------------------------------------------------------------------------------- David Schlegel, Princeton, 16 April 1999 Doug Finkbeiner, Berkeley ------------------------------------------------------------------------------- This directory contains data files for the following paper: "Maps of Dust IR Emission for Use in Estimation of Reddening and CMBR Foregrounds", Schlegel, D., Finkbeiner, D., & Davis, M., ApJ, 1998, 500, 525. This paper can be downloaded from one of the following sites: http://astro.princeton.edu/~schlegel/dust/index.html http://astro.berkeley.edu/dust/index.html This set of self-contained C routines is for reading the SFD dust maps. The maps are simple FITS files in Lambert (polar) projection, with one file for each hemisphere of the projection. The simplest interface is with "dust_getval". A more direct interface for reading Lambert-projection files is lambert_getval. You may link directly to dust_getval() in C (as done in dust_getval.c), or link to fort_lambert_getval() in Fortran (as done in testf_lambert.f). Report problems with the C interface to David Schlegel at "schlegel@astro.princeton.edu". Compilation ----------- Compile everything with "make all", which will put the binaries in the directory specified by the variable "DEST" in "Makefile". The "Makefile" is currently set up for compilation on a Sun, and should be edited for other platforms. Available maps -------------- The following maps are available by setting "map" in the call to "dust_getval": File names MAP --------------- ---- ---------------------------------------------------- SFD_i100*.fits I100 Reprocessed IRAS 100-micron map in MJy/sr SFD_xmap*.fits X X-map, temperature-correction factor SFD_temp*.fits T Temperature map in degrees Kelvin for n=2 emissivity SFD_dust*.fits Ebv E(B-V) in magnitudes SFD_mask*.fits mask 8-bit mask The default is to read the 4096^2 images, with a scale of approximately (2.37 arc min)^2 per pixel. Calling sequence ---------------- dust_getval gall galb map=map infile=infile outfile=outfile \ interp=interp noloop=noloop verbose=verbose ipath=ipath Simple examples --------------- Read the reddening value E(B-V) at Galactic (l,b)=(121,-21.5), interpolating from the nearest 4 pixels, and output to the screen: % dust_getval 121 -21.5 interp=y In this example, the returned value should be E(B-V) = 0.2926 mag. You may wish to know if there are any mask bits set for this position: % dust_getval 121 -21.5 map=mask 121.000 -21.500 2hcons OK OK OK OK big_obj OK This will report "2hcons", signifying that IRAS only scanned this position twice (out of a possible 3 HCONs). Also, the "big_obj" flag is reported as set, signifying that this position falls near the LMC, SMC or M31 (the latter in this case). Read the temperature map at positions listed in the file "dave.in", interpolating from the nearest 4 pixels, and output to file "dave.out". The path name for the temperature maps is "/u/schlegel/". % dust_getval map=T ipath=/u/schlegel/ interp=y \ infile=dave.in outfile=dave.out Optional inputs --------------- Either the coordinates "gall" and "galb" must be set, or these coordinates must exist in the file "infile". Output is written to standard output or the file "outfile". gall: Galactic longitude(s) in degrees galb: Galactic latitude(s) in degrees map: Set to one of the following (default is "Ebv"): I100: 100-micron map in MJy/sr X : X-map, temperature-correction factor T : Temperature map in degrees Kelvin for n=2 emissivity Ebv : E(B-V) in magnitudes mask: Mask values infile: If set, then read "gall" and "galb" from this file outfile: If set, then write results to this file interp: Set this flag to "y" to return a linearly interpolated value from the 4 nearest pixels. This is disabled if map=='mask'. noloop: Set this flag to "y" to read all values at once by reading the entire image into memory at once (or the smallest possible subimage. This is a faster option for reading a large number of values, but requires reading up to a 64 MB image at a time into memory. verbose: Set this flag to "y" for verbose output, printing pixel coordinates and map values ipath: Path name for dust maps; default to path set by the environment variable $DUST_DIR/maps, or to the current directory. Mask values ----------- The mask has 8 bits containing information about the processing history for each position on the sky. The lowest two bits contain a 2-bit integer of the number of HCON's in the IRAS/ISSA plates (0 through 3). The other bits are set to 1 if the following conditionals are true: Bit Name Comments --- --------- ------------------------------------------------------- 0-1 Number of HCON's in IRAS/ISSA plates 2 'asteroi' Asteroid has been removed from one HCON 3 'glitch ' Glitch has been removed from one HCON 4 'source ' Source (star or galaxy) has been removed 5 'no_list' No point source list available for this part of the sky 6 'big_obj' Position near