---
title: 'SIDES: Simulated Infrared Extragalactic Sky'
url: https://www.emergentmind.com/topics/simulated-infrared-extragalactic-sky-sides
type: topic
---

# SIDES: Simulated Infrared Extragalactic Sky

Searching arXiv for SIDES and closely related simulated infrared extragalactic sky papers.
SIDES is a public, empirically calibrated simulation framework for the dusty extragalactic sky, originally introduced as the **“Simulated Infrared Dusty Extragalactic Sky”** and later described in updated work as the **“Simulated Infrared Extragalactic Dusty Sky.”** Its defining objective is to connect halo-based galaxy populations, infrared spectral energy distributions, clustering, beam-convolved maps, and source-extraction procedures in the far-infrared to millimeter regime, where confusion and blending make observed catalogues differ substantially from intrinsic source populations [1703.08795][2509.15331].

## 1. Original formulation and physical construction

The canonical SIDES formulation was introduced to address a concrete observational tension: single-dish far-IR/submm surveys and high-angular-resolution interferometric follow-up do not yield the same number counts, because limited angular resolution blends several galaxies into one source. The original simulation therefore aimed to model the entire chain from galaxy evolution prescriptions to clustered sky maps and extracted catalogues. It covers **2 deg\(^2\)** over **\(0<z<10\)** and spans **70 \(\mu\)m to 1.2 mm** [1703.08795].

Its dark-matter backbone is the **Bolshoi-Planck** simulation, with box size \((250\, h^{-1}\,\mathrm{Mpc})^3\), particle mass \(1.5\times 10^8\, h^{-1} M_\odot\), and halos identified with **Rockstar**. SIDES constructs a **\(1.4^\circ \times 1.4^\circ\)** lightcone, corresponding to a comoving volume of **\(0.17\,\mathrm{Gpc}^3\)**, and uses periodic replication with a line of sight rotated by \(10^\circ\) about the \(y\)-axis and \(10^\circ\) about the \(z\)-axis to reduce repeated structure. Galaxies are assigned to halos and subhalos through **subhalo abundance matching** using the peak circular velocity \(v_{\rm pk}\), with an intrinsic scatter of **0.2 dex** in stellar mass at fixed \(v_{\rm pk}\). This is the mechanism by which SIDES inherits realistic large-scale structure and clustering rather than random sky positions [1703.08795].

On top of that halo lightcone, SIDES applies an updated **2SFM** population model. The stellar mass function is redshift dependent and parameterized as a double Schechter form. Galaxies are first split into passive and star-forming populations through a mass- and redshift-dependent star-forming fraction. Star-forming systems are then divided into **main-sequence** and **starburst** modes, with the starburst fraction set to **1.5% at \(z=0\)**, rising linearly to **3% at \(z=1\)** and remaining constant thereafter. The SFR distribution is lognormal, centered on the evolving main sequence for normal systems and on **\(5.3\,\mathrm{SFR}_{\rm MS}\)** for starbursts, with **0.3 dex** scatter in practice for both populations. SIDES also imposes a hard cap,
\[
\mathrm{SFR}<1000\,M_\odot\,\mathrm{yr}^{-1},
\]
motivated by interferometric follow-up showing that apparently extreme single-dish sources frequently deblend into multiple components [1703.08795].

Infrared luminosities are assigned through the Kennicutt conversion adjusted to a Chabrier IMF,
\[
\mathrm{SFR}=10^{-10}\,L_{\rm IR},
\]
with \(L_{\rm IR}\) in \(L_\odot\). SEDs are drawn from the **Magdis et al. (2012)** template library and depend on star-formation mode and the mean radiation field \(\langle U\rangle\). For main-sequence galaxies SIDES adopts
\[
\log_{10}\big[\langle U_{\rm MS}\rangle(z)\big]=\log_{10}\big[\langle U_{\rm MS}\rangle(z=0)\big]+\alpha_{\langle U\rangle}z,
\]
with \(\langle U_{\rm MS}\rangle(z=0)=5\) and \(\alpha_{\langle U\rangle}=0.25\), while starbursts use \(\langle U_{\rm SB}\rangle=31\). A **0.2 dex** scatter in \(\langle U\rangle\) is added. Lensing is included probabilistically, with strong lensing drawn from Hezaveh et al. and weak lensing approximated from Hilbert et al., but not tied self-consistently to the simulated foreground mass distribution [1703.08795].

