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.
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).
The GALACTICNUCLEUS Survey
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.