Interpreting the Spitzer/IRAC Colours of 7<z<9 Galaxies: Distinguishing
  Between Line Emission and Starlight Using ALMA
  Prior to the launch of JWST, Spitzer/IRAC photometry offers the only means of
studying the rest-frame optical properties of z>7 galaxies. Many such high
redshift galaxies display a red [3.6] - [4.5] micron colour, often referred to
as the "IRAC excess", which has conventionally been interpreted as arising from
intense [OIII]+Hbeta emission within the [4.5] micron bandpass. An appealing
aspect of this interpretation is similarly intense line emission seen in
star-forming galaxies at lower redshift as well as the redshift-dependent
behaviour of the IRAC colours beyond z~7 modelled as the various nebular lines
move through the two bandpasses. In this paper we demonstrate that, given the
photometric uncertainties, established stellar populations with Balmer (4000 A,
rest-frame) breaks, such as those inferred at z>9 where line emission does not
contaminate the IRAC bands, can equally well explain the redshift-dependent
behaviour of the IRAC colours in 7<z<9 galaxies. We discuss possible ways of
distinguishing between the two hypotheses using ALMA measures of [OIII] 88
micron and dust continuum fluxes. Prior to further studies with JWST, we show
that the distinction is important in determining the assembly history of
galaxies in the first 500 Myr.
