Galactic Dust Extinction Service Computes the foreground (Milky Way) extinction for a line of sight and/or region of the sky, returning the corresponding 100 um intensity, reddening maps, statistics and Galactic extinction.

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

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, the method works reasonably well, but with the distinct possibility of either multiple dust temerature distributions or variable grain sizes (most notably, large "fluffy" grains) the extinction results may be in error. The user of this service is advised to use caution when working deep in the Galactic Plane. (see cautionary Notes below).

Technical Summary

"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 um intensity images using COBE/DIRBE, Schlegel et al. (1998) use a technique by which the far infrared intensity ratio between 100 um and 240um is used to measure the dust temperture 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 reddening law and assuming a constant total-to-selective extinction parameter (Rv = 3.1). Hence, there are many instances in which systematics may creep into the resultant extinction maps. In particular, studies suggest that the Schlegel et al maps overestimate the extinction (or reddening) when the visual extinction is appreciable (cf. Arce & Goodman, 1999). (Caution: this link might crash Internet Explorer)   

• The effective resolution of the extinction maps (derived from ~few arminutes for the 100 um image, and ~1 degree for the dust temperature map) is considerably larger than molecular cloud structure observed in the interstellar medium. Consequently, one pixel value represents a smoothing or smearing of the 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 I100, 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 beware 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).

• For conversion from optical reddening to bandpass-specific extinction, the Galactic Dust Extinction Service assumes a standard extintion law (with Rv = 3.1). See above note.

• Region statistics are carried out on a small number of pixels (radius = 5 arcmin, by default) to minimize the smoothing effect due to low spatial resolution. Consequently, the statistics are subject to small number statistical fluctuations.

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)

• CMB Foreground Images -- LAMBDA data products, including allsky H-alpha maps, HI maps, Galactic Reddening (Schlegel et al dust maps)

• Schlegel dust maps (HealPIX FITS) and more served from LAMBDA. Note that the data served here is 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.

• NED Dust Extinction Utility

• Arce & Goodman (1999) (Caution: this link might crash Internet Explorer)    (ApJ, 512, L135), who compared the Schlegel et al. results with independent extinction measurments; their results suggest systematic discrepancy in the Schlegel maps for some regions of the sky (see cautionary notes).

• Atlas of Galactic Neutral Hydrogen by Hartmann & Burton (1997). Interstellar extinction may be derived from the column density of atomic hydrogen; the Dwingloo Survey has ~0.5 degree spatial resolution.