An Orphan No Longer? Detection of the Southern Orphan Stream and a Candidate Progenitor
Carl J. Grillmair, Lauren Hetherington, Raymond G. Carlberg, Beth Willman
aa r X i v : . [ a s t r o - ph . GA ] S e p Draft version October 27, 2018
Preprint typeset using L A TEX style emulateapj v. 5/2/11
AN ORPHAN NO LONGER? DETECTION OF THE SOUTHERN ORPHAN STREAM AND A CANDIDATEPROGENITOR
Carl J. Grillmair
Infrared Processing and Analysis Center, California Institute of Technology, Pasadena, CA 91125
Lauren Hetherington, Raymond G. Carlberg
Department of Astronomy and Astrophysics, University of Toronto, Toronto, ON M5S 3H4, Canada
Beth Willman
Haverford College, Department of Physics and Astronomy, Haverford, PA 19041
Draft version October 27, 2018
ABSTRACTUsing a shallow, two-color survey carried out with the Dark Energy Camera, we detect the southern,possibly trailing arm of the Orphan Stream. The stream is reliably detected to a declination of -38 ◦ ,bringing the total known length of the Orphan stream to 108 ◦ . We find a slight offset or “S” shapein the stream at δ ≃ − ◦ that would be consistent with the transition from leading to trailing arms.This coincides with a moderate concentration of 137 ±
25 stars (to g = 21 .
6) that we consider apossible remnant of the Orphan progenitor. The position of this feature is in agreement with previouspredictions.
Subject headings:
Galaxy: Structure — Galaxy: Halo INTRODUCTION
The Orphan stream was among the first stellar debrisstreams detected in the Sloan Digital Sky Survey (SDSS)(Belokurov et al. 2006; Grillmair 2006; Belokurov et al.2007). Populous and roughly 2 ◦ wide on the sky, thestream is clearly much broader and stronger than knownglobular streams such as Pal 5 (Odenkirchen et al. 2003;Grillmair & Dionatos 2006a). This, along with subse-quent findings of a metallicity dispersion of σ [Fe/H] =0.56 dex (Casey et al. 2013) and a metallicity gradientamplitude of 0.3 dex (Sesar et al. 2013) led researchers toconclude that the Orphan stream must be the remnant ofa dwarf galaxy. Early modeling efforts suggested that thestream might be related to the neutral hydrogen Com-plex A (Fellhauer et al. 2007; Jin & Lynden-Bell 2007),and that the progenitor of the stream might be thenearby dwarf galaxy UMa II (Fellhauer et al. 2007).However, subsequent work by Sales et al. (2008) andNewberg et al. (2010) does not support these ideas.Newberg et al. (2010) used measured positions and ve-locities to derive an orbit of the stream, and determinedthat the orbit is prograde, moderately inclined to theGalactic plane ( i ≈ ◦ ), fairly eccentric ( e ≈ . ≈
90 kpc from the Galactic center, andthat the portion of the stream visible in the SDSS foot-print is the leading arm. Based on the rising surfacedensity of the stream at the southern edge of the SDSSfootprint (in the direction of decreasing Galactocentricradius and far from apogalacticon), they also suggestedthat the progenitor would most likely be found betweendeclinations of 0 ◦ and − ◦ . [email protected]@[email protected]@gmail.com In this letter we describe the first results of a shallowimaging survey designed to trace the Orphan Stream wellsouth of the SDSS footprint. We briefly describe theobservations in Section 2. We analyze the spatial andcolor-magnitude characteristics of the stream in Section3. Concluding remarks are given in Section 4. OBSERVATIONS
Using the Orphan orbit estimation of Newberg et al.(2010) as a guide, we imaged a 9 ◦ to 15 ◦ -wide swath ofsky extending from the celestial equator to δ ≃ − ◦ and covering an area of 487 deg . This was carried outduring just two observing nights using the remarkablyefficient Dark Energy Camera (DECam) on the Blanco4-meter telescope at the Cerro Tololo Interamerican Ob-servatory (CTIO). Observations were made in g and i ,and exposures were kept to two 30 second dithers perfield to maximize the area covered while still reachingwell past the main sequence turnoff of the stream. Ob-servations were carried out over two observing seasons,with one night in March of 2014 and another in March of2015. Conditions were photometric during both nights,with typical seeing of 0 . ′′ in i and 1 − . ′′ seeing in g ,though with excursions of > ′′ for a short period duringthe 2014 run.The resulting 6.3 TB of data were processed usingthe 2015 version of the DECam Community Pipeline(Valdes et al. 2014). (2014 data were reprocessed withthe 2015 pipeline to take advantage of several improve-ments) The data were subsequently transferred to theUniversity of Toronto, where a photometry pipelinebased on SExtractor and PSFEx (Bertin & Arnouts1996) was constructed to photometer individual imagesusing point spread function (PSF) fitting.PSFs, aperture corrections, and 2nd order color termswere computed for each individual detector. The pho-tometry was calibrated against the SDSS catalog using ≃
20 deg of imaging in the Sloan footprint. Averageatmospheric extinction coefficients for CTIO were usedthroughout. Stars were typically observed at least twicein each filter (with the exception of a small number ofstars falling within the CCD gaps), and the individualphotometric measurements were combined over all rel-evant fields and over both observing runs. Within theSDSS footprint, calibration is good to 0.02 mags RMS.Perhaps owing to the variable nature of the PSFs overa field as large as that of DECam, we found that thestar/galaxy separation parameter “CLASS STAR” wasrather unreliable, with a spread that varied considerablyfrom the center to the edge of each field. Hence we re-lied primarily on the “FWHM WORLD” and “ELLIP-TICITY” parameters to excise sources that were clearlyextended. Imposing limits of FWHM WORLD < ′′ ,ELLIPTICITY < .
