Insights from HST into Ultramassive Galaxies and Early-Universe Cosmology

The early-science observations made by the James Webb Space Telescope (JWST) have revealed an excess of ultramassive galaxy candidates that appear to challenge the standard cosmological model ($\mathrm{\Lambda }\mathrm{CDM}$). Here, we argue that any modifications to $\mathrm{\Lambda }\mathrm{CDM}$ that can produce such ultramassive galaxies in the early Universe would also affect the UV galaxy luminosity function (UV LF) inferred from the Hubble Space Telescope (HST). The UV LF covers the same redshifts ($z\approx 7–10$) and host-halo masses ($M_{\mathrm{h}}\approx {10}^{10}–{10}^{12}{M}_{\odot }$) as the JWST candidates, but tracks star-formation rate rather than stellar mass. We consider beyond-$\mathrm{\Lambda }\mathrm{CDM}$ power-spectrum enhancements and show that any departure large enough to reproduce the abundance of ultramassive JWST candidates is in conflict with the HST data. Our analysis, therefore, severely disfavors a cosmological explanation for the JWST abundance problem. Looking ahead, we determine the maximum allowable stellar-mass function and provide projections for the high-$z$ UV LF given our constraints on cosmology from current HST data.

Figure 1

The dimensionless matter power spectrum, linearly extrapolated to $z=8$. The $\mathrm{\Lambda }\mathrm{CDM}$ prediction (black dotted) is not sufficient to explain the observed abundance of massive JWST galaxy candidates. We show the power enhancement [i.e., the bump described in Eq. (3)] that is necessary to address this abundance problem. The location and amplitude of the enhancement depend on the assumed star-formation efficiency $f_{\star }$. For instance, a lower $f_{\star }$ shifts the bump towards smaller $k$, as more massive halos are needed, and requires a larger amplitude due to the rarity of such halos. The gray shaded band represents the range of scales probed by the Hubble UV LFs, while the hatched areas indicate the scales at which other cosmological probes are sensitive, or when $f_{\star }>1$ is required.

Figure 2

Probability distributions of the average number of galaxies, calculated using Eq. (1) for the two brightest candidates in the JWST sample from [19]. The solid lines correspond to the results obtained using HST UV LF data with the gallumi pipeline, while the dashed lines are the [Poisson–cf., Eq. (2)] probability distributions inferred from the two observed JWST galaxies. It is clear that the HST posteriors do not reach the observed value of $N_{\mathrm{gal}}=2$ (black dotted line). We consider three different values of the star-formation efficiency $f_{\star }$, at or above the typical peak height of $f_{\star }=0.1$, to explore the impact of astrophysics.

Figure 3

Top: Forecast for the stellar mass function (SMF) at $z=7.5$, defined as the number density of galaxies above a stellar mass cut-off $M_{\star }^{\mathrm{obs}}$. The black curve is the $\mathrm{\Lambda }\mathrm{CDM}$ prediction, and the blue dotted curve indicates the enhancement allowed by HST UV LF data at 95% C.L., all assuming a constant $f_{\star }=0.3$ (which can be rescaled to any ${\tilde{f}}_{\star }$ by simply shifting the curves horizontally by a factor ${\tilde{f}}_{\star }/0.3$). Horizontal lines represent the fiducial volume (i.e., $1/\mathrm{Vol}$) of different surveys [1, 82, 83], whereas the red star is the inferred density from the $z=7.5$ galaxy of Ref. [19], which is far in excess of the SMF allowed by HST. Bottom: UV luminosity function at $z=12$, extrapolated from HST calibrations at $z=6–10$, along with JWST measurements at $z=12–13$ from [4, 5, 85, 86, 87]. The 68% and 95% confidence levels are obtained after marginalizing over our astrophysical parameters and the enhancement of power. We find that including a power boost alone is not enough to explain the JWST UV LFs.