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تسجيل مستخدم جديدAn Orphan No Longer? Detection of the Southern Orphan Stream and a Candidate Progenitor
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Using 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 $-38^circ$, bringing the total known length of the Orphan stream to $108^circ$. We find a slight offset or S shape in the stream at $delta simeq -14^circ$ that would be consistent with the transition from leading to trailing arms. This coincides with a moderate concentration of $137 pm 25$ stars (to $g = 21.6$) that we consider a possible remnant of the Orphan progenitor. The position of this feature is in agreement with previous predictions.
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We have developed a method for estimating the properties of the progenitor dwarf galaxy from the tidal stream of stars that were ripped from it as it fell into the Milky Way. In particular, we show that the mass and radial profile of a progenitor dwa
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lite Ursa Major II (UMa II) and the Orphan Stream. We provide numerical simulations of the disruption of UMa II that match the observational data on the position, distance and morphology of the Orphan Stream. We predict the radial velocity of UMa II as -100 km/s as well as the existence of strong velocity gradients along the Orphan Stream. The velocity dispersion of UMa II is expected to be high, though this can be caused both by a high dark matter content or by the presence of unbound stars in a disrupted remnant. However, the existence of a gradient in the mean radial velocity across UMa II provides a clear-cut distinction between these possibilities. The simulations support the idea that some of the anomalous, young halo globular clusters like Palomar 1 or Arp 2 or Ruprecht 106 may be physically associated with the Orphan Stream.
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