1. 1 Stephan’s Quintet (SQ): A Multi-galaxy Collision C. K. Xu IPAC, Caltech

2. 2 “Birth Certificate”: M.E. Stephan, 1876, CR Acad. Sci. Paris vol. 84, p641.

3. 3 Why is SQ interesting? (1)It looks fantastic in every waveband. (2)It reveals surprises every time being looked at by a new instrument. (3) Behind all the spectacular images and strange spectrographs, lies a very complex web of galaxy-galaxy and galaxy-IGM interactions. (4) It shows all kinds of interaction induced phenomena, including a large scale shock (~40 kpc), an IGM starburst, long tidal tails with tidal dwarf candidates, and a type II AGN. (5) How useful is the knowledge gained in studying this local (94 Mpc) system to our understanding of those multi-galaxy systems afar, such as multi-nuclei ULIRGs and multi-mergers in deep surveys? Who cares!

4. 4 6563 km/s786 km/s 57656620 km/s 6550 km/s 6583 km/s (Sbc sy2) (S0/a) (E) (Sbc) (E) (Sd) Gallagher et al. (2001, AJ 122,163) SQ seen in deep R band:

5. 5 1.4 GHz (VLA B-Array) Radio Continuum --- A gigantic shock front (~40 kpc) in the intragroup medium (IGM) Allan & Hartsuiker 1972, Nat. 239 1.4 GHz Westerbork Xu et al. 2003, ApJ 595, 665

6. 6 Trinchieri et al. (2003, A&A, 401, 173) X-ray (0.5-3 keV) on H  image X-ray (Chandra) on B-band image

7. 7 [OI] [SII] H  /[NII] [SII] Xu et al. 2003, ApJ 595, 665 Blue contours: H  (5700 km/sec comp.) Red contours: H  (6600 km/sec comp.) Spectroscopic confirmation of shock excitation: double spectrograph, Palomar 200”

8. 8 R-band ISOCAM 15  m A starburst in the IGM -- A product of high speed (900km/s) galaxy-IGM collision (Xu et al. 1999, ApJ 512, 178)

9. 9 Blue contours: H  (5700 km/sec comp., intruder) Red contours: H  (6600 km/sec comp., IGM) IGM starburst (“SQ-A”) (Xu et al. 1999, ApJ 512, 178)

10. 10 [OI] [SII] H  /[NII] [SII] H  /[NII] [SII] Shock front Regions: SQ-A region (IGM starburst) Blue contours: H  (5700 km/sec comp.) Red contours: H  (6600 km/sec comp.)

11. 11 SQ-A: A collision triggered starburst (Xu et al. 1999; 2003), or a tidal dwarf (Plana et al. 1999, ApJL 516, L69) ? Comparison: Obs. facts collision tidal dwarf 2 velocity (6600/6000 km/s) yes no (IGM/intruder) time scale OK (10 7 yr) too long (10 8 yr) age of the starburst: 10 7 yr OK no spatial link to the shock yes no

12. 12 6600 km/s HI 6000 km/s HI H2H2 H2H2 Bang!!! ~10 7 K Jog & Solomon (1992, ApJ 387, 152) model: IGMintruder

13. 13 SQ-A: CO velocity Lisenfeld et al. (2002, A&A 394, 823) (IRAM 30m) BIMA, Gao & Xu, 2000, ApJL, 542, L82. Molecular gas (CO) in the IGM starburst region (SQ-A): intruder IGM

14. 14 HI maps (VLA C/D) 6600 km/sec 6000 km/sec 5700 km/sec Williams et al. (2002, AJ, 123) total

15. 15 GALEX image (blue: FUV, yellow: NUV) Xu et al. 2005, ApJL 619, L95 star formation rate tidal features (tidal dwarf candidates, or tidally induced star formation regions). size of the 7318b UV disk: ~80 kpc SQ-tip L FUV (total)= 3.5 10 10 L  (ext. corr.) SFR (total)=6.7 M  /yr SFR(SQ-A)=1.3 M  /yr (20% of tot) SFR(N7319)=2.0 M  /yr (tail: ~15%) SFR(N7318b)=3.4 M  /yr FUV contours on R image old tail

16. 16 IRAM interferometer CO (beam=4.3x3.5”) Lisenfeld et al. (2002, A&A 394, 823) IRAM 30m CO (red) on 15um contours Molecular gas in tidal tail: SQ-A SQ-B (tidal dwarf) Lisenfeld et al. (2004, A&A 426, 471)

17. 17 New scenario: (Xu et al. 2005 ApJL 619, 95) 7319/7318a interaction -> Old theory: (Arp & Kormendy 1972, ApJL 178, 111; Moles et al 1997, ApJL 485, 69): two parallel passes of N7320c (the old intruder) Interaction history (before the intruder): young tail (1-2 10 8 yr) old tail (5-9 10 8 yr) N7319 N7318a young tail 6620 km/s 6583 km/s 6550 km/s 5900 km/s 105 kpc

18. 18 Toomre & Toomre (1972) equal mass encounter, t = 200 Myr Indeed NGC7319 and 7318b have nearly equal mass: NGC7319: K=10.0 mag NGC7318a: K=10.3 mag FUV contours on a R-band image

19. 19 X-ray (XMM) contours, r-band image R-band contours, XMM image Trinchieri et al. 2005, A&A, in press Connection of N7317 with the group: In a large R band and X-ray halo. X-ray colors: R: 0.3-1.5 keV G: 1.5-2.5 keV B: 2.5-6.0 keV N7317 (E) terminal shock?

20. 20 One word summary: we are just at the beginning of understanding SQ! Remaining Questions: Fate of the IGM gas (~10 10 M  ): falling back to parent galaxies (ULIRG-to-be?), or form new baby galaxies (tidal dwarfs)? reason for the huge (80kpc) UV/HI disk of the intruder: a head-on collision with 7318a (E) ~10 8 yrs ago (‘ring galaxy’ scenario)? will the ‘intruder’ N7318b be eventually captured by the group? secular evolution of galaxies in SQ: were 7318a (E) and 7320c (S0) late type galaxies ~ 1 Gyrs ago? Will N7319 (Sbc) evolve into a early type soon (it has no HI gas in the disk)? How is the type II AGN triggered? Is there any circum-nuclear starburst associated with the type II AGN? Need high resolution (sub-arcsec) IR observations because of the very high extinction (A V ~5). A challenge to simulators: Can this extremely complex system be eventually simulated?