Context
. The distribution of stars around a massive black hole (MBH) has been addressed in stellar dynamics for the last four decades by a number of authors. Because of its proximity, the centre of ...the Milky Way is the only observational test case where the stellar distribution can be accurately tested. Past observational work indicated that the brightest giants in the Galactic centre (GC) may show a density deficit around the central black hole, not a cusp-like distribution, while we theoretically expect the presence of a stellar cusp.
Aims
. We here present a solution to this long-standing problem.
Methods
. We performed direct-summation
N
-body simulations of star clusters around massive black holes and compared the results of our simulations with new observational data of the GC’s nuclear cluster.
Results
. We find that after a Hubble time, the distribution of bright stars as well as the diffuse light follow power-law distributions in projection with slopes of Γ ≈ 0.3 in our simulations. This is in excellent agreement with what is seen in star counts and in the distribution of the diffuse stellar light extracted from adaptive-optics (AO) assisted near-infrared observations of the GC.
Conclusions
. Our simulations also confirm that there exists a missing giant star population within a projected radius of a few arcsec around Sgr A*. Such a depletion of giant stars in the innermost 0.1 pc could be explained by a previously present gaseous disc and collisions, which means that a stellar cusp would also be present at the innermost radii, but in the form of degenerate compact cores.
Context. Nuclear star clusters (NSCs) are located at the photometric and dynamical centers of the majority of galaxies. They are among the densest star clusters in the Universe. The NSC in the ...Milky Way is the only object of this class that can be resolved into individual stars. The massive black hole Sagittarius A* is located at the dynamical center of the Milky Way NSC. Aims. In this work we examine the proper motions of stars out to distances of 1.0 pc from Sgr A*. The aim is to examine the velocity structure of the MW NSC and acquire a reliable estimate of the stellar mass in the central parsec of the MW NSC, in addition to the well-known black hole mass. Methods. We use multi-epoch adaptive optics assisted near-infrared observations of the central parsec of the Galaxy obtained with NACO/CONICA at the ESO VLT. Stellar positions are measured via PSF fitting in the individual images and transformed into a common reference frame via suitable sets of reference stars. Results. We measured the proper motions of more than 6000 stars within ~1.0 pc of Sagittarius A*. The full data set is provided in this work. We largely exclude the known early-type stars with their peculiar dynamical properties from the dynamical analysis. The cluster is found to rotate parallel to Galactic rotation, while the velocity dispersion appears isotropic (or anisotropy may be masked by the cluster rotation). The Keplerian fall-off of the velocity dispersion due to the point mass of Sgr A* is clearly detectable only at R $\la$ 0.3 pc. Nonparametric isotropic and anisotropic Jeans models are applied to the data. They imply a best-fit black hole mass of $3.6^{\rm +0.2}_{-0.4}$ $\times$ 106 $M_{\odot}$. Although this value is slightly lower than the current canonical value of 4.0 $\times$ 106 $M_{\odot}$, this is the first time that a proper motion analysis provides a mass for Sagittarius A* that is consistent with the mass inferred from orbits of individual stars. The point mass of Sagittarius A* is not sufficient to explain the velocity data. In addition to the black hole, the models require the presence of an extended mass of $0.5{-}1.5$ $\times$ 106 $M_{\odot}$ in the central parsec. This is the first time that the extended mass of the nuclear star cluster is unambiguously detected. The influence of the extended mass on the gravitational potential becomes notable at distances $\ga$0.4 pc from Sgr A*. Constraints on the distribution of this extended mass are weak. The extended mass can be explained well by the mass of the stars that make up the cluster.
Context.
The position of the Sun inside the disc of the Milky Way significantly hampers the study of the spiral arm structure given the high amount of dust and gas along the line of sight, and the ...overall structure of this disc has therefore not yet been fully characterised.
Aims.
We aim to analyse the spiral arms in the line of sight towards the Galactic centre (GC) in order to determine their distance, extinction, and stellar population.
Methods.
We use the GALACTICNUCLEUS survey, a JHK
s
high-angular-resolution photometric catalogue (0.2″) for the innermost regions of the Galaxy. We fitted simple synthetic colour-magnitude models to our data via
χ
2
minimisation. We computed the distance and extinction to the detected spiral arms. We also analysed the extinction curve and the relative extinction between the detected features. Finally, we studied extinction-corrected
K
s
luminosity functions (KLFs) to study the stellar populations present in the second and third spiral arm features.
Results.
We determined the mean distances to the spiral arms:
d
1
= 1.6 ± 0.2 kpc,
d
2
= 2.6 ± 0.2 kpc,
d
3
= 3.9 ± 0.3 kpc, and
d
4
= 4.5 ± 0.2 kpc, and the mean extinctions:
A
H
1
= 0.35 ± 0.08 mag,
A
H
2
= 0.77 ± 0.08 mag,
A
H
3
= 1.68 ± 0.08 mag, and
A
H
4
= 2.30 ± 0.08 mag. We analysed the extinction curve in the near-infrared for the stars in the spiral arms and find mean values of
A
J
/
A
H
= 1.89 ± 0.11 and
A
H
/
A
K
s
= 1.86 ± 0.11, in agreement with the results obtained for the GC. This implies that the shape of the extinction curve does not depend on distance or absolute extinction. We also built extinction maps for each spiral arm and find them to be homogeneous and that they might correspond to independent extinction layers. Finally, analysing the KLFs from the second and third spiral arms, we find that they have similar stellar populations. We obtain two main episodes of star formation: > 6 Gyr (∼60 − 70% of the stellar mass), and 1.5 − 4 Gyr (∼20 − 30% of the stellar mass), compatible with previous work. We also detect recent star formation at a lower level (∼10%) for the third spiral arm.
