On the Origin of Gas-stripping of Galaxies in Group Environments

TL;DR

This study uses the NewHorizon2 cosmological zoom-in simulation to investigate how low-mass group halos with $M_{\rm vir} \sim 10^{12-13}\,M_{\odot}$ affect the gas content of satellites with $M_{*}>10^{7}\,M_{\odot}$. By classifying gas into ISM, outflows, and surrounding gas and quantifying internal ($P_{\rm grav}$) and external ($P_{\rm ext}$) pressures, the authors show that gas stripping is efficient mainly for the lowest-mass satellites and often driven by external hydrodynamic pressures that can arise from hot ambient gas, cold dense gas from neighboring structures, or outflows; in some cases, tidal interactions contribute to disruption and gas loss. A key finding is a crossover around $M_{*}\sim10^{8}\,M_{\odot}$ where $P_{\rm ext}$ begins to dominate over $P_{\rm grav}$, largely due to the near-linear scaling of $P_{\rm ext}$ with halo mass versus the shallower scaling of $P_{\rm grav}$ with stellar mass. The results also show that starvation is ineffective for quenching in these group satellites, while tidal forces can lead to complete disruption, implying that studies focusing only on surviving galaxies may underestimate the true impact of environmental processes in groups. Overall, the work highlights the diverse and stochastic nature of gas removal in group environments and its relevance for understanding galaxy evolution outside clusters.

Abstract

We investigate how low-mass group environments ($M_{\rm vir} \sim 10^{12-13}\,M_{\odot}$) influence the gas content of their satellite galaxies with $M_* > 10^{7}\,M_{\odot}$ using the \NHtwo\ simulation. Many satellite galaxies preserve substantial gas reservoirs, yet show signs of outer gas stripping, reminiscent of jellyfish galaxies in clusters. In contrast, low-mass satellites ($<10^8 \, M_{\odot}$) are largely gas-deficient, and some of them undergo gas removal within their host group by external pressure triggered by either galaxy interactions or ram pressure exerted by the hot intragroup medium. Complete gas removal in these satellite galaxies occurs when the external hydrodynamic pressure exceeds the gravitational restoring force, typically due to stochastic events such as galaxy-galaxy interaction or nearby galactic outflows. The emergence of a characteristic stellar mass of $10^8 \, M_{\odot}$ which determines the efficiency of gas removal in groups, likely reflects the differing scaling relations of external pressure with halo mass and gravitational restoring force with stellar mass. While tidal interactions can be a significant cause of gas loss in satellite galaxies, those severe enough to affect the gas content in the central regions typically lead to the complete disruption of the galaxy. Consequently, gas loss driven by tidal interactions may be underestimated in the studies focusing solely on surviving galaxies. Group environments, where environmental effects are weaker and satellite galaxies tend to have lower restoring forces due to their low masses, exhibit complex manifestations of gas loss that are not seen in more massive environments such as clusters.

Figures (11)

• Figure 1: Left panel: Large-scale dark matter density map from the NewHorizon2 simulation surrounding the zoomed-in region. The boundary of the zoomed-in region is indicated by the blue solid line. The selected group halos ($M_{\rm vir}>10^{12}\,M_{\odot}$) are marked with the red circles. Right panel: Enlarged view of one of the group halos. The different components of the group halo are shown: dark matter in white, gas temperature in red, gas density in blue, and stellar density in green. The gray dashed circle indicates the region within the virial radius.
• Figure 2: Stellar mass--gas mass scaling relation for satellite galaxies at the final snapshot ($z=0.158$) in the NewHorizon2 simulation. Satellite galaxies are categorized as either normal ($f_{\rm gas}>0.01$; blue circles) or gas deficient ($f_{\rm gas}<0.01$; downward red arrows). For comparison, the scaling relation of the NewHorizon2 central galaxies is shown as the blue contours. Gas-rich normal satellite galaxies closely follow the scaling relation of the central galaxies, while gas-deficient satellites are predominantly low-mass systems. The stellar mass distribution of the satellite sample is shown as a histogram at the top. The observed scaling relations from galaxies in the local Universe (purple dotted line) and in low-mass groups (pink dotted line) are overplotted, both showing similar trends to those of the NewHorizon2 central galaxies.
• Figure 3: Evolution of gas fractions for normal (top) and gas-deficient satellite galaxies (bottom) over cosmic time. The colored solid lines in the top panel show the gas fraction of individual normal satellites over cosmic time from their infall to the final snapshot ($z=0.158$). In the bottom panel, the gas fraction evolution of the seven gas-deficient galaxies that lost their gas inside the host group is shown. The corresponding galaxy ID for each gas-deficient satellite is also displayed. In each line, the color intensity reflects the distance to the host group center: bolder colors indicate closer proximity (see color bars on the left side). Normal galaxies maintain a relatively constant gas fraction, while gas-deficient satellites experience a rapid loss of gas near the host group center.
• Figure 4: Stellar and gas morphologies of randomly selected normal satellites at their pericenter passages. Each horizontal triplet corresponds to one galaxy. Left panel: g-band surface brightness of a normal satellite galaxy with gas column density contours. Middle panel: Gas column density map. Right panel: Radial profiles of external pressure (pink solid line) and gravitational pressure (blue solid line), measured in spherical shells centered on the galaxy. In the left and middle panels, the red arrows indicate the direction of the motion of the galaxy. Galaxy ID and stellar mass are given at the top of the images. The stripping radius, defined as the location where $P_{\rm ext}=0.5\,P_{\rm grav}$, is marked by the red dashed vertical line in the pressure profile and by the circle in the gas column density map, if it exists.
• Figure 5: Time evolution of the ratio of external pressure to gravitational pressure for the seven gas-deficient satellites. Both pressures are computed using Equations \ref{['eq:pgv']} and \ref{['eq:prp']}, and smoothed over a $50\,{\rm Myr}$ window to reduce fluctuations. The black dashed horizontal lines mark the point at which $P_{\rm ext} = P_{\rm grav}$. The moment of gas removal for each galaxy is marked with the black arrow. For the galaxy with ID=236, three distinct sources of strong external pressures are illustrated in the top panels through gas column density maps: (A) fly-by interaction with another galaxy, (B) mild ram pressure, and (C) strong ram pressure. In each map, the black dashed circle indicates $2\,R_{\rm 90, gas}$, where $R_{\rm 90, gas}$ is the radius enclosing $90\%$ of the ISM gas mass within $3\,R_{\rm eff}$.