Rest-frame near-infrared sizes of galaxies at cosmic noon: objects in JWST's mirror are smaller than they appeared (2207.10655v2)

Abstract: Galaxy sizes and their evolution over cosmic time have been studied for decades and serve as key tests of galaxy formation models. However, at $z\gtrsim1$ these studies have been limited by a lack of deep, high-resolution rest-frame infrared imaging that accurately traces galaxy stellar mass distributions. Here, we leverage the new capabilities of the James Webb Space Telescope to measure the 4.4$\mu$m sizes of ${\sim}1000$ galaxies with $\log{\rm{M}*/\rm{M}\odot}\ge9$ and $1.0\le z \le 2.5$ from public CEERS imaging in the EGS deep field. We compare the sizes of galaxies measured from NIRCam imaging at 4.4$\mu$m ($\lambda_{\mathrm{rest}}\sim1.6\mu $m) with sizes measured at $1.5\mu$m ($\lambda_{\mathrm{rest}}\sim5500$A). We find that, on average, galaxy half-light radii are $\sim8$% smaller at 4.4$\mu$m than 1.5$\mu$m in this sample. This size difference is markedly stronger at higher stellar masses and redder rest-frame $V-J$ colors: galaxies with ${\rm M}* \sim 10^{11}\,{\rm M}\odot$ have 4.4$\mu$m sizes that are $\sim 25$% smaller than their 1.5$\mu$m sizes. Our results indicate that galaxy mass profiles are significantly more compact than their rest-frame optical light profiles at cosmic noon, and demonstrate that spatial variations in age and attenuation are important, particularly for massive galaxies. The trend that we find here impacts our understanding of the size growth and evolution of galaxies, and suggests that previous studies based on rest-frame optical light may not have captured the mass-weighted structural evolution of galaxies. This paper represents a first step towards a new understanding of the morphologies of early massive galaxies enabled by JWST's infrared window into the distant universe.

• The paper employs JWST/NIRCam to measure sizes of ~1000 galaxies in the rest-frame near-infrared at cosmic noon.
• It finds that galaxy half-light radii are on average 9% smaller at 4.4 microns, with up to 30% reduction in massive, red galaxies.
• The study suggests that mass profiles are more compact than optical light profiles, prompting revisions to galaxy size-mass relations.

Overview of "Rest-frame near-infrared sizes of galaxies at cosmic noon: objects in {\it JWST}'s mirror are smaller than they appeared"

The paper "Rest-frame near-infrared sizes of galaxies at cosmic noon: objects in {JWST}'s mirror are smaller than they appeared" by Katherine A. Suess et al., explores the structural properties of galaxies utilizing the capabilities of the James Webb Space Telescope (JWST). This paper aims to refine our understanding of galaxy evolution by examining the rest-frame near-infrared sizes of galaxies, particularly emphasizing the measurements captured during cosmic noon (1.0 ≤ z ≤ 2.5).

Key Findings

• Size Measurements: The authors employ JWST/NIRCam imaging to measure the sizes of approximately 1000 galaxies with stellar masses $\log{\rm{M}_*/\rm{M}_\odot} \ge 9$ within a redshift range of 1.0 to 2.5. The paper capitalizes on JWST's infrared capabilities to capture these measurements at a wavelength of 4.4 microns, comparing them with data captured at 1.5 microns.
• Size Reduction: The analysis reveals that galaxy half-light radii are, on average, 9% smaller at 4.4 microns compared to the 1.5 micron measurements. This size discrepancy is more pronounced in galaxies with higher stellar masses, particularly those exhibiting redder rest-frame V-J colors, where 4.4 micron sizes appear 30% smaller than the 1.5 micron counterparts.
• Implications for Galaxy Mass Profiles: The findings suggest that mass profiles of galaxies are more compact than their rest-frame optical light profiles at cosmic noon. This challenges previous conceptions derived from optical surveys, which might not have accurately captured the mass-weighted structural evolution of galaxies.
• Mass and Color Dependence: The paper finds a mass- and color-dependent trend where larger size discrepancies are observed in more massive and redder galaxies. This points to possible significant radial variations in age and dust attenuation in massive galaxies.

Implications and Future Directions

1. Revised Size-Mass Relations: This paper indicates a need to revise the traditional understanding of the size-mass relation, particularly for high-redshift galaxies. Prior inferences based on optical-based data might have overlooked substantial mass-structure differences.
2. Probing Stellar Assembly: The insights from this paper emphasize the importance of using infrared technologies in understanding how stellar mass is assembled within galaxies. The ability to resolve the rest-frame near-infrared structures allows for a closer approximation of actual mass distributions rather than light distributions.
3. Potential for New Discoveries: As JWST continues to provide high-resolution infrared images, the prospects for exploring stellar population distributions in distant galaxies increase. This will aid in refining models of galaxy evolution and understanding the drivers of structural transformations.
4. Exploration of Quenching Mechanisms: Understanding the size discrepancies in mass distribution provides a framework for investigating mechanisms behind galaxy quenching, offering clues to the process of star formation cessation and structural evolution.

This paper lays the groundwork for future research utilizing advanced space telescopes to challenge and refine existing paradigms in galaxy evolution studies. By leveraging the infrared capabilities of JWST, researchers have opened a new avenue for investigating the intricate structure and development of galaxies in the universe.