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تسجيل مستخدم جديدFinding halo streams with a pencil-beam survey: new wraps in the Sagittarius stream
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B. Pila-Diez
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We use data from two CFHT-MegaCam photometric pencil-beam surveys in the g and the r bands to measure distances to the Sagittarius, the Palomar 5 and the Orphan stream. We show that, using a cross-correlation algorithm to detect the turnoff point of the main sequence, it is possible to overcome the main limitation of a two-bands pencil-beam survey, namely the lack of adjacent control-fields that can be used to subtract the foreground and background stars to enhance the signal on the colour-magnitude diagrams (CMDs). We describe the cross-correlation algorithm and its implementation. We combine the resulting main sequence turnoff points with theoretical isochrones to derive photometric distances to the streams. Our results (31 detections on the Sagittarius stream and one each for the Palomar 5 and the Orphan streams) confirm the findings by previous studies, expand the distance trend for the Sagittarius faint southern branch and, for the first time, trace the Sagittarius faint branch of the northern-leading arm out to 56 kpc. In addition, they show evidence for new substructure: we argue that these detections trace the continuation of the Sagittarius northern-leading arm into the southern hemisphere, and find a nearby branch of the Sagittarius trailing wrap in the northern hemisphere.
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ant background galaxies to define a stationary reference frame. Five Orphan Stream stars are identified in one of the four fields based on combined color-magnitude and PM information. The average PM is consistent with the existing model of the Orphan stream by Newberg et al. In addition to the Orphan stream stars, we detect stars that likely belong to other stellar streams. To identify which stellar streams these stars belong to, we examine the 2-d bulk motion of each group of stars on the sky by subtracting the PM contribution of the solar motion (which is a function of position on the sky and distance) from the observed PMs, and comparing the vector of net motion with the spatial extent of known stellar streams. By doing this, we identify candidate stars in the Sagittarius and Lethe streams, and a newly-found stellar stream at a distance of ~17 kpc, which we tentatively name the Parallel stream. Together with our Sagittarius stream study (Sohn et al., 2015, ApJ, 803, 56), this work demonstrates that even in the Gaia era, HST will continue to be advantageous in measuring PMs of old stellar populations on a star-by-star basis, especially for distances beyond ~10 kpc.
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