Two types of Lyman-alpha emitters envisaged from hierarchical galaxy formation

Abstract

From multi-wavelength observations of LAEs,we know 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 CDM universe.As a result,it is shown that the age of simulated LAEs can spread over a wide range from 2*10^6yr to 9*10^8yr.Also,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 corresponds to early starburst in a young galaxy,whereas a Type 2 does to delayed starburst in an evolved galaxy,as a consequence of delayed accretion of a subhalo onto 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 1s are expected to be younger than 1.5*10^8yr,less dusty,and less massive with stellar mass M*<10^8 Msun,while Type 2s are older than 1.5*10^8yr,even dustier,and as massive as M*~10^8-10^10Msun.The fraction of Type 2s in all LAEs is a function of redshift.Type 2s discriminated clearly from Type 1s in two color diagram of z'-H vs J-K.We find that the brightness distribution of LyA in Type 2s is more extended than the main stellar component,in contrast to Type 1s.This is not only because delayed starbursts tend to occur in the outskirts of a parent galaxy,but also because LyA photons are effectively absorbed by dust in an evolved galaxy.Hence,the extent of LyA emission may be an additional measure to distinguish Type 2s from Type 1s

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