Effects on galaxy evolution: pair interactions versus environment

1. Effects on galaxy evolution: pair interactions versus environment S. Tonnesen & R. Cen arXiv: 1111.0636 Reviewed by Jianling Gan

2. Introduction • Gravitational interactions between galaxies are important as they can influence the gas content, star formation, color and morphology of galaxies. • Observation (lin et al. 2010) and simulation (Barton et al. 2007) have found that mergers are an important process in driving the color-density relation. • Plenty of work supported that pair interaction can affect the star formation rate (SFR). Pairs are bluer than non-paired galaxies because of higher SFR, and closer pairs are bluer with higher SFRs (Perez et al. 2006a, b).

3. Simulation • Cosmological simulation, code: Enzo (Bryan 1999; O’Shea et al. 2004; Joung et al. 2009) • Firstly, run a low resolution simulation, identify two regions at z=0: one centred on a cluster (C box), another a void (V box) • C box: 21*24*20 h-3Mpc3, cluster: M~2*1014M⊙, r₂₀₀~1.3 h-1Mpc; V box: 31*31*35h-3Mpc3, void size: 5h-1Mpc • Resimulate these two regions with high resolution • Cell size: 0.46h-1kpc; dark matter particle mass: 1.07*108h-1M⊙; stellar particle mass: 106M⊙

4. Pairs Selection Mass: M* > 109.6M⊙, massive Distance: d < 0.5 h-1Mpc, 0.25 h-1Mpc, close Energy: K<0.9|W|, bound pairs HOP: An algorithm for finding very close and strongly interacting galaxies

5. Result I: Where are pairs? • In C box, most pairs stay between 5~15Mpc from the cluster center; a few pairs stay within 5Mpc and decrease with decreasing distance • In V box, pairs is found to be common 10Mpc beyond the void center; there is large scatter because of the small number of galaxies in V box.

6. Result I: Where are pairs? Local density: d₅ is the distance to the fifth nearest neighbor V box C box • Pairs tend to reside in intermediate local density regions. • In cluster center, there is few pairs due to the high velocity dispersion (K>|W|). • Pairs are rare near void center because is almost no galaxies. total

7. Result II: Wet, Dry, and Mixed Mergers • In C box, wet pairs peak around 8Mpc; dry pairs peak around cluster center and 9Mpc; mixed pairs peak around 4Mpc and 9Mpc. • In V box, almost no dry pairs; no mixed pairs until nearly 20Mpc; plenty of wet pairs found from the void center to the edge.

8. Result II: Wet, Dry, and Mixed Mergers • All three types of pairs can be found in group environments (high local densiy), but only wet and mixed pairs are common in field environment (low local density). • Agree with Lin et al. (2010, observations)

9. Result III: Color, SFR and Gas Mass • C pairs are redder, V pairs are bluer; • C pairs are bluer than the total C population; • V pairs are redder than the total V population. blue red

10. Result III: Color, SFR and Gas Mass • SFR in C pairs is lower than in V pairs; • SFR in C pairs is higher than the total C population; • SFR in V pairs is lower than the total V population. • In C box, closer pairs have higher SFR low high

11. Result III: Color, SFR and Gas Mass • HI gas mass in C pairs is lower than in V pairs; • In C box, closer pairs are more gas-rich. low high

12. Conclusion • On large scales, the overdense region is dynamically more advanced(developed) than the underdense region, so the C box galaxies are more massive, redder and have less cold gas than the V box population. • On small scales, local density effectively determined the thermodynamic properties of gas and star formation. Higher local density corresponds to bluer color, less cold gas and lower SFR. • Close galaxy pair interactions induce shocks, gas compression and cooling of hot gas that leads to increasing of cold gas and star formation rate in pair galaxies.

13. Thank you!

14. Result IV: The local environment of bound galaxies • The SFR and HI gas mass of bound galaxies decrease with local density.