LMC, SMC, or M31; no sources removed 7 'no_IRAS' No IRAS data; filled with DIRBE data A call to "dust_getval" with "map=mask" returns the number of HCON's followed by the Name of a bit if it is set, or "OK" if not set. Data format ----------- All maps are stored as FITS files, in pairs of 4096x4096 (or MxM) pixel Lambert projections. The NGP projection covers the northern Galactic hemisphere, centered at b=+90 deg, with latitude running clockwise. The SGP projection covers the southern Galactic hemisphere, centered at b=-90 deg, with latitude running counter-clockwise. Galactic coordinates (l,b) are converted to pixel positions (x,y) via x = 2048 SQRT {1 - n sin(b)} cos(l) + 2047.5 y = - 2048 n SQRT{1 - n sin(b)} sin(l) + 2047.5 where n=+1 for the NGP, and n=-1 for the SGP. Pixel numbers are zero-indexed, with the center of the lower left pixel having position (x,y)=(0,0). These Lambert projections are minimally distorted at high Galactic latitudes, with the distortion approaching 40% at b=0 deg. The pixel size of (2.372 arcmin)^2 well-samples the FWHM of 6.1 arcmin. This projection is equivalent to the zenithal equal area (ZEA) projection of Griesen & Calabretta (1996). As of April 1999, our FITS header cards conform to their standard. Caveats ------- The caveats to using these maps to measure reddening or extinction can be summarized as follows: (1) Every effort has been made to remove both extragalactic sources and unresolved (Galactic) sources from the dust maps at |b| > 5 deg, and unconfused regions at lower latitudes. Some sources will remain owing to confusion or unusual FIR colors. No sources fainter than 0.6 Jy at 60 microns are removed, although these are not significant contaminants. (2) The IRAS satellite did not scan a strip amounting to 3% of the sky. In addition, we remove a circle of radius $2\degree$ centered at (l,b)=(326.28 deg,+51.66 deg) contaminated by Saturn. These regions are replaced with DIRBE data, and have no point sources removed. (3) The 100 micron passband for the DIRBE satellite is somewhat different than the 100 micron passband on IRAS. For a 20 K blackbody, the difference is small, but the color temperature of sources is sometimes very different than that of the cirrus. This sometimes results in artifacts in the temperature-correction term (on scales of one degree). Our remedy is described in Section 3.3 of the text. (4) The LMC, SMC and M31 are not removed from the maps, nor are sources within their Holmberg radius. Accurate reddenings THROUGH these galaxies is not possible since their temperature structure is not sufficiently resolved by DIRBE. Typical reddenings TOWARDS these galaxies is estimated from the median dust emission in surrounding annuli: E(B-V)=0.075 mag for the LMC, 0.037 mag for the SMC, and 0.062 mag for M 31. (5) At low Galactic latitudes (|b| < 5 deg), most contaminating sources have not been removed from the maps, and the temperature structure of the Galaxy is not well resolved. Furthermore, no comparisons between our predicted reddenings and observed reddening have been made in these regions. Thus, our predicted reddenings here should not be trusted, though inspection of the maps might be of some use. (6) The normalization of the dust column density to reddening has a formal uncertainty of 10\%. (7) Should one wish to change the DIRBE/IRAS dust zero-point to be consistent with the Burstein-Heiles maps, we suggest subtracting 0.020 mag in E(B-V). Extinction in Different Bandpasses ---------------------------------- Assuming an R_V=3.1 extinction curve, the dust maps should be multiplied by the value in the final column to determine the extinction in a given passband. The standard optical-IR bandpasses are represented by the CTIO and UKIRT transmission curves. For further details, see Appendix B of the text. Filter name LamEff A/A(V) A/E(B-V ---------------- ------ ------- ------- Landolt U 3372 1.664 5.434 Landolt B 4404 1.321 4.315 Landolt V 5428 1.015 3.315 Landolt R 6509 0.819 2.673 Landolt I 8090 0.594 1.940 CTIO U 3683 1.521 4.968 CTIO B 4393 1.324 4.325 CTIO V 5519 0.992 3.240 CTIO R 6602 0.807 2.634 CTIO I 8046 0.601 1.962 UKIRT J 12660 0.276 0.902 UKIRT H 16732 0.176 0.576 UKIRT K 22152 0.112 0.367 UKIRT L' 38079 0.047 0.153 Gunn g 5244 1.065 3.476 Gunn r 6707 0.793 2.590 Gunn i 7985 0.610 1.991 Gunn z 9055 0.472 1.540 Spinrad R 6993 0.755 2.467 APM b_J 4690 1.236 4.035 Stromgren u 3502 1.602 5.231 Stromgren b 4676 1.240 4.049 Stromgren v 4127 1.394 4.552 Stromgren beta 4861 1.182 3.858 Stromgren y 5479 1.004 3.277 Sloan u' 3546 1.579 5.155 Sloan g' 4925 1.161 3.793 Sloan r' 6335 0.843 2.751 Sloan i' 7799 0.639 2.086 Sloan z' 9294 0.453 1.479 WFPC2 F300W 3047 1.791 5.849 WFPC2 F450W 4711 1.229 4.015 WFPC2 F555W 5498 0.996 3.252 WFPC2 F606W 6042 0.885 2.889 WFPC2 F702W 7068 0.746 2.435 WFPC2 F814W 8066 0.597 1.948 DSS-II g 4814 1.197 3.907 DSS-II r 6571 0.811 2.649 DSS-II i 8183 0.580 1.893