HALO7D: Separating Sun-like Stars in the Milky Way Halo from Foreground White Dwarfs and Interesting Background Contaminants
Abstract
We present a study of Milky Way stars from the HALO7D survey based on deep spectroscopy with Keck II/DEIMOS and photometry from the Hubble Space Telescope (HST) to gain insight into the dark matter content and accretion history of the galaxy. Our observations focus on four fields from the CANDELS survey: GOODS-N, GOODS-S, EGS, and COSMOS. Because stars are known to generally form a linear sequence in multi-band photometric space (based almost solely on effective temperature) while the light of galaxies is comprised of a mix of stars, we construct color-color diagrams (2CDs) from different HST filters and create empirical moving medians using stellarity and brightness cuts to determine the stellar locus. By calculating the distance from the stellar loci in the 2CDs, we are able to distinguish stars from galaxies for objects up to 10^8-10^9 times fainter than those visible to the naked eye. Using Keck data to classify a small subset of stars, we confirm these spectroscopically-classified objects validate our 2CD analysis. We then identify a subset of unusual Milky Way stars within the HALO7D data set and conclude them to be in one of two groups. The first are white dwarfs (with Balmer absorption lines) whose distances, proper motions, and radial velocities we analyze to classify them as part of the Milky Way thin disk, thick disk, or halo with comparison to the synthetic Besancon model on white dwarfs. The other interesting background contaminants have a weak Paschen absorption line series, relatively bright apparent magnitudes, and small proper motions, indicating that these stars are likely not white dwarfs but rather more intrinsically luminous horizontal branch stars or blue stragglers in the remote Milky Way halo. The data analysis techniques developed in the course of this project could be potentially applicable to larger, more complex data sets that will be available in the future. This research was funded in part by the NSF, NASA, and STSCI. High school students AL, TJ, and JT conducted this research under the auspices of the Science Internship Program at UC Santa Cruz.
- Publication:
-
American Astronomical Society Meeting Abstracts #233
- Pub Date:
- January 2019
- Bibcode:
- 2019AAS...23326705L