A critical analysis of the ultraviolet continuum slopes (β) of high-redshift galaxies: no evidence (yet) for extreme stellar populations at z > 6

By J. Dunlop, R. McLure, Brant Robertson, Richard S. Ellis, D. Stark, M. Cirasuolo, L. Ravel. 2011

145 citations recorded by the source corpus.

Following the discovery of the first significant samples of galaxies at z > 6.5 with Wide Field Camera 3/Infra-Red (WFC3/IR) on board Hubble Space Telescope (HST), it has been claimed that the faintest high-redshift galaxies display extremely blue ultraviolet (UV) continuum slopes, with a UV power-law index β≃−3 (where f_λ∝λ^β). Such slopes are bluer than previously reported for any other galaxy population, and are most readily explained theoretically by extinction-free, young and very low metallicity stellar populations with a high ionizing photon escape fraction. Here we undertake a critical study of the evidence for such extreme values of β, combining three new WFC3/IR-selected samples of galaxies spanning nearly two decades in UV luminosity over the redshift range z≃ 4.5–8. We explore the impact of inclusion/exclusion of less robust high-redshift candidates and use the varying depths of the samples to explore the effects of noise and selection bias at a given UV luminosity. Simple data-consistency arguments suggest that artificially blue average values of β can result when the analysis is extended into the deepest ≃0.5 mag bin of these WFC3/IR-selected galaxy samples, regardless of the actual luminosity or redshift range probed. By confining attention to robust high-redshift galaxy candidates, with at least one 8σ detection in the WFC3/IR imaging, we find that the average value of β is consistent with 〈β〉=−2.05 ± 0.10 over the redshift range z= 5–7 and the UV absolute magnitude range −22 < M_(UV,AB) < − 18, and that 〈β〉 shows no significant trend with either redshift or MUV. We create and analyse a set of simple end-to-end simulations based on the WFC3/IR+ACS Hubble Ultra Deep Field (HUDF) and Early Release Science data sets which demonstrate that a bias towards artificially low/blue average values of β is indeed ‘expected’ when the UV slope analysis is extended towards the source detection threshold, and conclude that there is as yet no clear evidence for UV slopes significantly bluer than β≃−2, the typical value displayed by the bluest star-forming galaxies at more modest redshifts. A robust measurement of 〈β〉 for the faintest galaxies at z≃ 7 (and indeed z≃ 8) remains a key observational goal, as it provides a fundamental test for high escape fractions from a potentially abundant source of re-ionizing photons. This goal is achievable with HST, but requires still deeper WFC3/IR imaging in the HUDF.

Subfields: Astronomy and Astrophysics, Electrical and Electronic Engineering, Instrumentation

View Paper: https://www.semanticscholar.org/paper/ee572553a012502ef574b4a97aa56cbc67be387f

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In Astronomy and Astrophysics

1. Richard S. Ellis
2. R. Bouwens
3. P. Oesch
4. P. V. van Dokkum
5. M. Franx
6. J. Dunlop
7. G. Illingworth
8. I. Labbé

In Electrical and Electronic Engineering

1. Richard S. Ellis
2. J. Dunlop
3. R. Bouwens
4. D. Stark
5. P. Oesch
6. R. McLure
7. G. Illingworth
8. M. Ouchi

In Instrumentation

1. Richard S. Ellis
2. R. Bouwens
3. M. Franx
4. P. V. van Dokkum
5. J. Dunlop
6. I. Labbé
7. P. Oesch
8. G. Illingworth

Citations in the corpus

• THE MOSDEF SURVEY: MEASUREMENTS OF BALMER DECREMENTS AND THE DUST ATTENUATION CURVE AT REDSHIFTS z ∼ 1.4–2.6 (2015, background): “…and while it can be measured for individual galaxies to very faint magnitudes at high redshift (Bouwens et al. 2012; Finkelstein et al. 2012; Dunlop et al. 2012; Oesch et al. 2013), it is nonetheless also sensitive to age, metallicity, and star-formation history (e.g., Kong et al. 2004;…”
• NEW CONSTRAINTS ON COSMIC REIONIZATION FROM THE 2012 HUBBLE ULTRA DEEP FIELD CAMPAIGN (2013, background): “This conclusion is a ramification of the UDF12 UV spectral slope constraints by Dunlop et al. 11 We note that if we ignore the electron scattering optical depth constraint, the other datasets favor a UV luminosity density that declins more rapidly at high redshift than does our maximum likelihood…”
• UV-CONTINUUM SLOPES AT z  ∼  4–7 FROM THE HUDF09+ERS+CANDELS OBSERVATIONS: DISCOVERY OF A WELL-DEFINED UV COLOR–MAGNITUDE RELATIONSHIP FOR z ⩾ 4 STAR-FORMING GALAXIES (2011, methodology): “Dunlop et al. (2012) exclude faint sources, because their simulations suggested to them that β could not be measured in an unbiased way.”
• Rest-frame ultraviolet-to-optical spectral characteristics of extremely metal-poor and metal-free galaxies (2011, background): “The very blue ultraviolet (UV) colours of the HST/WFC3 LBGs suggest such a possibility (Bouwens et al. 2010a) (but see Dunlop et al. 2011).”
• Concordance models of reionization: implications for faint galaxies and escape fraction evolution (2012, background): “…UV continuum slopes recently reported for z ∼ 7 galaxies (Bouwens et al. 2010b) are also suggestive of weak nebular recombination emission, which would be consistent with very high escape fractions of ionizing photons (but see Dunlop et al. 2011 for a critical analysis of the UV continuum slopes).”
• SPECTROSCOPIC CONFIRMATION OF THREE z-DROPOUT GALAXIES AT z = 6.844–7.213: DEMOGRAPHICS OF Lyα EMISSION IN z ∼ 7 GALAXIES (2011, background): “By measuring the polarization of the cosmic background radiation, Dunkley et al. (2009) estimated the optical 13 See also Dunlop et al. (2011), who question this result. depth to reionization and concluded that, if it was a sudden event, reionization occurred at z = 11.0 ± 1.4 (see also, Komatsu et…”
• SPECTROSCOPIC CONFIRMATION OF z ∼ 7 LYMAN BREAK GALAXIES: PROBING THE EARLIEST GALAXIES AND THE EPOCH OF REIONIZATION (2011, background): “…awaiting the results of this effort. laura.pentericci@oa-roma.inaf.it increasing z (Finkelstein et al. (2010); Bouwens et al. (2010b) but see also Dunlop et al. (2011)) and that their stellar masses are on average smaller than those of their lower-z counterparts (Labbé et al. 2010), while the…”
• The UV continua and inferred stellar populations of galaxies at z ~7-9 revealed by the Hubble Ultra-Deep Field 2012 campaign (2012, methodology): “However, as discussed in detail by Dunlop et al. (2012) and Rogers et al. (2013), previous attempts to determine the UV spectral slope at faint magnitudes (MUV > −19) have inevitably been afflicted by bias.”