The blue UV slopes of z ~ 4 Lyman break galaxies: implications for the corrected star formation rate density
1 INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio, Italy
2 Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
3 INAF – Osservatorio Astronomico di Trieste, via G.B. Tiepolo 11, 34131 Trieste, Italy
Received: 8 September 2011
Accepted: 24 January 2012
Context. Study of the dust extinction in high-redshift galaxies is fundamental to obtaining an estimate of the corrected star formation rate density (SFRD) and to put constraints on galaxy evolution models.
Aims. We plan to analyse dust extinction in Lyman break galaxies (LBGs) by introducing a new and more reliable approach to their selection and to the characterization of their distribution of UV slopes β, using deep IR images from HST. We fully discuss the methodology and the results, focusing on a robust sample of z ~ 4 LBGs.
Methods. We exploit deep WFC3 IR observations of the ERS and HUDF fields over GOODS-South, combined with HST-ACS optical data, to select z ~ 4 LBGs through a new (B − V) vs. (V − H) colour diagram. The UV slope of the selected galaxies is robustly determined by a linear fit over their observed I,Z,Y,J magnitudes, coherently with the original definition of β. The same fit is used to determine their rest-frame UV magnitudes M1600 through a simple interpolation. We estimate the effect of observational uncertainties with detailed simulations that we also exploit, under a parametric maximum-likelihood approach, to constrain the probability density function of UV slopes PDF(β) as a function of rest-frame magnitude.
Results. We find 142 robust LBGs in the ERS and 25 in the HUDF field, limiting our sample to S/N(H) > 10 objects. Our newly defined criteria improve the selection of z ~ 4 LBGs and allow us to exclude red interlopers at lower redshifts, especially z ~ 3–3.5 objects. We show that using a linear fit to estimate β and accurately characterizing observational effects are required in this kind of analysis of flux-limited samples. We find that z ~ 4 LBGs are characterized by blue UV slopes, suggesting a low dust extinction: all L < L∗ galaxies have an average UV slope ⟨β⟩ ≃ −2.1, while brighter objects are only slightly redder (⟨β⟩ ≃ −1.9). We find an intrinsic dispersion ≃0.3 for PDF(β) at all magnitudes. The SFRD at z ~ 4 corrected according to these estimates turns out to be lower than previously found: log(SFRD) ≃ −1.09 M⊙/yr/Mpc3. Finally, we discuss how the UV slope of z ~ 4 galaxies changes as a function of the dust-corrected UV magnitude (i.e. SFR). We show that most galaxies with high SFR (≳80 M⊙/yr) are highly extincted objects. Among galaxies with lower SFR, we detect many with a much lower amount of reddening, although current observational limits prevent us from detecting those with high extinction, if they exist.
Key words: galaxies: distances and redshifts / galaxies: evolution / galaxies: high-redshift
© ESO, 2012