Galaxy groups & interactions: •! Galaxies are social creatures, almost all are found in pairs, groups, and clusters •! Groups have < 50 galaxies, sizes ~1-2 Mpc, ! ~ 100-500 km/s •! In contrast, clusters have 50 – several thousands of galaxies, sizes ~ few Mpc, ! ~ 700-1200 km/s •! Nearby groups: –! The Local Group! –! M81 group –! Sculptor group

•! Groups have HI gas and x-ray halos

Neighborhood of the Milky Way

Properties of groups: •! Prototypical group has 3-6 bright members and 30-40 faint ones •! Velocity dispersions ~100-500 km/s •! X-ray luminosities ~ 1042-1043 erg/s •! M/L ~200 !!! •! Groups show morphological segregation, spirals and irregulars tend to lie on the edges of the groups, dE’s and dSph’s are companions to massive galaxies •! Groups are dominated by spirals and irregulars

Map of the Local Group

Local Group to scale

The M81 Group

The Leo I (or M96) Group

X-ray gas in groups: •! In denser groups, we observe diffuse, ionized gas in the x-ray (free-free or Bremsstrahlung radiation) with temperatures of ~107 K •! Gas is probably a mixture of material –! Gas that never formed into galaxies –! Metal enriched gas that escaped from galaxies

•! Gas is confined by the gravitational pull of the group •! The virial theorem then implies that groups must have mass to light ratios of M/L ~ 150-500 –! More dark matter!! –! Either the individual galaxies halos extend much farther out or (more likely) there is mass between the galaxies forming a group gravitational potential

Compact groups: •! ~6 galxies in a very small volume, typical separations of only 20-40 kpc •! Compact groups are rare •! These can’t last long! Why not?

Stephan’s Quintet w/Chandra

Galaxy interactions:

Tidal tails

•! Not all galaxies fall on the Hubble sequence, many are peculiar! •! Arp made an Atlas of Peculiar Galaxies in 1966 based on the Palomar Sky Survey, later extended to the Southern hemisphere in 1982 •! What did he see – –! –! –! –! –! –! –! –!

Grossly distorted morphologies Tidal tails Polar rings around ellipticals Ring-shaped galaxies Rings of dust and/or gas Warps in spiral disks Shells and ripples around ellipticals Accretion of dwarf galaxies

Montage of interacting galaxies from John Hibbard

Dynamical friction: •! As a massive galaxy moves through a “sea” of stars, gas, (and the dark halo), it causes a wake behind it increasing the mass density behind it •! This increase in density causes the galaxy to slow and lose kinetic energy •! The galaxy will eventually fall in and merge with it’s companion HI contours over false color optical images Montage of interacting galaxies from John Hibbard

Dynamical friction

Dynamical friction: #G 2 M 2 " & Fdyn = C% ( 2 $ v m ' where –! C=depends on structure of both galaxies –! M=mass of galaxy falling in ! –! V=velocity of galaxy falling in –! "=density of stars (surrounding material)

•! Implications: –! The slower the galaxy’s speed, the stronger the dynamical force, the more intense the interaction –! The more massive the object, the greater the effect –! What does this mean for globular clusters vs. satellite galaxies? Galaxies in groups vs. galaxies in clusters?

Dynamical friction:

Dynamical friction: •! How long does it take? Consider the LMC falling in towards the Milky Way: –! For a DM halo that produces a flat rotation curve,

"(r) = –! Then,

•! How long does it take? Consider the LMC falling in towards the Milky Way:

–! Then,

v2 4 #Gr 2

L = Mvr (for a circular orbit) dL dr CGM 2 = Mv = r " Fdyn = #r " dt dt 4$ r 2

–! The angular momentum,

!

2

Fdyn =

!

CGM 4" r 2

!Integrating both sides,

–! The torque, #, is " = r # Fdyn but remember that torque is the rate of change of angular dL !momentum,

"=

rdr = "

dt

0

$ rdr = " R

!

–! Arrive at a merging time

!

CGM dt 4# v

CGM 4# v

T

$ dt 0

2" vR 2 T= CGM

! !

!

Dynamical friction: •! How long does it take? Consider the LMC falling in towards the Milky Way: 2

T=

2" vR CGM

–! For the LMC •! •! •! •! •!

M=2 x 1010 M! V=220 km/s R=50 kpc C=23 Merging time T =1.7 Gyr !!

!

–! Actually if we assume an elongated orbit, the answer is a much longer merger time (~5 Gyr) –! We can’t watch mergers happen, so how do we study them instead?

Galaxy interactions: •! We study galaxy interactions numerically via N-body simulations, attempting to include the affects of gas, star formation, and dark halo mass •! The field is heavily influenced by the 1972 paper of Toomre & Toomre who were able to reproduce many of the observed features of interacting galaxies with very simple simulations (only a few 100 particles, no gas, no dark matter) •! Some terminology (things that affect the observed interactions): –! Major merger – two similar mass galaxies, gives rise to tidal tails –! Minor merger – a satellite (dwarf?) galaxy merging with a larger massive galaxy, makes bridges, also tidal stripping –! Retrograde – galaxy is rotating in opposite direction of velocity of “intruder” –! Direct – galaxy is rotating in same direction as velocity of “intruder” –! Impact radius – distance between center of galaxy and intruder –! Inclination angle between galaxy and intruder –! Viewing angle – our line of sight to the merger

Retrograde encounter Toomre & Toomre (1972)

Effect of impact radius Toomre (1978)

Direct encounter Toomre & Toomre (1972)

Toomre & Toomre (1972) The Antennae

Chandra

M51 – the Whirlpool

Model of M51 interaction Toomre & Toomre (1972)

Starburst activity: •! •!

As we have seen, both direct mergers and more indirect interactions can trigger star formation in galaxies Caused by gas coming together, causing shocks which trigger star formation –! M81 group

•! •!

Gas which loses enough angular momentum will fall into the center (especially true if a bar is formed) These can lead to strong nuclear starbursts –! M82 is currently forming a few M!/year of stars (similar to a large spiral) in a nuclear area only 100 pc across! –! Starburst phases are short. Why?

•! •!

Powerful starbursts surrounded by dust will be very bright in the infrared We observe numerous ultraluminous infrared galaxies (ULIRGs), first discovered by the IRAS satellite, with L > 1012L! –! These galaxies are merging too!

M82

The Tadpole w/ HST/ACS

Toomre & Toomre The Mice

The Mice with HST/ACS

Simulation of the Mice

HI gas around Cartwheel

Multiwavelength Cartwheel