Properties of Fossil groups C. Mendes de Oliveira, IAG/U.Sao Paulo, Brazil

1. Main collaborators: Eduardo Cypriano, Laerte Sodré Jr., Ignacio de La Rosa, Roberto Cid Fernandez Properties of Fossil groups C. Mendes de Oliveira, IAG/U.Sao Paulo, Brazil A New Zeal for Old Galaxies Rotorua, New Zealand – Mar/2007

2. 1 - Sistems dominated by a single giant elliptical galaxy (m12 ≥ 2 mag) 2 – Extended X-Ray emission: LX,bol ≥ 1042 h50-2 erg/s Fossil Groups: what are they ?

3. Fossil groups: End products of merging of L* galaxies in low-density environments (Ponman et al. 94, Jones et al. 2003). Extreme case of dynamical friction • The most massive versions of today's CGs are the best candidate precursors of fossil groups: • CGs with • high s • neighborhoods • rich in low-luminosity • companions • E(s) Fossil group RXJ 0454.8-1806 @ z = 0.0314 (Mendes de Oliveira 2005). B image from Blanco+Mosaic (530 kpc) Red contours are ASCA (Yoshioka et al. 2004).

4. Poor groups of galaxies • small systems (a few L* galaxies) • More than 50% of the nearby structure in the universe (Tully 1987) are in groups of 3-20 members • A small fraction of these live in compact groups (CGs are responsible for 1% of the luminosity density of the universe) • Compact Groups of galaxies (CGs) • 3-7 bright gal. with projected g/g separation ~ galaxy diameter • Low velocity dispersions (~200 km/s) • Fairly isolated (by selection) • CG  high fraction of interacting members • evolve through dynamical friction • merge to form one single galaxy ? • Fossil group (Vikhlinin et al. 1999; Jones et al. 2003)

5. CGs are physical entities : • Diffuse intragroup matter is inferred in 75% of HCG => Physically bound (Hickson 1997, Mulchaey 2002,Ponman et al.) • The X-ray emitting CGs are the higher density groups and are dominated by a E. • CGs dominated by S do not show X-ray emission (Zabludoff & Mulchaey 1998) HCG 62 An X-ray Atlas of Groups of Galaxies Mulchaey, Davis, Mushotzky and Burstein (2003) http://www.ociw.edu/~mulchaey/Atlas/atlas.html

6. Do nearby CGs mimic the high redshift universe ? • Yes: high density + low s => high interaction rate • No: CGs are long-lived structures. Probably no isolated CGs in the • high z universe but CGs may fuel high z clusters? Produce fossil groups? CGs @ high z are difficult to detect (and are still to be discovered) 1 Mpc 18 galaxies within 2000 km/s • CG 6 @ z=0.22 • Lee et al. (2004) • = 700 km s Medium redshift example of CG? Core of a cluster ?

7. Are fossil groups results of objects such as these? 1 Mpc

8. Or these? 1 Mpc

9. Age vs. velocity dispersion of the galaxy • BCGs have overall older ages than: • field elliptical galaxies (already known), • first-ranked galaxies in fossil groups.

10. Metallicity vs. velocity dispersion of the galaxy First ranked in Fossil BCG Isolated Ellipticals BCGs are overall more metal poor than isolated ellipticals and than first-ranked galaxies in fossil groups

11. Mg/Fe vs. velocity dispersion of the galaxy BCG First ranked in Fossil Isolated Ellipticals

16. Formation scenarios: • Massive end of • the elliptical galaxy distribution • (Yoshioka et al. 2004) • 2.Merging of bright galaxies in • groups (Jones et al. 2003) • If the merger interpretation is correct, fossil groups have seen little infall of luminous • galaxies since their collapse.

