# Panic! At the Disks: First Rest-frame Optical Observations of Galaxy
Structure at $z>3$ with JWST in the SMACS 0723 Field

Leonardo Ferreira Centre for Astronomy and Particle Physics, University of
Nottingham, Nottingham, UK  Nathan Adams Jodrell Bank Centre for Astrophysics,
University of Manchester, Oxford Road, Manchester UK  Christopher J. Conselice
Jodrell Bank Centre for Astrophysics, University of Manchester, Oxford Road,
Manchester UK  Elizaveta Sazonova Department of Physics and Astronomy, Johns
Hopkins University, Baltimore, MD 21218, USA  Duncan Austin Jodrell Bank
Centre for Astrophysics, University of Manchester, Oxford Road, Manchester UK
Joseph Caruana Department of Physics, University of Malta, Msida MSD 2080
Institute of Space Sciences & Astronomy, University of Malta, Msida MSD 2080
Fabricio Ferrari Instituto de Matemática Estatística e Física, Universidade
Federal do Rio Grande, Rio Grande, RS, Brazil  Aprajita Verma Sub-department
of Astrophysics, University of Oxford, Denys Wilkinson Building, Keble Road,
Oxford, OX1 3RH.  James Trussler Jodrell Bank Centre for Astrophysics,
University of Manchester, Oxford Road, Manchester UK  Tom Broadhurst
Department of Physics, University of the Basque Country UPV/EHU, E-48080
Bilbao, Spain  DIPC, Basque Country UPV/EHU, E-48080 San Sebastian, Spain
Ikerbasque, Basque Foundation for Science, E-48011 Bilbao, Spain  Jose Diego
FCA, Instituto de Fisica de Cantabria (UC-CSIC), Av. de Los Castros s/n,
E-39005 Santander, Spain  Brenda L. Frye Department of Astronomy/Steward
Observatory, University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721,
USA  Massimo Pascale Department of Astronomy, University of California, 501
Campbell Hall #3411, Berkeley, CA 94720, USA  Stephen M. Wilkins Astronomy
Centre, University of Sussex, Falmer, Brighton BN1 9QH, UK  Institute of Space
Sciences and Astronomy, University of Malta, Msida MSD 2080, Malta  Rogier A.
Windhorst School of Earth and Space Exploration, Arizona State University,
Tempe, AZ 85287-1404  Adi Zitrin Physics Department, Ben-Gurion University of
the Negev, P. O. Box 653, Be’er-Sheva, 8410501, Israel

(Received July 28, 2022; Revised August 29, 2022)

###### Abstract

We present early results regarding the morphological and structural properties
of galaxies seen with the James Webb Space Telescope (JWST) at $z>3$ in the
Early Release Observations towards the SMACS 0723 cluster field. Using JWST we
investigate, for the first time, the optical morphologies of a significant
number of $z>3$ galaxies with accurate photometric redshifts in this field to
determine the form of galaxy structure in the relatively early universe. We
use visual morphologies and Morfometryka measures to perform quantitative
morphology measurements, both parametric with light profile fitting (Sérsic
indices) and non-parametric (CAS values). Using these, we measure the relative
fraction of disk, spheroidal, and peculiar galaxies at $3<z<8$. We discover
the surprising result that at $z>1.5$ disk galaxies dominate the overall
fraction of morphologies, with a factor of $\sim 10$ relative higher number of
disk galaxies than seen by the Hubble Space Telescope at these redshifts. Our
visual morphological estimates of galaxies align closely with their locations
in CAS parameter space and their Sérsic indices.

high redshift galaxies, JWST, Morphologies

††journal: ApJL

##  1 Introduction

The James Webb Space Telescope (JWST) was launched on December 25, 2021 with
its first operational image released to the public on July 11, 2022 by US
President Joe Biden. This first image is a very deep image of the RELICS
cluster SMACS J0723.3-732 (SMACS 0723, Ebeling et al., 2010; Repp & Ebeling,
2018; Coe et al., 2019). SMACS 0723 is massive cluster of galaxies at
$z=0.390$ which is also known to contain an extensive collection of strong
gravitational arcs with a measured and modelled mass profile (e.g., Golubchik
et al., 2022; Pascale et al., 2022). As this is the first JWST image to be
released for a field where many objects have existing accurate photometric
redshifts, it enables us to study the morphological evolution of galaxies with
the earliest JWST data available.

