fr Image T and h A NGC NGC Figu ttp eam om CS/WF cour ://h re Hib , 7252. 7469, of tak 1. u te bar 4 NGC C bb e sy Gal n image NGC Nor le d of Interacting and Merging Galaxies f site.org/n e rom axy 7252 t NASA, th al. s 4676 is s (c h ample (1994). i ttp: up ou s r an H. , rtes e e //h an stored wsc d F in d y ord Ar ub en eas of thi p bles ter/arc , f NASA, G. 299. t rom s is i s Illi te tu to . B Bottom dy org/news hiv ngw t - th he . and e/releas e T orth, le Hu op f 12 R t. ro bb an -ban ce M. M w es le n d os fr te /gal C d Heritage, midd t om Figure 1.4 Galaxy sample in this study. Top and middle row from left to right: Arp 256, r l am /arc images colored

axy/in NGC 7469, NGC 4676 and Arp 299. Bottom row from left to right: IC 883, NGC 2623 and l p e le f h in,

t NGC 7252. North is up, and east is to the left. Most colored images are restored from HST iv ro to e G. tak ACS/WFC images (courtesy of NASA, the Hubble Heritage, A. Evans and ESA, taken from w A. i t /r m ri e ractin f Hartig, e ages http://hubblesite.org/newscenter/archive/releases/galaxy/interacting/2008/16/image/a/, gh en Ev rom l e t: ase and courtesy of NASA, H. Ford, G. Illingworth, M. Clampin, G. Hartig, and the ACS Science an w ar i IC l s g/2008/16/image th s/2002/11/image Team, taken from http://hubblesite.org/newscenter/archive/releases/2002/11/image/d/). e e and f and t 883, CTIO r Image of NGC 7252 is restored from B- and R-band images taken with CTIO 4m telescope e to stored

ES from Hibbard et al. (1994). the ri NGC gh A, 4m A t: tak CS f rom teles 2623 Ar Figure 1.4 Galaxy sample in this study. Top and middle row from left to right: Arp 256, e Some galaxies don’t fit the elliptical/spiral/irregular classification. Scie n p

H 12 /d /a/, NGC 7469, NGC 4676 and Arp 299. Bottom row from left to right: IC 883, NGC 2623 and from cop 256, an n S /).

c NGC 7252. North is up, and east is to the left. Most colored images are restored from HST T d e e ACS/WFC images (courtesy of NASA, the Hubble Heritage, A. Evans and ESA, taken from http://hubblesite.org/newscenter/archive/releases/galaxy/interacting/2008/16/image/a/, and courtesy of NASA, H. Ford, G. Illingworth, M. Clampin, G. Hartig, and the ACS Science Team, taken from http://hubblesite.org/newscenter/archive/releases/2002/11/image/d/). Image of NGC 7252 is restored from B- and R-band images taken with CTIO 4m telescope from Hibbard et al. (1994).

Figure 1.4 Galaxy sample in this study. Top and middle row from left to right: Arp 256, NGCFigu7469,re 1.NGC4 Gal4676axy ansampled Arpin299.thisBottomstudy. roTopw franomd middleft toleriroghwt:fromIC 883,left NGCto righ2623t: Aranpd256, NGC 7469, NGC 4676 and Arp 299. Bottom row from left to right: IC 883, NGC 2623 and NGC 7252. North is up, and east is to the left. Most colored images are restored from HST 12 ACS/WFNGC C7252.imageNors (cthouisrtesupy, anofdNASA,east isthtoetHuhe bbleflet. Heritage,Most coloredA. Evimanagess andareESreA,storedtakefnromfromHST httpA://hCS/WFubbleCsite.org/nimages (cewscourtesenter/arcy of NASA,hive/releasthe Huesbb/galleaxy/inHeritage,teractinA. Evg/2008/16/imageans and ESA, tak/a/,en from andhcourttp://htesyuofbbNASA,lesite.org/nH. Fordewsc, G.enter/arcIllingwhivorth,e/releasM. Clesam/galpin,axy/inG. Hartig,teractinandg/2008/16/imagethe ACS Science/a/, Teamand, takcourentefromsy of hNASA,ttp://hH.ubFblesordi,teG..org/newsIllingworth,centeM.r/arcClamhivpein,/reG.leaseHartig,s/2002/11/imageand the ACS/dScie/). nce ImageTeamof NGC, take7252n fromis rhestoredttp://hfubromblesBi-teand.org/newsR-banced nimagester/arctakhivene/rweliethaseCTIOs/2002/11/image4m telescop/de /). fromImageHibbarofdNGCet al.7252(1994).is restored from B- and R-band images taken with CTIO 4m telescope from Hibbard et al. (1994).

