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Precision Distances to Dwarf Galaxies and Globular Clusters from Pan-Starrs1 3Π Rr Lyrae

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Precision Distances to Dwarf Galaxies and Globular Clusters from Pan-Starrs1 3Π Rr Lyrae DRAFT VERSION NOVEMBER 28, 2018 Typeset using LATEX twocolumn style in AASTeX61 PRECISION DISTANCES TO DWARF GALAXIES AND GLOBULAR CLUSTERS FROM PAN-STARRS1 3π RR LYRAE NINA HERNITSCHEK,1 JUDITH G. COHEN,1 HANS-WALTER RIX,2 EUGENE MAGNIER,3 NIGEL METCALFE,4 RICHARD WAINSCOAT,3 CHRISTOPHER WATERS,5 ROLF-PETER KUDRITZKI,3 AND WILLIAM BURGETT6 1Division of Physics, Mathematics and Astronomy, Caltech, Pasadena, CA 91125 2Max-Planck-Institut fur¨ Astronomie, Konigstuhl¨ 17, 69117 Heidelberg, Germany 3Institute for Astronomy, University of Hawaii at Manoa, Honolulu, HI 96822, USA 4Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK 5Department of Astrophysical Sciences, 4 Ivy Lane, Princeton University, Princeton, NJ 08544, USA 6GMTO Corporation (Pasadena), 465 N. Halstead Street, Suite 250, Pasadena, CA 91107, USA ABSTRACT We present new spatial models and distance estimates for globular clusters (GC) and dwarf spheroidals (dSphs) orbiting our Galaxy based on RR Lyrae (RRab) stars in the Pan-STARRS1 (PS1) 3π survey. Using the PS1 sample of RRab stars from Sesar et al.(2017) in 16 globular clusters and 5 dwarf galaxies, we fit structural models in (l; b; D) space; for 13 globular clusters and 6 dwarf galaxies, we give only their mean heliocentric distance D. We verify the accuracy of the period-luminosity (PL) relations used in Sesar et al.(2017) to constrain the distance to those stars, and compare them to period-luminosity-metallicity (PLZ) relations using metallicities from Carretta et al.(2009). We compare our Sesar et al.(2017) distances to the parallax-based Gaia DR2 distance estimates from Bailer-Jones et al.(2018), and find our distances to be consistent and considerably more precise. arXiv:1811.10808v1 [astro-ph.GA] 27 Nov 2018 Corresponding author: Nina Hernitschek [email protected] 2 HERNITSCHEK ET AL. 1. INTRODUCTION PL relations for both the RRab and RRc samples as well as In this paper, we exploit the opportunities provided by the for the combined RRab+RRc sample. In their 2017 paper Pan-STARRS1 (PS1) 3π RR Lyrae catalog (Sesar et al. 2017) (Neeley et al. 2017), they also present theoretical mid-IR to explore the Galactic halo. Using that large data set of PLZ relations for RR Lyrae stars at Spitzer and WISE wave- RRab stars covering 3/4 of the sky out to more than 130 kpc, lengths and show that the mid-IR PLZ relations can provide in Hernitschek et al.(2018) we looked at the seemingly distance estimates to individual RR Lyrae stars with uncer- smooth part of the stellar halo. We now focus on the RRab tainties better than 2%. This is comparable to the result by stars in globular clusters (GC) and dwarf galaxies in order to Sesar et al.(2017) derived for the RRab stars with optical determine their distances and constrain their structure. For photometry used in this paper. For the dSph Sculptor, which most of these objects, RRab stars may be the most precise lies outside of the PS1 3π footprint, Garofalo et al.(2018) distance tracers. As shown in Section8, all GC and dwarf derived mid-IR PLZ relations and calculated the distance to galaxies we study here are beyond any parallax precision of the dwarf galaxy as part of the Spitzer SHMASH project, Gaia DR2, which is according to Bailer-Jones et al.(2018) which targeted in total four dSph (Ursa Minor dSph, Bootes only valid within a heliocentric distance range of about 5 kpc. dSph, Sculptor dSph and Carina dSph). The main advantages of such a study are that due to the large This paper is structured as follows. In Section2, we intro- angular extent and depth of PS1 3π, the source of observa- duce the PS1 3π survey and lay out the properties of the PS1 tional data as well as the methodology are the same for all RRab stars. This is followed in Section3 by a description GC and dSph we analyze. The aim of this paper is to pre- of the spatial area covered by PS1 3π, and the list of GCs cisely constrain the position and extent of each of these over- and dSph that are included in it. In Section4, we present densities from RRab stars, where we determine these prop- the method of fitting a series of parameterized stellar den- erties for 29 GC and 11 dwarf spheroidal (dSph) galaxies, sity models and the probabilistic approach to constrain their and if this is not possible, to at least to give their mean he- model parameters. We describe fitting tests on mock data in liocentric distances. Furthermore, from the globular clusters Section 4.3. In Section5, the position and extent are esti- we demonstrate that the period-luminosity relations used by mated for each GC and dSph. We present various analyses Sesar et al.