Astro Tuesday: Samantha Usman and Taylor Hoyt

12:00–1:00 pm ERC 401

Samantha Usman "Classifying the Progenitor of the Stellar Stream 300S"

As dwarf galaxies and star clusters orbit in the Milky Way, they can tidally disrupt into extended stellar streams. The progenitor of one stellar stream, called 300S, has previously been claimed both to be a dwarf galaxy and to be a globular cluster. We aim to resolve this discrepancy by measuring the metallicities and chemical abundances of eight kinematically confirmed members of the 300S stream and comparing them to other dwarf galaxies and globular clusters. We measure a mean metallicity [Fe/H] of -1.5 with an unresolved metallicity dispersion, as would be expected for a globular cluster.  However, we also do not see any evidence for the light element dispersions (multiple populations) that are observed in nearly all intact Galactic globular clusters. Given its high luminosity, nonexistent metallicity dispersion, and accreted origin, 300S may suggest that the multiple populations phenomenon depends on a star cluster’s formation environment.

Taylor Hoyt "Recalibrating the Tip of the Red Giant Branch Distance Scale: Implications for the Hubble Constant"

The zero point calibration of the extragalactic distance scale is tied to any subsequent derivation of the Hubble constant (H0). Thus, it is imperative that calibration accuracy is continually improved for stellar standard candles. I discuss two new calibrations of the Tip of the Red Giant Branch (TRGB) distance scale. The first is based in the Magellanic Clouds and rectifies the shortcomings (e.g., contaminant stellar populations, biased TRGB measurements, and underestimated extinction estimates) of recent calibrations of the TRGB in that system. The second is based in the megamaser host galaxy NGC 4258 using brand new Hubble Space Telescope (HST) imaging (PI: Hoyt) that was designed to match the observing conditions of the Carnegie Chicago Hubble Program’s (CCHP) SN host observations, thereby reducing systematic uncertainties associated with the TRGB to negligible levels. Combined, the two results provide a 1% calibration of the TRGB distance scale, along with additional tests of the method’s dependence on metallicity and photometric color.

Event Type


May 31