---
title: Intermediate Band Imaging Survey (IBIS)
url: https://www.emergentmind.com/topics/intermediate-band-imaging-survey-ibis-3e38f234-18e7-46fb-b59d-2ce22a94a613
type: topic
---

# Intermediate Band Imaging Survey (IBIS)

Searching arXiv for recent papers on the Intermediate Band Imaging Survey (IBIS).
arXiv search: "Intermediate Band Imaging Survey"
The **Intermediate Band Imaging Survey (IBIS)** is a wide-field medium-band survey on **DECam at the Blanco telescope in Chile**, designed for the selection of high-redshift tracers, especially **Lyman-\(\alpha\) emitters (LAEs)**, in support of spectroscopic programs such as **DESI-II**. Its observing strategy combines intermediate-width filters, with \(\Delta\lambda \approx 250\,\text{\AA}\), and broad-band imaging from Subaru, HSC, and Rubin/LSST to isolate spectral breaks and emission-line excesses across the high-redshift interval \(2.3<z<3.5\). In current work, IBIS functions both as a survey for target construction and as a cosmological data set capable of supporting tomographic angular clustering and angular BAO analyses at \(z\gtrsim 2\) [2509.26467] [2512.06568].

## 1. Survey architecture and observational definition

IBIS is described as a **medium-band imaging survey on DECam at Blanco**. In the XMM-LSS field used for clustering validation, it provides **five adjacent medium bands spanning roughly \(4000<\lambda<5300\,\)Å**, each about **\(260\) Å** wide, and reaches a **\(5\sigma\) depth of \(\sim 25.5\) mag** in the deep field. More generally, the survey concept uses intermediate-width filters with \(\Delta\lambda \approx 250\,\text{\AA}\), together with broad-band Subaru and Rubin/LSST imaging, to isolate both continuum breaks and line excesses [2509.26467].

Published descriptions reflect more than one survey configuration. The clustering analysis describes IBIS as **planned to cover about \(3000\,\deg^2\)**, whereas the angular BAO forecasts evaluate a **fiducial \(5000\,\deg^2\)** LAE survey and then extend the same methodology to larger Stage-V configurations. This suggests that the literature distinguishes between the currently characterized survey and forecast survey realizations used for cosmological performance studies [2509.26467] [2512.06568].

| Quantity | Reported value | Context |
|---|---:|---|
| Instrument platform | DECam at Blanco | Survey implementation |
| Medium-band width | \(\Delta\lambda \approx 250\,\text{\AA}\) | General survey concept |
| XMM-LSS bands | 5 adjacent bands, \(4000<\lambda<5300\,\)Å | Clustering validation field |
| Deep-field depth | \(5\sigma \sim 25.5\) mag | XMM-LSS field |
| Stated footprint | \(\sim 3000\,\deg^2\) | Planned survey in clustering study |
| Fiducial BAO footprint | \(5000\,\deg^2\) | Forecast configuration |

The central observational rationale is that medium-band imaging occupies an intermediate regime between broadband photometry and spectroscopy. It yields photometric redshift information precise enough for **tomographic angular BAO**, while remaining wide-field enough to construct dense target samples for later spectroscopic use. At the same time, the photo-\(z\) precision is stated to be insufficient for **full 3D anisotropic BAO reconstruction**, which defines both the power and the limitation of the IBIS design [2512.06568].

## 2. Target selection and tracer populations

IBIS is intended to construct dense samples of high-redshift galaxies for large-scale-structure work, with emphasis on **LAEs** and, in practice, a broader population that includes **Lyman break galaxies (LBGs) with emission lines**. The target-selection strategy in the clustering analysis is explicitly exploratory and combines several medium-band color criteria: one class selects a flux peak in a central medium band relative to the two adjacent bands, another uses a synthetic \(g\)-band to identify Ly\(\alpha\) excess, and a third compares one band to a synthetic continuum built from the other four medium bands. These selections define redshift slices of width \(\Delta z\sim 0.2\) across **\(2.26<z<3.41\)** [2509.26467].

Spectroscopic validation is performed with **DESI ancillary spectroscopy**, which is used primarily to determine the true redshift distribution \(dN/dz\) and the interloper fractions \(f_{\rm int}\). Of the whole target sample, about **48% receive secure DESI redshifts**, sufficient to characterize the redshift distribution. The measured interloper fractions are described as generally modest, and for the higher-\(z\) slices the color cuts are refined to reduce interlopers from \(\lesssim 0.4\) to \(\lesssim 0.15\), at the cost of roughly halving the sample [2509.26467].

