Session 11 - LMC, Dwarf Galaxies.
Display session, Monday, January 15
North Banquet Hall, Convention Center

[11.05] Star Formation and Gas Densities in the Dwarf Irregular Galaxy Sextans A

J. D. Plummer (Northern Arizona U. and Washington State U.), D. A. Hunter (Lowell Obs.)

As a step in understanding the process of star formation in irregular galaxies, we have analysed the irregular galaxy Sextans A. Irregular galaxies provide star-forming systems that are unperturbed by spiral density waves. Sextans A is a tiny galaxy, 1.3 Mpc distant, just beyond the dynamic boundary of the Local Group. We studied the star formation properties of this galaxy using UBV and H\alpha images. Stars are not currently forming in the center of this galaxy, though they have in the past. The current star formation is in clumps in the outer parts of the galaxy and is not evenly distributed.

The total H\alpha luminosity found for Sextans A is 9 \times 10^38erg/s, which corresponds to a star formation rate of 6 \times 10^-3 M\mathordødot/yr for standard assumptions. The rate per unit area, within the Holmberg radius, is 6 \times 10^-10 M\mathordødot/yr/pc^2. Skillman et al. (1988) derived a total HI mass of 6 \times 10^7 M\mathordødot. At its present rate, Sextans A will use up all of its gas in 12 \times 10^9 yr, including the He contribution.

We have also compared the star formation and gas density in Sextans A to critical gas surface density models (Toomre 1964, Quirk 1972, Kennicutt 1988). We used a published rotation curve to calculate the critical gas density necessary for the instabilities that produce star-forming clouds (Skillman et al. 1988). The ratio of observed to critical gas density is low in Sextans A, at the low end of values found by Kennicutt (1988) for spiral galaxies. The current star formation is located in the region of the galaxy with higher radially averaged observed gas densities relative to the critical density. This suggests that Sextans A has a difficult time forming gas clouds, resulting in an observed low star formation rate. This research was funded by the REU program at Northern Arizona University.