Phlegethon, a Nearby $75\Deg $-Long Retrograde Stellar Stream
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Draft version September 12, 2018 Preprint typeset using LATEX style emulateapj v. 12/16/11 PHLEGETHON, A NEARBY 75◦-LONG RETROGRADE STELLAR STREAM Rodrigo A. Ibata1, Khyati Malhan1, Nicolas F. Martin1,2, and Else Starkenburg3 Draft version September 12, 2018 ABSTRACT We report the discovery of a 75◦ long stellar stream in Gaia DR2 catalog, found using the new STREAMFINDER algorithm. The structure is probably the remnant of a now fully disrupted globular cluster, lies ≈ 3:8 kpc away from the Sun in the direction of the Galactic bulge, and possesses highly retrograde motion. We find that the system orbits close to the Galactic plane at Galactocentric dis- tances between 4:9 and 19:8 kpc. The discovery of this extended and extremely low surface brightness −2 stream (ΣG ∼ 34:3 mag arcsec ) with a mass of only 2580 ± 140 M , demonstrates the power of the STREAMFINDER algorithm to detect even very nearby and ultra-faint structures. Due to its proximity and length we expect that Phlegethon will be a very useful probe of the Galactic acceleration field. Keywords: Galaxy: halo | Galaxy: stellar content | surveys | galaxies: formation | Galaxy: structure 1. INTRODUCTION through astrometric and photometric databases for The arrival of the second data release (DR2) of the stream-like structures. The first results of this algorithm Gaia mission has opened up the field of Galactic Archeol- applied to Gaia DR2 were presented in Malhan et al. ogy to exciting new endeavors that were previously com- (2018, hereafter paper II), but were limited to distances pletely out of reach. The excellent parallax and proper > 5 kpc, a choice that we made in order to reduce the motion measurements (Gaia Collaboration et al. 2018d) necessary calculation time. Here we describe a discovery of over a billion stars now allow the the dynamics of based upon re-running the algorithm searching for stellar the various constituents of our Galaxy to be studied in streams at distances between 0.5 and 5 kpc. great detail (Gaia Collaboration et al. 2018c,b), enabling The layout of this article is as follows. Section 2 progress towards the ultimate goal of understanding the explains the selection criteria that were applied to the formation of our Galaxy, its stellar components and the data and briefly summarizes the detection algorithm and dark matter. adopted parameters. The results of the analysis are re- Among the Galactic components, stellar streams ac- ported in Section 3. We present a refined fit to the stream count for only a very minor fraction of the total mass in Section 4, and our discussion and conclusions in Sec- budget, yet their astrophysical interest far exceeds their tion 5. small contribution to our Galaxy. This importance stems 2. DATA AND STREAM ANALYSIS partly from the fact that they represent fossil remnants of the accretion events that built up the Milky Way, giv- All the data used in the present analysis were drawn ing us a means to ascertain the number and provenance from the Gaia DR2 catalog (Gaia Collaboration et al. of the building blocks of the Galactic halo (Johnston 2016, 2018a). As described in paper II, we extinction- et al. 2008). Streams also roughly delineate orbits in corrected the survey using the Schlegel et al. (1998) dust the Galaxy, allowing one to probe the gradient of the maps, and kept only those stars with G0 < 19:5; this lim- gravitational potential (Ibata et al. 2001; Law & Ma- iting magnitude ensures a homogeneous depth over the sky, given the additional selection criterion of choosing jewski 2010; Varghese et al. 2011; K¨upper et al. 2015; ◦ Bovy et al. 2016; Bonaca & Hogg 2018) in a way that is to analyze the sky at jbj > 20 . independent of methods that make the often-unjustified A full exposition of the updated STREAMFINDER al- assumption of dynamical equilibrium of a tracer popula- gorithm and contamination model will be provided in tion. Furthermore, the low velocity dispersion and fine Ibata et al. (2018, in prep), which includes a demonstra- arXiv:1806.01195v2 [astro-ph.GA] 10 Sep 2018 transverse width of low-mass streams renders them ex- tion that real structures are easily distinguishable from cellent probes of the small-scale substructure of the dark false positives. Briefly, the algorithm works by examin- matter halo (Ibata et al. 2002; Johnston et al. 2002; Carl- ing every star in the survey in turn, sampling the possi- berg 2012; Erkal et al. 2016). ble orbits consistent with the observed photometry and In a previous contribution, we introduced a new algo- kinematics, and finding the most likely stream fraction given a contamination model and a stream model. The