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Thesis Format

Alternative Format

Degree

Doctor of Philosophy

Program

Astronomy

Supervisor

Barmby, Pauline

Abstract

Galaxies are complex systems of stars, gas, dust, and dark matter which evolve over billions of years, and one of the main goals of astrophysics is to understand how these complex systems form and change. Measuring the star formation history of nearby galaxies, in which thousands of stars can be resolved individually, has provided us with a clear picture of their evolutionary history and the evolution of galaxies in general.

In this work, we have developed the first public Python package, SFHPy, to measure star formation histories of nearby galaxies using their colour-magnitude diagrams. In this algorithm, an observed colour-magnitude diagram is modelled as a linear combination of many simple stellar populations with different ages and metallicities. This package treats metallicity as a free parameter, and the uncertainties are estimated by bootstrapping. This algorithm was tested on two different simulated populations and successfully recovered the input parameters. We have also measured the star formation history of the galaxy IC 1613 and found that the measured star formation history agrees with previous measurements for this galaxy.

Observing fainter phases of stellar evolution plays an essential role in the accuracy and precision of star formation history measurements for nearby galaxies. However, fainter sources are more contaminated by unresolved background galaxies. In the second project, we investigated the contamination effect of background galaxies in star formation history measurements by simulating stellar populations of nearby galaxies and a population of unresolved background galaxies. We found that deeper photometry helps to reduce the contamination only for distance moduli larger than (m-M)=23 or 0.4 Mpc. Most of the contamination effect comes from galaxies less than 2 magnitudes brighter than the photometry limit, and the contamination affects the older stellar populations more than younger populations. We also showed that including a model of background galaxies in the fitting process can result in a more accurate and precise measured star formation history compared to removing the contaminated part of the colour-magnitude diagram.

Summary for Lay Audience

Galaxies are complex systems of stars, gas, dust, and dark matter which evolve over billions of years. Galaxies are also very diverse in terms of their mass, size, shape and their contents. One of the main goals of astrophysics is to understand how these complex systems form and change. Nearby galaxies can be studied more thoroughly as their brightest stars can be resolved individually. Measuring the brightness and colour of stars in nearby galaxies helps us to measure the star formation history of these galaxies, which helps us better understand how galaxies evolve.

In this work, we have developed a public software package that can measure the star formation history of nearby galaxies using the measured brightness and colour of its individually resolved stars. This is done by comparing the observed stars with many simulated stars of different ages and stellar compositions. We tested the code on simulated populations, and the code successfully recovered the input parameters. We also used this code to measure the star formation history of a nearby galaxy, IC~1613, and our result matched the previous measurements.

In modelling nearby galaxies, observing the faintest stars is crucial for the accuracy and precision of our results. But the data may be contaminated by many faraway, small and faint background galaxies resembling individual stars. In this work, we also investigated the contamination effect of background galaxies in modelling the star formation history of nearby galaxies. We found a distance within which including faint sources would only increase the contamination effect. We found ranges in the distance, the brightness of stars, and stellar ages, which could be affected by the contamination. These findings emphasize the importance of the contamination effect for future nearby galaxy observations, e.g. with the Nancy Grace Roman Space Telescope.

Creative Commons License

Creative Commons Attribution-Noncommercial 4.0 License
This work is licensed under a Creative Commons Attribution-Noncommercial-Share Alike 4.0 License.

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