Suchergebnisse

[EN] The center of the Milky Way is the only galactic nucleus and the most extreme astrophysical environment that we can examine on scales of milli- parsecs. It contains the nearest example of a nuclear star cluster (NSC). NSCs have been found at the photometric and dynamical centers of the majority of galaxies in the local Universe. With e↵ective radii of a few parsecs and masses ranging between a few times 106–108 M!, they are among the densest known stellar structures. NSCs possess complex stellar populations and show clear signs of recurrent star formation, with the most recent event having occurred less than 100Myr ago in many of them. Moreover, NSCs can coexist with massive black holes (MBHs) at their centers. The Milky Way's nuclear star cluster (MWNSC) is located at a mere 8 kpc from Earth. While we can only study the integrated light in extragalactic NSCs, we can in the case of the Galactic center (GC) resolve physical scales on the order of a few milliparsecs (mpc) and thus study the properties, kinematics and even dynamics of individual stars. We possess unambiguous evidence for the existence of a 4 ⇥ 106 M! central black hole at the center of the MWNSC, thus making it an ideal target to study the interaction between a dense stellar cluster and a massive black hole. Due to the unique observational challenges – extreme crowding and extinction – the study of the NSC at the GC is confronted with unique difficulties. ; The high extinction limits imaging studies of the stars to the near-infrared (NIR). While this allows us, on the one hand, to use techniques like adaptive optics (AO) to obtain high angular resolution images from the ground, it poses, on the other hand, serious problems for stellar classification because the intrinsic colours of stars are small in the NIR. Because of the observational difficulties, our knowledge about the NSC still contains significant gaps, despite of several decades of observations. Existing work is a↵ected by either one or both of the following problems: (a) Low angular resolution, thus limiting it to the study of the brightest stars that represent less than one percent of the total population of the NSC; (b) limited spectral coverage, thus limiting their use for distinguishing between di↵erent types of stars. Spectroscopic studies with high angular resolution are necessarily limited to small numbers of stars or very small fields. They are therefore mostly limited to the central parsec. This thesis aims at providing answers to unresolved questions in our understanding of the GC, as well as provide new constraints on the structure of the MWNSC by using high resolution images and by improving the analysis. I examine images in a broad span of distances from the center of the Galaxy: from very large distances (⇠ 860 pc x 280 pc) to the innermost arcseconds where the supermassive black hole inhabits. Firstly, I aim to study the size and spatial structure of the MWNSC. I use data and methods that address potential shortcomings in previous work on the topic. I use 0.200 angular resolution Ks data to create a stellar density map in the central 86.4pc x 21 pc at the GC. I include data from selected AO assisted images obtained for the inner parsecs. In addition, I use IRAC/Spitzer MIR images. I model the Galactic bulge and the nuclear stellar disk in order to subtract them from the MWNSC. Finally, I fit a S´ersic model to the MWNSC and investigate its symmetry. I find that the MWNSC is flattened with an axis ratio of q = 0.71 ± 0.06, an e↵ective radius of Re = (5.1 ± 0.6) pc, and a S´ersic index of n = 2.2 ± 0.4. Its major axis may be tilted out of the Galactic plane by up to −10 degree. The distribution of the giants brighter than the Red Clump (RC) is found to be significantly flatter than the distribution of the faint stars. The MWNSC shares its main properties with other extragalactic NSCs found in spiral galaxies. The di↵erences in the structure between bright giants and RC stars might be related to the existence of not completely mixed populations of di↵erent ages. This may hint at recent growth of the MWNSC through star formation or cluster accretion. ; I also revisit the problem of inferring the innermost structure of the MWNSC, to clarify whether it displays a core or a cusp around the central black hole. The existence of dynamically relaxed stellar density cusp in dense clusters around massive black holes is a long-standing prediction of stellar dynamics, but it has so far escaped unambiguous observational confirmation. In order to study the distribution of stars around Sgr A*, I use data obtained with the NACO instrument at the ESO/Very Large Telescope (VLT) and I focus on two di↵erent methods to analyze three di↵erent stellar brightness ranges. I find that a Nuker law