Top Five Lessons Learned from the Colliding Antennae Galaxies Brad Whitmore December 1, 2011 Hubble Science Briefing Space Telescope Science Institute

ApJ 57, 137 (1923) CTIO B,V,R Image courtesy of John Hibbard Viewgraph courtesy of B. Rothberg. How did it get its name? 2

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1. Galaxies can merge (tidal tails are signature) 4

Movie Credits: Frank Summers (STScI), Chris Mihos (Case Western Reserve Univ.) & Lars Hernquist (Harvard) Download movie here: 5 “Cosmic Collisions” A Computer Simulation of Galaxies Colliding 5

2. When spirals merge they can form elliptical galaxies In 1977 (following up 1972 suggestion) Alar Toomre made the bold hypothesis that the remnants of merging galaxies are ellipticals. Graphic shows that if merger remnants are elliptical galaxies, there should be about 750 ellipticals in the NGC catalog, which there are ! 6

Toomre, “The Evolution of Galaxies and Stellar Populations”, 1977 NGC 4038/9 is the youngest and nearest galaxy in the Toomre sequence of merging galaxies, hence perhaps our best chance for understanding the formation of massive compact star clusters in mergers. 7

Artist’s rendering of what our night sky might look like at different stages of a merger with the Andromeda galaxy. Brad Whitmore STScI, July 28, Credit: James Gitlin/STScI

Sometimes you don’t know where your research will show up next. Brad Whitmore STScI, July 28,

3. Much of the star formation in mergers is in the form of young massive clusters Antennae - Whitmore et al. (1995, 1999, 2010) see also: NGC Holtzman et. al., 1992 NGC 7252, Whitmore & Schweizer 1993) 10

A daring prediction comes true in NGC Schweizer 1987 “It is suggested that galactic collisions or mergers may lead to globular-cluster formation as well as star formation.” Schweizer 1982 HST CTIO 4-Meter 11

Question - Why did it take HST to make this discovery ? Answer - Spatial resolution ! Ground-based Hubble - (ACS) 12

Perhaps the most convincing examples are these ~300 Myr clusters in the NW extension. NOTE - masses are 320,000, 270,000, and 260,000 Solar Masses ! 4. Some of these super star clusters have all the attributes expected of young globular clusters 13

10 Myr >>> wait 12 billion years >>> 12 Gyr Region S in the Antennae Globular cluster in NGC

Historically, young massive clusters were first found in merging galaxies. However, they have now been found in essentially all types of star-forming galaxies. 5. Cluster formation/destruction may be a “universal” phenomena NGC 1275 (giant elliptical) NGC 4214, M82, M83 (dwarf, peculiar, starburst) M51 (interacting/spiral galaxies) 15

Clusters formed in all star forming galaxies have similar luminosities, ages, and a wide range of other properties. > the formation (and destruction) of star clusters is a “universal” phenomena. Hence, while we started the project to better understand colliding galaxies, we ended up by learning about the properties of star formation in all galaxies. luminosity functionsage distributionsbrightest vs. number 16

Summary 1. Galaxies can merge (tidal tails are the signature). 2. When spirals merge they can form elliptical galaxies. 3. Much of the star formation is in the form of young massive clusters. 4. Some super star clusters have attributes of young globular clusters. 5. Cluster formation/destruction may be a “universal” phenomena., 17

BONUS – How to find color images of 87 merging galaxies in the Hubble Legacy Archive in less than 60 seconds Welcome to the Hubble Legacy Archive 18

BONUS – How to find color images of 87 merging galaxies in the Hubble Legacy Archive in less than 60 seconds 1. Type: hla.stsci.edu in the box at the top of your web browser and hit return. 2. Click on “Enter Site here”. 3. Type Arp 256 in the box and hit “Search” (i.e., start with a famous object). 4. Scroll over and notice that this is from proposal ID = (for HST/WFC). 5. Go to the top and click on “Reset”, then “advanced search”. 6. Enter in the Proposal ID box, and “Level 4” in the box under “Data Product” to just get color images. 7. Click on “Search”. 8. Click on the “Images” tab. Pick an object and click on “Interactive Display”. 9. Practice moving the image around, zooming in, using the Lighter/Darker, etc. 19

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