Accelerated Formation of Ultra-Massive Galaxies in the First Billion Years (2309.02492v2)

Abstract: Recent JWST observations have revealed an unexpected abundance of massive galaxy candidates in the early Universe, extending further in redshift and to lower luminosity than what had previously been found by sub-millimeter surveys. These JWST candidates have been interpreted as challenging the $\Lambda$CDM cosmology, but, so far, they have mostly relied only on rest-frame ultraviolet data and lacked spectroscopic confirmation of their redshifts. Here we report a systematic study of 36 massive dust-obscured galaxies with spectroscopic redshifts between $z_{\rm spec}=5-9$ from the JWST FRESCO survey. We find no tension with the $\Lambda$CDM model in our sample. However, three ultra-massive galaxies (log$M_{\star}/M_{\odot}$ $\gtrsim11.0$) require an exceptional fraction of 50% of baryons converted into stars -- two to three times higher than even the most efficient galaxies at later epochs. The contribution from an active nucleus is unlikely because of their extended emission. Ultra-massive galaxies account for as much as 17% of the total cosmic star formation rate density at $z\sim5-6$.

• The paper identifies 36 dust-obscured galaxies at z=5–9, including three ultra-massive galaxies with a 50% baryon-to-star conversion efficiency.
• It employs JWST FRESCO grism spectroscopy to secure precise redshifts and refine stellar mass estimates, overcoming limitations of previous optical studies.
• The findings challenge current ΛCDM models by suggesting re-evaluation of feedback processes and star formation mechanisms in early galaxy evolution.

Accelerated Formation of Ultra-Massive Galaxies in the First Billion Years

This paper presents a comprehensive examination of early massive galaxy candidates as revealed by recent JWST observations, with a particular focus on ultra-massive galaxies identified in the first billion years of cosmic time. The analysis leverages data from the JWST FRESCO survey to explore the formation and characteristics of these galaxies, offering new insights that challenge prior cosmological models and enhance our understanding of galaxy formation in the early universe.

Key Findings

The researchers identify 36 massive dust-obscured galaxies with spectroscopic redshifts ranging from $z_{\rm spec}=5-9$. Notably, within this sample, three ultra-massive galaxies exhibit stellar masses (log$M_{\star}/M_{\odot} \gtrsim 11.0$) that suggest a remarkable baryon-to-star conversion efficiency of 50%, which is two to three times higher than efficiencies observed in galaxies at later epochs. Despite this unprecedented efficiency, the paper finds no significant tension with current $\Lambda$CDM cosmology from their sample. These ultra-massive galaxies also contribute substantially to the cosmic star formation rate density at redshifts $z \sim 5-6$.

The JWST FRESCO survey, utilizing grism spectroscopy, enables a systematic characterization of emission-line galaxies. The survey detects emission lines such as H$\alpha$+[NII] and [OIII]+H$\beta$, providing crucial spectroscopic redshifts and allowing for refined stellar mass estimates. The research highlights that the dust-obscured nature of these galaxies, inferred from red colors and high attenuation, implies previous optical studies likely underestimated their prevalence and impact.

Theoretical and Practical Implications

The requirement of such a high baryon conversion efficiency for some early galaxies suggests a possible need to re-evaluate current galaxy formation models under the $\Lambda$CDM paradigm. The existence of these ultra-massive galaxies at lower redshifts than previously expected, combined with their efficient star formation, illustrates the dynamic range of early galaxy formation not captured by earlier models focused principally on the high-redshift universe.

From a practical perspective, the nuanced understanding provided by these JWST observations will guide future astronomical surveys and model refinements. These findings propose potentially missing physics or processes in existing simulations of galaxy formation, such as feedback mechanisms or starburst activities that are not adequately captured.

Future Prospects

The paper indicates that the spatial distribution and assembly processes of these ultra-massive systems at high redshift may be more complex than currently understood, urging further investigations into the role of environmental factors or cosmic variance. Further detailed spectral and spatial analysis with instruments like ALMA and enhanced JWST datasets could reveal more about the kinematics and chemical makeup of these early-universe galaxies.

In conclusion, this research illuminates a critical aspect of the early universe, addressing the unexpected efficiencies in massive galaxy formation and subsequently challenging some aspects of cosmological models. As JWST continues to provide unprecedented data, it is anticipated that our understanding of the high-redshift universe and the early stages of galaxy evolution will continue to evolve significantly.