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

Abstract

We present optical photometry of Hubble Space Telescope (HST ) ACS/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 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 main sequence stars to simulated stellar populations assuming a constant star formation rate over the dynamical timescale. The best-fitting IMF is deficient in high mass stars compared to a canonical Kroupa IMF, with a best-fit slope α = −2.45 and upper mass limit M U = 16 M ⊙ . This top-light IMF is consistent with predictions of the Integrated Galaxy-wide IMF theory. Combining the HST images with Hi data from The Hi Nearby Galaxy Survey Treasury (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 −7 M ⊙ yr −1 kpc −2 . This is depressed compared to the Kennicutt-Schmidt Star Formation Law, 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 Hα. 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% of the Hi in our field has Q ≤ 4 and this incorporates 96% of the observed MS stars. Hence 28% of the Hi in the field is largely dormant.

A new formulation of the external field effect in MOND and numerical simulations of ultra-diffuse dwarf galaxies – application to NGC 1052-DF2 and NGC 1052-DF4

“…This depends on the age and metallicity of the stellar population, the mass distribution of which also depends on the metallicity and star-formation rate (Kroupa et al 2013;Yan et al 2017;Jeřábková et al 2018). Galaxies with a high star formation rate are known to be producing stellar populations overabundant in massive stars (Gunawardhana et al 2011), while galaxies with a low star formation rate show a deficit of massive stars (Lee et al 2009;Watts et al 2018). Old, dormant galaxies also show significant variations of their stellar populations: elliptical galaxies may be dominated by very low mass stars (van Dokkum & Conroy 2010), while faint diffuse dwarf galaxies have a deficit of low mass stars (Gennaro et al 2018) when compared to the canonical stellar population (Yan et al 2017;Jeřábková et al 2018).…”

The radial distribution of supernovae compared to star formation tracers

“…The surveys have been used to measure the local star formation rate density, highlighting the importance of the contributions made from galaxy types that can be underrepresented in optically-selected samples: low Hi mass, low luminosity and low surface brightness galaxies (Hanish et al 2006;Audcent-Ross et al 2018). Evidence of possible reduced massive star formation has been detected in low luminosity and low surface brightness galaxies as well as in the outskirts of galaxies (Meurer et al 2009;Bruzzese et al 2015;Watts et al 2018;Bruzzese et al 2019).…”

“…This lack of Type II SNe in the outskirts could arise from a combination of Type II SN progenitors having masses towards the higher end of the mass range traced by UV and weak star formation in outer discs, apparently with an Initial Mass Function deficient in the most massive stars (e.g. Bruzzese et al 2015;Watts et al 2018;Bruzzese et al 2019). Metallicity gradients are unlikely to be important, with research showing that progenitor metallicity is not a significant factor in determining SN type (Anderson et al 2010).…”

“…a bottom-heavy IMF) in the low density, outer regions of galaxies (e.g. Thilker et al 2005;Bruzzese et al 2015;Watts et al 2018, Bruzzese et al 2019. Graur et al (2017) found SE SNe underrepresented by a factor of about 3 in low mass galaxies (M < 10 10 M ).…”

The ultra-diffuse dwarf galaxies NGC 1052-DF2 and 1052-DF4 are in conflict with standard cosmology

“…2 The effective radius along the semimajor axis of DF2 can be converted to a 3D circularized half-mass radius by R 1/2 ≈ 4 3 R e b/a, where b/a is the axis ratio being 0.85 for DF2 (Wolf et al 2010;van Dokkum et al 2018b). low SFR will have a top-light IGIMF compared to massive galaxies (Watts et al 2018). This would result in a lower number of giant stars, possibly making DF2 appear to be further away than it actually is (Zonoozi et al in prep).…”