Abundances in intragroup gas

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Abundances in intragroup gas

• Trevor Ponman
• University of Birmingham

Plus Jesper Rasmussen, Ria Johnson, Alexis
Finoguenov
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Abundances in intragroup gas

• Not all groups have detectable hot intergalactic
  gas
  
• This example is pretty, but unusually complex
• Typically, X-ray bright groups have hot gas
  centred on a central dominant early-type galaxy

X-ray/optical overlay
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Outline

• Abundance distributions in galaxy groups
• Chandra study of cool core groups
• XMM study including non-cool core systems
• Comparison to galaxy clusters
• Key points arising and some implications

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Abundances in groups Chandra study

• Rasmussen Ponman Chandra study of 15 X-ray
  bright groups, chosen to be reasonably relaxed.
  All but one have cool cores.
  
• Annular spectra extracted and fitted with hot
  plasma model (VAPEC), with Fe and Si abundances
  fitted.
  
• Groups scaled to r500 and stacked.

Rasmussen Ponman 2007
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Abundances in groups Chandra study

• Coadding the results for all 15 groups shows the
  average distributions of metal abundances much
  more clearly.

Rasmussen Ponman (in prep.)
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Abundances in groups Chandra study

• Central abundance peak at solar metallicity,
  with dip inside 0.01r500
  
• Outside this peak, iron abundance drops rapidly
  to 0.1-0.2 solar, by r0.4r500
  
• Si drops less rapidly, and so Si/Fe rises, from
  solar ratios in the core, to SNII-like ratio at
  large r.

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2dXGS study of group abundances

• 2D spectral mapping study of a sample of low
  redshift groups using XMM-Newton, by Alexis
  Finoguenov et al.
  
• Spectra extracted from regions with similar
  spectral properties and surface brightness, and
  fitted with APEC model (AG abundance system).
  
• Sample includes 17 cool core (CC) and 11
  non-cool-core (NCC) systems.

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2dXGS study of group abundances

• Sample includes 17 CC and 11 NCC systems
• Compare results for stacked CC systems to those
  from the Chandra study
  
• Abundances scaled up by factor 1.48 to correspond
  to Grevesse Sauval system used with Chandra
  data
  
• Good agreement on low group abundance at large r
• Central abundance dip not resolved in XMM data

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2dXGS study of group abundances
Comparing profiles for stacked CC and NCC
systems Abundance profiles are essentially flat
in NCC systems, in contrast to the pronounced
central abundance peak seen in CC groups.
Johnson et al (in prep.)
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Comparison to clusters

• Metallicity in clusters typically shows a central
  enhancement, outside which it drops to 0.2-0.4
  solar.
  
• XMM (e.g. Pratt et al 2007) and Chandra (e.g.
  Vikhlinin et al 2005, Baldi et al 2007) results
  confirm these features.
  
• The central peak may be plausibly explained by
  ejecta from the central galaxy - with
  predominantly SNIa origin (lower alpha/Fe) - i.e.
  similarly to what we find in groups.

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Comparison to clusters
Fe abundance in CC groups drops a factor 2 lower
than in clusters outside 0.4r500 (cf Buote et
al 2004).
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Comparison to clusters
Fe abundance in NCC groups appears to be a little
higher than in NCC clusters. No sign of central p
eak in groups.
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Key points on group abundances

• Abundance peak is present in CC groups

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Metal abundances in galaxy groups

• Decompose metals into SNIa and SNII
  contributions, assuming yields from Iwamoto et al
  (1999) WDD2 delayed detonation model for Ia, and
  Nomoto et al (2006) SNII model
• Central abundance peak arises predominantly from
  SNIa (cf Finoguenov et al 2000)
  
• The Z peak constrains mixing and cooling out of
  metal-rich gas
  
• AGN-induced mixing might account for flat
  abundance within the central 25 kpc, but not for
  a central dip in abundance

Rasmussen Ponman (in prep.)
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Metal abundances in galaxy groups

