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Extreme Stellar Populations in the Universe: Backsplash Dwarf Galaxies and Wandering Stars

Teyssier, Maureen Elizabeth

We demonstrate that stars beyond the virial radii of galaxies may be generated by the gravitational impulse received by a satellite as it passes through the pericenter of its orbit around its parent. These stars may become energetically unbound (escaped stars), or may travel to further than a few virial radii for longer than a few Gyr, but still remain energetically bound to the system (wandering stars). Larger satellites (10-100% the mass of the parent), and satellites on more radial orbits are responsible for the majority of this ejected population. Wandering stars could be observable on Mpc scales via classical novae, and on 100 Mpc scales via SNIa. The existence of such stars would imply a corresponding population of barely-bound, old, high velocity stars orbiting the Milky Way, generated by the same physical mechanism during the Galaxy's formation epoch. Sizes and properties of these combined populations should place some constraints on the orbits and masses of the progenitor objects from which they came, providing insight into the merging histories of galaxies in general and the Milky Way in particular. We distinguish between Local Group field galaxies which may have passed through the virial volume of the Milky Way, and those which have not, via a statistical comparison against populations of dark matter haloes in the Via Lactea II (VLII) simulation with known orbital histories. Analysis of VLII provides expectations for this escaped population: they contribute 13 per cent of the galactic population between 300 and 1500 kpc from the Milky Way, and hence we anticipate that about 7 of the 54 known Local Group galaxies in that distance range are likely to be Milky Way escapees. These objects can be of any mass below that of the Milky Way, and they are expected to have positive radial velocities with respect to the Milky Way. Comparison of the radius-velocity distributions of VLII populations and measurements of Local Group galaxies presents a strong likelihood that Tucana, Cetus, NGC3109, SextansA, SextansB, Antlia, NGC6822, Phoenix, LeoT, and NGC185 have passed through the Milky Way. Indeed, several of these galaxies -- especially those with lower masses -- contain signatures in their morphology, star formation history and/or gas content indicative of evolution seen in simulations of satellite/parent galactic interactions. Our results offer strong support for scenarios in which dwarfs of different types form a sequence in morphology and gas content, with evolution along the sequence being driven by interaction history. We use the Via Lactea II cosmological N-body simulation of the formation of Milky Way and M31 Analogues, to explore the expected properties of intergalactic light (light found beyond the virial radii of galaxies) in poor groups and around isolated galaxies. We find that the luminosity fraction of intergalactic light is ~1%. This is similar to observational measurements of intergalactic light in poor groups. We expect this result to be observationally verifiable through observations of supernovae Ia by blind, repeated surveys like Pan-STARRS and LSST. We find the major contributors to the intergalactic light are the largest mass satellite haloes due to the low stellar fraction expected in smaller mass haloes. The intergalactic light produced by the most massive satellites has a much smaller spatial extent than that produced by lower mass satellites, meaning that baryon prescriptions designed to supress star formation in low mass satellites also shrink the spatial extent of intergalactic light. It may be possible to use observations of the large quantity of intergalactic red giants, that we expect in the Local Group, to define the spatial extent of the intergalactic light, and thereby place limits on the total star formation in progenitor satellites in the Local Group.


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More About This Work

Academic Units
Thesis Advisors
Johnston, Kathryn V.
Ph.D., Columbia University
Published Here
September 18, 2013
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