Low Surface Brightness Galaxies in the SDSS: the link between environment, star-forming properties and AGN

Using the Sloan Digital Sky Survey (SDSS) data release 4 (DR 4), we investigate the spatial distribution of low and high surface brightness galaxies (LSBGs and HSBGs, respectively). In particular, we focus our attention on the influence of interactions between galaxies on the star formation strength in the redshift range $0.01 < z < 0.1$. With cylinder counts and projected distance to the first and fifth-nearest neighbor as environment tracers, we find that LSBGs tend to have a lack of companions compared to HSBGs at small scales ($<2$ Mpc). Regarding the interactions, we have evidence that the fraction of LSBGs with strong star formation activity increases when the distance between pairs of galaxies ($r_{p}$) is smaller than about four times the Petrosian radius ($r_{90}$) of one of the components. Our results suggest that, rather than being a condition for their formation, the isolation of LSBGs is more connected with their survival and evolution. The effect of the interaction on the star formation strength, measured by the average value of the birthrate parameter $b$, seems to be stronger for HSBGs than for LSBGs. The analysis of our population of LSBGs and HSBGs hosting an AGN show that, regardless of the mass range, the fraction of LSBGs having an AGN is lower than the corresponding fraction of HSBGs with an AGN. Also, we observe that the fraction of HSBGs and LSBGs having an AGN increases with the bulge luminosity. These results, and those concerning the star-forming properties of LSBGs as a function of the environment, fit with the scenario proposed by some authors where, below a given threshold of surface mass density, low surface brightness disks are unable to propagate instabilities, preventing the formation and evolution of massive black holes in the centers of LSBGs.

💡 Research Summary

The authors exploit the Sloan Digital Sky Survey Data Release 4 (SDSS‑DR4) to compare the spatial distribution, star‑forming activity, and active‑galactic‑nucleus (AGN) incidence of low‑surface‑brightness galaxies (LSBGs) and high‑surface‑brightness galaxies (HSBGs) in the redshift interval 0.01 < z < 0.1. A volume‑limited sample is constructed, and environment is quantified by two complementary methods: (i) counting neighbours inside a fixed cylindrical volume (radius = 1 Mpc, depth = ±1000 km s⁻¹) and (ii) measuring the projected distance to the first and fifth nearest neighbour (rₚ). The analysis shows that on scales smaller than ≈2 Mpc LSBGs have systematically fewer companions than HSBGs, indicating that LSBGs preferentially inhabit lower‑density regions.

To probe the effect of galaxy interactions on star formation, the authors examine pairs with decreasing projected separation. They use the birth‑rate parameter b = SFR/(M∗/t_H) as a proxy for recent star‑forming strength. When the separation falls below roughly four times the Petrosian radius r₉₀ of the primary galaxy, the fraction of LSBGs with elevated b rises, demonstrating that tidal encounters can trigger star formation even in low‑density disks. However, the increase in b for LSBGs is modest compared with HSBGs, whose star‑forming response to the same interaction strength is markedly stronger. This asymmetry is interpreted as a consequence of the higher disk stability of LSBGs: their low surface mass density suppresses the growth of non‑axisymmetric instabilities (bars, spirals) that would otherwise funnel gas inward.

The AGN analysis proceeds by cross‑matching the galaxy sample with the SDSS spectroscopic AGN catalogue (based on BPT diagnostics). Across the full stellar‑mass range, the fraction of LSBGs hosting an AGN is consistently lower than that of HSBGs. Both populations show an increasing AGN fraction with bulge luminosity (or equivalently bulge‑to‑total ratio), suggesting that the presence of a massive central concentration is a prerequisite for black‑hole growth regardless of disk surface brightness. The authors argue that the low surface‑mass density of LSBG disks hampers the development of dynamical instabilities that could transport gas to the nucleus, thereby limiting black‑hole feeding and AGN activity.

Overall, the study supports a picture in which LSBGs are not necessarily formed in isolation, but their survival and evolutionary pathways are strongly conditioned by their low‑density environments. Interactions can boost star formation, yet the response is muted relative to HSBGs, and the same structural stability that protects LSBGs from rapid gas consumption also curtails the fueling of central supermassive black holes. These findings reinforce theoretical models that propose a surface‑mass‑density threshold below which disk galaxies become dynamically “cold”, suppressing both large‑scale starbursts and AGN fueling. The work thus links environmental metrics, internal disk stability, and nuclear activity in a coherent framework for the divergent evolution of low‑ and high‑surface‑brightness galaxies.