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Science Spotlight from this week's observations

Proposal ID = 12304
Principle Investigator =  Jon A. Holtzman  -  New Mexico State University 
Title = "Metallicity Distribution Functions of 4 Local Group Dwarf Galaxies"
Time = Mar 24, 2012 10:37:03 - 14:56:47
Target =  LEO-II
Instrument = WFC3/UVIS, WFC3/IR

Background:

The most direct way to study the star formation history of the universe is to observe very distant galaxies. Since it takes billions of years for the light to reach us, we see galaxies as they appeared billions of years ago. In this way we can observe galaxies as they appeared when they were young, only a few hundred million years after the Big Bang in some cases. A problem with this direct approach is that only the brightest, most massive galaxies can be seen this far away. These extremely bright galaxies are not typical of average galaxies, including our own Milky Way galaxy. Fortunately, there is another way to study the star formation rate of more typical galaxies, namely by looking at the metallicity distribution of individual stars in dwarf galaxies in the Local Group (i.e., galaxies within 1 Mpc of the Milky Way). There is a strong correlation between metallicity and age, so observing the metallicity distribution provides a means of estimating star formation histories. It is believed that the Milky Way was largely formed by the merging of large numbers of dwarf galaxies. Hence, by determining the metallicity distribution of dwarf galaxies we are essentially studying the star formation history of the Milky Way. Dwarf galaxies therefore provide a "fossil record" of how star formation has proceeded in the universe.

Paraphrasing from the abstract:

We will measure metallicity distribution functions (MDFs) in four Local Group dwarf galaxies (Leo I, Leo II, Phoenix, and IC 1613) using a new medium band filter available with WFC3 that covers the Ca H and K lines -- the strongest metal absorption lines in the visible spectrum of cooler stars. In addition to the Ca filter (F390M), we will obtain broadband observations in several optical and near-IR filters. Together, these will yield metallicity measurements with an accuracy of about 0.2 dex for giants brighter than the red clump. Unlike broadband colors, our multi-band metallicity measurements will be insensitive to age variations in the population and reddening uncertainties or variations. The photometric metallicities will allow us to construct MDFs with almost an order of magnitude more stars than has been obtained from the ground for Leo I and Leo II, and hence will provide more stringent constraints on the number of rarer, more metal-poor stars; these will provide the first measurements of the metallicity distribution function in Phoenix and IC1613. The four galaxies span a wide range of different star formation histories. The MDFs will constrain models of cyhemical evolution and improve the derivation of the star formation histories.


You can find most of this information and more from the following webpage: http://www.stsci.edu/hst/ by entering "12304" in the Prop. ID box.