1. The interaction between galaxies and their environment Trevor Ponman University of Birmingham Jesper Rasmussen Carnegie Observatories

2. Environmental effects Morphology- density relation Goto et al 2003

3. Environmental effects Morphology- density relation Reduction in HI content Gavazzi et al 2006

4. Environmental effects Morphology- density relation Reduction in HI content Suppression of star formation Haines et al 2007

5. Cluster formation - simple model Galaxies form by baryon cooling within dark halos. These then cluster into groups.

6. Cluster formation - simple model RvRv Accretion shock Galaxies form by baryon cooling within dark halos. These then cluster into groups. Galaxy dark halos merge. Infalling gas is compressed and shocked at R v. The result is a virialised group or cluster.

7. Environmental effects - processes Gravitational Galaxy-galaxy interactions (mostly tidal) Tidal interaction with cluster potential Galaxy harassment Galaxy merging Hydrodynamical Ram pressure stripping Viscous stripping Strangulation Thermal evaporation Concentrate here on these, and examine the role of the group environment.

8. Ram pressure stripping Classic hydrodynamical gas removal process. Acts on early and late-type galaxies in clusters. Asymmetric extraplanar HI has been seen in many cluster spirals. Spectacular tails in the radio or X-ray are also seen (though much rarer). Wang et al 2004 Chandra emission (blue/green) superimposed on V band image (red). C153 The active disk-like galaxy C153, moving at v>1500 km s -1 through the complex merging cluster A2125.

9. Ram pressure stripping Gunn & Gott (1972) stripping formula Boselli & Gavazzi 2006 But v gal 2 dependence implies that this won’t work in groups. Most disk galaxies in nearby clusters (Virgo, A1367 & Coma) can be stripped during core crossing according to this formula. Coma Virgo & A1367

10. Why does stripping in groups matter? Only ~10% of galaxies are in clusters. Lots more are in groups. Strong environmental modification is seen in groups.

11. Morphological transformation begins at large radii in clusters Studies using large samples of galaxies from 2dF and SDSS suggests that galaxy modification sets in outside the virial radius of clusters, in groups or cluster fringes. Goto et al 2003

12. X-ray bright groups show a strong morphology-density relation Helsdon & Ponman 2003 Galaxy morphology in groups: Groups have a very wide range in spiral fraction X-ray bright groups show a morphology-density relation similar to clusters - but stronger Dressler clusters Groups Groups scaled to 3D density

13. HI deficiency is seen in groups Verdes-Montenegro et al studied the HI content of 72 Hickson compact groups, and found them to be significantly HI deficient, relative to HI content predicted by Haynes & Giovanelli (1984). Verdes-Montenegro et al 2001 HI deficiency: ∆ ≡ log M HI,pred. – log M HI,obs The deficiency was found to be correlated with the presence of a hot IGM (from Ponman et al 1996).

14. Why does stripping in groups matter? Only ~10% of galaxies are in clusters. Lots more are in groups. Strong environmental modification is seen in groups. Galaxies may be “preprocessed” in groups, before being incorporated into clusters.

15. Preprocessing of galaxies in groups Blue group with  ~170 km/s falling into A1367. Tidal disturbance, enhanced star formation, shell elliptical (likely merger), H  trails. Cortese et al 2006 R band HH

16. Why does stripping in groups matter? Only ~10% of galaxies are in clusters. Lots more are in groups. Strong environmental modification is seen in groups. Galaxies may be “preprocessed” in groups, before being incorporated into clusters.  Look for evidence of stripping operating within groups

17.  NGC2276 is a star-forming galaxy in the NGC2300 group  Embedded in T ≈ 10 7 K group gas  “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) Quilis et al. (2000) Simulation example Morphology strongly suggests ram pressure stripping.  Investigate with Chandra observations. Ram pressure in groups: NGC 2276

18. Chandra imaging 0.3-2 keV adaptively smoothed images 45 ks exposure (D = 37 Mpc).

19. Diffuse X-ray emission X-ray analysis X-ray contours over DSS image Shock conditions: M ≈ 1.7  v gal ≈ 850 km/s Thermal pressure shock IGM T=0.85 keV Hot ISM T=0.5 keV Shocked IGM T=1.1 keV

20. For v gal ≈ 850 km s -1, and n IGM =6x10 -4 cm -3 Results: Role of ram pressure Gas can be entirely stripped at r>9 kpc Trigger starburst + outflows Gas outflows dominated by ram pressure at z>2 kpc Peel off gas edge of disk via hydro-instabilities Iviscous stripping) Effects of ram pressure: Rasmussen, Ponman & Mulchaey 2006

21. Implications for galaxy evolution Current mass-loss rate of gas: ~ 5 M ⊙ yr -1. Remaining gas should be lost in another 1-2 Gyr. Star formation will cease “soon”. End product: No gas, little star formation. Larger bulge-to-disc ratio ~ S0 galaxy?

22. Modelling of stripping in groups Mayer, Mastropietro & Tran, in prep. Recent simulations by Kawata & Mulchaey (2007) and Mayer et al (in prep) suggest that hot gas can be stripped from group galaxies. Results from Vy Tran: 3D SPH simulations of galaxy (dark matter, disk+bulge stars and gas) with v rot =170 km/s, moving through a medium with n=10 -3 cm -3, and including cooling and star formation. v gal =450 km/s v gal =250 km/s Isolated galaxy Galaxy after 500 Myr

23. Modelling of stripping in groups Mayer, Mastropietro & Tran, in prep. Compression of the disk leads to cooling, instability and star formation. Some of the gas is stripped, and a large fraction is converted into stars. At present the simulations do not include the effects of feedback from SNe into the gas, so the effect of hot gas production and consequent stripping seen in NGC2276 is not included. gas mass total baryon mass

24. Intergalactic gas in groups So - could ram pressure stripping be responsible for all the HI deficiency seen in groups?  Look in detail at the HI-deficient HCGs. Verdes-Montenegro et al 2001

25. Intergalactic gas in groups X-ray plus VLA mapping of hot gas and HI in 8 of the most HI-deficient groups from the Verdes-Montenegro et al study. Verdes-Montenegro et al 2001 Chandra image of HCG40

26. Intergalactic gas in groups The X-ray properties are found to be very diverse. Several, such as HCG30, show no hot IGM. Ram pressure stripping cannot be the dominant effect in removing HI in many of these groups. Rasmussen et al, in prep.

27. Conclusions Galaxy groups must play a crucial role in removing gas from galaxies and establishing the morphology-density relation. Ram pressure stripping can operate in groups, when it acts in conjunction with starburst activity, which may be induced by galaxy interactions. A study of HI-deficient compact groups shows that ~half have no detectable hot IGM. Hence, ram pressure stripping cannot be the only mechanism leading to HI loss within groups. Other mechanisms include: tidal stripping, galaxy winds and gas exhaustion by star formation following removal of any gas reservoir (strangulation).