Low Surface Brightness Universe

• Low Surface Brightness Universe is a collection of diffuse galaxies, ultra-diffuse systems, and faint intracluster light that reveal the hidden baryonic structures in space.
• Modern observational techniques, such as adaptive sky subtraction and exponential-profile fitting, enable precise measurement of low HI densities and extended stellar disks.
• Research on LSB systems provides critical insights into galaxy formation, dark matter distribution, and environmental effects, shaping our cosmological models.

The low surface brightness (LSB) universe comprises the vast population of cosmic structures—galaxies, galactic outskirts, diffuse intracluster light, and cosmic web filaments—so faint in luminosity density per unit area that they evade detection with standard astronomical techniques. LSB systems, typically characterized by central surface brightnesses fainter than the night sky (e.g., μ₀(B) ≳ 22.5 mag arcsec⁻²), represent a major component of baryonic matter distribution and serve as critical laboratories for deciphering galaxy formation, dark matter properties, and environmental influences across cosmic time. Modern deep-wide surveys and next-generation instrumentation are revealing the ubiquity and diversity of LSB galaxies, ultra-diffuse galaxies, and diffuse cluster and intergalactic emissions.

1. Physical Properties of Low Surface Brightness Galaxies

LSB galaxies encompass a wide range of stellar masses and morphologies but are unified by their exceptionally low central and mean surface brightnesses. Structurally, their stellar distributions are best described by exponential or low-Sérsic-index ($n \lesssim 1$) profiles, with large scale lengths and diffuse, extended stellar and neutral atomic hydrogen (HI) disks. The stellar mass densities are low, and surface brightness profiles such as $I(r) = I_0 \exp(-r/\alpha)$ (exponential law) yield $\mu_0(B) > 22.5$–28 mag arcsec$^{-2}$ in typical samples (Du et al., 2015, Fu et al., 2023, Tanoglidis et al., 2020).

LSB disks are highly HI-rich: many exhibit $M_{\mathrm{HI}}\sim 10^{9}$–$10^{10}\,M_\odot$ with low gas surface densities ($\Sigma_{\mathrm{HI}}$ of a few $M_\odot$ pc$^{-2}$). Star formation rates (SFRs) are low and extended; the prolific HI content provides fuel, but star formation is inefficient due to the low midplane pressure and Toomre stability ($Q = \sigma\kappa/\pi G\Sigma$), as seen in giant LSB (GLSB) galaxies (Das, 2013, Cintio et al., 2019). Bulge components, when present, can be luminous and host low-luminosity AGN, but bulge and disk formation are often decoupled.

In the dwarf regime ($M_\ast < 10^9 \, M_\odot$), the LSB class includes ultra-diffuse galaxies (UDGs), which are quenched or gas-poor and exhibit large effective radii ($R_e \gtrsim 1.5$ kpc) and extremely low central surface brightness ($\mu_0 \gtrsim 24$ mag arcsec$^{-2}$) (Makarov et al., 2015, Jackson et al., 2020). LSB dwarfs show a broad spread ($\sim$3 mag arcsec$^{-2}$) in surface brightness at fixed luminosity, determined by assembly and feedback histories.

2. Formation Pathways and Evolution

LSB galaxies emerge through a variety of astrophysical processes that differ systematically with mass and cosmic epoch. For classical LSB disks ($M_\ast\sim 10^{9.5-10}\,M_\odot$), hydrodynamical simulations (e.g., NIHAO, EAGLE) show that co-planar, co-rotating mergers and the early aligned accretion of high-specific-angular-momentum gas are the dominant channels (Cintio et al., 2019, Stoppacher et al., 18 Apr 2025). This angular momentum builds extended, diffuse disks; perpendicular or misaligned mergers, conversely, generate more compact, high surface brightness (HSB) systems. The divergence in properties such as $j_*$ (the specific stellar angular momentum) between LSBGs and HSBGs occurs as early as $z\sim5$–7, with dynamical observables (e.g., $R_{\mathrm{Vmax}}$) bifurcating at $z\sim2$–3 (Stoppacher et al., 18 Apr 2025).

In the UDG and LSB dwarf domain ($M_\ast \lesssim 10^{9}\,M_\odot$), supernova-driven feedback and environmental effects are key. Early, rapid star formation in high-density dark matter environments results in strong outflows, flattening the central potentials and generating diffuse, puffed-up stellar distributions. Ongoing tidal heating and ram-pressure stripping (at $z\lesssim1$) further reduce central densities and SFRs, producing the observed diversity in LSB properties (Jackson et al., 2020).

Table: Dominant Physical Drivers Across LSB Mass Scales

Mass Regime	Dominant Formation Driver	Key Outcome
$M_\ast \gtrsim 10^9\,M_\odot$	High $j$, aligned mergers	Extended, disk-dominated LSBGs
$M_\ast \lesssim 10^9\,M_\odot$	Feedback + tidal effects	UDGs, diffuse quenched dwarfs

3. Dark Matter and Scaling Relations

LSB galaxies are universally dark matter-dominated, with high mass-to-light ratios throughout the disk and even at small radii. Rotation curves exhibit a slowly rising profile, reflecting the dominance of cored dark halos (e.g., Burkert profiles) with central surface densities $\Sigma_0 \sim 100\,M_\odot/\mathrm{pc}^2$—a value found to be remarkably stable across diverse disk populations (Paolo et al., 2018). Scaling relations link stellar disk scale length $R_d$ and mass $M_d$ to dark halo properties (core radius $R_c$, $\rho_0$), with empirical relations such as $\log R_c = 0.60 + 1.42 \log R_d$. The "stellar compactness" parameter ($C_*$) quantifies departures from the mean mass–size relation and reduces scatter in these scaling relations, with a corresponding "dark matter compactness" $C_{\mathrm{DM}}$ (Paolo et al., 2018).

