Do blue compact galaxies have red halos?

TL;DR

This paper investigates the origin of red halos around blue compact galaxies, suggesting they may be due to intermediate-metallicity stellar populations rather than extremely metal-rich halos, challenging previous assumptions.

Contribution

It demonstrates that recent spectral synthesis developments reduce the need for extreme metallicities to explain red halos, proposing a more standard metallicity origin for BCG outskirts.

Findings

Red halos in BCGs may originate from intermediate-metallicity stars.

Recent models lessen the need for extremely metal-rich populations.

Host metallicity is higher than gas metallicity in starburst regions.

Abstract

Red halos are faint, extended and extremely red structures that have been reported around various types of galaxies since the mid-1990s. The colours of these halos are too red to be reconciled with any hitherto known type of stellar population, and instead indicative of a very bottom-heavy stellar initial mass function (IMF). Due to the large mass-to-light ratios of such stellar halos, they could contribute substantially to the baryonic masses of galaxies while adding very little to their overall luminosities. The red halos of galaxies therefore constitute potential reservoirs for some of the baryons still missing from inventories in the low-redshift Universe. While most studies of red halos have focused on disk galaxies, a red excess has also been reported in the faint outskirts of blue compact galaxies (BCGs). A bottom-heavy IMF can explain the colours of these structures as well, but due to model degeneracies, stellar populations with standard IMFs and abnormally high metallicities have also been demonstrated to fit the data. Here, we show that due to recent developments in the field of spectral synthesis, the metallicities required in this alternative scenario may be less extreme than previously thought. This suggests that the red excess seen in the outskirts of BCGs may stem from a normal, intermediate-metallicity host galaxy rather than a red halo of the type seen around disk galaxies. The inferred host metallicity does, however, still require the host to be more metal-rich than the gas in the central starburst of BCGs, in contradiction with current simulations of how BCGs form.