Two types of Lyman α emitters envisaged from hierarchical galaxy formation

Abstract

In the last decade, numerous Lyman {α} (Ly{α}) emitters (LAEs) have been discovered with narrow-band filters at various redshifts. Recently, multiwavelength observations of LAEs have been performed and revealed that while many LAEs appear to be young and less massive, a noticeable fraction of LAEs possess much older populations of stars and larger stellar mass. How these two classes of LAEs are concordant with the hierarchical galaxy formation scenario has not been understood clearly so far. In this paper, we model LAEs by three-dimensional cosmological simulations of dark halo merger in a {Λ} cold dark matter ({Λ}CDM) universe. As a result, it is shown that the age of simulated LAEs can spread over a wide range from 2 {\times} 10^{6} to 9 {\times} 10^{8}yr. Furthermore, we find that there are two types of LAEs, in one of which the young half-mass age is comparable to the mean age of stellar component, and in the other of which the young half-mass age is appreciably shorter than the mean age. We define the former as Type 1 LAEs and the latter as Type 2 LAEs. A Type 1 LAE corresponds to early starburst in a young galaxy, whereas a Type 2 LAE does to delayed starburst in an evolved galaxy, as a consequence of delayed accretion of a subhalo on to a larger parent halo. Thus, the same halo can experience a Type 2 LAE phase as well as a Type 1 LAE phase in the merger history. Type 1 LAEs are expected to be younger than 1.5 {\times} 10^{8}yr, less dusty and less massive with stellar mass M_{star} {\lt}~ 5 {\times} 10^{8}M_{solar}, while Type 2 LAEs are older than 1.5 {\times} 10^{8}yr, even dustier and as massive as M_{star} ~ 5 {\times} 10^{8}-3 {\times} 10^{10}M_{solar}. The fraction of Type 2s in all LAEs is a function of redshift, which is less than 2 per cent at z {\gt}~ 4.5, ~30 per cent at redshift z = 3.1 and ~70 per cent at z = 2. Type 2 LAEs can be discriminated clearly from Type 1s in two-colour diagrams of z' - H versus J - K. We find that the brightness distribution of Ly{α} in Type 2 LAEs is more extended than the main stellar component, in contrast to Type 1 LAEs. This is not only because delayed starbursts tend to occur in the outskirts of a parent galaxy, but also because Ly{α} photons are effectively absorbed by dust in an evolved galaxy. Hence, the extent of Ly{α} emission may be an additional measure to distinguish Type 2 LAEs from Type 1 LAEs. The sizes of Type 2 LAEs range from a few tens to a few hundreds kpc. At lower redshifts, the number of more extended, older Type 2 LAEs increases. Furthermore, it is anticipated that the amplitude of angular correlation function for Type 2 LAEs is significantly higher than that for Type 1 LAEs, but comparable to that for Lyman break galaxies (LBGs). This implies that LBGs with strong Ly{α} line may include Type 2 LAEs.