Insights into Galaxy Morphology and Star Formation: Unveiling Filamentary Structures around an Extreme Overdensity at $z\sim 1.5$ Traced by [OII] Emitters

The environment of galaxies is known to play a significant role in shaping galaxy evolution by influencing factors such as morphology, gas content, and star formation rates. In the local universe galaxy properties vary noticeably along the cosmic web, from dense clusters to more isolated fields. Passive galaxies dominate dense regions, whereas star-forming galaxies are more prevalent in field environments (e.g., Dressler, 1980; Hashimoto et al., 1998; Poggianti et al., 1999; Couch et al., 2001; Goto et al., 2003; Kauffmann et al., 2004; Balogh et al., 2004; Capak et al., 2007; Morishita et al., 2017).

The influence of environment on the properties of galaxies has been observationally revealed across various redshift ranges. Parameters such as the star formation rate (SFR), galaxy morphology, and stellar mass are strongly influenced by the galaxy number density in their surrounding universe (Dressler, 1980; Kodama & Smail, 2001; Kodama et al., 2001; Smith et al., 2010; Peng et al., 2010). For instance, galaxies in lower-density environments exhibit higher median specific star formation rates (sSFR) at lower redshifts, indicating that environmental factors play a significant role in determining the fraction of quiescent galaxies. Further studies have demonstrated that this relationship evolves with redshift, with a notable reversal occurring around z $\geq$1, where star formation activity begins to correlate positively with the density of galaxies (Elbaz et al., 2007; Cooper et al., 2008; Lemaux et al., 2022). It was also found that blue-fraction of galaxies in clusters increased with redshifts (Butcher & Oemler, 1978, 1984). This evolution is intricately linked to the morphology–density relationship, where dense environments typically host quiescent, bulge-dominated galaxies, contrasting with the more active, star-forming galaxies in lower density environments (Dressler, 1980). This relationship is a pivotal aspect in understanding galaxy evolution, as it highlights the influence of the local environment on galaxy morphology and star formation activities.

The morphology-density relationship at higher redshifts remains a question. Recent studies extend this understanding to higher redshifts, exploring the morphology–density relationship in distant galaxy clusters (Newman et al., 2014; Paulino-Afonso et al., 2019). Tasca et al. (2009) found that the morphology- density relation is already in place at $z=$ 1, but the trend is flatter towards higher redshifts, indicating the morphological evolution of cluster galaxies since $z=$ 1. Sazonova et al. (2020) found that the environmental processes influencing galaxy shapes and densities were active as early as $z=$1.75, primarily affecting low-mass galaxies. Mei et al. (2023) found that the fraction of spheroid galaxies is higher in clusters, as well as the fraction of mergers. Star-forming galaxies in a $z\sim$2 protocluster exhibit morphologies similar to those in the general field, as reported in several studies (Peter et al., 2007; Naufal et al., 2023).

Numerical studies have enabled detailed morphological predictions (Schaye et al., 2015; Nelson et al., 2019), at even the highest redshifts Roper et al. (2022). However, reconciling these predictions with observational data presents challenges, especially in light of the James Webb Space Telescope (JWST) observations that show a diversity of galaxy structures at higher redshifts (Ferreira et al., 2022, 2023; Kartaltepe et al., 2023). This discrepancy underscores the need for a more thorough understanding of morphological evolution and its interplay with environmental factors at high redshifts. The JWST has revolutionized our view of the distant universe, offering unprecedented insights into galaxy assembly during this transformative epoch and allowing us to test key theoretical predictions about the early universe.

In this Letter, we report the discovery of an overdensity filamentary structures traced by [OII] emitters at $z\sim$1.5 in the COSMOS-UltraDeep field. By examining the morphologies of galaxies using recently acquired JWST NIRCam data, we investigate the impact of various environments on their ongoing star formation activities. Throughout this paper, the concordance $\Lambda$CDM cosmology (H₀ = 70 km s^-1 Mpc^-1 , $\Omega_{\Lambda}$ = 0.7 and $\Omega_{M}$ = 0.3) is adopted, and all magnitudes are reported on the AB system (Oke & Gunn, 1983). Finally, Chabrier (2003) initial mass function (IMF) is adopted.

The spatial distribution of the [OII] emitters is shown in Figure 1 (top). We observe a clear overdensity of [OII] emitters in a filament-like structure that extends from the north to southwest. Additionally, there is an extension of this filamentary structure to the east. To the northwest and southwest of the overdense core center, there are two regions of relatively high density. These features seem to be connected to the core of the structure, comprising a large-scale structure spanning over 50 cMpc, indicative of a cosmic web structure at this redshift.

