Star formation in the outskirts of DDO 154: A top-light IMF in a nearly dormant disc

We present optical photometry of Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS)/Wide Field Camera (WFC) data of the resolved stellar populations in the outer disc of the dwarf irregular galaxy DDO 154. The photometry reveals that young main sequence (MS) stars are almost absent from the outermost HI disc. Instead, most are clustered near the main stellar component of the galaxy. We constrain the stellar initial mass function (IMF) by comparing the luminosity function of the MS stars to simulated stellar populations, assuming a constant star formation rate over the dynamical time-scale. The best-fitting IMF is deficient in high-mass stars compared to a canonical Kroupa IMF, with a best-fitting slope α = -2.45 and upper mass limit M_U = 16M_⊙. This top-light IMF is consistent with predictions of the integrated galactic IMF theory. Combining the HST images with HI data from The HI Nearby Galaxy Survey (THINGS), we determine the star formation law (SFL) in the outer disc. The fit has a power-law exponent N = 2.92 ± 0.22 and zero-point A = 4.47 ± 0.65 × 10^-7M_⊙ yr^-1 kpc^-2. This is depressed compared to the Kennicutt- Schmidt SFL, but consistent with weak star formation observed in diffuse HI environments. Extrapolating the SFL over the outer disc implies that there could be significant star formation occurring that is not detectable in H a. Last, we determine the Toomre stability parameter Q of the outer disc of DDO 154 using the THINGS HI rotation curve and velocity dispersion map. 72 per cent of the HI in our field has Q ≤ 4 and this incorporates 96 per cent of the observed MS stars. Hence, 28 per cent of the HI in the field is largely dormant.