Census for the rest-frame optical and UV morphologies of galaxies at z = 4–10: First phase of inside-out galaxy formation

Abstract

We present the rest-frame optical and UV surface brightness (SB) profiles for 149 galaxies with M_opt < −19.4 mag at z = 4–10 (29 of which are spectroscopically confirmed with JWST NIRSpec), securing high signal-to-noise ratios of 10–135 with deep JWST NIRCam 1–|$5\, \mu$|m images obtained by the CEERS survey. We derive morphologies of our high-z galaxies, carefully evaluating the systematics of SB profile measurements with Monte Carlo simulations as well as the impacts of a) AGNs, b) multiple clumps including galaxy mergers, c) spatial resolution differences with previous HST studies, and d) strong emission lines, e.g., Hα and [O iii], on optical morphologies with medium-band F410M images. Conducting Sérsic profile fitting for our high-z galaxy SBs with GALFIT, we obtain effective radius ranges for optical r_e,opt and UV r_e,UV wavelengths of r_{e, opt} = 0.05–1.6 kpc and r_e,UV = 0.03–1.7 kpc that are consistent with previous results within large scatters in the size–luminosity relations. However, we find that the effective radius ratio, r_e,opt|$/$|r_e,UV, is almost unity, |$1.01^{+0.35}_{-0.22}$|⁠, over z = 4–10 with no signatures of past inside-out star formation such as found at z ∼ 0–2. There are no spatial offsets exceeding 3σ between the optical and UV morphology centers in cases of no mergers, indicative of major star-forming activity only being found near mass centers of galaxies at z ≳ 4 probably experiencing the first phase of inside-out galaxy formation.

1 Introduction

Investigating the evolution of galaxy morphologies over cosmic time offers valuable insights into the understanding of galaxy evolution (Conselice 2014). The launch of the James Webb Space Telescope (JWST; Gardner et al. 2023) has catalyzed significant advancements in characterizing the morphological properties of high-z star-forming galaxies (SFGs) predominantly selected based on the rest-frame UV spectral shapes (Yang et al. 2022; Treu et al. 2023; Ono et al. 2023; see also Langeroodi & Hjorth 2023). Several morphological studies have also been reported for other galaxy types including dusty SFGs and quiescent galaxies as well as galaxies selected more generally using photometric redshifts (Suess et al. 2022; Gillman et al. 2023; Ito et al. 2023; Le Bail et al. 2023; Yao et al. 2023; van der Wel et al. 2024). Furthermore, comprehensive morphological classifications have been conducted, encompassing visual classifications (Ferreira et al. 2022, 2023; Kartaltepe et al. 2023) and classifications employing machine-learning techniques (Robertson et al. 2023; Huertas-Company et al. 2023; Tohill et al. 2024).

To understand the evolution of galaxy morphologies, it is important to elucidate how galaxy sizes change across various wavelengths. In general, the rest-frame UV continuum predominantly traces young massive stars, whereas the rest-frame optical continuum captures even less massive stars. Notably, the rest-frame UV morphologies of nearby galaxies tend to be patchier compared to their rest-frame optical ones (e.g., Kuchinski et al. 2000, 2001; Windhorst et al. 2002). This trend extends to galaxies up to at least z ∼ 1 based on Hubble Space Telescope (HST) data (Papovich et al. 2005). Van der Wel et al. (2014) have investigated the wavelength dependence of SFG sizes up to z ∼ 2, finding that galaxy sizes decrease with longer wavelengths. Their results could be interpreted as a sign of inside-out growth, considering that longer wavelengths trace older less massive stars. However, some previous studies have reported little difference in morphologies in the rest-frame UV and optical for high-z galaxies (e.g., Dickinson 2000; Papovich et al. 2005; Shibuya et al. 2015). Nevertheless, it is worth noting that HST data probe the rest-frame optical morphologies of galaxies only up to z ∼ 3.

From a theoretical standpoint, the rest-frame UV and optical morphologies of higher-z galaxies have been investigated using cosmological simulations that properly take into account the baryonic physics including star formation and feedback processes. For instance, Ma et al. (2018) have used the results of the FIRE-2 cosmological simulations to study the rest-frame UV and optical sizes of z = 5–10 galaxies with halo masses of |$10^{8-12}\, M_{\odot }$| at z = 5. They have found that the rest-frame optical sizes are significantly larger compared to the rest-frame UV, because the rest-frame UV images are dominated by a limited number of bright clumps with young massive stars that are often not associated with a substantial stellar mass. Conversely, based on the results of the IllustrisTNG cosmological simulations, Costantin et al. (2023) have studied the morphologies of galaxies at z = 5–6 with stellar masses exceeding 10⁹M_⊙, and reported that their sizes in the rest-frame UV and optical are similar, probably due to their young ages. Similar results have also been obtained by Wu et al. (2020) based on the SIMBA cosmological hydrodynamic simulations.

