4,010 research outputs found
Biases in metallicity measurements from global galaxy spectra: the effects of flux-weighting and diffuse ionized gas contamination
Galaxy metallicity scaling relations provide a powerful tool for
understanding galaxy evolution, but obtaining unbiased global galaxy gas-phase
oxygen abundances requires proper treatment of the various line-emitting
sources within spectroscopic apertures. We present a model framework that
treats galaxies as ensembles of HII and diffuse ionized gas (DIG) regions of
varying metallicities. These models are based upon empirical relations between
line ratios and electron temperature for HII regions, and DIG strong-line ratio
relations from SDSS-IV MaNGA IFU data. Flux-weighting effects and DIG
contamination can significantly affect properties inferred from global galaxy
spectra, biasing metallicity estimates by more than 0.3 dex in some cases. We
use observationally-motivated inputs to construct a model matched to typical
local star-forming galaxies, and quantify the biases in strong-line ratios,
electron temperatures, and direct-method metallicities as inferred from global
galaxy spectra relative to the median values of the HII region distributions in
each galaxy. We also provide a generalized set of models that can be applied to
individual galaxies or galaxy samples in atypical regions of parameter space.
We use these models to correct for the effects of flux-weighting and DIG
contamination in the local direct-method mass-metallicity and fundamental
metallicity relations, and in the mass-metallicity relation based on
strong-line metallicities. Future photoionization models of galaxy line
emission need to include DIG emission and represent galaxies as ensembles of
emitting regions with varying metallicity, instead of as single HII regions
with effective properties, in order to obtain unbiased estimates of key
underlying physical properties.Comment: 37 pages, 29 figures, 4 tables. Accepted to ApJ. See Figures 15-17
for typical global galaxy biases in strong-line ratios, electron
temperatures, and direct-method metallicitie
JWST/NIRSpec Measurements of the Relationships Between Nebular Emission-line Ratios and Stellar Mass at z~3-6
We analyze the rest-optical emission-line ratios of star-forming galaxies at
2.7<=z<6.5 drawn from the Cosmic Evolution Early Release Science (CEERS)
Survey, and their relationships with stellar mass (M_*). Our analysis includes
both line ratios based on the [NII]6583 feature -- [NII]6583/Ha,
([OIII]5007/Hb)/([NII]6583/Ha) (O3N2), and [NII]6583/[OII]3727 -- and those
those featuring alpha elements -- [OIII]5007/Hb, [OIII]5007/[OII]3727 (O_32),
([OIII]4959,5007+[OII]3727)/Hb (R_23), and [NeIII]3869/[OII]3727. Given the
typical flux levels of [NII]6583 and [NeIII]3869, which are undetected in the
majority of individual CEERS galaxies at 2.7<=z<6.5, we construct composite
spectra in bins of M_* and redshift. Using these composite spectra, we compare
the relationships between emission-line ratios and M_* at 2.7<=z<6.5 with those
observed at lower redshift. While there is significant evolution towards higher
excitation (e.g., higher [OIII]5007/Hb, O_32, O3N2), and weaker nitrogen
emission (e.g., lower [NII]6583/Ha and [NII]6583/[OII]3727) between z~0 and
z~3, we find in most cases that there is no significant evolution in the
relationship between line ratio and M_* beyond z~3. The [NeIII]3869/[OII]3727
ratio is anomalous in showing evidence for significant elevation at 4.0<=z<6.5
at fixed mass, relative to z~3.3. Collectively, however, our empirical results
suggest that there is no significant evolution in the mass-metallicity
relationship at 2.7<=z<6.5. Representative galaxy samples and metallicity
calibrations based on existing and upcoming JWST/NIRSpec observations will be
required to translate these empirical scaling relations into ones tracing
chemical enrichment and gas cycling, and distinguish among the descriptions of
star-formation feedback in simulations of galaxy formation at z>3.Comment: 10 pages, 4 figures, ApJL, in pres
JWST/NIRSpec Balmer-line Measurements of Star Formation and Dust Attenuation at z~3-6
We present an analysis of the star-formation rates (SFRs) and dust
attenuation properties of star-forming galaxies at drawn from
the Cosmic Evolution Early Release Science (CEERS) Survey. Our analysis is
based on {\it JWST}/NIRSpec Micro-Shutter Assembly (MSA)
spectroscopic observations covering approximately m. Our primary
rest-frame optical spectroscopic measurements are H/H Balmer
decrements, which we use as an indicator of nebular dust attenuation. In turn,
we use Balmer decrements to obtain dust-corrected H-based SFRs (i.e.,
SFR(H)). We construct the relationship between SFR(H) and
stellar mass () in three bins of redshift (, , and ), which represents the first time the star-forming
main sequence has been traced at these redshifts using direct spectroscopic
measurements of Balmer emission as a proxy for SFR. In tracing the relationship
between SFR(H) and back to such early times (), it is
essential to use a conversion factor between H and SFR that accounts
for the subsolar metallicity prevalent among distant galaxies. We also use
measured Balmer decrements to investigate the relationship between dust
attenuation and stellar mass out to . The lack of significant redshift
evolution in attenuation at fixed stellar mass, previously confirmed using
Balmer decrements out to , appears to hold out to . Given
the rapidly evolving gas, dust, and metal content of star-forming galaxies at
fixed mass, this lack of significant evolution in attenuation provides an
ongoing challenge to explain.Comment: 9 pages, 4 figures, ApJ, in pres
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