Even before the release of the raw imaging, it was clear from the publicly
released promotional color image that this cluster contained a collection of
red and spiral galaxies that were not obviously present in the Hubble Space
Telescope (HST) imaging. These observations provide the ideal resource for a
first examination of the problem of how galaxy morphology changes from HST to
JWST, and how rest-frame optical morphologies appear in the un-probed region
$z>3$. To understand this question better, we have undertaken an early
analysis of this Early Release Observation (ERO) data released by the Space
Telescope Science Institute (STScI) on July 13, 2022 to the public.

This paper is a first-look study of the morphological evolution of galaxies
seen in the field around SMACS 0723, giving us our first redshift-based look
at how galaxy structure changes with redshift up to $z=8$. This is the first
field where this analysis can be performed due to the limited available
accurate photometric redshifts in other, early release observations.

Galaxy structure and morphology are one of the key aspects for understanding
galaxy evolution, and will be a key measurement that JWST will make throughout
its lifetime. Following the first servicing mission to Hubble, distant
galaxies started to have their structure resolved in the mid-nineties. This
revealed that faint, distant galaxies appear more peculiar and irregular than
local ones, and cannot be easily classified on the Hubble sequence (Griffiths
et al., 1994; Dressler et al., 1994; Driver et al., 1995; van den Bergh et
al., 1996). Why that is the case has remained a major topic of discussion for
almost three decades. These early observations, however, only showed that
galaxies became more peculiar at fainter magnitudes, which did not necessarily
correlate with further distances.

When redshifts became available, at first within the Hubble Deep Field, it was
clear that galaxy structures evolve strongly and systematically with redshift,
such that peculiar galaxies dominate the population at $z>2.5$ (e.g.,
Conselice, 2003; Papovich et al., 2005; Elmegreen & Elmegreen, 2005; Dahlen et
al., 2007; Buitrago et al., 2008; Conselice et al., 2008; Huertas-Company et
al., 2009; Buitrago et al., 2012; Mantha et al., 2018). It is now well-
established that galaxies as observed with HST become smaller and more
irregular/peculiar at higher redshifts, and this has been accounted for by the
merger process for a significant fraction (40-50%) of systems (Conselice,
2014).

However, although HST was revolutionary, morphological evolution measurements
still suffer some limitations. First among these is that due to HST’s limited
red wavelength coverage, we have not measured the rest-frame optical light of
galaxies within the first two Gyr after the Big Bang, that is at $z>3$. Very
few galaxies have been observed in the rest-frame optical bands at such
redshifted wavelengths, and most of these utilise ground-based adaptive
optics. This is due to the fact that the F160W band on HST only probes rest-
frame optical light up to $z\sim 2.8$, whereas JWST permits us to obtain this
information up to $z=8$ with F444W, and even beyond with MIRI. Second, HST
infrared imaging does not provide the necessary spatial resolution to resolve
most high-redshift objects. Furthermore, we have found in previous
observations (e.g., Conselice et al., 2005; Huertas-Company et al., 2009;
Mortlock et al., 2013; Huertas-Company et al., 2016) that the number of
galaxies that are classifiable as disks or spheroids (including ellipticals)
declines quickly when observing systems at higher redshifts, up to $z=3$.

Observations of galaxy structure and morphology at $z>3$ do show that in the
rest-frame UV, galaxies are peculiar and irregular (e.g., Elmegreen &
Elmegreen, 2005; Conselice & Arnold, 2009). Galaxies at these redshifts are
also often found to be clumpy, as seen with deep Wide Field Camera 3 (WFC3)
data (Elmegreen & Elmegreen, 2005; Oesch et al., 2010; Margalef-Bentabol et
al., 2018; Whitney et al., 2021; Margalef-Bentabol et al., 2022). Measurements
of galaxies in pairs also demonstrates that the merger rate and the fraction
of galaxies in mergers at $z\sim 6$ is as high as 50% (e.g., Conselice &
Arnold, 2009; Duncan et al., 2019). This implies that galaxy structure should
likewise be distorted accordingly (e.g., Duncan et al., 2019; Shibuya et al.,
2022). At the same time, we have believed for 30 years that the Hubble
sequence is established quite early at $z\sim 1$ (e.g., Mortlock et al., 2013;
Huertas-Company et al., 2016). However, all of these conclusions are based on
HST imaging, which has now been superseded in significant ways by the redder
bands, higher resolution, and better sensitivity of JWST.

