Transcript JWST and Chemical Enrichment at z~1-6 (lookback time: 8-13 billion yr) Alice Shapley (UCLA)

JWST and Chemical Enrichment at z~1-6
(lookback time: 8-13 billion yr)
Alice Shapley (UCLA)
Outline
• Why are heavy elements important?
• How do we measure them in star-forming galaxies?
• What will JWST do?
Galaxy Metallicities
• Fundamental metric of galaxy formation process, reflects gas
reprocessed by stars, metals returned to the ISM by SNe explosions.
• Measured from star-forming regions and stars in star-forming galaxies,
stars in early-types.
• Metallicity  Gas phase oxygen abundance in star-forming galaxies.
• Metal production rate reflects rate of high-mass star formation.
Chemical enrichment history entwined with build-up of stellar mass,
modulated by outflows/inflows of gas.
(Yang & Hester HST/WFPC2)
(Tremonti et al. 2004)
Galaxy Metallicities
• Fundamental metric of galaxy formation process, reflects gas
reprocessed by stars, metals returned to the ISM by SNe explosions.
• Measured from star-forming regions and stars in star-forming galaxies,
stars in early-types.
• Metallicity  Gas phase oxygen abundance in star-forming galaxies.
• Metal production rate reflects rate of high-mass star formation.
Chemical enrichment history entwined with build-up of stellar mass,
modulated by outflows/inflows of gas.
(Bouwens et al. 2010)
(Marchesini et al. 2009)
Universal Expansion and Redshift
• Our Universe is expanding, most famously discovered by Edwin Hubble in the 1920s.
Appears that external galaxies are receding from us, and more distant galaxies are
receding faster. This observation is consistent with a an increase in the overall scalefactor of the universe.
• The expansion of the universe leads to the redshift (z) of photons emitted by distant
sources. The universe stretches by a certain factor (1+z) in the time it take for the light
to reach us, and the wavelengths stretch by that same factor.
• For example, in the time it takes light to reach us from galaxies 11 billion l.y. away,
the universe has stretched by a factor of 4. Therefore, UV photons produced by the
galaxies are observed by us as optical photons.
Rest-frame Optical Spectra
[OII]
Hb [OIII]
Ha+
[NII] [SII]
• Optical spectrum provides a
“code” about the physical contents
of star-forming regions.
•At z > 1.4 (9 billion l.y. away),
bluest strong line moves past
9000Å. Becomes a problem for
near-IR spectroscopy.
• Problems from the ground:
atmospheric absorption, sky
background.
(Kennicutt 1998)
Ground-based Limitations
• Atmospheric absorption limits
the accessible redshift ranges for
various rest-frame optical features.
• In particular, beyond z~4 (12
billion l.y. away), it is not possible
to measure most emission lines.
Ground-based Limitations
Keck/NIRSPEC
H-band Sky Spectrum
• Significant fraction of near-IR
transmission bands is affected by
strong sky emission lines, leading
to severe systematics at affected
wavelengths. Beyond l~2.35 mm,
thermal background becomes
very high.
• With Keck/NIRSPEC (R~1500),
~1/3 of each near-IR bandpass is
affected by strong sky-lines. With
better resolution, less of an effect.
• In practice: very difficult to find
redshifts at which multiple lines
are free of sky contamination.
(Yuck!)
JWST/NIRSPEC and Metallicities
• With JWST/NIRSpec slits/MSA, we will be able to measure the
emission-line code for high-redshift galaxies, revealing the evolving heavyelement content of star-forming galaxies over a continuous swath of
cosmic times.
• With the JWST/NIRSpec/Integral Field Unit capability, we will map the
patterns in chemical abundances within individual galaxies, to test models
of their formation. We will also map the emission from large-scale
outflows and inflows.
(From JWST/NIRSpec site)