Effects of Dust Geometry in Lyα Galaxies at z = 4.41

Abstract

Equivalent widths (EWs) observed in high-redshift Ly{α} galaxiescould be stronger than the EW intrinsic to the stellar population ifdust is present residing in clumps in the interstellar medium (ISM). Inthis scenario, continuum photons could be extinguished, while theLy{α} photons would be resonantly scattered by the clumps,eventually escaping the galaxy. We investigate this radiative transferscenario with a new sample of six Ly{α} galaxy candidates in theGOODS CDF-S, selected at z=4.4 with ground-based narrowband imagingobtained at CTIO. Grism spectra from the HST PEARS survey confirm thatthree objects are at z=4.4, and that another object contains an activegalactic nucleus (AGN). If we assume the other five (non-AGN) objectsare at z=4.4, they have rest-frame EWs from 47 to 190 {Å}. Wepresent results of stellar population studies of these objects,constraining their rest-frame UV with HST and their rest-frame opticalwith Spitzer. Out of the four objects which we analyzed, three objectswere best fit to contain stellar populations with ages on the order of 1Myr and stellar masses from 3{\times}10^{8} to1{\times}10^{9} M_{solar}, with dust in the amount ofA_{1200}=0.9-1.8 residing in a quasi-homogeneous distribution.However, one object (with a rest EW~150 {Å}) was best fit by an 800Myr, 6.6{\times}10^{9} M_{solar} stellar population witha smaller amount of dust (A_{1200}=0.4) attenuating thecontinuum only. In this object, the EW was enhanced ~50% due to thisdust. This suggests that large EW Ly{α} galaxies are a diversepopulation. Preferential extinction of the continuum in a clumpy ISMdeserves further investigation as a possible cause of the overabundanceof large-EW objects that have been seen in narrowband surveys in recentyears.Based in part on observations taken at the Cerro Tololo Inter-AmericanObservatory.