Thanks to the near-infrared camera onboard JWST, NIRCam (Rieke et al. 2005), deep high-resolution images can now be obtained up to around |$5\, \mu$|m, allowing for the examination of galaxy morphologies in the rest-frame UV and optical up to z ≃ 9.5. Yang et al. (2022) have investigated the sizes of galaxies at z ∼ 7–10 in the rest-frame UV and optical based on the JWST GLASS data (Treu et al. 2022), and found that the average ratio of sizes in the rest-frame optical to UV is consistent with unity. Ono et al. (2023) have expanded individual high-z galaxy size measurements to faint galaxies by stacking their NIRCam images, thus further revealing that the size ratio is consistent with unity even down to fainter magnitudes. However, these studies have not covered a bright magnitude range exceeding −21 mag, which corresponds to the characteristic luminosity of z ∼ 3 galaxies, |$L^\ast _{z=3}$| (Steidel et al. 1999).

In this paper, we utilize JWST NIRCam images taken by the Cosmic Evolution Early Release Science (CEERS; Finkelstein et al. 2022) survey to determine the sizes of galaxies at z ≃ 4–10 in the rest-frame UV and optical across a broad luminosity range. We investigate sizes of spectroscopically identified galaxies at z_spec > 4.5 in the CEERS fields compiled by Nakajima et al. (2023) as well as galaxy candidates at similar redshifts selected based on photometric redshifts or the dropout technique (Shibuya et al. 2015; Bouwens et al. 2015; Finkelstein et al. 2023).

This paper is structured as follows. In section 2, we introduce the CEERS NIRCam data used in this study, and compile samples of galaxies at z ≃ 4–10 found in the CEERS fields in previous studies. In section 3, we describe our methodology for two-dimensional (2D) surface brightness (SB) profile fitting and conduct Monte Carlo (MC) simulations to correct systematic uncertainties in size and total magnitude measurements and to estimate statistical uncertainties in these measurements. In addition, we investigate the impact of strong emission lines on the rest-frame optical size measurements and the effect of the spatial resolution difference between the rest-frame UV and optical on the size measurements. Section 4 presents the results of our SB profile fittings for the galaxies at z ≃ 4–10 in the rest-frame UV and optical, and compares our results with those in previous work such as the size–luminosity relation and size ratio between the rest-frame UV and optical. Finally, we summarize this study in section 5. Throughout this paper, we employ magnitudes in the AB system (Oke & Gunn 1983). A flat universe with Ω_m = 0.3, |$\Omega _\Lambda = 0.7$|⁠, and H₀ = 70 km s⁻¹ Mpc⁻¹ is adopted. In this cosmological model, for instance, an angular dimension of |${1_{.}^{\prime \prime}0}$| corresponds to a physical dimension of 6.603 kpc at z = 4.5, 5.226 kpc at z = 7.0, and 4.463 kpc at z = 9.0 [equation (18) of Hogg 1999].

2 Data and samples

We investigate the sizes of galaxies at z ≃ 4–10 found in the CEERS fields. First, we use the spectroscopic sample of Nakajima et al. (2023), who have compiled spectroscopically identified galaxies in the CEERS fields (see also Arrabal Haro et al. 2023; Fujimoto et al. 2023; Tang et al. 2023). To increase the number of sources that we examine, we incorporate samples of galaxy candidates at similar redshifts selected based on photometric redshifts or the dropout technique. One is the Finkelstein et al. (2023) sample at z ∼ 9, which is selected from the JWST and HST data with photometric redshift values of z_photo ≃ 8–10. In addition, we include the Shibuya, Ouchi, and Harikane (2015) sample of dropout galaxies at z ∼ 5–7 selected in Harikane et al. (2016) from the HST data obtained by the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS; Grogin et al. 2011; Koekemoer et al. 2011). To compensate for the small number for z ∼ 7–8, we further add the z ∼ 7–8 samples from Bouwens et al. (2015), which are dropout galaxies selected based on the HST data. Note that there are overlaps between these samples; in this paper, we assign a unique ID to each source as listed in table 5 in appendix  1. The number of sources in each sample and the numbers of duplicated sources are summarized in table 1. Their redshift distributions are presented in figure 1.

To trace their rest-frame UV and optical continuum from galaxies at z ≃ 4–10, we make use of the JWST NIRCam F150W and F444W images, respectively (top panel of figure 2). We refer the reader to Harikane et al. (2023b) and Y. Harikane et al. (in preparation) for details about the imaging data sets. At the moment, our reduction of the F444W images for CEERS7 is suboptimal for reasons we do not yet fully understand; we therefore do not use it for our analyses. Although the wide wavelength coverage of F444W allows the capture of strong emission lines such as Hα and [O iii] 5008 at z ≳ 5 as demonstrated in the bottom panel of figure 2, this does not significantly affect measurements of galaxy sizes, as shown later in a comparison of measurements with F444W and F410M (subsection 3.3). The pixel scale of the NIRCam images is 0|${_{.}^{\prime\prime}}$|015 pix⁻¹. Their 10σ limiting magnitudes are summarized in table 2. Following the previous work (Mosleh et al. 2012; Ono et al. 2023), we select sources with a signal-to-noise ratio (S|$/$|N) larger than 10 for individual analyses in each band based on their apparent magnitudes measured in 0|${_{.}^{\prime\prime}}$|2 diameter circular apertures. For instance, if a galaxy shows an S|$/$|N larger than 10 in the F150W (F444W) image, its F150W (F444W) image is used for analysis. In cases where a galaxy has an S|$/$|N larger than 10 in F150W but has an S|$/$|N below 10 in F444W, only the F150W image is utilized for analysis. The number of sources in each catalog that have an S|$/$|N larger than 10 in each band is presented in table 1.