rithm (the STREAMFINDER; Malhan & Ibata 2018, here- after paper I) built specifically to search efficiently adopted contamination model is identical to that built for paper II. It is an empirical model in sky position, color- magnitude and proper motion space (i.e. six-dimensions) 1 Observatoire Astronomique, Universit´e de Strasbourg, constructed by spatially randomizing the Gaia counts CNRS, 11, rue de l'Universit´e,F-67000 Strasbourg, France; ro- with a 2◦ Gaussian. While the correlations between sky [email protected] 2 Max-Planck-Institut f¨ur Astronomie, K¨onigstuhl 17, D- position and color-magnitude are recorded as binned ar- 69117 Heidelberg, Germany rays, the color-magnitude and proper motion information 3 Leibniz Institute for Astrophyics Potsdam (AIP), An der is condensed by the use of a 100-component Gaussian Sternwarte 16, D-14482 Potsdam, Germany mixture model in each spatial bin. 2 Ibata, Malhan, Martin & Starkenburg 0 30 [Fe/H] = 1.4, retrograde 20 ] g 30 e d [ 60 Phlegethon b 10 ] 1 40 ] r g y 60 20 e s 0 d a [ m 0 0 [ b 1 [Fe/H] = 1.4, retrograde, vtan > 250 km s 20 10 ] g 30 e 20 d [ Phlegethon b 30 50 40 30 20 10 0 10 60 0 200 400 1 1 [mas yr ] vtan [km s ] 90 60 30 0 30 [deg] Figure 2. Proper motion distribution of the sample of stars in the lower panel of Figure 1. The color of the points marks Galactic longitude, which can be seen to vary continuously along the arc- Figure 1. Output of the STREAMFINDER software in the distance shaped feature in this distribution. This distinct structure was se- range [0:5; 5:0] kpc over the region of sky −40◦ < ` < 90◦, ◦ ◦ lected to lie within an irregular hand-drawn polygon in this proper −70 < b < −20 , assuming a stellar population of age 10 Gyr and motion plane; the resulting sample is shown with the larger (black [Fe=H] = −1:4. Only those stars are plotted that have retrograde circled) dots. orbital solutions, and for which the stream detection significance is greater than 8σ. The upper panel shows the full range of tan- gential motions, while the bottom panel retains only those stars −1 with vtan > 250 km s . A clear stream, labelled \Phlegethon", is To convert the kinematics of the integrated orbits detected as a continuous linear structure in the panels. The color of the points marks the tangential velocity of the stars, calculated into the space of observables, we assume that the Sun using the observed Gaia proper motion in conjunction with the lies 8:20 kpc from the Galactic center and 17 pc above distance solutions that the algorithm derives for every processed the Galactic mid-plane (Karim & Mamajek 2017), that star. −1 the local circular velocity is Vcirc = 240 km s and that the peculiar velocity of the Sun is (u ; v ; w ) = The adopted stream model is very simple: we inte- (9:0; 15:2; 7:0) km s−1 (Reid et al. 2014; Sch¨onrich et al. grate along the sampled orbits for a half-length of 10◦ 2010). (i.e. total length 20◦), and over this length the stream The main aim in developing the STREAMFINDER was counts have uniform probability. Perpendicular to the to try to find distant halo streams for which the Gaia stream model, the properties of all observables are taken parallax measurements are poor. To circumvent this de- to be Gaussian, so the model has a Gaussian physical ficiency, we use stellar populations models to convert the width (selected here to be 100 pc), a Gaussian dispersion measured photometry into trial line of sight distance val- in both proper motion directions equivalent to 3 km s−1, ues. To this end we adopted the PARSEC isochrone and a dispersion in distance modulus of 0:05 mag. These models (Bressan et al. 2012), covering a range in age width and velocity dispersion parameters are similar to and metallicity. Here, however, we report results us- the properties of known globular cluster streams such ing a single model with age 10 Gyr and metallicity of as the Palomar 5 stream (Dehnen et al. 2004; Ibata [Fe=H] = −1:4. et al. 2016), while the distance modulus dispersion was The stream solutions found by the STREAMFINDER au- adopted to allow for a small mismatch between the color- tomatically include the best-fit orbit over the trial stream magnitude behavior of the adopted stellar populations length, from which one can naturally obtain an estimate model and that of the real stellar population. of the sense of rotation of the survey stars with respect to The STREAMFINDER requires a model of the Galactic the Galaxy. This is possible because the algorithm finds potential in order to calculate orbits; for this we use the a value for the missing line of sight velocity information realistic Milky Way mass model of Dehnen & Binney of a star in the Gaia DR2 catalog by requiring continuity (1998), their model `1'.