URC (universal rotation curve) analysis of LSB samples shows a tight coupling between the embedded luminous and dark components, despite the disk and halo responding to different physical processes—a strong constraint on ΛCDM models.

4. Survey Methodologies and Instrumentation

The detection of LSB structures demands tailored instrumentation, sky subtraction, and data processing strategies. Traditional optical surveys (e.g., SDSS) can miss or bias against LSBGs due to sky oversubtraction and the reliance on circular Petrosian apertures (Du et al., 2015). Innovations include:

• Kron elliptical apertures and exponential-profile model fitting for total flux;
• Adaptive sky background estimation (e.g., polynomial fits, object masking, median tile subtraction);
• Machine-learning classifiers for artifact rejection and automated photometric measurement pipelines (Tanoglidis et al., 2020);
• Reproducible and automated NIR processing pipelines, such as NASIM, for removing large-scale systematics and preserving diffuse emission in VISTA legacy data (Saremi et al., 4 Aug 2025).

Instrumentation such as Dragonfly/ILMT (fast refractors with curving CCDs or liquid mirrors), and advanced survey designs (LSST, Euclid, DES, HSC) extend LSB detection down to $\sim 29.5$–$32$ mag arcsec$^{-2}$ (Borlaff et al., 2021, Brough et al., 2020, Lombardo et al., 2019, Fu et al., 2023). In the radio and UV, facilities such as ALFALFA, the ngVLA, and deep Ly$\alpha$ mapping campaigns reach the diffuse hydrogen and molecular gas in the cosmic web (Silva et al., 2016, Emonts et al., 2018, Du et al., 2015).

5. Environmental Effects and Population Statistics

A substantial fraction of LSBGs inhabit low-density environments and are found in isolation or at void boundaries, where the absence of tidal perturbation inhibits disk instability and promotes slow evolution (Das, 2013, Du et al., 2015). However, surveys identify a significant red (quenched) LSBG population, as well as UDGs and quenched dSphs in group and cluster environments (e.g., $\sim$33% red LSBGs in DES, (Tanoglidis et al., 2020); UDGs in Coma, Virgo, and at $z>1$, (Bachmann et al., 2021)). Dense environments facilitate quenching, size evolution, and ICL build-up, while field LSBGs retain gas and exhibit extended star formation. Cluster LSBGs and UDGs are found to evolve passively from $z\sim1$, with numbers increasing as a result of significant size growth over the past 8 Gyr.

For galaxy clusters, a quarter of systems in unbiased (optically-selected) samples show low central X-ray surface brightness ($\langle\mu_{300}\rangle$), but X-ray and SZ selection strongly miss these clusters, biasing cosmological constraints and the inferred baryon content (Andreon et al., 18 Apr 2024).

6. Implications for Galaxy Evolution, Cosmology, and Future Prospects

LSB galaxies numerically dominate the faint end of the galaxy luminosity function, with their diversity and abundance now being uncovered in deep-wide optical and near-infrared surveys (e.g., DES: 23,790 LSBGs in 5000 deg$^2$ (Tanoglidis et al., 2020); ILMT: 3,092 LSBGs (Fu et al., 2023)). The preponderance of LSB dwarfs especially at $M_\ast<10^9\,M_\odot$ and their role as the dominant dwarf population impacts solutions to the “missing satellites” and “too big to fail” problems.

LSB systems provide stringent constraints on the interplay between baryonic processes (feedback, star formation thresholds), dark matter halo structure (core-cusp, central surface density invariance), and environmental processing (tidal stirring, stripping, ram pressure). Advanced reduction pipelines (e.g., NASIM (Saremi et al., 4 Aug 2025)), next-generation survey data (LSST, Euclid), and high-resolution hydrodynamical simulations (EAGLE, NIHAO, NewHorizon) are essential for a statistical understanding of these faint systems.

Ultra-low surface brightness (ULSB) instrumentation—curved-focal-plane telescopes (Lombardo et al., 2019), refractors for sky-limited operation (Holwerda, 2018), and innovative space-based missions—will enable comprehensive mapping of stellar outskirts, ICL, tidal debris, and intergalactic filaments, opening new frontiers in the paper of galaxy assembly and the baryonic content of the universe.

7. Key Challenges and Future Directions

Major technical and scientific challenges for the low surface brightness universe include:

• Preserving faint, extended structures during sky subtraction and flat-fielding, particularly for near-infrared ground-based imaging limited by atmospheric background (Saremi et al., 4 Aug 2025).
• Accurate measurements of structural parameters in crowded or high-background environments, requiring robust masking and calibration techniques (Borlaff et al., 2021).
• Calibrating surface brightness selection effects in cluster and group studies, to enable complete census of LSB systems and unbiased determination of scaling relations (Andreon et al., 18 Apr 2024).
• Achieving high photometric-redshift accuracy ($\lesssim10\%$) in the LSB and dwarf regime for statistically robust cosmological analyses (Kaviraj, 2020).
• Theoretical modeling requires higher mass and spatial resolution to resolve the diffuse, low-mass end of the galaxy population and their LSB features (Kaviraj, 2020).

These challenges are being met with concerted efforts in instrumentation (e.g., LSST’s ultra-high étendue and deferred sky-subtraction; Euclid’s unique combination of spatial resolution, depth, and calibration strategy (Brough et al., 2020, Borlaff et al., 2021)) and in simulation (e.g., zoom-in hydrodynamics of galaxy assembly and feedback in the LSB regime).

As the available data and analysis techniques continue to deepen, the low surface brightness universe will offer unprecedented insight into galaxy evolution, the dark matter distribution, and the baryonic assembly across cosmic time.