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

1. Main collaborators: Probing structure formation & evolution with galaxy groupsJesper Rasmussen (Univ. of Birmingham) T. Ponman S. Raychadhury T. Miles (Birmingham) E. D'Onghia(MPE) J. Mulchaey(Carnegie) J. Sommer-Larsen K. Pedersen (DARK, Copenhagen)

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

3. - act as precursors to clusters in hierarchical structure formation 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!

4. XMM IMACS (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. Goal: Understand nature+evolution of galaxy population in groups and its connection to global group properties. Strategy: XI Project: XMM + - BVR imaging w/ Las Campanas 100” - Multi-object spectroscopy w/ IMACS @ Baade/Magellan.

5. Sample and analysis 25 groups selected at random: Drawn from the 2dF galaxy group catalogue (Merchan & Zandivarez 2002). Selection criteria: 0.060 < z < 0.063 -so Rvir ≈ 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).

6. 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)

7. LX, σ, Tall indicate depth of gravitational potential. X-ray groups obey an LX - 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 targetinga class of groups not previously studied in detail. Comparison to X-ray selected groups

8. 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%. Why are the XI groups X-ray underluminous? 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). But: Large σ'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 group+cluster samples (e.g. Girardi & Giuricin 2000). - consistent with cosmological simulations of hierarchical structure formation - consistent with absence of central, dominant elliptical

9. 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. 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.

10. (II) Metallicity structure in relaxed galaxy groups • 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:

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

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

13. 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 : 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).

14. 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 Fe and Si profiles

15. Metal production dominated by SN Ia in central regions. Si/Fe: In group cores generally consistent with local (Solar) SN mixture and IMF. 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

16. 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) Combining the results... All 200 measurements:

17. 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? ....and binning them too

18. 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 MFe/LB 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? Implications • 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?

19. 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. Summary & outlook

20. NGC 6482 “Definition” of fossils: Large isolated elliptical galaxy with LX > 1042 erg/s and Δm12≥ 2 within 0.5 rvir. Extent and T of X-ray gas indicate group-like total mass. Show lack of L* galaxies. 2 x 0.5 rvir (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?

21. N-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

22. Formation of fossil vs non-fossil group Sample divided into 2, according to whether Δm12≥ 2 (FG's) or Δm12 < 2 (non-FG's).

23. Results Stellar mass of BG1 and BG2 in FG and non-FG. Composite lum. function Δm12and“formation” redshift

24. Interpretation Fossils form via dynamical friction. Drag acceleration (Binney & Tremaine 1987) in SIS potential: adyn ∝−Mgalρ× f(V). Infall time-scale: tinf∝r0 VH2 VS-3 ~ H0-1 for L* galaxy at r0=100 kpc in M=1014 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.

25. 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 ∝VH3 so fossil clusters shouldn't exist. FG's reside preferentially in low-density environments. (should be easily observationally testable). Summary Simulations suggest:

26. XI groups: In the process of collapse. Tenuous IGM, low LX, high spiral fraction, no central, dominant elliptical. An evolutionary sequence? 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?