## 2. Map-making, confusion, and the reinterpretation of observed counts

A distinguishing feature of SIDES is that it does not stop at intrinsic galaxy catalogues. It produces beam-convolved maps in Herschel/PACS, Herschel/SPIRE, SCUBA-2-like, AzTEC-like, and NIKA2-like bands, and then runs source extraction in a manner intended to mimic observational practice. Herschel/PACS/SPIRE catalogues are extracted with **FASTPHOT** using **24 \(\mu\)m positions as priors**, retaining only the brightest 24 \(\mu\)m source within **0.5 FWHM** to limit degeneracies, whereas **850 \(\mu\)m** and **1.2 mm** sources are extracted blindly above **\(5\sigma\)** and deboosted following Geach et al. The SPIRE confusion noise measured in the simulated maps is **6.0 mJy at 250 \(\mu\)m**, **6.5 mJy at 350 \(\mu\)m**, and **6.0 mJy at 500 \(\mu\)m**, consistent within \(2\sigma\) with Nguyen et al. [1703.08795].

The central scientific result is that limited angular resolution fundamentally reshapes observed continuum counts. SIDES shows that at **350 and 500 \(\mu\)m**, Herschel number counts between **5 and 50 mJy** are biased high by about a **factor 2**, and that the corresponding redshift distributions are biased toward low \(z\). At **500 \(\mu\)m**, the brightest galaxy contributes only about **58–60%** of the measured flux, whereas in a NIKA2-like **1.2 mm** survey the brightest galaxy contributes about **95%**. This is why intrinsic counts match interferometric measurements while source-extracted counts from beam-smoothed maps match single-dish measurements: the two observing modes are probing different effective source definitions [1703.08795].

SIDES also quantifies how clustering propagates into standard analysis products. The relative stacking excess,
\[
\frac{S_{\rm stack}-\langle S_{\rm cat}\rangle}{\langle S_{\rm cat}\rangle},
\]
is **13 ± 1%** at **250 \(\mu\)m**, **21 ± 1%** at **350 \(\mu\)m**, and **34 ± 1%** at **500 \(\mu\)m**. In the SPIRE pixel histogram \(P(D)\), clustering changes the histogram by less than about **5%** at **250 \(\mu\)m** and by up to **15%** at **350 and 500 \(\mu\)m**. The shot-noise level is written as
\[
\sigma_{\rm Poisson}^2=\int_0^{S_{\nu,\rm cut}} S_\nu^2 \frac{d^2N}{dS_\nu\,d\Omega}\,dS_\nu,
\]
and SIDES shows that previously reported discrepancies between measured CIB shot noise and shot noise inferred from observed SPIRE counts can be resolved once one uses intrinsic rather than blend-boosted counts [1703.08795].

The same framework also reinterprets “red Herschel sources.” For the colour and flux cuts used by Asboth et al., the intrinsic clustered **2 deg\(^2\)** SIDES catalogue contains **no** such source, while a larger **274 deg\(^2\)** simplified unclustered catalogue contains only **18** intrinsically selected objects, all strongly lensed. After beam smoothing and noise, however, the simulation yields **\(1.7_{-0.9}^{+1.9}\)** red detections in **2 deg\(^2\)**, scaling to **\(229_{-121}^{+258}\)** over **274 deg\(^2\)**, consistent within \(1\sigma\) with the observed **477** detections. The implication stated in the paper is that the apparent excess can be an observational artifact produced by noise, confusion, weak lensing, and steep underlying colour and flux distributions [1703.08795].

## 3. Spectroscopic SIDES and the pySIDES implementation

The 2022 CONCERTO-oriented extension transformed SIDES from a continuum-focused dusty-sky simulation into a spectroscopic sub-mm/mm framework that includes the dominant lines around **1 mm**: **[\(\mathrm{CII}\)] 158 \(\mu\)m**, the **CO rotational ladder**, and **[CI]**. It uses the original SIDES halo lightcone and semi-empirical population model, but adds line-emission prescriptions, a spectral-cube generator, and a full public Python rewrite, **pySIDES** [2204.12827].