17. What’s the origin of fossil groups? Merging of L* galaxies… • In the field.Andromeda and Milky Way when they merge - will they form a fossil group? Unlikely, the total mass will be 5x10^12 Msun and no X-ray gas • In Compact Groups. Multiple merger of L* (Ponman et al. 1994) D‘Onghia & Lake, 2004 ApJ,612,628

18. All fossil groups known to date 15 Mendes de Oliveira, Cypriano & Sodré 2005, astro-ph/0509884

19. Members Low S/N Non Members CMD – RXJ 1552.2+2013 sigma=795 km/s Gemini/GMOS data MdO, Cypriano and Sodré 2005

20. Photometric lum. function Spectroscopic completeness function Spectroscopic lum. function Luminosity Function of RX J1552.2+2013 Mdo, Cypriano, Sodré Jr. 2005

21. Dynamical friction does not deplete the faint-end for Given a mass function only the high mass end will be affected in fossil groups by dynamical friction and major merging. for Dynamical friction will not affect the mass function below

22. In favour of merger hypothesis: • gap in luminosity function at L* • high L of central galaxy • strong correlation between LX of groups and Lopt of central galaxy Against the merger hypothesis: M/L ratios seem to be much higher in fossil groups than in clusters and groups (Yoshioka et al. 2004) • Khosroshahi et al. find no boxy galaxies in a sample of 7 first-ranked galaxies in fossil groups, indicating that these galaxies may have grown by accretion either than merger

23. Summary/Conclusions. • Nearby CGs are very complex systems, tracing their history is a challenge (e.g. Stephan’s Quintet). Groups are in different evolutionary phases. • CGs are bound structures (X-rays) showing numerous signs of interaction. They are sites of formation of tidal dwarf galaxies, young clusters and intergalactic HII regions. • CGs infalling into clusters may provide a mechanism to form clusters @ high z • Do CGs mimic the high redshift universe ? Open question. • Yes: high density + low s => high interaction rate • No: CGs are long-lived structures. Probably no isolated CGs at high z • Interpretation of kinematics of distant galaxies (and TF relation) may need nearby sample of galaxies to disentangle beam-smearing from evolutionary effects. • There are less than two dozen fossil groups known. A fraction of them are fossil clusters. A search in the SDDS-DR4 yielded five candidate fossil clusters. • First ranked galaxies – mostly disky, favours accretion either than merging. A few are cD galaxies, most have ionized gas and half have very old ages. • The groups with determined luminosity functions have either flat or declining functions. • Groups like HCG 31, Stephan’s quintet could not have been the precursors of FGs • More high-redshift compact groups and fossil groups/clusters badly needed.

25. Procedure • Selection of elliptical galaxies from SDSS (from the Luminous Red Galaxies catalog) • Cross-match with Rosat (only extended source) using OpenSkyQuery • Cone search, 0.5 Mpc around the elliptical, to find the neighbors • Constrain the neighbors using photometric redshift - check if they form a group with the elliptical • Check the photometric condition to be a fossil, in the R band: M1 - Mi > 2 • Get information about the elliptical galaxies, group members, images, etc

26. Cone search for each candidate with a 0.5 Mpc radius Select the neighbors at the same redshift, using the uncertainty of the photometric redshifts as a range Check the photometric condition to be a fossil, in the R band: M1 - Mi > 2 All these were done using SQL Selection of fossil groups candidates

27. 34 fossil groups candidates were found Results

29. Chandra image

31. Analysis of the members 11051 z - i z

33. Discussion • Selection by using bit flags in SDSS database • Photometric redshift in SDSS available for DR5, but OpenSkyQuery has up to DR3 • Nevertheless, one can use SDSS SQL Server (Casjobs) and OpenSkyQuery

35. Search for new fossil groups/clusters • Search for fossil groups in the SDSS-DR5 • Check profiles and colours to ensure that galaxy M1 (taken from LRG sample, Eisenstein 2001) is an early-type object. • A cone search is made within 1 Mpc to look for neighbours with M2 > M1+2 with accordant z-phot. • A cross-match with ROSAT all-sky Survey sources is made to choose only X-ray extended sources. dos Santos et al. 2006

36. Dwarf galaxies in HCG 68 Study of individual groups, taking into account detailed morphology give alpha = -1.2 (H68 and H44) Bomans et al. (2006) find very steep composite luminosity function of five groups using statistical background subtraction

37. H42  =-1.7 photometric LF Photometric luminosity function alpha = -1.7 Spectroscopic luminosity function alpha= -1.2 Carrasco, Mendes de Oliveira and Infante 2006  =-1.2 spectroscopic LF

38. Several groups detected in the background of HCG 42