Studying the gaseous outskirts of galaxy groups with coherent Ly α absorption patterns^⋆

¹ Institut für Physik und Astronomie, Universität Potsdam, Karl-Liebknecht-Str. 24/25, 14476 Golm, Germany
² Department of Astronomy & Astrophysics, 525 Davey Lab, The Pennsylvania State University, University Park, PA 16802, USA
³ AURA for ESA, Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ Department of Physics & Astronomy, Nieuwland Science Hall, The University of Notre Dame, Notre Dame, IN 46556, USA
⁵ Eureka Scientific, Inc., 2452 Delmer Street, Oakland CA 94602, USA

^⋆⋆ Corresponding author: prichter@astro.physik.uni-potsdam.de

Received: 23 November 2024
Accepted: 15 July 2025

Abstract

Aims. In this study, we explore the properties of diffuse intergalactic gas residing on the outskirts of the four nearby, low-mass galaxy groups NGC 1052, NGC 5866, NGC 4631, and NGC 3992 (all at cz ≤ 2000 km s⁻¹) beyond their group virial radii.

Methods. Using archival ultraviolet absorption spectra of bright active galactic nuclei (AGNs) observed with the Hubble Space Telescope (HST) and its Cosmic Origins Spectrograph (COS), we searched for H I Ly α absorption near the groups’ recession velocities along 35 sightlines that pass the outer group medium (OGrM) at normalized impact parameters of ρ/R_vir = 1 − 3. We derived H I column densities of the absorbers and constrained the physical conditions in the gas (thermal pressure, density, neutral gas fraction, absorption path-length) by using a hydrostatic toy model of the groups’ gas environment and assuming photoionization.

Results. H I Ly α absorption near the groups’ recession velocities is detected along 19 sightlines with H I column densities in the range of log (N(H I)/cm⁻²) = 12.50 − 14.34, implying a high OGrM detection rate of more than 50 percent. We transform this value into an incidence rate of OGrM absorbers per unit redshift of d𝒩/dz = 232 ± 58 for absorbers with log (N(H I)/cm⁻²)≥13.2 and ρ/R_vir = 1 − 3. This is 25 percent above the value derived for the general population of Ly α absorbers within z = 0 filaments and more than twice the value for the z = 0 Ly α forest (considering the same column-density range). From the modeling, we obtain lower limits for the gas densities from log (n_H/cm⁻³) =  − 5.00 to −3.72, comparable to densities found in the overall Ly α forest.

Conclusions. Our study unveils a large cross section and overdensity of Ly α absorbers on the outskirts of these four nearby groups. Such an overdensity is in line with a previously proposed scenario in which AGN feedback lifts gaseous material to large distances beyond the virial radius of groups into the OGrM. However, a larger survey of OGrM absorbers and a comparison with hydrodynamical simulations will be necessary to constrain the cosmological mass density of OGrM absorbers and pinpoint their role in cosmological structure formation and galaxy and group evolution.