The complex, variable near-infrared extinction towards the Nuclear Bulge

Abstract

Using deep J-, H- and K S -band observations, we have studied the near-infrared extinction of the Nuclear Bulge, and find significant, complex variations on small physical scales. We have applied a new variable near-infrared colour excess (V-NICE) method to measure the extinction; this method allows for variation in both the extinction law parameter a and the degree of absolute extinction on very small physical scales. We see significant variation in both these parameters on scales of 5 arcsec. In our observed fields, representing a random sample of sight lines to the Nuclear Bulge, we measure a to be 2.64 ± 0.52, compared to the canonical 'universal' value of 2. Our measured levels of A KS are similar to previously measured results (1 ≤ A KS ≤ 4.5); however, the steeper extinction law results in higher values for A J (4.5 ≤ A J ≤ 10) and A H (1.5 ≤ A H ≤ 6.5). Only when the extinction law is allowed to vary on the smallest scales can we recover self-consistent measures of the absolute extinction at each wavelength, allowing accurate reddening corrections for field star photometry in the Nuclear Bulge. The steeper extinction law slope also suggests that previous conversions of near-infrared extinction to A V may need to be reconsidered. Finally, we find that the measured values of extinction are significantly dependent on the filter transmission functions of the instrument used to obtain the data. This effect must be taken into account when combining or comparing data from different instruments.