HUBBLE-SPACE-TELESCOPE OBSERVATIONS OF THE UNUSUAL BLUE COMPACT DWARFGALAXY MARKARIAN-996

We present Hubble Space Telescope Wide Field Planetary Camera 2 V and I images and Faint Object Spectrograph UV and optical spectrophotometr y of the unusual blue compact dwarf galaxy Mrk 996. We found Mrk 996 t o have a disk structure with a small scale length (similar to 0.4 kpc) and a two-arm spiral-like pattern confined to the inner 160 pc in rad ius. Nearly all the star-forming activity occurs in an extremely compa ct nuclear H II region of radius similar to 315 pc. A central density greater than 10(6) cm(-3), together with a density gradient, are neede d to explain the emission-like intensities. The density gradient may b e caused by a mass outflow, which would also account for the much broa der line widths (FWHM similar to 900 km s(-1)) of the high-excitation lines originating in the inner region as compared with the low-excitat ion lines originating in the outer region. There is a large population of WR stars that may drive the mass outflow and heavily pollute the c entral region with nitrogen, producing a nitrogen abundance gradient f rom the inner to the outer parts. The nitrogen abundance in the center is about 25 times larger than that derived for other blue compact gal axies. It decreases by a factor of about 6 from the inner high-excitat ion zone to the outer low-excitation zone. The number ratio of WNL sta rs to O stars is greater than or equal to 0.1, larger by a factor of g reater than or equal to 5 than predictions by evolutionary synthesis m odels. The ratio of the number of WC stars to the total number of WR s tars is similar to 0.11. The oxygen abundance of Mrk 996 is similar to 1/10 that of the Sun, its C/O ratio is 0.25, and its helium mass frac tion Y = 0.26. There is a system of old (similar to 10(10) yr) globula r clusters distributed asymmetrically around Mrk 996, mainly to the so uth of the galaxy; an asymmetry probably related to the one seen in th e outer isophotes of Mrk 996. The globular cluster luminosity function in Mrk 996 is very similar to that of the Milky Way, with a suggestio n of a slightly larger number of clusters at the bright end, in the M( v) range between -9 and -10.4.