Thomas Davison
Career Stage
Student (postgraduate)
Poster Abstract
In the past decade Integral Field Spectroscopy has expanded with the development of large FoV spectrographs. These instruments enable the simultaneous measurement of high signal-to-noise spectra out to large galactocentric distances, delivering unprecedented spatially resolved spectroscopic data. This, combined with recent advancements in full-spectral fitting techniques, grants us incredible insights into derived stellar populations present within a galaxy, including the inferred fractions of accreted material. Using the EAGLE simulations, we build predictions of populations properties that we expect to see when applying these novel techniques to IFU data. Furthermore, we use MUSE IFU observations to test these predictions, and investigate the accreted fractions of galaxies with respect to galactocentric radius as well as the galaxy mass-size plane.
Plain text summary
Title: Growing a Galaxy, Exploring Merger and Accretion History with Galactic Archaeology.
Author: Thomas Davison
This poster gives a brief overview of an ongoing PhD project, which explores methods of tracing galaxy merger history by identifying stars which were not born in the galaxy in question. Galaxies are well known to experience frequent mergers during their lifetime, with simulations leading the way in showing the high frequency with which galaxies consume smaller galaxies. These simulations show how galaxies are often built with a dense core of stars born within the galaxy (in-situ) surrounded by stars that originated in other galaxies (ex-situ). This is shown at the bottom of the page in a figure from simulations described in Pillepich, A. et al, 2015.
Simulations of galaxy growth have been used to show the high numbers of mergers that most galaxies experience. Work with the EAGLE simulations has shown that the largest simulated galaxies are composed of 90% ex-situ stars, meaning the majority of their stellar mass has been consumed from smaller galaxies, rather than formed inside the galaxy. The figure shows the ex-situ fraction in colour, for a mass-size plane of galaxies. We see from the colour that ex-situ fraction increases independently both with galaxy mass and galaxy radius. The top two panels show the inner regions of the galaxies, whilst the lower two panels look at the outer regions of the galaxies. We see that the outer regions of galaxies have higher ex-situ fractions than the centres, suggesting that ex-situ material is preferentially deposited onto the outskirts during mergers and fly-by accretion.
We wish to explore the same features using observational data from the MUSE instrument. Unlike in simulated data, observational data has no time dimension, and so histories of galaxies must be derived from current spectra and structure seen in the galaxy today. This can be achieved using full spectral fitting. In this technique, models of stars are combined until the spectrum of a galaxy is reproduced. This allows us to estimate the ages and the chemical compositions of all the stars in the galaxy. Galaxies follow well understood rules in how the metal content of the stars must increase over time, according to the galaxy mass. As such, we can look for any groups of stars that do not follow the expected relation. Any of these stars are unlikely to have originated in the current galaxy, and so must have arrived via accretion or merging. This is shown in the lower figure, which shows estimates of ex-situ fraction as colour, overlaid on the original galaxy image.
The MUSE instrument gives us spectral information for a galaxy, but it does so over a spatial field. What this means is we are able to build up a map of spectral properties across a 2D image of the galaxy. In the figure we show a few of these properties for NGC1404. As mentioned previously, we now want to use these properties for many galaxies to estimate ex-situ fractions, and to locate these fractions in the galaxies. This should give us an idea of the trends in ex-situ fraction with galaxy size and mass, as well as with the galactocentric radius of the galaxy. We will then be able to compare these results to the simulated versions and explore any differences of similarities. Work is ongoing.
Author: Thomas Davison
This poster gives a brief overview of an ongoing PhD project, which explores methods of tracing galaxy merger history by identifying stars which were not born in the galaxy in question. Galaxies are well known to experience frequent mergers during their lifetime, with simulations leading the way in showing the high frequency with which galaxies consume smaller galaxies. These simulations show how galaxies are often built with a dense core of stars born within the galaxy (in-situ) surrounded by stars that originated in other galaxies (ex-situ). This is shown at the bottom of the page in a figure from simulations described in Pillepich, A. et al, 2015.
Simulations of galaxy growth have been used to show the high numbers of mergers that most galaxies experience. Work with the EAGLE simulations has shown that the largest simulated galaxies are composed of 90% ex-situ stars, meaning the majority of their stellar mass has been consumed from smaller galaxies, rather than formed inside the galaxy. The figure shows the ex-situ fraction in colour, for a mass-size plane of galaxies. We see from the colour that ex-situ fraction increases independently both with galaxy mass and galaxy radius. The top two panels show the inner regions of the galaxies, whilst the lower two panels look at the outer regions of the galaxies. We see that the outer regions of galaxies have higher ex-situ fractions than the centres, suggesting that ex-situ material is preferentially deposited onto the outskirts during mergers and fly-by accretion.
We wish to explore the same features using observational data from the MUSE instrument. Unlike in simulated data, observational data has no time dimension, and so histories of galaxies must be derived from current spectra and structure seen in the galaxy today. This can be achieved using full spectral fitting. In this technique, models of stars are combined until the spectrum of a galaxy is reproduced. This allows us to estimate the ages and the chemical compositions of all the stars in the galaxy. Galaxies follow well understood rules in how the metal content of the stars must increase over time, according to the galaxy mass. As such, we can look for any groups of stars that do not follow the expected relation. Any of these stars are unlikely to have originated in the current galaxy, and so must have arrived via accretion or merging. This is shown in the lower figure, which shows estimates of ex-situ fraction as colour, overlaid on the original galaxy image.
The MUSE instrument gives us spectral information for a galaxy, but it does so over a spatial field. What this means is we are able to build up a map of spectral properties across a 2D image of the galaxy. In the figure we show a few of these properties for NGC1404. As mentioned previously, we now want to use these properties for many galaxies to estimate ex-situ fractions, and to locate these fractions in the galaxies. This should give us an idea of the trends in ex-situ fraction with galaxy size and mass, as well as with the galactocentric radius of the galaxy. We will then be able to compare these results to the simulated versions and explore any differences of similarities. Work is ongoing.
Poster file
Poster Title
Growing a Galaxy, Exploring Merger and Accretion History with Galactic Archaeology
Url
https://www.star.uclan.ac.uk/~tdavison/