2, FLAGS=0, and 16 < g < . g = 21 . g ≈
23, others (with airmasses ≥
2) arecomplete to only g ≈ .
7. These issues will be furtherexplored in a forthcoming contribution. For our presentpurposes, we avoid these issues by simply cutting off oursample at g = 21 . ANALYSIS
We used a matched filter to optimally separate themetal poor stars of the Orphan Stream from the muchlarger population of foreground disk stars (Rockosi et al.2002; Grillmair 2009). This technique has been usedto detect several streams at surface densities as low as10 stars deg − (Grillmair & Dionatos 2006a,b; Grillmair2006, 2009, 2011; Bonaca et al. 2012). We generateda filter based on the Padova database of theoreti-cal stellar isochrones (Marigo et al. 2008; Girardi et al.2010), selecting for stars with [Fe/H] = -1.6. Allstars with 16 < g < . − ◦ . The 18 kpc distanceused in Figure 1 corresponds to the strongest streamsignal and roughly matches the 19-21 kpc range of dis-tances expected on the basis of an orbit fit to Newberget al. (2010)’s data compilation for the northern Orphanstream. Differences may be due to inaccurate matching of the DECam g and i photometry to the Sloan filtersassumed by the Padova isochrones, or possibly a metal-licity gradient in the Orphan Stream (Sesar et al. 2013).It may also be that 18 kpc is the correct distance of thestream in this region, and that the actual orbit of streamstars in this region needs to be refined.The northern 10 ◦ of the detected stream matches nicelywith the portion of the stream detected in the Sloan foot-print. A full-width-at-half-maximum of ≈ . − ◦ is alsoconsistent with that observed in the northern stream.The stream appears to be reliably detected to δ ≃ − ◦ ,below which the character of the distribution changessignificantly (see below). This brings the known lengthof the stream to ≈ ◦ . Over the southern interval − ◦ < δ < − ◦ , we find the stream is well fit (towithin 0 . ◦ ) by a polynomial of the form: α = 163 . − . × δ + 0 . × δ (1)Figure 2 shows the distribution of E ( B − V )over our survey area from the maps ofSchlegel, Finkbeiner, & Davis (1998). A compari-son of Figures 1 and 2 shows that the pattern of thestar counts in the region − ◦ > δ > − ◦ closelymatches the filamentary distribution of dust emissionand enhanced reddening. Dereddening our photometryhas evidently pulled an excess of fainter and redderstars into the sample. Whereas E ( B − V ) is fairlyuniform and ranges from 0.02 to 0.06 over the northernhalf of the survey area, the filamentary structures at δ ≈ − ◦ show color excesses ranging from 0.2 to0.3. Arbitrarily scaling down the Schlafly & Finkbeiner(2011) absorption coefficients reduces the effect, butdoes not yield any convincing signatures of an under-lying stream. Tracing the stream through this regionwould presumably benefit in the near term from a deep,near-infrared survey, though it should ultimately bedetected in Gaia proper motion data.Figure 3 shows a color-magnitude distribution of starschosen to lie within the ± ◦ of the center of the streamnorth of δ = − ◦ . Over plotted are isochrones for pop-ulations with Z = 0.0001 ([Fe/H] = -2.1) and Z=0.0005([Fe/H] = -1.6). Z=0.0005 appears to match the mainsequence somewhat better than Z=0.0001, which corre-sponds to the metallicity found by Newberg et al. (2010)for the blue horizontal branch stars. The value [Fe/H] =-1.6 used in Figure 1 matches a measurement of [Fe/H]= -1.63 found by Casey et al. (2013) for red giants. Notethat Sesar et al. (2013) see evidence for a metallicitygradient in the northern stream, with the nearer, moresoutherly stars being ≃ . δ ≈ − ◦ . There are a number of possible reasonsfor the offset: ( i ) the orbit calculation did not take intoaccount the southern stream (which as yet has no ve-locity information), ( ii ) the effects of halo flattening ortriaxiality have not been considered, or ( iii ) we may belooking at the trailing arm of the stream.If we use the northern orbit fit as a guide, we see thatwhile it appears to fit the stream reasonably well northof δ ≈ − ◦ , an eastward offset of ≈ . ◦ begins rathersuddenly south of δ = − ◦ , and stays roughly constantto δ = − ◦ . At (R.A., dec) = (167 ◦ , -14 ◦ ), midwaybetween the northern