Thus, in this paper we explore the morphological properties of the earliest
galaxies through an approach based on galaxy classification and measurement.
We demonstrate that these early galaxies have a more normal morphology than
expected, with classifications showing that disk galaxies are much more common
than previous observations suggested (e.g., Conselice et al., 2005; Huertas-
Company et al., 2009; Conselice, 2014; Huertas-Company et al., 2016; Margalef-
Bentabol et al., 2022). Overall, we argue that the formation of the Hubble
sequence appears to be ongoing much earlier than we had anticipated based on
HST observations.

This paper is organized as follows: in §2 we describe the data and our methods
and outline. In §3 we describe the morphological results of our study, §4 is a
short discussion of our results, and §5 is an overall summary. Throughout this
paper we assume a $\Lambda$ cold dark matter cosmological model with
$\Omega_{\Lambda}=0.7$, $\Omega_{\textrm{M}}=0.3$ and $H_{0}=70$ km s-1 Mpc-1.
All magnitudes are given in the AB system (Oke, 1974; Oke & Gunn, 1983).

##  2 Data Reduction and Products

The data we use for this analysis originates from the Early Release
Observations of SMACS 0723 (Pontoppidan et al., 2022) and include observations
taken with the Near Infrared Camera (NIRCam; Rieke et al., 2005, 2008, 2015).
The images were obtained on June 06, 2022 (PI: Pontoppidan; Program ID 2736)
in the F090W, F150W, and F200W short-wavelength (SW) bands, and F356W, F277W,
and F444W long-wavelength (LW) bands. The total integration time for this
target is 12.5 hr. Figure 1 shows the combined color image of SMACS 0723 which
we created from our own reduction.

We reprocess the uncalibrated lower-level JWST data products following a
slightly modified version of the JWST official pipeline. This is because the
initial release of the higher-level data products have been found to contain
WCS alignment issues as well as sub-optimal background subtraction. The key
differences are as follows: (1) We use version 1.5.2 of the pipeline as
opposed to version 1.5.3, which was the most up-to-date version at the time of
writing. This is because version 1.5.3 has a significant bug in the background
subtraction step that led to sub-optimal
performance.111https://github.com/spacetelescope/jwst/issues/6920 (2) We apply
the CEERS 1/F noise and flat field correction (Bagley et al. in prep) between
stages 1 and 2 of the official pipeline. (3) We extract the SkyMatchStep from
stage 3 and run it independently on each NIRCam frame, allowing for quicker
assessment of the background subtraction performance and fine-tuning. (4)
After Stage 3, we align the final science images onto a GAIA-derived WCS using
tweakreg, part of the DrizzlePac python
package222https://github.com/spacetelescope/drizzlepac. We then pixel match
the images with the use of astropy
reproject333https://reproject.readthedocs.io/en/stable/. Finally, we re-align
the RELICS SMACS 0723 HST imaging to the GAIA DR2 catalog due to large $~{}1$″
offsets, and match it with astropy reproject as well. We then apply
astrometric corrections to the positions of sources available in the RELICS
catalogs.

Overall, this data set allows us to probe the rest-frame optical images of
galaxies out to $z=8$. In Figure 2, we show the rest-frame wavelength probed
by each individual filter and how they can be combined together for up to
$z=8$ optical rest-frame coverage. In addition to this, we combine our
observations with HST Archival data in the WFC3 F160W band.

Figure 1: JWST color image of SMACS 0723 showing the overall distribution of
galaxy shapes and morphologies, including the lensing arcs. This image was
produced from our reduced data products via a composite of data in 6 bands:
F090W, F150W, F200W, F277W, F356W, and F444W. F090W and F150W were assigned
blue colours, F200W and F277W green, and F356W and F444W orange and red
respectively.

We employ two different approaches to the SMACS 0723 data: first a
quantitative analysis using Morfometryka (Ferrari et al., 2015), where we
measure non-parametric morphology estimates such as concentration, asymmetry,
and smoothness (CAS; Conselice et al., 2003); Gini-M20 (Lotz et al., 2004),
various sizes, as well as light profile fitting, which is described in detail
§2.2. Second, we provide simple visual classifications for all sources with
$S/N>10$ in their optical rest-frame filters, described in detail in §2.3.

