Trinh X. Thuan — Page 2

arXiv1997

I ZW 18 -- a New Wolf-Rayet Galaxy

We report the discovery of broad Wolf-Rayet emission lines in the Multiple Mirror Telescope (MMT) spectrum of the NW component of I Zw 18, the lowest-metallicity blue compact dwarf (BCD) galaxy known. Two broad Wolf-Rayet (W-R) bumps at the wavelengths $λ$4650 and $λ$5800 are detected indicating the presence of WN and WC stars. The total numbers of WN and WC stars inferred from the luminosities of the broad He II $λ$4686 and C IV $λ$5808 lines are equal to 17(+/-)4 and 5(+/-)2, respectively. The W-R to O stars number ratio is equal to about 0.02, in satisfactory agreement with the value predicted by massive stellar evolution models with enhanced mass loss rates. The WC stars in the northwest component of I Zw 18 can be responsible for the presence of the nebular He II $λ$4686 emission line, however the observed intensity of this line is several times larger than model predictions, and other sources of ionizing radiation at wavelengths shorter than 228Åare necessary.

astro-ph↗

arXiv1996

HST observations of the blue compact dwarf SBS 0335-052: a probable young galaxy

We present HST WFPC2 V and I images and GHRS UV spectrophotometry of the spectral regions around Ly$_alpha$ and OI 1302 of the extremely metal-deficient (Z~Zsun/41) blue compact dwarf (BCD) galaxy SBS 0335-052. All the star formation in the BCD occurs in six super-star clusters (SSC) with ages =< 3-4 Myr. Dust is clearly present and mixed spatially with the SSCs. There is a supershell of radius ~380 pc, delineating a large supernova cavity. The instantaneous star formation rate is ~0.4 Msun yr^-1. Strong narrow Ly$α$ emission is not observed. Rather there is low intensity broad (FWZI = 20 A) Ly$α$ emission superposed on even broader Ly$α$ absorption by the HI envelope. This broad low-intensity emission is probably caused by resonant scattering of Ly$α$ photons. The BCD appears to be a young galaxy, undergoing its very first burst of star formation. This conclusion is based on the following evidence: 1) the underlying extended low-surface-brightness component is very irregular and filamentary, suggesting that a significant part of the emission comes from ionized gas; 2) it has very blue colors (-0.34 =< (V-I)$_0$ =< 0.16), consistent with gaseous emission colors; 3) the OI 1302 line is not detected in absorption in the GHRS spectrum, setting an upper limit for N(O)/N(H) in the HI envelope of the BCD of more than 3000 times smaller than the value in Orion.

astro-ph↗