Galaxy Size and Mass Build-up in the First 2 Gyrs of Cosmic History from Multi-Wavelength JWST NIRCam Imaging (2410.16354v1)

Abstract: The evolution of galaxy sizes in different wavelengths provides unique insights on galaxy build-up across cosmic epochs. Such measurements can now finally be done at $z>3$ thanks to the exquisite spatial resolution and multi-wavelength capability of the JWST. With the public data from the CEERS, PRIMER-UDS, and PRIMER-COSMOS surveys, we measure the sizes of $\sim 3500$ star-forming galaxies at $3 \leqslant z<9$, in 7 NIRCam bands using the multi-wavelength model fitting code GalfitM. The size-mass relation is measured in four redshift bins, across all NIRCam bands. We find that, the slope and intrinsic scatter of the rest-optical size-mass relation are constant across this redshift range and consistent with previous HST-based studies at low-z. When comparing the relations across different wavelengths, the average rest-optical and rest-UV relations are consistent with each other up to $z=6$, but the intrinsic scatter is largest in rest-UV wavelengths compared to rest-optical and redder bands. This behaviour is independent of redshift and we speculate that it is driven by bursty star-formation in $z>4$ galaxies. Additionally, for $3\leqslant z<4$ star-forming galaxies at $\rm M_* > 10^{10} M_{\odot}$, we find smaller rest-$\rm 1\rm\,\mu m$ sizes in comparison to rest-optical (and rest-UV) sizes, suggestive of colour gradients. When comparing to simulations, we find agreement over $\rm M_* \approx 10^{9} - 10^{10} M_{\odot}$ but beyond this mass, the observed size-mass relation is significantly steeper. Our results show the power of JWST/NIRCam to provide new constraints on galaxy formation models.

• The paper shows that the rest‐optical size-mass relation remains constant in slope and scatter across redshifts, extending earlier Hubble results.
• The analysis of ~3500 star-forming galaxies with GalfitM highlights increased intrinsic scatter in rest-UV measurements due to bursty star formation at high redshifts.
• The study reveals that massive (M* > 10^10 M☉) galaxies at redshifts 3–4 exhibit smaller sizes at 1μm wavelengths, indicating significant color gradients and denser core structures.

Overview of the Study on Galaxy Size and Mass Build-up Using JWST NIRCam Imaging

This paper focuses on investigating the evolution of galaxy sizes and mass during the first two billion years of the universe, specifically for galaxies with a redshift greater than 3. The research utilizes the James Webb Space Telescope's (JWST) NIRCam imaging capabilities across multiple wavelengths—a technological advancement enabling unprecedented spatial resolution and depth for early universe studies. This work is based on data sourced from prominent JWST surveys such as CEERS, PRIMER-UDS, and PRIMER-COSMOS.

Key Objectives and Methodology

The primary objective of this research is to extend the measurements of galaxy sizes past the capabilities of the Hubble Space Telescope to redshifts greater than 3. This involves measuring the sizes of approximately 3500 star-forming galaxies within the redshift range of 3 to 9, distributed across several NIRCam bands. The authors employ the multi-wavelength model-fitting code GalfitM for this task, conducting a detailed analysis of the size-mass relation in four redshift bins across the rest-optical and rest-UV spectrum.

Significant Findings

1. Size-Mass Relation Consistency: The slope and intrinsic scatter of the rest-optical size-mass relation remain constant across the investigated redshift range, aligning with findings from previous Hubble-based studies at lower redshifts.
2. Wavelength Comparisons: The size-mass relations in rest-optical and rest-UV wavelengths show consistency up to a redshift of 6. However, there is a notable increase in intrinsic scatter at rest-UV wavelengths compared to rest-optical and redder bands. This is attributed to bursty star formation in galaxies with a redshift greater than 4.
3. Mass Variation at Different Wavelengths: For star-forming galaxies within the redshift range of 3 to 4, those with stellar masses exceeding $10^{10} M_{\odot}$ exhibit smaller sizes at rest $\rm 1\mu m$ wavelengths in comparison to rest-optical and rest-UV sizes. This suggests the presence of color gradients within these galaxies.

Implications of the Research

The findings from this paper suggest several theoretical and practical implications for understanding galaxy evolution. The observed consistency in the size-mass relation across different wavelengths points to underlying physical processes that may influence galaxy growth independently of their environment. Moreover, the agreement between observed and simulated size-mass relations for certain mass ranges emphasizes the potential accuracy and utility of current galaxy formation models in capturing these evolutionary trends.

The ability of NIRCam to identify variations in size at specific wavelengths reveals insights into star formation processes and internal galaxy structures that were previously inaccessible. These observations lay a foundation for refining models of galaxy evolution by accounting for the effects of phenomena such as color gradients and dust distribution.

Future Directions

Further investigations can explore more profound impacts of the findings on galaxy formation models, especially regarding the observed steeper size-mass relations at high stellar masses. Additional high-redshift data, potentially from upcoming JWST deep field surveys, could help delineate the slope of these relations more clearly at even higher redshifts. Greater focus on the interplay between star formation, dust obscuration, and galaxy morphology will be critical in understanding the development of the earliest cosmic structures.

Overall, this research significantly contributes to astrophysics by showcasing JWST's capability to unravel complexities in galaxy size and mass evolution, offering new pathways for understanding the universe's formative epochs.