Kristen B. Wingfield McQuinn
Recent Publications
Below I show a few abstracts and key figures from some of my recent papers and those of members of my group.
-
The first paper focuses on empirically determining the properties of the lowest metallicity massive O stars in the local universe.
-
The second paper focuses on the evolutionary pathway of galaxies that are extremely metal-poor.
-
The third paper measures the properties of galactic winds driven by supernova explosions from low-mass galaxies. The punch line is that the observed winds have much less material than predicted theoretically and needed in simulations to reproduce realistic galaxies.
-
The last paper studies how the tip of the red giant branch could be used as a standard candle in near-infrared wavelengths for precision distances.
For a full list of my publications, go to : McQuinn Publications on NASA ADS
Far-ultraviolet Spectra of Main-sequence O Stars at Extremely Low Metallicity; Telford et al. (2021)
Metal-poor massive stars dominate the light we observe from star-forming dwarf galaxies and may have produced the bulk of energetic photons that reionized the universe at high redshift. Yet, the rarity of observations of individual O stars below the 20% solar metallicity (Z ⊙) of the Small Magellanic Cloud (SMC) hampers our ability to model the ionizing fluxes of metal-poor stellar populations. We present new Hubble Space Telescope far-ultraviolet (FUV) spectra of three O-dwarf stars in the galaxies Leo P (3% Z ⊙), Sextans A (6% Z ⊙), and WLM (14% Z ⊙). We quantify equivalent widths of photospheric metal lines and strengths of wind-sensitive features, confirming that both correlate with metallicity. We infer the stars' fundamental properties by modeling their FUV through near-infrared spectral energy distributions and identify stars in the SMC with similar properties to each of our targets. Comparing to the FUV spectra of the SMC analogs suggests that (1) the star in WLM has an SMC-like metallicity, and (2) the most metal-poor star in Leo P is driving a much weaker stellar wind than its SMC counterparts. We measure projected rotation speeds and find that the two most metal-poor stars have high v sin(i) ≥ 290 km s-1, and estimate just a 3%-6% probability of finding two fast rotators if the metal-poor stars are drawn from the same v sin(i) distribution observed for O dwarfs in the SMC. These observations suggest that models should include the impact of rotation and weak winds on ionizing flux to accurately interpret observations of metal-poor galaxies in both the near and distant universe.
Extremely metal-poor (XMP) galaxies are low-mass, star-forming galaxies with gas-phase oxygen abundances below 12 + log(O/H) = 7.35 (∼ 1/20 Z☉). Galaxy evolution scenarios suggest three pathways to form an XMP: (1) secular evolution at low galaxy masses, (2) slow evolution in voids, or (3) dilution of measured abundances from infall of pristine gas. The recently discovered XMP galaxy Leoncino, with an oxygen abundance below 3% Z {}☉, provides an opportunity to explore these different scenarios. Using Hubble Space Telescope imaging of the resolved stellar populations of Leoncino, we measure the distance to the galaxy to be D={12.1}-3.4+1.7 Mpc and find that Leoncino is located in an underdense environment. Leoncino has a compact morphology, hosts a population of young, massive stars, has a high gas-to-star mass ratio, and shows signs of interaction with a galaxy nearby on the sky, UGC 5186. Similar to nearly all XMP galaxies known in the nearby universe, Leoncino is offset from the Luminosity-Metallicity (LZ) relation. However, Leoncino is consistent with the stellar Mass-Metallicity (MZ) relation defined by Local Volume galaxies. Thus, our results suggest that the offset from the LZ relation is due to higher recent star formation, likely triggered by a minor interaction, while the low oxygen abundance is consistent with the expectation that low-mass galaxies will undergo secular evolution marked by inefficient star formation and metal loss via galactic winds. This is in contrast to XMP galaxies that are outliers in both the LZ and MZ relations; in such cases, the low oxygen abundances are best explained by dilution due to the infall of pristine gas. We also discuss why quiescent XMP galaxies are underrepresented in current surveys.
