Cold Gas Infall onto A Brightest Group Galaxy via A Gas-Rich Minor Merger

Dust and cold gas are not uncommon in nearby early-type galaxies (ETGs), and represent an important aspect of their evolution. However, their origin has been debated for decades. Potential sources include internal processes (e.g., mass loss from evolved stars), external mechanisms (e.g., minor mergers or cooling flows), or a combination of both. Gas-rich minor mergers have long been proposed as an important channel for cold gas fueling in both observations and simulations, but direct evidence of cold gas transportation via gas-rich minor mergers remains elusive, particularly in galaxy groups and clusters where environmental effects are prevalent. In this letter, we present the first unambiguous case of direct cold gas transportation onto a brightest group galaxy (BGG) at $z=0.25$, driven by an ongoing close-separation gas-rich minor merger with a mass ratio of $\sim1:56$. High-resolution JWST imaging reveals a heavily obscured, low-mass satellite that is barely visible at restframe optical wavelengths. Tidal stripping from this satellite deposits gas and dust onto the BGG, forming prominent $\sim$10 kpc dust lanes in situ. Cosmological simulations indicate that such interactions preferentially occur in gas-rich satellites undergoing their first infall in highly eccentric orbits. Our results highlight the pivotal role of gas-rich minor mergers in replenishing cold gas reservoirs and shaping the evolution of central ETGs in galaxy groups.

💡 Research Summary

This paper presents the first unambiguous detection of cold gas being transferred onto a brightest group galaxy (BGG) through a gas‑rich minor merger. The target system, CW‑BGG‑1 at redshift z = 0.2475 in the COSMOS field, is a massive early‑type galaxy that hosts a heavily obscured low‑mass satellite (CW‑BGG‑1‑C) with a stellar mass ratio of roughly 1:56. Using high‑resolution JWST NIRCam imaging in four filters (F115W, F150W, F277W, F444W) together with HST ACS F606W and F814W data, the authors perform a detailed two‑dimensional decomposition with GALFITM. The BGG is modeled with four Sérsic components, while the satellite is described by three components (bulge, disk, extended envelope). The satellite is virtually invisible in the optical bands but emerges strongly in the longest‑wavelength JWST filter (F444W), indicating extreme dust obscuration.

A joint spectral energy distribution (SED) fit is carried out using the FSPS framework, MIST isochrones, MILES stellar libraries, and the Draine & Li (2007) dust emission templates. The fitting includes a delayed exponential star‑formation history, Calzetti attenuation, and, for the satellite, nebular emission lines. Bayesian inference with the dynesty nested sampler yields robust physical parameters. The BGG has a current stellar mass of log M★/M⊙ ≈ 11.3, while the satellite’s stellar mass is log M★/M⊙ ≈ 9.2. The satellite’s dust mass is ∼10⁶ M⊙ and its infrared luminosity L_IR ≈ 10⁴³ erg s⁻¹, consistent with the ∼10 kpc dust lanes observed in the BGG. The satellite’s stellar population is young (≤ 1 Gyr) and metal‑poor (Z ≈ 0.3 Z⊙), implying a gas‑rich, recently accreted system.

To interpret the observations, the authors turn to the IllustrisTNG50 cosmological magneto‑hydrodynamical simulation. They select group‑scale halos containing a massive central galaxy and a low‑mass companion with a similar mass ratio. The simulation shows that when such a satellite makes its first infall on a highly eccentric orbit (e ≈ 0.9), strong tidal forces strip gas and dust from the satellite, depositing them onto the central galaxy’s inner regions. The stripped material forms extended dust lanes and cold gas clouds that match the observed morphology and scale. The group environment’s relatively low intragroup medium pressure and long ram‑pressure stripping timescales (∼3 Gyr) allow the satellite’s cold gas to survive long enough to be transferred, unlike in richer clusters where ram pressure would quickly remove it.

The authors argue that this case provides direct evidence that gas‑rich minor mergers are a dominant channel for replenishing cold gas and dust in central early‑type galaxies within groups. This challenges the view that internal stellar mass loss or cooling flows are the primary sources of cold gas in such systems. Because satellite velocity dispersions are lower in groups than in clusters, the frequency of minor mergers is higher, making this mechanism especially relevant for the long‑term stellar mass growth, AGN fueling, and overall evolution of BGGs. The paper concludes by recommending follow‑up high‑resolution spectroscopy (e.g., ALMA, JWST NIRSpec, MUSE) to map the kinematics and metallicity of the transferred gas, and by suggesting that future simulations refine the treatment of AGN feedback and cooling to better capture the fate of accreted material.