1. Jesper Rasmussen (Univ. of Birmingham)
Metallicity structure in X-ray bright galaxy groups - Radial distribution of metals in dynamically relaxed groups
Jesper Rasmussen (Univ. of Birmingham)
with Trevor Ponman

2. Background X-ray spectroscopy of hot group gas – issues to address:
Metal abundances in clusters well-studied, situation in groups much more unclear.
But significant fraction of baryons (and galaxies) are in groups:
Chemical evolution of the Universe ↔ content and distribution of metals in groups.
X-ray spectroscopy of hot group gas – issues to address:
Behaviour of SN II products outside group cores?
Fe-content in outskirts? Need to determine ZFe at large radii, to estimate total iron masses & compare to enrichment models.
Abundance profiles: Also signatures of galactic feedback – can we disentangle AGN (redistribution of gas) from supernova (source of metals) feedback ?

3. Sample and analysis
Basis: Chandra archival data of GEMS groups (Osmond & Ponman 2004).
Selection criteria:
“brightness” > 6000 photons
to enable detailed spatially resolved spectroscopy.
D > 20 Mpc
to go well outside the group core.
Undisturbed morphology
to exclude groups with recent merger activity.
1-T and 2-T plasma model fits to spectra in radial bins with ≥ 2000 net cts.
Free parameters: T, ZFe, ZSi, Zothers
(vapec model in xspec with solar abundances from Grevesse & Sauval 1998).
All radii converted into r/r500, using (Evrard et al. 1996)
r500 = (124/H0) × (‹TX›/10 keV)1/2

4. Surface brightness & temperature structure
Groups relaxed (supports use of 1-D profiles),
have a cool core extending beyond central galaxy.
0.3-2 keV adaptively smoothed images.

5. A correlation between ‹TX› and ‹Z› ?
Do groups show lower abundances than clusters?
Real effect (e.g. ejection of metals from group potentials) or induced by systematics ?
- gas in clusters detected to relatively larger radii.
- importance of Fe bias increasing at low TX (Buote 2000).
<T> and <Z> measured within r500 :
Chandra + XMM results for 22
groups:
Correlation test: Kendall's τ = 0.12 (significance: 0.8σ).
So no indication that Fe
preferentially ejected from
lower-mass systems within this
TX-range.

6. Fe and Si profiles Fe profiles: Central excesses.
Profiles bottoming out towards ~ 0.1 Z, lower than in clusters (Böhringer et al. 2004; Tamura et al. 2004).
Si profiles:
Similar to ZFe(r) in group cores.
Smaller radial variation at large r.
Increase in outer parts in some groups.

7. Silicon-to-iron ratio
SN II
ZSi/ZFe: signature of relative importance of SN II vs SN Ia.
Adopted SN model abundances: Baumgartner et al. (2005).
Based on yields from Nomoto et al. (1997) + Salpeter IMF.
SN Ia
Metal production dominated by SN Ia in central regions.
Si/Fe: In group cores generally consistent with local (Solar) SN mixture and IMF.

8. Combining the results... Fe declines outside group core at
All 200 measurements:
Fe declines outside group core at
> 4σ significance, with
log (ZFe) ∝ −0.7 log (r/r500).
Value at r500 is ~ 0.1 Z
Si is almost constant with r
outside core (declines at 0.6σ)

9. ....and binning them too In group cores: Both Fe, Si roughly uniform.
SN Ia dominate, probably from central, bright galaxy.
SN II contribution required at all radii, dominate at large radii.
Due to early enrichment from less massive galaxies?

10. Implications
Although <Z> ≈ 0.3Z, as in clusters, Fe abundance at large radii lower than in clusters by factor of ~2.
Total MFe in gas mainly determined by ZFe at large r,
 MFe/LB smaller in groups than clusters (Renzini 1997).
But <Z> does not correlate with depth of grav. potential (TX)
&
baryon fractions in some T ~ 1-2 keV groups are near-cosmic
(Buote et al. 2004, Rasmussen & Ponman 2004)
Ejection of (enriched) gas via AGN/SN winds not globally important?
Significant fraction of Fe in groups not accounted for?
non-central enrichment inefficient?
ejection of metals only efficient in outskirts and/or accompanied by very low “mass-loading”, independently of TX ?

11. Summary & outlook
Fe profiles show central excesses, but flatten out to ~ 0.1 Z, lower than in clusters (e.g. Tamura et al. 2004). Si nearly constant with r outside group core.
“Global” mean of ZSi/ZFe ≈ 1.3 solar – agrees with cluster results. But clear dichotomy in Si/Fe distribution.
Enrichment in group cores marginally dominated by SN Ia.
SN II contribution required at all radii, and dominates strongly in outer parts.
Low Z at large radii challenging simple enrichment models if baryon fractions
are near-cosmic (Buote et al. 2004).
Planned work:
- Investigate correlations with radio luminosity of central galaxy.
- perform detailed tests of enrichment/feedback models.