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 JHKs 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 AKs ∼ 1.20 ± 0.08 mag and ∼1.48 ± 0.10 mag, respectively. We present Ks 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 Ks 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.
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 × 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 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°. 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.
Context. Due to the extreme extinction towards the Galactic centre (AV ∼ 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 (αλ) 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 JHKs, 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 α 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 αJH = 2.43 ± 0.03 and αHKs = 2.23 ± 0.03. The dependence of the extinction index on the wavelength could explain the differences obtained for αλ in the literature since it was assumed constant for the NIR regime.
We present the results of a large-scale proper motion study of the central ∼36′ × 16′ of the Milky Way, based on our high angular resolution GALACTICNUCLEUS survey (epoch 2015) combined with the HST ...Paschen-
α
survey (epoch 2008). Our catalogue contains roughly 80 000 stars, an unprecedented kinematic dataset for this region. We describe the data analysis and the preparation of the proper motion catalogue. We verify the catalogue by comparing our results with measurements from previous work and data. We provide a preliminary analysis of the kinematics of the studied region. Foreground stars in the Galactic disc can be easily identified via their low reddening. Consistent with previous work and with our expectations, we find that stars in the nuclear stellar disc have a smaller velocity dispersion than inner bulge stars, in particular in the direction perpendicular to the Galactic plane. The rotation of the nuclear stellar disc can be clearly seen in the proper motions parallel to the Galactic plane. Stars on the near side of the nuclear stellar disc are less reddened than stars on its far side. Proper motions enable us to detect co-moving groups of stars that may be associated with young clusters dissolving in the galactic centre that are difficult to detect by other means. We demonstrate a technique based on a density clustering algorithm that can be used to find such groups of stars.
We present improved relative astrometry for stars within the central half parsec of our Galactic Center (GC) based on data obtained with the 10 m W. M. Keck Observatory from 1995 to 2017. The new ...methods used to improve the astrometric precision and accuracy include correcting for local astrometric distortions, applying a magnitude-dependent additive error, and more carefully removing instances of stellar confusion. Additionally, we adopt jackknife methods to calculate velocity and acceleration uncertainties. The resulting median proper motion uncertainty is 0.05 mas yr−1 for our complete sample of 1184 stars in the central 10″ (0.4 pc). We have detected 24 accelerating sources, 2.6 times more than the number of previously published accelerating sources, which extend out to 4″ (0.16 pc) from the black hole. Based on S0-2's orbit, our new astrometric analysis has reduced the systematic error of the supermassive black hole (SMBH) by a factor of 2. The linear drift in our astrometric reference frame is also reduced in the north-south direction by a factor of 4. We also find the first potential astrometric binary candidate S0-27 in the GC. These astrometric improvements provide a foundation for future studies of the origin and dynamics of the young stars around the SMBH, the structure and dynamics of the old nuclear star cluster, the SMBH's properties derived from orbits, and tests of general relativity in a strong gravitational field.
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.
ABSTRACT Recent broadband 34 and 44 GHz radio continuum observations of the Galactic center have revealed 41 massive stars identified with near-IR (NIR) counterparts, as well as 44 proplyd candidates ...within 30″ of Sgr A*. Radio observations obtained in 2011 and 2014 have been used to derive proper motions of eight young stars near Sgr A*. The accuracy of proper motion estimates based on NIR observations by Lu et al. and Paumard et al. have been investigated by using their proper motions to predict the 2014 epoch positions of NIR stars and comparing the predicted positions with those of radio counterparts in the 2014 radio observations. Predicted positions from Lu et al. show an rms scatter of 6 mas relative to the radio positions, while those from Paumard et al. show rms residuals of 20 mas. We also determine the mass-loss rates of 11 radio stars, finding rates that are on average ∼2 times smaller than those determined from model atmosphere calculations and NIR data. Clumpiness of ionized winds would reduce the mass loss rate of WR and O stars by additional factors of 3 and 10, respectively. One important implication of this is a reduction in the expected mass accretion rate onto Sgr A* from stellar winds by nearly an order of magnitude to a value of a few yr−1. Finally, we present the positions of 318 compact radio sources within 30″ of Sgr A*, 45 of which have stellar counterparts in the NIR Ks (2.18 m) and L′ (3.8 m) bands.
We present high-angular-resolution radio observations of the Arches cluster in the Galactic centre, one of the most massive young clusters in the Milky Way. The data were acquired in two epochs and ...at 6 and 10 GHz with the
Karl G. Jansky
Very Large Array. The rms noise reached is three to four times better than during previous observations and we have almost doubled the number of known radio stars in the cluster. Nine of them have spectral indices consistent with thermal emission from ionised stellar winds, one is a confirmed colliding wind binary, and two sources are ambiguous cases. Regarding variability, the radio emission appears to be stable on timescales of a few to ten years. Finally, we show that the number of radio stars can be used as a tool for constraining the age and/or mass of a cluster and also its mass function.