provides an adequate description of the nuclear cluster's intrinsic shape (assuming spherical symmetry). I find that the stellar density decreases with a 3D power-law index inside the range ! = 1.1 − 1.4 for distances smaller than the influence radius of Sgr A* (⇠ 3 pc). We can rule out a flat core with high confidence. The cusp is shallower than the predicted one by theory, but it can be explained if the star formation history of the NSC is taken into acount (Baumgardt et al., 2018). Finally, I peek at the very center of the Galaxy by improving the reduction and analysis of existing data. I use holographic technique and bootstrapping analysis. This has led to the detection of NIR emission of Sgr A* in epochs prior to 2003, when the first measurements were taken due to the development of AO. That allows Chen et al. (in preparation) to study the long-term NIR activity and explore the accretion flow onto the black hole in epochs where it had not been detected so far. ; [ES]El centro de la V´ıa L´actea es el ´unico n´ucleo gal´actico y el entorno astrof´ısico m´as extremo que podemos examinar en escalas de milli-parsecs. Contiene el ejemplo m´as cercano de un c´umulo estelar nuclear (o NSC, del ingl´es nuclear star cluster). Los NSCs han sido encontrados en los centros fotom´etricos y din´amicos de la mayor´ıa de las galaxias en el Universo local. Con radios efectivos de unos pocos parsecs y masas que van entre unas pocas veces 106–108 M!, se encuentran entre las estructuras estelares m´as densas conocidas. Adem´as, los NSCs pueden coexistir con agujeros negros masivos (o MBHs, del ingl´es massive black holes) en sus centros. El c´umulo estelar nuclear de la V´ıa L´actea (o MWNSC, del ingl´es Milky Way's NSC) est´a situado a tan s´olo 8 kpc de la Tierra. Mientras que podemos estudiar ´unicamente la luz integrada en NSCs extragal´acticos, en el caso del centro gal´actico (o GC, del ingl´es Galactic center) podemos resolver escalas f´ısicas del orden de unos pocos milli-parsecs (mpc) y, por lo tanto, estudiar las propiedades, cinem´atica, e incluso la din´amica de estrellas individuales. Contamos con evidencia inequ´ıvoca de la existencia de un agujero negro de masa 4⇥106 M! en el centro del MWNSC, convirti´endolo as´ı en un objetivo ideal donde estudiar la interacci´on entre un c´umulo estelar nuclear y un agujero negro masivo. Debido a sus desaf´ıos observacionales ´unicos - extremo hacinamiento y extinci´on - el estudio del NSC en el GC se enfrenta a dificultades ´unicas. La alta extinci´on limita los estudios de im´agenes de las estrellas al infrarrojo (o NIR, del ingl´es near-infrared). Mientras que esto nos permite el uso de t´ecnicas como la ´optica adaptativa (o AO, del ingl´es adaptive optics) para obtener im´agenes con alta resoluci´on angular desde la tierra por un lado, por otro lado tenemos serios problemas para la clasificaci ´on estelar debido a que el color intr´ınseco de las estrellas son peque˜nos en el NIR. A causa de las dificultades observacionales, nuestro conocimiento del NSC contiene todav´ıa brechas significativas, a pesar de varias d´ecadas de observaciones. El trabajo existente est´a afectado por uno o ambos de los siguientes problemas: (a) Baja resoluci´on angular, limit´andolo as´ı al estudio de las estrellas m´as brillantes, que representan menos que el uno por ciento de la poblaci´on total del NSC; (b) cobertura espectral limitada, lo que limita su uso para distinguir entre diferentes tipos de estrellas. Los estudios espectrosc´opicos con alta resoluci´on angular est´an necesariamente restringidos a peque˜nos n´umeros de estrellas o campos muy peque˜nos. Por lo tanto, est´an en su mayor´ıa limitados al parsec central. Esta tesis tiene como objetivo proporcionar respuestas a preguntas sin resolver en nuestra comprensi´on del GC, as´ı como proporcionar nuevas restricciones en la estructura del MWNSC mediante el uso de im´agenes de alta resoluci´on y la mejora del an´alisis. Examino las im´agenes en un amplio rango de distancias desde el centro de la Galaxia: desde distancias muy grandes (⇠ 860 pc x 280 pc) hasta los arco-segundos m´as profundos donde habita el agujero negro supermasivo. ; En primer lugar, mi objetivo es estudiar el tama˜no y la estructura espacial del MWNSC. Utilizo datos y m´etodos que abordan posibles deficiencias en trabajos anteriores sobre el tema. Utilizo 0.200 resoluci´on angular Ks datos para crear un mapa de densidad estelar en el GC en los 86.4 pc x 21 pc centrales. Incluyo datos de im´agenes asistidas por AO obtenidas para los parsecs internos. Adem´as, utilizo im´agenes del medio infrarrojo de IRAC/Spitzer. Modelo el bulbo gal´actico y el disco estelar nuclear para subtraerlos del MWNSC. Finalmente, ajusto un modelo S´ersic a el MWNSC e investigo su simetr´ıa. Encuentro que