Figure 2: Rest-frame wavelength at a given redshift for the F200W, F277W,
F356W, and F444W filters. The hatched regions show the areas where NIRCam
filters probe the optical rest-frame for $z>3$, with the color corresponding
to the respective optimal filter for the redshift range. To follow galaxies’
optical rest-frames we use F277W for $3<z<4$, F356W for $4<z<6$, and F444W for
$6<z<8$.

###  2.1 Photometric Redshifts

We use photometric redshifts derived through the Bayesian photo-z code (BPz)
(Benítez, 2000; Benítez et al., 2004; Coe et al., 2006) by the RELICS program
(Salmon et al., 2020), which used HST imaging in 7 bands for 41 clusters,
including SMACS 0723, and archival Spitzer IRAC measurements to measure
photometric redshifts of galaxies up to $z=8$. The BPz code compares RELICS
fluxes to 11 templates for ellipticals, spirals and starburst galaxies. The
overall method for this is described in detail in Coe et al. (2019).

For our analysis, we selected 355 galaxies with RELICS photometric redshifts
from the JWST footprint of the SMACS 0723 field, restricting our sample to
$1.5\leq z\leq 8$. The distribution of redshifts across the SMACS 0723 field
is shown in Figure 3. From these 355 sources 280 were considered classifiable,
excluding stars and point sources.

As an additional sanity check of these redshifts, and to see if they could be
improved upon, we conduct our own SED fitting procedure with the use of
LePhare (Arnouts et al., 1999; Ilbert et al., 2006). Within LePhare, we use
the COSMOS galaxy templates (Ilbert et al., 2009) which are based on the
commonly used BC03 template set (Bruzual & Charlot, 2003). These templates are
modified with dust attenuation up to $E(B-V)=1.5$ (Calzetti et al., 2000) and
attenuation from the IGM following Madau (1995). We initially run the SED
fitting process on the original RELICS photometry and obtain strong agreement
with their BPz based redshifts. We then add in photometry from an F200W
selected NIRCam catalogue which is cross matched to the RELICS catalogue with
a 0.5 arcsecond tolerance. To be consistent with the photometry derived in the
RELICS catalogues, we use isophotal magnitudes as measured by SExtractor
(Bertin & Arnouts, 1996) for our sources. We find that the photo-z’s are
consistent with the original RELICS estimations when the bluest NIRCam bands
available (F090W and F150W) are added to the SED fitting procedure. However,
when photometry from F200W and redwards are added, we find that some
originally high redshift sources ($z>3$) in the RELICS catalogue are given new
solutions at $z<1$, this is found to be the result of a lack of a strong
Balmer break at $\sim 2\mu$m. Examining these sources in detail reveal them to
be classified as a star-forming disk in our later analysis. Estimations of the
proper size and absolute magnitudes of these objects reveal many of the new
low-z solutions to give extremely small ($\leq 0.1\ $pkpc) and faint
($M_{F415W}>-16$) properties for the sources444The results presented here are
robust against putting these sources at lower redshifts since they represent a
small fraction of the overall sample.. Additionally, the current dataset was
reduced prior to the calibration updates based on on-flight observations,
which had large offsets on zero-points of up to $\sim 0.4$ mag (Adams et al.,
2022; Rigby et al., 2022). This observation, combined with the subsequent disk
classification lead us to proceed with the original RELICS BPz redshifts for
all sources.

Figure 3: The distribution of redshifts across the SMACS 0723 field. Different
redshift regimes, where we use our methodology to investigate the
morphological evolution, are shown as differently-colored markers.

###  2.2 Quantitative morphologies: Morfometryka

Figure 4: Plots showing the morphological evolution of the galaxies found in
the SMACS 0723 field up to $z=6$. These show the fraction of the total number
of galaxies, within a given redshift bin, which has the given type as
determined by visual morphologies. We also include the morphological evolution
which has been derived from HST observations of the CANDELS fields in Mortlock
et al. (2013). Circled markers denote the JWST observations at higher
redshifts. We note that the increase in spheroids can be attributed to smaller
sizes with increasing redshift $z$ as discussed in § 3.1.1.