A central result of the validation work is that the IBIS-selected sample is **not a pure LAE sample**. Instead, the modeling indicates that it is composed of an **overlapping mixture of LAEs and LBGs with emission lines**. In the \(z\approx 3\) sample, about **46.1%** of the medium-band-selected galaxies are also recovered in a traditional \(u\)-dropout LBG catalog. The sample therefore occupies the continuum between LAE and LBG selection rather than defining a disjoint tracer class. This is important for interpreting both its clustering amplitude and its utility for future spectroscopic target selection [2509.26467].

The BAO forecast work adopts a more idealized tracer definition. There, the fiducial IBIS forecast is based on a **pilot-survey-informed LAE sample** spanning **\(2.3<z<3.4\)**, divided into five redshift bins defined by medium-band filters: roughly \([2.26,2.56]\), \([2.47,2.77]\), \([2.68,2.98]\), \([2.89,3.20]\), and \([3.10,3.41]\), with mean redshifts \(z_{\rm mean}\approx 2.41, 2.62, 2.83, 3.05,\) and \(3.26\). The forecast uses **LAEs rather than LBGs** because LBGs have broader redshift distributions that would smear the BAO feature too strongly after projection [2512.06568].

## 3. Clustering measurements and halo interpretation

Because DESI fiber assignment in the ancillary program is not random and introduces complicated angular selection effects, the current IBIS analysis does not attempt a conventional full 3D clustering measurement. Instead, it measures **angular clustering** within the narrow medium-band shells and projects the signal into \(w(\theta)\). The basic relation is written as
\[
w(\theta)=\int d\chi_1\,d\chi_2\,f(\chi_1)f(\chi_2)\, \xi\!\left(\sqrt{\chi_1^2+\chi_2^2-2\chi_1\chi_2\cos\theta}\right),
\]
with the narrow-shell approximation
\[
w(\theta)\approx \left[\int d\chi\,f^2(\chi)\right] \int d\Delta\,\xi\!\left(\sqrt{\chi_0^2\widetilde{\omega}^2+\Delta^2}\right)=\mathcal{L}^{-1}w_p(R=\chi_0\widetilde{\omega}),
\]
and the estimator
\[
w(\theta)=\frac{DD-2DR+RR}{RR}.
\]
This formalism allows the target sample to be used directly while avoiding the full complexity of DESI fiber assignment [2509.26467].

The observed clustering is detected clearly in the **\(z\sim 3\)** sample and in the **total \(z\sim 2.5\)** sample, while the wide \(z\sim 2.5\) sample is only marginally detected. Using a power-law real-space correlation function,
\[
\xi(r)=\left(\frac{r_0}{r}\right)^\gamma,\qquad \gamma=1.8,
\]
the analysis infers **\(r_0\sim 3\)–\(4\,h^{-1}\) Mpc**, with values around **\(3.1\)–\(4.1\,h^{-1}\) Mpc**, and a corresponding **linear bias \(b\sim 1.8\)–\(2.5\)**. These values are reported as consistent with earlier LAE and faint LBG measurements [2509.26467].

The physical interpretation is developed with both **HOD modeling** and **one-loop Eulerian perturbation theory**. The HOD model adopts the standard five-parameter form
\[
\langle N_{\rm cen}(M)\rangle=\frac{1}{2}\,\mathrm{erfc}\!\left(\frac{\ln(M_{\rm cut}/M)}{\sqrt{2}\sigma}\right),
\]
\[
\langle N_{\rm sat}(M)\rangle=\langle N_{\rm cen}(M)\rangle \left(\frac{M-\kappa M_{\rm cut}}{M_1}\right)^\alpha,\qquad M>\kappa M_{\rm cut}.
\]
The best-fitting models are reported to give good fits, generally within about **\(1\sigma\)**, with **satellite fractions \(f_{\rm sat}\sim 0.03\)–\(0.12\)**. The inferred halo occupation suggests that the \(z\sim 2.5\) sample resembles typical LAEs, whereas the \(z\sim 3\) sample leans more toward **LBG-like halo masses around \(\sim 10^{12}\,h^{-1}M_\odot\)** [2509.26467].

