. The user of this service is advised to use caution when working deep in the Galactic Plane.

• The effective resolution of the extinction maps (~few arminutes like the 100 um image; the dust temperature map is ~1 degree) is considerably larger than molecular cloud structure observed in the interstellar medium. Consequently, one pixel value can smear underlying structure -- which would tend to underestimate the detailed extinction (of dense structures). The extinction maps are a proxy to the average extinction along the line of sight, and not to the detailed extinction for fine structure in the interstellar medium.

• As noted above in the technical summary, the zodiacal foreground emission and bright stars have been removed from the 100 um image, reddening and dust temperature maps. The maps do, however, include the LMC, SMC and M31 extragalactic sources (which were removed by Schlegel et al. in their original data release). The user should be aware that the "reddening" derived from pointings toward these and other bright extragalactic sources do not necessarily reflect the foreground ambient dust (in fact, the dust temperature and reddening is not reliable in these cases).

• Region statistics are carried out on a small number of pixels (radius = 5 arcmin, by default). Consequently, the statistics are subject to small-number statistics.

• For conversion from optical reddening to bandpass-specific extinction, Schlegel et al. assume a single standard extinction law (with Rv = 3.1). This is also true of the Schlafly and Finkbeiner results, though the extinction law is different, and the assumed object template is different.

Links

• DIRBE/IRAS Dust Maps, the Schlegel et al. web site. The downloadable data is stored in regular FITS, but using a polar projection only (type: GLON-ZEA, GLAT-ZEA)

• Foreground Images -- LAMBDA data products, including all-sky H-alpha maps, HI maps (e.g. Leiden-Dwingloo survey).

• Schlegel dust maps (Healpix FITS) and more served from LAMBDA. Note that the data served here are in Healpix format.

• 94GHz Model Dust Map, Using a combination of COBE and IRAS data, Finkbeiner, Davis & Schlegel (1999) derived models for thermal dust emission at microwave and submillimeter frequencies (see Microwave Dust Emission). This map is based on their best-fit two-component Model #8. See more on this extrapolation method here and at LAMBDA.

• Planck All-Sky Maps. These products from Planck mission data include all-sky HEALPix maps of the thermal dust component and dust opacity in E(B-V).

• Other All-Sky Maps. These HEALPix formatted maps show the distribution of structures primarily associated with the interstellar medium in the Milky Way as viewed in a number of surveys, including those of Balmer line emission of ionized hydrogen, 21 cm emission from neutral hydrogen, CO from dense molecular gas, and thermal emission of cold dust that is significant at submillimeter wavelengths.

• NED Dust Extinction Utility

References

• Cardelli, J.A., Clayton, G.C., & Mathis, J. 1989, ApJ 345, 245.

• Fitzpatrick, E.L. 1999, PASP 111, 63.

• Indebetouw, R. et al. 2005, ApJ 619, 931.

• O'Donnell, J.E. 1994, ApJ 422, 1580.

• Schlafly, E.F. & Finkbeiner, D.P. 2011, ApJ 737, 103 (SandF).

• Schlegel, D.J., Finkbeiner, D.P. Davis, M. 1998, ApJ 500, 525 (SFD).

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