The slope of the near-infrared extinction law

We determine the slope of the near-infrared extinction power law (A(lambda) proportional to lambda-alpha) for eight regions of the Galaxy between l similar to 27 degrees and similar to 100 degrees. UKIDSS Galactic Plane Survey data are compared, in colour-colour space, with Galactic population synthesis model data reddened using a series of power laws and convolved through the UKIDSS filter profiles. Monte Carlo simulations allow us to determine the best-fitting value of alpha and evaluate the uncertainty. All values are consistent with each other giving an average extinction power law of alpha = 2.14+0.04(-0.05). This is much steeper than most laws previously derived in the literature from colour excess ratios, which are typically between 1.6 and 1.8. We show that this discrepancy is due to an inappropriate choice of filter wavelength in conversion from colour excess ratios to alpha and that effective rather than isophotal wavelengths are more appropriate. In addition, curved reddening tracks, which depend on the spectral type and filter system, should be used instead of straight vectors.