The perturbative description uses **velocileptors** and writes the galaxy field as
\[
\delta_g \approx b_1\delta + \frac{b_2}{2}\delta^2 + b_s s^2,
\]
with counterterms and stochastic terms added in the power spectrum. The fitted linear biases are around **\(b_1\simeq 1.8\)** at \(z=2.5\) and **\(b_1\simeq 2.4\)–\(2.6\)** at \(z=3\). The analysis reports only modest scale dependence at \(z=2.5\), somewhat stronger scale dependence at \(z=3\), and good agreement between auto-bias and cross-bias, implying that the IBIS-selected galaxies behave as well-defined biased tracers rather than as a pathological selection [2509.26467].

## 4. Angular BAO formalism and forecast performance

The principal cosmological use case developed for IBIS is **angular BAO**. Because medium-band photo-\(z\) uncertainties remove most radial information, the forecast is built from **angular power spectra** \(C_\ell^{ij}\) between redshift bins \(i\) and \(j\), rather than from full anisotropic 3D clustering. The BAO signal is parameterized by the dilation parameter \(\alpha\), with
\[
P_{\rm m}(k,z;\alpha)=P_{\rm nw}(k,z)+P_{\rm w}\!\left(\frac{k}{\alpha},z\right),
\]
\[
\alpha=\frac{(D_A(z)/r_d)}{(D_A(z)/r_d)_{\rm fid}},
\]
and
\[
\ell_{\rm BAO}\approx \chi(z)/r_d.
\]
Smooth broadband residuals are marginalized using cubic spline nuisance terms,
\[
\mathcal{D}_\ell^{ij}\approx \sum_m \beta_m^{ij}S_m(\ell),
\]
with knot spacing **\(\Delta \ell_{\rm spline}=350\)**, while the Gaussian covariance includes shot noise
\[
N_\ell^{ij}=\delta_{ij}/\bar n_i^{2D}.
\]
The analysis is performed in the **flat-sky/plane-parallel approximation** rather than the Limber approximation because the bin widths are only a few times the BAO scale [2512.06568].

The fiducial tracer model is anchored to the pilot survey analysis of Ebina et al. The LAE bias is assumed to rise smoothly from **\(b=2.0\) to \(b=2.5\)** across the redshift range. The comoving number density is varied over \(\bar n=(1.5\text{--}3.0)\times10^{-4}\,h^3\,\mathrm{Mpc}^{-3}\), corresponding to projected surface densities of about **625–1250 \(\deg^{-2}\)** for the full sample, or **125–250 \(\deg^{-2}\)** per redshift bin. Interloper-free baseline forecasts are supplemented with a more realistic case in which **\(f_{\rm int}=10\%\)** [2512.06568].

The headline forecast is that a **\(5000\,\deg^2\)** IBIS-like LAE survey can constrain \(\alpha\) at **\(z_{\rm eff}=2.8\)** with **\(2.6\%\)** precision for a tracer density of **\(2\times10^{-4}\,h^3\,\mathrm{Mpc}^{-3}\)** and **\(f_{\rm int}=10\%\)**. In the no-interloper baseline, the forecasts show that for **\(A_{\rm surv}\ge 2000\,\deg^2\)** and \(\bar n_{\rm LAE}\gtrsim 625\,\deg^{-2}\), the BAO position can already be recovered to **better than \(5\%\)**, while for the **\(5000\,\deg^2\)** footprint the precision falls below **\(3\%\)**, and at the highest assumed density **\(\bar n_{\rm LAE}=1250\,\deg^{-2}\)** it reaches **\(1.6\%\)**. Marginalizing over the spline broadband degrades \(\sigma(\alpha)\) by only about **\(2.5\%\)** overall, and the study finds that including multipoles up to **\(\ell_{\rm max}\sim 1000\)** captures most of the BAO information [2512.06568].

A key methodological conclusion is that these BAO forecasts are **shot-noise dominated**. The precision scales approximately as
\[
\sigma(\alpha_{\rm BAO}) \propto A_{\rm surv}^{-1/2}\,\bar n^{-1/2},
\]
and is therefore more sensitive to increases in area and tracer density than to moderate changes in bias. Interlopers are modeled as a suppression of the signal,
\[
C_\ell \to C_\ell(1-f_{\rm int})^2,
\]
so contamination primarily worsens the ratio of shot noise to recovered clustering signal rather than inducing a qualitatively different regime [2512.06568].