When measuring galaxy sizes, it is imperative to consider image smearing by point spread functions (PSFs). To address this, we utilize empirical PSFs generated by stacking bright point sources in the actual NIRCam images. Within each field, we select four to nine unsaturated bright point sources with ≃22–24 mag. The PSF FWHM values are listed in table 2.

3 Analyses

3.1 Surface brightness profile fitting

We measure the half-light radii of the high-z galaxies in our compiled sample through fitting the Sérsic profile (Sersic 1968) to the observed 2D SB profiles. The Sérsic profile is defined as follows:

where Σ_e is the surface brightness at the half-light radius r_e, and n signifies the Sérsic index. The value of b_n is specified such that r_e encapsulates half of the total flux.

We deploy GALFIT version 3 (Peng et al. 2002, 2010) for the profile fitting. This software convolves a galaxy surface brightness profile with a PSF profile and optimizes the fits using the Levenberg–Marquardt algorithm to minimize χ². The resultant parameters from GALFIT comprise the centroid coordinates, total magnitude, radius along the semi-major axis (a), Sérsic index (n), axis ratio (b|$/$|a), and position angle of the fitted object. The circularized half-light radius, |$r_e = a \sqrt{b/a}$|⁠, is calculated by utilizing the radius along the semi-major axis and the axis ratio, which is commonly used in galaxy size measurements in previous studies (e.g., Newman et al. 2012; Mosleh et al. 2012; Ono et al. 2013; Shibuya et al. 2015; Kawamata et al. 2018). SExtractor is deployed to obtain initial parameters for the GALFIT profile fitting. With the exception of the Sérsic index, all parameters are set to vary during the profile fitting. Noise images are derived from the inverse square root of the weight maps and employed to assign weights to individual pixels during the profile fitting. Segmentation images generated by SExtractor are used for masking extraneous objects around the sources of interest.

We fix the Sérsic index at n = 1.5, which corresponds to the median value of SFGs with similar UV luminosities to those of our sources reported in the previous work (Shibuya et al. 2015).¹ In fact, when we run GALFIT with the Sérsic index left as a free parameter for 29 bright spectroscopically confirmed galaxies in our sample, the median values of the best-fitting Sérsic indices are 1.4 in F150W and 1.0 in F444W (see also Sun et al. 2024; for lower redshifts, see Martorano et al. 2023). However, the uncertainties of the Sérsic indices for individual sources would not be small. For illustration, figure 3 displays the observed radial profiles for C01_02, C01_06, and C03_04, together with the best-fitting results where the Sérsic index n is treated as a free parameter and where n is fixed at 1.5 or 4.0. All of these cases fit the observed data points well, suggesting that it is challenging to determine the Sérsic index simultaneously with the other parameters using the currently available data. Therefore, we fix the value of the Sérsic index in the SB profile fittings, consistent with the approach taken in the previous studies.

3.2 Monte Carlo simulations

Following previous work (e.g., Ono et al. 2013, 2023; Shibuya et al. 2015), we execute a series of MC simulations to quantify systematic and statistical uncertainties in the profile fitting with GALFIT. We use the F150W and F444W images, which probe the rest-frame UV and optical emission from our galaxies at z ≃ 4–10. In addition, we utilize the F410M images as detailed in subsection 3.3, as well as smoothed F150W images whose PSF sizes are comparable to those of the F444W images as described in subsection 3.4. As demonstrated in table 2, the depths and PSF FWHMs are almost the same across different pointings of CEERS. For simplicity, we conduct MC simulations for each band with respect to CEERS2 and apply the results to correct systematic uncertainties and estimate statistical uncertainties for all the pointings from CEERS1 to CEERS10. We use GALFIT to generate galaxy images with a fixed Sérsic index n of 1.5. The half-light radius r_e and total magnitude are randomly selected within ranges of 0.5–27.0 pixels and 24.5–30.0 mag, respectively. These galaxy images are then convolved with a PSF image, which is a composite of bright, unsaturated stars selected from each band. The PSF-convolved galaxy images are subsequently inserted into blank regions of the actual NIRCam images and analyzed in an identical manner to our high-z galaxies.

We present the results of size measurements for our MC simulated galaxies in figure 4, and those of total magnitude measurements in figure 5. The general trends seen in these figures are the same as those in our previous results of Ono et al. (2023). Our MC simulations confirm a systematic underestimation of half-light radii and overestimation of total magnitudes in GALFIT measurements for fainter objects. It is also found that the MC simulated galaxies with faint magnitudes and large sizes do not meet the S|$/$|N > 10 criterion for aperture magnitude. Since our sample is limited to those with an aperture magnitude S|$/$|N larger than 10, the systematic effects are not large.² We rectify these systematic effects and also use statistical uncertainties in size and total magnitude measurements in the same manner as our previous work; briefly, we utilize the MC simulation outcomes within the same output magnitude (size) bins as our high-z galaxies to correct for the output sizes (magnitudes) by the median differences between the input and output values and to assign the 68th percentiles in these bins as statistical uncertainties [see section 3 of Ono et al. (2023) for details].

3.3 Impact of strong emission lines

As mentioned in section 2, strong emission lines such as Hα and [O iii] from galaxies at z ≳ 5 are included in F444W due to its wide wavelength coverage. To assess the impact of the strong emission lines on our galaxy size measurements, we use images with the medium-band filter F410M. As shown in the bottom panel of figure 2, for the redshift range of z ≃ 5.63–6.63, F410M does not encompass the strong emission lines, allowing us to primarily probe the rest-frame optical continuum.

