The goal of our project is to provide a breakthrough in our knowledge of the stellar structures and stellar populations around the massive black hole at the heart of the Milky Way.

The Galactic Centre: A Unique Astrophysical Laboratory

Galactic nuclei are the dense inner cores of galaxies, where stellar densities can reach values over a million times the one in the Sun´s neighbourhood. In galaxies similar to our Milky Way the nucleus is dominated by a massive black hole, with a mass between a hundred thousand to a few ten million solar masses, surrounded by a nuclear star cluster of a few hundred thousand to a few tens of million solar masses. To explore these environments adequately, we need images and spectroscopy with the highest possible angular resolution. With the currently existing biggest ground-based telescopes (8-10m primary mirrors) we can reach angular resolutions of the order 0.05”. This corresponds to a physical scale of 0.002 pc in the centre of our own Galaxy, but already to 0.2 pc in the case of the Andromeda galaxy, the nearest galaxy similar to the Milky Way. Assuming a modest surface density of one million stars per square parsec, one resolution element thus comprises roughly a few  stars in the Galactic Centre (GC), but a few tens of thousands of  stars in Andromeda´s nucleus. This simple back-of-the-envelope estimate demonstrates in an impressive way that the centre of the Milky Way is a unique place in the Universe where we can explore the interaction between stars, interstellar medium and a massive black hole.

The left panel shows a UVI image of the nucleus of the Andromeda Galaxy (M31) with WFPC2/Hubble Space Telescope (HST). The right panel shows a pseudo-colour near-infrared image of the nucleus of the Milky Way with WFC3/HST. Both images have exactly the same size of 11.65" on each side. This corresponds to a physical length scale of 45 pc in case of the Andromeda galaxy, but of only 0.45 pc in case of the Milky Way, where we can discern hundreds of stars.

The Fingerprint of a Galactic Nucleus

Although the Galactic Centre (GC) is the nearest galactic nucleus and a unique laboratory, our knowledge of its stellar population is severely limited because of the unique combination of observational challenges - extreme extinction that varies on scales of a few arcseconds combined with an extremely high source density and a high dynamic range of the target . Although major astronomical surveys, such as 2MASS, UKIDSS, or VVV cover this region, their angular resolution is >=0.7”. Surveys with the Hubble Space Telescope cover only small regions or have a very limited wavelength coverage.Comparison between Ks band images of the VVV and GALACTICNUCLEUS surveys.

Our survey GALACTICNUCLEUS will fill this blind spot on the sky and provide 0.2” angular resolution images for a region of >3000 pc2 around the Milky Way’s central black hole, Sagittarius A*, plus about 1000 pc2 in several separate fields in the nuclear stellar disk. Because of the high angular resolution, GALACTICNUCLEUS will be about 10 times less confused than any other existing multi-wavelength survey of the region. We will provide JHK photometry for about one million stars. We use a technique called speckle holography to obtain the necessary high angular resolution from the ground.  ESO is supporting this effort with a 160 hour Large Programme with the near-infrared imager HAWK-I at the VLT (LP 195.B-0283; PI: Schödel).



We are acquiring JHK imaging data with HAWK-I/VLT  on a 35.3’ x15.5’ (83 pc x 36 pc) field centred on Sgr A* plus ten individual fields of 7.5’x3.2’ (17.5 pc x 7.5 pc), along the major axis of the nuclear bulge and a Bulge field offset to the Galactic north.


Overview of the GALACTICNUCLEUS survey.
The survey fields are superposed on a Spitzer 3.6 µm image of the Galactic Centre region (Stolovy et al. 1996).

Key to obtaining accurate photometry of the stars in the Galactic Centre is the use of an angular resolution as high as possible. For this purpose we apply the speckle holography algorithm (see, e.g., Primot et al. 1990, Petr et al. 1998, Schödel et al. 2013, and references therein).

To obtain the necessary series of short exposures, we use HAWK-I/VLT in its FASTJITTER mode with an exposure time of 1.2s. This short exposure time forces us to window the detector so that we can only use 1/3 of HAWK-I's field-of-view, corresponding to about 7.5’x3’. The following picture shows a comparison between a field in the Galactic Centre as imaged with WFC3 at the Hubble Space Telescope (HST) and with HAWK-I at the VLT. As we can see, we can obtain data of equivalent angular resolution. The advantage of the HST is a more stable PSF and sensitivity than in our ground-based observations. HAWK-I/VLT, on the other hand, provides us with a much larger instrumental field-of-view, easier access to the observatory, and the possibility to perform observations in the K-band (~2.2 µm), which is crucial to correctly estimate the interstellar extinction in this field.

Comparison between images of the same GC region from WFC3/HST and HAWK-I/VLT speckle holography.

Our main aims are defined by the following questions:

  1. What is the structure, mass and light distribution of the nuclear star cluster )NSC)?
  2. What is the radius of influence of the black hole?
  3. What is the relation between the NSC and the surrounding disk-like part of the nuclear bulge (the so-called nuclear disk? What is the formation history of the NSC?
  4. Did recent star formation take place only in the central parsec or throughout the cluster? Models of the recent star formation history at the Galactic centre predict a large number of young clusters, but we only know of three of them (Arches, Quintuplet, Central Cluster). Where are the missing clusters? Have they already dissolved?
  5. The Hills-mechanism predicts that the central black hole will break up binary stars. Thus, the so-called S-stars, B-dwarfs tightly bound to Sagittarius A*, may have been deposited at their current location. In parallel, hypervelocity stars are ejected from the Milky Way. This requires a source population of B-dwarf binaries distributed throughout the central parsecs. Does it exist?

Speckle Holography

Speckle holography allows one to obtain the best estimate of an astronomical object from long image time series, with exposure times shorter than the coherence time of atmospheric turbulence. The latter is on the order of a few 0.01s at wavelengths around 2 µm, but our experience shows that this requirement can be relaxed substantially, in particular if we do not need to reach the full diffraction limit of a given telescope (or cannot reach it because a camera’s pixel scale is too large). In practice, we use exposure times as long as 1 s.

Each short exposure is the convolution of the astronomical object with the instantaneous point spread function (PSF), which is dominated by the atmospheric turbulence and is continuously changing. In Fourier space this can be expressed as Ij = Pj * O, where Ij is the Fourier transform of the image, Pj the one of the instantaneous PSF and O the one of the astronomical object. The speckle holographic reconstruction is obtained from the series of short exposures by a combination of averaging (to suppress the noise) and division in Fourier space: O = <Ij Ij*>/<Pj Pj*>, where <> denotes the average over all the exposures and the asterisk denotes the complex conjugate. After reverse Fourier transform into image space, the image is convolved with a reconstruction PSF (we typically use a Gaussian) that suppresses noise at high frequencies that cannot be passed by the telescope. Key to high-fidelity image reconstruction is a correct and high signal-to-noise extraction of the instantaneous PSF from each short exposure. We use an improved version of the algorithm suggested by Schödel et al. (2013), which is based on the PSF extraction algorithm used by the software StarFinder (Diolaiti et al. 2000).

Single short exposure of the GC (0.1s exposure time) obtained with NACO/VLT in the Ks-band. The inset in the upper left corner shows the instantaneous PSF extracted from the short exposure. The right panel shows a speckle holographic reconstruction of the field, using over 10,000 short exposures.