1. Galaxy Group Studies in Future 2011. 06.07 Jubee Sohn

2. Contents What is the galaxy group? Why the galaxy group? How to study galaxy group?

3. Galaxy Group? Definition : More than two distinct galaxies that are bound gravitationally for periods of the order of 10^9 yrs (de Vaucouleur’s, 1975) Smallest aggregates of galaxies within 1 to 2 Mpc Divide into ‘Compact’ Group and ‘Loose’ Group Compact group : systems of N > 3, whose magnitude differences less than 3.

4. Why the Galaxy Group? A Group is our local environment : Our Milky Way lives in a small loose group The Local Group :

5. Why the Galaxy Group? A Group is our local environment : Our Milky Way lives in a small loose group The Local Group : The Local group comprises more than 30 galaxies Its gravitational center located somewhere between MW & M31 Total Mass ~ 1.3 x 10^12 Msun The Local group is the part of the Virgo Superclusters -> The evolution of Milky Way should be interpreted in terms of galaxy evolution in galaxy group

6. Why the Galaxy Group? Groups are the most common galaxy environment About the 70% of galaxies in local universe reside in galaxy group In local universe, the stellar populations are resolved From the studies of resolved stellar population, we can derived galaxy evolution -> 1. Why the galaxy groups are dominant in local universe ? 2. How about the properties of resolved stellar population in galaxy group environment ?

7. Why the Galaxy Group? Groups are the most favored environment for galaxy mergers Galaxy density : similar to the core of galaxy cluster Velocity dispersion : ~ 250 km/s for both compact & loose group The perfect environment of galaxy interaction Mamon (2000) concluded that the merger rate is 100-fold higher in compact groups than in rich clusters -> How the galaxy interaction affected to galaxy evolution ?

8. Local Group Study Makarov & Karachentsev (2011) presented an all-sky catalog of nearby galaxy groups (z ~ 0.01) Mean projected radius ~ 270 kpc virial mass ~ 2.4 x 10^12 Msun

9. Compact Group Studies The first example : Stephan’s Quintet Arp (1966) contains several groups Rose (1977) and Hickson (1982) produced the first catalogs of compact group Several compact group catalogs based on SDSS : e.g. > Lee et al. 2004 McConnachie et al. 2009

10. Compact Group Studies Star clusters in compact group : Star cluster survey for HCG 7, 16, 22, 31, 79, 87, 90, 92 ( OTL ) have done using HST data. Some HCGs, which underwent galaxy interaction, shows strong star cluster formation activities AGN statistics in compact groups : Shimada et al. (2000) presented that HCGs does not show any preference for AGN Martinez et al. (2010), however, reported that large fraction (~42%) of HCGs are AGNs

11. Problems of group studies Hard to draw the statistical properties Galaxy groups have mostly less than 10 member galaxies In order to draw the statistical information similar to galaxy clusters, we need much more observation Also, each galaxy group have characteristic environment (e.g. evolutionary stage), so we cannot just compile the each galaxy group properties to draw the general properties.

12. Problems of group studies Hard to draw the statistical properties

13. Problems of group studies Membership contamination by foreground or background galaxy The most famous example of foreground contamination of Stephan’s Quintet Only 60% of original HCGs are actually physically relavant galaxy group According to our preliminary investigation on galaxy group catalog of SDSS DR7, only 100 among 7,700 groups are confirmed as genuine group

14. Group Study strategy 1. Solution for exist problems Make the new & large group catalog Large area survey Deep photometric observation : galaxy group environment for high redshift region in order to divide group evolutionary stage -> Large FoV, good resolution, ability for survey Make the ‘genuine’ group catalog Need redshift information for all member galaxies -> ability for multi-object spectroscopy

15. Group Study strategy 2. Understanding of galaxy properties in group Star formation activity Bar fraction AGN fraction

