arXiv:1209.5391v1 [astro-ph.GA] 24 Sep 2012 h aatcptnil hsajitaayi fmany the of constrain to analysis method joint promising a a thus provides on potential, streams constraints Galactic Each provide independent will the potential. provides halo streams stream Galactic distant Galactic individual the more on the of constraints by analysis similar dominated potential An Way is Milky disk. which the kpc strong of 10-15 full, a flattening between placing the the GD-1, analyzed on for (2010) constraint space al. its phase et for Koposov available six–dimensional are its motions stars. sufficiently proper to member is that 2006) as GD- kpc) Dionatos The (10 well & nearby 2011). as (Grillmair al. stream 2012), tidal et of al. 1 Yoon 2004; et shape 2009; Lux Helmi (Carlberg and 2005; ’lumpiness’ (e.g., strength al. potential et the Johnston gravitational to Galactic sensitive the are and tances Orphan 2002; 2007). al. al. recognition et et beyond iden- Newberg Belokurov disrupted yet Stream, Stream, tidally not 5; al. Monoceros is been et (e.g., Sollima object has Palomar progenitor or isolated the tified as 14; (e.g., which identified in are Palomar clusters however, features streams, Some 2003, globular dwarf 2011). Sagittarius al. with the objects et al. Odenkirchen et of or known Belokurov system 2003, Many al. with 2006) tail et (SDSS). associated (Majewski tidal galaxy Survey surveys spheroidal the are Sky photometric as streams Digital such wide-field these Sloan to the of due as is such streams hierarchically. part, lar in evidence least at providing formed, clusters, are galaxies globular that or disrupting 2008). from galaxies al. originate Mart´ınez-Delgado to dwarf et presumed 2001; are M31) al. streams al. These et (hereafter et Belokurov Ibata Andromeda (e.g., galaxies Way, 2006; massive nearby Milky other the and of halos 61;[email protected] 06511; rpittpstuigL using 2021 typeset 31, Preprint August version Draft tla tem a xedoe ag hscldis- physical large over extend can streams Stellar stel- Way Milky numerous of discovery recent The galactic the throughout found are streams Stellar 1 eateto srnm,Yl nvriy e ae,CT Haven, New University, Yale Astronomy, of Department itneo 26 of distance lse.Aclrmgiuedarmo h temrgo hw nov ( an old shows an region to stream corresponds the turn-off, of sequence diagram main magnitude color A cluster. tdsacsbten8ad4 p.Avsa erho hs asr w M31/M33 maps of these direction of the search in stream visual 0 spans new A stream a wh The as techn maps kpc. stream. well 40 density matched-filter as and stellar streams, a creating 8 stellar Using by between 8 distances Release galaxies. at Data M33) Survey Sky and Digital (M31 Triangulum and ouain uueknmtcsuiso hsadsmlrcl tem w streams potential. cold gravitational similar Galactic headings: and the Subject this of shape of the studies on kinematic Future population. epeeteiec o e ik a tla ia temi h dire the in stream tidal stellar Way Milky new a for evidence present We ODMLYWYSELRSRA NTEDRCINO TRIANGULU OF DIRECTION THE IN STREAM STELLAR WAY MILKY COLD A 1. A INTRODUCTION T ± E tl mltajv 5/2/11 v. emulateapj style X p.Tewdho h temi ossetwt en h ia remn tidal the being with consistent is stream the of width The kpc. 4 aay ao—Glx:structure Galaxy: — halo Galaxy: n Bonaca Ana . 2 ◦ y12 by ◦ 1 ntesy r7 cb . p npyia nt ihabs fitting best a with units physical in kpc 5.5 by pc 75 or sky, the on rf eso uut3,2021 31, August version Draft al Geha Marla , ABSTRACT ∼ 2Gr n ea-or([Fe/H] metal-poor and Gyr) 12 1 npoeto gis 3 n 3.In M33. found and stream, M31 Triangulum against the projection name in we which stream, potential. Way Milky full eainhpt te aatcsbtutrsin substructures Galactic other to In relationship technique. matched-filter and § analysis data SDSS the 1,woeodaeadlwmtliiy(13 metallicity cluster globular low Fil- the and on age based population. old are stream whose work a this M13, of in used part higher ters a being with of substruc- preferentially space, probability Way color-magnitude by in works Milky stars, filtering of weighting Matched context searches. the ture in technique filter their by distinguished age. be and can and composition compared disk chemical as Way typi- Milky populations substructures be- the stellar in to different stars Stars to of disk. belong Way majority cally Milky the hemi- the magnitudes, Galactic to long these Southern At the in sphere. imaging released newly eetdlcly ewe 45 between locally, selected