Very Blue UV-Continuum Slope β of Low Luminosity z ~ 7 Galaxies from WFC3/IR: Evidence for Extremely Low Metallicities?
Abstract
We use the ultra-deep WFC3/IR data over the Hubble Ultra Deep Field and the Early Release Science WFC3/IR data over the CDF-South GOODS field to quantify the broadband spectral properties of candidate star-forming galaxies at z ~ 7. We determine the UV-continuum slope β in these galaxies, and compare the slopes with galaxies at later times to measure the evolution in β. For luminous L* z=3 galaxies, we measure a mean UV-continuum slope β of -2.0 ± 0.2, which is comparable to the β ~ -2 derived at similar luminosities at z ~ 5-6. However, for the lower luminosity 0.1L* z=3 galaxies, we measure a mean β of -3.0 ± 0.2. This is substantially bluer than is found for similar luminosity galaxies at z ~ 4, just 800 Myr later, and even at z ~ 5-6. In principle, the observed β of -3.0 can be matched by a very young, dust-free stellar population, but when nebular emission is included the expected β becomes >=-2.7. To produce these very blue βs (i.e., β ~ -3), extremely low metallicities and mechanisms to reduce the red nebular emission seem to be required. For example, a large escape fraction (i.e., f esc gsim 0.3) could minimize the contribution from this red nebular emission. If this is correct and the escape fraction in faint z ~ 7 galaxies is gsim0.3, it may help to explain how galaxies reionize the universe.
Based on observations made with the NASA/ESA Hubble Space Telescope, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programs #11563, 9797.- Publication:
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The Astrophysical Journal
- Pub Date:
- January 2010
- DOI:
- 10.1088/2041-8205/708/2/L69
- arXiv:
- arXiv:0910.0001
- Bibcode:
- 2010ApJ...708L..69B
- Keywords:
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- galaxies: evolution;
- galaxies: high-redshift;
- Astrophysics - Cosmology and Nongalactic Astrophysics
- E-Print:
- 5 pages, 5 figures, accepted for publication in Astrophysical Journal Letters