Figure 1.4 Galaxy sample in this study. Top and middle row from left to right: Arp 256, NGC 7469, NGC 4676 and Arp 299. Bottom row from left to right: IC 883,12NGC 2623 and 12 NGC 7252. North is up, and east is to the left. Most colored images are restored from HST ACS/WFC images (courtesy of NASA, the Hubble Heritage, A. Evans and ESA, taken from http://hubblesite.org/newscenter/archive/releases/galaxy/interacting/2008/16/image/a/, and courtesy of NASA, H. Ford, G. Illingworth, M. Clampin, G. Hartig, and the ACS Science Team, taken from http://hubblesite.org/newscenter/archive/releases/2002/11/image/d/). Image of NGC 7252 is restored from B- and R-band images taken with CTIO 4m telescope from Hibbard et al. (1994).

12 How Galaxies Collide

If galaxies move away from each other as the universe expands, how can they ever collide?

Interacting Galaxy UGC 9618

The gravitational attraction of two massive galaxy halos can locally reverse the expansion and cause a collision.

Most interacting pairs probably fell together “recently”. 1972ApJ...178..623T Tides between disk galaxies create filaments of stars.

SPIN

–0.5 0 0.5 1

1.5 2 2.5 3

Galactic Bridges and Tails The Mice: Two Colliding Spirals

"The Mice": Colliding Galaxies

NGC 4676: True-Color RGB Image Simulation of the Mice

The Mice at Play Simulation of the Mice

The Mice at Play Orbit Decay

+

1. Spherical halos approach Orbit Decay

+

1. Spherical halos approach 2. Tidal forces stretch halos Orbit Decay

+

1. Spherical halos approach 2. Tidal forces stretch halos 3. Elongated halos are torqued Orbit Decay

+

1. Spherical halos approach 2. Tidal forces stretch halos 3. Elongated halos are torqued 4. Halos spin up as orbit decays Why do Galaxies Merge?

Tidal forces transform the organized orbital motion of galaxies into random motions of stars and dark matter.

• This is a form of friction — it slows galaxies down.

• Dark matter plays critical role — absorbs momentum. What Kind of Galaxy Results from a Merger?

Random stellar orbits can naturally account for the oval shapes and slow rotation of elliptical galaxies.

• Merger hypothesis: spiral galaxies merge to form elliptical galaxies.

• Estimated merger rates can produce right number of elliptical galaxies.

• Need additional star formation in mergers to form cores of elliptical galaxies. The Antennae

Super Star Clusters in the Antennae Galaxies Rapid star formation is common in merging spiral galaxies! NGC 4038/4039 Simulation of the Antennae

Antennae Simulation With Star Formation Antennae Simulation With Star Formation A Recent Merger Remnant: NGC 7252