antd iga CD ihhg otatbetween contrast high with (CMD) diagram magnitude M92). of (e.g., clusters isochrones globular accommodates used tol- which commonly generous other following 1c), a region (Figure within a isochrone erance to population’s confined target and the smoothed is filter nal radius a within se- 15 catalog of are photometric of stars diagram DR8 target the Hess filter from The a lected by 1b). (Figure 1a) foreground (Figure the population target SDSS, the by required. observed are is transformations M13 filter no streams. thus stellar cold known = [Fe/H] uvyDt ees Ahr ta.21,hereafter 2011, than brighter deg al. stars 14,555 et select (Aihara the We 8 DR8). Release SDSS Data Survey iy Kallivayalil Nitya , ,w eemn h tempoete,addsusits discuss and properties, stream the determine we 3, eew rsn vdnefranwMlyWystellar Way Milky new a for evidence present we Here oks ta.(02 rtitoue h matched- the introduced first (2002) al. et Rockosi u prahesrsta ein ntecolor- the in regions that ensures approach Our of diagram Hess a dividing by constructed is filter Our esac o tla tem nteSonDgtlSky Digital Sloan the in streams stellar for search We ′ rmM3scne.Tefrgon ouainis population foreground The center. M13’s from − 1 . ;VneBr 00 r iia oother to similar are 2000) VandenBerg 6; 2 rest hc,i dnie sa as identified if which, erdensity cvr l ftemjrknown major the of all ecovers DScvrg,o hc 20deg 5200 which of coverage, SDSS c rb h ik a halo Way Milky the probe ich ihw aeteTriangulum the name we hich 2. l rvd ih constraints tight provide ill 1 qe esac h Sloan the search we ique, AAANALYSIS DATA to fteAndromeda the of ction ′ − ∼ n 75 and n faglobular a of ant 1 . e)stellar dex) 0 ′ rmM3 h fi- The M13. from M § ediscuss we 2 r . y and Gyr 5 2over 22 = § 3.3. 2 is the target and foreground population are given the most TABLE 1 weight in filtering. The filter is most heavily weighted Triangulum stream properties near the main sequence turn-off (Figure 1c), where nu- merous M13 stars are present, but few Galactic stars are as blue in color. The main sequence is less weighted be- Property Value RA (deg)a [21.346, 23.975] cause of the large population of Milky Way stars present a Dec (deg) [34.977, 23.201] redward of the turn-off. The subgiant and giant branches l (deg) [130.756, 136.023] are weighted the least because of the small number of b (deg) [−27.378, −38.538] M13 stars which occupy those areas of the CMD. Width 0.2◦ (75 pc) A total of four matched filters (in r vs. g−r, g vs. g−r, Length 12◦ (5.5 kpc) g vs. g − i and g vs. g − z) were applied to the whole Distance 26 ± 4 kpc SDSS survey area for a distance range between 8kpc and [Fe/H] −1.0 dex 40 kpc. The inner distance limit was chosen to avoid the Age 12 Gyr Milky Way disk, while the outer limit is the distance at which the main sequence turnoff falls below the SDSS aStream boundary points, see Equation 1 for a relation between 95% completeness limit (Juri´cet al. 2008). Each filter RA and Dec. was applied to the full SDSS photometric sample and the resulting density maps were coadded. The left column The Triangulum stream is nearly linear on the sky, of Figure 2 shows the final density maps for the North whose coordinates satisfy the equation: ◦ ◦ Galactic hemisphere, while right panels show the South Dec( )= −4.4 × RA( )+128.5 (1) Galactic hemisphere2. The SDSS footprint in the North has been extensively searched for halo substructure and is in the range RA≃ 21◦ − 24◦ (Table 1). We define a new known as the “field of streams” (Belokurov et al. 2006). coordinate system such that the stream runs along a y- We recover both compact and diffuse known objects in axis. This system is an equatorial system rotated 11◦ this field. Globular clusters and dwarf galaxies are visible counterclockwise around RA = 22◦ and Dec = 32◦. A as small, concentrated overdensities. At a distance of stellar density map of the stream region in this new co- 8 kpc (Figure 2, top) we detect the cold GD-1 stream, ordinate system is shown in Figure 3d. To assess the as well as the much wider signatures of the Monoceros statistical significance of this overdensity, we create a stream and leading Sagittarius tidal arm. At a distance one–dimensional stellar density profile of the stream (re- of 26 kpc (Figure 2, bottom) we detect the tidal tails of gions −9◦ 2 M13 Foreground Filter = M13 / Foreground (a) (b) (c) 16 18 r 20 22 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 0.2 0.4 0.6 0.8 g - r g - r g - r Fig. 1.