• Using the low z SNIa rates of Mannucci et al
  2005) - hence an underestimate at larger z - the
  central excess could arise from the central
  galaxy alone
• The rather flat distribution in SNII products
  suggest they were injected before cluster
  formation, at z2

Fe mass in the central peak normalised to K band
luminosity of central galaxy
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Key points on group abundances

• Abundance peak is present in CC groups
• Predominantly from SNIa products, and could be
  provided by the central galaxy
  
• Central dip in abundance is seen within the
  inner 5 kpc in many of these
  
• Not well understood, but seen also in many
  clusters (cf Andy Fabians talk) . Metal
  transport? He sedimentation? Resonant scattering?
  Fe bias?
• NCC groups have flat abundance profiles
• Suggests that NCC systems have been strongly
  mixed in the fairly recent past, which conflicts
  with models (e.g. McCarthy et al 2008, Burns et
  al 2008) in which NCC systems result from entropy
  injection at high z.
• Compared to clusters, groups have low abundance
  at large r, and substantially lower lower MFe/LB
  
  

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Metal abundances in galaxy groups

• Integrating the iron mass within r500, we find a
  strong trend with system mass in the
  iron-mass-to-light ratio.

Rasmussen Ponman (in prep.)
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What happened to metals in groups?
Bulk gas loss from groups?
Hot gas fractions do appear to drop in groups.
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What happened to metals in groups?
Bulk gas loss from groups?
Clusters
Hot gas fractions do appear to drop in groups.
But if gas were ejected from outer regions then
mean abundance of remaining gas should be raised,
whereas it is lower in groups.
Rasmussen Ponman (in prep.) Values within r500.
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What happened to metals in groups?
Less efficient star formation produces less
metals in groups?
No - the fraction of the baryons in stars is
actually higher in groups, which should lead to
more enrichment of the gas.
Rasmussen Ponman (in prep.) Values within r500.
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What happened to metals in groups?
Metals cool out at the centre?
Maybe - mass to light ratios do appear to be
lower in groups. Cooling at low z should deplete
primarily SNIa products, and is limited by the
observation of the central Fe peak.
Cooling flows seem to be suppressed in groups, as
in clusters.
Parker et al (2005)
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What happened to metals in groups?
Preferential loss of enriched gas?
Enriched by SNIa or SNII? Which is missing?
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What happened to metals in groups?
Preferential loss of enriched gas?
Enriched by SNIa or SNII? Which is missing? S
plit IMLR plot into Ia and II products.
? Both are substantially deficient!
SNIa
SNII
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Preheating in filaments - a mechanism for losing
SNII products?
Preheating at z3 blows up the baryons in
filaments, increasing the entropy of gas accreted
by clusters, and accounting for the high excess
entropies seen to large radii in groups and
clusters. Could SNII products could escape from
the small filaments which feed groups?
? Could account for reduced IMLR in groups, and
the low metallicity of gas outside the group core.
Borgani et al 2005 - baryons at z2 with low and
high feedback
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What happened to metals in groups?

• Bulk gas loss from groups? No
• Less efficient star formation produces less
  metals in groups? No
  
• Metals cool out at the centre? Maybe
• Loss of SNII products - preheating prior to
  cluster collapse ? SNII-enriched high entropy gas
  blown out of feeder filaments? Maybe
  
• Loss of SNIa products - cooling at halo centre
  /or loss via AGN-heated bubbles? Maybe
  

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Conclusions

• Galaxy groups, like cool core clusters, have
  central abundance peaks dominated by SNIa
  products, which could be provided by the central
  galaxy.
• The presence of this peak limits cooling rates
  (naively 10 M? yr-1) and mixing in the IGM.
  
• Flat distribution of SNII metals ? injection
  before group forms.
  
• Uniform abundance distribution in NCC systems
  suggests recent efficient mixing - induced by AGN
  or mergers?
  
• Fe abundances in groups drop below those of
  clusters outside the core.
  
• Integrated iron mass to light ratios are lower in
  groups by up to a factor 10.
  
• This shortfall applies to both SNIa and SNII
  products.
  
• Requires preferential loss of metal-rich gas two
  mechanisms?