1. Baryon content of galaxy groups Ming Sun (University of Virginia) ‏ M. Voit, M. Donahue (MSU) A. Vikhlinin, W. Forman, C. Jones (CfA) N. Sehgal (KIPAC) C. Sarazin (Virginia)

2. Groups? Why do we care? 10 15 M ʘ 10 13 M ʘ group cluster Galaxy groups are the smallest halos where the bulk of baryons are accounted for. Mass function group cluster Detected baryons (McGaugh + 2010)‏

3. Groups are : 1) Ideal systems to study baryonic physics (e.g., SN winds, cooling, AGN heating), which dominates the systematic uncertainties for cluster cosmology. 2)The same baryonic physics is important to understand the formation and evolution of galaxies (most galaxies are in groups). Stellar fraction (McGaugh + 2010)

4. Are groups gas / baryon poor ? Are groups metal poor ?

5. Sun + 2009 ; Vikhlinin + 2009, Chandra samples (r 2500 ~ 1/3 r vir, r 500 ~ 2/3 r vir ) Groups are gas poor around the center cosmic baryon fraction

6. L X - T Groups (Sun+09), Chandra + REXCESS (Pratt+09, +10), XMM (both samples also agree well on other scaling relations) Bolometric < r 500 Bolometric (0.15 r 500, r 500 ) 2.91 2.85

7. Are groups gas poor at large radii ? Vikhlinin + 09; REXCESS (Pratt + 09); Sun + 09

8. Humphrey et al. 2011, Chandra Deep observations are required for gas at large radii ! L X, 2500 / L X, 500 ~ 4/5 vs. M gas, 2500 / M gas, 500 ~ 1/4 10 12 10 13 10 14 10 15 M 2500 or M 500 (M ʘ )

9. Pressure content in groups Sun, Sehgal + 2011 The key to understand the SZ power spectrum (from SPT and ACT) as ~ half of the SZ power at l = 3000 comes from low-mass systems ! Low SZ power measured from SPT ( ~50% of expected for  8 = 0.8) Lueker + 2010 and Shirokoff + 2011

10. What about baryon content in groups? 1) f gas  at r > r 500 (NFW, β=0.5, ~ 80% increase from r 500 to r 101 ) (but, slope change? mass bias? sample bias? clumping?) 2) Intragroup light (IGL) ? cosmic baryon fraction (Lin+03; Gonzalez+07; Giodini+09 Sun+09; Vikhlinin+09) stars gas + stars

11. Are groups gas / baryon poor ? Are groups metal poor ?

12. Baumgartner et al. 2005 Are groups metal (iron) poor ? Emission-weighted (ASCA) M gas -weighted Sun et al. 2011 (also see Rasmussen & Ponman, 2009; Bregman + 2010)

13. Why ? metal loss at early time (e.g., SN winds, Quasars) late time (e.g., radio AGN) Group gas is also iron poor around the center If all metals produced were kept … abundance of clusters

14. Looking forward … 1) Hydrostatic equilibrium mass to be calibrated 2) Selection bias ? --- (try different samples) 3) Extending to lower masses ! (low nH and back.) 4) Go beyond r 500 (e.g., stacking) 5) More theory / simulation work

15. Conclusions: 1) Groups are gas poor around the center (e.g., < 1/3 r vir ) but not necessarily gas poor at large radii. The gas distribution has important implications on e.g., feedback and SZ power spectrum. 2) Groups have a higher SF efficiency than clusters but the group stellar fraction (including IGL) needs to be better constrained. 3) Groups are also metal (iron) poor around the center (e.g., < 1/3 r vir ).

16. The End

17. Entropy scaling relations ( Sun et al. 2009, 43 groups; Vikhlinin et al. 2009, 14 clu.; Pratt et al. 2010, 31 clu.)‏

18. Halo mass Stellar mass / Halo mass Halo Stellar Fraction

19. Sehgal et al. 2010 Arnaud et al. 2010 (REXCESS) Battaglia et al. 2010 Shaw et al. 2010 Trac et al. 2010 Sun, Sehgal et al. 2011

20. Why are groups metal poor? Systematic issues: 1) “iron bias” (Buote 2000) for group cool cores, but NOT the reason here 2) “inverse iron bias” (Rasia et al. 2008) for 2 – 3 keV systems, up to 40% Physical reasons: 1) re-accretion of pristine gas (e.g., Renzini 1997) ? 2) metal loss at early times (SN winds)? related to pre-heating (see Rasmussen & Ponman 2009) 3) late-stage metal loss (AGN? Kirkpatrick et al.; Simionescu, Werner et al.)

21. 1) No pretty Chandra images 2) Hydrostatic equilibrium mass 3) Selection bias 4) Correlated errors

22. Are groups gas poor at large radii ? Vikhlinin + 09; REXCESS (Pratt + 09); Sun + 09 @ r 500

23. L X - T Groups (Sun+09), Chandra + REXCESS (Pratt+09, +10), XMM Bolometric < r 500 0.5-2 keV < r 500 Bolometric (0.15 r 500, r 500 ) 0.5-2 keV (0.15 r 500, r 500 ) 2.91 2.85 2.60 2.55