## 5. Role in DESI-II and Stage-V survey strategy

IBIS is framed not only as a source of imaging-based target selection, but also as a **precursor cosmology survey**. For **DESI-II**, which is expected to target LAEs and LBGs over **\(2.3<z<3.5\)**, IBIS is identified as a central imaging component alongside Subaru and Rubin imaging. In this role, early angular BAO measurements from IBIS can calibrate selection functions, identify the redshift bins and tracers with the best leverage, and expose potential systematics before the full spectroscopic analyses are carried out [2512.06568].

The same logic is extended to **Spec-S5** and later Stage-V configurations. The forecast study considers a Spec-S5-like medium-band program with **seven filters spanning roughly \(3750<\lambda<5500\) Å**, corresponding to **\(2.1<z<3.5\)**, and a target LAE density of **\(3000\,\deg^{-2}\)** over **\(11000\,\deg^2\)**. Under those assumptions, the combined seven-bin angular BAO measurement reaches about **\(0.7\%\)** precision on \(\alpha\). The paper also considers a longer-wavelength HSC-like extension to **\(3.5<z<5.2\)** and finds that combined constraints can beat **\(3\%\)** for survey areas above **\(2000\,\deg^2\)** if tracer densities remain sufficient [2512.06568].

Within this framework, IBIS serves as a demonstrator for a broader observational strategy: medium-band imaging can be used not merely to feed spectroscopy, but to extract useful cosmological information before the spectroscopic program is complete. A plausible implication is that IBIS-like imaging can reduce survey-design uncertainty by testing tracer density, interloper control, binning choices, and BAO observability directly on the imaging sample itself [2512.06568].

## 6. Limitations, systematics, and simulation requirements

The most explicit limitation of the IBIS approach is radial resolution. The forecast study states that medium-band photo-\(z\) errors are **not good enough for full 3D anisotropic BAO reconstruction**, and that **BAO reconstruction is not attempted for IBIS** because the photometric redshift errors remove radial modes and limit displacement-field estimation. The angular analysis therefore trades away radial information in exchange for robustness in projected clustering [2512.06568].

On the sample-construction side, the medium-band approach accepts somewhat higher contamination than traditional narrow-band LAE selection because spectroscopy can clean the sample later. The clustering study shows that interloper control is therefore a practical, not merely formal, part of the survey design: improving the higher-\(z\) selection from \(\lesssim 0.4\) to \(\lesssim 0.15\) materially sharpens the characterization of \(dN/dz\) and the subsequent clustering interpretation, even at the cost of roughly halving the sample [2509.26467].

The simulation requirements implied by IBIS-selected tracers are unusually demanding. Using AbacusSummit-based mocks, the clustering paper argues that the current simulations **barely have enough mass resolution** to host the halos implied by the data. To resolve the halos hosting most of the selected galaxies under the standard HOD form, halos down to about **\(\sim 0.2 M_{\rm cut}\)** are needed, which translates into particle masses of order **\(10^9\,h^{-1}M_\odot\)** or lower for the \(z\sim 3\) sample. A realistic survey-area mock over **\(\sim 1000\,\deg^2\)** at \(z=3\) would require a volume larger than **\(2\,h^{-1}\) Gpc** on a side and roughly **\(10^{12}\)** particles. This places mock generation and covariance control among the principal technical challenges for using IBIS-selected galaxies in precision cosmology [2509.26467].

Taken together, these constraints define the present status of the Intermediate Band Imaging Survey. IBIS already functions as an empirically validated medium-band selection program for high-redshift galaxies, with measured clustering consistent with \(r_0\simeq 3\)–\(4\,h^{-1}\) Mpc and \(b\sim 1.8\)–\(2.5\), while forecast work indicates that an IBIS-like survey can deliver **sub-\(3\%\)** angular BAO precision at \(z_{\rm eff}=2.8\) over a \(5000\,\deg^2\) footprint. Its scientific role is therefore dual: it is both a target-selection infrastructure for future spectroscopy and a high-redshift angular-clustering experiment in its own right [2509.26467] [2512.06568].

Source: https://www.emergentmind.com/topics/intermediate-band-imaging-survey-ibis-3e38f234-18e7-46fb-b59d-2ce22a94a613