We perform SB profile fittings for galaxies with spectroscopic redshifts of z_spec = 5.63–6.63 in our sample using the F410M and F444W images. Although the number of sources with spectroscopic redshifts within this range is 13 regardless of their S|$/$|N values, successful convergence of the SB profile fitting is achieved for three sources each in the CEERS3 and CEERS6 fields. For the remaining sources, the SB profile fitting encounters numerical convergence issues probably due to the low S|$/$|N values (for details, see section 10 of the GALFIT user’s manual). Figure 6 displays the results of the SB profile fittings. Among them, C03_01, C06_02, and C06_07 are relatively faint, but still appear to fit well with the Sérsic profiles. Given the limited number of sources explored here, we include these faint ones in the plots as well.

The top and bottom panels of figure 7 compare the size and total magnitude measurement results, respectively. We confirm a broad agreement in sizes derived from F410M and F444W. While individual size measurements differ by about 1–4σ between F410M and F444W, suggesting that sizes may vary to this extent for individual sources, the overall average does not seem to be significantly impacted. Moreover, particularly for relatively brighter sources indicated with filled circles, the difference in size is within 20%. On the other hand, the total magnitudes measured with F444W are systematically brighter than those measured with F410M mostly by ≃ 0.2–0.4 mag, which is likely due to the strong Hα emission line probed with F444W. Indeed, since the FWHM of the F444W filter transmission³ is about |$1.1\, \mu$|m, these magnitude differences roughly correspond to rest-frame Hα emission line equivalent widths (EWs) of 320–700 Å for galaxies at z = 6. These values roughly align with the average EW values for high-z SFGs reported in previous studies: |$555^{+332}_{-311}$| Å for z = 4.9 galaxies with a median M_UV = −21.4 mag (table 4 of Harikane et al. 2018) and 453 ± 84 Å for z = 3.8–5.3 galaxies with −20.5 < M_UV < −19.5 mag (table 3 of Lam et al. 2019; see also Maseda et al. 2020; Prieto-Lyon et al. 2023).⁴ Because the Hβ + [O iii] EWs are not very different from that of Hα (the median Hβ + [O iii] EW value for galaxies at z ∼ 6.5–8 with slightly fainter magnitudes is 780 Å; Endsley et al. 2023), the impact of strong line emission from Hα and Hβ + [O iii] should be comparable.⁵ Since the size and magnitude differences due to the strong emission lines are not very large owing to the wide wavelength coverage of F444W, from the next subsection onwards, we utilize the F444W images to measure the rest-frame optical sizes and total magnitudes of our galaxies over a more extensive redshift range.

Note that Zhang et al. (2023) have selected sources with spatially extended Hβ + [O iii] emission by combining two NIRCam broad-band filters, one of which incorporates the Hβ + [O iii] emission and the other does not. The focus of their study is primarily on the spatially extended SB distributions substantially beyond the effective radii. In contrast, our size measurements mainly rely on SB profiles within the effective radii with relatively high S|$/$|N values. Therefore, there is no conflict between our confirmation and the approach of Zhang et al. (2023). Moreover, it is worth noting that sources exhibiting spatially extended Hβ + [O iii] emission are rare, and hence unlikely to significantly impact our general measurements of high-z galaxy sizes.

3.4 Impact of different spatial resolutions in the rest-frame UV and optical

One of the primary topics of interest in this study is the comparison between the rest-frame UV and optical sizes. For this purpose, the difference in the spatial resolutions of the F150W and F444W images needs to be taken into account. For instance, as presented in figure 17 in appendix  2, C01_05 has two components near the center of the F150W cutout image, and the SB profile fitting is performed for one of the two components. However, in the F444W image, these two components are considered as one blended source, and the SB profile fitting is performed for the blended source accordingly. As such, for fair comparisons of the SB profile fitting results with the F150W and F444W images, we need to consider the difference in the spatial resolutions.

We create smoothed F150W images by applying Gaussian kernels to the original F150W images so that the PSF sizes of the smoothed F150W images match those of the F444W images. We then perform SB profile fittings using GALFIT on the PSF-matched F150W images. We also carry out MC simulations for the PSF-matched F150W images following the same methodology outlined in subsection 3.2. The results of size and total magnitude measurements for our MC simulated galaxies are shown in figures 4 and 5, respectively. Based on these MC simulation results, we correct systematic effects associated with the SB profile fittings and evaluate statistical uncertainties in the measurements.

By visually inspecting the output images of the SB profile fittings, we assign a flag about blendedness to each object as listed in table 6 in appendix  2. When the component fitted in the smoothed F150W image and/or the F444W image is a single component in the original F150W image, we set the flag to 1 (single). When the component fitted in the smoothed F150W image and/or the F444W image is multiple components in the original F150W image, we set the flag to 2 (multiple). In other words, we assign a multiple flag to a source if visual inspection of the residual image from a single component fit for it reveals the presence of additional components. These cases often show multiple segments in segmentation maps created by SExtractor. Typically, the centers of these multiple components are spatially offset from each other. For comparisons of the SB profile fitting results in the rest-frame UV and optical, we use the results from the original (PSF-matched) F150W images for the single (multiple) flagged sources. This approach ensures a fair comparison even for data with different spatial resolutions.

