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Making and Analysing Simulated Images of Globular Clusters for MICADO @ ELT

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Making and Analysing Simulated Images of Globular Clusters for MICADO @ ELT Radboud University Master's Thesis Making and Analysing Simulated Images of Globular Clusters for MICADO @ ELT Author: Supervisor: Luc IJspeert Dr. Søren Larsen A thesis submitted in fulfilment of the requirements for the degree of Master of Science in the Department of Astrophysics Faculty of Science Radboud University July 2019 Contents 1 Abstract 3 2 Introduction4 2.1 A new size class...................................4 2.2 Globular Clusters..................................5 2.3 Goals........................................5 3 Theory 7 3.1 Globular clusters..................................7 3.2 The telescope....................................8 4 Methods 10 4.1 StarPopSim: basis and functionality....................... 10 4.1.1 Generating masses............................. 10 4.1.2 Radial distributions............................ 11 4.1.3 Stellar isochrones.............................. 13 4.1.4 Spectra and remnants........................... 14 4.1.5 Basic functions and input......................... 15 4.1.6 The simulated data............................. 17 4.2 SimCADO...................................... 17 4.2.1 Overview of the simulation........................ 17 4.2.2 Adaptive optics............................... 19 4.2.3 Using SimCADO.............................. 19 4.3 Photometry..................................... 20 4.3.1 Use of IRAF................................ 21 4.3.2 The obtained data............................. 22 4.4 Further analysis................................... 24 4.4.1 Coordinate systems............................ 24 4.4.2 Calibration of the magnitudes....................... 25 4.4.3 Identification and outliers......................... 26 4.4.4 Distance and age estimates........................ 29 5 Results 31 5.1 Error measures................................... 31 5.2 Measured positions................................. 32 5.3 Magnitude accuracy................................ 33 5.4 Distance, age and metallicity........................... 36 6 Conclusion & Discussion 39 6.1 Strange features.................................. 40 6.2 Outlook....................................... 41 References 43 A StarPopSim functions 46 B Cluster input properties 50 C IRAF parameters 52 1 List of abbreviations AGB Asymptotic Giant Branch AO Adaptive Optics CDF Cumulative Distribution Function CMD Colour-Magnitude Diagram ELT Extremely Large Telescope HB Horizontal Branch HLR Half-Light Radius HRD Hertzsprung-Russel Diagram IMF Initial Mass Function LTAO Laser Tomography Adaptive Optics MCAO Multi-Conjugate Adaptive Optics MS Main Sequence NGS Natural Guide Star PDF Probability Density Function PSF Point Spread Function SCAO Single Conjugate Adaptive Optics 2 1 Abstract The Extremely Large Telescope (ELT) is in the process of being built at this moment, and is scheduled for first light in 2025. It will be the largest optical telescope in the world for years to come after that. At this moment, the largest telescopes that cover visible and in- frared wavelengths are a little over ten meters in diameter: the ELT will almost quadruple that. This leads to high expectations, especially in the astronomical community. Some of the expectations of this impressive device are quantified, within the field of globular clus- ters. Models of globular clusters are built to obtain realistic positions and magnitudes that can subsequently be used to make synthetic images of these models. The images are simu- lated with SimCADO, taking into account the atmosphere, the telescope performance and the imaging instrument at first light (MICADO). The generated clusters cover a range of distances of the order of nearby galaxies, and vary in mass and size. The age is set to a typical 10 Gyr for globulars, and the metallicity of about 0.1Zj is also representative of this type of cluster. The images are made with exposure times of 30 minutes and a pixel scale of 4 milli-arcseconds. Photometry is performed with IRAFs implementation of the DAOPhot algorithms by Stetson (1987). The position measurements show good agreement with the model clusters, and outliers can succesfully be identified based on a large deviation of the measured and real positions. The magnitude accuracy is a strong function of both magnitude and radius. Both are not unexpected in crowded stellar fields due to the difficulty of measuring the light from one individual star where many others are present. In the centre of clusters, magnitudes can deviate by many magnitudes, while on the far outskirts, a deviation of up to half a magnitude is more typical. Some strange features are identified in the photometry, in both the difference between measured and real magnitude and in the colours of the stellar populations. The cause remains unknown, although errors in either photometry or image simulations seem most likely at this point. Future projects could expand on the parameters used in this research, while saving on time by using the code written here to produce the globular clusters. Most gain can probably be made by investigating a range of total exposure times, making use of short individual exposures that are added together later. 