Morfometryka was designed to perform several structural measurements on galaxy
images, in an automatic non-interactive way (Ferrari et al., 2015; Lucatelli &
Ferrari, 2019). It was devised mainly to measure non-parametric morphometric
quantities, but also performs single-component Sérsic model fitting. It takes
as input the galaxy and point spread function (PSF) images, estimates the
background with an iterative algorithm, deblends the sources and defines which
one is the target. Then, it filters out external sources using
GalClean555GalClean – https://github.com/astroferreira/galclean (de Albernaz
Ferreira & Ferrari, 2018). From the segmented region it calculates basic
geometrical parameters (e.g. center, position angle, axial ratio) using image
moments. Following this, it performs photometry, measuring fluxes in ellipses
with the aforementioned parameters. Along the way, it masks point sources over
the ellipse annulus with a sigma clipping criterion.

From the luminosity growth curve it establishes the Petrosian radius and the
Petrosian Region, inside which all measurements are made. The 1D Sérsic fit is
performed on the luminosity profile. For robustness, the 1D outputs are used
as inputs for a 2D Sérsic fit done with the galaxy image and JWST PSF images
generated with the official package WebbPSF666https://webbpsf.readthedocs.io/.
Morfometryka uses the PSF to produce the Sérsic profiles and to mask an area
of the size of the PSF FWHM from the central region of the source stamp for
non-parametric morphology calculations. Even though these simulated PSFs are
realistic, we note that deviations from the true PSF might exist. However, as
we are interested in extended sources, effects of this type are neglible.
Finally, Morfometryka measures several morphometric parameters (concentration;
asymmetry; Gini; M20; entropy, spirality, and curvature, among others).

###  2.3 Visual Classification

All galaxies in the sample were classified by three co-authors of this paper
with experience with galaxy structure and classification (CC, LF, ES). The
visual classification scheme that we use is described in detail in Ferreira et
al. (2022; in prep). Here we give a quick summary. In general, we use four
categories for visual classifications, following Mortlock et al. (2013); these
are defined as described below, and differ slightly from a traditional Hubble
classification scheme, but are generally very similar.

  * •

Class 0: Unclassifiable: Galaxies too small and/or too faint to classify, and
images with artifacts.

  * •

Class 1: Spheroids: These galaxies are resolved, symmetrically and centrally
concentrated, with a smooth profile and are round/elliptical in shape.

  * •

Class 2: Disks: This category includes galaxies that exhibit a resolved disk
in the form of an outer area of lower surface brightness with a regularly
increasing brightness towards the center of the galaxy. This classification
does not depend on there being a spiral pattern in the system, although one
can be present in this classification.

  * •

Class 3: Peculiar. This class is for well-resolved galaxies with a morphology
which is dominated by a disturbance or peculiarity and has no obvious disc or
spheroid component.

Each galaxy is further classified as smooth or structured, where structured
galaxies have features standing out from the smooth stellar envelope, such as
star-formation clumps, tidal features, and merger signatures. Galaxies with
distinct disk and bulge components were also classified as structured.
Finally, the classifiers were able to provide additional notes on each source
to aid in future analysis. We do not however use these further detailed
morphologies in this paper.

Figure 5: Comparison HST vs JWST images for 9 objects in each class within our
sample. Left columns shows the HST F160W image, whilst the middle panel shows
the JWST view at the same orientation, in the rest-frame optical. The far
right panel shows the color image of this system as seen through the JWST
NIRCam F277W, F356W and F444W filters, generated by TRINITY (Coe et al.,
2012). The IDs refer to the original RELICs catalogs identification.

These classifications were all carried out separately and then combined into
an average, which we then use throughout this work. Our process was such that
we only classified each galaxy in our sample in the wavelength which most
closely matches the rest-frame optical wavelength of the observations. This
allows us to match the classifications at different redshifts to determine how
morphological evolution is occurring. We find that the classifications by the
three classifiers agree $2/3$ in $\sim 63\%$ (177) of the sources while
perfect agreement $3/3$ happens $\sim 33\%$ (87) of the time. Catastrophic
classifications where all 3 classifiers disagree happens only in $\sim 5\%$
(16) of the cases. Table 1 shows the average of the fractions for each of the
three classifiers for the three different types (sph, disk and peculiar). Also
listed as the error-bar on these averages is the standard deviation of the
fractions among the classifiers, showing that these are always relatively
small and in some cases the agreement is to within a few percent.

