PAH Emission in Cluster Galaxies

Updated 12 November 2025

PAH emission in cluster galaxies is a set of mid-IR features from carbonaceous grains, signifying active star formation and interstellar conditions.
Observations using instruments like JWST/NIRCam and AKARI enable measurements of PAH luminosities and band ratios to assess dust grain properties.
Environmental processes such as ram-pressure stripping and quenching modulate PAH survival, offering insights into galaxy evolution in dense clusters.

Polycyclic aromatic hydrocarbons (PAHs) are small, carbonaceous dust grains responsible for prominent mid-infrared emission features in star-forming galaxies. PAH emission traces the physical conditions of the interstellar medium (ISM) and is intimately linked to star formation. In galaxy clusters, the properties and survival of PAHs are shaped by both internal mechanisms—such as star formation and feedback—and external cluster-specific processes including ram-pressure stripping, the hot intracluster medium (ICM), and environmental quenching. Recent advances with JWST and deep infrared spectroscopy have allowed precise spatially-resolved and statistical analyses of PAH emission in cluster environments, revealing both the universality and the environmental modulation of PAH carriers.

1. Observational Diagnostics of PAH Emission in Cluster Galaxies

PAH emission manifests as a set of broad features at key mid-IR wavelengths (notably at 3.3, 6.2, 7.7, 8.6, 11.3, and 17 μm). In clusters, these features are targeted via both broad- and medium-band imaging (e.g., AKARI/S7/L15, JWST/NIRCam F335M, F430M, MIRI F770W/F1130W) and low-resolution spectroscopy (e.g., Spitzer/IRS, PRIMA/FIRESS). Emission in the rest-frame 8 μm window, traced by AKARI/L15 at z∼0.8 or JWST/MIRI filters at lower redshift, is dominated by the 7.7 μm + 8.6 μm complexes, while the 3.3 μm feature is isolated via rest-frame medium band (F430M at z=0.3–0.4).

Key diagnostic metrics include:

Quantity	Definition/Role	Instrument/Band
$L_8$	$\nu L_\nu$ at rest 8 μm, PAH-dominated	AKARI L15, JWST MIRI
$L_{3.3}$	Integrated luminosity of 3.3 μm PAH feature	JWST F335M/F430M
$L_\mathrm{IR}$	Total IR luminosity (8–1000 μm), star formation proxy	Herschel, Spitzer
$R_\mathrm{PAH}$	Ratio $L_8/L_\mathrm{IR}$ , PAH strength per unit dust continuum	Field & cluster SFGs
$R_{3.3/7.7}$ etc.	PAH band ratios, inform on PAH size and ionization	JWST NIRCam/MIRI

Emission line ratios (e.g., PAH 7.7 μm/11.3 μm) also quantify PAH charge state and processing. Tracers of the local radiation field ([O III]/Hβ, Brα/Paα) and stellar population age (from SED fitting) are combined with PAH maps to disentangle feedback, metallicity, and environmental effects (Murata et al., 2015, Dale et al., 17 Jan 2025, Dale et al., 2022, Cheng et al., 26 Jun 2025, Benotto et al., 7 Nov 2025).

2. Environmental Trends and Spatial Distribution within Clusters

Cluster galaxies show a distinct radial and environmental dependence in their PAH emission properties:

In the galaxy cluster RX J0152.7–1357 at $z=0.84$ (AKARI/L15), $L_8/L_\mathrm{IR}$ for cluster galaxies aligns with the field, showing no significant environmental offset. Starbursts with depressed $R_\mathrm{PAH}$ are confined to $R>1$ Mpc, i.e., the cluster outskirts; the core contains only main-sequence (MS) star-forming galaxies with elevated $R_\mathrm{PAH}$ (Murata et al., 2015).
In Abell 2744 ( $z=0.306$ ), spatially-resolved JWST studies detect 3.3 μm PAH emission predominantly in galaxies at projected cluster distances $R>500$ kpc, specifically outside the high-mass-density core. These PAH-bright galaxies have SFRs consistent with the $z=0.3$ field MS and display extended, asymmetric PAH morphologies; higher asymmetry suggests early-stage ram-pressure interaction or lopsided gas inflow (Cheng et al., 26 Jun 2025, Benotto et al., 7 Nov 2025).

