Draft version August 11, 2021 Preprint typeset using LATEX style emulateapj v. 12/16/11 STAR FORMATION HISTORIES OF ULTRA-FAINT DWARF GALAXIES: ENVIRONMENTAL DIFFERENCES BETWEEN MAGELLANIC AND NON-MAGELLANIC SATELLITES? ? Elena Sacchi1;2;3, Hannah Richstein4, Nitya Kallivayalil4, Roeland van der Marel1;5, Mattia Libralato6, Paul Zivick4, Gurtina Besla7, Thomas M. Brown1, Yumi Choi1, Alis Deason8;9, Tobias Fritz10;11, Marla Geha12, Puragra Guhathakurta13, Myoungwon Jeon14, Evan Kirby15;16, Steven R. Majewski4, Ekta Patel17;18, Joshua D. Simon19, Sangmo Tony Sohn1, Erik Tollerud1, and Andrew Wetzel20 1Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 2 Leibniz-Institut f¨urAstrophysik Potsdam, An der Sternwarte 16, 14482 Potsdam, Germany; [email protected] 3INAF{Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Via Gobetti 93/3, I-40129 Bologna, Italy 4University of Virginia, Department of Astronomy, 530 McCormick Road, Charlottesville, VA 22904, USA 5Center for Astrophysical Sciences, Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD 21218, USA 6AURA for the European Space Agency (ESA), ESA Office, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA 7Steward Observatory, University of Arizona, 933 North Cherry Avenue, Tucson, AZ 85721-0065, USA 8Institute for Computational Cosmology, Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK 9Centre for Extragalactic Astronomy, Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK 10Instituto de Astrof´ısicade Canarias, Calle Via L´acteas/n, 38206, La Laguna, Tenerife, Spain 11Universidad de La Laguna (ULL), Departamento de Astrof´ısica,30206, La Laguna, Tenerife, Spain 12Department of Astronomy, Yale University, 52 Hillhouse Ave., New Haven, CT 06520, USA 13UCO/Lick Observatory, Department of Astronomy & Astrophysics, University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064, USA 14School of Space Research, Kyung Hee University, 1732 Deogyeong-daero, Yongin-si, Gyeonggi-do 17104, Korea 15Department of Astronomy, California Institute of Technology, 1200 E California Boulevard, Pasadena, CA 91125, USA 16Department of Physics, University of Notre Dame, Notre Dame, IN 46556, USA 17Department of Astronomy, University of California, Berkeley, 501 Campbell Hall, Berkeley, CA, 94720, USA 18Miller Institute for Basic Research in Science, 468 Donner Lab, Berkeley, CA 94720, USA 19Observatories of the Carnegie Institution for Science, 813 Santa Barbara Street, Pasadena, CA 91101, USA 20Department of Physics & Astronomy, University of California, Davis, CA 95616, USA Draft version August 11, 2021 ABSTRACT We present the color-magnitude diagrams and star formation histories (SFHs) of seven ultra-faint dwarf galaxies: Horologium 1, Hydra 2, Phoenix 2, Reticulum 2, Sagittarius 2, Triangulum 2, and Tucana 2, derived from high-precision Hubble Space Telescope photometry. We find that the SFH of each galaxy is consistent with them having created at least 80% of the stellar mass by z 6. For all galaxies, we find quenching times older than 11.5 Gyr ago, compatible with the scenario∼ in which reionization suppresses the star formation of small dark matter halos. However, our analysis also reveals some differences in the SFHs of candidate Magellanic Cloud satellites, i.e., galaxies that are likely satellites of the Large Magellanic Cloud and that entered the Milky Way potential only recently. Indeed, Magellanic satellites show quenching times about 600 Myr more recent with respect to those of other Milky Way satellites, on average, even though the respective timings are still compatible within the errors. This finding is consistent with theoretical models that suggest that satellites' SFHs may depend on their host environment at early times, although we caution that within the error bars all galaxies in our sample are consistent with being quenched at a single epoch. Keywords: galaxies: ultra faint dwarf { galaxies: evolution { galaxies: star formation { galaxies: stellar content { galaxies: kinematics and dynamics { Local Group { Magellanic Clouds 1. INTRODUCTION ing Lambda Cold Dark Matter (ΛCDM) cosmological arXiv:2108.04271v1 [astro-ph.GA] 9 Aug 2021 Ultra-faint dwarf (UFD) galaxies are interesting and scenario, an important channel for mass growth of DM peculiar objects, representing many extremes in terms of halos is hierarchical accretion. Indeed, simulations show galaxy properties. Their population includes the least that even low-mass host halos have substructures down luminous, least chemically-enriched, most dark matter to their resolution limit (Wetzel et al. 2016; Dooley et al. (DM) dominated, and oldest satellite galaxies of the 2017; Besla et al. 2018; Jahn et al. 2019; Wang et al. Milky Way (MW). UFDs could be the relics of the first 2020). UFDs would also undergo hierarchical growth, galaxies believed to form, and therefore provide us with and are thus precious tools to study the physics of galaxy a fossil record of the conditions for star formation in the assembly in the early