The results of Multiple Mirror Telescope ( MMT ) spectrophotometry of the extremely low-metallicity blue compact galaxy ( BCG ) SBS 0335–052 ( SBS – the Second Byurakan Survey ) are presented .
The oxygen abundance in central brightest part of the galaxy is found to be 12 + log ( O/H ) = 7.33 \pm 0.01 , only slightly greater than the oxygen abundance in the most metal-deficient BCG I Zw 18 .
We show that the N/O , Ne/O , S/O and Ar/O abundance ratios in SBS 0335–052 are close to those derived in other BCGs , suggesting that heavy element enrichment in the HII region is due to massive star evolution .
However , we find an O/Fe abundance ratio close to that in the Sun , in variance with values derived for other BCGs .
The helium abundance derived from the HeI \lambda 4471 , 5876 and 6678 emission lines , taking into account of collisional and fluorescent enhancement , is Y = 0.245 \pm 0.006 , close to the value of the primordial helium abundance Y _ { p } =0.243 \pm 0.003 derived by Izotov , Thuan & Lipovetsky .
We detect auroral [ OIII ] \lambda 4363 emission in the inner part of HII region with a diameter of 14 ^ { \prime \prime } or 3.6 kpc and find that the HII region inside this diameter is hot , T _ { e } \sim 20000K .
The oxygen abundance in this region is nearly constant ( 12 + log ( O/H ) = 7.1 – 7.3 ) with a gradual decrease to the outer part of HII region , implying effective mixing of ionized gas on short time-scales .
We study the distribution of the nebular HeII \lambda 4686 emission line and find it is not produced by main-sequence O-stars or Wolf-Rayet stars .
Possible excitation mechanisms of this line , such as massive X-ray binaries and shocks , are discussed .
We also discuss the origin of blue underlying extended low-intensity emission detected in SBS 0335–052 on V , R and I images .
The blue ( V - I ) and ( R - I ) color distributions suggest that a significant contribution to the extended low-intensity envelope is due to ionized gas emission .
This is evidence that SBS 0335–052 is a young galaxy experiencing its very first burst of star formation .
However , we find that the observed equivalent width of H \beta emission in the extended envelope is 2–3 times lower than the value expected in the case of pure gaseous emission .
Furthermore , we find that the widths of H \gamma and H \beta are narrower than the instrumental profiles ; this could be explained by presence of underlying stellar absorption from A stars .
These findings suggest that , along with the blue young ( \sim 10 ^ { 7 } yr ) stellar clusters in the center of the galaxy , an older stellar population with age \sim 10 ^ { 8 } yr may be present in the extended envelope of SBS 0335–052 , having a total mass \sim 10 ^ { 7 } M _ { \odot } , two orders of magnitude smaller than the neutral gas mass but comparable with the total mass of stars in blue young stellar clusters observed in the center of the galaxy .
We conclude that SBS 0335–052 is a young nearby dwarf galaxy with age \sim 10 ^ { 8 } yr .