Hubble Space Telescope Observations of the Unusual Blue Compact Dwarf Galaxy Markarian 996

We present Hubble Space Telescope Wide Field Planetary Camera 2 V and I images and Faint Object Spectrograph UV and optical spectrophotometry of the unusual blue compact dwarf galaxy Mrk 996. We found Mrk 996 to have a disk structure with a small scale length (0. 4 kpc) and a two-arm spiral-like pattern confined to the inner 160 pc in radius. Nearly all the star-forming activity occurs in an extremely compact nuclear H II region of radius ~315 pc. A central density greater than 10⁶^ cm^-3^, together with a density gradient, are needed to explain the emission-like intensities. The density gradient may be caused by a mass outflow, which would also account for the much broader line widths (FWHM ~ 900 km s^-1^) of the high-excitation lines originating in the inner region as compared with the low-excitation lines originating in the outer region. There is a large population of WR stars that may drive the mass outflow and heavily pollute the central region with nitrogen, producing a nitrogen abundance gradient from the inner to the outer parts. The nitrogen abundance in the center is about 25 times larger than that derived for other blue compact galaxies. It decreases by a factor of about 6 from the inner high- excitation zone to the outer low-excitation zone. The number ratio of WNL stars to O stars is >= 0. 1, larger by a factor of >= 5 than predictions by evolutionary synthesis models. The ratio of the number of WC stars to the total number of WR stars is ~0. 11. The oxygen abundance of Mrk 996 is ~1/10 that of the Sun, its C/O ratio is 0. 25, and its helium mass fraction Y = 0. 26. There is a system of old (~10¹0^ yr) globular clusters distributed asymmetrically around Mrk 996, mainly to the south of the galaxy; an asymmetry probably related to the one seen in the outer isophotes of Mrk 996. The globular cluster luminosity function in Mrk 996 is very similar to that of the Milky Way, with a suggestion of a slightly larger number of clusters at the bright end, in the MV_ range between -9 and -10. 4.