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The characterisation of the extinction curve in the near infrared (NIR) isfundamental to analyse the structure and stellar population of the Galacticcentre (GC), whose analysis is hampered by the extreme interstellar extinction($A_V\sim 30$ mag) that varies on arc-second scales. Recent studies indicatethat the behaviour of the extinction curve might be more complex thanpreviously assumed, pointing towards a variation of the extinction curve as afunction of wavelength. We aim at analysing the variations of the extinctionindex, $\alpha$, with wavelength, line-of-sight, and absolute extinction,extending previous analysis to a larger area of the innermost regions of theGalaxy. We analysed the whole GALACTICNUCLEUS survey, a high-angular resolution($\sim 0.2''$) $JHK_s$ NIR survey specially designed to observe the GC inunprecedented detail. It covers a region of $\sim 6000$\,pc$^2$, comprisingfields in the nuclear stellar disc, the inner bulge, and the transition regionbetween them. We applied two independent methods based on red clump (RC) starsto constrain the extinction curve and analysed its variation supersedingprevious studies. We used more than 165,000 RC stars and increasedsignificantly the size of the regions analysed to confirm that the extinctioncurve varies with the wavelength. We estimated a difference $\Delta\alpha =0.21\pm0.07$ between the obtained extinction indices, $\alpha_{JH}=2.44\pm0.05$and $\alpha_{HK_s} = 2.23\pm0.05$. We also concluded that there is nosignificant variation of the extinction curve with wavelength, with theline-of-sight or the absolute extinction. Finally, we computed the ratiosbetween extinctions, $A_J/A_H = 1.87\pm0.03$ and $A_{H}/A_{K_s} = 1.84\pm0.03$,consistent with all the regions of the GALACTICNUCLEUS catalogue.

astronomy and astrophysics

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