GALACTICNUCLEUS: A high angular-resolution JHK_s imaging survey of the Galactic centre

Abstract

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 Δα = 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.

Distance to three molecular clouds in the central molecular zone

“…If the RC stars in our CMDs lie at the GC, then they should scatter around this reddening line. We assumed the extinction curve used in Zoccali et al (2021), A K s = 1.308 ± 0.05 × E(H − K s ) (Minniti et al 2020), and the extinction curve A K s = 1.19 ± 0.04 × E(H − K s ) (Nogueras-Lara et al 2020b). Figure 4 shows the results.…”

“…2. We applied the techniques described in Nogueras-Lara et al (2018b, 2019b, 2020b to distinguish between an individual or a double RC in the data. Namely, we divided the features into colour bins (∼0.1 mag, depending on the density of the points for each region) and fitted a single-and a double-Gaussian model to the K s distribution for each colour bin.…”

“…Therefore, the analysed RC features do not include any significant contribution from stars at different distances given that there are not any points significantly outside the general slope trend in the RC features, within the uncertainties. In this way, the slopes of the RC features indicate the direction of the reddening vector and can be translated into the extinction curve (e.g., Nishiyama et al 2009;Nogueras-Lara et al 2020b). We obtained the reddening free magnitude for the RC features by computing the corresponding linear fit and following the equation:…”

“…where K s0 is the reddening free magnitude, K s and H are the observed magnitudes, (H − K s ) 0 is the intrinsic colour of RC stars (0.07 ± 0.03; e.g., Nogueras-Lara et al 2020a), and A H /A K s is the extinction curve (1.84 ± 0.03, Nogueras-Lara et al 2020b). Figure 3 shows the underlying distributions for the analysed regions where we identified the double RC structure obtained in the previous section.…”

“…Observations along the line-of-sight towards the GC are hampered by extreme extinction (A V 30 mag, A K s 2.5 mag; e.g., Nishiyama et al 2008;Schödel et al 2010;Nogueras-Lara et al 2018a, 2020b and crowding. Given these limitations, the majority of stars that can be observed are red giants.…”