el MWNSC est´a aplanado con una relaci´on de eje de q = 0.71 ± 0.06, un radio efectivo de Re = (5.1 ± 0.6) pc, y un ´ındice S´ersic de n = 2.2 ± 0.4. Su eje mayor se puede inclinar desde el plano gal´actico hasta −10 grados. La distribuci´on de las estrellas gigantes m´as brillantes que el grupo rojo (o RC, del ingl´es red clump) es significativamente m´as plano que la distribuci ´on de las estrellas d´ebiles. El MWNSC comparte sus principales propiedades con otros NSCs extragal´acticos encontrados en galaxias espirales. Las diferencias en la estructura entre gigantes brillantes y las estrellas RC podr´ıan estar relacionadas con la existencia de poblaciones de diferentes edades no completamente mezcladas. Esto puede indicar un crecimiento reciente del MWNSC a trav´es de la formaci´on de estrellas o la acreci´on de c´umulos. Tambi´en reviso el problema de inferir la estructura m´as interna del MWNSC, para aclarar si muestra un coraz´on o una c´uspide alrededor del agujero negro central. La existencia de una c´uspide de densidad estelar din´amicamente relajada en grupos densos alrededor de agujeros negros masivos es una antigua predicci´on de la din´amica estelar, pero hasta ahora ha escapado de la confirmaci´on observacional sin ambig¨uedades. Para estudiar la distribuci´on de estrellas alrededor de Sgr A*, uso los datos obtenidos con el instrumento NACO en el telescopio ESO/Very Large Telescope (VLT) y me centro en dos m´etodos diferentes para analizar tres rangos de brillo estelar diferentes. Encuentro que una ley de Nuker proporciona una descripci´on adecuada de la forma intr´ınseca del c´umulo nuclear (asumiendo simetr´ıa esf´erica). Encuentro que la densidad estelar disminuye con un ´ındice 3D de ley de potencia dentro del rango ! = 1.1−1.4 para distancias m´as peque˜nas que el radio de influencia de Sgr A* (⇠ 3 pc). Podemos descartar un coraz´on plano con alta confianza. La c´uspide es menos profunda que la pronosticada por la teor´ıa, pero puede explicarse si se tiene en cuenta la historia de formaci´on estelar del NSC (Baumgardt et al., 2018). Finalmente, me asomo al mismo centro de la Galaxia mejorando la reducci´on y al an´alisis de los datos existentes. Gracias a la t´ecnica hologr´afica y el an´alisis bootstrapping, detectamos la emisi´on de NIR de Sgr A* en ´epocas anteriores a 2003, cuando se tomaron las primeras mediciones debido al desarrollo de AO. Esto permite a Chen et al. (in preparation) estudiar la actividad en el NIR a largo plazo y explorar el flujo de acreci´on en el agujero negro en ´epocas donde no se hab´ıa detectado hasta el momento. ; 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]. ; Peer reviewed

Esta tesis tiene como objetivo proporcionar respuestas a preguntas sin resolver en nuestra comprensión del GC, así como proporcionar nuevas restricciones en la estructura del MWNSC mediante el uso de imágenes de alta resolución y la mejora del análisis. Examino las imágenes en un amplio rango de distancias desde el centro de la Galaxia: desde distancias muy grandes (~860 pc x 280 pc) hasta los arco-segundos más profundos donde habita el agujero negro supermasivo. En primer lugar, mi objetivo es estudiar el tamaño y la estructura espacial del MWNSC [1]. Utilizo datos y métodos que abordan posibles deficiencias en trabajos anteriores sobre el tema. Utilizo 0.2'' resolución angular Ks datos para crear un mapa de densidad estelar en el GC en los 86.4 pc x 21 pc centrales. Incluyo datos de imágenes asistidas por AO obtenidas para los parsecs internos. Además, utilizo imágenes del medio infrarrojo de IRAC/Spitzer. Modelo el bulbo galáctico y el disco estelar nuclear para subtraerlos del MWNSC. Finalmente, ajusto un modelo Sérsic a el MWNSC e investigo su simetría. Encuentro que el MWNSC está aplanado con una relación de eje de q=0.71± 0.06, un radio efectivo de Re = (5.1± 0.6) pc, y un índice Sérsic de n = 2.2 ± 0.4. Su eje mayor se puede inclinar desde el plano galáctico hasta -10 grados. La distribución de las estrellas gigantes más brillantes que el grupo rojo (o RC, del inglés red clump) es significativamente más plano que la distribución de las estrellas débiles. El MWNSC comparte sus principales propiedades con otros NSCs extragalácticos encontrados en galaxias espirales. Las diferencias en la estructura entre gigantes brillantes y las estrellas RC podrían estar relacionadas con la existencia de poblaciones de diferentes edades no completamente mezcladas. Esto puede indicar un crecimiento reciente del MWNSC a través de la formación de estrellas o la acreción de cúmulos. También reviso el problema de inferir la estructura más interna del MWNSC, para aclarar si muestra un corazón o una cúspide alrededor del agujero negro central. La existencia de una cúspide de densidad estelar dinámicamente relajada