Constraints on the star-formation rate ofz~ 3 LBGs with measured metallicity in the CANDELS GOODS-South field

Abstract

Aims. We aim to constrain the assembly history of high-redshift galaxies and the reliability of UV-based estimates of their physical parameters from an accurate analysis of a unique sample of z ∼ 3 Lyman-break galaxies (LBGs). Methods. We analyse 14 LBGs at z ∼ 2.8-3.8 constituting the only sample where both a spectroscopic measurement of their metallicity and deep IR observations (CANDELS+HUGS survey) are available. Fixing the metallicity of population synthesis models to the observed values, we determine best-fit physical parameters under different assumptions about the star-formation history (SFH) and also consider the effect of nebular emission. For comparison, we determine the UV slope of the objects, and use it to estimate their SFR UV99 by correcting the UV luminosity. Results. A comparison between star-formation rate (SFR) obtained through SED-fitting (SFR fit ) and the SFR UV99 shows that the latter are underestimated by a factor of 2-10, regardless of the assumed SFH. Other SFR indicators (radio, far-IR, X-ray, recombination lines) coherently indicate SFRs a factor of 2-4 larger than SFR UV99 and in closer agreement with SFR fit . This discrepancy is due to the solar metallicity implied by the usual β−A 1600 conversion factor. We propose a refined relation, appropriate for subsolar metallicity LBGs: A 1600 = 5.32 + 1.99 * β. This relation reconciles the dust-corrected UV with the SED-fitting and the other SFR indicators. We show that the fact that z ∼ 3 galaxies have subsolar metallicity implies an upward revision by a factor of ∼1.5-2 of the global SFRD, depending on the assumptions about the age of the stellar populations. We find very young best-fit ages (10-500 Myr) for all our objects. From a careful examination of the uncertainties in the fit and the amplitude of the Balmer break we conclude that there is little evidence of the presence of old stellar population in at least half of the LBGs in our sample, suggesting that these objects are probably caught during a huge star-formation burst, rather than being the result of a smooth evolution.

Extreme emission-line galaxies out toz~ 1 in zCOSMOS

“…In our study this hypothesis is supported by the fact that the SFR derived from the SED (which takes into account the metallicity of the galaxy to choose the best-fit model) is always systematically offset to higher values by ∼0.2 dex. In agreement with the results of Castellano et al (2014) if this offset is only due to the dust attenuation correction it would imply that Meurer's zeropoint should be corrected upwards by this quantity for typical EELGs, resulting in a median dust attenuation in the FUV of about ∼10−20% higher. Although a rigorous analysis of the discrepancies between different SFR indicators is far from the scope of this paper, we caution about using dust attenuation corrections for lowmetallicity galaxies under the assumption of models and calibrations which are valid for solar metallicity environments.…”

“…Since standard stellar population synthesis models do not include nebular emission this may have an impact on the SED fitting and, in particular, on the computed total stellar masses (e.g., Krueger et al 1995;Papaderos et al 1998;Schaerer & de Barros 2009;Atek et al 2011;Curtis-Lake et al 2013;Stark et al 2013;Castellano et al 2014;Santini et al 2015;Pacifici et al 2015). In order to overcome this potential systematic effect for the sample of EELGs, an additional set of fits were computed after removing the contribution of emission line fluxes to the observed broadband magnitudes.…”

“…Recent work by Castellano et al (2014) suggests that for young, low-metallicity galaxies at high redshift the SFRs derived from dust-corrected UV luminosities can be underestimated by up to a factor of 2−10 regardless of the assumed star formation history. Such a discrepancy is due to the solar metallicity implied by the usual β UV -A FUV conversion factor.…”

The ASTRODEEP Frontier Fields catalogues

“…Demagnified number counts in the cluster fields investigated in this work compared with the previous two FFs from Castellano et al (2016). Schaerer & de Barros (2009) under the assumption of an escape fraction of ionizing photons fesc = 0.0 (see also Castellano et al 2014). The FFs allow us to probe the galaxy distribution down to very low masses and SFRs, including objects with M ∼ 10 7 M and S FR ∼ 0.1-1 M yr −1 at z > 6, depending on magnification.…”

SPITZER ULTRA FAINT SURVEY PROGRAM (SURFS UP). II. IRAC-DETECTED LYMAN-BREAK GALAXIES AT 6 ≲ z ≲ 10 BEHIND STRONG-LENSING CLUSTERS

“…To calculate the expected β, we use the Calzetti et al (2000) dust attenuation law to calculate the amount of dust attenuation at restframe 1600 Å from E B V ( ) - ) to calculate the expected β. We show the Meurer et al (1999) relation as a solid line and the Castellano et al (2014) relation as a dashed line in Figure 13. The scatter of the measured β of our sample is larger than the difference between the two empirical calibrations, but the distribution is roughly consistent with both calibrations.…”