The Next Generation Virgo Cluster Survey. XXXVII. Distant RR Lyrae Stars and the Milky Way Stellar Halo out to 300 kpc
arxiv(2024)
摘要
RR Lyrae stars are standard candles with characteristic photometric
variability and serve as powerful tracers of Galactic structure, substructure,
accretion history, and dark matter content. Here we report the discovery of
distant RR Lyrae stars, including some of the most distant stars known in the
Milky Way halo, with Galactocentric distances of approximately 300 kpc. We use
time-series u*g'i'z' Canada-France-Hawaii Telescope/MegaCam photometry from the
Next Generation Virgo Cluster Survey (NGVS). We employ a template light curve
fitting method based on empirical Sloan Digital Sky Survey (SDSS) Stripe 82 RR
Lyrae data to identify RR Lyrae candidates in the NGVS data set. We eliminate
several hundred suspected quasars and identify 180 RR Lyrae candidates, with
heliocentric distances of approximately 20–300 kpc. The halo stellar density
distribution is consistent with an r^(-4.09 +/- 0.10) power-law radial profile
over most of this distance range with no signs of a break. The distribution of
ab-type RR Lyrae in a period-amplitude plot (Bailey diagram) suggests that the
mean metallicity of the halo decreases outwards. Compared to other recent RR
Lyrae surveys, like Pan-STARRS1 (PS1), the High Cadence Transient Survey
(HiTS), and the Dark Energy Survey (DES), our NGVS study has better
single-epoch photometric precision and a comparable number of epochs but
smaller sky coverage. At large distances, our RR Lyrae sample appears to be
relatively pure and complete, with well-measured periods and amplitudes. These
newly discovered distant RR Lyrae stars are important additions to the few
secure stellar tracers beyond 150 kpc in the Milky Way halo.
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