Does radiative feedback make faint z > 6 galaxies look small?

Abstract

Recent observations of lensed sources have shown that the faintest (M UV ≈ −15 mag) galaxies observed at z = 6 − 8 appear to be extremely compact. Some of them have inferred sizes of less than 40 pc for stellar masses between 10 6 and 10 7 M , comparable to individual super star clusters or star cluster complexes at low redshift. High-redshift, low-mass galaxies are expected to show a clumpy, irregular morphology and if star clusters form in each of these well-separated clumps, the observed galaxy size would be much larger than the size of an individual star forming region. As supernova explosions impact the galaxy with a minimum delay time that exceeds the time required to form a massive star cluster, other processes are required to explain the absence of additional massive star forming regions. In this work we investigate whether the radiation of a young massive star cluster can suppress the formation of other detectable clusters within the same galaxy already before supernova feedback can affect the galaxy. We find that in low-mass (M 200 10 10 M ) haloes, the radiation from a compact star forming region with an initial mass of 10 7 M can keep gas clumps with Jeans masses larger than ≈ 10 7 M warm and ionized throughout the galaxy. In this picture, the small intrinsic sizes measured in the faintest z = 6 − 8 galaxies are a natural consequence of the strong radiation field that stabilises massive gas clumps. A prediction of this mechanism is that the escape fraction for ionizing radiation is high for the extremely compact, high-z sources.

Low-luminosity galaxies in the early universe have observed sizes similar to star cluster complexes

“…While SNe winds might be one possible mechanism (i.e., 4 Myr feedback time scale for wind speeds of 50 km/s and source sizes of ∼200 pc: see Bastian 2008), another mechanism for inhibiting star formation in other parts of lower mass galaxies would be through radiative feedback. Ploeckinger et al (2019) show that this mechanism could well be more effective at inhibiting star formation in other regions of lower-mass galaxies. Zick et al (2018) looked in detail at the probable appearance of forming globular clusters in the Fornax dwarf galaxy using information in the color magnitude diagrams of field stars in Fornax and stars in its 5 globular clusters.…”

“…The star cluster complex forms out of overdense baryonic material. As feedback from the star cluster complex could temporarily inhibit star formation in other regions of the galaxy (e.g., Bastian 2008;Ploeckinger et al 2019), galaxies may be much smaller in terms of their readily-visible spatial extent than they actually are. The spatial size of distant star-forming galaxies would be made to look smaller than they are due to dominant impact of the youngest star cluster complexes on the U V morphologies of dwarf galaxies (e.g., Overzier et al 2008;Ma et al 2018), as occurs e.g.…”

“…From a theoretical perspective, addressing this question properly would require high resolution hydrodynamical simulations (e.g., Ma et al 2018Ma et al , 2020Ploeckinger et al 2019). Nevertheless, one plausible scenario is to imagine the collapse of an overdensity resulting in the formation of a star cluster complex and feedback from that star cluster complex preventing star formation from occurring at any other position in a dwarf galaxy (Figure 4).…”

“…The purpose of the present work is to revisit these comparisons, utilizing the large selection of z = 6-8 galaxies with size measurements from Bouwens et al (2021a) while leveraging a variety of new results -both from simulations (Ploeckinger et al 2019;Pfeffer et al 2019) and from local observations (Zick et al 2018) -to allow for an improved interpretation of the observational results. Two aspects are of particular interest: (1) comparing the sizes of sources in these selections with star cluster complexes like 30 Doradus in the LMC and (2) using the Bouwens et al (2021a) searches to set constraints on proto-globular cluster formation models such as described by Boylan-Kolchin (2017), Pfeffer et al (2019), J.…”

z~2-9 Galaxies magnified by the Hubble Frontier Field Clusters I: Source Selection and Surface Density-Magnification Constraints from >2500 galaxies

“…Given the above considerations and our formal constraints on δ, it appears that δ must be close to 0, and that a significant fraction of the lower luminosity z = 2-9 galaxy population have very small rest-U V sizes. One potential explanation for a potentially steeper sizeluminosity relation in the rest-U V is the possibility that only only a small part of a fainter star-forming galaxies may be experiencing prominent star formation at a time, resulting in a much smaller apparent physical size (Overzier et al 2008;Ma et al 2018;Ploeckinger et al 2019;Bouwens et al 2017cBouwens et al , 2021.…”