Universe. era of reionization (Bovill & Ricotti 2009; Wheeler et al. Recent simulations have illustrated in detail that star 2019, and references therein). According to the prevail- formation (SF) in UFDs is impacted by the local back- ground UV ionizing field and stellar feedback, with both acting as effective quenching mechanisms at such low- ? Based on observations obtained with the NASA/ESA Hubble Space Telescope at the Space Telescope Science Institute, which mass scales (e.g., Jeon et al. 2017, 2021; Wheeler et al. is operated by the Association of Universities for Research in 2019; Applebaum et al. 2021). The timescales in these Astronomy under NASA Contract NAS 5-26555. simulations are such that shortly after the epoch of reion- 2 Sacchi et al. ization ends (z 6), UFDs are effectively quenched, al- scenario, they identified Carina 2, Carina 3, Horologium though the exact∼ duration of SF can vary depending on 1, and Hydrus 1 as long-term Magellanic satellites, and the halo mass, even for a fixed ionization background Reticulum 2 and Phoenix 2 as recently captured Magel- (Jeon et al. 2017), and often some residual interstellar lanic satellites. medium remains in the galaxies and can fuel SF for Although these works provide fundamental contribu- another 1 - 2 Gyr (Wheeler et al. 2019). Analyses of tions to our understanding of the MW satellites' dy- zoom-in simulations of UFDs in MW-like environments namics, we need star formation histories (SFHs) to fully have found that traditional environmental effects (e.g., explore their properties and understand the impact of tidal field, ram pressure) are not primary factors in the environment and reionization on such low mass galax- quenching timescale (Applebaum et al. 2021). ies. Within the Local Group, the Hubble Space Telescope Observational data support a ubiquitous quenching (HST ) is able to resolve individual stars in galaxies down timescale for UFDs around the time of reionization to several magnitudes below the oldest main sequence (Brown et al. 2014; Weisz et al. 2014). Given that the (MS) turnoff, allowing us to measure their ancient SFHs MW accreted these systems at different times (Fritz et al. and explore differences in the SF quenching behavior (as 2018), this indicates a global rather than a local phys- in, e.g., Brown et al. 2014). A detailed analysis and com- ical explanation for the common quenching timescale. parison of UFDs residing in different environments at However, other studies (e.g., Wetzel et al. 2016; Joshi early times is also one way to discover variations in the et al. 2021) have shown a dependence of the quenching ionization field over large scales. timescale on host mass, presumably because the strength Here we present an analysis of the optical color- of the background and local UV field changes depending magnitude diagrams (CMDs) and SFHs of seven UFD on the environment (more massive hosts would have more galaxies part of the HST Treasury Program 14734 SF and thus produce more ionizing radiation). (PI: Kallivayalil). They are listed in Table1, together UFDs are hard to identify due to their low luminosi- with their distances, in the range 30 150 kpc, 4 ∼ − ties MV > 8, implying stellar masses M? . 10 M , V -band magnitudes, between 1:8 and 5:2, and and generally− old (> 10 Gyr) stellar populations (Simon possible association with the Magellanic− Clouds.− Our 2019), which lack bright young stars that would ease analysis is based on high-precision photometry from their discovery and identification. However, a great ef- the Advanced Camera for Surveys (ACS) and liter- fort has been made in the past few years to increase the ature spectroscopic measurements, and employs the statistics of satellites around the MW, and many new synthetic CMD method to derive the SFH of each galaxy. UFDs were discovered thanks to wide-field surveys such as PAN-STARRS (Laevens et al. 2015), DES (Bechtol et al. 2015; Drlica-Wagner et al. 2015; Koposov et al. 2. DATA AND PHOTOMETRY 2015), and ATLAS (Torrealba et al. 2016). Observations of a total of 30 UFDs were performed us- Many of these new satellites were found in the proxim- ing the F606W and F814W filters of the HST ACS Wide ity of the Magellanic Clouds (MC), a region targeted by Field Channel (Treasury Program 14734; PI: Kallivay- several deep imaging surveys (e.g., Drlica-Wagner et al. alil). The basic observing strategy included collecting 2015; Martin et al. 2015; Nidever et al. 2017; Koposov four dithered 1100 s exposures in both filters for each et al. 2018; Torrealba et al. 2018). These surveys pro- target. vide a great opportunity to test the self-similarity of The images were processed through the current ACS ΛCDM, which predicts that MW satellites, such as the pipeline, which corrects for charge-transfer inefficiency Large Magellanic Cloud (LMC), should also have their (CTI), and the separate dithers were combined using own satellites, which fell into the MW potential with the the DRIZZLE package to create the drc files.