GALACTICNUCLEUS: A high-angular-resolution JHKs imaging survey of the Galactic centre III. Evidence for wavelength dependence of the extinction curve in the near-infrared

Context. The characterisation of the extinction curve in the near-infrared (NIR) is fundamental to analysing the structure and stellar population of the Galactic centre (GC), whose analysis is hampered by the extreme interstellar extinction (AV ~ 30 mag) that varies on arc-second scales. Recent studies indicate that the behaviour of the extinction curve might be more complex than previously assumed, pointing towards a variation of the extinction curve as a function of wavelength.
Aims: We aim to analyse the variations of the extinction index, α, with wavelength, line-of-sight, and absolute extinction, extending previous analyses to a larger area of the innermost regions of the Galaxy.
Methods: We analysed the whole GALACTICNUCLEUS survey, a high-angular resolution (~0.2″) JHKs NIR survey specially designed to observe the GC in unprecedented detail. It covers a region of ~6000 pc2, comprising fields in the nuclear stellar disc, the inner bulge, and the transition region between them. We applied two independent methods based on red clump (RC) stars to constrain the extinction curve and analysed its variation superseding previous studies.
Results: We used more than 165 000 RC stars and increased the size of the regions analysed significantly to confirm that the extinction curve varies with the wavelength. We estimated a difference ∆α = 0.21 ± 0.07 between the obtained extinction indices, αJH = 2.44 ± 0.05 and αHKs = 2.23 ± 0.05. We also concluded that there is no significant variation of the extinction curve with wavelength, with the line-of-sight or the absolute extinction. Finally, we computed the ratios between extinctions, AJ/AH = 1.87 ± 0.03 and AH/AKs = 1.84 ± 0.03, consistent with all the regions of the GALACTICNUCLEUS catalogue.