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)

Groups? Why do we care?

1015 Mʘ 1013 M

ʘ

groupcluster

Galaxy groups are the smallest halos where the bulk of baryons are accounted for .

Mass function

groupcluster

Detected baryons(McGaugh + 2010)

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)

Are groups gas / baryon poor ?

Are groups metal poor ?

Sun + 2009 ; Vikhlinin + 2009, Chandra samples (r2500 ~ 1/3 rvir , r500 ~ 2/3 rvir)

Groups are gas poor around the center

cosmic baryon fraction cosmic baryon fraction

LX - T

Groups(Sun+09), Chandra

+

REXCESS(Pratt+09, +10), XMM

(both samples also agree well on other scaling relations)

Bolometric< r500

Bolometric(0.15 r500, r500)

2.91

2.85

Are groups gas poor at large radii ?

Vikhlinin + 09; REXCESS (Pratt + 09); Sun + 09

Humphrey et al. 2011, Chandra

Deep observations are required for gas at large radii !

LX, 2500 / LX, 500 ~ 4/5

vs.

Mgas, 2500 / Mgas, 500 ~ 1/4

1012 1013 1014 1015

M2500 or M500 (Mʘ)

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

What about baryon content in groups?

1) fgas at r > r500

(NFW, β=0.5, ~ 80% increase from r500

to r101

)

(but, slope change? mass bias? sample bias? clumping?) 2) Intragroup light (IGL) ?

cosmic baryon fractioncosmic baryon fraction

(Lin+03; Gonzalez+07; Giodini+09 Sun+09; Vikhlinin+09)

stars gas + stars

Are groups gas / baryon poor ?

Are groups metal poor ?

Baumgartner et al. 2005

Are groups metal (iron) poor ?

Emission-weighted (ASCA) Mgas

-weighted

Sun et al. 2011

(also see Rasmussen & Ponman, 2009; Bregman + 2010)

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

Looking forward …

1) Hydrostatic equilibrium mass to be calibrated2) Selection bias ? --- (try different samples)3) Extending to lower masses ! (low nH and back.)4) Go beyond r500 (e.g., stacking)5) More theory / simulation work

Conclusions:

1) Groups are gas poor around the center (e.g., < 1/3 rvir) 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 rvir) .

The End

Entropy scaling relations

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

Halo mass

Ste

llar

mas

s / H

alo

mas

sHalo Stellar Fraction

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

Why are groups metal poor?

Systematic issues:1) “iron bias” (Buote 2000) for group cool cores, but NOT the reason here2) “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.)

1) No pretty Chandra images

2) Hydrostatic equilibrium mass

3) Selection bias

4) Correlated errors

Are groups gas poor at large radii ?

Vikhlinin + 09; REXCESS (Pratt + 09); Sun + 09

@ r500

LX - T

Groups(Sun+09), Chandra

+

REXCESS(Pratt+09, +10), XMM

Bolometric< r500

0.5-2 keV< r500

Bolometric(0.15 r500, r500)

0.5-2 keV(0.15 r500, r500)

2.91

2.85

2.60

2.55