HALO7D Extension Survey: Analysis of Keck DEIMOS Spectra and Velocities of Milky Way Halo Stars in the COSMOS Field

The stellar halo of the Milky Way (MW), an extended sparse component that surrounds the main disk, is composed mostly of old stars and contains vital clues about the MW’s accretion history. In this research project, we analyzed DEIMOS multislit spectra of stars in the COSMOS field from the HALO7D survey extension, a multidimensional HST+Gaia+Keck study of the MW’s stellar halo. Based on Gaia-based apparent magnitudes of the target stars in the B, G, and R bands, which match the range of wavelengths covered by the DEIMOS spectra, we determined the spectroscopic flux calibration constant C for each of the three bandpasses for each multislit mask. The C values tend to decrease with increasing airmass, particularly in the B band (scattering/absorption of starlight by dust in the Earth’s atmosphere), and increasing seeing FWHM (slit losses). A fraction of the spectra are affected by instrumental artifacts and appear as outliers in plots of calibrated spectroscopic flux vs. Gaia B/G/R apparent magnitude. We present line-of-sight velocity distributions (LOSVD) of stars grouped in two different ways: (1) by spectral type, and (2) by color-magnitude diagram (CMD) location. We compare the COSMOS field LOSVDs to the prediction of the Besançon model, a simple smooth analytical model of the MW, with model stars grouped by CMD location like the COSMOS data. The observed LOSVD gets broader and the mean velocity more positive as one goes from low temperature M dwarf stars to warmer K dwarfs to hotter main sequence turnoff stars; the model comparison indicates that we are sampling the MW thin disk, thick disk, and halo, respectively. There is generally good agreement between the COSMOS data and Besançon model thin and thick disk predictions, but, as has previously been noted, the model halo density profile is shallower than observed. These kinds of data and model comparisons provide insight into the structure and substructure (departure from smoothness) of the MW, revealing information about its accretion history. EU and RC conducted their research under the auspices of the Science Internship Program at the University of California Santa Cruz. This research was funded in part by the National Science Foundation and NASA/STScI.