Article

Modelling the dynamical evolution of the Bootes dwarf spheroidal galaxy

Monthly Notices of the Royal Astronomical Society (Impact Factor: 5.11). 01/2008; 385(2). DOI: 10.1111/j.1365-2966.2008.12921.x
Source: arXiv

ABSTRACT

We investigate a wide range of possible evolutionary histories for the recently discovered Bootes dwarf spheroidal galaxy,
a Milky Way satellite. By means of N-body simulations, we follow the evolution of possible progenitor galaxies of Bootes for a variety of orbits in the gravitational
potential of the Milky Way. The progenitors considered cover the range from dark matter-free star clusters to massive, dark
matter-dominated outcomes of cosmological simulations. For each type of progenitor and orbit, we compare the observable properties
of the remnant after 10 Gyr with those of Bootes observed today. Our study suggests that the progenitor of Bootes must have
been, and remains now, dark matter-dominated. In general, our models are unable to reproduce the observed high velocity dispersion
in Bootes without dark matter. Our models do not support time-dependent tidal effects as a mechanism able to inflate significantly
the internal velocity dispersion. As none of our initially spherical models is able to reproduce the elongation of Bootes,
our results suggest that the progenitor of Bootes may have had some intrinsic flattening. Although the focus of this paper
is the Bootes dwarf spheroidal, these models may be of general relevance to understanding the structure, stability and dark
matter content of all dwarf spheroidal galaxies.

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    ABSTRACT: We present a new suite of photometric and spectroscopic data for the faint Bootes II dwarf spheroidal galaxy (dSph) candidate. Our deep photometry, obtained with the Isaac Newton Telescope/Wide Field Camera, suggests a distance of 46 kpc and a small half-light radius of 4.0' (56 pc), consistent with previous estimates. Follow-up spectroscopy obtained with the Gemini/GMOS instrument yielded radial velocities and metallicities. While the majority of our targets covers a broad range in velocities and metallicities, we find five stars that share very similar velocities and metallicities and that are all compatible with the colors and magnitudes of the galaxy's likely red giant branch. We interpret these as a spectroscopic detection of the Bootes II system. These stars have a mean velocity of –117 km s–1, a velocity dispersion of (10.5 ± 7.4) km s–1, and a mean [Fe/H] of –1.79 dex, with a dispersion of 0.14 dex. At this metallicity, Boo II is not consistent with the stellar-mass-metallicity relation for the more luminous dwarf galaxies. Coupled with our distance estimate, its high negative systemic velocity rules out any physical connection with its projected neighbor, the Bootes I dwarf spheroidal, which has a velocity of ~+100 km s–1. The velocity and distance of Bootes II coincide with those of the leading arm of Sagittarius, which passes through this region of the sky, so that it is possible that Bootes II may be a stellar system associated with the Sagittarius stream. Finally, we note that the properties of Bootes II are consistent with it being the surviving remnant of a previously larger and more luminous dSph galaxy.
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