orbit fit and the run of Equation1, there is a moderate but significant, 1 . ◦ -wide overden-sity of stars that is somewhat larger and stronger thanthe clumps to the immediate north or south. This clumpappears to be the extended, northern portion of a fea-ture found by Newberg et al. (2010) in an “outrigger”SEGUE stripe at δ ≈ − ◦ . We hypothesize that thetransition from the northern to the southern portionsof the stream is the “S-shape” signature expected from aprogenitor losing stars from its first and second Lagrangepoints. We further suggest that this clump of stars couldbe the remnant of the progenitor of the Orphan stream.Based on the rise and fall of stream surface density withposition along the stream, Newberg et al. (2010) pre-dicted that the progenitor of the Orphan stream shouldbe situated between δ = 0 ◦ and δ = − ◦ . This is con-sistent with the position of our overdensity at δ ≈ − ◦ .Moreover, Newberg et al. (2010) determined that thenorthern portion of the Orphan stream must be the lead-ing arm. Tidal stripping in a constant- v c potential re-quires that the leading arm should be made up of starsreleased from progenitor’s first Lagrange point, into or-bits of lower Galactocentric radius R . Conversely, thetrailing arm will be made up of stars lost from the sec-ond Lagrange point, falling behind the progenitor andorbiting at larger R . This is consistent with Figure 4;the westward offset of the southern portion of the streamtakes it further away from the Galactic center, which isto the left in the figure.At a distance of 18 kpc, a 1 . ◦ offset corresponds to ≈
470 pc. The L1 and L2 lagrange points will alwaysbe aligned along a radial to the Galactic center. At thecurrent position of the putative progenitor, we would beviewing it at an angle of ≈ ◦ from the L1 - L2 radial.If indeed the northern and southern Orphan streams areleading and trailing arms, respectively, then the impliedphysical separation would be 1.2 kpc. We consequentlytake the upper limit on the tidal radius of the progenitorto be 600 pc.The number of stars within the putative progenitor isnot large. Examining a square region 1 . ◦ on a side andcentered on (R.A., dec) = (167.125 ◦ ,-14.273 ◦ ) and com-paring with background fields to the east and west, wecount stars with 0 . < g − i < .
44 and 19 . < g < .
6. Scaling by the area ratios, we find a background-subtracted count of 137 ±
24 stars. Integrating over theluminosity function of Omega Cen (de Marchi 1999), wearrive at an approximate total population of 2100 ± r − (0 . ± . , making itunlikely that the feature could be gravitationally bound.By definition, the tidal radius r t = ( M p / M G ( R )) R in a flat rotation curve, where M G ( R ) is the mass ofthe Galaxy within Galactocentric radius R , and M p isthe mass of the progenitor. If we take R = 21 kpc and M G ( R ) = 1 − × M ⊙ , we arrive at an upper limiton the progenitor’s recent mass of ∼ . − . × M ⊙ .Depending on the number of red giants, the luminosityof the object could range from 1 × to 4 × L ⊙ . Ifa bound object remains, then M/L ∼ − M ⊙ /L ⊙ .Using the luminosity-metallicity relation ofKirby et al. (2011), the [Fe/H] = -1.6 measurementof Casey et al. (2013) suggests a total luminosity ofthe original progenitor of 2 . × L ⊙ . On the otherhand, Newberg et al. (2010)’s value of [Fe/H] = -2.1implies 6 × L ⊙ . Our luminosity estimate above wouldsuggest that the progenitor has lost between 94% and100% of its original mass.There are other surface density peaks evident in Figure4 but we are less inclined to consider these as progenitorcandidates as they do not show the morphological indica-tors (e.g. offsets) we would associate with the transitionfrom leading to trailing arms. Given the orientation ofthe Orphan stream and our view of it, such a featureshould be readily apparent.We note also that near the southernmost end of thesurvey area is the globular cluster Ruprecht 106. Thiscluster is situated along the plausible extension of theOrphan stream. However, while its metallicity of [Fe/H]= -1.67 (Harris 1996) is similar to that of the Orphanstream, its distance of 12 kpc and radial velocity of -44km s − are at odds with values of 21 kpc and +72 kms − predicted by the orbit fit to the northern Orphanstream. We conclude that Ruprecht 106 is unlikely to bephysically associated with the stream. CONCLUSIONS
