The initial mass function and star formation law in the outer disc of NGC 2915

TL;DR

This study uses HST data to analyze stellar populations in NGC 2915's outskirts, constraining the initial mass function and star formation law, revealing a steep IMF slope and a significant contribution of the outer disc to overall star formation.

Contribution

It provides the first detailed constraints on the upper-end IMF and star formation law in the outer disc of NGC 2915 using resolved stellar populations and HI data.

Findings

IMF slope α = -2.85 with upper mass limit 60 M☉ from luminosity function

Upper mass limit ≤ 20 M☉ if all Hα emission is from HII regions

Outer HI disc contributes 11-28% of total recent star formation

Abstract

Using Hubble Space Telescope (HST) ACS/WFC data we present the photometry and spatial distribution of resolved stellar populations in the outskirts of NGC 2915, a blue compact dwarf with an extended HI disc. These observations reveal an elliptical distribution of red giant branch stars, and a clumpy distribution of main-sequence stars that correlate with the HI gas distribution. We constrain the upper-end initial mass function (IMF) and determine the star formation law (SFL) in this field, using the observed main-sequence stars and an assumed constant star formation rate. Previously published H{\alpha} observations of the field, which show one faint HII region, are used to provide further constraints on the IMF. We find that the main-sequence luminosity function analysis alone results in a best-fitting IMF with a power-law slope {\alpha}=-2.85 and upper-mass limit M$_\rm{u}$ = 60 M$_\odot$. However, if we assume that all H{\alpha} emission is confined to HII regions then the upper-mass limit is restricted to M$_\rm{u}$ $\le$20 M$_\odot$. For the luminosity function fit to be correct we have to discount the H{\alpha} observations implying significant diffuse ionized gas or escaping ionizing photons. Combining the HST photometry with HI imaging we find the SFL has a power law index $N=1.53 \pm 0.21$. Applying these results to the entire outer HI disc indicates that it contributes 11--28% of the total recent star formation in NGC 2915, depending on whether the IMF is constant within the disc or varies from the centre to the outer region.