Excavating the fossil record of spiral galaxies

Peterken, Thomas (2020) Excavating the fossil record of spiral galaxies. PhD thesis, University of Nottingham.

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Despite spiral galaxies being extremely common in our local neighbourhood, their formation, growth, and dynamics are not fully understood. However, the spatial variation in the properties of stellar populations contained within spiral galaxies are expected to bear the imprint of some of the past and present physical processes driving global and local structure and dynamics. Combining modern integral-field spectroscopic galaxy surveys with spectral fitting methods offers an unprecedented opportunity to peer into the stellar population "fossil record" and how it varies between and within spiral galaxies. By obtaining a full star-formation history at every location, it is possible to construct images denoting the spatial distribution of stars of different ages across the galaxy. In this thesis we explore how such a "time slicing" technique can be applied to data from the SDSS-IV MaNGA survey, and show that this approach can provide insights into the formation and the internal structure of spiral galaxies.

While the defining features of spiral galaxies are the beautiful arms that they display, the exact nature of such structure is still an open question. It has been widely assumed that spiral arms in "grand design" systems are the products of density waves that propagate around the disk with an approximately constant angular speed Om_P. We show that it is possible to measure an offset between young stars of a known age and the spiral arm in which they formed in a grand-design spiral galaxy, consistent with predictions of a density wave model. By measuring how this offset varies with radius, we obtain a direct measure of Om_P at a range of radii, and show that the spiral pattern in this galaxy is consistent with being quasi-stationary.

We then investigate how the azimuthal structures of the barred spiral galaxy MCG+07-28-064 vary when traced by stars of different ages. Decomposing this galaxy into "time slices", we find evidence for the ongoing growth of the bar, and for the most recent star formation occurring on its leading edge. We also show that spiral arms can be traced in stellar populations as old as 2 Gyr, providing further evidence for the density wave model of spiral structure.

In preparation to apply time slicing analyses to a large population of galaxies, we refine and test the spectral fitting methods. We show that the stellar population fitting techniques employed in this thesis must be carefully interpreted. For example, we find that accurately extracting the very youngest (<=30 Myr) stellar populations is not feasible, due to the limitations of modelling template spectra. However, reassuringly, we demonstrate that most populations can be reliably modelled in all of the conditions typically found in MaNGA galaxies.

Finally, we perform a fossil record analysis for a large population of low-redshift spiral galaxies, thereby making use of the full power of MaNGA's sample size. By measuring the mean stellar ages and formation times as a function of galactic radius --- and also the radial profiles of different time slices --- we find evidence for inside-out growth being a generic feature of spiral galaxies, and most significant in massive galaxies. By interpreting the radial profiles of time~slices as indicative of the size of the galaxy at the time those populations had formed, we are able to use the stellar population fossil record to quantitatively trace the simultaneous growth in mass and size of the spiral galaxies over the last 10 Gyr. Despite finding that the evolution of the measured light-weighted radius is consistent with inside-out growth in the majority of spiral galaxies, we observe that an equivalent mass-weighted radius has changed little over the same time period. Since radial migration effects are likely to be small, we conclude that although the growth of disks in spiral galaxies has occurred predominantly through an inside-out mode, this has not had anywhere near as much impact on the distribution of stellar mass within spiral galaxies.

These studies show that there is a wealth of untapped information on the spatial variation of the stellar populations which is now available to exploit with the current generation of integral-field spectroscopic galaxy surveys. A time-slicing approach to studying the fossil record is therefore an extremely powerful technique to answer some open questions on the structure and dynamics of spiral galaxies.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Aragón-Salamanca, Alfonso
Merrifield, Michael
Keywords: Spiral galaxies, Stellar populations
Subjects: Q Science > QB Astronomy
Faculties/Schools: UK Campuses > Faculty of Science > School of Physics and Astronomy
Item ID: 60728
Depositing User: Peterken, Thomas
Date Deposited: 28 Jul 2020 11:11
Last Modified: 28 Jul 2020 11:11
URI: https://eprints.nottingham.ac.uk/id/eprint/60728

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