1. Caught in the act Caught in the act - witnessing the transformation of a spiral galaxy Jesper Rasmussen + Trevor Ponman, John Mulchaey

2. >50% of galaxies live in groups & clusters. Dominant baryonic component: Hot gas, not galaxies. Galaxy population in clusters (& groups): Fewer spirals, more ellipticals and S0's. Interpretation: Spirals → E + S0's... Background “Morphology-density relation”: Thomas & Katgert (2006)

3. ● Galaxy-galaxy interactions ??? Galaxy transformations ?? Galaxy-gas interactions: Ram pressure P = ρ gas v 2 gal

4. Focus has been on clusters (higher densities & galaxy velocities)... Helsdon & Ponman (2003)...but much transformation activity occurs in groups. Galaxy-gas interactions (P = ρ gas v 2 gal ) not important? Galaxy transformations II

5. ● Embedded in T ≈ 10 7 K group gas ● Bow-shock like morphology ● “Tail” of HI+radio continuum emission ● 55% less HI than typical isolated spirals ● High star formation rate (~ 5 M ⊙ /yr) 1.49 Ghz Hummel & Beck (1995) H α on R Gruendl et al. (1993) Quilis et al. (2000) Simulation example Observe in X-rays: Determine ram pressure NGC 2276

6. = + Chandra NGC 2276

7. Shock conditions: M ≈ 1.7  v gal ≈ 850 km/s Results M ≈ 1.5 Mach cone Trigger starburst + outflows Sweep back stellar outflows Peel off gas from edge of disk via hydro-instabilities Effects of ram pressure:

8. Implications Current mass-loss rate of gas: ~ 5 M ⊙ /yr. Gas loss for 1-2 Gyr, remaining supply lost in another 1-2 Gyr. Star formation will cease “soon”. End product: No gas, little star formation. Large bulge-to-disc ratio + larger bulge. ~ S0 galaxy.

9. Summary Spirals transformed into ellipticals+S0's in dense environments. ● Unclear which processes cause this. ● NGC 2276: Gas-galaxy interactions can modify morphology of galaxies in groups (perhaps via induced starburst activity). ● Could provide important route to forming S0's and establish morph.-density relation in groups.