The physical properties of Lyαemitting galaxies: not just primeval galaxies?

Abstract

Aims. We have analyzed a sample of Lyman break galaxies from z ∼ 3.5 to z ∼ 6 selected from the GOODS-S field as B, V, and i-dropouts, and with spectroscopic observations showing that they have the Lyα line in emission. Our main aim is to investigate their physical properties and their dependence on the emission line characteristic and to shed light on the relation between galaxies with Lyα emission and the general LBG population. Methods. The objects were selected from their optical continuum colors and then spectroscopically confirmed by the GOODS collaboration and other campaigns. From the public spectra we derived the main properties of the Lyα emission such as total flux and rest frame EW. We then used complete photometry, from U band to mid-infrared from the GOODS-MUSIC database, and through standard spectro-photometric techniques we derived the physical properties of the galaxies, such as total stellar mass, stellar ages, star formation rates, and dust content. Finally we investigated the relation between emission line and physical properties. Results. Although most galaxies are fit by young stellar populations, a small but non negligible fraction has SEDs that cannot be represented well by young models and require considerably older stellar component, up to ∼1 Gyr. There is no apparent relation between age and EW: some of the oldest galaxies have high line EW, and should be also selected in narrow-band surveys. Therefore not all Lyα emitting galaxies are primeval galaxies in the very early stages of formation, as is commonly assumed. We also find a range of stellar populations, with masses from 5 × 10 8 M to 5 × 10 10 M and SFR from few to 60 M yr −1 . Although there is no net correlation between mass and EW, we find a significant lack of massive galaxies with high EW, which could be explained if the most massive galaxies were either dustier and/or if they contained more neutral gas than less massive objects. Finally we find that more than half of the galaxies contain small but non negligible amounts of dust: the mean E(B − V) derived from the SED fit and the EW are well-correlated, although with a large scatter, as already found at lower redshift.

Influence of physical galaxy properties on Lyαescape in star-forming galaxies

“…-Looking at the Lyα equivalent width as a function of extinction, we do not find the trend commonly observed in high-redshift samples (e.g. Shapley et al 2003;Pentericci et al 2009). We explain the absence of correlation to the decoupling of the attenuation affecting the UV continuum and the multiparameter process responsible for the attenuation the Lyα line.…”

“…Composite spectra of z ∼ 3 LBGs (Shapley et al 2003) showed a trend between EW Lyα and E(B − V) where objects with strong Lyα emission also have steeper UV continua. Similar trends have been observed more recently (Vanzella et al 2009;Pentericci et al 2009;Kornei et al 2010) where LBGs that exhibit Lyα in emission tend to have bluer UV slopes than those with Lyα in absorption. Following Shapley et al (2003), Pentericci et al (2009) of extinction and noted the same trend between the mean values of EW Lyα versus stellar extinction.…”

“…Similar trends have been observed more recently (Vanzella et al 2009;Pentericci et al 2009;Kornei et al 2010) where LBGs that exhibit Lyα in emission tend to have bluer UV slopes than those with Lyα in absorption. Following Shapley et al (2003), Pentericci et al (2009) of extinction and noted the same trend between the mean values of EW Lyα versus stellar extinction. Again, all the observed correlations listed here are model-dependent and may be affected by several uncertainties.…”

“…With the aim of understanding the different parameters governing the Lyα escape and observability, many studies, both observational and theoretical, have found an apparent variation of the Lyα strength with the UV luminosity (Shapley et al 2003;Ando et al 2006;Ouchi et al 2008;Verhamme et al 2008;Vanzella et al 2009;Pentericci et al 2009;Balestra et al 2010;Stark et al 2010;Schaerer et al 2011;Shimizu et al 2011). In particular, the data show a lack of high Lyα equivalent width in luminous LBGs.…”

Ly$\mathsf{\alpha}$ emitters in the GOODS-S field

“…Since the restframe UV images obtained with the ACS on HST show a very small angular size, the implication is that this is an example of an ultradense spheroid at a very early epoch. A significant fraction of other Lyα emitters from GOODS, showing apparent Balmer breaks, have also been interpreted as evolved, massive stellar populations (Pentericci et al 2009). …”