Galactic Cosmology: Dark Matter and the Faint End of Galaxy Formation

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Our understanding of Cosmology rests on two fundamental puzzles“dark matter” and “dark energy” whose solution will redefine the way we understand Physics. The nature of dark matter, in particular, is key to extending the Standard Model of Particle Physics and to understanding the hierarchical nature of structure formation in the Universe. Studies of the Cosmic Microwave Background radiation and of galaxy clustering have led to a true paradigm, where dark matter takes the form of weakly-interacting elementary particles that emerge from the early Universe with negligible thermal velocities. This Lambda-Cold-Dark-Matter (LCDM) scenario has now matured into a full, falsifiable theory where detailed theoretical predictions may be directly confronted with observation and experiment. ******A cornerstone prediction of LCDM is that dark matter halos (non-linear structures where galaxies form) exist in vast numbers and span a very large range in mass. The mass function of LCDM halos, however, differs markedly from the galaxy mass function: galaxy masses and halo masses must thus be related by a strongly non-linear function that contains signatures of the astrophysical processes that govern galaxy formation. This relation has particularly intriguing consequences in the regime of faint (dwarf) galaxies. Since low-mass halos vastly outnumber dwarfs, reconciling LCDM with observation requires that luminous galaxies have only been able to form in a small fraction of low-mass halos. Most low-mass halos must thus be “dark”, and dwarf galaxy formation must effectively be cut off below a characteristic halo mass of order ~109 solar masses. This “threshold” halo mass should leave clear observational imprints in the faintest dwarfs which, if properly elucidated, could serve to validate or falsify some of the basic tenets of the cold dark matter paradigm. One example are halos just below the threshold, which, although unable to form stars, should still retain part of their primordial gas content. Detecting this population of gas-filled, star-less “mini-halos” filled would be a major discovery and a resounding success for LCDM on small scales. ******The faintest dwarfs are best studied in the Local Group of Galaxies, since they can only be detected out to a few Mpc. Some dwarfs, in addition, may have melded into the Galaxy at early times, leaving clues to their origin in the kinematics, metallicities, and abundance patterns of Galactic stars. I propose an ambitious plan to turn the Milky Way (the "Galaxy") into a powerful cosmological probe, combining supercomputer simulations, data mining, and direct observational surveys to decipher the cosmological clues buried in the Galaxy and its immediate neighbours. The aim of this “Galactic Cosmology” is to learn how to use the faintest, and most puzzling, specimens of the galaxy population to pry open a unique astronomical window into one of the defining Physics challenges of our erathe nature of dark matter.