Left panel: Luminosity vs. 12+log(O/H) from Berg et al. (2012) with the addition of other XMP galaxies with oxygen abundances below 5% Zsun from various studies measured with the direct-method. All XMP galaxies are offset from the LZ relation, with the exception of the Leo~P galaxy whose low metallicity has been shown to driven by inefficient star formation and galactic winds McQuinn et al. (2015) and J1005+3772 (Senchyna et al. 2019). If the young, upper main sequence stars in Leoncino are excluded, Leoncino agrees with the LZ relation based on the estimated fainter luminosity. Uncertainties in oxygen abundance are smaller than some of the points. Right panel: Log stellar mass vs. 12+log(O/H). The stellar mass of Leoncino is based the 3.6 micron flux, a M/L ratio of 0.47, and the adopted distance of 12.1 Mpc. Typical, star-forming low-mass galaxies with direct-method oxygen abundances and robust distances from the Local Volume Legacy sample form a tight relation in the MZ plane with less dispersion than the LZ relation (Berg et al. 2012; black triangles). The best-fitting line is shown as a solid line with the 1-sigma uncertainties shaded in grey. A number of XMP galaxies that were offset in the LZ plane now agree with the MZ relation, including Leoncino, and there is significantly less scatter when considering all the additional XMP galaxies shown. Plot symbols are labeled in the legend with references for the oxygen abundances; a second reference is provided if the stellar mass was obtained from a separate study.
Stellar-feedback-driven outflows are predicted to play a fundamental role in the baryon cycle of low-mass galaxies. However, observational constraints of winds in nearby dwarf galaxies are limited, as outflows are transient, intrinsically low surface brightness features and thus difficult to detect. Using deep Hα observations, we search for winds in a sample of 12 nearby dwarfs (M_* ∼ 10^7─10^9.3 Msun) that host ongoing or recent starbursts. We detect features that we classify as winds in six galaxies, fountain candidates in five galaxies, and diffuse interstellar medium (ISM) in one system. Winds are found preferentially in galaxies with centrally concentrated star formation, while fountains are found in galaxies with spatially distributed star formation. We suggest that the concentration of star formation is a predictor for whether a low-mass galaxy will develop a wind. The spatial extent of all detected ionized gas is limited (<1/10 virial radius) and would still be considered the ISM by cosmological simulations. Our observations suggest that the majority of material expelled from dwarfs does not escape to the intergalactic medium but remains in the halo and may be recycled to the galaxies. Derived mass-loading factors range from 0.2 to 7 (with only a weak dependency on circular velocity or stellar mass), in tension with higher values in simulations needed to reproduce realistic low-mass galaxies and resolve discrepancies with ΛCDM. The sample is part of the panchromatic STARBurst IRegular Dwarf Survey—STARBIRDS—designed to characterize the starburst phenomenon in dwarf galaxies. We also report a previously uncatalogued nearby galaxy (J1118+7913).
Mass-loading factors as a function stellar mass; points are color-coded by the concentration of star formation as shown in the side colorbar. Overplotted in the left panel are observationally measured mass-loading factors from Martin (1999; blue triangles). The best-fitting relation Eta ~ M_*^(0.04) is shown as a solid black line. Overplotted in the right panel are predicted mass-loading factors from hydrodynamical simulations from Vogelsberger et al. (2013; triangles), Ford et a;. (2014; upside down triangles), Muratov et al. (2015; pentagons), Christensen et al. (2016; squares), and Hu (2019; diamonds). In contrast to predictions, the mass-loading factors do not exhibit a strong dependency on stellar mass. Instead, higher mass-loading factors are associated with galaxies with centrally concentrated star formation.
The tip of the red giant branch (TRGB) is a well-established standard candle used to measure distances to nearby galaxies. The TRGB luminosity is typically measured in the I-band, where the luminosity has little dependency on stellar age or stellar metallicity. As the TRGB is brighter at wavelengths redder than the I-band, observational gains can be made if the TRGB luminosity can be robustly calibrated at longer wavelengths. This is of particular interest given the infrared capabilities that will be available with the James Webb Space Telescope and an important calibration consideration for using TRGB distances as part of an independent measurement of the Hubble constant. Here, we use simulated photometry to investigate the dependency of the TRGB luminosity on stellar age and metallicity as a function of wavelength (λ 475 nm-4.5 μm). We find intrinsic variations in the TRGB magnitude to increase from a few hundredths of a magnitude at λ800-900 nm to ∼0.6 mag by λ1.5 μm. We show that variations at the longer infrared wavelengths can be reduced to 0.02-0.05 mag (1%-2% accuracy in distance) with careful calibrations that account for changes in age and metal content. These represent the minimum uncertainties; observational uncertainties will be higher. Such calibration efforts may also provide independent constraints of the age and metallicity of stellar halos where TRGB distances are best measured. At 3.6 and 4.5 μm, the TRGB magnitude is predicted to vary by ∼0.15 mag even after corrections, making these wavelengths less suitable for precision distances.
Intrinsic TRGB magnitude recovered from the synthetic photometry as a function of wavelength for an as- sumed distance of 1 Mpc. The different color-symbol combi- nations show the TRGBs for different stellar age and metal- licity combinations.