The statistically robust lack of cluster-center starbursts with low $R_\mathrm{PAH}$ supports a scenario where the cluster environment governs the fraction of star-forming, PAH-bright galaxies rather than intrinsically modifying PAH emission in individual main-sequence systems.

3. Physical Interpretation: PAH Survival, Destruction, and Shielding

The fate of PAH carriers in cluster environments is a function of both ISM physics and environmental processes:

PAHs primarily reside in photodissociation regions (PDRs) where their strength is determined by the local UV radiation field, destruction by hard radiation, and balance between ionization and recombination (Dale et al., 17 Jan 2025, Dale et al., 2022).
In brightest cluster galaxies (BCGs) of cool-core clusters, Spitzer/IRS spectra show normal PAH band ratios and size distributions even in close proximity to the hot ($1$–$10$ keV) ICM, consistent with significant shielding within dense molecular clouds ( $N_\mathrm{H}\sim10^{21-22}$ cm $^{-2}$ , $A_V\sim1$ –$10$). This self-shielding prevents wholesale PAH destruction by X-rays or suprathermal electrons (Donahue et al., 2011).
Ram pressure stripping (RPS) in cluster outskirts can physically remove PAH-carrying ISM from galaxy disks and deposit it in the wake and tails. High-resolution JWST imaging of A2744 resolves PAH emission in both disk-truncated and downstream clump structures up to 40 kpc from the main galaxy. These findings directly confirm that small neutral PAHs can be stripped and survive in the ICM, challenging models that assume rapid grain destruction in shocks or hot ICM (Benotto et al., 7 Nov 2025).

A plausible implication is that the survival of PAHs in cluster galaxies, and their presence in stripped tails, is facilitated by a combination of molecular cloud shielding, rapid migration with cold gas, and environmental modulation via gas accretion and stripping flows.

4. PAH Emission as a Star-Formation Tracer in Clusters

PAH luminosity calibrations have emerged as effective tracers of recent star formation, particularly in dust-obscured environments:

On resolved (∼40–800 pc) scales in field spirals and cluster galaxies, the 3.3 μm feature ( $L_{3.3}$ ) correlates tightly with SFR measured by hydrogen recombination lines (Brα, Paα), albeit with a sub-linear relation (power-law index $\alpha\sim0.75$ ), and scatter $\sim 0.14$ –$0.4$ dex driven by IMF sampling, local age, and PAH destruction. At galaxy-integrated scales, the relation becomes nearly linear (Gregg et al., 15 May 2024, Cheng et al., 26 Jun 2025, Benotto et al., 7 Nov 2025).
PAH-based SFRs in cluster galaxies (A2744) match SED-based (UV–FIR) estimates across a factor of 2–3, confirming robust extinction-immunity: $L_{3.3}$ is unaffected by dust that suppresses optical tracers (Cheng et al., 26 Jun 2025).
The utility of PAH emission as a star-formation probe in clusters is supported by its ability to recover the entire SFR budget, including the heavily obscured, Herschel-bright population, with substantially less observational cost than full FIR campaigns.

This suggests that medium-band JWST imaging and PAH mapping can systematically inventory obscured star formation in diverse cluster environments, quantify quenching onset, and identify galaxies with ongoing gas accretion.

