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Stephan’s Quintet (SQ):A Multi-galaxy Collision

C. K. Xu

IPAC, Caltech

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

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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!

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6563 km/s786 km/s

5765 6620 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:

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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

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Trinchieri et al. (2003, A&A, 401, 173)

X-ray (0.5-3 keV) on H imageX-ray (Chandra) on B-band image

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[OI]

[OI][SII]

H/[NII]

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”

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R-bandISOCAM 15m

A starburst in the IGM -- A product of high speed (900km/s) galaxy-IGM collision

(Xu et al. 1999, ApJ 512, 178)

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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)

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[OI]

[OI][SII]

H/[NII]

H/[NII]

[SII]

H/[NII]

[SII]

Shock frontRegions:

SQ-A region(IGM starburst)

Blue contours: H (5700 km/sec comp.)Red contours: H (6600 km/sec comp.)

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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 (107 yr) too long (108 yr)

• age of the starburst: 107 yr OK no

• spatial link to the shock yes no

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6600 km/s HI

6000 km/s HIH2

H2

Bang!!!

~107K

Jog & Solomon (1992, ApJ 387, 152) model:

IGM intruder

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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

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HI maps(VLA C/D)

6600 km/sec

6000 km/sec

5700 km/sec

Williams et al.(2002, AJ, 123)

total

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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

LFUV (total)= 3.5 1010 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

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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)

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New scenario:(Xu et al. 2005ApJL 619, 95)7319/7318ainteraction ->

Old theory:(Arp & Kormendy1972, 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 (1-2 108 yr)

old tail (5-9 108 yr)

N7319

N7318a

young tail

6620 km/s

6583 km/s6550 km/s

5900 km/s

105 kpc

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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

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X-ray (XMM) contours, r-band image

R-band contours, XMM image

Trinchieri et al. 2005, A&A, in press

Connection of N7317with the group:In a large R band and X-ray halo.

X-ray colors:R: 0.3-1.5 keVG: 1.5-2.5 keV B: 2.5-6.0 keV

N7317 (E)

terminal shock?

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One word summary: • we are just at the beginning of understanding SQ!

Remaining Questions:• Fate of the IGM gas (~1010 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) ~108 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 (AV~5). • A challenge to simulators: Can this extremely complex system be eventually simulated?