Suchergebnisse

This work has made use of BaSTI web tools. The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement n◦ [614922]. This work is based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programmes IDs 195.B-0283 and 091.B-0418. We thank the staff of ESO for their great efforts and helpfulness. F N-L acknowledges financial support from a MECD pre-doctoral contract, code FPU14/01700. We acknowledge support from the State Agency for Research of the Spanish MCIU through the Centre of Excellence Severo Ochoa Award of the Instituto de Astrofísica de Andalucía (CSIC) (SEV-2017-0709). ; The centre of the Milky Way is a fundamental astrophysical target since it is the closest galaxy nucleus (∼ 8 kpc from Earth) and the only one where we can study individual stars with high angular resolution. It is an extreme environment characterised by extreme conditions such as: very high stellar densities (∼ 105−7 pc−3, Launhardt et al., 2002; Schödel et al., 2007, 2018), a tidal field so intense that even massive, young clusters dissolve into the background in less than 10 Myr (Portegies Zwart et al., 2002), high turbulence and temperature of the interstellar medium (Morris & Serabyn, 1996), a strong magnetic field (Crocker, 2012), and intense UV radiation (Launhardt et al., 2002). Despite, or possibly because of these extreme properties, the Galactic centre (GC) is the Galaxy's most prolific massive star forming environment (Mauerhan et al., 2010b; Schödel et al., 2007; Yusef-Zadeh et al., 2009). Moreover, it possesses a supermassive black hole, SgrA*, in its centre. Therefore, it is a unique laboratory to improve our understanding about the inner regions of the galaxies. Nevertheless, in spite of its importance only ∼ 1% of its total area has been studied with the sufficient wavelength coverage and high angular resolution to analyse its stellar structure and population. In addition, the high extinction (and its strong variation on arc second scales) and the extreme source crowding of the central regions of the Milky Way pose great difficulties to the study of the GC. In this way, all the existing surveys that contain the GC (2MASS, UKIDSS, VVV, SIRIUS) fail at covering adequately the GC area. This thesis aims at improving the state of the art in order to enable us to perform a detailed study of the stellar population of the GC. For this, we carried out the GALACTICNUCLEUS survey during this PhD thesis. This is a high angular near infrared (NIR) survey (JHKs bands) with the HAWK-I instrument located at the VLT UT4 (ESO, Paranal-Chile). In this dissertation, I explain the whole process to perform the survey from the observing strategy to the first data release of the catalogue. The development of the data reduction (highly non standard) pipeline and the analysis of the data to obtain accurate photometry and astrometry, lie at the heart of this thesis. To achieve the necessary high angular resolution to analyse the stellar population of the GC, we use the speckle holographic technique described in Schödel et al. (2013), which has been optimised for crowded fields. To assess the data quality, we compare GALACTICNUCLEUS with the up-to-now best surveys for the GC: VVV, SIRIUS and NICMOS (HST) data. ; El centro de la Vía Láctea constituye un objetivo fundamental para la astrofísica, puesto que es el núcleo galáctico más cercano (∼ 8 kpc desde la Tierra) y el único donde es posible estudiar las estrellas individuales con alta resolución angular. Es un entorno extremo caracterizado por condiciones límite como: densidades estelares muy elevadas (∼105−7 pc−3, Launhardt et al., 2002; Schödel et al., 2007, 2018), un campo de marea tan intenso que incluso los cúmulos jóvenes y masivos se disuelven en menos de 10 Myr entre la población estelar circundante (Portegies Zwart et al., 2002), alta turbulencia y temperatura del medio interestelar, un fuerte campo magnético (Crocker, 2012), y una intensa radiación ultravioleta (Launhardt et al., 2002). A pesar de estas condiciones, o precisamente debido a ellas, el centro Galáctico (CG) es el entorno estelar más prolífico en nuestra galaxia para la formación de estrellas masivas (Mauerhan et al., 2010b; Schödel et al., 2007; Yusef-Zadeh et al., 2009). Además, su centro está ocupado por un agujero negro supermasivo, SgrA*. Por lo tanto, es un laboratorio único para mejorar nuestro conocimiento sobre las regiones internas de las galaxias. Sin embargo, a pesar de su extraordinaria importancia, solo ∼ 1% de su área total ha sido estudiada con la suficiente resolución angular y cobertura en diferentes longitudes de onda para analizar su estructura y población estelar. Además, la alta extinción (y su fuerte variación en escalas de segundos de arco) y la extrema densidad de fuentes de las regiones centrales de la Vía Láctea, constituyen serios obstáculos para su estudio. De esta forma, todos los catálogos existentes que contienen el CG (2MASS, UKIDSS, VVV, SIRIUS) fracasan al cubrir este área en detalle. El objetivo fundamental de esta tesis es contribuir a la mejora del estado del arte, para permitir un estudio detallado de la población estelar del bulbo nuclear de la Vía Láctea. Para ello, hemos desarrollado un nuevo catálogo, GALACTICNUCLEUS, durante esta tesis doctoral. Este catálogo se caracteriza por estar diseñado especialmente para la observación del CG en el infrarrojo cercano (bandas JHKs) con alta resolución angular. Los datos fueron tomados con el instrumento HAWK-I situado en el UT4 del VLT (ESO, Paranal-Chile). En esta disertación, se trata detalladamente el proceso completo desde la elaboración de la estrategia de observación hasta la primera publicación de los datos del catálogo. El desarrollo de un procedimiento para la reducción de los datos (altamente no estándar) y el análisis de los mismos para obtener fotometría y astrometría precisas constituyen una parte fundamental de la tesis. Para conseguir la resolución angular necesaria para analizar la población estelar del CG, utilizamos la técnica de holografía descrita en Schödel et al. (2013), que ha sido optimizada para campos con altas densidades estelares. Para comprobar la calidad de los datos, llevamos a cabo una comparación de GALACTICNUCLEUS con los mejores catálogos existentes hasta la fecha para el CG: VVV, SIRIUS y NICMOS (HST). ; Tesis Univ. Granada. ; European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement n◦ [614922] ; MECD pre-doctoral contract, code FPU14/01700 ; State Agency for Research of the Spanish MCIU through the Centre of Excellence Severo Ochoa Award of the Instituto de Astrofísica de Andalucía (CSIC) (SEV-2017-0709)