THE [O III]NEBULA OF NGC 7252 3

Figure 1. Groundbased B and narrow-band [O III]andHα images of NGC 7252 obtained with the du Pont 2.5-m telescope. (a)High-contrastdisplayofB image, showing 6.!2 × 5.!7(120× 110 kpc2)fieldofview;theboxmarksthe3.!1 × 2.!85 (60 × 55 kpc2)areadisplayedinpanels(b)–(d).Noticethetwotidal tails connecting to loops around the main body. (b)PortionofB image displayed at twice enlarged scale relative to Panel (a). (c)and(d)Continuum-subtracted [O III] λ5007 and Hα emission-line images, reproduced at the same scale as Panel (b) and showing the distribution of ionized gas. Notice the [O III]nebulaSW of the nucleus in Panel (c). (e)and(f)Same[OIII]andHα images as in panels (c) and (d), but twice enlarged and reproduced at lower contrast. Details of the [O III]nebulaandoffiveHII regions in the W loop are better visible. Notice also the brightly Hα-emitting disk centered on the nucleus (black circular area). This ionized- and molecular-gas disk corresponds to the central “minispiral” observed on HST images; for details, see text. the area shown 2× enlarged in Figures 1b–1d.Thebottom ∼ 14 kpc west of the nucleus, best visible in the same two Hα two panels are enlarged 2× more, showing a region half the panels. size (and one quarter the area) of the box drawn in Figure 1a. The [O III]nebulosity(hereafter‘[OIII]nebula’)coversa Both the [O III] λ5007 and the Hα images had the galaxy con- projected area of about 10.1 × 6.6kpc2 (31.!!5 × 20.!!6) in the tinuum subtracted as described above. They are displayed at east–west and north–south directions, respectively, and ap- relatively high contrast in the middle two panels, and at lower pears brightest at a point lying about 4.0 kpc (12.!!4) west and contrast in the bottom two panels to show more details. No- 3.6 kpc (11.!!2) south of the nucleus. The apparent structure of tice the striking [O III]emissionnebulosityslightlybelowthe this [O III]nebulaisdiscussedinmoredetailin§3.1below. centers of panels (c) and (e), the bright central disk of Hα emission surrounding the nucleus of the galaxy (circular black areas in panels d and f), and the string of five H II regions 2.2. Spectroscopy NGC 7252: Data vs. Model

M = 21.22 M = 20.68 B B U B = 0.17 U B = 0.16 B V = 0.66 B V = 0.60 V R = 0.74 V R = 0.52 NGC 7252: Star Formation History Starburst Galaxies

Starburst Galaxy M82

Star formation rate: ~10 × Milky Way’s.

Gas outflow driven by supernovae

Galaxy Wars: M81 versus M82 Arp 299: Supernova Factory

First encounter ~700 Myr ago

Ultra-Luminous Infrared Galaxy 12 (L > 10 L⊙):

Lopt ≈ 0.01 Lir

Interacting Galaxy NGC 3690 Galaxies: Spectral7.1 Energy Groups: the Distributions homes of disk galaxies (SEDs) 291

Starlight “Hot” Dust Far IR Visible Mid. IR Near IR Near UV

Fig. 7.7. Normal elliptical and disk galaxies are brightest in the visible and near-infrared, at λ < 2 µm. MostDust dust peak grains areat λ cooler ~ 50 than μ 30m K, ⇒ and T their ~ 60 emission K peaks beyond 100 µm. In the starburst M82 and the ultraluminous infrared galaxy, dust intercepts far more of the light, and it is hotter, radiating mainly at λ < 100 µm. See Figure 2.24 for details of emission lines – ISO: P. Chanial and G. Lagache.

the rate

LFIR 1 ˙ " yr− . (7.11) M ∼ 6 109 L M# × # 13 So the most powerful ULIRGs, radiating LFIR >10 L , give birth to 1000 # ∼ M# of new stars each year. About half of these have∼ an active nucleus (see Section 9.1) in addition to a ferocious starburst. Arp 220, a merging pair of galaxies with characteristic tidal tails, is only 1 75 Mpc away, giving us a close-up view of a ULIRG. It is making 200 yr− ∼ M# of new stars, but only a few percent of their light escapes directly. The rest is 12 absorbed by dust, so we see LFIR 1.5 10 L . Radio and infrared observations ∼ × # that can see through the dust reveal two nuclei, each surrounded by a gas disk 8 3 3 20 pc in radius with 6 10 of gas at densities n(H2) 10 cm− . These ∼ × M# ∼ in turn are embedded in a kiloparsec-sized disk with >109 of dense gas. M# Typically a ULIRG contains (5–10) 109 of dense molecular gas, mainly × M# in a rotating disk or ring in the central kiloparsec. The Milky Way has half as much gas (see Table 2.4), but it is spread over ten times the area and forms stars a hundred times more slowly. Within a gigayear after two disk galaxies have merged, the excitement is mostly over. The hot massive stars made from gas compressed during the merger Arp 220: Merger Remnant

Core contains as much gas as entire Milky Way!

A Collision In The Heart Of A Galaxy

Star formation rate: ~100 × Milky Way’s!

Active nucleus as well as stars.

Interacting Galaxy Arp 220