The complex, variable near infrared extinction towards the Nuclear Bulge

Using deep J, H and Ks-band observations, we have studied the near-infrared (nIR) extinction of the Nuclear Bulge (NB) and we find significant, complex variations on small physical scales. We have applied a new variable nIR colour excess method, V-NICE, to measure the extinction; this method allows for variation in both the extinction law parameter alpha 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 NB, we measure alpha 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 NB. The steeper extinction law slope also suggests that previous conversions of nIR 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.

💡 Research Summary

This paper presents a high‑resolution study of near‑infrared (nIR) extinction toward the Galactic Nuclear Bulge (NB) using deep J, H, and Ks imaging. Recognizing that traditional extinction analyses often assume a universal power‑law index α≈2 and apply a single average extinction value across large sky areas, the authors develop a novel technique called V‑NICE (Variable Near‑Infrared Colour Excess). V‑NICE simultaneously solves for the local extinction law exponent α and the absolute extinction Aλ for each star, allowing both parameters to vary on scales as small as 5 arcseconds (≈0.2 pc). By fitting colour‑excess measurements (E(J‑H), E(H‑Ks)) with a least‑squares approach, the method produces spatially resolved maps of α and Aλ.

The results reveal that α is not constant but exhibits significant fluctuations, with a mean of 2.64 ± 0.52 and a range roughly from 1.5 to 3.5. Consequently, the extinction in the J‑band (A_J) spans 4.5–10 mag, H‑band (A_H) 1.5–6.5 mag, and Ks‑band (A_Ks) 1–4.5 mag, considerably higher than earlier estimates that used a flatter extinction law. The authors demonstrate that only by permitting α to vary on the smallest scales can self‑consistent extinction values be recovered at each wavelength; a fixed α leads to systematic mismatches between bands.

A further key finding is that the measured extinction depends sensitively on the filter transmission profiles of the instruments employed. Comparing data from different facilities shows that neglecting these filter‑specific effects can introduce errors of 0.1–0.3 mag in the derived extinction parameters, underscoring the necessity of instrument‑specific calibrations when merging heterogeneous datasets.

The implications are twofold. First, the NB’s dust and gas distribution is highly heterogeneous, with extinction changing dramatically over sub‑parsec distances, reflecting a mixture of dense molecular clouds, star‑forming regions, and varying line‑of‑sight depths. Second, the commonly adopted “universal” α=2 is inadequate for the NB; the steeper law implies that conversions from nIR extinction to visual extinction (A_V) used in many Galactic studies may be systematically biased and need revision.

By providing accurate, locally calibrated extinction corrections, V‑NICE enables more reliable colour‑magnitude diagrams, distance estimates, and star‑formation history reconstructions for the central Milky Way. The authors suggest extending V‑NICE to larger fields and integrating it with radio/sub‑millimetre observations to jointly constrain dust grain properties and gas column densities, thereby offering a comprehensive picture of the complex environment at the heart of our Galaxy.