Context. The environment of Sagittarius A* (Sgr A*), the central black hole of the Milky Way, is the only place in the Universe where we can currently study the interaction between a nuclear star cluster and a massive black hole and infer the properties of a nuclear cluster from observations of individual stars.Aims. This work aims to explore the star formation history of the nuclear cluster and the structure of the innermost stellar cusp around Sgr A*.Methods. We combined and analysed multi epoch high quality AO observations. For the region close to Sgr A* we apply the speckle holography technique to the AO data and obtain images that are >= 50% complete down to K-s approximate to 19 within a projected radius of 5 '' around Sgr A*. We used H-band images to derive extinction maps.Results. We provide K-s photometry for roughly 39 000 stars and H-band photometry for similar to 11 000 stars within a field of about 40 '' x40 '', centred on Sgr A*. In addition, we provide K-s photometry of similar to 3000 stars in a very deep central field of 10 '' x10 '', centred on Sgr A*. We find that the K-s luminosity function (KLF) is rather homogeneous within the studied field and does not show any significant changes as a function of distance from the central black hole on scales of a few 0.1 pc. By fitting theoretical luminosity functions to the KLF, we derive the star formation history of the nuclear star cluster. We find that about 80% of the original star formation took place 10 Gyr ago or longer, followed by a largely quiescent phase that lasted for more than 5 Gyr. We clearly detect the presence of intermediate-age stars of about 3 Gyr in age. This event makes up about 15% of the originally formed stellar mass of the cluster. A few percent of the stellar mass formed in the past few 100 Myr. Our results appear to be inconsistent with a quasi-continuous star formation history. The mean metallicity of the stars is consistent with being slightly super solar. The stellar density increases exponentially towards Sgr A* at all magnitudes between K-s=15-19. We also show that the precise properties of the stellar cusp around Sgr A* are hard to determine because the star formation history suggests that the star counts can be significantly contaminated, at all magnitudes, by stars that are too young to be dynamically relaxed. We find that the probability of observing any young (non-millisecond) pulsar in a tight orbit around Sgr A* and beamed towards Earth is very low. We argue that typical globular clusters, such as they are observed in and around the Milky Way today, have probably not contributed to the nuclear cluster's mass in any significant way. The nuclear cluster may have formed following major merger events in the early history of the Milky Way. © ESO 2020 ; The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement no: [614922]. RS, FNL, EGC, ATGC, and BS acknowledge financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). ATGC, BS, and RS acknowledge financial support from national project PGC2018-095049-B-C21 (MCIU/AEI/FEDER, UE). F. N.-L. gratefully acknowledges funding by the Deutsche Forschungsgemeinschaft (DFG, German Research Foundation) - Project-ID 138713538 - SFB 881 ("The Milky Way System", subproject B8). This work is based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programmes IDs 083.B-0390, 183.B-0100 and 089.B-0162. We thank the staff of ESO for their great efforts and helpfulness. ; Peer reviewed

