Probing Large-scale Structure and the Multi-Phase IGM at the Cosmic Noon -- Insights from a Joint Survey with Euclid, CSST, JPCam, and JUST

Abstract

We present scientific and technical justifications of a potential coordinated Euclid/CSST/JPCam/JUST survey of the Euclid Deep Field North (EDF-N), aimed at probing the multi-phase circumgalactic and intergalactic medium (CGM/IGM) at the cosmic noon over ~20 deg$^2$. The survey is structured around three connected goals: (1) improving photometric redshift (photo-z) accuracy through the combination of broad- and narrow-band photometry, enabling reliable identification of large-scale structures; (2) probing extended CGM emission with dedicated narrow-band imaging; and (3) mapping foreground IGM via absorption-line spectroscopy of background galaxies. Together, these components establish an integrated observational framework to investigate galactic ecosystems -- linking galaxies to their circumgalactic and intergalactic environments -- at cosmic noon. We show that the J-PAS-like narrow-band system used in JPCam substantially improves photo-z accuracies from only the Euclid/CSST broad-band data, especially for star-forming galaxies at z~1.0-1.4. This enables the identification of galaxy groups and (proto-)clusters directly from photo-z measurements. Stacked JPCam narrow-band imaging should also detect extended [O II]-emitting CGM halos. We then construct mock 3D gas distribution model and realistic galaxy catalog, and further construct mock CSST and JUST background galaxy spectra adding Lyalpha and Mg II absorptions. The reconstructed 3D H I field from CSST Lyalpha forest reliably recovers large-scale structures; however, our simulations indicate that detecting diffuse IGM Mg II absorption with JUST is infeasible, either through spectral stacking or via the two-point correlation function method. We conclude that constraining the metallicity of the diffuse IGM will require significantly deeper and higher-resolution spectroscopy expected from future facilities such as the 39 m E-ELT.

Figures (16)

• Figure 1: Cumulative number of galaxies (solid lines) and AGNs (dashed lines) above a given redshift $z_{\rm min}$ for several magnitude limits ($m < 22.0-25.0$), computed over a $1.4\rm~deg^2$ field corresponding to the FOV of a single JUST pointing. The horizontal dashed line marks the total number of spectroscopic fibers available per pointing ($N_{\rm fibers} = 2000$), indicating the effective upper limit on the number of background sources that can be targeted simultaneously.
• Figure 2: Example of the forward modeling SED fitting procedure. The grey curve shows the intrinsic model spectrum generated from our stellar continuum + empirical emission-line prescription (see text in § \ref{['subsec:ExpPhotometry']} for details). Colored points indicate the synthetic photometry with noise added according to the survey detection limits in each filter (CSST, Euclid, and a selected subset of 14 OAJ/JPCam narrow-bands). Horizontal bars show the effective wavelength range contributing to each bandpass. The best-fit photometric SED model, obtained by minimizing $\chi^2$ over redshift, stellar mass, SFR, and dust attenuation, is shown as a solid red curve. The inset text lists the input ("True") and recovered ("Best fit") physical parameters, including their $1\sigma$ uncertainties derived from bootstrap resampling of the observed photometry.
• Figure 3: Photometric redshift precision as a function of Euclid $J$-band magnitude for galaxies at two fiducial redshifts ($z_{\rm true}=1.2$ and 2.0). In each panel, the vertical axis shows the redshift uncertainty $\sigma_z/(1+z)$ in logarithmic scale, derived from bootstrap resampling of noisy synthetic photometry. Colors represent different SFR scaling factors relative to the star-forming main sequence, while marker shapes encode different stellar masses. Dashed lines correspond to fits using only broad-band data (CSST + Euclid), whereas solid lines include the 14 JPCam narrow-band filters that bracket redshifted [OII] and the Balmer break at $z\sim1.0-1.4$. The two horizontal lines denote the corresponding velocity measurement accuracy of $v=100\rm~km~s^{-1}$ and $v=1000\rm~km~s^{-1}$.
• Figure 4: Star-forming main sequence and SFR factor tracks at $z\simeq1.2$. The solid line shows the SF main sequence with the SFR factor $f_{\rm SFR}=\mathrm{SFR}/\mathrm{SFR_{MS}}=1$, while dashed curves correspond to SFR factors $f_{\rm SFR}=0.2,\,0.5,\,2,$ and $5$. The left axis gives the SFR, and the right axis shows the corresponding [OII] luminosity $L_{\rm [O\,II]}$ linearly scaled to the SFR via Eq. \ref{['eq:OIISFR']}.
• Figure 5: Predicted detectability of extended [OII] CGM emission in JPCam narrow-band imaging at $z\simeq1.2$. (a) Radial [OII] surface brightness profiles (solid curves; left axis) and the corresponding single galaxy S/N (dashed curves; right axis) for galaxies with $\mathrm{SFR}=1,\,2,\,5,\,10,\,20,\,50,$ and $100~\mathrm{M_\odot~yr^{-1}}$. We adopt the JPCam J0820 filter ($\sim10\rm~h$ exposure, $m_{\rm AB,lim}\simeq24.6$ at $\rm S/N\simeq5$), a [OII]-SFR conversion using Eq. \ref{['eq:OIISFR']}, and simply assume that a fraction $f_{\rm CGM}=0.3$ of the total [OII] luminosity emerges in an exponential CGM with scale radius $r_{\rm s}=30~\mathrm{kpc}$. A PSF of $\sim1^{\prime\prime}$ has been adopted. Both axes are shown on a logarithmic scale. (b) Number of galaxies required to stack, $N_{\rm req}(r)$, in order to reach a target $\mathrm{S/N}=3$ in the azimuthally averaged [OII] radial profile, for the same set of SFRs. Vertical dashed lines mark $r=50$ and $100~\mathrm{kpc}$, respectively.