Table 1: The filters used in this study with the redshift ranges used within that filter and the average rest-frame wavelength in which that filter probes at that redshift. Also shown are the classification fractions for our three main types of galaxies; spheroids, disks, and peculiars. These are the averages of the three classifiers with the standard deviation listed as the value $\sigma$. Sérsic indices for each class and redshift bin is also provided, showing the mean of the distribution together with $15\%$ and $85\%$ percentile scatter. Filter | z |  $<$Rest-$\lambda>$ | class (sph) |  $\sigma$(sph) | class (disk) |  $\sigma$(disk) | class (pec) |  $\sigma$(pec) | $n_{sph}$ | $n_{disk}$ | $n_{pec}$
---|---|---|---|---|---|---|---|---|---|---|---
F090W | 1.5-3.0 | 0.36-0.22 $\mu m$ | 0.18 | 0.02 | 0.38 | 0.10 | 0.43 | 0.12 | $1.32_{1.14}^{1.6}$ | $1.12_{0.58}^{1.62}$ | $0.76_{0.37}^{1.54}$
F227W | 3.0-4.0 | 0.69-0.55 $\mu m$ | 0.21 | 0.08 | 0.48 | 0.05 | 0.31 | 0.03 | $1.22_{1.12}^{1.32}$ | $1.04_{0.8}^{1.3}$ | $1.04_{0.31}^{1.53}$
F356W | 4.0-6.0 | 0.71-0.50 $\mu m$ | 0.37 | 0.01 | 0.40 | 0.02 | 0.23 | 0.03 | $1.58_{1.08}^{1.81}$ | $1.11_{0.81}^{1.29}$ | $1.67_{1.26}^{1.09}$

##  3 Results

###  3.1 Distribution of Morphology with Redshift

One of the main questions that we can investigate with this early imaging from
JWST is the distribution of morphological types with redshift. Given the
redshifts we have from HST and the morphologies from JWST we can make the
first measurement of the morphological distribution of galaxies up to $z<8$.

In Figure 4 we plot the morphological distribution of our sample of galaxies
with morphological classifications from the JWST imaging. As can be seen, we
find a remarkable increase in the number of spiral galaxies over what was
thought to exist in previous analysis of the deepest HST imaging in the NIR,
which found that that there were very few disk galaxies at $z>1.5$, with a
rapid decline in the numbers at higher redshifts (e.g., Conselice & Arnold,
2009; Mortlock et al., 2013; Huertas-Company et al., 2016).

More generally, a decline in spheroids was also seen, but the morphological
change with redshift was not as pronounced as it was for the disk galaxies.
Figure 5 shows some examples of different galaxy types and how they appear
differently in the JWST vs. the HST imaging, revealing that morphologies are
often much easier to make out within the JWST data. There is however, a
propensity for these disks and spheroids to contain peculiar features, such as
tidal features and clumpy regions, that can differentiate them from $z=0$
examples. We however do not investigate these further in this paper.

Overall, we find that the disk galaxy population makes up about half of the
galaxies that are identified within the field of SMACS 0723 at $z>2.5$. This
is a remarkable result, as it shows that galaxies such as the Milky Way could
potentially have retained the same overall morphological state for over 12
billion years if these distant disk galaxies are similar to the ancestor
galaxy of the Milky Way.

####  3.1.1 Morphology at $z>4$

The morphologies of $z>4$ galaxies, as probed by the F356W filter, differ from
the rest-frame UV morphologies determined from HST imaging (Conselice &
Arnold, 2009). Despite what might have been expected from HST observations,
the morphologies of at least the brightest galaxies are much less distorted
than had been previously been thought based on HST observations. Also, the
galaxies at this epoch are often very tiny, such that their size in a NIRCam
image is just a bit larger, or within, the PSF of JWST. In fact, the larger
number of spheroids/compact objects is in part due to the fact that so many of
these systems are unresolved, an indication that their sizes are quite tiny.
Future studies carefully measuring sizes with the use of the JWST PSF will
examine these sizes and their evolution.

###  3.2 Quantitative Morphologies

Figure 6: Concentration vs. Asymmetry. The abundance of disks results in their
populating the area originally used for selecting late-type galaxies. Decision
thresholds shown here are drawn from (Bershady et al., 2000). Figure 7: Plot
showing the Sérsic index evolution in bins of redshift. Shown are the
morphological types and their mean values for these indices alongside error
bars representing the $15\%$ and $85\%$ percentiles of the distribution. We
can see that the average hovers about $n=1$, but that the spheroids appear to
have higher values even at the higher redshifts.

We quantify the structures of these galaxies based on the NIRCam imaging. This
can be done in a number of ways, and will be the focus of future dedicated
papers. We give a broad overview of quantitative morphology for our sample and
leave it to future papers to elaborate on these issues.