Dong, Hui et. al. ; Because of strong and spatially highly variable interstellar extinction and extreme source crowding, the faint (K ≥ 15) stellar population in the Milky Way's nuclear star cluster is still poorly studied. RR Lyrae stars provide us with a tool to estimate the mass of the oldest, relative dim stellar population. Recently, we analysed HST/WFC3/IR observations of the central 2.3 × 2.3 arcmin2 of the Milky Way and found 21 variable stars with periods between 0.2 and 1 d. Here, we present a further comprehensive analysis of these stars. The period- luminosity relationship of RR Lyrae is used to derive their extinctions and distances. Using multiple approaches, we classify our sample as 4 RRc stars, 4 RRab stars, 3 RRab candidates and 10 binaries. Especially, the four RRab stars show sawtooth light curves and fall exactly on to the Oosterhoff I division in the Bailey diagram. Compared to the RRab stars reported by Minniti et al., our new RRab stars have higher extinction (AK > 1.8) and should be closer to the Galactic Centre. The extinction and distance of one RRab stars match those for the Milky Way's nuclear star cluster given in previous works. We perform simulations and find that after correcting for incompleteness, there could be not more than 40 RRab stars within the Milky Way's nuclear star cluster and in our field of view. Through comparing with the known globular clusters of the Milky Way, we estimate that if there exists an old, metal-poor (-1.5 < [Fe/H] < -1) stellar population in the Milky Way nuclear star cluster on a scale of 5 × 5 pc, then it contributes at most 4.7 × 105 M⊙, i.e. ~18 per cent of the stellar mass. © 2017 The Authors. ; The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no [614922]. The work is also supported partly by NASA via the grant GO-14589, provided by the Space Telescope Science Institute. FN-L acknowledges financial support from an MECD predoctoral contract, code FPU14/01700. This work uses observations made with the NASA/ESA HST and the data archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc. under NASA contract NAS 5-26555. ; Peer reviewed

The proper motion catalogue is only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/632/A116 ; Proper motion studies of stars in the centre of the Milky Way have typically been 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 comprises 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 the 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 interrelationships between 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 are able to trace low-mass or partially-dissolved young clusters in the GC that can hardly be discovered by any other means. Our pilot study for this work is based on a field in the nuclear bulge associated with HII regions that show the presence of young stars. We have detected the first group of co-moving stars coincident with an H II region. Using colour–magnitude diagrams, we have inferred that the co-moving stars are consistent with the post-main sequence stars with ages of few Myr. 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 Myr.© ESO 2019 ; The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no [614922]. B.Sh., R.S., F.N.L., E.G.C., and A.T.G.C. acknowledge financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (SEV-2017-0709). F. N. L. acknowledges financial support from a predoctoral contract of the Spanish Ministerio de Educacion, Cultura y Deporte, code FPU14/01700. ; Peer reviewed