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
≈ 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 ∼11 000 stars within a field of about 40″ × 40″, centred on Sgr A*. In addition, we provide
K
s
photometry of ∼3000 stars in a very deep central field of 10″ × 10″, 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.
Context.
The determination of absolute and relative distances of molecular clouds along the line-of-sight towards the central molecular zone (CMZ) is crucial for inferring its orbital structure and ...dynamics and for understanding star formation in the clouds.
Aims.
Recent work has suggested that the G0.253+0.016 cloud (the Brick) does not belong to the CMZ. This motivated us to crosscheck those results, computing the absolute and relative distances to the Brick as well as to another two molecular clouds (the 50 km s
−1
and the 20 km s
−1
clouds), and discuss their CMZ membership.
Methods.
We used the colour magnitude diagrams
K
s
versus
H
−
K
s
to compare stars detected towards the target clouds with stars detected towards three reference regions in the nuclear stellar disc (NSD) and the Galactic bulge. We used red clump (RC) stars to estimate the distance to each region.
Results.
We found that all the clouds present a double RC feature. Such a double RC has been reported in previous work for the NSD, but not for the bulge adjacent to it. We exclude the possibility that the different RC features are located at significantly different distances. The obtained absolute and relative distances are compatible with the Galactic centre distance (∼8 kpc).
Context. The GC is the closest galactic nucleus, offering the unique possibility of studying the population of a dense stellar cluster surrounding an SMBH. Aims. The goals of this work are to develop ...a new method of separating early and late type stellar components of a dense stellar cluster based on narrow band filters, applying it to the central parsec of the GC, and conducting a population analysis of this area. Methods. We use AO assisted observations obtained at the ESO VLT in the NIR H-band and 7 intermediate bands covering the NIR K-band. A comparison of the resulting SEDs with a blackbody of variable extinction then allows us to determine the presence and strength of a CO absorption feature to distinguish between early and late type stars. Results. This new method is suitable for classifying K giants (and later), as well as B2 main sequence (and earlier) stars that are brighter than 15.5 mag in the K band in the central parsec. Compared to previous spectroscopic investigations that are limited to 13–14 mag, this represents a major improvement in the depth of the observations and reduces the needed observation time. Extremely red objects and foreground sources can also be reliably removed from the sample. Comparison to sources of known classification indicates that the method has an accuracy of better than ~87%. We classify 312 stars as early type candidates out of a sample of 5914 sources. Several results, such as the shape of the KLF and the spatial distribution of both early and late type stars, confirm and extend previous works. The distribution of the early type stars can be fitted with a steep power law ($\beta_{1''}$ = -1.49 ± 0.12), alternatively with a broken power law, $\beta_{1-10''}$ = -1.08 ± 0.12, $\beta_{10-20''}$ = -3.46 ± 0.58, since we find a drop in the early type density at ~10″. We also detect early type candidates outside of 0.5 pc in significant numbers for the first time. The late type density function shows an inversion in the inner 6″, with a power-law slope of $\beta_{R<6''}$ = 0.17 ± 0.09. The late type KLF has a power-law slope of 0.30 ± 0.01, closely resembling the KLF obtained for the bulge of the Milky Way. The early type KLF has a much flatter slope of (0.14 ± 0.02). Our results agree best with an in-situ star formation scenario.
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 ± 0.02
stat
± 0.05
sys
, 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
K
s
≈ 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 ± 0.03
model
± 0.05
sys
. 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 × 10
7
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.
ABSTRACT
We present stellar metallicity measurements of more than 600 late-type stars in the central 10 pc of the Galactic Centre. Together with our previously published KMOS data, this data set ...allows us to investigate, for the first time, spatial variations of the nuclear star cluster’s metallicity distribution. Using the integral-field spectrograph KMOS (VLT), we observed almost half of the area enclosed by the nuclear star cluster’s effective radius. We extract spectra at medium spectral resolution and apply full spectral fitting utilizing the PHOENIX library of synthetic stellar spectra. The stellar metallicities range from M/H = −1.25 dex to M/H > +0.3 dex, with most of the stars having supersolar metallicity. We are able to measure an anisotropy of the stellar metallicity distribution. In the Galactic north, the portion of subsolar metallicity stars with M/H < 0.0 dex is more than twice as high as in the Galactic south. One possible explanation for different fractions of subsolar metallicity stars in different parts of the cluster is a recent merger event. We propose to test this hypothesis with high-resolution spectroscopy and by combining the metallicity information with kinematic data.
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 ± 0.1
sys
. 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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