MCAO near-IR photometry of the globular cluster NGC 6388: MAD observations in crowded fields

Abstract

Context. Deep photometry of crowded fields, such as Galactic globular clusters, is severely limited by the resolution of ground-based telescopes. On the other hand, the Hubble Space Telescope does not have the near-infrared (NIR) filters needed to allow large color baselines. Aims. In this work we demonstrate how ground based observations can reach the required resolution when using Multi-Conjugated Adaptive Optic (MCAO) devices in the NIR, such as the experimental infrared camera (MAD) available on the VLT. This is particularly important since these corrections are planned to be available on all ground-based telescopes in the near future. Methods. We demonstrate this by combining the infrared photometry obtained by MAD/VLT with ACS/HST optical photometry of our scientific target, the bulge globular cluster NGC 6388, in which we imaged two fields. In particular, we constructed colormagnitude diagrams with an extremely wide color baseline in order to investigate the presence of multiple stellar populations in this cluster.Results. From the analysis of the external field, observed with better seeing conditions, we derived the deepest optical-NIR CMD of NGC 6388 to date. The high-precision photometry reveals that two distinct sub-giant branches are clearly present in this cluster. We also use the CMD from the central region to estimate the distance [(m − M) • = 15.33] and the reddening (E(B − V) = 0.38) for this cluster. We estimate the age to be (∼11.5 ± 1.5 Gyr). The large relative-age error reflects the bimodal distribution of the SGB stars. Conclusions. This study clearly demonstrates how MCAO correction in the NIR bands implemented on ground based telescopes can complement the high-resolution optical data from HST.

A double stellar generation in the globular cluster NGC 6656 (M 22)

“…In addition, stars from the two groups of different Na content populate two distinct RGB sequences in the U vs. U − B CMD. Moreover, no split in either the MS or SGB has been iden- As in M 22, in NGC 1851 (Milone et al 2008) and NGC 6388 (Piotto 2008;Moretti et al 2009;Piotto 2009) the presence of multiple populations is inferred from a split in the SGB. Unfortunately, for both of these clusters, the available chemical measurements from high resolution spectroscopy are limited to seven RGB stars for NGC 6388 (Carretta et al 2007) and eight RGB stars for NGC 1851 (Yong & Grundahl 2008).…”

An E-ELT case study: colour–magnitude diagrams of an old galaxy in the Virgo cluster

“…The MAD engineering grade detector had a pixel scale of 0.028 arcsec per pixel over a ∼1 arcmin square field. Several studies of Galactic stellar fields have been published from this successful experiment (e.g., Momany et al 2008;Moretti et al 2009;Bono et al 2009;Ferraro et al 2009;Sana et al 2010). The Large Magellanic Cloud (LMC) was the most complex and crowded extra-galactic stellar population studied with MAD (e.g., Campbell et al 2010, andFiorentino et al 2011, submitted).…”

Astrometry with the MCAO instrument MAD. An analysis of single-epoch data obtained in the layer-oriented mode

“…In Table 1, the observations are summarized together with performance indicators such as the FWHM of the fitted PSF and the seeing measured by the DIMM monitor. The same data were also analyzed to derive photometry by Moretti et al (2009). …”