The structure and stellar population of the Nuclear Bulge of the Milky Way

Abstract

[EN]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. The catalogue covers a total region of ∼ 0.3 ◦ (∼ 6,000 pc2 ) located at the nuclear bulge, the inner bulge and the transition region between them. We obtained accurate JHKs photometry of ∼ 3.3×106 stars in the GC detecting around 20 % in J, 65 % in H and 90 % in Ks . The GALACTICNUCLEUS survey reaches 5 σ detection limits 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.

[ES] 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, Sgr A*. 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