Context. This is the second of three papers that search for the predicted stellar cusp around the Milky Way’s central black hole, Sagittarius A*, with new data and methods. Aims. We aim to infer the ...distribution of the faintest stellar population currently accessible through observations around Sagittarius A*. Methods. We used adaptive optics assisted high angular resolution images obtained with the NACO instrument at the ESO VLT. Through optimised PSF fitting we removed the light from all detected stars above a given magnitude limit. Subsequently we analysed the remaining, diffuse light density. Systematic uncertainties were constrained by the use of data from different observing epochs and obtained with different filters. We show that it is necessary to correct for the diffuse emission from the mini-spiral, which would otherwise lead to a systematically biased light density profile. We used a Paschen α map obtained with the Hubble Space Telescope for this purpose. Results. The azimuthally averaged diffuse surface light density profile within a projected distance of R ≲ 0.5 pc from Sagittarius A* can be described consistently by a single power law with an exponent of Γ = 0.26 ± 0.02stat ± 0.05sys, similar to what has been found for the surface number density of faint stars in Paper I. Conclusions. The analysed diffuse light arises from sub-giant and main-sequence stars with Ks ≈ 19−22 with masses of 0.8−1.5 M⊙. These stars can be old enough to be dynamically relaxed. The observed power-law profile and its slope are consistent with the existence of a relaxed stellar cusp around the Milky Way’s central black hole. We find that a Nuker law provides an adequate description of the nuclear cluster’s intrinsic shape (assuming spherical symmetry). The 3D power-law slope near Sgr A* is γ = 1.13 ± 0.03model ± 0.05sys. The stellar density decreases more steeply beyond a break radius of about 3 pc, which corresponds roughly to the radius of influence of the massive black hole. At a distance of 0.01 pc from the black hole, we estimate a stellar mass density of 2.6 ± 0.3 × 107 M⊙ pc-3 and a total enclosed stellar mass of 180 ± 30 M⊙. These estimates assume a constant mass-to-light ratio and do not take stellar remnants into account. The fact that a flat projected surface density is observed for old giants at projected distances R ≲ 0.3 pc implies that some mechanism may have altered their appearance or distribution.
Context. The existence of dynamically relaxed stellar density cusps in dense clusters around massive black holes is a long-standing prediction of stellar dynamics, but it has so far escaped ...unambiguous observational confirmation. Aims. In this paper we aim to revisit the problem of inferring the innermost structure of the Milky Way’s nuclear star cluster via star counts, to clarify whether it displays a core or a cusp around the central black hole. Methods. We used judiciously selected adaptive optics assisted high angular resolution images obtained with the NACO instrument at the ESO VLT. Through image stacking and improved point spread function fitting we pushed the completeness limit about one magnitude deeper than in previous, comparable work. Crowding and extinction corrections were derived and applied to the surface density estimates. Known young, and therefore dynamically not relaxed stars, are excluded from the analysis. Contrary to previous work, we analyse the stellar density in well-defined magnitude ranges in order to be able to constrain stellar masses and ages. Results. We focus on giant stars, with observed magnitudes K = 12.5−16, and on stars with observed magnitudes K ≈ 18, which may have similar mean ages and masses than the former. The giants display a core-like surface density profile within a projected radius R ≤ 0.3 pc of the central black hole, in agreement with previous studies, but their 3D density distribution is not inconsistent with a shallow cusp if we take into account the extent of the entire cluster, beyond the radius of influence of the central black hole. The surface density of the fainter stars can be described well by a single power-law at R < 2 pc. The cusp-like profile of the faint stars persists even if we take into account the possible contamination of stars in this brightness range by young pre-main sequence stars. The data are inconsistent with a core-profile for the faint stars. Finally, we show that a 3D Nuker law provides a good description of the cluster structure. Conclusions. We conclude that the observed density of the faintest stars detectable with reasonable completeness at the Galactic centre, is consistent with the existence of a stellar cusp around the Milky Way’s central black hole, Sagittarius A*. This cusp is well developed inside the influence radius of Sagittarius A* and can be described by a single three-dimensional power-law with an exponent γ = 1.43 ± 0.02 ± 0.1sys. This corroborates existing conclusions from Nbody simulations performed in a companion paper. An important caveat is that the faint stars analysed here may be contaminated significantly by dynamically unrelaxed stars that formed about 100 Myr ago. The apparent lack of giants at projected distances of R ≲ 0.3 pc (R ≲ 8′′) of the massive black hole may indicate that some mechanism may have altered their distribution or intrinsic luminosity. We roughly estimate the number of possibly missing giants to about 100.
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