The discovered overdensity highlights the clustering of star-forming galaxies, possibly indicating the initial phase of galaxy cluster formation. This contrasts with the nearby universe, where dense environments have inactive red galaxies and star-forming galaxies are in low-density environments. We calculate the local density environment using the Nth nearest neighbor approach. For defining local density, we consider a radius encompassing the 10th nearest neighbor object ($\Sigma_{\mathrm{10th}}$), combining both [OII] emitters and quiescent galaxies. This density distribution pattern is found consistent even when varying the Nth value in our analysis in the range of 5-10.

Based on the measured local density, we here classify galaxies that are within the $r<1.\!^{\prime}5$ from the densest central coordinates $(\alpha=9^{\text{h}}59^{\text{m}}54^{\text{s}},\delta=+02^{\circ}20^{\prime}00.75^{\prime\prime})$ as core members. Those situated between $1.\!^{\prime}5$ and 5^′ are classified as outskirt galaxies. Figure 1 (bottom-left) shows the zooming view of the core and outskirts region. Those in the lower 45th percentile of the $\Sigma_{\mathrm{10th}}$ distribution are classified as field galaxies. The classification of field galaxies remains robust even when considering the lower 25th percentile.

We evaluate the overdensity of [OII] emitters in the core. The observed average surface density of 1447 [OII] emitters in the total survey area of 4932 arcmin², is $(2.93\pm 0.08)\times 10^{-1}\,\text{arcmin}^{-2}$ where the uncertainty reflects the Poisson noise. In the most overdense core region, spanning roughly 7 arcmin² in which 22 [OII] emitters are enclosed, the number density is $(3.11\pm 0.66),\text{arcmin}^{-2}$, signifying an 11-fold increase compared to the overall surveyed region. The probability of the observed overdensity resulting from Poisson fluctuations is $\sim 10^{-15}$. This suggests a highly unlikely scenario under the assumption of a Poisson distribution, indicating that overdensity is likely due to a significant underlying structure.

From the Chandra COSMOS Legacy Survey (Civano et al., 2016), which catalogs 4016 X-ray point sources across 1.5 deg² of the COSMOS field, we identified 29 spectroscopically confirmed AGN between redshifts 1.45 and 1.49, and associated host galaxies as reported by Marchesi et al. (2016). The bottom-right panel of Figure 1 presents the local density $\Sigma_{\mathrm{10th}}$ [1/Mpc²] for different galaxy populations, including the X-ray AGNs. The median value of the X-ray AGN $\Sigma_{\mathrm{10th}}$ distribution positions itself within the interquartile range of 50–75%. Of the 29 X-ray AGNs, 11 are situated within the highest density quartile (75–100%), 7 fall within the 50–75% range, 2 are categorized between 25–50%, and 9 are found within the least dense quartile (0–25%). Within these distributions, four Xray AGNs reside in the outskirts, while the core contains only one. Additionally, cross-matching within a 1 arcsecond radius identified two of our [OII] emitters as X-ray AGNs, with one belonging to the outskirts sample. These have been excluded from our [OII] emitter sample for subsequent analysis. It is generally acknowledged that AGNs can significantly influence star formation in galaxies (Hopkins et al., 2014; Somerville & Davé, 2015; Beckmann et al., 2017; Penny et al., 2018). While existing observations at low redshifts indicate a lower prevalence of AGN activity in dense environments (Gisler, 1978; von der Linden et al., 2010; Mo et al., 2018), suggesting a potential reversal of this relationship at higher redshifts (Lehmer et al., 2009; Digby-North et al., 2010; Alberts et al., 2016; Krishnan et al., 2017; Huang et al., 2022). A comprehensive examination of the link between AGN activity and environmental density is beyond the scope of this study.

This study has presented a pilot analysis of [OII] emitters within the COSMOS UltraDeep field at $z\sim 1.5$, leveraging narrowband data from the Hyper Suprime-Cam Subaru Strategic Program and high-resolution imaging from the JWST NIRCam as part of the COSMOS-Web survey. Our investigation reveals large filamentary structures traced by [OII] emitters, surrounding an extremely overdense core. By taking advantage of the two unique datasets, we have shed light on the critical role of environment on galaxy evolution at $z\sim$1.5.

