Peter Linde

The usage of Stromgren photometry in studies of local group dwarf spheroidal galaxies - Application to Draco: a new catalogue of Draco members and a study of the metallicity distribution function and radial gradients

Aims. In this paper we demonstrate how Stromgren uvby photometry can be efficiently used to: 1. identify red giant branch stars that are members in a dwarf spheroidal galaxy; 2. derive age-independent metallicities for the same stars and quantify the associated errors. Methods. Stromgren uvby photometry in a 11 x 22 arcmin field centered on the Draco dwarf spheroidal galaxy was obtained using the Isaac Newton Telescope on La Palma. Members of the Draco dSph galaxy were identified using the surface gravity sensitive c(1) index which discriminates between red giant and dwarf stars. Thus enabling us to distinguish the (red giant branch) members of the dwarf spheroidal galaxy from the foreground dwarf stars in our galaxy. The method is evaluated through a comparison of our membership list with membership classifications in the literature based on radial velocities and proper motions. The metallicity sensitive m(1) index was used to derive individual and age-independent metallicities for the members of the Draco dSph galaxy. The derived metallicities are compared to studies based on high resolution spectroscopy and the agreement is found to be very good. Results. We present metallicities for 169 members of the red giant branch in the Draco dwarf spheroidal galaxy (the largest sample to date). The metallicity distribution function for the Draco dSph galaxy shows a mean [Fe/H] = -1.74 dex with a spread of 0.24 dex. The correlation between metallicity and colour for the stars on the red giant branch is consistent with a dominant old, and coeval population. There is a possible spatial population gradient over the field with the most metal-rich stars being more centrally concentrated than the metal-poor stars. (Less)

An Integrated Model of the European Extremely Large Telescope

Integrated models including optics, structures, control systems, and disturbances are important design tools for Extremely Large Telescopes (ELTs). An integrated model has been formulated for the European ELT and it includes telescope structure, main servos, primary mirror segment control system, wind, optics, wavefront sensors, deformable mirror, and an AO reconstructor and controller. There are three model phases: Initialization, execution of a solver to determine time responses, and post-processing. In near future, the model will be applied for performance studies and design trade-offs for the European ELT.

Clusters as probes of distance, evolution, and chemistry of galaxies

Stellar clusters are highly useful as tools for determination of distances, ages and abundances of heavy elements of galaxies, also at larger distances. Their utility for these purposes has, so far, been severely limited, mainly due to image crowding. The introduction of Extremely Large Telescopes (ELTs) with full adaptive optics (AO) and near diffraction limited performance should imply a drastic improvement concerning the usefulness of clusters and the limiting distances of high quality data. We have made a study of stellar clusters as probes of distance, evolution and chemistry of galaxies at distances from one to twenty Mpc. From data on the Stromgren uvby system, partly from direct measurements taken from the literature, we have synthesized test clusters, one open and one globular, as well as galactic backgrounds. The clusters have been embedded in the backgrounds and located at distances between one and twenty Mpc. Here, vby data have been measured, reduced and analyzed. Color-magnitude diagrams (CMDs), metallicity diagrams (MDs) and luminosity functions (LFs) have been constructed. They have been evaluated absolutely and compared to the corresponding template data. We conclude that with a 50 m AO ELT, for open as well as globular clusters, MDs are of high quality for clusters out to and beyond 5 Mpc and useful out to 10 Mpc. CMDs are of very high quality well beyond 5 Mpc. They are of high scientific value out to and beyond 10 Mpc and valuable for clusters even out to 20 Mpc. LFs are highly informative well beyond 10 Mpc and still rather valuable at 20 Mpc. With sufficient measurement data available, LFs are useful for clusters in galaxies even beyond 20 Mpc.

High time resolution astrophysics and ELTs: Which wavelength?

Observational High Time Resolution Astrophysics differs from conventional astrophysics in regard to the detectors employed which have a time resolution less than that obtainable through CCD with a normal readout τ < a few minutes. This paper looks at the implications for HTRA from extremely large telescopes and specifically, as an exemplar its possible impact on pulsar astrophysics. We demonstrate, by using the derived point-spread-function from models of the Euro50 telescope, the possible effects active and adaptive mirrors have on observing rapidly varying astronomical objects.

ELTs, AO wavelengths and science output

A study is presented of the impact on science data from extremely large telescopes of a transformation of wavelength base for optimization of actuator architecture from 2 200 to 1 250 nm. From the optical path difference (OPD) data for Euro50, we transform to E-ELT OPDs. We compute the corresponding power spectrum, in which we simulate a higher actuator density via high-pass filtering to convert from K to J band actuator-pitch optimization. From the modified power spectrum we derive the correspondingly modified OPDs, PSFs and Strehl ratios. A massive improvement is demonstrated resulting from converting from AO@K to AO@J. This result is followed up by model-based E-ELT imaging in a field in a galactic disc at a distance of 4 Mpc. The improvements in image quality, background and limiting magnitude are very large as are the increases in photometric precision derived from the field imaging. Further, the great science benefit and large opportunities provided by partial AO is demonstrated. In conclusion, while admittedly challenging, pushing AO optimization to wavelengths as short as possible is of prime concern for the science output of ELTs.

