A Tale Twice Told: The Luminosity Profiles of the Sagittarius Tails

Abstract

The Sagittarius dwarf galaxy is the archetype of a tidally disrupting system. Both leading and trailing tails can be observed across at least 180 degrees of the sky and measurements of their luminosity density profiles have recently become available. Using numerical simulations, we explore the factors that control the appearance of such profiles. We use two possible models for the Sgr progenitor. The first is a one-component Plummer model, which may represent either a dark matter free progenitor, or one in which pre-existing dark matter has already been largely stripped. The second is a two-component model in which the stars are represented by a Hernquist sphere embedded in a cosmologically modish Navarro-Frenk-White dark halo. Disruption of the models in the Milky Way galaxy provides us with two tellings of the tale of the formation of the Sgr stream. The initial disintegration of the baryons proceeds more slowly for the two-component models because of the protective cocoon of dark matter. Once this has been stripped, though, matters proceed apace. In both cases, the profiles after ~6 pericentric passages provide good matches to the observational data, but the tails are more extended for the two-component models. The leading and trailing tails are symmetric at apocentre or pericentre. At other orbital phases, asymmetries are present, as tails are compressed as they approach apocentre and stretched out as they approach pericentre. There may exist density enhancements corresponding to such pile-ups which may be observable in current survey data. We re-visit the calculation of Niederste-Ostholt et al. (2010) and slightly revise upwards the luminosity of the Sgr progenitor to 9.9-14.4 x 10^7 solar luminosities based on insights from the simulations.

8 pages, 4 figures, accepted for publication in MNRAS

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