We report on a superdense star-forming region with an effective radius (R e ) smaller than 13 pc identified at z=6.143 and showing a star-formation rate density Σ SF R ∼ 1000 M yr −1 kpc −2 (or conservatively > 300 M yr −1 kpc −2 ). Such a dense region is detected with S/N 40 hosted by a dwarf extending over 440 pc, dubbed D1. D1 is magnified by a factor 17.4(±5.0) behind the Hubble Frontier Field galaxy cluster MACS J0416 and elongated tangentially by a factor 13.2±4.0 (including the systematic errors). The lens model accurately reproduces the positions of the confirmed multiple images with a r.m.s. of 0.35 . D1 is part of an interacting star-forming complex extending over 800 pc. The SED−fitting, the very blue ultraviolet slope (β −2.5, F λ ∼ λ β ) and the prominent Lyα emission of the stellar complex imply that very young (< 10 − 100Myr), moderately dust-attenuated (E(B-V)<0.15) stellar populations are present and organised in dense subcomponents. We argue that D1 (with a stellar mass of 2 × 10 7 M ) might contain a young massive star cluster of M 10 6 M and M U V −15.6 (or m U V = 31.1), confined within a region of 13 pc, and not dissimilar from some local super star clusters (SSCs). The ultraviolet appearance of D1 is also consistent with a simulated local dwarf hosting a SSC placed at z=6 and lensed back to the observer. This compact system fits into some popular globular cluster formation scenarios. We show that future high spatial resolution imaging (e.g., E−ELT/MAORY-MICADO and VLT/MAVIS) will allow us to spatially resolve light profiles of 2-8 pc.

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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.

Context. BL Lac objects are low-power active nuclei exhibiting a variety of peculiar properties caused by the presence of a relativistic jet and orientation effects. Aims. We present adaptive optics near-IR images at high spatial resolution of the nearby BL Lac object PKS 0521-365, which is known to display a prominent jet both at radio and optical frequencies. Methods. The observations were obtained in Ks-band using the ESO multi-conjugated adaptive optics demonstrator at the Very Large Telescope. This allowed us to obtain images with 0.1 arcsec effective resolution. We performed a detailed analysis of the jet and its related features from the near-IR images, and combined them with images previously obtained with HST in the R band and by a re-analysis of VLA radio maps. Results. We find a remarkable similarity in the structure of the jet at radio, near-IR, and optical wavelengths. The broad-band emission of the jet knots is dominated by synchrotron radiation, while the nucleus also exhibits a significant inverse Compton component. We discovered the near-IR counterpart of the radio hotspot and found that the near-IR flux is consistent with being a synchrotron emission from radio to X-ray wavelengths. The bright red object (red-tip), detached but well aligned with the jet, is well resolved in the near-IR and has a linear light profile. Since it has no radio counterpart, we propose that it is a background galaxy not associated with the jet. Conclusions. The new adaptive optics near-IR images and previous observations at other frequencies allow us to study the complex environment around the remarkable BL Lac object PKS 0521-365. These data exemplify the capabilities of multi conjugate adaptive optics observations of extragalactic extended sources.

Aims. We present a study aimed at deriving constraints on star formation at intermediate ages from the evolved stellar populations in the dwarf irregular galaxy UKS 2323-326. These observations were also intended to demonstrate the scientific capabilities of the multi-conjugated adaptive optics demonstrator (MAD) implemented at the ESO Very Large Telescope as a test-bench of adaptive optics (AO) techniques. Methods. We perform accurate, deep photometry of the field using J and K s band AO images of the central region of the galaxy. Results. The near-infrared (IR) colour-magnitude diagrams clearly show the sequences of asymptotic giant branch (AGB) stars, red supergiants, and red giant branch (RGB) stars down to ∼1 mag below the RGB tip. Optical-near-IR diagrams, obtained by combining our data with Hubble Space Telescope observations, provide the best separation of stars in the various evolutionary stages. The counts of AGB stars brighter than the RGB tip allow us to estimate the star formation at intermediate ages. Assuming a Salpeter initial mass function, we find that the star formation episode at intermediate ages produced ∼6 × 10 5 M of stars in the observed region.

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Over the last few years increasing consideration has been given to the study of laser guide stars (LGS) for the measurement of the disturbance introduced by the atmosphere in optical and near‐infrared (near‐IR) astronomical observations from the ground. A possible method for the generation of a LGS is the excitation of the sodium layer in the upper atmosphere at approximately 90 km of altitude. Since the sodium layer is approximately 10 km thick, the artificial reference source looks elongated, especially when observed from the edge of a large aperture. The spot elongation strongly limits the performance of the most common wavefront sensors. The centroiding accuracy in a Shack–Hartmann wavefront sensor, for instance, decreases proportionally to the elongation (in a photon noise dominated regime). To compensate for this effect, a straightforward solution is to increase the laser power, i.e. to increase the number of detected photons per subaperture. The scope of the work presented in this paper is twofold: an analysis of the performance of the weighted centre of gravity algorithm for centroiding with elongated spots and the determination of the required number of photons to achieve a certain average wavefront error over the telescope aperture.

Abstract. MAORY is the multi-conjugate adaptive optics module for the European Extremely Large Telescope.A conceptual design study is in progress and is approaching the final review. A description of the current design and of the expected performance is presented.