Exploring the links between star formation and minor companions around isolated galaxies


E-mail: jedman@uci.edu


Previous studies have shown that galaxies with minor companions exhibit an elevated star formation rate. We reverse this inquiry, constructing a volume-limited sample of ∼L (Mr≤−19.5 + 5log h) galaxies from the Sloan Digital Sky Survey Data Release 6 that are isolated with respect to other luminous galaxies. Cosmological simulations suggest that 99.8 per cent of these galaxies are alone in their dark matter haloes with respect to other luminous galaxies. We search the area around these galaxies for photometric companions. Matching strongly star forming [EW(Hα) ≥ 35 Å] and quiescent [EW(Hα) < 35 Å] samples for stellar mass and redshift using a Monte Carlo resampling technique, we demonstrate that rapidly star forming galaxies are more likely to have photometric companions than other galaxies. The effect is relatively small; about 11 ± 1 per cent of quiescent, isolated galaxies have minor photometric companions at radii ≤60 kpc h−1 kpc, while about 16 ± 1 per cent of strongly star forming ones do. Though small, the cumulative difference in satellite counts between strongly star forming and quiescent galaxies is highly statistically significant (PKS= 1.350 × 10−3) out to radii of ∼100 h−1 kpc. We discuss explanations for this excess, including the possibility that ∼5 per cent of strongly star forming galaxies have star formation which is causally related to the presence of a minor companion.