STScI Logo

Hubble Space Telescope
The HubbleObserver Corner for HST Observer Monthly

Visit the HubbleObserver Archive.

Science Spotlight from this week's observations

Proposal ID = 12549
Principle Investigator =  Thomas Brown  - Space Telescope Science Institute
Title = "The Formation History of the Ultra-Faint Dwarf Galaxies"
Time = May 6, 2012 01:57:28 - 06:22:12
Instrument = ACS/WFC, WFC3/UVIS


The standard paradigm for the evolution of galaxies begins with the formation of large numbers of low mass dwarf galaxies. The vast majority of these small galaxies then merge together, forming larger and larger galaxies. Remnants of this original population of dwarfs can still be observed in the local universe, and it is the faintest of these (the ultra-faint dwarf galaxies) that are the focus of this proposal. Since the galaxies are nearby, we can observe the individual stars in detail and determine when they were formed. A key question that motivates this proposal is whether most of the stars have the same age (i.e., their formation was synchronized), and whether that age indicates that the stars formed before the reionization of the universe, as cosmological models predict. These models also predict that there should be hundreds of dwarfs that are satellites around larger galaxies; the leftovers that have not merged with other systems. However, there appear to be only a few dozen dwarfs galaxies around our own Milky Way galaxy. This is known as the "missing satellite" problem, and is another focus of this proposal.

Paraphrasing from the abstract:

We propose a deep, high-precision, photometric survey of six ultra-faint dwarf galaxy satellites of the Milky Way, in order to discern any cosmologically-driven synchronization of their formation histories. This new class of galaxy, recently discovered in the Sloan Digital Sky Survey, appears to be an extension of the classical dwarf spheroidals to low luminosities, offering a new front in the efforts to understand the missing satellite problem. Because they are the least luminous, most dark matter dominated, and least chemically evolved galaxies known, the ultra-faint dwarfs are the best candidate fossils from the early universe. Hierarchical models of galaxy formation predict that the ultra-faint dwarfs formed the bulk of their stars before the epoch of reionization. Relying upon relative age measurements with sub-Gyr resolution, we will characterize the age dispersion within each galaxy, the age variation among these galaxies, and the ages of these galaxies relative to those of the Galactic globular clusters. These measurements will provide the only meaningful test of the synchronized star formation predicted by current models. The program will be an important test of our current assumptions regarding the hierarchical assembly of galaxies at the low-mass end of the power spectrum, and place this new class of galaxy in a cosmological context.

You can find most of this information and more from the following webpage: by entering "12549" in the Prop. ID box.