ELTs, adaptive optics, and wavelengths

A number of Extremely Large Telescopes for visual-infrared and adjacent wavelengths are in various degrees of progress. All have primary mirrors with equivalent diameters larger than 20 m and are intended for operation with adaptive optics systems. We discuss several ELT observing parameters as functions of wavelength. Stellar energy distributions and atomic line spectra are inspected as are the transmission of the Earths atmosphere, the emissivity of the sky and telescope and instruments as well as detector sensitivity, resolution and signal-to-noise ratio. The spatial resolution depending on the size of the diffraction limited adaptive optics point spread function is discussed. We have evaluated the ELT efficiency in terms of Johnson V to N band photometry, simulating diffraction-limited ELT images of a stellar field at 4 Mpc and 4 kpc, respectively. We conclude that the information content at shorter wavelengths is of dominant nature and that there is every reason to do the utmost to include shorter wavelengths in the AO regime. We propose to adopt a short-wavelength goal of 1 000 nm for first light AO with later updates reaching down to visual wavelengths.

Sampling distant stellar populations on the evolution of galaxies

Modelling and simulation, based on observational data, were used to examine the potential of ELT photometry for studies of the evolution of distant galaxies. An open cluster, a globular cluster and two mixed field populations were employed. Colour-magnitude and metallicity diagrams were examined. For younger populations, excellent turn-off-point age data and abundance data can be obtained even beyond 20Mpc. Higher population age weakens data if not improved with longer exposures. Still, the great potential of ELT photometry for studies of the evolution of galaxies is confirmed. Comments are given on adaptive optics and photometry.

Evolution of distant galaxies from cluster photometry: a program for a 50-m ELT

The formation, shaping and evolution of galaxies are processes of high interest but poorly known. This is true also for our closest galactic neighbours. Of key importance is a representative sample of all major types of galaxies with solid evolutionary parameters. Stellar clusters are excellent probes of galactic evolution, albeit so far useful at smaller distances only, mainly due to the limiting effects of image crowding. With Extremely Large Telescopes (ELTs) with full adaptive optics (AO) and near diffraction limited performance, the effects of image crowding will be drastically reduced. Thus, the excellence of clusters of stars as evolutionary probes can be extended out to cosmological distances. We have studied this extension. With data on the Strömgren uvby system, based on direct measurements taken from the literature, we synthesised an open test cluster as well as a galactic background field. The cluster was embedded in the background and located at distances between one and 500 Mpc. y and b data were measured with a 50 m ELT, reduced and analysed, out to 20 Mpc for individual cluster members and between 10 and 500 Mpc for the properties of the integrated cluster. Based on individual stellar data, colour-magnitude diagrams (CMDs) and luminosity functions (LFs) were constructed and studied for age parameters. For the integral cluster data, we studied the cluster colour index (b-y). We conclude that with a 50 m AO ELT, turn-off point (TOP) determination in CMDs provide ages of excellent quality out to 8 Mpc, of high quality out to 16 Mpc and of reasonable quality to 20 Mpc. At this distance level, the luminosity function provides good support. The integral colour, (b-y), as an age parameter for the clusters, can be determined with high precision out to at least 300 Mpc and with somewhat reduced accuracy to 500 Mpc. Thus, using a 50 m ELT and observing stellar clusters, we can study evolutionary parameters of galaxies out to 20 Mpc and obtain age parameters out to around 500 Mpc from the Sun.

Distributed observing facility for remote access to multiple telescopes

The REMOT (Remote Experiment Monitoring and conTrol) project was financed by 1996 by the European Community in order to investigate the possibility of generalizing the remote access to scientific instruments. After the feasibility of this idea was demonstrated, the DYNACORE (DYNAmically, COnfigurable Remote Experiment monitoring and control) project was initiated as a REMOT follow-up. Its purpose is to develop software technology to support scientists in two different domains, astronomy and plasma physics. The resulting system allows (1) simultaneous multiple user access to different experimental facilities, (2) dynamic adaptability to different kinds of real instruments, (3) exploitation of the communication infrastructures features, (4) ease of use through intuitive graphical interfaces, and (5) additional inter-user communication using off-the-shelf projects such as video-conference tools, chat programs and shared blackboards.

Point object recognition — Some single- and multi-channel applications

Abstract High-precision photometric analysis of images of crowded stellar fields needs sophisticated algorithms. The photometric precision is, however, a strong function of the completeness of source detection. We discuss several aspects of this problem, both with relation to single- and multi-channel applications. In single images, we separate detection into two phases, source image enhancement and actual detection. Comparative tests show a point spread function pixel fitting technique to give the best results. For the fraction of undetectable stars still affecting image statistics, we have developed a technique to extract information about their effect on the faint-end luminosity function. We give two examples of multi-channel image fusion applications: (1) a combination of low- and high-resolution images and (2) removal of undersampling effects by sub-pixel image displacements. Preliminary results show considerable potential for these techniques.