en grupos densos alrededor de agujeros negros masivos es una antigua predicción de la dinámica estelar, pero hasta ahora ha escapado de la confirmación observacional sin ambigüedades. Para estudiar la distribución de estrellas alrededor de SgrA*, uso los datos obtenidos con el instrumento NACO en el telescopio ESO/Very Large Telescope (VLT) y me centro en dos métodos diferentes para analizar tres rangos de brillo estelar diferentes [2, 3]. Encuentro que una ley de Nuker proporciona una descripción adecuada de la forma intrínseca del cúmulo nuclear (asumiendo simetría esférica). Encuentro que la densidad estelar disminuye con un índice 3D de ley de potencia dentro del rango gamma=1.1-1.4 para distancias más pequeñas que el radio de influencia de SgrA* (~3 pc). Podemos descartar un corazón plano con alta confianza. La cúspide es menos profunda que la pronosticada por la teoría, pero puede explicarse si se tiene en cuenta la historia de formación estelar del NSC [4]. Finalmente, me asomo al mismo centro de la Galaxia mejorando la reducción y al análisis de los datos existentes. Gracias a la técnica holográfica y el análisis bootstrapping, detectamos la emisión de NIR de SgrA* en épocas anteriores a 2003, cuando se tomaron las primeras mediciones debido al desarrollo de AO. Esto permite a [5] estudiar la actividad en el NIR a largo plazo y explorar el flujo de acreción en el agujero negro en épocas donde no se había detectado hasta el momento. ; This thesis aims at providing answers to unresolved questions in our understanding of the GC, as well as provide new constraints on the structure the MWNSC by using high resolution images and by improving the analysis. I examine images in a broad span of distances from the center of the Galaxy: from very large distances (~860 pc x 280 pc) to the innermost arcseconds where the supermassive black hole inhabits. Firstly, I aim to study the size and spatial structure of the MWNSC. I use data and methods that address potential shortcomings in previous work on the topic. I use 0:200 angular resolution Ks data to create a stellar density map in the central 86.4pc x 21 pc at the GC. I include data from selected AO assisted images obtained for the inner parsecs. In addition, I use IRAC/Spitzer MIR images. I model the Galactic bulge and the nuclear stellar disk in order to subtract them from the MWNSC. Finally, I fit a Sérsic model to the MWNSC and investigate its symmetry. I find that the MWNSC is flattened with an axis ratio of q = 0:71 _ 0:06, an effective radius of Re = (5:1 _ 0:6) pc, and a Sérsic index of n = 2:2 _ 0:4. Its major axis may be tilted out of the Galactic plane by up to -10 degree. The distribution of the giants brighter than the Red Clump (RC) is found to be significantly flatter than the distribution of the faint stars. 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 diferent ages. This may hint at recent growth of the MWNSC through star formation or cluster accretion. I also revisit the problem of inferring the innermost structure of the MWNSC, to clarify whether it displays a core or a cusp around the central black hole. The existence of dynamically relaxed stellar density cusp in dense clusters around massive black holes is a long-standing prediction of stellar dynamics, but it has so far escaped unambiguous observational confirmation. In order to study the distribution of stars around Sgr A*, I use data obtained with the NACO instrument at the ESO/Very Large Telescope (VLT) and I focus on two different methods to analyze three different stellar brightness ranges. I find that a Nuker law provides an adequate description of the nuclear cluster's intrinsic shape (assuming spherical symmetry). I find that the stellar density decreases with a 3D power-law index inside the range ϒ =1:1 - 1:4 for distances smaller than the influence radius of Sgr A* (~ 3 pc). We can rule out a at core with high confidence. The cusp is shallower than the predicted one by theory, but it can be explained if the star formation history of the NSC is taken into acount (Baumgardt et al., 2018). Finally, I peek at the very center of the Galaxy by improving the reduction and analysis of existing data. I use holographic technique and bootstrapping analysis. This has led to the detection of NIR emission of Sgr A* in epochs prior to 2003, when the first measurements were taken due to the development of AO. That allows Chen et al. (in preparation) to study the long-term NIR activity and explore the accretion flow onto the black hole in epochs where it had not been detected so far. ; Tesis Univ. Granada. ; 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 programmes IDs 183.B-0100 and 089.B-0162

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

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