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Current star formation rate

The star formation rate (SFR) of the current SF episode can be estimated from the total H$\alpha$ luminosity. The H$\alpha$ flux was measured within the slit, and was corrected for the missing light outside the slit. A correction factor of 2.33 was calculated, using the brightness profile along the slit and assuming circular symmetry. For the derived total flux F(H$\alpha$)=5.8 $\times10^{-14}$ erg s^-1cm^-2 we obtain a total H$\alpha$ luminosity of 10³⁹ erg s^-1.

This H$\alpha$ luminosity corresponds to a current SFR of $\sim$ 0.007 $M_{\odot}$ yr^-1 (Hunter & Gallagher [1986]) assuming a Salpeter IMF with a 0.1 $M_{\odot}$ lower mass cutoff. The gaseous mass converted into the stars during a 3 Myr burst is $\sim$ 2$\times$10⁴ $M_{\odot}$. The unknown contribution of the gaseous emission to the light from the LSB component further complicates the stellar mass estimate. If only stars with age $\le$ 100 Myr contribute to the LSB luminosity, then the total mass of stars in the underlying disc with $M_{B,{\rm disc}}$=-123 is 1.3$\times$10⁶ $M_{\odot}$ (from e.g., Leitherer et al. [1999]). This is much smaller than the total neutral gas mass of $\approx$3.0$\times$10⁷ $M_{\odot}$, accounting for a helium mass fraction of 0.25.

The total mass of ionized gas can be obtained from the average mass density inside the H II region and its volume. An average mass density corresponding to the average electron density $N_{\rm e}$ of 1 cm^-3 within a volume with a diameter of 0.5 kpc yields a total ionized gas mass of 10⁶$M_{\odot}$.

The above estimates show that baryonic matter in HS 0822+3542 is dominated by the gaseous component.