I Zw 18 as morphological paradigm for rapidly assembling high-z galaxies

TL;DR

This paper investigates IZw18, a prototypical blue compact dwarf galaxy, revealing its extended nebular emission and implications for understanding high-redshift galaxy morphology and observational biases.

Contribution

It demonstrates that nebular emission significantly influences galaxy morphology, affecting surface brightness profiles and potentially leading to misclassification of distant galaxies.

Findings

Nebular emission extends to ~16 stellar scale lengths in IZw18.

Extended nebular emission can mimic profiles of massive galaxy spheroids.

Caution is needed when interpreting high-redshift galaxy structures due to nebular effects.

Abstract

IZw18, ever since regarded as the prototypical blue compact dwarf (BCD) galaxy, is, quite ironically, the most atypical BCD known. This is because its large exponential low-surface brightness envelope is not due to an old stellar host but entirely due to extended nebular emission (ne) (Papaderos et al. 2002; P02). We study IZw18 and IZw18C down to an unprecedently faint surface brightness level using HST ACS data. We argue that the properties of IZw18C can be consistently accounted for by propagating star formation over the past ~100 Myr, in combination with stellar diffusion and the associated radial stellar mass filtering effect (P02). As for IZw18, we find that ne extends out to ~16 stellar scale lengths and provides at least 1/3 of the total optical emission. The case of IZw18 suggests caution in studies of distant galaxies in dominant stages of their evolution, rapidly assembling their stellar mass at high specific star formation rates (SSFRs). It calls attention to the fact that ne is not necessarily cospatial with the underlying ionizing and non-ionizing stellar background, neither has to scale with its surface density. The prodigious energetic output during dominant phases of galaxy evolution may result in large exponential ne envelopes, extending much beyond the still compact stellar component, just like in IZw18. Therefore, the morphological paradigm of IZw18, while probably unique in the nearby Universe, may be ubiquitous among high-SSFR galaxies at high redshift. Using IZw18 as reference, we show that extended ne may introduce substantial observational biases and significantly affect fundamental galaxy relations. Among others, we show that the surface brightness profiles of distant morphological analogs to IZw18 may be barely distinguishable from Sersic profiles with an exponent 2<n<5, thus mimicking the profiles of massive galaxy spheroids. (abridged)