Due to the extreme extinction towards the Galactic centre (A(v) similar to 30 mag), its stellar population is mainly studied in the near-infrared (NIR) regime. Therefore, a proper analysis of the NIR extinction curve is necessary to fully characterise the stellar structure and population of the inner part of the galaxy. Aims. We studied the dependence of the extinction index (alpha(lambda)) in the NIR on the line of sight, wavelength, and extinction. Methods. We used the GALACTICNUCLEUS imaging survey, a high angular resolution catalogue (0.2 '') for the inner part of the Galaxy in JHK(s) and studied the spatial variation in the extinction index. We also applied two independent methods based on red clump stars to compute the extinction index between different bands and its variation with wavelength. Results. We did not detect any significant line-of-sight or extinction variation in a within the studied region in the nuclear stellar disc. The extinction index between JH and HKs differs by 0.19 +/- 0.05. We obtained mean values for the extinction indices alpha(JH)( )= 2.43 +/- 0.03 and alpha(HK), = 2.23 +/- 0.03. The dependence of the extinction index on the wavelength could explain the differences obtained for a d in the literature since it was assumed constant for the NIR regime.© ESO 2019. ; The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement no [614922]. This work is based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programme IDs 195. B-0283 and 091. B-0418. We thank the sta ff at ESO for their great e ff orts and helpfulness. F. N.-L. acknowledges financial support from a MECD pre-doctoral contract, code FPU14/01700. F. N. acknowledges financial support through Spanish grants ESP2015-65597C4-1-R and ESP2017-86582-C4-1-R (MINECO/FEDER). We acknowledge support from the State Agency for Research of the Spanish MCIU through the Centre of Excellence Severo Ochoa Award of the Instituto de Astrofisica de Andalucia (CSIC) (SEV-2017-0709). N.N. acknowledges support from Sonderforschungsbereich SFB 881 "The Milky Way System" (subproject B8) of the German Research Foundation (DFG). ; Peer reviewed

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 86.4 pc x 21 pc at the Galactic center. We include data from selected adaptive-optics-assisted images obtained for the inner parsecs. In addition, we use Spitzer/IRAC 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 Sersic 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=0.710.10, an effective radius of R-e=(5.1 +/- 1.0) pc, and a Sersic 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 gamma=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.© ESO 2020 ; The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement n degrees[614922]. This work is based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programmes IDs 195.B-0283 and 091.B-0418. We thank the staff of ESO for their great efforts and helpfulness. N.N. acknowledges support by the Sonderforschungsbereich SFB 881 "The Milky Way System" of the German Research Foundation (DFG). F.N.-L. acknowledges financial support from a MECD pre-doctoral contract, code FPU14/01700. We acknowledge support by the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofisica de Andalucia (CSIC) (SEV-2017-0709). ; Peer reviewed

Micron-sized particles (microbeads) dispersed in a suspension of magnetic nanoparticles, i.e., ferrofluids, can be assembled into different types of structures upon application of an externalmagnetic field. This paper is devoted to theoretical modeling of a relative motion of a pair of microbeads (either soft ferromagnetic or diamagnetic) in the ferrofluid under the action of applied uniform magnetic field which induces magnetic moments in the microbeads making them attracting to each other. The model is based on a point-dipole approximation for the magnetic interactions between microbeads mediated by the ferrofluid; however, the ferrofluid is considered to possess an anisotropic magnetic permeability thanks to field-induced structuring of its nanoparticles. The model is tested against experimental results and shows generally better agreement with experiments than the model considering isotropic magnetic permeability of ferrofluids. The results could be useful for understanding kinetics of aggregation of microbeads suspended in a ferrofluid. From a broader perspective, the present study is believed to contribute to a general understanding of particle behaviors in anisotropic media. ; This work was supported by Project No. FIS2013-41821-R (Ministerio de Economía y Competitividad, Spain; cofunded by ERDF, European Union). P.K. acknowledges financial support of the French government, piloted by the National Research Agency (ANR) in the framework of the project Future Investments UCAJEDI, Ref. No. ANR-15-IDEX-01 (ImmunoMag and RheoGels). A.Z. acknowledges the Russian Fund for Basic Research (Project Ref. 16-58-12003), as well as the Program of the Russian Federation Ministry of Science and Education (Project Ref. 3.1438.2017/4.6).

