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

TL;DR

This study demonstrates direct cold-gas transport onto a brightest group galaxy (CW-BGG-1) at $z=0.2475$ via a gas-rich minor merger with a satellite of mass ratio $ ext{≈}1{:}56$, guaranteed by high-resolution JWST/HST imaging that reveals $ ext{~10 kpc}$ dust lanes formed by tidal stripping. Joint UV–FIR SED modeling shows the satellite is a low-mass ($M_* obreak\sim ext{10}^{9.7}\,M_\odot$), heavily obscured system contributing the bulk of the system’s FIR emission, with total dust mass $M_{ m dust} obreak= ext{10}^{8.4\

Abstract

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.

Figures (7)

• Figure 1: Single-band images (left) and pseudo-color image (right) of the CW-BGG-1 system. Label in each panel indicates the name of the band: HST ACS F606W and F814W in the top row; JWST NIRCam F115W, F150W, F277W, and F444W in the middle and bottom rows. The pseudo-color image is generated using F814W (blue), F115W (green), and F444W (red). The white arrow in each panel indicates the location of the satellite CW-BGG-1-C. The bottom-left scale bar indicates 10 kpc. North is up and east is left.
• Figure 2: Two-dimensional modeling of CW-BGG-1 and CW-BGG-1-C. Columns from left to right show the data, best-fit BGG model, data$-$BGG model, bulge model of the CW-BGG-1-C, and data$-$BGG model $-$ CW-BGG-1-C bulge model in six HST/ACS and JWST/NIRCam bands. BGG model is constructed by fitting four Sérsic components to CW-BGG-1 after masking dust lanes, CW-BGG-1-C and its tidal tails, and neighboring sources using GALFITM. CW-BGG-1-C is modeled using three Sérsic components (one for the bulge, one for the disk, and one for the extended envelope) in data$-$BGG model image after masking tidal tails. Red ellipses denote the aperture used to measure the flux of CW-BGG-1-C.
• Figure 3: Joint spectral energy distribution modeling of the CW-BGG-1 system (left) and posterior distributions of the derived physical parameters (right). The black dots represent the total SED of the CW-BGG-1 system while the red dots represent SED of CW-BGG-1-C after decomposing emission from CW-BGG-1. The empty gray and red squares are the model photometric fluxes at the corresponding observed bands of the data for CW-BGG-1 and CW-BGG-1-C, respectively. The gray, blue, and red curves are the median total, median CW-BGG-1, and median CW-BGG-1-C model SEDs from our Bayesian nested sampling posteriors, respectively. The shaded regions indicate the 16th-to-84th percentile confidence ranges. The lower panel displays the residual data with respect to the median models in the unit of observed uncertainties. Most of the data points are within 1$\sigma$ deviations. The right panel displays the posterior distribution of the physical parameters derived from the joint SED fitting. The first four parameters are the stellar mass and $V$-band extinction ($A_V$) of the two galaxies, respectively. The last two parameters are the total star formation rate (SFR) based on the total infrared (integrated over 8-1000 $\mu$m) luminosity and total dust mass of the CW-BGG-1 system.
• Figure 4: SED data (top) and posterior distributions of model fitting parameters (bottom) to the bulge of CW-BGG-1-C. The upper panel displays the SED data (black points) and the model median (gray curve) and 16-to-84 confidence interval (gray shaded region) of the fitting to CW-BGG-1-C bulge SED, with fitting residual showing underneath. The bottom panel displays the posterior distributions of model fitting parameters and the derived physical parameters.
• Figure 5: Dust mass ($M_{\rm dust}$) and star formation rate (SFR) versus stellar mass ($M_*$). The red star denotes the CW-BGG-1 system while the blue dots represent dust-detected early-type galaxies at $0.2<z<0.3$ from 2023ApJ...953...27L. Error bars indicate 1$\sigma$ uncertainties. The black dashed curve and gray shaded stripe denotes the galaxy star-forming main sequence at $z=0.25$ taken from equation 14 in SFMS and its $\pm0.5$ dex dispersion, respectively. CW-BGG-1 is located on the galaxy star-forming main sequence and is among the dustiest ETGs at $0.2<z<0.3$ in the massive end ($M_*>10^{10.5}\,M_{\odot}$).