---
title: Red-Herschel Sources in FIR Surveys
url: https://www.emergentmind.com/topics/red-herschel-sources
type: topic
---

# Red-Herschel Sources in FIR Surveys

Red-Herschel sources are a rare population identified via their distinct far-infrared (FIR) colors in Herschel/SPIRE data: rising flux densities through the 250, 350, and 500 μm bands—typically referred to as "500 μm-risers." These sources efficiently isolate dusty star-forming galaxies (DSFGs) at high redshifts (z ≳ 2–6), including protoclusters, galaxy overdensities, and lensed ultraluminous infrared galaxies (ULIRGs). Red-Herschel selection methods underpin extragalactic surveys tracing the earliest peaks of dust-embedded star formation, and form the observational backbone for galaxy evolution studies at cosmic noon and beyond.

## 1. Selection Criteria and Definitions

Red-Herschel sources are characterized by monotonically rising SPIRE flux densities, formalized as $S_{500} > S_{350} > S_{250}$, where $S_{250}$, $S_{350}$, $S_{500}$ are the measured flux densities at 250 μm, 350 μm, and 500 μm, respectively [1601.02665][1912.04354][2406.15729]. For practical catalog construction, additional thresholds are imposed:
- Minimum $S_{500}$: typically $> 52$ mJy for robust selection [1601.02665], or $> 30$ mJy for efficiency in smaller fields [1310.7583].
- No radio-loud AGN or blazar counterparts, established via cross-matches with FIRST/NVSS [1801.07266].
- Band-merged catalogs and confusion-noise suppression via matched filtering are standard, with selection often performed in a difference map, $D = 0.92M_{500} - 0.392M_{250}$, which reduces confusion to $\sigma_{\text{conf},D} \sim 3.5$ mJy [1601.02665].

Extensions to longer wavelengths (e.g., SCUBA-2 850 μm) identify "SPIRE dropouts," sources undetected in SPIRE but bright at 850 μm, indicative of $z\gtrsim 6$ [1912.04354].

The table summarizes principal selection criteria:

| Criterion                    | Typical Value/Range      | Context                              |
|------------------------------|-------------------------|--------------------------------------|
| SPIRE color                  | $S_{500} > S_{350} > S_{250}$ | Core definition                      |
| Flux cut ($S_{500}$)         | $> 52$ mJy, $> 30$ mJy, $> 20$ mJy | Varies by survey, field              |
| Confusion noise (D-map)      | $\sim 3.5$ mJy          | Matched-filter map                   |
| Ancillary AGN rejection      | NVSS/FIRST non-detection | Purity control                       |

These color criteria select galaxies whose FIR SEDs peak longward of observed wavelengths, a signature of cold, dust-enshrouded starbursts redshifted into the SPIRE regime.

## 2. Source Multiplicity, Deblending, and Physical Association

Herschel’s SPIRE beam ($\sim$18–36″ FWHM) blends multiple DSFGs, particularly in overdense regions or along lines of sight through massive halos. ALMA interferometric follow-up shows that:
- 73% of red-Herschel detections are single (point-like) sources at $\sim$1″ resolution; 20% are genuine multiples ($\geq$2 sources separated by $>$3″), and ~5% are candidate lenses or close mergers [2406.15729][2512.00822].
- In double/multiple systems, only 13% of doubles and 8% of triples are likely physically associated ($\Delta z < 0.01$), yet 47–67% of triple/quadruple systems contain at least one potentially associated pair [2512.00822].
- The brightest component generally dominates the flux, contributing 64% (doubles), 48% (triples), and 42% (quads).

This multiplicity analysis suggests that the enhanced SFRs typical of red-Herschel sources are primarily internally driven rather than the result of large-scale interactions, though the catalogs serve as potential proto-cluster targets [2512.00822].

## 3. Redshift, SED Properties, and Star Formation Activity

Red-Herschel sources have extreme infrared luminosities and high redshifts:
- ALMA+SPIRE SED modeling yields median $L_{\text{IR}} \sim 8.9 \times 10^{12}\ L_\odot$, SFR $\sim 1.3 \times 10^{3}\ M_\odot\,\text{yr}^{-1}$, and $M_{\text{gas}} \sim 4 \times 10^{11}\ M_\odot$ [2406.15729].
- The redshift distribution for single systems peaks at $z \approx 2.8$, lensed systems at $z \approx 3.3$, with detections out to $z \sim 6$ [2406.15729][1310.7583][1912.04354].
- Depletion times are short ($\sim 0.3$ Gyr), consistent with a starburst population.

For the brightest confirmed sources, SED fits return $T_d \sim 50$–65 K, $L_{\text{IR}} \sim 1$–$6 \times 10^{13} L_\odot$, and SFRs up to $10^4\ M_\odot\,\text{yr}^{-1}$ [1310.7583]. For protostellar sources (PACS Bright Red sources or PBRs), modified black-body fits to $\lambda \geq 70\,\mu$m yield dust temperatures $T_{MBB} = 16$–27 K, envelope masses $0.2$–$2\,M_\odot$, and luminosities $0.7$–$10\,L_\odot$ [1302.1203].

