Probing structure formation Probing structure formation & evolution with galaxy & evolution with galaxy groups groups Jesper Rasmussen (Univ. of Birmingham) Main collaborators: T. Ponman S. Raychadhury T. Miles (Birmingham) J. Sommer- Larsen K. Pedersen (DARK, Copenhagen) E. D'Onghia (MPE) J. Mulchaey (Carnegie)
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Probing structure formation & evolution with galaxy groups Probing structure formation & evolution with galaxy groups Jesper Rasmussen (Univ. of Birmingham)
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Probing structure formation & evolution Probing structure formation & evolution with galaxy groupswith galaxy groups
Jesper Rasmussen (Univ. of Birmingham)
Main collaborators:
T. PonmanS. RaychadhuryT. Miles(Birmingham)
J. Sommer-LarsenK. Pedersen(DARK, Copenhagen)
E. D'Onghia (MPE)
J. Mulchaey (Carnegie)
OutlineOutline
Projects unrelated but each provide a specific view of the baryonic component in different stages of group evolution.
(I) The XI project: Studying an unbiased sample of galaxy groupsThe nature of the group population – deep X-ray and optical observations of groups.
(II) Metallicity structure of hot gas in dynamically relaxed groupsThe Chandra view of chemical enrichment and redistribution of X-ray gas.
(III) Formation of “fossil groups” in a hierarchical UniverseThe nature and origin of fossil groups.
- Cosmological importance of galaxy groups
Why study groups?Why study groups?
Galaxy groups
– contain majority of galaxies (Eke et al. 2004) and baryons (Fukugita et al. 1998) in the local Universe
– i.e. are the characteristic structures formed at the present epoch
– act as precursors to clusters in hierarchical structure formation
– can serve as laboratories for the study of - galaxy evolution (galaxy-galaxy interactions efficient, most gal's in groups) - non-gravitational processes in structure formation
Groups cosmologically
important!
(I) The (I) The XIXI Groups Project Groups Project
Goal: Understand nature+evolution of galaxy population in
groups and its connection to global group properties.
XMM
IMACS
Motivation:
X-ray obs. of groups: Heterogeneous samples of hand-picked systems. X-ray selection may build in serious bias.
Currently no unbiased census of properties of - hot gas (= intragroup medium; IGM) - dynamics of galaxieswithin groups.
Smoothed 0.3-2 keV XMM mosaic images,19' x 19' (~ 1.3 x 1.3 Mpc)
Exposure-corrected surface brightness profiles of unsmoothed data.
Comparison to X-ray selected groupsComparison to X-ray selected groups
LX, σ, T all indicate depth of gravitational
potential.X-ray groups obey an L
X - σ relation: 2 out of 4 cases: No hot intragroup
medium detected.
In MZ9014: Only faint irregular IGM emission. Disturbed X-ray morphology,
Mgas
~ 4 x 1011 M
All 4 groups X-ray underluminous relative to expectations from X-ray bright groups.
Suggests we are targeting a class of groups not previously studied in detail.
Why are the Why are the XIXI groups X-ray underluminous? groups X-ray underluminous?
1) Many collapsed groups contain very little intragroup gas.Many collapsed groups contain very little intragroup gas.E.g. due to strong galactic feedback. - why can feedback reduce L
X by 2 orders of mag in systems with similar potential wells?
- Ellipticals generate more feedback, but XI spiral fraction is large, ~65%.
Groups grav. bound: All have number density contrasts δρ/<ρ> ≥ 80. Leaves at least 3 possible explanations:
2) Gas not heated to X-ray temperatures (grav. potentials too shallow).Gas not heated to X-ray temperatures (grav. potentials too shallow).But: Large σ's indicating deep potentials.Two groups do show X-ray emission. Large t
cool → density, rather than T, is low.