— Steps in creating an M13-based filter for selection of old, metal-poor stars: the left panel shows a Hess diagram of SDSS stars detected in the inner region of globular cluster M13, while that of a surrounding area, representative of Milky Way foreground, is given in the middle panel. A ratio of the two produces the desired filter, which preferentially selects stars that are in the region of the color-magnitude diagram with the greatest contrast between M13 and Milky Way stellar populations. In order to reduce clumpiness, the filter is smoothed (right panel). The resulting filter is most heavily weighted near the main sequence turn-off. Red lines, derived from an isochrone matching the M13 population, trace the width of the filter on all three panels. North hemisphere South hemisphere M31 60 40 M33 40 20 GD-1 Monoceros Monoceros Dec (deg) Dec 20 (deg) Dec 0 0 -20 60 40 New stream 40 20 Sagittarius Dec (deg) Dec 20 Sagittarius (deg) Dec 0 Palomar 5 Orphan 0 -20 260 240 220 200 180 160 140 120 80 60 40 20 0 -20 -40 -60 -80 RA (deg) RA (deg) Fig. 2.— Stellar density maps obtained via matched filtering at distances of 8 kpc (top) and 26 kpc (bottom). The left columns show the SDSS footprint in the northern Galactic hemisphere, while maps on the right are in the south. The stretch is linear, with darker areas corresponding to higher stellar density regions. The most prominent features are tidal arms of the Sagittarius dwarf galaxy, bright Milky Way globular clusters, and brightest neighboring galaxies. Regions not imaged by the SDSS are shown in white. The maps are available online. lates with progenitor mass, while the length of a stream Belokurov et al. 2006, Majewski et al. 2003). The width is related to the time since the stars became unbound of the Triangulum stream suggests that its progenitor is (Johnston & Bullock 2004). The average width of the a globular cluster, consistent with the age and metallicity Triangulum stream, σ = 10′ (75pc), is comparable to determined below. other globular cluster streams in the literature. The GD- Our estimated length for the Triangulum stream, 12◦ 1 stream, whose progenitor is undetected but presumed (5.5 kpc), is likely an underestimate given the incom- to be a globular cluster, is σ = 12′ (35 pc; Koposov et al. pleteness of the SDSS data. This is shorter, but compara- 2010), while the globular cluster Pal 5 is σ = 18′ (120 pc; ble to the Palomar 5 stream (22◦, 9 kpc). Deeper imaging Odenkirchen et al. 2003). In contrast, dwarf galaxies along the Triangulum stream may reveal a longer extent produce much broader streams: the Orphan stream than detected in the SDSS data. is ∼ 2◦ wide (730 pc; Grillmair 2006; Belokurov et al. 2006), while the Sagittarius stream is 20◦ or more (7kpc; 3.2. Age and Metallicity of the Triangulum Stream 3 Matched filter Dust Galaxies 80 10 0.20 20 (e) (a) (b) (c) 60 34 9 0.18 18 weight 40 N 10 8 0.16 16 (d) 32 M 33 7 0.14 14 5 6 0.12 12 30 5 0.10 10 Dec (deg) Dec 28 4 0.08 8 0 y (deg) y 3 0.06 6 M 33 26 2 0.04 4 -5 24 1 0.02 2 0 0.00 0 -10 24 22 20 24 22 20 24 22 20 -4 -2 0 2 4 RA (deg) RA (deg) RA (deg) x (deg) Fig. 3.— (a) Matched filter map of the Triangulum stream region in equatorial coordinates. (b) Dust map of the same region. Color scale represents reddening E(B − V ). (c) Blue galaxy number counts in the stream region. (d) Matched filter map of the wider stream region in rotated coordinates. Stream regions are boxed in blue and red, the dashed ellipse shows the extent of RGB stars around the M33 disk. Red lines on panels a-d bracket stream region. (e) 1D profiles of the stream in the top and bottom boxes from the panel (d) (blue and red, respectively) and their average (black). We estimate the age and metallicity of the Triangulum [Fe/H] = −1.25. A range of younger and more metal rich stream using the best-fitting distance of 26 ± 4kpc. Due isochrones fails to match the shape of the overdensity in to the small number of stars associated with the stream, the Hess diagram. the CMD in this region is dominated by Milky Way Given the proximity of the stream to the M33 foreground stars. We therefore follow Koposov et al. galaxy, we have also overplotted a range of old and (2010) in constructing a statistically subtracted CMD. metal-poor isochrones at the distance of M33 (809 First, we create a 1D profile of all stars satisfying the kpc; McConnachie et al. 2005) to test whether the color-magnitude cut 20 4 South. Even though the latter structure (whose origin remains controversial) has a large metallicity spread from [Fe/H] = −1.6 to [Fe/H] = −0.4 that could be consistent with the stream discovered here, it seems to be at much closer heliocentric distance at ∼ 11 kpc (Yanny et al. 2003; Crane et al. 2003; Chou et al. 2010). 