Young Galaxies

High-redshift galaxy studies rely in large part on samples selected using color techniques or extreme emission-line properties.  For example, Lyman Break Galaxies (LBGs) are star-forming z>3 galaxies selected on the basis of their rest-frame UV colors, which are indicative of significant absorption at wavelengths below 912 A.  However, it is very difficult to know how such galaxy populations relate to one another or how they fit into the overall scheme of galaxy evolution.

A classic example of a highly valued population whose relationship to other samples is unclear is the Lyman alpha emitter (LAE) population.  Lyman alpha emission-line searches have been widely used to find the highest redshift galaxies (z>6).  This line is the only spectroscopic signature that can be used to confirm the redshift of a galaxy selected on the basis of its color properties.  However, Lyman alpha is a difficult line to interpret.  Because the line is resonantly scattered by neutral hydrogen, determining its escape path and hence its destruction by dust is an extremely complex problem.  Thus, our understanding of what determines the fraction of galaxies with Lyman alpha emission is weak.

Ideally, we would like to know whether the presence of Lyman alpha emission is related to other properties of the galaxy, such as its metallicity, extinction, morphology, or kinematics - i.e., what controls the escape of Lyman alpha photons, and how the properties of the highest redshift LAEs are modified by the intergalactic gas.

Due to the difficulty with observing in the UV, until recently we had much more information on the z~2-3 LAEs and how their properties related to those of other UV selected galaxies at these redshifts (LBGs) than we did on lower-redshift samples.  However, substantial samples of z~0.3 and z~1 LAEs have now been found with the Galaxy Evolution Explorer (GALEX) grism spectrographs.  These samples have many advantages.  The galaxies are bright and can be easily studied at other wavelengths.  Perhaps even more importantly, they can be integrated into comprehensive studies of galaxies at the same redshifts to understand some of the selection biases.  Moreover, by comparing the Lyman alpha properties of the low-redshift galaxies with their optical properties, including their H alpha line strengths, we can calibrate the conversion of Lyman alpha luminosity to star formation rate.

We are learning a lot about LAEs from these samples and from the comparison of these samples to those at high-redshifts.  For example, it appears that LAEs represent an early stage in a starburst when the star-forming gas is still relatively pristine and the primary star-forming region is small.  It also appears that there is a time sequence, with the Lyman alpha emission line dying away and the metallicity of the gas rising as the galaxy evolves.  We hope to make significant advances in placing the LAEs into context by studying these objects with the SALT spectrograph.

Another young galaxy population was discovered through imaging with narrowband filters built to find high-redshift LAEs.  These "late bloomers" are undergoing their first episodes of star formation quite recently (z~0.8) and were discovered by their unusually strong [OIII] emission.  They generally have very low metallicities, which means we may be able to obtain clues about the first stages of galaxy formation in the universe by studying in detail a large sample. We plan to conduct a wide-area search with the One Degree Imager on WIYN that will turn up thousands of these galaxies.  We will then be able to explore whether these newly forming galaxies contain some minimum amount of heavy elements, as might be expected if the intergalactic gas is relatively uniformly enriched with metals made in previous galaxy formation.  If extremely low metallicity galaxies are found, then we can conclude that portions of the intergalactic gas have remained relatively pristine.

UW-Madison Astronomy Home