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.

Strategy:

XI Project: XMM +

- BVR imaging w/ Las Campanas 100”- Multi-object spectroscopy w/ IMACS @ Baade/Magellan.

Sample and analysisSample and analysis

Drawn from the 2dF galaxy group catalogue(Merchan & Zandivarez 2002).

Selection criteria:

0.060 < z < 0.063 - so R

vir ≈ 1 Mpc matches FOV ≈ 30'.

Ngal

≥ 5 - to avoid 'spurious' groups.

σ < 500 km/s - need poor systems (most common, dyn.

evolution most rapid, dispersion in properties greatest).

25 groups selected at random:

Group Ngal SigmaMZ5383 9 385MZ9014 13 260MZ4577 5 223MZ9307 7 401

XIXI: First X-ray results: First X-ray results

Rasmussen et al., MNRAS, submitted.

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:

Sample and analysisSample and analysis

Selection criteria:

● “brightness” > 6000 photonsto enable detailed spatially

resolved spectroscopy.

● D > 20 Mpcto go well outside the group core.

● Undisturbed morphologyto exclude groups with recent

merger activity.

1-T and 2-T model fits to spectra with ≥ 2000 net cts. Free parameters: T, ZFe

, ZSi, Z

others

(vapec model in xspec with solar abundances from Grevesse & Sauval 1998).

All radii converted into r/r500

, using (Evrard et al. 1996)r

500 = (124/H

0) × (‹T

X›/10 keV)1/2

Basis: Chandra archival data of GEMS groups (Osmond & Ponman 2004).

Surface brightness & temperature structureSurface brightness & temperature structureGroups relaxed (supports use of 1-D profiles),

have a cool core extending beyond central galaxy.

0.3-2 keV adaptively smoothed images.

A correlation between A correlation between ‹‹TTXX›› and and ‹‹ZZ›› ? ?

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.

<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.

Show lack of L* galaxies.

NGC 6482

2 x 0.5 rvir

N-body + hydro-simulationsN-body + hydro-simulations

Basis: Cosmological ΛCDM 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

Formation of fossil vs non-fossil groupFormation of fossil vs non-fossil group

Sample divided into 2, according to whether Δm12

≥ 2 (FG's) or Δm12

< 2 (non-FG's).

ResultsResults

Composite lum. function

Stellar mass of BG1 and BG2 in FG and non-FG.

Δm12 and “formation” redshift

InterpretationInterpretation

Fossils form via dynamical friction.Drag acceleration (Binney & Tremaine 1987) in SIS potential:

adyn

∝ −Mgal

ρ × f(V).

Infall time-scale: tinf

∝ r

0 V

H2 V

S-3

~ H0

-1 for L* galaxy at r

0=100 kpc in M=1014 M group.

Drag acceleration ∝ Mgal

so dwarfs experience less dyn.

friction.

But timescales long and ∝VH

3

(why fossil clusters don't exist).

Infall along filaments required to build fossils.

SummarySummary

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. Δm12

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 ∝V

H3 so fossil clusters shouldn't exist.

FG's reside preferentially in low-density environments. (should be easily observationally testable).

Simulations suggest:

An evolutionary sequence?An evolutionary sequence?

XI groups: In the process of collapse. Tenuous IGM, low L

X, 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 L

B. Endpoint of dynamical evolution

(eventually also in clusters!). Relation to BCGs?