The brightest galaxies at Cosmic Dawn

Abstract

Recent JWST observations suggest an excess of 𝑧 10 galaxy candidates above most theoretical models. Here, we explore how the interplay between halo formation timescales, star formation efficiency and dust attenuation affects the properties and number densities of galaxies we can detect in the early universe. We calculate the theoretical upper limit on the UV luminosity function, assuming star formation is 100% efficient and all gas in halos is converted into stars, and that galaxies are at the peak age for UV emission (∼ 10 Myr). This upper limit is ∼ 4 orders of magnitude greater than current observations, implying these are fully consistent with star formation in ΛCDM cosmology. In a more realistic model, we use the distribution of halo formation timescales derived from extended Press-Schechter theory as a proxy for star formation rate (SFR). We predict that the galaxies observed so far at 𝑧 10 are dominated by those with the fastest formation timescales, and thus most extreme SFRs and young ages. These galaxies can be upscattered by ∼ 1.5 mag compared to the median UV magnitude vs halo mass relation. This likely introduces a selection effect at high redshift whereby only the youngest ( 10 Myr), most highly star forming galaxies (specific SFR ∼ > 30 Gyr −1 ) have been detected so far. Furthermore, our modelling suggests that redshift evolution at the bright end of the UV luminosity function is substantially affected by the build-up of dust attenuation. We predict that deeper JWST observations (reaching 𝑚 ∼ 30) will reveal more typical galaxies with relatively older ages (∼ 100 Myr) and less extreme specific SFRs (∼ 10 Gyr −1 for a 𝑀 UV ∼ −20 galaxy at 𝑧 ∼ 10).

On the ages of bright galaxies $\sim 500$ Myr after the Big Bang: insights into star formation activity at $z \gtrsim 15$ with JWST

“…This would allow large numbers of 𝑧 ∼ 15 galaxies to be bright without also requiring their descendants at 𝑧 ∼ 9 to be old. This picture of bright 𝑧 12 galaxies becoming increasingly likely to be observed during bursts of star formation is consistent with the evolution towards younger ages we have observed between 𝑧 ∼ 7 and 𝑧 ∼ 9, as well as the theoretical picture proposed by Mason et al (2022). Alternatively, if 𝑧 ∼ 15 galaxies become significantly dust reddened or quench between 𝑧 ∼ 15 and 𝑧 ∼ 8.5 − 11, this may have a similar effect on the UV luminosity evolution (bright at early times, followed by periods of being at lower luminosities), perhaps even leading to 𝑧 ∼ 9 descendants that do not even enter our sample.…”

Stress Testing $Λ$CDM with High-redshift Galaxy Candidates

“…This requirement does not rely on assumptions such as abundance matching but rather is simply a statement about the baryonic reservoir associated with halos that contain enough baryonic material to form the galaxies in question. It is also more stringent than the requirement that the observed galaxy UV luminosity function not exceed the theoretical maximum coming from a nearly instantaneous (10 Myr) conversion of a halo's full baryonic reservoir into stars (Mason et al 2022), as it is an integral constraint as opposed to a differential one.…”

Schrodinger's Galaxy Candidate: Puzzlingly Luminous at $z\approx17$, or Dusty/Quenched at $z\approx5$?

“…DISCUSSION 4.1. Implications of the z ≈ 17 scenario If CEERS-1749 is confirmed to lie at z ≈ 17, it would force a major revision of early galaxy evolution models, and potentially even our underlying cosmological framework (see, e.g., Steinhardt et al 2016;Mason et al 2022;Boylan-Kolchin 2022). It is very challenging to produce such extraordinarily luminous and massive galaxies only ∼200 Myrs after the Big Bang under standard assumptions in the framework of ΛCDM cosmology (see also the candidates reported in Atek et al 2022;Yan et al 2022).…”