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Stellar Metallicities from Skymapper Photometry I: a Study of the Tucana II Ultra-Faint Dwarf Galaxy

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Stellar Metallicities from Skymapper Photometry I: a Study of the Tucana II Ultra-Faint Dwarf Galaxy Draft version January 22, 2020 Typeset using LATEX twocolumn style in AASTeX62 Stellar metallicities from SkyMapper photometry I: A study of the Tucana II ultra-faint dwarf galaxy Anirudh Chiti,1 Anna Frebel,1 Helmut Jerjen,2 Dongwon Kim,3 and John E. Norris2 1Department of Physics and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA 2Research School of Astronomy and Astrophysics, Australian National University, Canberra, ACT 2611, Australia 3Astronomy Department, University of California, Berkeley, CA, USA (Received September 2019; Revised December 2019; Accepted January 2019) Submitted to ApJ ABSTRACT We present a study of the ultra-faint Milky Way dwarf satellite galaxy Tucana II using deep photom- etry from the 1.3 m SkyMapper telescope at Siding Spring Observatory, Australia. The SkyMapper filter-set contains a metallicity-sensitive intermediate-band v filter covering the prominent Ca II K feature at 3933.7 A.˚ When combined with photometry from the SkyMapper u; g, and i filters, we demonstrate that v band photometry can be used to obtain stellar metallicities with a precision of 0:20 dex when [Fe/H] > 2:5, and 0:34 dex when [Fe/H] < 2:5. Since the u and v filters bracket the∼ Balmer Jump at 3646 A,˚− we also find∼ that the filter-set can be− used to derive surface gravities. We thus derive photometric metallicities and surface gravities for all stars down to a magnitude of g 20 within 75 arcminutes of Tucana II. Photometric metallicity and surface gravity cuts remove nearly∼ all foreground∼ contamination. By incorporating Gaia proper motions, we derive quantitative membership probabilities which recover all known members on the red giant branch of Tucana II. Additionally, we identify multiple likely new members in the center of the system and candidate members several half-light radii from the center of the system. Finally, we present a metallicity distribution function derived from the photometric metallicities of likely Tucana II members. This result demonstrates the utility of wide-field imaging with the SkyMapper filter-set in studying UFDs, and in general, low sur- face brightness populations of metal-poor stars. Upcoming work will clarify the membership status of several distant stars identified as candidate members of Tucana II. Keywords: galaxies: dwarf galaxies: individual (Tuc II) Local Group stars: abundances 1. INTRODUCTION modeling of their early chemical evolution (e.g., Kirby The Milky Way's satellite dwarf galaxies test paradigms et al. 2011; Venn et al. 2012; Romano et al. 2015; Escala of the formation and evolution of the local universe. et al. 2018) and the nature and properties of the earliest These systems are thought to be similar to those that nucleosynthesis events (e.g., Ji et al. 2016a). were accreted to form the old Milky Way halo (Frebel Of particular interest in this regard are the Milky arXiv:2001.07684v1 [astro-ph.GA] 21 Jan 2020 & Norris 2015). Consequently, studying the stellar con- Way's ultra-faint dwarf galaxies (UFDs). These systems tent of these ancient dwarf galaxies and comparing their have stellar populations that are old (& 10 Gyr) and metal-poor (average [Fe/H] 2:0) (see Simon 2019 stars to those in the Milky Way halo can probe poten- . − tial connections between the these stellar populations for a review), where a metal-poor star is defined as hav- ing an iron abundance [Fe/H] 1, in which [Fe/H] = (e.g., Kirby et al. 2008; Frebel et al. 2010). The rela- ≤ − tively simple nature of dwarf galaxies also enables the log10(NFe=NH)? log10(NFe=NH) . Hence, UFDs are particularly interesting− targets both from the perspec- tive of chemical evolution, since they are thought to Corresponding author: Anirudh Chiti have undergone only a few cycles of chemical enrich- [email protected] ment and star formation, and from a cosmological per- 2 spective, since at least some of them are hypothesized work from the Pristine Survey (Starkenburg et al. 2017), to be surviving first galaxies (Frebel & Bromm 2012). which uses a narrow-band filter centered on the Ca II K However, the faintness of UFDs makes it difficult line at 3933.7A˚ to obtain photometric metallicities, has to study their stellar population in detail. Each sys- been successful in applying this technique to find halo tem has only a handful of stars that are bright enough metal-poor stars (e.g., Youakim et al. 2017; Starken- (V . 