For **[\(\mathrm{CII}\)]**, the default model adopts the De Looze et al. relation,
\[
\log_{10}\left(\frac{L_{\rm [CII]}^{DL14}}{L_\odot}\right)=7.06+\log_{10}\left(\frac{\rm SFR}{M_\odot\,yr^{-1}}\right),
\]
with an intrinsic scatter of **0.2 dex** and no duty cycle, so every star-forming SIDES galaxy emits according to the adopted relation. An alternative Lagache et al. prescription is also implemented. For CO, the simulation derives \(L'_{\rm CO(1-0)}\) from \(L_{\rm IR}\) using the Sargent et al. relation, adds **0.2 dex** scatter, and lets the CO SLED vary from galaxy to galaxy through the radiation field parameter \(\langle U\rangle\). For [CI], empirical relations are calibrated from the Valentino et al. compilation, with **0.2 dex** scatter for [CI](1–0) and **0.19 dex** scatter for [CI](2–1) [2204.12827].

Mock **CONCERTO-like** cubes are generated over **125–305 GHz**, with **1 GHz** spectral channels, **5 arcsec** angular pixels, and the underlying **\(1.4\times1.4\) deg\(^2\)** SIDES field. The total cube is the sum of continuum and all line cubes. The released products are designed for both intensity mapping and sub-mm continuum and line surveys, and the paper is explicit that the code and products are public [2204.12827].

Validation is broad. The earlier SIDES continuum model was already shown to reproduce mid-IR to mm number counts, redshift distributions, Herschel map pixel histograms, and CIB power spectra. The spectroscopic extension adds validation against CO luminosity functions from COLDz and ASPECS/PdBI, [CI] luminosity functions from ASPECS, [\(\mathrm{CII}\)] luminosity-function constraints from ALPINE, and **mmIME** shot noise near **100 GHz**. The paper states that SIDES is **always within the 1-\(\sigma\)** observational ranges for the CO luminosity functions shown, and predicts an average total CO+[CI] shot noise of
\[
0.059\ {\rm Jy}^2\,{\rm sr}^{-1}\,{\rm GHz}^{-1},
\]
consistent with the mmIME measurements quoted in the paper [2204.12827].

The main forecasting result is that **[\(\mathrm{CII}\)] intensity mapping at \(z\sim 6\)** is highly uncertain and strongly foreground limited. Depending on the adopted \([\mathrm{CII}]\)-SFR relation and high-\(z\) star-formation history, the predicted **\(z\sim 6\)** [\(\mathrm{CII}\)] power spectrum varies by **about two orders of magnitude**. The continuum is the brightest component by a factor of **3 to 100** depending on frequency, and at **300 GHz** the **CO foreground power spectrum is higher than the [\(\mathrm{CII}\)] one** for the base scenario. Masking galaxies detectable in deep surveys can reduce foregrounds to about **20% of the [\(\mathrm{CII}\)] power spectrum up to \(z\sim 6.5\)**, but the paper states that this strategy will not be sufficient at higher redshift [2204.12827].

## 4. JWST/MIRI-era SIDES: stellar light, quiescent galaxies, and obscured AGN

A 2025 update explicitly reworks SIDES for the **JWST/MIRI** regime. The paper keeps the original population backbone of pySIDES—Bolshoi-Planck halos, abundance matching via \(v_{\rm pk}\), and the 2SFM division into quiescent, main-sequence, and starburst populations—but modifies the SED and AGN treatment so that the simulation can reproduce MIRI number counts, redshift distributions, and AGN fractions [2509.15331].

The most important change is that main-sequence and starburst templates no longer consist only of dust emission. The updated model adds stellar emission using **SWIRE** templates: **Sd** for main-sequence galaxies and **M82** for starbursts. Quiescent galaxies, which were omitted from the original far-IR SIDES because they were assumed irrelevant for far-IR counts, are now added explicitly using the **Ell5** template from the SWIRE library, normalized through a redshift-dependent stellar mass-to-\(J\)-band light ratio. AGN and composite systems are introduced using the **Kirkpatrick et al. (2015)** template library, binned by the mid-IR AGN fraction \(f_{\rm AGN,MIR}\), with additional SWIRE-based stellar components stitched onto the templates at wavelengths chosen not to alter the \(8\)–\(1000\,\mu\mathrm{m}\) normalization [2509.15331].

