Galaxies

My Research

the evolution of S0 galaxies

My main area of research is studying the star-formation histories of bulges and discs of lenticular galaxies in nearby clusters in order to better understand their evolution and how closely related they are to spiral galaxies. There are many theories for the transition from spiral to S0, most of which focus on explanations for the truncation of star formation in these galaxies. Such ideas range from secular evolution, arising from internal processes using up all the gas necessary for star formation, to interactions with neighbouring galaxies, in which the gas is used up in a rapid starburst event, and even to interactions with the intracluster medium, which can strip the galaxy of its gas as it travels through the cluster. All of these influences would affect the bulge and disc in different ways, and we therefore aim to study these components individually for clues to the evolution of these galaxies.

As part of my PhD I developed a method to apply bulge-disc decomposition to longslit spectra of S0 galaxies, in which the two-dimensional spectrum is decomposed wavelength-by-wavelength into bulge and disc components, thus allowing separate one-dimensional spectra for each to be constructed. With these spectra we could obtain estimates of the relative ages and metallicities of the stellar populations in each component, which tells us where the most recent star formation activity occurred and for how long it went on. An example of our initial results can be seen in the plot below, where it can be seen that the bulges contain systematically younger and more metal rich stellar populations than the discs, suggesting that the final star formation episode within these galaxies occurred in the bulge. We have also been able to measure colour, age and metallicity gradients within the bulge and disk as well, which can tell us about whether the most recent star formation activity happened throughout the entire bulge or disc, or only in the central or outer regions.

age metallicity plot

However, with long-slit spectra alone we cannot determine whether these young stellar populations are distributed throughout the bulge or only in the centre of the galaxy. Similarly, with only cluster galaxies in the sample, it is uncertain to what extent environment plays a role in the young central stellar populations. Therefore, I have further developed the spectroscopic bulge-disc decomposition technique in order to apply it to IFU datacubes of a larger sample of S0s. Bulge-Disc Decomposition of IFU data, or BUDDI, is the result. BUDDI uses GALFITM, a modified form of Galfit that was developed by the MEGAMORPH team to fit multi-waveband images simultaneously, to obtain wavelength-dependent fit parameters for each component from which the decomposed spectra can be created. Since GALFITM uses the spectral and spatial information from over the whole datacube, the S/N of the data is boosted over that of any individual image. However, applying GALFITM to a datacube would prove computationally very expensive, and so BUSSI uses the following multi-step process to find the best fit to the galaxy.

BUDDI sequence

The paper outlining the method used by BUDDI, the tests we carried out and some example analyses will appear on the arXiv on the 3rd November. BUDDI will be available via GitHub in the coming months. If you are interested in using BUDDI, please contact me.

NGC 4550- a galaxy with two counter-rotating stellar discs

Not all S0s formed in the same way, and so a study of their evolution must also consider the more unusual S0s with different formation histories. As part of this, I have recently been looking at NGC 4550, an unusual S0 with two extended counter-rotating stellar disks.

I have been able to decompose a high-quality long-slit spectrum of this galaxy into two counter-rotating stellar components and a gaseous component in order to study both their kinematics and stellar populations. I found that the secondary disk, which co-rotates with the gas, contains significantly younger stars and is also brighter than the primary disk, both of which suggest that this disk underwent more recent star formation. From this we were able to conclude that this glaxy formed through one of two possible scenarios- either through a carefully controlled merger of two fully-formed galaxies, in which the different ages reflect the ages of the progenitors; or through accretion of gas from an external source which settled into a disk and underwent a star formation event that produced the second stellar component.

Below is a video made by my supervisor, Professor Mike Merrifield, describing the background of this galaxy and summarising what we have found. Alternatively the video can be seen on YouTube.