The structure and accretion history of the Milky Way from the combination of photometry and spectroscopy of halo RR Lyrae stars

Monday, 15 April 2024 9 a.m. — 10 a.m. MST

AURA Lecture Hall

NOIRLab South Colloquia
Gustavo Medina Toledo (University of Toronto)
In the Lambda CDM framework, the extended halo of the Milky Way and the kinematics of halo stars retain key information to place our Galaxy in a broad cosmological context and to understand the physical processes that govern galaxy and stellar evolution. RR Lyrae stars (RRLs) are pulsating variable stars among the most used outer halo tracers, as they are intrinsically bright distance indicators ubiquitous in the halo. Therefore, studying the physics of their pulsations and their chemodynamics of halo with dedicated and large photometric and spectroscopic surveys is crucial to reconstruct their evolution, the accretion history of our Galaxy and its mass distribution. 
In this talk, I will discuss the outcomes of a search for faint RRLs using Dark Energy Camera (DECam) data over ~400 sq. deg., as part of the Halo Outskirts With Variable Stars (HOWVAST) survey. We identify over 500 RRL candidates with heliocentric distances between 7 and 270 kpc and report the tantalizing detection of 27 stars beyond 100 kpc from the Galactic center. From the radial distribution of our sample we infer the shape of the halo, contrast accretion models with our observations, and check for asymmetries resulting from the dynamical response of the Galaxy to the infall of massive satellites. HOWVAST represents our effort to carry out frontier Galactic science with RRLs, and should only be surpassed once the Rubin Observatory Legacy Survey of Space and Time (LSST) begins scientific operations. I will also report the results of our spectroscopic follow-up campaigns and what we learnt from the Milky Way accretion history from a 6-D analysis of these data. Additionally, I will discuss a recent exploration of the rich dataset provided by the Dark Energy Spectroscopic Instrument (DESI) survey, that contains ~6,000 RR Lyrae stars with homogeneously-derived spectroscopic properties (radial velocities and atmospheric parameters). We use the reported properties not only to investigate the physics of their pulsation, but also to study the formation and general properties of the Milky Way. I will present several applications of the DESI RR Lyrae catalog, such as studying the metallicity dependence of their pulsational properties (periods and amplitudes) and using DESI to measure the mass of the Milky Way within 250 kpc. I will highlight the tantalizing potential of DESI and its complementarity with other large spectroscopic and photometric surveys, all of which will allow us to shed new lights on key aspects of stellar astrophysics and the formation and evolution of our Galaxy.