[Paper Review] Enhanced Sub-kpc Scale Star-formation: Results From A JWST Size Analysis of 341 Galaxies At 5

Takahiro Morishita, M. Stiavelli+8 more|arXiv (Cornell University)|Aug 9, 2023

Galaxies: Formation, Evolution, Phenomena4 citations

TL;DR

This study analyzes 341 high-redshift galaxies (5 < z < 14) from JWST Cycle 1 fields using deep NIRCam imaging and spectroscopic data, revealing a slow size evolution with redshift and identifying compact, sub-kpc-scale star-forming galaxies with extreme star formation surface densities (>10 M☉ yr⁻¹ kpc⁻²), suggesting intense early star formation activity without evidence of AGN feedback.

ABSTRACT

We present a comprehensive search and analysis of high-redshift galaxies in a suite of nine public JWST extragalactic fields taken in Cycle 1, covering a total effective search area of $\sim358{ m arcmin^2}$. Through conservative ($8σ$) photometric selection, we identify 341 galaxies at $51.5\,σ$ below the derived size-mass slope. These compact sources exhibit a high star formation surface density $Σ_{ m SFR}&gt;10\,M_\odot\,{ m yr^{-1}\,kpc^{-2}}$, a range in which only $&lt;0.01\,\%$ of the local star-forming galaxy sample is found. For those with available NIRSpec data, no evidence of ongoing supermassive black hole accretion is observed. A potential explanation for the observed high [OIII]-to-Hbeta ratios could be high shock velocities, likely originating within intense star-forming regions characterized by high $Σ_{ m SFR}$. Lastly, we find that the rest-frame UV and optical sizes of our sample are comparable. Our results are consistent with these early galaxies building up their structures inside-out and yet to exhibit the strong color gradient seen at lower redshift.

Motivation & Objective

• To characterize the rest-frame UV size-stellar mass relation of high-redshift galaxies (5 < z < 14) using JWST data.
• To investigate the evolution of galaxy sizes across cosmic time at z > 5, particularly the rate of size growth.
• To identify and analyze compact, sub-kpc-scale star-forming galaxies with extreme star formation surface densities.
• To assess the role of star formation and feedback processes in shaping early galaxy structures, particularly in the absence of AGN activity.
• To test whether early galaxies exhibit color gradients or structural features typical of lower-redshift systems.

Proposed method

• Conducted a conservative photometric selection (8σ) of 341 galaxies in nine public JWST extragalactic fields covering ~358 arcmin².
• Used rest-frame UV and optical photometry from NIRCam to measure effective radii (Reff) and stellar masses (M*) for the sample.
• Applied regression analysis to derive the Reff ∝ M*^0.19±0.03 size-mass relation and size-redshift evolution (Reff ∝ (1+z)^-0.4±0.2).
• Identified compact sources via residual analysis, flagging those >1.5σ below the size-mass relation (13% of sample).
• Evaluated star formation surface density (ΣSFR) using UV luminosities and effective radii, comparing to local galaxy samples.
• Analyzed available NIRSpec spectroscopy to search for AGN signatures (e.g., high-ionization lines) and assessed [O iii]/Hβ ratios for shock or starburst origins.

Figure 1: Radial flux profiles of webbpsf with various $\sigma_{\rm jitter}$ values (dashed lines) generated for the JADESGDS F115W image, where the adopted profile ( $\sigma_{\rm jitter}=0.022$ ) is color-coded in cyan. The profile of an example bright star taken from the image is shown for compari

Experimental results

Research questions

• RQ1 How does the rest-frame UV size-stellar mass relation of galaxies at 5 < z < 14 compare to lower-redshift populations?
• RQ2 What is the rate of size evolution of high-redshift galaxies over cosmic time, and how does it compare to prior literature estimates?
• RQ3 What fraction of high-redshift galaxies are compact (sub-kpc scale) and exhibit extreme star formation surface densities?
• RQ4 Is there evidence of active galactic nucleus (AGN) activity in the most compact, high-SFR galaxies?
• RQ5 What physical mechanisms could explain the high [O iii]/Hβ line ratios observed in some compact, high-SFR galaxies?

Key findings

• The size-stellar mass relation for high-redshift star-forming galaxies follows Reff ∝ M*^0.19±0.03, similar to z ~ 3 star-forming galaxies but scaled down by ~0.7 dex in size.
• The size evolution is slower than previously thought, with Reff ∝ (1+z)^-0.4±0.2, indicating minimal size growth over the redshift range 5 < z < 14.
• Approximately 13% of the sample are compact sources with effective radii <100 pc, located >1.5σ below the size-mass relation.
• These compact galaxies exhibit extreme star formation surface densities (ΣSFR > 10 M☉ yr⁻¹ kpc⁻²), a regime found in less than 0.01% of local star-forming galaxies.
• No evidence of ongoing supermassive black hole accretion was found in compact sources with available NIRSpec data.
• High [O iii]/Hβ ratios in some compact galaxies may be driven by high shock velocities from intense star formation, not AGN activity.

Figure 2: Sample distribution in the star formation rate–stellar mass plane (circles). Those with spectroscopic redshift (blue square) and flagged as compact (magenta; Sec 4.4 ) are marked accordingly. Those flagged in galfit results (Sec. 3.3 ) are shown by open symbols. The SFMS slope at $z=5$ (bl