The catalogues described in this paper and specified in Table 1 are only at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/cat/J/A+A/631/A20. The catalogues follow the structure presented in Table 2. ; Context. The high extinction and extreme source crowding of the central regions of the Milky Way are serious obstacles to the study of the structure and stellar population of the Galactic centre (GC). Existing surveys that cover the GC region (2MASS, UKIDSS, VVV, SIRIUS) do not have the necessary high angular resolution. Therefore, a high-angular-resolution survey in the near infrared is crucial to improve the state of the art. Aims. Here, we present the GALACTICNUCLEUS catalogue, a near infrared JHKs high-angular-resolution (0.2″) survey of the nuclear bulge of the Milky Way. Methods. We explain in detail the data reduction, data analysis, calibration, and uncertainty estimation of the GALACTICNUCLEUS survey. We assess the data quality comparing our results with previous surveys. Results. We obtained accurate JHKs photometry for ∼3.3 × 106 stars in the GC detecting around 20% in J, 65% in H, and 90% in Ks. The survey covers a total area of ∼0.3 deg2, which corresponds to ∼6000 pc2. The GALACTICNUCLEUS survey reaches 5σ detections for J ∼ 22 mag, H ∼ 21 mag, and Ks ∼ 21 mag. The uncertainties are below 0.05 mag at J ∼ 21 mag, H ∼ 19 mag, and Ks ∼ 18 mag. The zero point systematic uncertainty is ≲0.04 mag in all three bands. We present colour–magnitude diagrams for the different regions covered by the survey.© ESO 2019 ; The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement n° [614922]. This work is based on observations made with ESO Telescopes at the La Silla Paranal Observatory under programmes IDs 195.B-0283 and 091.B-0418. We thank the staff of ESO for their great efforts and helpfulness. We acknowledge financial support from the State Agency for Research of the Spanish MCIU through the "Center of Excellence Severo Ochoa" award for the Instituto de Astrofísica de Andalucía (SEV-2017-0709). F.N.-L. acknowledges financial support from a MECD pre-doctoral contract, code FPU14/01700. R.S., A.T.G.-C., B.S. acknnowledge financial support from the national grant PGC2018-095049-B-C21 (MCIU/AEI/FEDER, UE). F.N. acknowledges financial support through Spanish grants ESP2015-65597-C4-1-R and ESP2017-86582-C4-1-R (MINECO/FEDER). N.N. acknowledges support by Sonderforschungsbereich SFB 881 "The Milky Way System" (subproject B8) of the German Research Foundation (DFG). ; Peer reviewed

We used 4-yr baseline Hubble Space Telescope/Wide Field Camera 3 IR observations of the Galactic Centre in the F153M band (1.53 μm) to identify variable stars in the central ~2.3 arcmin × 2.3 arcmin field.We classified 3845 long-term (periods from months to years) and 76 short-term (periods of a few days or less) variables among a total sample of 33 070 stars. For 36 of the latter ones, we also derived their periods (< 3 d). Our catalogue not only confirms bright long period variables and massive eclipsing binaries identified in previous works but also contains many newly recognized dim variable stars. For example, we found δ Scuti and RR Lyrae stars towards the Galactic Centre for the first time, as well as one BL Her star (period < 1.3 d). We cross-correlated our catalogue with previous spectroscopic studies and found that 319 variables have well-defined stellar types, such as Wolf-Rayet, OB main sequence, supergiants and asymptotic giant branch stars. We used colours and magnitudes to infer the probable variable types for those stars without accurately measured periods or spectroscopic information. We conclude that the majority of unclassified variables could potentially be eclipsing/ellipsoidal binaries and Type II Cepheids. Our source catalogue will be valuable for future studies aimed at constraining the distance, star formation history and massive binary fraction of the Milky Way nuclear star cluster. © 2017 The Authors. ; The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement no [614922]. The work is also supported by NASA via the grant AR-14589, provided by the Space Telescope Science Institute. FN-L acknowledges financial support from an MECD predoctoral contract, code FPU14/01700. ; Peer reviewed