The question of whether the sizes and total magnitudes obtained from the original F150W images can be reproduced when the spatial resolution is as low as that of the F444W images is intriguing. Figure 8 compares the size and total magnitude results derived for the original and PSF-matched F150W images. The top panel illustrates the comparison of the size measurements. For single sources, the results are in good agreement across a wide range of sizes. This indicates that, even when the spatial resolution is as large as in the F444W image, small sizes derived for the original F150W image can still be obtained in the PSF-matched F150W image. On the other hand, for multiple sources, the obtained sizes tend to be larger after PSF matching as expected, because more components are fitted in the SB profile fitting after PSF matching.

The bottom panel of figure 8 demonstrates the magnitude measurements with the F150W images before and after PSF matching. As expected, the results for the single sources align well, while the multiple sources tend to be systematically brighter after PSF matching.

4 Results and discussion

4.1 SB profile fitting results for the rest-frame UV and optical continuum

We perform SB profile fittings for the galaxies at z ≃ 4–10 in the CEERS fields, which are compiled in section 2, to determine their sizes and total magnitudes in the rest-frame UV and optical. The F150W images are used to probe the rest-frame UV continuum, whereas the F444W images are used for the rest-frame optical. The SB profile fittings are conducted only for sources with S|$/$|N values exceeding 10 (section 2). Among them, the SB profile fittings with the F150W (F444W) images successfully converge for 29 and 120 (22 and 96) sources with and without spectroscopic confirmations, respectively. Their distributions on the rest-frame UV absolute magnitude (M_UV) vs. redshift and the rest-frame optical absolute magnitude (M_opt) vs. redshift planes are presented in figure 9. The best-fitting parameters obtained from the SB profile fittings for the individual sources are summarized in table 6 and the individual fitting result images are shown in figure 17 in appendix  2. Pseudo-color cutout images of those galaxies whose spectroscopic redshifts have been compiled by Nakajima et al. (2023) are shown in figure 18 in appendix  2.⁶

The morphologies of some of these sources have been investigated in previous work. C06_06 is the same as CEERS-AGN-z5-1 in Onoue et al. (2023) and CEERS 1670 in Kocevski et al. (2023), which is selected as an AGN candidate and confirmed by follow-up spectroscopy. Onoue et al. (2023) have reported that the central SB profile of C06_06 is dominated by the PSF in the F115W image, suggesting a size of less than 0.13 kpc. Our obtained size for this source in F150W is 0.09 ± 0.01 kpc, which is consistent with these previous results. Note that the other source reported in Kocevski et al. (2023), CEERS 3210, is too faint to be investigated in this study.

In addition to C06_06, C03_04 is selected as a broad-line AGN by Harikane et al. (2023a) based on the broad component detections only in the permitted Hα line.⁷ In addition, C02_02 (CEERS 1465) and C08_01 (CEERS 1019) are selected as possible broad-line AGN candidates in Harikane et al. (2023a) (see also Larson et al. 2023). Harikane et al. (2023a) have reported that C06_06 shows a compact morphology, while the other three sources have extended morphologies. In our SB profile fittings, although all of these sources are fitted with Sérsic profiles, C06_06 shows the smallest size.⁸ In this sense, our results are consistent with their findings. In what follows, we mark these AGNs and AGN candidates with a special symbol in the figures as necessary. As mentioned in subsection 4.3, these broad-line AGNs and AGN candidates tend to be more compact than normal SFGs. It should also be noted that the remaining sources at z > 4.5 in their study are not investigated in this paper due to their faintness or encountering numerical convergence issues in the SB profile fittings.

Very recently, based on the data from public extragalactic fields obtained in JWST Cycle 1, Morishita et al. (2024) have compiled a catalog of SFGs at z = 5–14 and measured their sizes in the rest-frame UV. Their catalog encompasses 20 of the galaxies that we investigate. A comparison of the size measurement results for these galaxies is shown in figure 10. Because they only provide the effective radius along the semi-major axis, our comparison here is also based on the effective radius along the semi-major axis. Based on the single or multiple flags established in subsection 3.4, we differentiate the symbols for the data points. As can be seen from figure 10, when considering only the galaxies labeled single, our results align well with theirs. However, there are some multiple flagged galaxies that deviate significantly. Specifically, for three objects, their results are larger by about 0.3–0.5 dex compared to ours; their IDs are C01_05, C06_01, and C08_01 (figure 17 in appendix  2). It is likely that Morishita et al. (2024) have treated them as a single blended source in their SB profile fittings, while we perform SB profile fittings for one of these components, which could explain the significant size discrepancy. Despite these exceptions, the results from both studies generally align well.

In Zhang et al. (2023), galaxies with spatially extended Hβ + [O iii] emission are selected based on the emission line images created by subtracting NIRCam broad-band images with and without emission line contributions. Among the selected sources, CEERS 792 is the same as C06_04. However, due to the low S|$/$|N, we do not conduct SB profile fitting for this particular source in this study.

4.2 Comparisons between HST and JWST measurement results

Based on the HST WFC3 images, Shibuya, Ouchi, and Harikane (2015) have performed SB profile fittings for the galaxies in their sample and estimated their sizes and total magnitudes in the rest-frame UV. Because the spatial resolutions of HST and JWST are different, it raises a concern whether it is appropriate to compare these SB profile fitting results.

