Investigating the Chemical Homogeneity of Low-Metallicity Blue Compact Dwarf Galaxies Using Integral Field Spectroscopy

Abstract by By Bethan James 
The study of the chemical and physical evolution of Blue Compact Dwarf (BCDs) galaxies is fundamental in our understanding of galaxy formation in the early Universe. Being nearby, BCDs serve as excellent laboratories to study chemical enrichment processes in often unblemished environments, in comparison to primordial galaxies at high-redshift. It has been claimed in the past that in low-metallicity BCDs (12+log(O/H)<8.3), the N/O value is independent of metallicity (O/H ratio), implying the need to invoke a primary production of nitrogen in intermediate-mass stars, in addition to the secondary nitrogen produced from the CNO cycle in high-mass stars.  In order to better understand this controversial issue, more extensive spatial information than that offered by traditional longslit spectroscopic methods, is required.

We undertook an integral field spectroscopic study of the nebular gas within a sample of BCDs previously thought to have anomalously high N/O values. Here we present the results of this study for 3 BCDs: two with anomalous N/O values (Mrk996 and UM420) and one with more normal N/O values (UM462). We describe in detail how we derived the physical conditions (Te, Ne) as a function of position within the galaxy, and as a consequence, how this revealed both revised metallicities and normal N/O ratios. Abundances for other elements will also be discussed. In the case of Mrk996, which displays both narrow and broad component emission lines, we associated an increased N/O ratio with the broad component in the nebular gas clearly correlated with WR emission features. This is one of the first clear evidence of nitrogen self-enrichment of an HII region from WR stars.

All the above mentioned results suggest that the nitrogen enrichment seen in the low-metallicity gas of these BCDs is not due to primary nitrogen production, as suggested by current theories. This investigation is also a direct proof that performing spatially resolved spectroscopy, and conducting a separate analysis of broad and narrow emission line components, are vital in deriving the 'true' chemical abundances of BCD galaxies.

Probing the stellar populations of z ~ 1.5-3.5 galaxies using rest-frame UV and optical morphology

Abstract by Nicholas Bond
I discuss some of my recent research studying the properties of the stellar populations of high-redshift galaxies by correlating their morphological properties with their spectral energy distributions. Using samples of star-forming galaxies at z ~ 1.5 - 3.5, including BX, BzK, Lyman break galaxies, and Lyman Alpha Emitters (LAEs), we implement comparative morphological diagnostics to spatially differentiate the young and old stars. Although the morphological differences between rest-frame UV and rest-frame optical light are generally small, they are statistically significant for the majority of star-forming galaxies at z >~ 2, and larger than for early-type galaxies at similar redshifts.

We then connect this work to LAEs, which are thought be among the most pristine of the high-redshift galaxies. Despite the common belief that a young, dust-free environment is needed to produce an LAE, there is increasing evidence for an underlying population of older stars in many LAEs, especially towards lower redshifts (z ~ 2).

Date: March 4th, 2010 (Friday)
Location: STScI, room N420  

http://adsabs.harvard.edu/abs/2011ApJ...729...48B
http://adsabs.harvard.edu/abs/2011arXiv1101.3017G
http://adsabs.harvard.edu/abs/2010ApJ...716L.200B
http://adsabs.harvard.edu/abs/2010arXiv1005.3006G
http://adsabs.harvard.edu/abs/2010ApJ...714..255G
http://adsabs.harvard.edu/abs/2009ApJ...705..639B
http://adsabs.harvard.edu/abs/2003ApJ...598..827P