Context. The study of the inner region of the Milky Way bulge is hampered by high interstellar extinction and extreme source crowding. Sensitive high angular resolution near-infrared imaging is needed to study stellar populations and their characteristics in such a dense and complex environment. Aims. We aim at investigating the stellar population in the innermost Galactic bulge, to study the star formation history in this region of the Galaxy. Methods. We used the 0.2″ angular resolution JHK data from the GALACTICNUCLEUS survey to study the stellar population within two 8.0′×3.4′ fields, about 0.6° and 0.4° to the Galactic north of the Milky Way centre and to compare it with the one in the immediate surroundings of Sagittarius A∗. We also characterise the absolute extinction and the extinction curve of the two fields. Results. The average interstellar extinction to the outer and the inner field is A ∼ 1.20 ± 0.08 mag and ∼1.48 ± 0.10 mag, respectively. We present K luminosity functions that are complete down to at least two magnitudes below the red clump (RC). We detect a feature in the luminosity functions that is fainter than the RC by 0.80 ± 0.03 and 0.79 ± 0.02 mag, respectively, in the K band. It runs parallel to the reddening vector. We identify the feature as the red giant branch bump. Fitting α-enhanced BaSTI luminosity functions to our data, we find that a single old stellar population of ∼12.8 ± 0.6 Gyr and Z = 0.040 ± 0.003 provides the best fit. Our findings thus show that the stellar population in the innermost bulge is old, similar to the one at larger distances from the Galactic plane, and that its metallicity is about twice solar at distances as short as about 60 pc from the centre of the Milky Way, similar to what is observed at about 500 pc from the Galactic Centre. Comparing the obtained metallicity with previous known values at larger latitudes (|b| > 2°), our results favour a flattening of the gradient at |b| < 2°. As a secondary result we obtain that the extinction index in the studied regions agrees within the uncertainties with our previous value of α = 2.30 ± 0.08 that was derived for the very Galactic centre.© ESO 2018. ; The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013) / ERC grant agreement no [614922]. F. N.-L. acknowledges financial support from a MECD pre-doctoral contract, code FPU14/01700. F. N. acknowledges financial support through Spanish grants ESP2015-65597-C4-1-R and ESP2017-86582-C4-1-R (MINECO/FEDER). ; Peer Reviewed

Context. The Galactic centre (GC) is of fundamental astrophysical interest, but existing near-infrared surveys fall short covering it adequately, either in terms of angular resolution, multi-wavelength coverage, or both. Here we introduce the GALACTICNUCLEUS survey, a JHK imaging survey of the centre of the Milky Way with a 0.2 angular resolution. Aim. The purpose of this paper is to present the observations of Field 1 of our survey, centred approximately on SgrA∗ with an approximate size of 7.95 × 3.43. We describe the observational set-up and data reduction pipeline and discuss the quality of the data. Finally, we present the analysis of the data. Methods. The data were acquired with the near-infrared camera High Acuity Wide field K-band Imager (HAWK-I) at the ESO Very Large Telescope (VLT). Short readout times in combination with the speckle holography algorithm allowed us to produce final images with a stable, Gaussian PSF (point spread function) of 0.2 FWHM (full width at half maximum). Astrometric calibration is achieved via the VISTA Variables in the Via Lactea (VVV) survey and photometric calibration is based on the SIRIUS/Infrared Survey Facility telescope (IRSF) survey. The quality of the data is assessed by comparison between observations of the same field with different detectors of HAWK-I and at different times. Results. We reach 5σ detection limits of approximately J = 22, H = 21, and K = 20. The photometric uncertainties are less than 0.05 at J ≤ 20, H ≤ 17, and K ≤ 16. We can distinguish five stellar populations in the colour-magnitude diagrams; three of them appear to belong to foreground spiral arms, and the other two correspond to high- and low-extinction star groups at the GC. We use our data to analyse the near-infrared extinction curve and find some evidence for a possible difference between the extinction index between J - H and H - K. However, we conclude that it can be described very well by a power law with an index of αJHK = 2.30 ± 0.08. We do not find any evidence that this index depends on the position along the line of sight, or on the absolute value of the extinction. We produce extinction maps that show the clumpiness of the ISM (interstellar medium) at the GC. Finally, we estimate that the majority of the stars have solar or super-solar metallicity by comparing our extinction-corrected colour-magnitude diagrams with isochrones with different metallicities and a synthetic stellar model with a constant star formation.© ESO 2018. ; The research leading to these results has received funding from the European Research Council under the European Union's Seventh Framework Programme (FP7/2007-2013)/ERC grant agreement No. [614922].F.N.-L. acknowledges financial support from a MECD pre-doctoral contract, code FPU14/01700. F.N. acknowledges Spanish grants FIS2012-39162-C06-01, ESP2013-47809-C3-1-R and ESP2015-65597-C4-1-R. ; Peer Reviewed