Observations

The observations of the selected globular clusters and stars were obtained in service mode in the period October 2006 – January 2007 with SINFONI in three near-IR bands – J, H, and K. As a starting point we decided to analyse the data in the K-band and from here on we will discuss only this sub-set. The results from the other two filters will be presented in a following study.

The SINFONI K-band grating covers the wavelength range from 1.95 to 2.45 µm at a dispersion of 2.45 Å/pix. The spectral resolution around the centre of the filter is R≃ 3500, as measured from arc lamp frames. Due to the relatively large apparent sizes of LMC GCs on the sky, they could not be observed within one single SINFONI FoV, even the largest one available, covering 8′′

×₈′′. In order to be able to sample at least one effective radius for the majority of the clusters we decided to use a3×₃mosaic of the largest FoV of SINFONI, thus covering the central24′′_×

24′′

for each cluster. Given the goal to sample the total light, and not to get the best possible spatial resolution, all the observations were performed in natural seeing mode, i.e. with no AO correction. The integration time was chosen based on the requirement to achieve a signal-to-noise ratio of at least 50 in the final integrated spectra. The optimal exposure time for one pointing of the mosaic was 150 s, divided into three integrations of 50 s, dithered by 0′′.

25to reject bad pixels. With the short integration time we assure less sensitivity to the very bright and variable near-IR night sky.

As described in Section 2.4, in order to correct for the effects of the night sky, we have to observe empty sky regions very close in time and space to our scientific ob- servations. Fig 4.6 gives an overview of the pattern, which we have used to design one observation block (OB). First we start with 50 s integration on a ”sky” region, then the telescope moves to the centre of the cluster and executes three 50 s integrations, dithered by 1 spatial pixel (0′′.

25) in each direction. Then it goes back to the sky field and does not observe the same point, as the previous ”sky”, but a different point, shifted by 8′′

, and goes again to the cluster, where it makes the next three integrations on the next mosaic pointing (also shifted by 8′′

with respect to the centre of the cluster). This pattern is repeated nine times in total, with the only difference being the chang- ing direction of the 8′′

shift. At the end we observed one more, ”closing” sky frame. After that the OB continues with the bright stars. For them we used the same sequence of one sky frame plus three object integrations. The difference here is the integration time, which was 10 s per individual integration, leading to a total on source time of 30 s. The same exposure time was used for the sky frames, associated with the stars. The sky fields for each cluster are located outside its tidal radius (see Table 4.2) and were checked by eye to be devoid of bright stars.

Figure 4.1: Colour-magnitude diagrams and spatial distribution of the bright stars, selected to be observed around the old and metal poor clusters. Magnitudes and po- sitions are extracted from the 2MASS Point Source Catalogue (Skrutskie et al. 2006). The filled circles represent all the stars with reliable photometry located within the tidal radius of the clusters. The red filled circles represent the stars which are within the SINFONI24′′_×

24′′

FoV, centred on the clusters. The triangular symbols mark the additional bright stars, which were observed.

Figure 4.2: The same figure as Figure 4.1, but for the intermediate age and more metal rich clusters.

NGC 1754 NGC 2005

NGC 1806 NGC 2162 NGC 2173

NGC 2019

Figure 4.3: OpticalR-band images of our cluster sample (taken from DSS). The black boxes and crosses represent our SINFONI24′′_×

24′′

mosaic FoV and centring, respec- tively. The red squares mark the closest of the additional bright stars we have observed around each cluster. The green circles and crosses represent the 20′′

radius aperture and the centre used in the photometry by Pessev et al. (2006). The cyan circles on the images of the intermediate age clusters (bottom line) represent the90′′

radius aperture of Mucciarelli et al. (2006). The size of each image is3.′

NGC 1754 NGC 2005 NGC 2019

NGC 1806 NGC 2162 NGC 2173

Figure 4.4: K-band 2MASS images of our cluster sample. Colour and symbol coding is the same as on Figure 4.3. North is up, east – to the left. Note the very bright star at south-east from the central SINFONI mosaic for NGC 1754. This star was identified as a foreground, Milky Way star, and was excluded from data analysis. Note also the difference between the optical, Figure 4.3, and near-IR images of the clusters. The image sizes are the same.

Figure 4.5: Colour-magnitude diagram for the additional stars and central mosaics of our sample. The photometry of the six GCs (coloured square symbols) comes from the catalogue by Pessev et al. (2006). Data about the additional bright stars (diamond symbols) come from the 2MASS Point Source Catalogue (Skrutskie et al. 2006). Colour coding of the symbols for the bright stars matches the cluster in whose vicinity they were observed.

central parts of each cluster (27 object exposures plus 10 sky exposures) and 4 frames for each additional star (3 star plus one sky). The total execution time for the longest OB did not exceed 1.5 hours. This gives us security that the telluric correction, de- rived from the telluric stars, observed after each OB (see Table 4.4) will be sufficiently accurate. The telluric stars were observed at similar airmass as the clusters.