RR lyrae stars in the newly discovered ultra-faint dwarf galaxy centaurus I

Abstract

We report the detection of three RR Lyrae (RRL) stars (two RRc and one RRab) in the ultra-faint dwarf (UFD) galaxy Centaurus I (Cen I) and two Milky Way (MW) δ Scuti/SX Phoenicis stars based on multi-epoch giz DECam observations. The two RRc stars are located within two times the half-light radius (rh) of Cen I, while the RRab star (CenI-V3) is at ∼6 rh. The presence of three distant RRL stars clustered this tightly in space represents a 4.7σ excess relative to the smooth distribution of RRL in the Galactic halo. Using the newly detected RRL stars, we obtain a distance modulus to Cen I of μ0 = 20.354 ± 0.002 mag (σ = 0.03 mag), a heliocentric distance of De = 117.7 ± 0.1 kpc (σ = 1.6 kpc), with systematic errors of 0.07 mag and 4 kpc. The location of the Cen I RRL stars in the Bailey diagram is in agreement with other UFD galaxies (mainly Oosterhoff II). Finally, we study the relative rate of RRc+RRd (RRcd) stars (fcd) in UFD and classical dwarf galaxies. The full sample of MW dwarf galaxies gives a mean of fcd = 0.28. While several UFD galaxies, such as Cen I, present higher RRcd ratios, if we combine the RRL populations of all UFD galaxies, the RRcd ratio is similar to the one obtained for the classical dwarfs (fcd ∼ 0.3). Therefore, there is no evidence for a different fraction of RRcd stars in UFD and classical dwarf galaxies. ; Fermilab LDRD project L2019-011 NASA Fermi Guest Investigator Program Cycle 9 91201 United States Department of Energy (DOE) National Science Foundation (NSF) Spanish Government UK Research & Innovation (UKRI) Science & Technology Facilities Council (STFC) UK Research & Innovation (UKRI) Higher Education Funding Council for England (HEFCE) National Center for Supercomputing Applications at the University of Illinois at Urbana-Champaign University of Chicago Ohio State University Mitchell Institute for Fundamental Physics and Astronomy at Texas AM University Financiadora de Inovacao e Pesquisa (Finep) Fundaco Carlos Chagas Filho de Amparo a Pesquisa do Estado do Rio de Janeiro Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPQ) Spanish Government German Research Foundation (DFG) Dark Energy Survey United States Department of Energy (DOE) University of California at Santa Cruz University of Cambridge, Centro de Investigaciones Energeticas, Medioambientales y Tecnologicas-Madrid University of Chicago, University College London DES-Brazil Consortium University of Edinburgh ETH Zurich United States Department of Energy (DOE) University of Chicago University of Illinois at Urbana-Champaign Institut de Ciencies de l'Espai (IEEC/CSIC) Institut de Fisica d'Altes Energies, Lawrence Berkeley National Laboratory Ludwig-Maximilians Universitat Munchen associated Excellence Cluster Universe University of Michigan, NSF's NOIRLab University of Nottingham Ohio State University OzDES Membership Consortium University of Pennsylvania University of Portsmouth Stanford University United States Department of Energy (DOE) Stanford University University of Sussex Texas AM University United States Department of Energy (DOE) DE-AC02-07CH11359 ; Versión publicada - versión final del editor