16. Galaxy Interaction The most important issue for galaxy group is the high interaction rate between member galaxies How the galaxy interaction affect to galaxy evolution ? 1. Through the star formation It has been known that galaxy interaction induced strong star formation ( cf. Di Matteo et al. 2007 ) 2. Bar structure formation Tidal perturbation by neighbor galaxy may induce bar structure ( cf. Miwa & Noguchi 1998, Berentzen 2004 ) 3. AGN - host galaxy connection Galaxy interaction may produce the gas inflow to the center of galaxy, which induce the AGN ( cf. Storchi - Bergmann 2008 )

17. Group Study strategy 2. Understanding of galaxy properties in group Star formation activity high resolution photometry (?) observation for several star formation indicator (e.g. H alpha, IR observation ) Bar fraction high resolution to resolve the bar structure AGN fraction high resolution spectroscopy, X-ray observation

18. Group Study strategy Demands : Large FoV for survey -> LSST ability for multi-object spectroscopy -> SDSS-III, GMT High resolution for both imaging & spectroscopy -> LSST, GMT Various filter systems including IR -> WISE, GMT, etc Combination of several future instrument will give us great chance to expand group studies

19. Group Study strategy 1. Make the new catalog using LSST & SDSS-III Let’s find galaxy group from the imaging data from LSST It is WIDE, DEEP, FAST survey project ! Scanning the whole southern sphere in 3 days Possible to detect high z galaxy group ( even beyond z ~ 3) However, there is no spectroscopic data, so we need to use phot-z for group membership confining SDSSLSST

20. Group Study strategy 1. Make the new catalog using LSST & SDSS-III Let’s find galaxy group from the imaging data from SDSS-III SDSS-III will collect data from 2008 ~ 2014 It will collect two million spectra for the northern hemisphere

21. Group Study strategy 2. Compensate redshift information Using several telescopes with multi-object spectroscopic instruments NIRMOS for GMT : 7’x7’ FoV / R ~ 3,000 -> enough to get spectra of all member galaxy for single exp. GMOS : 5.5’ x 5.5’ FoV / R ~ 2,500 Using several telescopes with multi-object spectroscopic instruments This process is severe to select genuine group catalog

22. Group Study strategy 3. Investigation of group properties Obtain the morphological information and bar fraction from the high resolution imaging from LSST In local universe, galaxy groups contains more early type galaxies than late type galaxies -> Is this a result of continuous merging of galaxies in group environment? Is bar fraction in galaxy group higher than other environments? local groupHCGsFieldGroupCluster 28 (61) %25%17%12%25%

23. Group Study strategy 3. Investigation of group properties AGN population statistics If the AGN triggered by galaxy interaction, high AGN fraction of galaxy group is required, but it is controversial with HCG samples More spectroscopic survey data is useful to investigate AGN fraction Other wavelength data is also very helpful to study AGN population ( Chandra - X-ray AGN fraction / FIRST - Radio loud AGN fraction )

24. Group Study strategy 3. Investigation of group properties AGN population statistics Referencespure AGNAGN + TO Martinez et al. (2010) 45%48% Sohn et al. (2011, in prep.) 40%63%

25. Group Study strategy 3. Investigation of group properties Star formation activities How many galaxy group shows active star formation which triggered by galaxy interaction How many stars or star clusters formed in group? Do the stellar population of galaxy group differ from the field or cluster?

26. Group Study strategy 3. Investigation of group properties Star formation activity Are these typical star cluster distribution of groups? SQ HCG31

27. Group Study strategy 3. Investigation of group properties Star formation activities ? Despite of higher interaction ratio, not many starburst galaxies are reside in galaxy group ( SDSS + WISE ) How about the high-z group? or southern hemisphere group? 텍스트

28. Summary Galaxy group studies still do not have enough sample for statistical interpretation Large survey for both imaging & spectroscopic will give us a great opportunity to study galaxy group Making a new, large galaxy group catalog Understanding an evolution of galaxies in group environment 1. morphology evolution & Bar fraction 2. AGN fraction 3. Triggered star formation activity by galaxy interaction?