19) to obtain detailed chemical abundances. Thus, burg et al. 2018) and also study UFDs to derive their the number of foreground stars generally outnumbers metallicity distribution functions (Longeard et al. 2018, the number of bright(er) UFD stars in images of the 2019). galaxy. This makes identifying UFD member stars for A goal of this paper is to demonstrate that the follow-up observations time-consuming, since stars along SkyMapper filter set (Bessell et al. 2011) can be used the giant branch in a color-magnitude diagram (CMD) to chemically characterize UFDs via photometric mea- must first be spectroscopically followed up with low surements of stellar parameters. Besides metallicities or medium-resolution spectroscopy to measure veloci- from the v filter, the relative flux through the SkyMap- ties (and metallicities) to identify true member stars. per u and v filters is sensitive to the surface gravity Only for those confirmed member stars it is useful to (log g) of stars since those filters surround the Balmer obtain reliable chemical abundances, usually from high- Jump at 3646 A˚ (e.g., Murphy et al. 2009). The log g of resolution spectroscopy. stars is of additional use as a discriminant when studying One can principally bypass the time-intensive interme- UFDs, since their member stars that are bright enough diate step of low or medium-resolution spectroscopy by for spectroscopy are generally on the red giant branch deriving metallicities from photometry, since UFD stars (RGB) and should thus have low surface gravities. We have collectively been shown to be metal-poor ([Fe/H] note that we do not analyze horizontal branch stars in . 2:0), and thus generally more metal-poor than fore- this study, as it is difficult to discriminate their metallic- ground− halo stars (An et al. 2013). Indeed, Pace & ities due to their relatively high effective temperatures. Li(2019) demonstrated that one can increase the ef- Given this difficulty in discriminating metallicities, pho- ficiency in identifying member stars of UFDs by us- tometry with higher precision than the data presented ing metallicity-sensitive colors in Dark Energy Survey in this paper would be required to derive photometric photometry. Deriving reliable metallicities of individ- metallicities for stars on the horizontal branch of Tucana ual metal-poor stars from photometry is a relatively re- II (g 19.2). cent techique (i.e., Starkenburg et al. 2017), building The∼ Tucana II UFD was discovered in the Dark En- on previous studies that demonstrated that photometry ergy Survey (Koposov et al. 2015; Bechtol et al. 2015). could be used to identify metal-poor stars (Anthony- Tucana II is relatively nearby (57 kpc) and has a half- Twarog et al. 1991). Photometric metallicities are gen- light radius of 9.830 (Koposov et al. 2015). It was con- erally computed by using a narrow-band imaging filter firmed as a UFD by Walker et al.(2016), who mea- −1 that is sensitive to the overall metallicity of the star due sured a large velocity dispersion (σvlos = 8:6 km s ), a to the presence of a prominent metal absorption feature low mean metallicity of [Fe/H] = 2.23, and identified (i.e., the Ca II K line) within the bandpass of the filter eight stars as probableh members.i Ji− et al.(2016b) and (e.g., Keller et al. 2007; Starkenburg et al. 2017; Whitten later Chiti et al.(2018) presented chemical abundances et al. 2019). Photometry has the additional benefit of from high-resolution spectroscopy of seven stars in Tu- being able to provide information on all stars within the cana II. Interestingly, two of the stars from the Chiti field of view of the camera, whereas in spectroscopy, one et al.(2018) sample were new member stars that were is limited by e.g., slit arrangements, number of fibers, or approximately two half-light radii from the center of Tu- pixels in the CCD mosaic. cana II. These stars were originally selected for spectro- The SkyMapper Southern Sky Survey (Keller et al. scopic follow-up based on the SkyMapper photometry 2007; Wolf et al. 2018) pioneered the search for described here. If not for these data, they would likely metal-poor stars using a filter set that contains an have been missed by traditional low or medium resolu- intermediate-band 300 A˚ wide v filter that encom- tion spectroscopic selection techniques. passes the Ca II K line∼ within its bandpass, making the We obtained deep images (down to g 22) of the flux through the v filter dependent on the overall stellar Tucana II UFD using the u; v; g, and i filters∼ on the metallicity. This narrow-band v filter has already been 1.3 m SkyMapper telescope to demonstrate that we can used to identify a number of extremely metal-poor stars 1) use the photometry to efficiently identify bright mem- and several stars with [Fe/H] < 6 (Keller et al. 2014; bers for follow-up high resolution spectroscopy, and 2) Jacobson et al. 2015; Nordlander− et al. 2019). Recent use the photometric metallicities of the member stars Photometric metallicities of Tucana II members 3 to derive a metallicity distribution function (MDF) of by-row basis using the overscan region that was 50 pix- dwarf galaxies.
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