Variability of the near-infrared extinction curve towards the Galactic centre

et al. 2020

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Context. The Milky Way nuclear star cluster (MWNSC) is a crucial laboratory for studying the galactic nuclei of other galaxies, but its properties have not been determined unambiguously until now. Aims. We aim to study the size and spatial structure of the MWNSC. Methods. This study uses data and methods that address potential shortcomings of previous studies on the topic. We use 0.2 angular resolution K s data to create a stellar density map in the central 86.4pc x 21pc at the Galactic center. We include data from selected adaptive-optics-assisted images obtained for the inner parsecs. In addition, we use IRAC/Spitzer mid-infrared (MIR) images. We model the Galactic bulge and the nuclear stellar disk in order to subtract them from the MWNSC. Finally, we fit a Sérsic model to the MWNSC and investigate its symmetry. Results. Our results are consistent with previous work. The MWNSC is flattened with an axis ratio of q = 0.71 ± 0.10, an effective radius of R e = (5.1 ± 1.0) pc, and a Sérsic index of n = 2.2 ± 0.7. Its major axis may be tilted out of the Galactic plane by up to −10 degrees. The distribution of the giants brighter than the Red Clump (RC) is found to be significantly flatter than the distribution of the faint stars. We investigate the 3D structure of the central stellar cusp using our results on the MWNSC structure on large scales to constrain the deprojection of the measured stellar surface number density, obtaining a value of the 3D inner power law of γ = 1.38 ± 0.06 sys ± 0.01 stat . Conclusions. The MWNSC shares its main properties with other extragalactic NSCs found in spiral galaxies. The differences in the structure between bright giants and RC stars might be related to the existence of not completely mixed populations of different ages. This may hint at recent growth of the MWNSC through star formation or cluster accretion.

Section: Appendix A1: Extinction and Completenessmentioning

confidence: 99%

New constraints on the structure of the nuclear stellar cluster of the Milky Way from star counts and MIR imaging

Gallego-Cano

et al. 2020

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“…Nevertheless, recent studies have found some evidence of a wavelength-dependence of the extinction index (Nogueras-Lara et al 2018a;Hosek et al 2018). In particular, Nogueras-Lara et al (2019b) analysed the extinction law in the NIR towards the central region of the NSD and found that the extinction index depends on wavelength, but not on the line-ofsight. This dependence has strong implications on the derivation A&A 641, A141 (2020) Fig.…”

Section: Introductionmentioning

confidence: 99%

“…of the structure of the innermost part of the Galaxy and the identification of the stellar type via NIR photometry, since a small change in the extinction index (∼10−15%) leads to a significant change in absolute extinction (∼0.3 mag, e.g. Matsunaga et al 2016;Nogueras-Lara et al 2019b) that might be translated into a biased estimation of the distance.…”

Section: Introductionmentioning

confidence: 99%

“…In this work, we analyse, in detail, the extinction curve towards the GC using JHK s photometry from the GALAC-TICNUCLEUS catalogue. We follow up the study initiated by Nogueras-Lara et al (2019b) and extend it to the whole survey (∼6000 pc 2 ) by increasing the analysed area by a factor of four. Our study includes regions that belong to the GC and the inner bulge of the Galaxy.…”

Section: Introductionmentioning

confidence: 99%

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

Neumayer

et al. 2020

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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.

“…The extreme and spatially highly variable interstellar extinction toward the GC (Nishiyama et al 2008;Fritz et al 2011;Nogueras-Lara et al 2019b) makes it practically impossible to identify young clusters in the near-infrared (NIR) colour-magnitude diagrams (CMDs), which are almost fully degenerate because of reddening (Nogueras-Lara et al 2018). However, stellar proper motions can provide us with an alternative way of detecting young clusters, which are expected to show coherent proper motions.…”

Section: Introductionmentioning

confidence: 99%

First results from a large-scale proper motion study of the Galactic centre

Shahzamanian

et al. 2019

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Proper motion studies of stars in the centre of the Milky Way have been typically limited to the Arches and Quintuplet clusters and to the central parsec. Here, we present the first results of a large-scale proper motion study of stars within several tens of parsecs of Sagittarius A* based on our 0.2 angular resolution GALACTICNUCLEUS survey (epoch 2015) combined with NICMOS/HST data from the Paschen-α survey (epoch 2008). This study will be the first extensive proper motion study of the central ∼ 36 × 16 of the Galaxy, which is not covered adequately by any of the existing astronomical surveys such as Gaia because of its extreme interstellar extinction (AV 30 mag). Proper motions can help us to disentangle the different stellar populations along the line-of-sight and interpret their properties in combination with multi-wavelength photometry from GALACTICNUCLEUS and other sources. It also allows us to infer the dynamics and interrelationship between the different stellar components (Galactic bulge, nuclear stellar disk, nuclear stellar cluster) of the Galactic Centre (GC). In particular, we use proper motions to detect co-moving groups of stars which can trace low mass or partially dissolved young clusters in the GC that can hardly be discovered by any other means. Our pilot study in this work is on a field in the nuclear bulge associated by HII regions that show the presence of young stars. We detect the first group of co-moving stars coincident with an HII region. Using colour-magnitude diagrams, we infer that the co-moving stars are consistent with being the post-main sequence stars with ages of few Myrs. Simulations show that this group of stars is a real group that can indicate the existence of a dissolving or low to intermediate mass young cluster. A census of these undiscovered clusters will ultimately help us to constrain star formation at the GC in the past few ten Myrs.