Sean McGee, Research Astronomer

Research Interests

My research is principally driven by three general questions:
  1. How do groups, clusters and the growth of structure with cosmic time affect the formation of galaxies?
  2. How does gas get into galaxies and how is it distributed within the ISM and CGM?
  3. How were the first generation of stars and black holes formed?
Roughly, my current focus for each of these questions are in the following areas:

1. Groups, clusters and the growth of structure with cosmic time

a) Evolution of galaxy properties in groups and clusters

Much of my research is focussed on the role groups and clusters play in altering the morphology and star formation properties of galaxies. This work has been done within the Group Environment Evolution Collaboration with samples at z=0.4 (GEEC) and z=1.0 (GEEC2) and will be continuing to z=1.5 within the GoGreen survey

I also use simple theoritical models to try to understand how the galaxy properties arise in cosmological structure formation and what their properties can tell us about the baryon cycle of all galaxies.

b) High Redshift Protoclusters

GTC Lyman alpha large programme - I am working on a narrow band imaging survey of 10 high redshift radio galaxies with the Gran Telescopio Canarias. This program is an ESO/GTC large programme (PI: G. Miley). Observations are ongoing.

Herschel overdensity searchs - We have begun a program to find high redshift protoclusters in large area Herschel surveys. The initial candiates have been determined and we are now verifying the selection using deep radio observations.

HzRG Herschel survey - Deep Herschel observations (PI: Rottgering) have been taken of ~ 30 high redshift radio galaxies with the intention of quantifiying the environments of those galaxies. I have been obtaining optical images of these fields while radio observations are being executed.

2. The distribution of gas in galaxies

a) Searching for the most extreme Lyman alpha blobs

Nearly 15 years ago, the first 'Lyman alpha blob' was serendipitously discovered using a narrow band imaging filter. Such blobs are fascinating objects, as the detected Lyman alpha emission can be extended over 100 kpc scales. This emission effectively illuminates the gas within the cosmic web, allowing the quantity and distribution of the normally invisible gas to be studied. Thus, finding these rare objects can give us a unique window on the gas cycle of galaxies. I am working on a systematic survey for such objects. By the time the survey is complete, this program will have searched an order of magnitude more volume than previous LAB surveys. We will find the rarest, and thereby most informative, LABs that exist.

b) Reionization and the escape fraction of ionizing photons

I've recently become interested in how ionizing photons escape the opaque surroundings of their birthplace. It is thought that the reionization of the universe was principally caused by ionizing photons produced by star forming galaxies, however, this requires close to 50% of the photons to escape the galaxy. Unfortunately, numerous studies find that the escape fraction is significantly lower than that. I am undertaking a systematic study of z = 1 galaxies in the deepest GALEX images to directly constrain how the escape fraction depends on galaxy properties.

3. The first stars and black holes

a) Extremely metal poor stars

Extremely metal poor stars, thought to have formed from nearly pristine gas in the early Universe, are sensitive probes of the formation of the first stars and their subsequent chemical pollution of the environment. Recently, a star was located in the Milky Way halo with the lowest known metal content ([Fe/H] < -7). Detailed abundance measurements were used to infer that the first generation of stars had typical masses of 50-60 solar masses and resulted in relatively feeble supernovae which did not widely disperse their Fe cores (Keller et al. 2014). Unfortunately, this is a measurement of the birthplace of a particular star and not universally applicable. For this reason, I am extending the search for extremely metal poor stars to a wide field around dwarf galaxies. Observations of this program are underway to image very wide fields around each dwarf galaxy

b) Direct collapse black holes

The appearance of massive black holes at z > 6, when the Universe was only 800 Myrs old, is a great puzzle in astronomy. Given the short timescale and the expected physical limits to black hole growth, it is unlikely that these black holes formed from the remnants of normal stars. One possible mechanism for the formation of these objects is through direct collapse in the small halos in the regions of more massive halos. These more massive halos provide direct Lyman Werner radiation which prevents the cooling of molecular hydrogen and inhibits fragmentation in the smaller halos, ultimately resulting in massive direct collapse black holes. I am working on the observational signatures of these DCBHs.

4. Physically varying initial mass function and the efficiency of galaxy formation

With my LEAPS summer student, Ryosuke Goto, I am exploring physically motivated models for the variation in the stellar initial mass function. Using these models as the basis for suites of stellar population models to derive physical parameters via spectral energy distribution fitting. We have published a first paper examining the result of employing empirical models of IMF variation and how that alters the local stellar mass function and efficiency of galaxy formation.