The AGN contribution is assigned probabilistically. The total IR AGN fraction is defined as
\[
f_{\rm AGN}=\frac{L_{\rm IR,AGN}}{L_{\rm IR,tot}},
\qquad
L_{\rm IR,tot}=L_{\rm IR,AGN}+L_{\rm IR,SF},
\]
with the accretion-disk luminosity written as
\[
L_{\rm disk}=\epsilon\,\dot M\,c^2,
\]
using \(\epsilon=0.11\), and the IR AGN luminosity parameterized as \(L_{\rm IR,AGN}=\alpha L_{\rm disk}\). The paper tests \(\alpha=0.3,\ 0.5,\ 1.0\) and finds that **\(\alpha=0.5\)** gives the best agreement with observed MIRI AGN fractions, which is then adopted as the fiducial model. The AGN fraction is capped at **\(f_{\rm AGN}=0.9\)** [2509.15331].

This revised SIDES reproduces observed integral number counts across **8 of the 9 MIRI imaging filters**, with **F1130W omitted** because constraints were sparse. It also preserves SIDES’s earlier successes at Herschel wavelengths. The scientific picture reported in the paper is that **main-sequence galaxies dominate the faint end** of the MIRI counts, while **AGN dominate at bright flux densities \((S_\nu \gtrsim 20\,\mu{\rm Jy})\)**. The paper further notes that the brightest MIRI AGN at roughly **\(20\)–\(50\,\mu{\rm Jy}\)** and above are likely AGN-dominated, whereas fainter AGN-selected sources are more often composites. The abstract also states that surveys below **\(25\,\mathrm{arcmin}^2\)** suffer from **\(\sim 30\%\)** uncertainty in bright AGN counts because of cosmic variance [2509.15331].

## 5. SIDES as a survey-forecasting platform: PRIMAger and polarized confusion

A 2024 PRIMAger study uses the **2 deg\(^2\)** SIDES realization as the astrophysical sky model for a baseline assessment of confusion in both intensity and polarization. In this application SIDES supplies the source positions, flux densities, redshifts, stellar masses, and clustering; polarization is then added through a phenomenological extension rather than a native SIDES prediction [2404.04320].

The simulated PRIMAger maps include only confusion noise, with no instrumental noise, no photon noise from diffuse backgrounds, and no Galactic cirrus in the baseline case. Gaussian beams are adopted, with FWHM from **4.1 arcsec at 25 \(\mu\)m** to **27.6 arcsec at 235 \(\mu\)m**. The classical confusion variance is written as
\[
\sigma_{\rm conf}^2=\int \int b^2 d\Omega \times \int_0^{S_{\rm lim}} S^2 \frac{dN}{dS}\, dS,
\]
but the paper estimates the confusion level directly from maps by iterative clipping and then defines the detection threshold as
\[
S_{\rm det}=5\sigma_{\rm conf}.
\]
For polarization, each SIDES galaxy is assigned an integrated polarization fraction drawn from a Gaussian with mean **\(\mu_p=1\%\)** and standard deviation **\(\sigma_p=0.3\%\)**, with a uniform polarization angle \(\alpha\in[0,\pi)\), and the Stokes parameters are defined by
\[
Q=pI\cos 2\alpha,\qquad U=pI\sin 2\alpha,\qquad P=\sqrt{Q^2+U^2}=pI.
\]
The maps are built in \(Q\) and \(U\) first, then converted to \(P\), because vector cancellation makes polarized confusion fundamentally different from intensity confusion [2404.04320].

The resulting confusion limits rise rapidly with wavelength in intensity: **0.021 mJy at 25.0 \(\mu\)m**, **4.6 mJy at 96.3 \(\mu\)m**, **12 mJy at 126 \(\mu\)m**, **28 mJy at 172 \(\mu\)m**, and **46 mJy at 235 \(\mu\)m**. In polarization, assuming mean \(p=1\%\), the confusion limits are **0.03 mJy at 96.3 \(\mu\)m**, **0.078 mJy at 126 \(\mu\)m**, **0.17 mJy at 172 \(\mu\)m**, and **0.25 mJy at 235 \(\mu\)m**. The paper states that polarization confusion is therefore **more than 100x lower** than intensity confusion and that simply multiplying intensity confusion by a mean polarization fraction is not adequate [2404.04320].