First we show the concentration-asymmetry diagram, which has been used to
classify galaxies and has a broad correspondence with galaxy types in the
nearby universe (e.g., Conselice et al., 2000, 2003, 2008; Bluck et al., 2012;
Whitney et al., 2021). We present the concentration-asymmetry diagram in the
SMACS 0723 field in Figure 6 as measured by Morfometryka. What we find is that
there is no great distinction between the disks and the spheroids, but we do
find that the peculiars are in the region of high asymmetry where mergers are
located (Conselice et al., 2000; Whitney et al., 2021). We also find that
there are few galaxies with very high concentration values, consistent with
previous work that found even massive galaxies to have low light
concentrations (Buitrago et al., 2012).

Another avenue of investigation is the examination of the light profiles of
our galaxies, which we have also measured. Previous work has shown that almost
all massive galaxies at $z>2$ have Sérsic indices which are $n\sim 1$, which
differs for galaxies at lower redshifts where $n\sim 4$ for similar mass
galaxies (e.g., Buitrago et al., 2012; Bluck et al., 2014). We show a basic
view of the average Sérsic index evolution for our sample in Figure 7, which
demonstrates that many of our galaxies contain indices with $n\sim 1$, with
most disks around this value, as expected. We also find many spheroids at this
Sérsic index, but on average these spheroids have a larger $n$ value.The mean
values together with $15\%$ and $85\%$ percentiles of the distributions for
each redshift bin and class are displayed in Table 1.

###  3.3 Formation of the Hubble Sequence

One of the primary goals of galaxy morphology and structural analysis is to
determine when the Hubble sequence was established. By Hubble sequence we mean
the establishment of spheroids (e.g., ellipticals) and spiral galaxies as we
see in the case of the most massive galaxies in the nearby universe. We know
for certain that there are fewer ellipticals and spirals at high redshift.
However, an important question is: when did the first spheroids and disk
galaxies form?

It is important to be clear about what we mean by this, as a definition of
these galaxy types is neither trivial nor simple. By a ‘spheroid’ we mean a
galaxy that exhibits a round or elliptical shape with a classical, steep light
profile and a smooth structure. A ‘disk galaxy’ is one that is either a
smooth, disk-like object, or something with visible spiral arms.

The trend of galaxy type with redshift has been measured by several different
papers. We include the analysis in Mortlock et al. (2013) as the basis for our
understanding of the morphological evolution at $z<3$. As Figure 4 shows, we
have not reached the limit of where the first ellipticals and spheroids have
formed. We will need to probe even higher redshifts to find when and if there
are no spheroids or disk galaxies. Thus, at least some aspect of the Hubble
sequence was in place at $z\sim 6$. It is, however, important to point out
that these classifications are done purely by visual estimates in one band. We
have not used colour or other features to classify galaxies, and we know from
work with WFC3 that galaxy structure and physical properties of their stars
becomes decoupled at higher redshifts $z>1.5$ (Conselice et al., 2011). It
remains to be seen how the physical properties of our ‘Hubble types’ here
correlate with the underlying stars in these systems.

##  4 Discussion

This is one of the earliest papers on the morphologies of galaxies at high
redshift with JWST, and thus our conclusions will be revisited by others in
the months and years to come. However, it does appear from an initial analysis
that there are far more disk galaxies at high redshift than originally thought
with HST. We in fact find that at the highest redshifts probed by HST there
are in fact up to 10 times more disk galaxies than we had thought, based on
the JWST visual morphologies.

This implies that disk galaxies have existed in large numbers for quite a
significant amount of time. This may mean that the morphologies of some disk
galaxies, such as the Milky Way, have remained in their current form for over
10 billion years. This would challenge our ideas about mergers being a very
common process, and it might be the case that mergers are only a dominant
process for forming the stellar masses of certain types of galaxies, namely
spheroids, which have a relatively constant merger fraction at $z>2.5$ at
around 10%. Although on average galaxies should go through multiple mergers
over cosmic time (Duncan et al., 2019), it is not clear how these mergers
would affect disk morphologies or if there are only certain galaxies that go
through mergers multiple times while others, such as the disks we find here,
do not undergo these mergers very often or at all at $z<6$.

