Star formation history and metallicity in the Galactic inner bulge revealed by the red giant branch bump

Abstract

Context. The study of the inner region of the Milky Way's 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 use the 0.2 ′′ angular resolution JHK s 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's 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 Ks ∼ 1.20±0.08 mag and ∼ 1.48±0.10 mag, respectively. We present K s luminosity functions that are complete down to at least 2 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 s -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 increases down to distances as short as about 60 pc from the centre of the Milky Way. 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 the value of α = 2.30 ± 0.08, derived in Nogueras-Lara et al. (2018) for the very Galactic centre. Conclusions.

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

“…An overabundance of bright stars has been found in the Nuclear Bulge (NB) formed by the NSD + the MWNSC compared with the stellar population of the GB/Bar (Blum et al 1996;Philipp et al 1999). This result is a consequence of different star formation histories of both entities since the GB has not experienced any significant amount of recent star formation, contrary to the NB (see Nogueras-Lara et al 2018b). The low angular resolution of previous studies made any potential differences between the distribution of RC stars and brighter stars difficult to study.…”

“…We use K s-band data obtained with the near-infrared camera High Acuity Wide field K-band Imager (HAWK-I) at the ESO VLT from the GALACTICNUCLEUS survey, a JHKs imaging survey of the center of the Milky Way (see Nogueras-Lara et al 2018a, 2019a. We use the central 14 fields of the survey described in detail in Nogueras-Lara et al (2019a) that together compose a region of about 84.4 pc x 21 pc (see the green rectangle in Fig.…”

“…All the parameters that we obtain in the fits are consistent with the final results in Table 2 at the 3σ level. We repeat the procedure to study the effect on the final NSC parameters of the extinction index uncertainty from Nogueras-Lara et al (2018b, 2019c) ( α = 0.08). We use α = 2.38 and α = 2.22, respectively.…”

“…We use a field of view (FOV) of 86 pc x 20.2 pc and we use two different data sets: high resolution data with a point spread function (PSF) of 0.2 full width at half maximum (FWHM) from the GALACTICNUCLEUS survey (Nogueras-Lara et al 2018a, 2019a and even higher resolution data (0.05 ) from Gallego-Cano et al (2018) for the inner parsecs, the most crowded region near Sgr A*. Moreover, we use a larger field from Nishiyama et al (2013) of the central ∼ 860 pc x 280 pc of our Galaxy to explicitly take into account the influence of the different Galactic components that appear superposed on the line-of-sight toward the NSC.…”

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

“…2. Firstly, we divided the RC region in the CMD into small vertical bins to apply the SCIKIT-LEARN python function GaussianMixture (GMM, Pedregosa et al 2011) to compare a one-Gaussian model with a two-Gaussian model to fit the K s stellar distribution for each bin (Nogueras-Lara et al 2018b). Using the Bayesian information Notes.…”