Figure 11 displays a comparison of the rest-frame UV sizes and total magnitudes for the galaxies in the Shibuya, Ouchi, and Harikane (2015) sample obtained with the HST and JWST images. Because of the difference in spatial resolution between HST and JWST, some galaxies appearing as single sources in the HST images are resolved into multiple components in the JWST images. Conveniently, the spatial resolution of HST WFC3 is similar to those of the red filters of JWST NIRCam. Therefore, for this comparison with the HST results, we use the JWST results from the original or PSF-matched F150W images depending on whether the sources are flagged as single or multiple in subsection 3.4.

As depicted in the top panel of figure 11, the size results obtained from HST and JWST images roughly coincide. Likewise, as presented in the bottom panel of figure 11, the total magnitude results also largely agree. Although individual outliers exist that deviate significantly, such sources are rare, and on average the results show good agreement. In the following subsections, we compare our JWST results with previous results obtained from the HST images, along with previous JWST results.

4.3 Size–luminosity relation in the rest-frame UV and optical

We investigate the relationship between the size and luminosity of our galaxies. The top (bottom) panel of figure 12 shows the rest-frame UV (optical) sizes and total magnitudes for galaxies at z ≃ 4.5–9.5, which are estimated from our SB profile fittings with the F150W (F444W) images in this study. Here we directly use the optical total magnitudes obtained with the F444W images; these measurements are systematically brighter due to the influence of strong emission lines such as Hα and [O iii] as examined in subsection 3.3.

Both panels confirm the trend that fainter sources tend to be smaller in size reported in the literature (Oesch et al. 2010; Grazian et al. 2012; Ono et al. 2013, 2023; Huang et al. 2013; Jiang et al. 2013; Kawamata et al. 2015, 2018; Holwerda et al. 2015, 2020; Shibuya et al. 2015; Curtis-Lake et al. 2016; Bowler et al. 2017; Bouwens et al. 2017, 2022; Bridge et al. 2019; Yang et al. 2022). Additionally, the spectroscopically confirmed sources appear to be more compact, probably because the more compact a source is, the easier it is to detect its emission lines by spectroscopy. The broad-line AGNs and AGN candidates except for C02_02 appear to be relatively compact, which can be attributed to the bright emission from the central core compared to the host galaxy components.

The previous work has shown that the sizes of galaxies decrease with increasing redshift on average (Ferguson et al. 2004; Bouwens et al. 2004, 2006, 2017, 2022; Hathi et al. 2008; Oesch et al. 2010; Grazian et al. 2012; Ono et al. 2013, 2023; Huang et al. 2013; Jiang et al. 2013; Kawamata et al. 2015, 2018; Holwerda et al. 2015, 2020; Shibuya et al. 2015; Curtis-Lake et al. 2016; Allen et al. 2017; Bowler et al. 2017). Thus, we show the rest-frame UV sizes and total magnitudes divided into three redshift ranges in figure 13. For each redshift bin, we split the samples into three or two groups based on the total magnitude and calculate the median sizes and its 68th percentiles. These values are plotted together and summarized in table 3. For all of these subsamples, we confirm the trend that fainter sources have smaller sizes, although this trend is not very apparent in the median values for the 6.5 < z < 9.5 subsample. For comparison, we also show the results of Shibuya, Ouchi, and Harikane (2015), who have investigated the rest-frame UV sizes of high-z galaxies using HST images, as well as the results of Yang et al. (2022) and Ono et al. (2023) based on JWST images. We find that our results are broadly consistent with these previous results when comparing at similar total magnitudes. Note that the results from Morishita et al. (2024) are not plotted here, because their measurements for the circularized radius are not available.

In figure 14, we show the rest-frame optical sizes and total magnitudes divided into three redshift ranges in the same way as figure 13. For the 4.5 < z < 5.5 subsample, we confirm the trend that fainter sources are smaller in size. However, such a trend is not significantly seen in the remaining subsamples. For the 6.5 < z < 9.5 subsample, we find that our results are broadly consistent with the previous results of Yang et al. (2022) and Ono et al. (2023) at around M_opt ≃ −21 mag. It should be noted that, in these previous studies, SB profile fittings have been performed for galaxies fainter than those investigated in this study thanks to the GLASS data, which are deeper than the CEERS data used in this study. Additionally, we confirm that our results are on average in broad agreement with the very recent results of Sun et al. (2024).

4.4 Size ratio and spatial offset between the rest-frame optical and UV

The main topic of this study is to compare the sizes of high-z galaxies in the rest-frame UV and optical. When comparing galaxy sizes in different wavelengths, it is necessary to properly consider the wavelength dependence of the spatial resolution.⁹ As described in subsection 3.4, to ensure a fair comparison of the SB profile fitting results in the rest-frame UV and optical, we use the results from the original (PSF-matched) F150W images for the single (multiple) flagged sources.

