Exploring the outskirts of the Milky Way's ultra-faint dwarf galaxies
The Milky Way is surrounded by dozens of ancient dwarf galaxies that formed in the first 1 billion years of the universe. Some of these systems are known as "ultra-faint" dwarf galaxies, and are likely analogs of some of earliest galaxies. In a few papers (1, 2, 3), I discovered that one of these galaxies had a previously hidden, ancient component of stars that extended out to 1 kiloparsec. This feature may be a signature of one of the first galactic mergers, and was the first direct evidence that these relic galaxies may inhabit extended dark matter distributions. This component was uncovered by implementing a novel imaging technique to identify ancient, metal-poor stars. I am currently extending this to other ultra-faint dwarf galaxies to constrain formation pathways of early systems. This work has made mainstream press and was featured on the Science Channel.
Distant stars uncovered around an ancient dwarf galaxy, from Figure 1a in Chiti et al. (2021, Nat. Astron., 5, 392).
Environmental variations in early Chemical Evolution
I am interested the consistency between early chemical evolution in the Milky Way and its surrounding dwarf galaxies, as revealed by the chemistry of their most ancient, metal-poor stars. This question is linked to environmental variations in early star formation, since differences in e.g., the early stellar initial mass function can translate to different chemical abundance signatures. I am working on this question by performing searches for the most metal-poor stars in the Milky Way's dwarf galaxies. In two systems so far, I have significantly increased the sample of the most metal-poor stars, providing insights into the early formation of carbon (1, 2, 3).
The carbon abundances of the most metal-poor stars in the Sagittarius dwarf galaxy, presented in Figure 6 in Chiti et al. (2020, ApJ, 901, 164).
The nucleosynthetic yields of the first stars
Stars in the lowest metallicity regimes ([Fe/H] < -4.0) are plausibly only enriched by ~1 supernova event. Consequently, the detailed chemical abundances of these stars can be related to yields from the first generation of supernovae. However, fewer than 20 stars are known in this metallicity regime, due to their rarity. I am working with collaborators on increasing the known sample of stars in this regime through spectroscopic follow-up of stars identified as low-metallicity candidates (1, 2).
Supernova yield model fits to the chemical abundances of a star with [Fe/H] < -6.0, from Figure 4 in Frebel et al. (2019, ApJ, 871, 146)
Mapping low-metallicity stars in the Milky Way
The Milky Way host populations of ancient (aka. "metal-poor") stars that formed in the early universe, but still exist today. These stars provide a local window to the early universe, since their chemical composition reflects yields from the earliest generations of stars. Their spatial distribution also provides insights on the formation of the Milky Way. In two papers (1, 2), I used public photometry from the SkyMapper survey + the Gaia mission to uncover the stars that make up the ancient component of the Milky Way by generating a map out to ~5 kpc of metal-poor stars down to the lowest metallicity regime ([Fe/H] ~ -3.0).
A low-metallicity map of the Milky Way, presented in Figure 2 of Chiti et al. (2021, ApJL, 911, 23)
The formation and early evolution of the Milky Way ecosystem
I am working with collaborators on understanding the early formation of each of the Milky Way's components, using the large sample of metal-poor stars that I identified in the SkyMapper survey (1, 2). In a recent paper (3), my collaborators led a study that used this sample of metal-poor stars to investigate the early evolution of the Milky Way's disk system. In another paper (4), I found that some of the carbon-enhanced metal-poor stars in the Milky Way's halo may have originated from ancient, accreted analogs of current dwarf galaxies. Currently, I am working with the DELVE collaboration to find faint, extended structures around globular clusters, which may provide insights on the assembly of the Milky Way and probe its underlying dark matter halo.
The distribution of metallicities of metal-poor stars in the Milky Way as a function of location in the |Z| direction, from Figure 5 in Chiti et al. (2021, ApJL, 911, 23)
Methods to find the most metal-poor stars
During my PhD, I developed and implemented techniques to derive precise metallicities from metallicity-sensitive imaging filters. This work enabled many of the above projects by leading to a huge increase in the efficiency of identifying the most metal-poor stars in the Milky Way and its surrounding dwarf galaxies relative to traditional spectroscopic techniques. I am interested in extending this work to other filters or datasets (e.g., Gaia XP spectra), and applying this work to galaxies outside the Milky Way ecosystem.
Bandpass of the metallicity-sensitive SkyMapper v filter, taken from Figure 2 in Chiti et al. (2020, ApJ, 891, 8).