Are dSph galaxies Galactic building blocks?

Abstract

Dwarf spheroidal galaxies (dSph) are frequently assumed to represent surviving examples of a vast now destroyed population of small systems in which many of the stars now forming the Milky Way were formed. Ongoing accretion and considerable sub-structure in the outer Galactic halo is direct evidence that there is some role for stars formed in small galaxies in populating the (outer) galaxy. The evidence from stellar populations is however contradictory to this. dSph stellar populations are unlike any stars found in significant numbers in the Milky Way. The dSph are indeed small galaxies, formed over long times with low rates of star formation. Most of the stars in the Milky Way halo however seem to have formed quickly, at higher star formation rate, in gas mixed efficiently on kpc scales. The overwhelming majority of Milky Way stars, those in the Galactic thick disk and thin disk, seem to have nothing at all to do with dwarf galaxy origins. There is overwhelming and irrefutable evidence that large scale structure in the Universe is well- described by the CDM model. In this a (primordial) spectrum of perturbations with equal power on all scales generates density fluctuations which grow under self-gravity, with the dominant matter being both non-relativistic at early times, and weakly or un-coupled to the radiation which dominates the Universe at those early times. Thus the dominant matter cannot be baryonic, which couples efficiently to radiation through Thompson scattering. It is non-relativistic, does not couple to, or emit, radiation, and dominates the local gravitational field. Hence Cold Dark Matter, CDM. This model describes the Universe well on large scales, in fact on all scales large enough that the growth of density perturbations remains linear, so that local dark matter particle physics and baryonic physical processes remain unimportant. The baryonic physics of gas cooling becomes very important at scales of order 10 6 M� . There must also be some general small-scale limit, when the physical properties of whatever types of particles make up the CDM become important, and dominate the scale-free initial perturbations. Finding that scale will provide our first information on the types of matter which make up CDM, and so is of very considerable interest. A standard approach to discovering that scale is to examine when an assumed absence of scale violates observation. That is, one extrapolates the model to smaller and smaller scales until observations indicate discordance. This is the particular interest of dwarf galaxies, which form the smallest systems in which dark matter has been detected. Are they consistent with extrapolated gravitational CDM and baryons, with no evidence of additional physics? The simplest, and most extreme, CDM extrapolation posits structure in gravitationally bound CDM potential wells on all scales, with the number of such systems increasing as mass −2 . This is divergent, so must cut off somewhere. But at what scale does