Understanding the starburst mode of star formation in dwarfs has proven a multi-decade challenge. Building on the successful program characterizing the properties of starbursts (see Past Scientific Contributions), I have created an holistic program to study the lifecycle and evolution of starbursts in low-mass systems – STARBIRDS. I have been awarded programs on several telescope facilities to support this ambitious goal including Chandra, GALEX, multiple sites on KPNO, HST, Spitzer, VLA, GBT, and Parkes. Through analysis of these rich, complex data sets, I will address the following questions: What triggers a starburst? Is this a potential phase of evolution or are they uniquely externally triggered? Do starbursts ubiquitously drive galactic winds? Are these outflows unbound? If not, what is the timescale of the gas-recycling from the outflows? I have built a multi-national team with expertise in both data and simulations to ensure the highest quality deliverables for the survey, including a public data archive to be hosted by MAST, the archive for space telescope data. The simpler dynamics and histories of dwarf galaxies reduce the ambiguities inherent in interpreting results of more complicated, massive galaxies. Yet, the potential impact of this work from resolved systems extends to broader questions of galaxy evolution including how material is recycled in galaxies and how galaxies interact with their halos and their larger-scale environments.
I am leading the international collaboration to obtain the first JWST detailed observations of resolved stellar populations of nearby galaxies to derive JWST star formation histories in the early release science (ERS) program. The nearby galaxy community is uniting to bring a comprehensive JWST program on select targets in pre-cycle 1 observations. In preparation for ERS, I am already developing observing strategies for the nearby galaxy community (including linking filter and photometric depth choices with quantified uncertainties) using simulations of JWST resolved stellar populations (McQuinn et al. in prep).
There are a growing number of discoveries of Milky Way satellite galaxies with extremely low masses (i.e., ultra-faint dwarfs or UFDs, M* < 10^7 Msun), all of which are gas-poor. Their gas-rich counter-parts outside the Local Group are the local counterparts to galaxies predicted to populate the turn-over in the galaxy luminosity function in the epoch of reionization. However, due to their intrinsic faintness and smaller angular size, these types of galaxies have remained hidden from detection in most local surveys. By searching for their gas component (instead of their stars), we are now discoverying and studying these enigmatic galaxies the faint-end of the luminosisty fuction in detail. Working in this new parameter space (the SHIELD project; Cannon et al. 2013), I have measured the distances to 30+ extremely low-mass gas-rich galaxies and characterized their star formation properties from their resolved stellar populations (McQuinn et al. 2014, 2015a). As a lead collaborator, my results show that the star-formation main sequence continues to lower masses with smaller instrinsic scatter.
One of the highlights from this research is the discovery and study of Leo P, a star-forming galaxy located just outside the Local Group with a stellar mass of 5 x 10^5 Msun (Mdyn ~ 10^7 Msun). Leo P has one of the lowest gas-phase metallicities ever measured (12+log(O/H) = 7.04 ± 0.04), and contrary to many theoretical predictions, is unquenched at the present day and forming stars at a fairly constant rate. The star formation history of Leo P derived from HST imaging reveals hints that this galaxy may have experienced a quelling of star formation post-reionization (McQuinn 2015b). A detailed accounting of the oxygen atoms in Leo P shows that the galaxy has lost 95% of its oxygen – presumably due to stellar feedback – over its lifetime (McQuinn et al. 2015c). The retention fraction of 5% is the first empirical anchor for outflows and chemical evolution scenarios in this mass regime.
The widely used UV star formation rate (SFR) scaling relation (i.e., Kennicutt 1998) is based solely on theoretical spectra calibrated in conjunction with the photosphere models of a small number of high-mass stars in the Galaxy. Using SFRs derived from resolved stars in nearby dwarf galaxies, I have re-calibrated the UV SFR scaling with measured uncertainties. This is the first independent calibration of the UV SFR using a data set that a priori meets the assumptions necessary for a UV scaling relation, that is minimally impacted by UV attenuation, and that has quantified uncertainties. My results show a 50% disagreement with previous calibrations indicating that UV SFRs based on theoretical spectra are under-measuring the star formation in galaxies (McQuinn et al. 2015d,e). I am currently working with population synthesis modeler Gustavo Bruzual to diagnosis the source of the discrepancy in the models. These results have the potential to impact a wide range of astrophysics as many studies rely on our understanding of young, massive, bright stars and the calibration of the UV emission in galaxies, both locally and at high redshift.
The Spitzer Infrared Nearby Galaxies Survey (SINGS) and its offspring programs (e.g., THINGS, HERACLES, KINGFISH) have resulted in a fundamental change in our view of star formation and the ISM in galaxies, and together they represent the most complete multi-wavelength data set yet assembled for a large sample of nearby galaxies. These great investments of observing time have been dedicated to the goal of understanding the interstellar medium, the star formation process, and, more generally, galactic evolution at the present epoch. Nearby galaxies provide the basis for which we interpret the distant universe, and the SINGS sample represents the best studied nearby galaxies.
Accurate distances are fundamental to interpreting observations of galaxies. Surprisingly, many of the SINGS spiral galaxies have numerous distance estimates resulting in confusion. Using new observations from the Hubble Space Telescope, I have measured the distance to 8 of the SINGS spiral galaxies using the highly accurate tip of the red giant branch method. The sample includes well known galaxies such as M51 (the Whirlpool), M63 (the Sunflower), M104 (the Sombrero), and M74 (the archetypal grand design spiral).