By Elijah Bernstein-Cooper, February 25, 2015, 0 comments.

Jouni Kainulainen shared 2MASS Av image with us today. He provided the following warning:

Attached is a near-IR-derived visual extinction map (A_V) that more-or-less stitches together the maps of California, Taurus, and Perseus from Kainulainen et al. (2009). It is not quite a perfect coverage, but you can have a look and decide if it is adequate.

I did not quite get the full picture of what you are planning to do… But in any case, I think I should emphasise that maps such as these are derived by comparing stellar colours to some reference field. In other words, the maps measure the relative extinction compared to that reference field, not absolute extinction (this is, to my knowledge, true for all near-IR maps out there).

Since this kind of large maps span several degrees on the sky, the reference field colours are likely to change within the map. Therefore, I have used several reference fields and interpolated between them to compute the “zero-point” of the extinction in the map.

Whether the above affects your analysis or not, I don’t now, but I think it is an important point to remember when considering the ability of the map to trace various gas components.

Below is a screenshot of the Av image with the Planck Av 4 and 8 mag contours.

K09 Av