Using a large, shallow DECam survey, we have tracedthe Orphan stream from the celestial equator to δ ≃− ◦ . The stream appears to be roughly 18 kpc dis-tant, and its trajectory generally agrees with expecta-tions based on orbit fits to the northern stream. Thecolor magnitude distribution is clearly metal poor andappears similar to that of the northern Orphan stream.We find a stellar concentration and apparent offsets inthe stream that would be consistent with a remnant pro-genitor.This southern extension of the Orphan stream shouldenable significant improvements in constraining the over-all orbit, and ultimately the shape of the Galactic po-tential. This is particularly interesting in that the Or-phan stream passes through quadrants of the halo notprobed by the Sagittarius stream. Slightly deeper thanthe present survey, the Pan-STARRS survey may en-able us to improve the signal-to-noise ratio somewhatfor δ > − ◦ . For more southerly regions, where weare strongly affected by reddening, a deep, near-infraredsurvey may help to trace the stream still further south.We gratefully acknowledge Jonathan Hargis for help-ful suggestions in the course of developing our pho-tometry pipeline. This project used data obtainedwith the Dark Energy Camera (DECam), which wasconstructed by the Dark Energy Survey (DES) col-laboration. Funding for the DES Projects has beenprovided by the DOE and NSF(USA), MISE(Spain),STFC(UK), HEFCE(UK). NCSA(UIUC), KICP(U.Chicago), CCAPP(Ohio State), MIFPA(Texas A&M),CNPQ, FAPERJ, FINEP (Brazil), MINECO(Spain),DFG(Germany) and the collaborating institutions inthe Dark Energy Survey, which are Argonne Lab,UC Santa Cruz, University of Cambridge, CIEMAT-Madrid, University of Chicago, University College Lon-don, DES-Brazil Consortium, University of Edinburgh,ETH Zurich, Fermilab, University of Illinois, ICE (IEEC-CSIC), IFAE Barcelona, Lawrence Berkeley Lab, LMU Munchen and the associated Excellence Cluster Universe,University of Michigan, NOAO, University of Notting-ham, Ohio State University, University of Pennsylvania,University of Portsmouth, SLAC National Lab, StanfordUniversity, University of Sussex, and Texas A&M Uni-versity.ms Facilities:
CTIO:Blanco (DECam).
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Fig. 1.—
Filtered surface density map of our Decam survey area, overlaid on the SDSS DR10 footprint. The stretch is linear, with lighterareas indicating higher surface densities. The map is the result of a filter based on a Padova isochrone with [Fe/H] = -1.6, an age of 12Gyr, and shifted to a distance of 18 kpc. The Sloan data have been smoothed with a 0 . ◦ Gaussian kernel while the DECam map, owing toits somewhat shallower depth, has been smoothed with a 0 . ◦ kernel. Seeing was 0 . − . ′′ over most of the survey area, with two stripes( − ◦ > δ > − ◦ , − ◦ > δ > − ◦ ) having seeing in excess of 2 ′′ . The highest airmasses ( > .
8) occurred at δ > − ◦ . Fig. 2.—
Distribution of E ( B − V ) over the field shown in Figure 1. Lighter areas indicate higher color excesses. Values of the colorexcess range from 0.02 in the darkest, northern reaches of the survey, to 0.3 in the brightest filaments at δ ≈ − ◦ . Fig. 3.—
Hess diagram of stars lying within 1 ◦ of the centerline of the Orphan stream north of δ = − ◦ , after subtraction of thedistribution of stars along the edge of the survey area. The result has been convolved with a 0.05 mag Gaussian kernel. Lighter areasindicate higher surface densities.The solid lines shows a Padova isochrone with [Fe/H] =-2.1 (blue, left) and -1.6 (red, right), age 12 Gyrs,and shifted to distances of 20 and 18 kpc, respectively. Fig. 4.—
A more detailed view of the DECam filtered surface density map in Figure 1, again smoothed with a Gaussian kernel of width0 . ◦ . The white curve traces an orbit fit to the position and velocity data of Newberg et al. (2010) for the northern Orphan stream. Theblack curve extending to δ = − ◦ is the fit to the southern portion of the stream given by Equation 1. The black contour in the southernhalf of the survey area corresponds to E ( B − V ) = 0 ..