Long exposure photograph of the night sky from the Southern Hemisphere (credit: the awesome Dr. Hannah Krug). The inset images are three-color composites of infrared wavelength images (from the SAGE and HERITAGE collaborations)  showing emission by dust throughout the galaxies.

The Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) at a mere 50 kpc and 62 kpc away, metallicities of ~1/2 and ~1/5 solar, and stellar masses of  2x109 and 3x108 solar masses, these galaxies present an ideal system to study the physics relevant to star formation in galaxies in the early universe.

The Large-scale Relationship Between Molecular Gas and Star Formation

Star formation plays a critical role in shaping how galaxies form and evolve. Understanding the molecular gas content of low-metallicity galaxies is key to solve the cosmological evolution of gas mass fractions and H2 density as a function of redshift (e.g., Tacconi et al. 2010, 2013; Genzel et al. 2012), and the dependence of the star formation efficiency on galaxy mass and metallicity (e.g., Bolatto et al. 2011; Saintoge et al. 2011; Krumholz et al. 2011; Glover & Clark 2012; Clark & Glover 2013).

Using the dust emission from HERITAGE Herschel data (Meixner et al. 2013) to map the molecular gas in the Magellanic Clouds we avoid the known biases of CO emission as a tracer of H2 and detect more extended molecular gas (see the left panel of Figure 1). Using our dust-based molecular gas estimates, we find molecular gas depletion times (the ratio of the molecular gas to the star formation rate) that are within the range found for normal disk galaxies, but shorter than the average value, which could be due to recent bursts in star formation. We find no evidence for a strong intrinsic dependence of the molecular gas depletion time on metallicity. We study the relationship between gas and star formation rate across a range in size scales from 20 pc to 1 kpc, including how the scatter in molecular gas depletion time changes with size scale, and discuss the physical mechanisms driving the relationships (see right panel of Figure 1). We compare the metallicity-dependent star formation models of Ostriker, McKee, & Leroy (2010) and Krumholz (2013) to our observations and find that they both predict the trend in the data, suggesting that the inclusion of a diffuse neutral medium is important at lower metallicity. However, the predictions do not capture the full extent of the scatter in the relationship between gas and star formation, which simulations suggests could be due to the time-averaging effect of star formation rate tracers (e.g., Hα; Feldmann et al. 2011).

The Physical Conditions of Warm H2 in the SMC

Modeling of the spatially-resolved mid-IR Spitzer IRS spectroscopy of rotational H2 line emission in 5 regions in the SMC allows us to understand the temperature and column density of the warm molecular gas at low metallicity. We find the properties of the warm molecular gas in the SMC are similar to nearby galaxies (e.g., Roussel et al. 2007). One possible interpretation is that the low metallicity environment is not significantly affecting the heating and cooling of the H2 gas. The ~10% warm molecular gas fractions when compared to the dust-based estimates (Jameson et al. in prep, Bolatto et al. 2011) indicate that our dust-based molecular gas estimates are predominately cool, potentially star-forming gas.

The Effect of Metallicity on the Structure of the Molecular Gas

and Photodissociation Regions in the SMC

We study how the CO emission traces the molecular gas at low metallicity and high resolution by comparing new ALMA ACA 12CO, 13CO, and C18O (2-1) (resolution ~ 7” ~ 2 pc, PI: Jameson) and APEX 12CO (2-1) data (PI: Rubio) to estimates of the total amount of molecular gas from Herschel [CII] and [OI] spectroscopic maps for 4 star-forming regions in the SMC. We also have a highly ranked Cycle 3 proposal to obtain high resolution (resolution ~ 1” ~ 0.5 pc) maps from the 12m-array that were not observed in Cycle 2. In the outskirts of the molecular clouds, we find that the CO-to-H2 conversion factor (X(CO)) is much higher than the canonical Milky Way value (see the left panel of Figure 2), likely a consequence of enhanced photodissociation as shown by 3D simulations (Glover & Mac Low 2011; Shetty et al. 2011). By studying the variation in X(CO) throughout the regions, we can provide a calibration of the fraction of CO-faint gas. This is key to any ALMA CO observation of the high-z universe, as we need to infer molecular mass from a few bright spectral lines (CO, [CII]). 

My Interest in Astronomy

Curriculum Vitae: Jameson_CV.pdf

The Hubble Ultra Deep Field is one of the most extraordinary images I have ever seen. The beauty in astronomy is what first grabbed my attention. The reason I have continued to study Astronomy is the incredibly amount of information that lies within the beauty, and the challenge of deciphering the data. Ultimately, the question that fundamentally motivates my research is:

How can we explain why the universe looks the way we see it today?


Currently, I’m interested in tackling one small aspect of this question: how do the low-metallicity* environments affect the molecular gas and star formation? This question can only be answered in nearby systems, but the answer is needed to correctly interpret observations of the low-metallicity galaxies in the early universe. Not only that, but these nearby systems (like the Magellanic Clouds) are pretty neat systems that warrant study on their own!

I am working with Prof. Alberto Bolatto to address this question for my thesis.

*Being silly astronomers, we define “metallicity” as any element heavier than He. Heavier elements are produced through nuclear fusion within stars and in supernovae (the death of some stars). The more stars a galaxy has produced over time, the more heavy elements (metals) it will have produced.

Dissertation Research:

“The Effect of Metallicity on the Molecular Gas and Star Formation

in the Magellanic Clouds”

Hubble Ultra Deep Field 3 color image - almost everything in the image is a galaxy. The red box next to the Moon is showing the coverage of the Ultra Deep Field on the sky.