3) XIXI groups are collapsing for the first time. groups are collapsing for the first time.- consistent with X-ray/optical studies of large group+cluster samples (e.g. Girardi & Giuricin 2000).- consistent with cosmological simulations of hierarchical structure formation- consistent with absence of central, dominant elliptical
Summary & outlookSummary & outlook
Low LX, disturbed X-ray morphology, no dominant elliptical:
Observed groups not virialised - systems only now collapsing.
With our z-selected sample we are catching groups at a different stage than those previously studied.
Current X-ray studies of galaxy groups may be biased towards dynamically old (and perhaps rather uncommon?) systems.
Eventual key outcomes● state of collapse of groups● reliable estimate of fraction of optically selected groups which contain a hot IGM.
X-ray (+ radio) status:
6 more XMM data sets coming up - 15 more proposed for. HI imaging too...Soon: 10 groups in X-rays with complete optical coverage. Will allow us to cut sample in 2, study differences.
(II) Metallicity structure in relaxed galaxy (II) Metallicity structure in relaxed galaxy groupsgroups
● Fe-content in outskirts? Need to determine ZFe
at large radii, to estimate total iron masses.
● Behaviour of SN II products outside group cores?
● Abundance profiles: Also signatures of galactic feedback – can we disentangle AGN (redistribution of gas) from supernova (source of metals) feedback ?
Background:
Metal abundances in clusters well-studied, situation in groups much more unclear. But majority of galaxies are in groups →chemical evolution of the Universe ↔ metals in groups
X-ray spectroscopy of hot group gas - issues to address:
So no indication that Fe preferentially ejected from lower-mass systems within
this TX-range.
<T> and <Z> measured within 0.1-0.3 r500 :
Do groups show lower abundances than clusters? Correlation induced by systematics? - gas in clusters detected to relatively larger radii.- importance of Fe bias increasing at low T
X (Buote 2000).
Fe and Si profilesFe 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
Silicon-to-iron ratioSilicon-to-iron ratio
Metal production dominated by SN Ia in central regions.
Si/Fe: In group cores generally consistent with local (Solar) SN mixture and IMF.
SN Ia
SN II ZSi/Z
Fe: 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.
Combining the results...Combining the results...
Fe declines outside group core at > 4σ significance, with log (Z
Fe) ∝ −0.7 log (r/r
500).
Value at r500
is ~ 0.1 Z
Si is almost constant with routside core (declines at 0.6σ)
All 200 measurements:
....and binning them too....and binning them too
Both Fe, Si roughly constant within group core. SN II contribution required at all radii.
SN Ia in group cores, probably from central, bright galaxy.SN II at large radii – early enrichment from less massive galaxies?
ImplicationsImplications
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, confirming that M
Fe/L
B smaller in groups than clusters (Renzini 1997).
But <Z> does not correlate with depth of grav. potential (TX): Ejection of enriched gas via AGN/SN winds not important?
If baryon fractions in T ~ 1-2 keV groups are near-cosmic (Buote et al. 2004, Rasmussen & Ponman 2004):
• significant fraction of Fe in groups not accounted for?
• ejection of metals accompanied by very low “mass-loading”, independently of TX ?• non-central enrichment is inefficient?
Summary & outlookSummary & 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 const. with r.
“Global” mean of ZSi/Z
Fe ≈ 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 fractionsare near-cosmic (Buote et al. 2004).
Planned work:- Investigate correlations with radio luminosity of central galaxy.- perform detailed tests of enrichment/feedback models.
(III) Cosmological simulations of galaxy (III) Cosmological simulations of galaxy groupsgroups
- Investigating the origin of “fossil” groups
FG's: Comprise nearly all “field” ellipticals with MR ≤ −22.5. Locally as numerous as
poor and rich clusters combined (10-20% of all systems of comparable LX).
Origin not clear. Early studies indicated high M/L ratios.
Recent obs. indicate high NFW concentration parameters → early formation epoch?
Product of mergers or of an unusual galaxy luminosity function?
“Definition” of fossils:
Large isolated elliptical galaxy with LX
> 1042 erg/s and Δm
12 ≥ 2 within 0.5 r
vir.
Extent and T of X-ray gas indicate group-like total mass.