Because SIDES contains the full underlying source population, the PRIMAger study can identify the main counterpart as the brightest galaxy within the beam half-light radius. It finds that in intensity the brightest galaxy contributes **more than 91%** of the measured flux in the PHI bands, but only **0.72** in **PPI4**, so other objects contribute about **\(\sim 30\%\)** at **235 \(\mu\)m**. Clustering has a mild impact on intensity confusion, up to **25%**, and negligible impact in polarization under the assumptions of random polarization angles and no intrinsic alignments. The paper also states that a basic blind extraction is sufficient to detect galaxies at the knee of the luminosity function up to **\(z\sim 3\)** and **\(10^{11}\,M_\odot\)** main-sequence galaxies up to **\(z\sim 5\)**. In polarization, for a conservative sensitivity, the forecast is **\(\sim 8{,}000\)** detections up to **\(z=2.5\)** [2404.04320].

## 6. Related frameworks, boundaries of the SIDES concept, and persistent limitations

Several adjacent simulated-sky frameworks are often compared with SIDES, but those comparisons are sometimes explicitly inferential because the papers do not always mention SIDES directly. The central distinction is whether the simulation is designed primarily as a **dusty far-IR/submm extragalactic sky** or for a different wavelength regime and science case [2501.05632][2011.07074][2001.08787][1002.1112].

| Framework | Main regime | Relation to SIDES |
|---|---|---|
| OpenUniverse2024 | Optical to near-IR Rubin/Roman sky | Matched multi-survey synthetic sky, not a far-IR/submm dusty-sky simulation |
| SPRITZ | Panchromatic UV-to-radio/X-ray, IR-LF anchored | Conceptually similar in goal, but luminosity-function-first and strongly AGN-aware |
| Websky | Full-sky microwave/CIB with tSZ, kSZ, lensing | Complementary CMB-oriented framework with a CIB component, not a detailed IR-galaxy mock |
| Wilman et al. (2010) IR-radio simulation | Mid-IR to sub-mm extension of a radio sky | Precursor and related semi-empirical mock, not the canonical SIDES model |

OpenUniverse2024 is particularly important as a boundary case. The paper itself states that it should be understood as a matched Rubin/LSST and Roman synthetic sky with improved optical-to-near-infrared galaxy realism, not as a dedicated far-IR/submm extragalactic background simulation in the style of SIDES. SPRITZ, by contrast, is explicitly SIDES-adjacent: it starts from Herschel population-separated IR luminosity functions and builds a panchromatic spectro-photometric mock including AGN, low-resolution IR spectroscopy, radio properties, and X-ray properties, but it is **luminosity-function-first** rather than halo-lightcone-first. Websky is complementary rather than equivalent: it simulates the full extragalactic microwave sky for CMB analyses, including a CIB/DSFG component, but emphasizes full-sky maps and cross-correlations with tSZ, kSZ, and lensing rather than detailed infrared galaxy-population realism. The Wilman et al. infrared-radio simulation is best regarded as a precursor in the lineage of semi-empirical extragalactic sky simulations rather than as SIDES itself [2501.05632][2011.07074][2001.08787][1002.1112].

Within SIDES proper, several limitations recur across releases. In the original continuum framework, the clustered simulation covers only **2 deg\(^2\)**, environmental quenching is not modeled explicitly, lensing is probabilistic rather than spatially self-consistent, high-\(z\) SMF, sSFR, and SED behavior are extrapolated, halos below **\(10^{10}\,M_\odot\)** are absent, and the original continuum model omitted AGN mid-IR contributions. The spectroscopic cubes omit instrumental noise and detailed line profiles in the main forecast products. The MIRI update imports AGN fractions statistically from **SC SAM** rather than modeling black-hole growth within pySIDES itself. The PRIMAger polarization forecasts rely on a simple integrated polarization prescription with no wavelength dependence, no AGN-specific treatment, and no intrinsic alignment of polarization angles. These limitations do not negate the utility of SIDES; rather, they define the framework’s proper domain as an empirically anchored, survey-oriented family of mock skies in which map-making, confusion, and selection effects are treated as first-order scientific variables rather than post-processing details [1703.08795][2204.12827][2509.15331][2404.04320].

Source: https://www.emergentmind.com/topics/simulated-infrared-extragalactic-sky-sides