Probing structure formation

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Probing structure formation evolution with
galaxy groups Jesper Rasmussen (Univ. of
Birmingham)

• Main collaborators

T. Ponman S. Raychadhury T. Miles (Birmingham)
E. D'Onghia (MPE) J. Mulchaey (Carnegie)
J. Sommer-Larsen K. Pedersen (DARK, Copenhagen)
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Outline
- Cosmological importance of galaxy groups
(I) The XI project Studying an unbiased sample
of galaxy groups The nature of the group
population deep X-ray and optical observations
of groups.
(II) Metallicity structure of hot gas in
dynamically relaxed groups The Chandra view of
chemical enrichment and redistribution of X-ray
gas.
(III) Formation of fossil groups in a
hierarchical Universe The nature and origin of
fossil groups.

• Projects unrelated but each provide a specific
  view of the baryonic component in different
  stages of group evolution.

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Why study groups?
- act as precursors to clusters in hierarchical
structure formation

• 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!
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(I) The XI Groups Project
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
galaxies within groups.
Strategy XI Project XMM - BVR imaging w/
Las Campanas 100 - Multi-object spectroscopy w/
IMACS _at_ Baade/Magellan.

• Goal Understand natureevolution of galaxy
  population in groups and its connection to global
  group properties.

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Sample and analysis
25 groups selected at random
Drawn from the 2dF galaxy group
catalogue (Merchan Zandivarez 2002).
Selection criteria 0.060 lt z lt 0.063 - so Rvir
1 Mpc matches FOV 30'. Ngal 5 - to avoid
'spurious' groups. s lt 500 km/s - need poor
systems (most common, dyn. evolution most rapid,
dispersion in properties greatest).
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XI First X-ray results

• Rasmussen et al., MNRAS, submitted.

Exposure-corrected surface brightness profiles of
unsmoothed data.
Smoothed 0.3-2 keV XMM mosaic images, 19' x 19'
( 1.3 x 1.3 Mpc)
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Comparison to X-ray selected groups

• LX, s, T all indicate depth of gravitational
  potential.
• X-ray groups obey an LX - s relation

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Why are the XI groups X-ray underluminous?
Groups grav. bound All have number density
contrasts d?/lt?gt 80. Leaves at least 3 possible
explanations

• 1) Many collapsed groups contain very little
  intragroup gas.
• E.g. due to strong galactic feedback.
• - why can feedback reduce LX by 2 orders of mag
  in systems with similar potential wells?
• - Ellipticals generate more feedback, but XI
  spiral fraction is large, 65.

2) Gas not heated to X-ray temperatures (grav.
potentials too shallow). But Large s's
indicating deep potentials. Two groups do show
X-ray emission. Large tcool ? density, rather
than T, is low.
3) XI groups are collapsing for the first time. -
consistent with X-ray/optical studies of large
groupcluster samples (e.g. Girardi Giuricin
2000). - consistent with cosmological simulations
of hierarchical structure formation - consistent
with absence of central, dominant elliptical
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Summary 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.
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(II) Metallicity structure in relaxed galaxy
groups
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

• 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 ?

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Sample and analysis
Basis Chandra archival data of GEMS groups
(Osmond Ponman 2004).

• Selection criteria
• brightness gt 6000 photons
• to enable detailed spatially resolved
  spectroscopy.
• D gt 20 Mpc
• to go well outside the group core.
• Undisturbed morphology
• to exclude groups with recent merger activity.

1-T and 2-T model fits to spectra 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
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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.
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A correlation between TX and Z ?
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 TX (Buote
2000).

• Chandra XMM results for 22 groups
• Correlation test Kendall's t 0.12
  (significance 0.8s).
• So no indication that Fe preferentially ejected
  from lower-mass systems within this TX-range.

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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

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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.

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Combining the results...
All 200 measurements

• Fe declines outside group core at
• gt 4s 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 0.6s)

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....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?

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Implications

• Although ltZgt 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 MFe/LB smaller in groups than
  clusters (Renzini 1997).
• But ltZgt 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?

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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 const. with r.
• 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.

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(III) Cosmological simulations of galaxy groups-
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?

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N-body hydro-simulations
Basis Cosmological ?CDM dark-matter simulation,
starts at z 39 (Sommer-Larsen et al. 2005).
Randomly selected 12 isolated groups with M
1014 M? for SPH re-simulation (D'Onghia et al.
2005) Study cosmologically representative sample.

• SPH code incorporates
• star formation
• chemical evolution
• metal-dependent radiative cooling
• cosmic UV field
• galactic starburst winds

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Formation of fossil vs non-fossil group
Sample divided into 2, according to whether ?m12
2 (FG's) or ?m12 lt 2 (non-FG's).
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Results
Stellar mass of BG1 and BG2 in FG and non-FG.
Composite lum. function
?m12 and formation redshift
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Interpretation
Fossils form via dynamical friction. Drag
acceleration (Binney Tremaine 1987) in SIS
potential adyn ? -Mgal ? f(V). Infall
time-scale tinf ? r0 VH2 VS-3 H0-1 for L
galaxy at r0100 kpc in M1014 M? group.
Drag acceleration ? Mgal so dwarfs experience
less dyn. friction. But timescales long and ?VH3
(why fossil clusters don't exist). Infall
along filaments required to build fossils.
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Summary
Simulations suggest

• Cosmological simulations can reproduce the
  formation of fossil groups.
• Fraction (2-4 out of 12) agrees with obs.
  estimates.
• Fossil groups form via dyn. friction. ?m12 scales
  with formation redshift.
• ? FG's are old systems, should have high dark
  matter concentration.
• Formation of FG's requires low impact parameters
  accretion through filaments.
• Timescales ?VH3 so fossil clusters shouldn't
  exist.
• FG's reside preferentially in low-density
  environments.
• (should be easily observationally testable).

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An evolutionary sequence?

• XI groups In the process of collapse. Tenuous
  IGM, low LX, high spiral fraction, no central,
  dominant elliptical.

Relaxed groups. X-ray luminous, contain dominant
E which has affected its surroundings.
Fossils X-ray luminous. Central elliptical
completely dominates LB. Endpoint of dynamical
evolution (eventually also in clusters!).
Relation to BCGs?