New detections of Ly α emission in young galaxies

Abstract

We report IUE observations of two extremely low-metallicity H n galaxies. Lya emission was detected in both galaxies. Our results confirm previous findings that young or unevolved galaxies exhibit weak or absent Lya emission combined with a strong UV continuum, and that the Ly a/H/? ratio and metal content are correlated in the sense that the lower the abundance, the larger the ratio. This supports the hypothesis that in the higher metallicity systems Lya is destroyed by dust absorption. At redshifts of between 2 and 3, Lya emission comparable to the strongest detected in Hii galaxies [L(Lya)~5 x 10 42 erg s _1 ] would be barely visible; this fact can explain the relative lack of success in detecting Lya emission associated with low-metallicity damped Lya systems in QSOs. Our results cast doubt on claims of detection of high-redshift young galaxies associated with active galaxies, or with strong Lya and both C iv and He n, since they may represent very extended narrow-line regions photoionized by radiation with a power-law spectrum rather than by normal hot stars.