Our findings suggest that galaxies in denser environments, particularly core regions, not only demonstrate higher star formation rates but also exhibit more structural disturbances, implying active galaxy formation processes such as gas cooling and mergers. The observed trends hint at a possible link between the level of structural disturbance and the extent of enhanced star formation (e.g., Hoyos et al., 2016). The enhanced star-forming activities within this overdense filamentary structure could be due to the increased efficiency of gas accretion. Such a process may result in a higher supply of cold gas to galaxies, subsequently elevating their star formation rates. Furthermore, some simulations also suggest that the influx of cold gas along the cosmic web’s filaments supplies the necessary fuel for galaxies, thereby driving up their star formation rates at high redshifts (e.g., Kereš et al., 2005; Dekel & Birnboim, 2006; Genel et al., 2008). Continuous gas inflow is likely to contribute to disk instability (Dekel et al., 2009), indicating environmental variations in gas accretion mechanisms. Such mechanisms may result in a higher prevalence of disturbed or irregular galaxy morphologies in dense areas than in field regions, which is consistent with our findings. Unlike galaxy clusters in the local universe that are dominated by their passive population, this structure appears to be in an early stage of vigorous assembly with a higher gas fraction, as reported in (e.g., D’Amato et al., 2020; Castignani et al., 2020). Therefore, interaction with the hot gas may not yet be significant in this structure, which could potentially not affect star-forming galaxies through mechanisms such as ram pressure stripping. Several studies indicate that star formation in dense environments at high redshifts is enhanced compared to field galaxies, attributed to the increased rates of gas accretion and mergers (Alberts et al., 2014; Monson et al., 2021; Lemaux et al., 2022; Shi et al., 2024; Pérez-Martínez et al., 2024). Shimakawa et al. (2018) also found that star-forming galaxies in dense groups at $z\sim$ 2.5 had elevated specific star formation rates (sSFRs) relative to their counterparts in less dense environments, likely due to their substantial HI gas reserves in the proto-cluster (Lee et al., 2016; Cai et al., 2017; Mawatari et al., 2017). Our overdense structure may also have a large amount of HI gas. Future observations of molecular gases are essential to confirm the HI gas content in this structure and further understand its influence on galaxy evolution.

Our study has focused on analyzing the morphology and star formation activities of star-forming galaxies in different environments. While the exact impact of our results is yet to be fully understood, we found evidence of environmental impact on the morphology and star-formation. Our key findings can be summarized as follows:

1. 1.

   The [OII] emitters have unveiled a substantial filamentary structure over 50 cMpc, surrounding a core of extreme galaxy overdensity, with a galaxy number density approximately $11\times$ higher than the field average. The core of this structure, characterized by a pronounced overdensity of star-forming galaxies, potentially marks an early phase of galaxy cluster formation. This contrasts with typical associations in the nearby universe, where dense environments harbor inactive red galaxies, and star-forming galaxies are more prevalent in less dense areas.

2. 2.

   A significant increase in morphological disturbances has been observed in core region galaxies compared with those in outskirts and field environments. This finding highlights the profound impact of environmental density on galaxy morphology, especially in denser areas where interactions and mergers become more frequent.

3. 3.

   Core galaxies exhibited higher SFR and sSFR compared to field galaxies. This, along with similar stellar mass distributions across environments, emphasizes the role of environmental density in influencing star formation efficiency, suggesting accelerated processes such as gas cooling and mergers in denser environments.

4. 4.

   While no statistically significant correlation was found between structural disturbance and sSFR, a trend was observed where galaxies with greater degrees of disturbance displayed a marginally higher sSFR. This trend, observed across the core, outskirts, and field regions, suggests a potential connection between structural disturbance and enhanced star formation.

Those galaxies in the overdense core may be undergoing merger and are thought to play a major role in the growth of massive galaxies. It is important to recognize that the findings of this study regarding the impact of environment on galaxies within this overdense structure might not be universally applicable to every cluster or overdense region at this epoch in the universe. Outcomes could significantly vary based on the developmental stage of each cluster, even if they share the same redshift. Clearly, more extensive samples of galaxies at this and other redshifts are needed to generalize these results and to validate the observed differences in morphology. In future work, we aim to broaden the scope of our research by incorporating a larger dataset, including the additional data from ongoing and upcoming surveys. This expansion will not only enhance our sample size but also enable more robust statistical inferences. This will allow for a more comprehensive analysis that considers additional variables influencing galaxy evolution, such as interactions between galaxies and the effects of AGN feedback on star formation and morphology. This approach will enrich our understanding of the complex interplay between environmental conditions and galaxy evolution.

We are grateful to the anonymous reviewer for their insightful comments and suggestions that improved the manuscript. This work is based on data collected at the Subaru Telescope and retrieved from the HSC data archive system, which is operated by Subaru Telescope and Astronomy Data Center (ADC) at NAOJ.

R.L. was previously supported by MEXT and currently by Graduate Program on Physics for the Universe (GPPU), Tohoku University. R.L. acknowledges support from JST SPRING, grant No. JPMJFS2102, and KAKENHI International Leading Research (Grant Number 22K21349). T.K. acknowledges financial support from JSPS KAKENHI Grant Number 18H03717.