The top panels of figure 15 plot the sizes in the rest-frame UV and optical of the z ≃ 4–10 galaxies estimated from the SB profile fittings with the F150W and F444W images. The bottom panels of figure 15 present the size ratio against the total magnitude in the rest-frame UV for these galaxies. In these plots, we divide the sample into three redshift ranges to examine any difference due to redshift. Based on these panels, we find that the size ratio is around unity, with some galaxies being larger than 1 and others smaller. In a similar manner to figures 13 and 14, the sample is divided in half based on size or total magnitude, and their median sizes or median size ratios with its 68th percentiles are also shown in figure 15. The median size ratios and their corresponding total magnitudes are summarized in table 4. In these redshift ranges, we find that the average size ratios align with unity regardless of M_UV. For the highest redshift range, our results are in agreement with the previous results obtained for galaxies at z ≃ 7–9 (Yang et al. 2022; Ono et al. 2023). Our results demonstrate that the average size ratios of galaxies at lower redshifts of z ≃ 4.5–6.5 are also unity. The median size ratio of all the galaxies at z ≃ 4–10 investigated in this study is |$r_{\rm e,opt}/r_{\rm e,UV} = 1.01^{+0.35}_{-0.22}$|⁠.¹⁰ Very recently, Morishita et al. (2024) have reported that the sizes in the rest-frame UV and optical for z = 5–10 galaxies are comparable, which is consistent with our results.

Utilizing the HST CANDELS data, van der Wel et al. (2014) have demonstrated that the sizes of SFGs at z ∼ 0–2 decrease with increasing wavelength. Interestingly, the difference in size becomes smaller at higher redshifts. Their results can be attributed to younger average ages of SFGs at higher redshifts, resulting in a convergence of the spatial distributions of young massive stars traced in the UV and somewhat older less massive stars traced in the optical (see also Shibuya et al. 2015). The combination of our results with the previous JWST results indicate that there is no significant difference in the sizes of SFGs at z ≃ 4–10 in the rest-frame UV and optical across a wide magnitude range, which is in line with the idea of younger average ages of SFGs at higher redshifts. Indeed, recent spectral energy distribution (SED) analyses for SFGs at comparable redshifts have reported remarkably young ages (e.g., Papovich et al. 2023; Fujimoto et al. 2023).

Furthermore, figure 16 illustrates the spatial offsets between the positions of the rest-frame UV and optical emission of 45 galaxies in our sample for which sizes are successfully measured with both the F150W and F444W images showing no signs of merger activity (red filled circles). The determination of the presence of signs of mergers or tidal interactions is based on visual inspection. Primarily, sources flagged with the multiple component flag as presented in table 6 in appendix  2 are considered to show signs of mergers or tidal interactions. Additionally, sources that are also resolved into multiple components in the smoothed F150W images and/or F444W images are classified as exhibiting signs of mergers or tidal interactions as well, such as C2_20 and C09_01.¹¹ We find that, although 16 sources exhibit spatial offsets exceeding 1σ, none surpass the 3σ threshold. If the positional offset follows a Gaussian distribution centered at zero, the expected numbers of sources within and outside the 1σ range are 45 × 0.683 = 30.735 and 45 × 0.317 = 14.265, respectively, resulting in a chi-squared goodness of fit of 0.309. Since the chi-squared value for a 95% confidence level with one degree of freedom is 3.84, our results do not contradict the assumption of a Gaussian distribution centered at zero, implying on average no significant spatial offset between the rest-frame UV and optical. This suggests that, for SFGs at z = 4–10 with no clear signs of galaxy merger, the primary star-forming activity occurs near the mass centers of the galaxies.

Our results demonstrate that, for SFGs at z = 4–10, the sizes and positions traced in the rest-frame UV and optical continuum are not significantly different. This contrasts with observations at lower redshifts z ≲ 2, where galaxies exhibit smaller sizes at longer wavelengths, suggesting inside-out growth (e.g., van der Wel et al. 2014). Based on this concept, our results might imply that SFGs at z = 4–10 are in the early stages of inside-out galaxy formation, where the distribution of young massive and less massive stars is not yet differentiated, indicative of the first phase of this process. More precisely, future studies might offer clear insights through spatially resolved SED analyses (e.g., Giménez-Arteaga et al. 2023; Boyett et al. 2024) for galaxies over a wide range of magnitudes and redshifts.

In figure 16, we additionally present the results of the spatial offsets in the rest-frame UV and optical for sources whose sizes are measured with both the F150W and F444W images exhibiting possible signs of merger and/or tidal interactions (depicted as black open circles). Among the 67 sources that we analyze, 48 have an offset from the origin greater than 1σ, yielding a chi-squared goodness of fit of 49.4. This rejects the hypothesis that the data follow a Gaussian distribution centered at the origin at a confidence level of 95%, possibly implying that SFGs at z = 4–10 accompanied by multiple components on average show significant offsets between the positions in the rest-frame UV and optical. Although the observed offsets of these sources seem to be less conspicuous than those reported in the simulation results of Ma et al. (2018) (see their figure 2), a quantitative comparison in the future might provide valuable insights into the physical processes involved in galaxy formation.

