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?
cluster
1015 Mʘ
group
cluster
group
1013
Mʘ
Mass function
Detected baryons
Galaxy groups are the
smallest halos where the
bulk of baryons are
accounted for .
(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 ?
Groups are gas poor around the center
cosmic baryon fraction
cosmic baryon fraction
Sun + 2009 ; Vikhlinin + 2009, Chandra samples
(r2500 ~ 1/3 rvir , r500 ~ 2/3 rvir)
Bolometric
< r500
LX - T
Groups
(Sun+09), Chandra
+
REXCESS
(Pratt+09, +10), XMM
(both samples also agree well
on other scaling relations)
2.91
Bolometric
(0.15 r500, r500)
2.85
Are groups gas poor at large radii ?
Vikhlinin + 09; REXCESS (Pratt + 09); Sun + 09
Deep observations are required for
gas at large radii !
LX, 2500 / LX, 500 ~ 4/5
vs.
Mgas, 2500 / Mgas, 500 ~ 1/4
1013
1014
1015
M2500 or M500 (Mʘ)
Humphrey et al. 2011, Chandra
1012
Pressure content in groups
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
Sun, Sehgal + 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 fraction
stars
(Lin+03; Gonzalez+07; Giodini+09
cosmic baryon fraction
gas + stars
Sun+09; Vikhlinin+09)
Are groups gas / baryon poor ?
Are groups metal poor ?
Are groups metal (iron) poor ?
Emission-weighted (ASCA)
Baumgartner et al. 2005
Mgas-weighted
Sun et al. 2011
(also see Rasmussen & Ponman, 2009; Bregman + 2010)
Group gas is also iron poor around the center
If all metals
produced
were kept …
abundance of
clusters
Why ?
metal loss at early time (e.g., SN winds, Quasars)
late time (e.g., radio AGN)
Looking forward …
𝑝 𝑀1 𝑀true)
𝑝 𝑀1 obs 𝑀obs, 𝑂)
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 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.)
Stellar mass / Halo mass
Halo Stellar Fraction
Halo mass
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 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.)
1) No pretty Chandra images
2) Hydrostatic equilibrium mass
3) Selection bias
4) Correlated errors
Are groups gas poor at large radii ?
@ r500
Vikhlinin + 09; REXCESS (Pratt + 09); Sun + 09
LX - T
Bolometric
< r500
Groups
(Sun+09),
Chandra
+
REXCESS
(Pratt+09,
+10),
XMM
0.5-2 keV
< r500
2.60
2.91
Bolometric
(0.15 r500, r500)
0.5-2 keV
(0.15 r500, r500)
2.85
2.55