Alternatively, it is also possible that these high redshift disks undergo
major mergers, but reform their disks after the disruptive event. This is a
process that is found to happen in simulations of gas-rich mergers (e.g.,
Sparre & Springel, 2017; Peschken et al., 2020).

There are a few caveats with this study that future studies will be able to
flesh out in much more detail. The first is that we only use the visual rest-
frame optical morphology of a galaxy to determine whether or not it is a
spheroid, disk, or a peculiar. These systems, however, are more obvious than
they were in the HST imaging, implying that in the rest-frame optical we are
seeing the underlying morphology in a much clearer way than we are in the
rest-frame UV, despite strongly star-forming galaxies having a very similar
appearance in the UV and optical, at least at $z<3$ (Windhorst et al., 2002;
Papovich et al., 2005; Taylor et al., 2015; Mager et al., 2018). It would
appear that at least disk galaxies are not easily seen in the UV, and this is
an indication that their stellar population and star formation histories are
spatially segregated (old/young stars in bulges/disks, for example), just as
they are at lower redshifts. Future studies will certainly be able to study
these resolved structures in more detail to learn about the detailed process
of disk formation, as done for HST observations at $z<3$ (Huertas-Company et
al., 2016; Margalef-Bentabol et al., 2022).

Finally, there is also the fact that this was conducted in a small field of
view area of 2.2’x2.2’, around a lensing cluster. In the future we will probe
non-cluster regions and larger areas to allow for a more detailed comparison
with previous results from HST imaging, such as CANDELS (Grogin et al., 2011;
Koekemoer et al., 2011).

##  5 Summary and Conclusions

In this paper we present a morphological and structural analysis on some of
the earliest galaxies imaged by the JWST, which has provided rest-frame
optical morphologies and structures for a statistically significant number of
galaxies at $z>3$ for the first time. We also examine the structures of
galaxies at $1.5<z<3$, where HST has not had the depth and resolution to infer
galaxy morphology correctly. Three of the authors classified 280 galaxies
visually at $1.5<z<8$ to determine basic morphological types - spheroid, disk,
peculiar, at rest-frame optical wavelengths given by JWST. We also ran
quantitative parametric and non-parameteric morphologies on these galaxies.

Our key findings are:

I. The morphological types of galaxies changes less quickly than previously
believed, based on precursor HST imaging and results. That is, these early
JWST results suggest that the formation of normal galaxy structure was much
earlier than previously thought.

II. A major aspect of this is our discovery that disk galaxies are quite
common at $z\sim 3-6$, where they make up $\sim 50$% of the galaxy population,
which is over 10 times as high as what was previously thought to be the case
with HST observations. That is, this epoch is surprisingly full of disk
galaxies, which observationally we had not been able to determine before JWST.

III. Distant galaxies at $z>3$ in the rest-frame optical, despite their
appearance in the HST imaging, are not as highly clumpy and asymmetric as once
thought. This effect has not been observed before due to the nature of
existing deep imaging with the HST which could probe only ultraviolet light at
$z>3$. This shows the great power of JWST to probe rest-frame optical where
the underlying mass of galaxies can now be traced and measured.

This study is the first examination of the problem of distant galaxy
morphology with JWST, and specifically the formation of galaxy structure at
$z>3$. Our results suggest many directions for immediate future study. We have
not included any new JWST galaxies that were not seen with HST, and have not
examined the structural properties as a function of stellar mass or other
physical properties. All of these will need to be fully examined in the
future.

The present study, however, shows the importance of JWST for understanding the
structural evolution of galaxies, which is now open for detailed
investigation.

We thank the anonymous referee and Dan Coe for comments that led to
improvements in this manuscript. We thank Anthony Holloway, Sotirios Sanidas
and Phil Perry for critical and timely help with computer infrastructure that
made this work possible. We acknowledge support from the ERC Advanced
Investigator Grant EPOCHS (788113), as well as a studentship from STFC. This
work is based on observations made with the NASA/ESA Hubble Space Telescope
(HST) and NASA/ESA/CSA James Webb Space Telescope (JWST) obtained from the
Mikulski Archive for Space Telescopes (MAST) at the Space Telescope Science
Institute (STScI), which is operated by the Association of Universities for
Research in Astronomy, Inc., under NASA contract NAS 5-03127 for JWST, and NAS
5–26555 for HST. These observations are associated with program 14096 for HST,
and 2736 for JWST. LF acknowledges financial support from Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior - Brazil (CAPES) in the form of a
PhD studentship.

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