3 2 Introduction Telescopes have become bigger and more powerful over the course of history and as techno- logical advancements allowed. The two main motivations for building larger main telescope mirrors1 are on the one hand to get a larger light collecting area and thus the capability to see fainter objects, and on the other hand to get better resolving power. The smallest angular separation a telescope can resolve is theoretically related to its size with the simple formula θ ≈ λ©D, where λ is the observed wavelength, D is the diameter of the aperture and θ is the diffraction limit in radians. Unfortunately, if a telescope is placed on the surface of earth, the atmosphere interferes2. This will limit the angular resolution to the seeing at the site of observation, which is generally about 1 arcsecond (also denoted with ¬¬) or 0.4¬¬ in optimal conditions. The diameter at which a telescope reaches this resolution for red light is under half a meter; increasing the aperture size beyond that would not increase the angular resolution. So if we want better observations, and we do, then we might try one of two things: lift our telescopes into space above the atmosphere or find another creative solution. In space there is no atmosphere to inhibit the propagation of our valuable photons. However, it is still very expensive to get large, heavy satellites into orbit or further out into space. Plus, while cost is a factor, it is overshadowed by the immense practical difficulty of getting something so delicate launched safely, after which it would have to be assembled or preferably assemble itself. On top of that, it is a non-trivial, if not impossible task to adapt, upgrade or even repair such a machine. The perfect example here is the largest space telescope in assembly today: the James Webb Space Telescope (JWST) (NASA, website 2019[a]) with its 6.5 meter primary mirror that folds up to fit inside a rocket. The JWST will not orbit the earth; instead it will be positioned in the second Lagrangian point (L2) of the Earth-Sun system. This will mean that it is unfeasible to reach it for servicing missions, something that has happened several times for the Hubble Space Telescope (NASA, website 2019[b]). This brings me back to the point of technological advancements. Fairly recently in the history of telescopes, so called adaptive optics (AO) have become reality. These systems aim to negate the blurring effect of the atmosphere by analysing the wave fronts of the incoming light and changing the shape of one or more of the telescope mirrors to compensate for the deformation. This of course is easier said than done, which is why this is still a field of ongoing development at this time. The results, however, should bring us close to the diffraction limit of the telescope it is applied to. This is why ground based telescopes have grown in size far past the limit imposed on them based on their earthly tethers. 2.1 A new size class There are already several established implementations of AO in observatories like the Keck II telescope and the Very Large Telescope (Wizinowich et al., 2000; Hippler, 2019). In these cases, the AO was added some time after they were taken into operation. The next generation of large ground based telescopes will be built with AO systems as an integral part of their design, enabling diffraction limited observations from first light. The now under construction (European) Extremely Large Telescope (ELT) (Gilmozzi and Spyromilio, 2007) will be the largest among the first extremely large telescopes, alongside the planned Thirty Meter Telescope (Sanders, 2013) and Giant Magellan Telescope (Johns et al., 2012). One of the two first light instruments of ELT is the Multi-adaptive optics Imaging Camera for Deep Observations (MICADO) (Davies et al., 2010) which will get an advanced AO module with several modes of operation. There are numerous interesting science cases to be explored in more depth or for the first time with this new size class. With a 39 meter primary mirror and the AO to fully make use of its resolving power, the prominent science cases are those 1Lenses quickly become impractical above a certain size. 2Pun intended. 4 of high angular resolution. To give an idea of what is to come, I list a few examples from Fiorentino et al. (2017): research on (proto-)planets and sub-stellar objects, stellar kinematics in globular clusters, looking for intermediate
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