Variable stars in Local Group Galaxies -- V. The fast and early evolution of the low-mass Eridanus II dSph galaxy

Abstract

We present a detailed study of the variable star population of Eridanus II (Eri II), an ultra-faint dwarf galaxy that lies close to the Milky Way virial radius. We analyze multi-epoch $g,r,i$ ground-based data from Goodman and the Dark Energy Camera, plus $F475W,F606W,F814W$ space data from the Advanced Camera for Surveys. We report the detection of 67 RR Lyrae (RRL) stars and 2 Anomalous Cepheids, most of them new discoveries. With the RRL stars, we measure the distance modulus of Eri II, ${\mu }_0=22.84\pm 0.05$ mag (D${}_{\odot }=370\pm 9$ kpc) and derive a metallicity spread of 0.3 dex (0.2 dex intrinsic). The colour distribution of the horizontal branch (HB) and the period distribution of the RRL stars can be nicely reproduced by a combination of two stellar models of [Fe/H]=($-2.62$, $-2.14$). The overall low metallicity is consistent with the red giant branch bump location, 0.65 mag brighter than the HB. These results are in agreement with previous spectroscopic studies. The more metal-rich RRL and the RRab stars have greater central concentration than the more metal-poor RRL and the RRc stars that are mainly located outside $\sim 1$ r${}_{\mathrm{h}}$. This is similar to what is found in larger dwarf galaxies such as Sculptor, and in agreement with an outside-in galaxy formation scenario. This is remarkable in such a faint dwarf galaxy with an apparently single and extremely short ($<1$ Gyr) star formation burst. Finally, we have derived new and independent structural parameters for Eri II and its star cluster using our new data that are in very good agreement with previous estimates.

20 pages, 14 figures, 8 tables. Accepted for publication in MNRAS

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Cited by in corpus (8)

• Metallicity Distribution Functions of 13 Ultra-Faint Dwarf Galaxy Candidates from Hubble Space Telescope Narrowband Imaging
• RR Lyrae-based Distances for 39 Nearby Dwarf Galaxies Calibrated to Gaia eDR3
• Metallicity Distribution Function of the Eridanus~II Ultra-Faint Dwarf Galaxy from Hubble Space Telescope Narrow-band Imaging
• Testable predictions of outside-in age gradients in dwarf galaxies of all types
• On the Reionization-Era Globular Cluster in Low-Mass Galaxy Eridanus II
• EDGE: the puzzling ellipticity of Eridanus II's star cluster and its implications for dark matter at the heart of an ultra-faint dwarf
• DELVE Milky Way Satellite Galaxy Census I: Satellite Population and Survey Selection Function in DES, DELVE, and Pan-STARRS
• New theoretical Period-Luminosity-Metallicity relations for RR Lyrae in the Rubin-LSST filters