## 4. Spatial Density, Clustering, and Protoclusters

Extensive surveys quantify the sky density and clustering of red-Herschel sources:
- In HerMES/HeLMS, surface densities range from $1.7$–$8.2$ deg$^{-2}$ at $S_{500} \geq 52$–$20$ mJy [1601.02665][1912.04354]. At $S_{500} \geq 30$ mJy, densities are $3.3 \pm 0.8$ deg$^{-2}$ [1310.7583].
- Lensed ULIRGs are rare, comprising $<1$–$5$\% of surveyed fields, while $20$–$21$\% of fields host significant protocluster candidates, characterized by $>3\sigma$ galaxy or red-galaxy overdensities [2204.06752][1503.08773].
- Planck–Herschel studies find median overdensity contrasts of $\delta_{500} \sim 5$, often with $10$ SPIRE sources per structure, yielding aggregate SFRs up to $7 \times 10^{3}\ M_\odot\,\text{yr}^{-1}$ [1503.08773].

Stacked profiles indicate widely distributed star formation over projected Mpc scales at $z\sim2$; the highest-z protocluster peaks are both more intense and more extended than those found in lower-z cluster surveys [2204.06752].

## 5. Comparison to Galaxy Evolution Models and Implications

Observed red-Herschel number counts and source properties systematically exceed predictions from established backward-evolution models (Béthermin+, Franceschini+, Valiante+):
- Measured $dN/dS$ for red sources at $S_{500} \sim 60$ mJy exceed model predictions by factors of $10$–$20$ [1601.02665][1310.7583].
- Even invoking extreme lensing boosts, models do not reproduce the observed bright, high-z population [1310.7583].
- Simulations show that noise boosting (Eddington bias) and blending have significant effects, but corrections via Monte Carlo injection-recovery analyses can self-consistently match observed counts with phenomenological models (e.g., Gruppioni+13 luminosity function, SIDES) once confusion and selection biases are incorporated [1801.07266][1912.04354].

The abundance and properties of red-Herschel sources necessitate revisions to evolutionary scenarios, including the population of highly dust-obscured starbursts at $z \gtrsim 4$ and the role of gravitational lensing at high flux densities.

## 6. Lensing, Contamination, and AGN Fraction

Gravitational lensing is pervasive among the brightest red-Herschel sources:
- All galaxies with $S_{500} > 100$ mJy or $S_{1.3\,\text{mm}} \geq 13$ mJy are found to be gravitationally amplified [2406.15729].
- Statistical analyses reveal a $\sim3\times$ excess of foreground WISE/SDSS sources within $7''$ of red-Herschel fields, indicating weak or strong lensing in up to $\sim50$\% of bright DSFGs [1801.07266].
- Machine-learning classifiers trained on SPIRE fluxes (input: $S_{250}$, $S_{350}$, $S_{500}$) efficiently separate high-SFR lenses ($>100$ mJy) from protoclusters and normal star-formers [2204.06752].

Radio-loud AGN contamination is low ($\lesssim1.2\%$), and visual inspection of maps removes 10–15\% of severe blends [1912.04354].

## 7. Variants: PACS Red Sources and Low-Redshift Analogs

“Red” source selection is applied in other Herschel bands and contexts:
- PACS Bright Red Sources (PBRs) are defined by $\log_{10}[\lambda F_{\lambda,70}/\lambda F_{\lambda,24}]>1.65$, mapping to extreme Class 0 protostars in the Orion complex [1302.1203]. These sources exhibit SEDs peaking beyond 70 μm, cold dust envelopes, and high mass-infall rates.
- At low redshift ($z \leq 0.2$), optically “red” galaxies detected in SPIRE include both dust-reddened inclined spirals and passive ellipticals with external cold dust reservoirs; these form $\sim$4.2\% of massive submm detections [1511.08018].

Additionally, PACS 160 μm ("Red band") surveys reach deep flux thresholds and resolve up to 60% of the cosmic infrared background, with LIRGs dominating the counts in the $5$–$100$ mJy regime [1809.03990].

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Red-Herschel sources form a unique window into the dusty, star-forming universe at early epochs, constraining the evolutionary trajectory of massive galaxies, the relative contribution of obscured starbursts to cosmic star formation rates, and a critical testbed for cosmological structure formation models. Their selection, physical properties, and multiplicity are intimately tied to survey methodologies and instrumental capabilities, with ongoing ALMA/JWST follow-up promising refined measurements of clustering, lensing, and SFR activity across cosmic time.

Source: https://www.emergentmind.com/topics/red-herschel-sources