Dissertation: Investigating Brown Dwarf Atmospheres: Gravity, Dust Content, Cloud Structure and Metallicity

(Cassiopeia – Autumn/l’automne 2017)

by Kendra Kellogg
Thesis defended on July 13, 2017
Department of Physics and Astronomy, Western University
Thesis advisor: Dr. Stanimir Metchev

Brown dwarfs are the lowest mass products of star formation. Their low masses don’t allow them to sustain, or sometimes even begin, the thermonuclear processes that provide stars with internal energy and the thermal pressure necessary to maintain hydrostatic equilibrium. Thus, their radii and effective temperatures decrease as they age, continually changing their spectral classification. However, it is now a well-known fact that the spectral appearance of ultra-cool dwarfs is governed by more than just temperature. Factors such as gravity, metallicity and cloud distribution play an important role in the structure and composition of ultra-cool dwarf atmospheres and ultimately their spectra.

Pinning down the effects of some of the contributing factors to the structure and evolution of brown dwarf atmospheres has been the goal of my thesis research. Through a joint positional and colour cross-match of optical, near-infrared and mid-infrared all-sky surveys, I have identified 20 new brown dwarfs with “peculiar” photometric colours which are candidates for having unusual atmospheric properties. I have determined that a number of these objects have atypical surface gravities and/or atmospheric dust content using near-infrared spectroscopic observations. I have also determined the timescales for the various peculiarities by comparing these objects to the population of “normal” objects. In addition, I have studied in detail a few of the most peculiar objects in order to understand how conditions on individual objects affect their atmospheric structure and composition.

Dissertation: Lights in Dark Places: Inferring the Milky Way Mass Profile using Galactic Satellites and Hierarchical Bayes

(Cassiopeia – Autumn/l’automne 2017)

by Gwendolyn Eadie
Thesis defended on July 18, 2017
Department of Physics and Astronomy, McMaster University
Thesis advisor: Dr. William Harris

Despite valiant efforts by astronomers, the mass of the Milky Way (MW) Galaxy is poorly constrained, and not known within a factor of two. A range of techniques have been developed and different types of data have been used to estimate the MW’s mass. One of the most promising and popular techniques is to use the velocity and position information of satellite objects orbiting the Galaxy to infer the gravitational potential, and thus the total mass. Using these satellites, or Galactic tracers, presents a number of challenges: 1) much of the tracer velocity data are incomplete (i.e. only line-of-sight velocities have been measured), 2) our position in the Galaxy complicates how we quantify measurement uncertainties of mass estimates, and 3) the amount of available tracer data at large distances, where the dark matter halo dominates, is small. The latter challenge will improve with current and upcoming observational programs such as Gaia and the Large Synoptic Survey Telescope (LSST), but to properly prepare for these data sets we must overcome the former two. In this thesis work, we have created a hierarchical Bayesian framework to estimate the Galactic mass profile. The method includes incomplete and complete data simultaneously, and incorporates measurement uncertainties through a measurement model. The physical model relies on a distribution function for the tracers that allows the tracer and dark matter to have different spatial density profiles. When the hierarchical Bayesian model is confronted with the kinematic data from satellites, a posterior distribution is acquired and used to infer the mass and mass profile of the Galaxy.

This thesis walks through the incremental steps that led to the development of the hierarchical Bayesian method, and presents MW mass estimates when the method is applied to the MW’s globular cluster population. Our best estimate of the MW’s virial mass is M(vir) = 0.87 x 1012 Solar masses with a 95% credible range of (0.67 – 1.09) x 1012 Solar masses. We also present preliminary results from a blind test on hydrodynamical, cosmological computer-simulated MW-type galaxies from the McMaster Unbiased Galaxy Simulations. These results suggest our method may be able to reliably recover the virial mass of the Galaxy.

Dissertation: The Effects of Environment on the Atomic and Molecular Gas Properties of Star-Forming Galaxies

(Cassiopeia – Autumn/l’automne 2017)


by Angus King Fai Mok
Thesis defended on July 31, 2017
Department of Physics and Astronomy, McMaster University
Thesis advisor: Dr. Christine Wilson

Where a galaxy is located has a strong effect on its properties. The dense cluster environment is home to a large population of red, quiescent elliptical galaxies, whereas blue, star-forming, spiral galaxies are common in lower-density environments. This difference is intricately linked to the ability of the galaxy to form new stars and therefore ultimately to the fuel for star formation, the atomic and molecular gas. In this thesis, I use two large JCMT surveys to explore the effects of environment on the atomic gas, molecular gas, and star formation properties of a large sample of nearby gas-rich galaxies.

From the NGLS and follow-up studies, I select a sub-sample of 98 HI-flux selected spiral galaxies. I measure their total molecular gas mass using the CO J=3-2 line and combine this data with measurements of their total atomic gas mass using the 21-cm line and star formation rate using attenuation-corrected H-alpha luminosity. I find an enhancement in the mean H2 mass and a higher H2-to-HI ratio for the Virgo Cluster sample. Virgo Cluster galaxies also have longer molecular gas depletion times (H2/SFR), which suggests that they are forming stars at a lower rate relative to their molecular gas reservoirs than non-Virgo galaxies.

Next, I collect VLA 21 cm line maps from the VIVA survey and follow-up VLA studies of selected galaxies in the NGLS. I measure the surface density maps of the atomic gas, molecular gas, and star formation rate in order to determine radial trends. I find that the H2 distribution is enhanced near the centre for Virgo Cluster galaxies, along with a steeper total gas (HI + H2) radial profile. I suggest that this is due to the effects of moderate ram pressure stripping, which would strip away low-density gas in the outskirts while enhancing high-density gas near the centre. There are no trends with radius for the molecular gas depletion times, but the longer depletion times for the Virgo Cluster sample is still present.

Finally, I use 850 micron continuum observations for 105 star-forming galaxies and CO J=2-1 line observations for 35 galaxies in the initial data release (DR1) of the JINGLE survey. I match the JINGLE galaxies to a SDSS group catalogue and measure environmental parameters such as the host halo mass, environment density, and location in phase space. I find that the molecular gas masses estimated from the 850 micron and CO J=2-1 line observations are well-correlated. The H2-to-HI ratio and the molecular gas depletion times do not appear to vary with stellar mass. I did not find any significant variation with environment in the DR1 sample, but I will apply this framework to the full JINGLE sample once the complete dataset is available.