(2017) to determine the distances of the RRab based on these fitted parameters. In Section6, we calculate stars are precise and their predicted magnitudes well within mean distances for those dwarf galaxies and GC which don’t the claimed uncertainties. have enough sources for the fitting process. We also discuss RR Lyrae stars (RRL) as distance indicators have the ad- the implications on period-luminosity relations in Sec.7, and vantage of being low-mass stars found in both early- and late- compare our distance estimates to those derived using the type stellar systems (van den Bergh 1999). When periods, or second Gaia data release in Sec.8. A discussion and sum- periods and metallicities, for RRL are known, their distances mary of our results is given in Section9. The main part of can be estimated using period-luminosity (PL) or period- the paper is followed by a large Appendix containing figures luminosity-metallicity (PLZ) relations. Visual magnitude- as well as tables. metallicity relations are described for example in Chaboyer 2. RR LYRAE STARS FROM THE PS1 SURVEY et al.(1996), Bono et al.(2003), Cacciari & Clementini (2003). In the infrared, the extinction is smaller and the am- As in our previous papers, e.g. Hernitschek et al.(2017) plitude of variation is smaller. In the near infrared (NIR), and Hernitschek et al.(2018), our analysis is based on a well-defined PL relations can be found (Longmore et al. sample of highly likely RRab stars, as selected by Sesar et al. 1986; Bono et al. 2001, 2003; Catelan et al. 2004; Braga (2017) from the Pan-STARRS1 3π survey. Here, we briefly et al. 2015; Sesar et al. 2017), extending to the mid-infrared describe the pertinent properties of the PS1 3π survey and the with only small scatter (Madore et al. 2013; Klein et al. RR Lyrae light curves obtained, and recapitulate briefly the 2014). process of selecting the likely RRab for the catalog of PS1 These individual distance estimates can then be used to cal- RRab stars, as laid out in Sesar et al.(2017). We also briefly culate distances to and the shape of globular clusters as well characterize the obtained candidate sample. as dwarf galaxies. Dwarf galaxies orbiting our Milky Way The Pan-STARRS 1 (PS1) survey (Kaiser et al. 2010) col- are not only interesting objects per se, but can also provide lected multi-epoch, multi-color observations undertaking a us with important information to determine the Milky Way’s number of surveys, among which the PS1 3π survey (Cham- total mass and dynamics. bers et al. 2016) is currently the largest. It has ob- ◦ Dambis et al.(2014) used WISE as well as zero-points served the entire sky north of declination −30 in five fil- from HST parallaxes to derive PL relations from 360 RRL ter bands (gP1; rP1; iP1; zP1; yP1) with a 5σ single epoch belonging to 15 globular clusters. depth of about 22.0, 22.0, 21.9, 21.0 and 19.8 magnitudes More recent work on the distances of globular clusters was in gP1; rP1; iP1; zP1, and yP1, respectively (Stubbs et al. carried out by e.g. Neeley et al.(2015) who investigated 2010; Tonry et al. 2012). 9 mid-IR period-luminosity-metallicity (PLZ) relations for the Starting with a sample of more than 1:1 × 10 PS1 3π globular cluster NGC 6121 (M4), which is not in our sample sources, Hernitschek et al.(2016) and Sesar et al.(2017) as it lies outside of the PS1 3π footprint. Their work contains subsequently selected a sample of 44,403 likely RRab stars, of which ∼17,500 are at Rgc ≥ 20 kpc, by applying PRECISION DISTANCES TO DWARF GALAXIES AND GLOBULAR CLUSTERS FROM PAN-STARRS1 3π RR LYRAE 3 machine-learning techniques based on light-curve charac- Catalogue of Variable Stars in Globular Clusters. As shown teristics. RRab stars are the most common type of RR Lyrae, in Fig. 12, we find that for each of the 11 globular clusters making up ∼91% of all observed RR Lyrae (Smith 2004), available in both catalogs, our catalog misses most RRab in and displaying the steep rises in brightness typical of RR the globular cluster’s central regions. The dSph galaxies are Lyrae. sufficiently diffuse that this is not a significant issue. The identification of the RRab stars is highly effective, and the sample of RRab stars is pure (90%), and complete 4. DENSITY FITTING (≥ 80% at 80 kpc) at high galactic latitudes. Distances to In this section we lay out a forward-modeling approach to these stars were calculated based on flux-averaged i mag- P1 describe the spatial distribution of RRab stars within and near nitudes, corrected for dust extinction using extinction coeffi- overdensities such as dwarf galaxies and globular clusters.
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