4. CONCLUSIONS We have searched the SSDS DR8 photometric database for Milky Way stellar streams using a matched filtering technique. We recover all known streams in the current literature. Based on a visual search of our stellar density maps, we present evidence for a new stream discovered in the southern Galactic hemisphere in the direction of M33 which we name the Triangulum stream. We did not find any other stream-like structures in the dataset. The Triangulum stream spans 0.2◦ by 12◦ (75pc by 5.5 kpc) on the sky with a best fitting distance of 26 ± 4 kpc. We estimate the age and metallicity of the stream via a statistically subtracted color-magnitude di- agram of the region, which suggests a 12Gyr, metal-poor ([Fe/H] = −1.0) stellar population. The narrow width of the Triangulum stream is comparable to the GD-1 and Palomar 5 streams, from which we conclude that the Triangulum stream is also a disrupted globular clus- ter. While we do not observe the progenitor object, the stream extends beyond the SDSS photometric coverage. We note that the PAndAS team (McConnachie et al. 2009) has independently detected the Triangulum stream at the same sky position but at a higher significance due to deeper photometric data (M. Fardal, priv. communi- cation). Searches for cold stellar streams in the halo are moti- vated by the desire to map the Galactic gravitational po- Fig. 4.— Top: Hess diagram of the stream region with statisti- tential. Further characterizing the size and shape of the cally subtracted background. Bottom: Contours of the smoothed Triangulum stream via deeper and more extended imag- Hess diagram with overplotted 12 Gyr isochrones spanning a metal- licity range of [Fe/H] = −0.5 to −1.5 at the stream distance (red) ing will improve the usefulness of this stream as a Galac- and [Fe/H] = −1.1 to −2.1 at the M33 distance (gray). tic halo probe. Radial velocities and proper motions of Triangulum stream stars will provide tighter constraints on the nature of its stellar populations and orbital prop- ferred age of 8 Gyr (Chou et al. 2011). The metallicity erties, providing new constraints on the Galactic gravi- of TriAnd is higher than that inferred for the Triangu- tational potential. lum stream and we estimate a population which is 4 Gyr older. Martin et al. (2007) have detected a signature of ACKNOWLEDGMENTS another overdensity behind the TriAnd, at heliocentric The authors wish to thank Jeremy Bradford, Nhung distance ∼ 28kpc. Both features are far more extended Ho, Mario Juri´cand Beth Willman for providing use- than Triangulum, however, the Triangulum stream could ful comments on the manuscript. We also thank Nikhil be related to these overdensities as a localized feature at Padmanabhan for help in the early stages of this project. the outer edge of a much larger diffuse merger remnant. This work was supported by NSF grant AST-0908752 Spectroscopic and/or astrometric follow-up is required to and the Alfred P. Sloan Foundation. explore this possibility. Funding for SDSS-III has been provided by the Alfred Recently, it has been argued that the ‘bifurcation’ in P. Sloan Foundation, the Participating Institutions, the the Sagittarius stream (Belokurov et al. 2006) is in fact National Science Foundation, and the U.S. Department a distinct stream with its own progenitor and kinemat- of Energy Office of Science. The SDSS-III web site is ics (Koposov et al. 2012). The so-called Cetus stream, http://www.sdss3.org/. first discovered by Newberg et al. (2009), is fainter, thin- SDSS-III is managed by the Astrophysical Research ner and more metal-poor than the Sagittarius stream. Consortium for the Participating Institutions of the Its derived orbit is in the direction of the Triangulum SDSS-III Collaboration including the University of Ari- stream, however, it has lower metallicity ([Fe/H] = −2.1) zona, the Brazilian Participation Group, Brookhaven Na- and appears to be at larger heliocentric distance (34 kpc; tional Laboratory, University of Cambridge, Carnegie Newberg et al. 2009). 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A Cold Milky Way Stellar Stream in the Direction of Triangulum
