With this paper we aim at constraining 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).
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 on the star-formation history and considering also the effect of nebular emission. For comparison we determine the UV slope of the objects, and use it to estimate their SFRUV by correcting the UV luminosity under standard assumptions.
Best-fit ratio M_early-burst/M_total,where M_early-burst is the stellar mass formed during an “early- burst” at age=1.3-1Gyr and the total stellar mass M_total = M_late-burst + M_early-burst includes a “late- burst” at age=300Myrs.
A comparison between SFR obtained through SED-fitting (SFRfit) and the SFRUV shows that the latter are underestimated by a factor 2-10, regardless of the assumed SFH. Other SFR indicators (radio, far-IR, X-ray, recombination lines) coherently indicate SFRs a factor 2-4 larger than SFRUV and in closer agreement with SFRfit. Such discrepancy is due to the assump- tion of solar metallicity in the usual β − A1600 conversion factor. We propose a refined relation, appropriate for sub-solar metallicity LBGs: A1600 = 5.32 + 1.99 ∗ β. This relation reconciles the dust-corrected UV with the SED-fitting and the other SFR indicators. It also implies a revision by a factor ∼2 of the global SFRD: ≃ 0.37 M⊙/yr/Mpc3. We find very young best-fit ages (10-500 Myrs) for all our objects. From a careful examination of the uncertainties in the fit and the ampli- tude of the Balmer break we conclude that there is little evidence for the presence of old stellar population in at least half of the LBGs in our sample, suggesting that these objects are probably caught during huge star-formation burst, rather than being the result of a smooth evolution.
Fig.2: Maximum allowed age assuming different SFHs, as a function of the relevant best-fit age.
In Fig.2 we present the maximum allowed age assuming different SFHs, as a function of the relevant best-fit age. When assuming declining of constant SFHs all objects have a maximum age ≤ 500Myr (i.e. a formation redshift z_form ≤ 6) . On the other hand, half of the LBGs is compatible with age ≥ 1.0 Gyr (z_form ∼10) when assuming rising star-formation histories. We verified that this results do not significantly depend on our definition of age as the onset of the star-formation episode. Consistently with our previous findings on the amplitude of the Balmer break, this test indicates that the assumed parametric form for the SFH largely affects constraints on the presence of old stellar populations: while rising and “rising- declining” SFH allow for a high formation redshift within the best-fit uncertainty, both declining and constant SFH univocally indicate ages of few 100Myrs.
We also build a double-component library of BC03 models (shown in the main figure above), i.e. we assume that the SEDs originate from two different bursts with constant SFH of different intensity: an “early burst” at age=1.3-1Gyr, and a “late-burst” started at age=300 Myrs. We then fit our objects determining the best-fit ratio of the stellar mass formed during the early burst and the total stellar mass. The results can be seen in the image above where we plot the best fit burst ratio and its uncertainty as a function of the best-fit age of single-component constant SFH models. These results coherently point to a minor contribution from older stellar populations to the SEDs of our objects, although significant uncertainties remain, in particular because of the poor constraints on the SFH.
One key conclusion is confirmation that the appropriate metallicity to use in the fitting is ~0.1 times the solar value. Adopting this metallicity, and then fitting a range of star‐formation histories to the combined optical‐infrared photometry, yields star‐formation rates for these objects which suggest that the star‐formation density at z~3 is even higher than previously thought. We note that this paper also makes use of the new near‐infrared data in the GOODS-‐South field provided by members of our team through the VLT Hawk‐I HUGS program, and thus provides another example of the importance of coherently combining ground‐based and space‐based data.