5 Summary

In this study, we have performed SB profile fittings for the 149 galaxies at z ≃ 4–10 with S|$/$|N values exceeding 10, 29 of which have been spectroscopically confirmed, using the deep JWST NIRCam images obtained by the CEERS survey. The F150W and F444W images allow for probing their rest-frame UV and optical continuum emission. Although the Sérsic index is typically around 1.0–1.5 in the rest-frame UV and optical, we have fixed it in the SB profile fittings in the same manner as previous work due to the large uncertainties for individual galaxies. Following our previous approach (Ono et al. 2023), we have performed MC simulations to correct for the systematic underestimates of galaxy sizes and total luminosities, particularly for faint sources. Our primary results are as follows:

1. We have carefully examined the impact of strong emission lines on galaxy size measurements with galaxies at z_spec = 5.63–6.63, utilizing images with the medium-band filter F410M and the broad-band filter F444W, one of which incorporates strong emission lines and the other does not. We have concluded that the size differences due to the effect of strong emission lines are minor, enabling the use of the F444W images to measure galaxy sizes across a more extensive redshift range.

2. We have also taken into account the difference in spatial resolutions between the F150W and F444W images for a fair comparison between the rest-frame UV and optical sizes of galaxies, by smoothing the F150W images to match the PSF sizes of the F444W images. The SB profile fitting results from the original F150W images and the PSF-matched F150W images have demonstrated that, even when the spatial resolution differs by a factor of about three, the size and total magnitude measurements are in good agreement.

3. Based on the comparisons between the rest-frame UV sizes and total magnitudes of galaxies observed by the HST and JWST, considering the difference of their spatial resolutions, these two measurements broadly align with each other with few outliers, confirming the availability of the previously obtained HST results (Shibuya et al. 2015) in comparison with JWST measurements.

4. We have investigated the relationship between the size and luminosity of galaxies at z = 4–10 both in the rest-frame UV and optical, confirming the trend that fainter sources are smaller in size on average as reported in the previous studies (e.g., Oesch et al. 2010; Grazian et al. 2012; Ono et al. 2013).

5. After careful consideration of the spatial resolution differences in the rest-frame UV and optical images, we have found that the optical-to-UV size ratios of galaxies at z = 4–10 are on average around unity across a wide range of magnitudes, likely because of the young average ages of SFGs at high redshifts.

6. We have found no significant spatial offsets between the rest-frame UV and optical for z = 4–10 SFGs with no clear signs of merger, implying that the primary star-forming activity in these galaxies is likely occurring near their mass centers, which can be explained by the possibility that these galaxies are experiencing the initial stages of inside-out galaxy formation.

Ackonowledgments

This work is based on observations made with the NASA/ESA/CSA James Webb Space Telescope. The data were obtained from the Mikulski Archive for Space Telescopes at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST. This work was partially performed using the computer facilities of the Institute for Cosmic Ray Research, the University of Tokyo. This research made use of GALFIT (Peng et al. 2002, 2010), SExtractor (Bertin & Arnouts 1996), IRAF (Tody 1986, 1993),¹² SAOImage DS9 (Joye & Mandel 2003), Numpy (Harris et al. 2020), Matplotlib (Hunter 2007), Scipy (Virtanen et al. 2020), Astropy (Astropy Collaboration 2013, 2018),¹³ and Ned Wright’s Javascript Cosmology Calculator (Wright 2006).¹⁴ This work was supported by the World Premier International Research Center Initiative (WPI Initiative), MEXT, Japan, as well as KAKENHI Grant Numbers 15K17602, 15H02064, 17H01110, 17H01114, 19K14752, 20H00180, 21H04467, 21J20785, and 22K03670 through the Japan Society for the Promotion of Science (JSPS). This work was partially supported by the joint research program of the Institute for Cosmic Ray Research (ICRR), the University of Tokyo.

Appendix 1. IDs of the galaxies analyzed in this study

In section 2, we compiled galaxies with redshifts ranging from z ≃ 4 to 10 in the CEERS fields found in previous studies (Shibuya et al. 2015; Bouwens et al. 2015; Nakajima et al. 2023; Finkelstein et al. 2023). Table 5 summarizes their unique IDs assigned in this study, their original IDs in the previous catalogs, and their overlaps between the previous catalogs. Additionally, we include a flag in table 5 to indicate if these galaxies are also listed in catalogs from the following studies by different groups: Donnan et al. (2023), Fujimoto et al. (2023), Bouwens et al. (2023), Tang et al. (2023), Harikane et al. (2024). One should be aware that this additional flag is not comprehensive, as it does not cover all published literature to date. It should also be noted that these catalogs have a preference for focusing exclusively on higher-z (z ≳ 7) sources compared to the redshift range covered in our study.

Appendix 2. SB fitting results for individual galaxies

In section 4, we have performed SB profile fittings for the high-z galaxies in the CEERS fields. The sizes and total magnitudes obtained from the SB profile fittings for individual galaxies are summarized in table 6. In addition, the SB profile fitting result images for each object are presented in figure 17. Moreover, the pseudo-color images of those galaxies whose spectroscopic redshifts are compiled by Nakajima et al. (2023) are shown in figure 18.

Upon scrutinizing the output images of the SB profile fittings, we allocate a blendedness designation for each subject as listed in table 6. This process involves identifying whether the fitted component in either the smoothed F150W and/or the F444W image corresponds to a single source or multiple sources in the original F150W image. For instances where the fitted component in the F444W image is a single component in the original F150W image, we label it as “single” with a flag value of 1. In contrast, when the fitted component in the F444W image consists of multiple components in the original F150W image, the classification becomes “multiple,” marked by a flag value of 2. Essentially, this means that a source is marked as having multiple components if the residual image after a single component fit indicates the existence of other components. This approach ensures a fair comparison even for data with different spatial resolutions.

Footnotes

References