5. Impact of Local ISM Conditions: Cluster vs. Field Comparisons

Comparisons of star-forming cluster members and their field analogs elucidate the universality and environmental modulation in PAH behavior:

Across redshifts $z=0$ –$2$, the $L_8/L_\mathrm{IR}$ ratio decreases with increasing $L_\mathrm{IR}$ and sSFR relative to the main sequence, regardless of environment. Both field and cluster SFGs populate the same $R_\mathrm{PAH}$ – $L_\mathrm{IR}$ and $R_\mathrm{PAH}$ –sSFR $_\mathrm{rel}$ scaling relations within formal uncertainties (Murata et al., 2015).
JWST/PHANGS observations of PAH feature ratios (e.g., $R_{3.3/7.7}$ , $R_{3.3/11.3}$ ) around 30,000 stellar associations in 19 galaxies show a systematic suppression of the smallest PAHs (3.3 μm) relative to longer-wavelength features in diffuse ISM regions, with ratios $\sim0.10$ –$0.15$ dex lower than in clusters and associations (Dale et al., 17 Jan 2025). The observed band ratios are best explained by models incorporating large, ionized PAH populations subjected to hard UV radiation fields.
Both in clusters and the field, hard radiation—quantified by high [O III]/Hβ—anti-correlates with PAH ratios tracing small grains, confirming that the size and charge state of PAHs are primarily controlled by the local stellar and ISM conditions, not the macroscopic cluster environment (Dale et al., 2022, Dale et al., 17 Jan 2025).

6. Environmental Quenching, Gas Flows, and Feedback

Cluster-specific processes that modulate PAH emission are tightly coupled to galaxy evolution and feedback:

The lack of PAH-bright galaxies (with starburst-like $R_\mathrm{PAH}$ ) in high-density cluster cores, and their prevalence in infall regions or filament endpoints, is consistent with rapid environmental quenching (ram-pressure, tidal stripping, strangulation) preferentially suppressing the number of obscured SFGs, not the intrinsic PAH properties of those which remain active (Murata et al., 2015, Cheng et al., 26 Jun 2025, Benotto et al., 7 Nov 2025).
Asymmetric PAH morphologies and off-center tails signal early-stage gas stripping and/or accretion events along cosmic filaments (Cheng et al., 26 Jun 2025).
In cool-core BCGs, the coexistence of strong PAH bands, excess H $_2$ line emission, and non-linear forbidden neon lines ([Ne II], [Ne III]) points to a two-component heating model: UV-star formation dominating PAH/small grain excitation and suprathermal electrons from AGN/ICM contributing to molecular/ionized gas heating (Donahue et al., 2011).

This environmental regulation of PAH emission provides a tracer for both quenching and gas accretion, and opens new avenues for using PAH mapping to measure the efficiency and timescale of feedback-driven star-formation suppression in clusters.

7. High-Redshift Clusters and Prospects for Future Surveys

Advances in the sensitivity and mapping speed of far-IR observatories (PRIMA/FIRESS) enable efficient PAH-based surveys of high-redshift cluster and protocluster galaxies:

PRIMA/FIRESS low-resolution spectroscopy will detect 7.7, 11.2, and 6.2 μm PAH features at $z\gtrsim5$ for $L_\mathrm{IR}\sim10^{13} L_\odot$ sources, with $\gtrsim10\times$ efficiency compared to CO(1–0) molecular gas mapping (Yoon et al., 2 Sep 2025).
PAH band ratios, when combined with full spectral fitting, directly probe ISRF strength and the relative fractions of gas-rich, star-forming, or quenching cluster members, and allow robust redshift determination in complex protoclusters.
Systematic differences between full spectral and flux-clipping PAH band measurements underscore the necessity of multi-band or spectroscopic coverage to disambiguate continuum and ISRF effects.

The extension of these techniques to large cluster samples will reveal the prevalence of obscured SF throughout cluster assembly, quantify the universality of PAH–SFR scaling, and constrain the impact of the ICM and large-scale structure on dust and gas survival at cosmic noon and reionization.

PAH emission serves as a unique, physically motivated tracer of star formation, ISM processing, feedback, and environmental effects in cluster galaxies across cosmic time, bridging the small-scale physics of dust grain survival with the large-scale ecology of galaxy transformation in massive halos.