Information on resource 'Reddening and Extinction maps of the Magellanic Clouds'

A catalogue of E(V-I) extinction values is presented for 3174 (LMC) and 693 (SMC) fields within the Magellanic Clouds. The extinction values were computed by determining the (V-I) colour difference of the red clump from Optical Gravitational Microlensing Experiment (OGLE III) observations in the V and I bands and theoretical values for unreddend red clump colours.

The Optical Gravitational Microlensing Experiment (OGLE III) has monitored both Magellanic Clouds in the V and I band. Using the public stellar photometry database of OGLE III in order to derive spatially resolved reddening maps, we subdivide the OGLE III observations of the Large Magellanic Cloud (LMC) in 3174 subfields. The size of the fields varies from 4.5 x 4.5 square arcmin to 36 x 36 square arcmin in order to ensure that each field contains at least 300 stars and no more than 4800 stars. I.e., in dense regions our reddening resolution is higher.

In each of these subfields the mean (V-I) colour of the red clump is determined. A theoretical colour of (V-I):sub:0 = 0.92 mag is adopted for the LMC and subtracted from the measured value. The difference is defined as reddening E(V-I). To provide you directly useable data we calculated the reddening for different filters by adopting Schlegels list of conversion factors (Table 6 in Schlegel et al. 1998, AJ, 500, 525). To use this table the extinction has to be recalculated from E(B-V) to E(V-I). For this purpose we adopted E(V-I) = 1.38 × E(B-V) (Tammann et al. 2003, A&A, 404, 423).

For the Small Magellanic Cloud (SMC) the same procedure is used. The OGLE III data are subdivided into 693 contiguous subfields. The reddening of each subfield is determined adopting a theoretical colour of (V-I)0 = 0.89 mag.

The following table (containing values taken from 1998ApJ...500..525S) shows the laws adopted to compute extinction values in the various bands:

Filter name λeff A/A(V) A/E(B-V) A/E(V-I)
Landolt U 3372 1.66 5.43 3.94
Landolt B 4404 1.32 4.32 3.13
Landolt V 5428 1.02 3.32 2.4
Landolt R 6509 0.82 2.67 1.94
Landolt I 8090 0.59 1.94 1.41
Gunn g 5244 1.07 3.48 2.52
Gunn r 6707 0.79 2.59 1.88
Gunn i 7985 0.61 1.99 1.44
Gunn z 9055 0.47 1.54 1.12
Stromgren u 3502 1.6 5.23 3.79
Stromgren b 4676 1.24 4.05 2.93
Stromgren v 4127 1.39 4.55 3.3
Stromgren beta 4861 1.18 3.86 2.8
Stromgren y 5479 1 3.28 2.37
Sloan u' 3546 1.58 5.16 3.74
Sloan g' 4925 1.16 3.79 2.75
Sloan r' 6335 0.84 2.75 1.99
Sloan i' 7799 0.64 2.09 1.51
Sloan z' 9294 0.45 1.48 1.07

Crossmatching with your tables

If you want to add extinctions to your own tables, use TAP. Our TAP server has the ivo-id ivo://org.gavo.dc/__system__/tap/run. You will need a TAP client to do this. Recent versions of TOPCAT, for example, allow you use TAP. To add a column with the reddening, load the table into TOPCAT and note its id (that's the number in front of the colon in the Table List in TOPCAT's main window; if you've just started TOPCAT, that's 1).

Then, select TAP from the TOPCAT's VO menu, at the bottom ("TAP URL") enter and click "Enter Query".

In the resulting dialog, enter:

SELECT ev_i, mine.* FROM
        mcextinct.exts AS exts
        JOIN tap_upload.t1 AS mine
        ON (1=CONTAINS(POINT('ICRS', mine.raj2000, mine.dej2000),exts.bbox))

If your table has id 15, say, change tap_upload.t1 to tap_upload.t15. You will also need to fix the column names for RA and Dec (written as raj2000 and dej2000 here).

To compute extinctions in specific bands, use the factors given in the A/E(V-I) column in the table above above. To, e.g., add the extinctions in Sloan u' and Sloan z', say:

SELECT ev_i*3.74 as A_sloan_u, ev_i*1.07 as A_sloan_z, mine.* FROM
        mcextinct.exts AS exts
        JOIN tap_upload.t1 AS mine
        ON (1=CONTAINS(POINT('ICRS', mine.raj2000, mine.dej2000),exts.bbox))

For batch processing, similar operations can be done with STILTS or GAVO's tapsh. For the latter, you could use a shell script like the following to add an ev_i column to the file named in the first command line argument argument (note the backup); again, you'll need to fix the column names for RA and Dec:

cp $1 $1.bak
tapsh <<EOF
server ivo://org.gavo.dc/__system__/tap/run
SELECT ev_i, mine.* FROM                mcextinct.exts AS exts          JOIN tap_upload.mytable AS mine                 ON (1=CONTAINS(POINT('ICRS', mine.raj2000, mine.dej2000),exts.bbox))
upload "$1" as mytable
save "$1"

Services defined within this resource descriptor

Tables defined within this resource descriptor

If you use this data, please cite Haschke, R., Grebel, E., Duffau, S., 2011: "New Optical Reddening Maps of the Large and Small Magellanic Clouds", AJ 141, 158, 2011AJ....141..158H

The photometric data of the red clump is provided by the OGLE collaboration.

Part of this research was supported by SFB 881 "The Milky Way System".

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